Symposium ES02 : Next-Generation Intercalation Batteries

Lithium-rich (LR) layered oxides are among the most promising high-energy cathode candidates for next-generation Li-ion batteries LIBs, with excess capacity enabled by oxygen redox (OR). Recent studies show that their actual OR reversibility is largely determined by the local structural ordering, especially stacking faults. However, due to the challenge in quantitative determination of stacking faults, there has been intense debate on how stacking faults affect the OR in LR layered oxides. In this work, we made quantitative analysis of the dependence of stacking faults on the Li contents in Li1+x Ni0.13Co0.13Mn0.54O2 through synchrotron X-ray and neutron diffraction measurements coupled with structure refinements using FAULTS program. Results from the refinements using a composite 2-phase model (C2-m and R-3m phases) match well with experimental data, and so allowing us to make quantitative investigation on the the correlation between the probability of stacking faults and OR reversibility. Finding from this study, along with its implication to rational design of LR layered oxide cathodes will be discussed.

High capacity cathode material is key to meet high expectation on lithium ion battery for electrical transportation and smart grids. Cumulatively cationic and anionic redox has been identified as a promise mechanism for next generation cathode materials. The peroxo-/superoxo- like O-O bonding is believed to play important role in anionic redox. ¹However, fundamental understanding of the mechanisms such as the origin and reversibility of peroxo-/superoxo- like O-O dimer is still lacking². So far, theoretical modeling are limited and fail to show peroxo-/superoxo- like O-O dimers in delithiated phases ^{3, 4}, which has been detected during electrochemical performances^{5 6}.This talk will show the evolution of peroxo-/superoxo- like O-O dimers during the delithiation process with first-principles modeling. Several morphologies of transition metal peroxo/superoxo complex are found in the partially delithiated phase of Li_2-xMnO₃ (3/4≤x≤5/4) and oxygen release is predicted to happen when x≥3/2. There are two categories of O-O dimers, one links the MnO₆ in same Mn layer or adjacent Mn layers. In the other category, one or two O-O dimer(s) belong(s) to one MnO₆ octahedron. The bond length of O-O dimer ranges from 1.25 Å to 1.45 Å, close to the O-O bonds in peroxides and superoxides. Crystal orbital Hamilton population (COHP) analysis shows strong anti-bonding character near to Fermi level and partial charge density indicates π-like anti-bonding charge hole compensate the charge deficiency in delithiated phases during 3/4≤x≤5/4. Surprisingly, peroxo-like dimers are also found in the second lithium delithiation of polyanionic cathode Li₂CoSiO₄, challenging the long understanding that only two-electrons conventional cationic redox is recognized in orthogonal silicate cathode family. This study may open a new venture to understand the anionic redox, helpful to rational design of new generation cathode to achieve high energy-density.
1. G. Assat and J. M. Tarascon, Nature Energy, 2018, 3, 373-386.
2. S. K. Jung and K. Kang, Nature Energy, 2017, 2, 912-913.
3. M. Saubanere, E. McCalla, J. M. Tarascon and M. L. Doublet, Energy & Environmental Science, 2016, 9, 984-991.
4. D. H. Seo, J. Lee, A. Urban, R. Malik, S. Kang and G. Ceder, Nature Chemistry, 2016, 8, 692-697.
5. E. McCalla, A. M. Abakumov, M. Saubanere, D. Foix, E. J. Berg, G. Rousse, M.-L. Doublet, D. Gonbeau, P. Novak, G. Van Tendeloo, R. Dominko and J.-M. Tarascon, Science, 2015, 350, 1516-1521.
6. A. J. Perez, Q. Jacquet, D. Batuk, A. Iadecola, M. Saubanère, G. Rousse, D. Larcher, H. Vezin, M.-L. Doublet and J.-M. Tarascon, Nature Energy, 2017, 2, 954-962.

High capacity cathode materials are a key to the success of high performance lithium ion battery to emerging technologies for electrical transportation and smart grids. Accumulating cationic and anionic redox in transition metal peroxo/superoxo complex has been identified as the key mechanism to achieve superior reversible capacity in multi-lithium compounds. However its associated lattice oxygen releases are a fundamental threat to battery safety and long term performance stability. Here we present a prototype study of Co-Mn lithium silicate composites, in which Co-peroxo/superoxo redox is observed in tetrahedral structured polyanionic materials for high reversible capacity over 230 mAh/g accompanied by zero gas release. Two lithium reactivity and its structure evolution are carefully studied by in operando synchrotron X-ray absorption spectroscopy and powder diffraction. First-principles molecular dynamics are employed to provide the first of its kinds understanding of Co-peroxo/superoxo interaction in the domain of two lithium reactivity. Because of the importance of polyanoinic composition in the high safety category of lithium cathode, this study may open a new venture to design rationally a new generation cathode that will achieve high energy-density and high safety simultaneously.

To comply with the ever-growing demand of energy to power portable electronics and electric vehicles, increasing the energy density of lithium-ion batteries (LIB) has become a major bottleneck. Li-ion battery cathode materials have relied solely on cationic redox reactions, until the recently discovered anionic redox chemistry has become a new approach to design new cathode materials with high energy density, for example, Li_1.2Ni_0.2Mn_0.6O₂, and Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (Li-rich NMC), etc.^1–3 However, due to certain drawbacks, Li-rich oxides are still far away from commercialization. While intense research is currently devoted to understand this issue, we take a different strategy to contribute to the current understanding of anionic redox, by moving to Li-rich layered sulfides.
While the pure Li_1.33Ti_0.67S₂ was reported to be electrochemically inactive, we have designed new phases of nominal composition Li_1.33-2y/3Ti_0.67-y/3Fe_yS₂ with y = 0.1 – 0.5, that are electrochemically active and exhibit electrochemical activity comparable to Li-rich layered oxides. The voltage profile begins upon charge with a sloped oxidation, associated to the cationic Fe^2+/3+ oxidation followed by a plateau due to the contribution of the anionic 2S^2– / (S₂)^n– (n < 2) redox process. Maximum reversible discharge capacity up to 245 mAh/g could be obtained for the y = 0.3 (Li_1.13Ti_0.57Fe_0.3S₂) composition. Via detail study on Li_1.13Ti_0.57Fe_0.3S₂, we show a very small voltage hysteresis of compared to Li-rich NMC. Concerning the issue of kinetics, oxygen redox is found to be severely affected by sluggish kinetics. However in Li_1.13Ti_0.57Fe_0.3S₂, we found that the charge-transfer resistance is very small and remains nearly constant throughout the cycle as opposed to oxides where the resistance builds up drastically with deeper oxidation of oxygen. Despite these positive effects, voltage fading still persists in Li-rich sulfides, though the voltage fade is very less and tends to stabilize after few cycles. The voltage fade is merely ~40 mV in Li_1.13Ti_0.57Fe_0.3S₂, whereas in Li-rich NMC the fading is ~150 mV. Thus, in short we demonstrate that increasing the covalency of metal-ligand bonds by moving to Li-rich sulfides can show paths to mitigate practical bottlenecks of anionic redox.


References
1. Assat, G. & Tarascon, J. M. Fundamental understanding and practical challenges of anionic redox activity in Li-ion batteries. Nat. Energy 3, 373–386 (2018).
2. Yabuuchi, N. Solid-state Redox Reaction of Oxide Ions for Rechargeable Batteries. Chem. Lett. 46, 412–422 (2017).
3. Luo, K. et al. Anion Redox Chemistry in the Cobalt Free 3d Transition Metal Oxide Intercalation Electrode Li[Li 0.2 Ni 0.2 Mn 0.6 ]O 2 Oxidation of O 2− on charging is associated with the generation. J. Am. Chem. Soc 138, 12 (2016).

Batteries, as one of the most versatile energy storage device, are key enablers for the decarbonisation of both the transport and power sector. Li-ion batteries being light weight and capable of providing high energy density have conquered the transport sector, therefore, there is a sorely need for low cost batteries for grid applications linked to the use of renewable energies. In this regard, Na-ion battery technology that utilizes more sustainable materials is rapidly developing as a possible alternative to the Li-ion for massive electrochemical energy storage applications.¹ Several prototype Na-ion batteries using various chemistries have already been demonstrated; the most feasible being the two types of technologies based either on sodium layered oxides Na_xMO₂ (x ≤ 1, M= transition metal ion(s)) or polyanionic compound such as Na₃V₂(PO₄)₂F₃ as positive electrode and carbon as negative electrode.^2,3 Nevertheless, both technologies suffer with poor specific energy in comparison to their lithium counterparts, the major limitation being the poor achievable capacity and/or the redox voltage of the positive electrode. The polyanionic phases suffer with high molecular weight and hence poor capacity, while the sodium layered oxides cannot achieve their theoretical capacity due to their structural instability.^4,5 In order to increase the specific energy of the sodium ion cells, at first, it is essential to understand the origin of such limitations. Hence, we have studied the poly anionic Na₃V₂(PO₄)₂F₃ and several sodium layered oxides systems and bench marked their performances in practical Na-ion full cells.⁶ The results will be discussed by considering the fundamental aspects such as structure-stability-electrochemical performance relationship of the electrode material, moisture stability, interfacial reactivity and thermal stability of the charged electrode, hence safety of the Na-ion cells. Finally, a new material design approach will be discussed with the hope that they will help the battery community to develop high capacity electrode materials that can overpass the limitations of the present generation sodium ion batteries.
References:
(1) Deng, J.; Luo, W.-B.; Chou, S.-L.; Liu, H.-K.; Dou, S.-X. Sodium-Ion Batteries: From Academic Research to Practical Commercialization. Adv. Energy Mater. 2018, 8 (4), 1701428.
(2) Kubota, K.; Kumakura, S.; Yoda, Y.; Kuroki, K.; Komaba, S. Electrochemistry and Solid-State Chemistry of NaMeO ₂ (Me = 3d Transition Metals). Adv. Energy Mater. 2018, 8 (17), 1703415.
(3) Fang, Y.; Zhang, J.; Xiao, L.; Ai, X.; Cao, Y.; Yang, H. Phosphate Framework Electrode Materials for Sodium Ion Batteries. Adv. Sci. 2017, 4 (5), 1600392.
(4) Dugas, R.; Zhang, B.; Rozier, P.; Tarascon, J. M. Optimization of Na-Ion Battery Systems Based on Polyanionic or Layered Positive Electrodes and Carbon Anodes. J. Electrochem. Soc. 2016, 163 (6), A867–A874.
(5) Sathiya, M.; Jacquet, Q.; Doublet, M.-L.; Karakulina, O. M.; Hadermann, J.; Tarascon, J.-M. A Chemical Approach to Raise Cell Voltage and Suppress Phase Transition in O3 Sodium Layered Oxide Electrodes. Adv. Energy Mater. 2018, 8 (11), 1702599.
(6) Sathiya, M.; Qing, W.; Tarascon, J.-M. Will sodium layered oxides ever be competitive for sodium ion battery applications? Submitted, J. Electrochem. Soc.

Room temperature Na-ion batteries are highly considered as an alternative technology to Li-ion batteries and are projected to be manufactured at a much lower cost. The development of high power high energy density Na-ion batteries is contingent upon carefully choosing high performance positive and negative electrodes. Among the candidate phosphate materials, sodium vanadium fluorophosphate (Na_1.5VPO_4.8F_0.7) has gained considerable interest with its high redox potential of the material (operating voltage ~3.8 V vs. Na/Na⁺) and high theoretical capacity (130 mAh^.g^-1). To date, the primary synthesis approach for obtaining Na_1.5VPO_4.8F_0.7 is high temperature solid state reaction, however, to drive down the cost of SIBs, it would be beneficial to use a low cost and low energy consumption synthesis approach. In this study, we aim to implement a two-step solution phase synthesis (Pechini and hydrothermal) to consume less energy and minimize our impact on the environment to prepare Na_1.5VPO_4.8F_0.7 nanoparticles which is demonstrated as a high-rate high cycle life positive electrode material. At various charging rates, Na_1.5VPO_4.8F_0.7 achieved a high capacity of 130 mAhg^-1 (C/10) and up to 80 mAhg^-1 (20C). The high rate capability of this material is owed to its nanometric size and its small volume expansion of 3% upon continuous charge-discharge cycles. For these reasons, the cycling behavior showed superior stability where after 100 cycles, 93% of the initial capacity was maintained. Na_1.5VPO_4.8F_0.7 was then paired with an Sb-reduced graphene oxide negative electrode and in its full cell format achieved a capacity of 115 mAhg^-1 (1C) and 68 mAhg^-1 (20C). This full cell devices represents a feasible Na-ion battery devices capable of performing on par with commercial Li-ion batteries (LiCoO₂//graphite).

Layered sodium transition metal oxide system of NaTMO₂ (TM=3d transition metal ions and their mixture) forms a unique materials platform to study the transition metal, oxygen and sodium interactions. Specifically, Fe and Mn, which are not stable in LiTMO₂, become much more stable in NaTMO₂ due to the increased alkaline layer distance. In this talk, it is shown that the interplay between these TM and Na ions at lower Na compositions modulates the electrochemical performance of Na_xTMO₂ through novel high voltage phases. Combined in situ (synchrotron) XRD, atomic resolution (scanning) TEM, neutron diffraction and density functional theory (DFT) simulation techniques are used to understand the structural and functional details. Specifically, for the Fe containing mixed transition metal system of Na(Fe_xTM_1-x)O₂, a rippling phase at high voltage is identifed, caused by the unique electronic property of FeO₆ octahedra, while at low voltage it goes back to the normal flat phase through an asymmetric discharge structural evolution pathway from charge, giving high discharge rate capability. For NaMnO₂ a unique phenomenon of super charge separation is reported, where Mn charge superplane related to the collective Jahn-Teller activity of Mn³⁺ is found to dominate the entire electrochemical evolution pathway, giving asymmetric charge discharge behavior and high capacity. The underlying connection between the two systems of Na(Fe_xTM_1-x)O₂ and NaMnO₂ will be discussed to further facilitate the design of new cathode materials.
References: 1. Super charge separation and high voltage phase in Na_xMnO₂, Advanced Functional Materials, 1805105 (2018)
2. Reversible flat to rippling phase transition in Fe containing layered battery electrode materials, Advanced Functional Materials, 1803896 (2018)

Na- and K-ion batteries offer cheaper, safer alternatives to their Li-ion counterparts. However, additional structural phase transitions upon cycling can lead to faster mechanical degradation in their electrode materials. This is largely because Na⁺/K⁺ ions, unlike Li⁺ ions, are stabilized not only in octahedral coordination but also in prismatic coordination. The latter allows for more complex intercalant orderings due to the availability of a second triangular sublattice. The ordering of Na/K within a given structure greatly influences the voltage profile and ion transport, but can be difficult to resolve experimentally. We have performed first-principles statistical mechanics studies of the layered Na_xCoO₂ and K_xCoO₂ cathode materials, which exhibit several stable host structures with Na/K in either octahedral or prismatic coordination, related by gliding of CoO₂ sheets. Phase stability between these structures and stable Na/K orderings within each structure are presented and compared to experiments. We have identified several "Devil’s staircases" of infinite hierarchical orderings that Na_xCoO₂ displays over its composition range. K_xCoO₂ stabilizes similar orderings for intermediate K concentration and a new family of structures with mixed octahedral and prismatic coordination in the same layer at higher concentrations. This mixed phase brings about significant distortions of the CoO₂ layers and may facilitate stacking sequence changes at the end of discharge. Potential mechanisms for ion migration in these materials are discussed, as well as implications for Na/K ordering in other layered transition-metal oxides.

Modern developments of ultra-high efficiency mapping of Resonant Inelastic X-ray Scattering (mRIXS) has opened up this fundamental-physics technique for energy material researches with much improved throughput and superior chemical sensitivity beyond conventional spectroscopy. This has become a timely and critical solution to clarify the unusual redox states involved in high energy-density batteries, including both the cationic and anionic redox reactions in intercalation-type battery electrodes.
This presentation will not discuss the complex physics involved in the RIXS process (Tutorial on RIXS planned in this MRS meeting); instead, mRIXS will be introduced from the chemistry point of view to the material science community with examples on both transition-metal and oxygen states. We show that mRIXS could distinguish spectroscopic signatures of a novel Mn1+ state in batteries from conventional Mn states, based on the extra dimension of information that is completely missing in x-ray absorption spectroscopy [1]. We then elaborate the problems and challenges of probing oxygen redox reactions in battery electrodes, and demonstrate that, based on the same technical improvement beyond X-ray absorption, mRIXS is capable to not only detect the lattice oxygen redox states in battery electrodes [2], but also precisely quantify the lattice oxygen redox contributions and its reversibility/cyclability upon electrochemical cycling. The contrasts between different systems provide important clarifications to understand the mysterious oxygen activities, and suggest that oxygen release and lattice oxygen redox are independent processes involved in intercalation batteries, a relationship that has been indiscreetly taken for granted [3].
[1] Firouzi et al., Nat Comm 9, 861 (2018)
[2] Yang & Devereaux, J Power Sources 389, 188 (2018)
[3] (commentary) Yang, Nat Energy 3, 619 (2018)

Much efforts are undertaken in lithium ion battery research for the identification of processes taking place at the surfaces of electrode materials and components.
However, for an intercalation battery also the bulk properties are relevant because the bulk is the host of the lithium ions, notwithstanding that it is the surface where the lithium enters and exits the positive and negative electrodes.
The functionality of the battery electrodes is at large reflected by their electronic structure as it amounts from the crystallographic arrangement of the ions in the electrode materials. In the last 30 years it has become popular to investigate the electronic structure of battery materials and components with x-ray methods, including their operando and in situ assessment, and including synchrotron raidation centers as the necessary x-ray sources.
It has turned out that the soft x-rays are particularly suited for the elucidation of the electronic structure of the electrodes, particularly the oxide based positive electrodes.
It is an unfortunate weakness of the soft x-rays that they have a relatively low penetration depth and thus relatively low information depth. Practically, the attentuation of soft x-rays is around 1 micrometer, whereas the particulates of spinel based cathodes in lithium secondary batteries range in the order of 25 micrometers. Bulk specificity and soft x-rays rule thus each other out. It is thus vitually impossible to get a soft x-ray spectrum at the 3d metal 2p absorption threshold or an oxygen 1s spectrum from the bulk of such industry grade battery material, neither ex situ nor in situ: unless you use a spectroscopyic trick which has matured to real life applicability since only recently.
I will demonstrate how the battery in situ cell can be charged and discharged and during this process hard x-rays with 11 keV energy can penetrate the entire cell, and the X-ray Raman contribution from the fluoressence signal can be extracted for example for the Mn 2p features.
It was therefore possible to record a large number of Mn2p soft x.ray multiplets which showed characteristic changes in their relative spectral weight of L2 and L3 peaks while lithoum ions are being inserted in the electrode and extracted from the electrode.
Very interestingly it was possible to tickle out the spin states of the Mn ions which shows that there is an intermediate Mn3+ phase which undergoes a high spin and a low spin transition, which hitherto was not known in the condensed matter, solid dtate chemsitry and battery technology community.
The design of the in situ cell and the spectroscopic principle of the X-ray Raman method, available only since few years for the battery expert, are explained as well.

References:
[Braun 2015] Braun A, Nordlund D, Song S-W, Huang T-W, Sokaras D, Liu X, Yang W, Weng T-C, Liu Z: Hard X-rays in–soft X-rays out: An operando piggyback view deep into a charging lithium ion battery with X-ray Raman spectroscopy. Journal of Electron Spectroscopy and Related Phenomena 2015, 200:257-263.doi: 10.1016/j.elspec.2015.03.005.
[Braun 2017] Braun A: X-ray Studies on Electrochemical Systems - Synchrotron Methods for Energy Materials. Berlin/Boston: Walter De Gruyter GmbH; 2017. 470 pages.

Li₄Ti₅O₁₂(LTO)-based negative electrode for lithium-ion batteries is of interest for electrical vehicles due to its safety, low cost and cycling stability [1]. In this study, the effect of the positive electrode on the electrochemical performances of LTO electrodes, in relation with the Solid Electrolyte Interphase (SEI) properties, has been investigated [2]. Full cells LTO/LiNi_3/5Co_1/5Mn_1/5O₂ (NMC) and LTO/LiMn₂O₄ (LMO) were cycled at 40°C over 100 cycles and the electrodes were analyzed by XPS, Scanning Auger Microscopy (SAM) and Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) after one and 100 cycles. Moreover, LTO/LTO symmetrical cells were also analyzed in order to be free of the positive electrode impact. For each system, LTO electrodes are homogeneously covered by surface layers since the first cycle which induces an irreversible capacity loss. This latter is more important for LTO/LMO compared to LTO/NMC and LTO/LTO. Both SEI layers are composed of organic (polyethylene oxides, oxalates) and inorganic species (LiF, phosphates and fluorophosphates) but in different proportions and with different 2D and 3D spatial distributions: fluorine species are detected deeper in the electrode than organic species and in higher quantities for LTO/LMO for instance [3]. Moreover, the SEI is thicker on the LTO electrode when cycled versus LMO compared to NMC and contains small amounts of manganese, homogeneously spread over the surface and deeply inserted in the SEI, which entails an increase of the system impedance. In conclusion, a thick SEI associated with the presence of metallic species could alter the passivating role of the SEI and explain the less efficient electrochemical performance of LTO/LMO cells. Analysis after cycling at higher voltage are currently in progress to study the evolution of interfacial layers composition and thickness and to better understand the interactions between the two electrode materials.

References:
[1] Tarascon et al. Nature 2001, 414, 359–367.
[2] El Ouatani et al. Journal of The Electrochemical Society, 156(6): A468, 2009.
[3] Nicolas Gauthier et al. Journal of The Electrochemical Society, 165(13): A2925-A2934, 2018.

Outgassing of active materials in Li-ion batteries provides a route to quantitatively study degradation processes that occur during cycling. In particular, we are primarily interested in quantifying the individual and coupled decomposition/transformations of the cathode – a lithiated transition metal oxide (TMO) – and the electrolyte – most commonly carbonate blends (ethylene carbonate, diethyl carbonate, etc.) with lithium hexafluorophosphate (LiPF₆) as the salt. Previous observations of high-voltage instabilities include TMO surface reconstruction, transition metal dissolution, electrolyte decomposition, and formation of surface species. However, this picture is still incomplete, with the dependence on electrolyte and TMO composition not yet fully understood. We will present results in which isotopic labeling of ¹⁸O in Ni-rich and Li/Mn-rich NMCs is combined with quantitative gas evolution analysis to show that residual solid lithium carbonate (Li₂CO₃) on the surface of TMOs has a direct impact on electrolyte and electrode degradation. In particular, oxygen release from the TMO lattice is related to the amount of Li₂CO₃ present in the cathodes. Our results suggest that the role of impurities on interfacial reactivity in batteries is critically important and should be a key parameter considered in similar future studies.

The push for higher energy density and concern about safety has driven increasing interest in all solid-state lithium batteries. The thin film formats in which these devices are typically made allow only a fraction of the theoretical capacity to be realized due to the low areal capacity of the cathodes, and require expensive vacuum deposition technologies. For vehicle applications, where high energy density and low cost are priorities, it will be necessary to design cells with thicker positive electrodes, similar to what is used in lithium ion batteries (~ 100 μm). To overcome electronic and ionic transport limitations, these electrodes will need to be fabricated in composite form, which include the solid electrolyte and an electronically conductive additive. We are developing a method for doing this by first freeze tapecasting porous scaffolds of Al-substituted LLZO (Li₇La₃Zr₂O₁₂) and then infiltrating the unidirectional pores of these structures with active material and conducting polymer. By changing processing variables, it is possible to fine-tune the porosity, pore size, and thickness of the scaffolds. For this talk, we will discuss the freeze tapecasting process and the challenges of developing solid state batteries based on LLZO.

Fast lithium solid electrolytes are critical components of all-solid-state batteries (SSB) which promise safer operation and higher power and energy densities compared to commercial Li ion batteries. The highly polarizable anionic sulfide sub-lattice of thiophosphates, prominently represented by the tetragonal Li₁₀GeP₂S₁₂ (LGPS) family of compounds, provides a rather flat energy 3D landscape with low activation energy paths for lithium ion movement, rendering thiophosphates and, more generally, sulfides, the currently most promising class of materials for the discovery of ultrahigh ionic conductivities in solids [1,2,3]. However, LGPS-type thiophosphates are known to readily decompose under ambient conditions, often leading to electronically conductive or insulating products with sluggish Li ion dynamics. In addition, there is an undisputed need to replace the precious Germanium by more earth-abundant elements whilst maintaining high Lithium conductivities of > 1 mS/cm. We will discuss several strategies to tackle these challenges. First, our recent efforts in exploring the solid solution system Li11-xSi2–xP1+xS12 will be described, which led to the discovery of new solid electrolytes in the LSiPS family displaying conductivities of > 1 mS/cm [2,4]. While fast in absolute terms, we will show that these materials are in fact glassy ceramics, carrying amorphous side phases which reduce the apparent Lithium conductivities. Further, the solid solution system Li-Sn-S will be discussed as a source of new sulfide-based superionic Lithium conductors with layered structures. Li_0.6[Li_0.2Sn_0.8S₂] exhibits a lithium ion diffusivity of σ_NMR = 9.3 mS/cm at room temperature as determined by pulsed field gradient (PFG) NMR [5,6]. We will show that Li_0.6[Li_0.2Sn_0.8S₂] hydrates in air while maintaining high Li diffusivities and, hence, exhibits greatly improved environmental stability as compared to most sulfide-based solid electrolytes. We will further discuss the exfoliation—restacking of lithium tin sulfide nanosheets into conformal thin films and their strongly hydration-dependent in-plane conductivity, which bodes well for the use of these environmentally robust sulfide solid electrolytes not only in SSBs, but also as solution-processable humidity sensors.

[1] A. Kuhn, V. Duppel, B.V. Lotsch, Energy Environ. Sci. 2013, 6, 3548–3552.
[2] A. Kuhn, O. Gerbig, C. Zhu, F. Falkenberg, J. Maier, B.V. Lotsch, Phys. Chem. Chem. Phys. 2014, 16, 14669−14674.
[3] A. Kuhn, J. Köhler, B.V. Lotsch, Phys. Chem. Chem. Phys. 2013, 15, 11620–11622.
[4] S. Harm, A. Hatz, I. Moudrakovski, R. Eger, A. Kuhn, C. Hoch, B. V. Lotsch, submitted.
[5] A. Kuhn, T. Holzmann, J. Nuss, B. V. Lotsch, J. Mater. Chem. A 2014, 2, 6100-6106.
[6] T. Holzmann, L. M. Schoop, M. Ali, I. Moudrakovski, G. Gregori, J. Maier, R.J. Cava, B.V. Lotsch, Energy Environ. Sci. 2016, 9, 2578–2585.

Closo-borate salts are comprised of loosely packed, boron-derived polyatomic anions. Recently, these materials have been demonstrated to exhibit extraordinary ionic conductivity and good electrochemical stability, making them attractive as novel solid electrolytes for next-generation lithium and sodium batteries. In addition to their practical potential, these materials also present wide tunability in structure and composition, making them an excellent platform for exploring underlying mechanisms and developing new descriptors for ion conduction. In this talk, I will show how extensive diffusion simulations and computational “experiments” have been applied to closo-borates in order to systematically isolate factors such as stoichiometry, strain, alloy composition, and crystal structure in the determination of ionic conductivity. Our results highlight key connections between frustration, correlation, and ultrafast diffusion. Different types of frustration present in closo-borates will be discussed, arising from factors such as off-stoichiometry, competition between interstitial site occupancies, symmetry incompatibilities between local bonding character and lattice geometry, and dynamical frustration coupled to anharmonic processes. Physicochemical origins of the relevance of these factors for cation mobility will be explored, with a view towards developing design rules for engineering faster room-temperature ionic conductors based on closo-borate salts.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Lithium metal anode has been regarded as the "Holy Grail" of next-generation battery technologies. In order to stabilize the electrodeposited Li metal for safe and high energy batteries, the use of novel electrolyte systems and advanced separators has been studied to suppress the formation of high surface area Li dendritic structure. (1-2) In this talk, we report on our use of mesoporous silica thin films with small pore size on macroporous alumina membrane as a novel separator for lithium metal batteries. Symmetric Li electrochemical cells with the ceramic separator exhibit long-term stability at a high current density of 5 mA cm^-2 and a capacity of 2.5 mAh cm^-2. The effect of porous ceramic separator on the mechanism of Li growth is studied in detail.
Highly concentrated electrolytes (solvate electrolytes) have been proposed to suppress the formation of Li dendritic structure and polysulfide dissolution in lithium-sulfur (Li-S) batteries.(3-5) We use in situ transmission X-ray microscopy (TXM) to study the effect of solvate electrolyte on Li plating/stripping process. In situ TXM images show that Li particles with uniform density are formed in ether-based solvate electrolyte during initial plating process. Additionally, in situ spectroscopy including Raman and X-ray spectroscopy (X-ray diffraction and X-ray absorption spectroscopy) are used to investigate sulfur reaction mechanism and the interaction between polysulfide and solvate electrolyte. We found that the sulfur species formed in the ether-based solvate electrolyte are different from the sulfur species formed in conventional DOL/DME electrolyte. These results suggest that solvate electrolyte changes the sulfur reaction mechanism. We next propose different solvate electrolytes with low polysulfide solubility and high stability toward Li metal to enhance the capacity retention of Li-S batteries.

References:
[1] W. Xu et al., Energy Environ. Sci. 2014, 7 (2), 513.
[2] Y. Xiang et al., ChemSusChem 2016, 9 (21), 3023.
[3] M. Cuisinier et al., Energy Environ. Sci., 2014, 7, 2697.
[4] Y. Yamada et al., J. Electrochem. Soc. 2015, 162 (14), A2406.
[5] L. Cheng et al., ACS Energy Lett. 2016, 1, 503.

In secondary batteries, the anode/electrolyte interphase plays a key role in the electrochemical performances. As the liquid organic electrolyte undergoes degradation in the electrochemical potential window of a cycling battery, a Solid Electrolyte Interphase (SEI) is formed upon cycling. This interphase layer leads to a double-edged problematic: the formation of the SEI lowers the coulombic efficiency and causes irreversible capacity loss, but it also passivates the electrode from the electrolyte and prevents further aging processes. Knowing this, any modification of the SEI should be performed with parsimony as it could break the balance between the positive and negative aspect of the SEI. By synthetizing a chemisorbed thin fluorinated layer upon anode material, we managed to improve the passivating power of the SEI leading to enhanced electrochemical performances. We also determine that very low quantities of fluorine on the active electrode material surface leads to several beneficial effects.
We aimed to prospect the influence of the surface fluorination on different aspect of a Li-ion battery, from the active material to the electrolyte interphase, thanks to a multi-scale probing approach. The chemical nature of the surface layer was describe by the mean of the XPS, as well as the fluorine distribution on the surface with both AES and SAM. The fluorine has been quantified around 10 at. % of the extreme surface of the Li₄Ti₅O₁₂ (LTO) material, without diffusion in particles bulks. The bulk and sub-surface properties of fluorinated LTO (LTO-F) were also investigated by coupling XRD, Raman Spectroscopy and NMR ¹⁹F, showing no modifications of the crystallographic structure. The influence of the surface fluorination on the electrochemical performance was investigate by galvanostatic cycling and by coupling XPS and SAM on cycled electrodes. We had a specific attention to the impact of the fluorination on the SEI thickness and stability in charge and discharge. Indeed, LTO-F exhibit a new reactivity toward the electrolyte, leading to a thinner and stabilized SEI. Finally, the gas generation of the LTO-F electrodes has been investigate by Gas Chromatography – Mass Spectrometry (GC-MS), as gassing is known to be a roadblock to the commercialization of LTO^1,2. We demonstrate that the CO₂ outgassing is reduced by the surface fluorination.
(1) He, Y.-B.; Li, B.; Liu, M.; Zhang, C.; Lv, W.; Yang, C.; Li, J.; Du, H.; Zhang, B.; Yang, Q.-H.; et al. Gassing in Li(4)Ti(5)O(12)-Based Batteries and Its Remedy. Sci. Rep. 2012, 2, 913.
(2) Zhang, L.; Zhang, S.; Zhou, Q.; Snyder, K.; Miller, T. Electrolytic Solvent Effects on the Gassing Behavior in LCO||LTO Batteries. Electrochimica Acta 2018, 274, 170–176.

There has been considerable interest in developing low-cost, high-energy electrodes for batteries. However, synthesizing materials with the desired phases and properties has proven difficult due to the complexity of the reactions involved in chemical synthesis. Additional challenge comes from the fact that synthesis is often undertaken under non-equilibrium conditions and, hence, the process is hard to be predicted by theoretical computations. In situ, real-time probing of synthesis reactions allows for identification of intermediates and determination of thermodynamic/kinetic parameters governing kinetic reaction pathways, thereby enabling synthetic design of materials with desired structure and properties. In this presentation, we will report our recent results from technique development and application to in situ probing and synthetic control of local structural ordering and stoichiometry during synthesis of high-Ni layered LiNi_1-x (MnCo)_xO₂ (1-x>0.7). Findings from this study, along with its implication to designing surface-stabilized high-Ni layered oxide cathodes, will be discussed.
ACKNOWLEDGMENT. This work was supported by the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Office, Contract No. DE-SC0012704.

The practicality issue of the lithium rich layered oxide materials originate from irreversible structure transformation and voltage decay upon cycling—processes in which defect electrochemistry plays a vital role. Understanding the correlation between defect generation and voltage decay is essential for implementing rational design strategies to improve voltage stability and long term cycling. In this talk I will demonstrate the unique metastable structure of cycled Li-rich layered oxide resulting from defect formation. Modification can be applied to drive the cycled material back to a stable state and subsequently lead to the voltage recovery. This study of structure metastability and reversibility opens up new opportunities for resolving the voltage decay issue in high-capacity layered oxides, more importantly the insights might help the researchers to improve other aspects of the anion redox active materials.

The discovery of oxygen redox in Li-rich NMC cathode materials has revived the hopes for high-energy density batteries. [1-2] These materials are derived from the parent compound, Li₂MnO₃ [3-4], which was initially considered electrochemically inactive due to its Mn⁴⁺ oxidation state in octahedral coordination. However, reversible capacities were demonstrated for Li₂M<span style="font-size:10.8333px">n</span>O₃ upon “activation” at high voltages however the exact charge compensation mechanism remains unclear [5]. Recent fundamental studies of oxygen redox have largely focused on 4d and 5d-based model systems, [1] which are prone to less gas evolution. However, elucidating the origin of the excess reversible capacities in LR-rich NMC electrodes requires a fundamental understanding of the nature of oxygen participation within Li₂MnO₃.

We report the chemical and structural evolution of Li₂MnO₃ electrodes upon cycling using a suite of characterization techniques to evaluate the extent of oxygen release in activating the Mn redox. In addition, oxygen-release is also found to be responsible for gradual structural disordering that occurs upon cycling. These results are discussed in the context of Li-rich NMC electrodes; especially the role of oxygen loss at the cathode-electrolyte interface [2, 6].

G. Assat and J.-M. Tarascon. Nat. Energy 3, 1-14 (2018)
B. Qiu, M. Zhang, L. Wu et al. Nat Comm. 7, 1-10 (2016)
Z. Lu, D. D. MacNeil, and J. R. Dahn. Electrochemical and Solid-State Lett. 4, A191-A194 (2001)
M. M. Thackeray, S.-H. Kang, C. S. Johnson et al. J. Mater. Chem. 17, 3112-3125 (2007)
J. Rana, M. Stan, R. Kloepsch et al. Adv. Energy Mater. 4, 1-12 (2014)
E. Hu, X. Yu, R. Lin et al. Nat. Energy 3, 690-698 (2018)

In the pursuit of high energy density Li-ion battery cathodes, Li-rich systems have demonstrated high reversible capacities that are considered accessible through oxygen redox. Investigation into the oxygen redox mechanism and identification of attractive oxygen redox candidates has driven increased utilization of x-ray spectroscopy techniques that directly probe the oxygen environment. In particular, resonant inelastic x-ray scattering (RIXS) at the O K-edge has emerged as a prime technique to provide sensitivity to the oxygen chemical environment. Sharp RIXS features have been observed in a range of transition metal (TM) 3d systems, including Li-rich NMC [1], Na_2/3[Mg_0.28Mn_0.72]O₂ [2]_, and Li_1.9Mn_0.95O_2.05F_0.95 [3], that are considered signatures of oxidized oxygen. Yet, even for the model Li-rich NMC systems, uncertainty in the interpretation of these RIXS features remains as well as questions on the local environment and long-term stability of oxidized oxygen states.

Here, we focus on the stability of Li[Li_0.144Ni_0.136Mn_0.544Co_0.136]O₂, a model LR-NMC system, at high degrees of delithiation under the x-ray exposure conditions needed to conduct RIXS measurements. Combining sXAS and RIXS, our studies demonstrate the sensitivity of surface transition metal and bulk oxygen states to aggressive x-ray beam exposure in LR-NMC systems. In addition to the surface photoreduction of transition metals, we find a strong loss of inherent oxidized oxygen states with x-ray exposure. Our studies demonstrate the utilization of RIXS for the identification of oxidized oxygen states, while providing new insight into the nature of oxidized oxygen environments.

This work was supported as part of NECCES, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0012583

[1] W. E. Gent, K. Lim, Y. Liang, Q. Li, T. Barnes, S.-J. Ahn, K. H. Stone, M. McIntire, J. Hong, J. H. Song, Y. Li, A. Mehta, S. Ermon, T. Tyliszczak, D. Kilcoyne, D. Vine, J.-H. Park, S.-K. Doo, M. F. Toney, W. Yang, D. Prendergast, & William C. Chueh. Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides. Nature Communications, 8, 2091 (2017)
[2] Urmimala Maitra, Robert A. House, J. W. Somerville, N. Tapia-Ruiz, J. G. Lozano, N. Guerrini, R. Hao,K. Luo, L. Jin, M. A. Pérez-Osorio, F. Massel, D. M. Pickup, S. Ramos, X. Lu, D. E. McNally, A. V. Chadwick, F. Giustino, T. Schmitt, L. C. Duda, M. R. Roberts, and P. G. Bruce. Oxygen redox chemistry without excess alkali-metal ions in Na_2/3[Mg_0.28Mn_0.72]O₂. Nature Chemistry, 10, 288-295 (2018)
[3] R. A. House, L. Jin, U. Maitra, K. Tsuruta, J. W. Somerville, D. P. Forstermann, F. Massel, L. Duda, M. R. Roberts, and P. G. Bruce. Lithium manganese oxyfluoride as a new cathode material exhibiting oxygen redox. Energy Environ. Sci. 11, 926-932 (2018)

Lithium-rich layered oxides (LRLO) have lately emerged as leading candidates to replace the classical stoichiometric insertion oxides owing to their increased energy densities provided by the oxygen redox activities.¹ Despite such a positive attribute, LRLO still awaits commercial success due to practical obstacles appearing during the first cycle (partial O₂ release) or upon cycling (voltage fade).² Though the origin of voltage fade has been discussed from various structural perspectives (i.e., cation trapping through tetrahedral sites, formation of partial dislocations and/or microstructure defects), a consensus has been reached on the strong coupling between structural dynamics and oxygen redox chemistry.^3-6 Nonetheless, establishing an explicit structure-properties scenario has proved challenging due to characterization limitations as well as the inherent complexity of the coupled transition metal and oxygen redox processes themselves. Herein, we investigated the structure evolution of several LRLO materials during their initial cycles via an arsenal of characterization techniques including both in-situ and ex-situ X-ray diffraction (XRD), electron microscopic imaging and a novel developed method enabling to spot O₂ release. Based on these techniques, we could identify the role of oxygen redox chemistry in the formation of a new metastable phase upon charging LRLOs. Its consequence on the practical aspects of LRLO electrodes will be discussed together with chemical/physical means to prevent it.

References:
1. J. Wang, et al. Advanced Energy Materials, 6 (21), 1600906 (2016).
2. G. Assat and J.-M. Tarascon, Nature Energy, 3 373-386 (2018).
3. M. Sathiya, et al. Nature Materials, 14 230-238 (2014).
4. A. Singer, et al. Nature Energy, 3 (8), 641-647 (2018).
5. E. Hu, et al. Nature Energy, 3, 690–698 (2018).
6. W. E. Gent, et al. Nature communications, 8 (1), 2091 (2017).

Certain molten solvates of Li salts can be regarded as solvate ionic liquids (SILs). A typical example is equimolar mixtures of glymes (G3: triglyme and G4: tetraglyme) and Li[TFSA]([TFSA]=[NTf₂]) ([Li(glyme)][TFSA]). The amount of free glyme estimated by Raman spectroscopy and MD simulation is a trace in [Li(glyme)]X with perfluorosulfonylamide-type anions such as [TFSA]^-, and thereby can be regarded as solvate ionic liquids. The activity of free glyme in the glyme-Li salt mixtures evaluated by measuring EMF of the concentration cells drastically diminishes at a higher concentration of Li salt, leading to a drastic increase in the electrode potential. Unlike conventional electrolytes, the solvation of Li⁺ by the glyme forms stable and discrete solvate ions ([Li(glyme)]⁺) in the solvate ionic liquids.

An intriguing aspect of the SILs is unusual solubility. The theoretical capacity of the S cathode is 10 times higher than that of conventional cathode materials used in current Li–ion batteries. However, Li–S batteries suffer from the dissolution of lithium polysulfides, which are formed by the redox reaction at the S cathode. In the SILs, [Li(glyme)][TFSA], both cations and anions are weakly coordinating ions with low Lewis acidity and basicity, respectively. The [Li(glyme)][TFSA] molten complexes do not readily dissolve other ionic solutes due to the weak coordinating nature of the cation and anion, which leads to the stable operation of the Li–S battery. Another interesting feature is electrochmical intercalation of graphite, which becomes possible in the SILs without using a typical SEI former. The mechanism for this intercalation reaction has not been clarified yet, but the activity of free solvent in electroytes definetely correlates with this phenomenon.

Further, polymer electrolytes composed of ABA-triblock copolymers and [Li(glyme)][TFSA] SILs are proposed to simultaneously achieve high ionic conductivity, thermal stability, and a wide potential window. Different block copolymers, consisting of a SIL-incompatible A segment (polystyrene, PSt) and SIL-compatible B segments (poly(methyl methacrylate) (PMMA) and poly(ethylene oxide) (PEO)) are utilized. The SILs can be solidified with the copolymers through physical crosslinking by the self-assembly of the PSt segment. The thermal and electrochemical properties of the polymer electrolytes are significantly affected by the stability of the [Li(glyme)]⁺ complex in the block copolymer B segments, and the preservation of the SILs contributes to their thermal stabilities and oxidation stabilities greater than 4 V vs. Li/Li⁺. The [Li(glyme)]⁺ complex cation is unstable in the PEO matrix, whereas the complex structure of [Li(glyme)]⁺ is stable in the PMMA-based polymer electrolyte. By using the PMMA-based polymer electrolytes, a 4-V class Li batteries with a LiCoO₂ cathode and a Li metal anode can be stably operated; in contrast, this is not possible using the PEO-based electrolyte.

Layered transition-metal oxides continue to be an exciting area of new cathode materials. For instance, Li-excess materials appear to produce larger capacities but currently suffer from poor cyclability. Furthermore, additions of Al, Mg, and other elements into current cathodes are used to cycle batteries at higher voltages to extract more energy from the same cathode materials. In both cases, new compositions are introduced in the transition-metal layer, replacing some of the Co, Ni, or Mn. As these increasingly complex systems are (dis)charged to the extremes of their capabilities, many (not yet well understood) processes can result in battery degradation. Importantly, the layered cathode materials tend to break down when most of the active cation has been deintercalated at the end of charge, resulting in poor capacity retention and voltage fade. As Li atoms deintercalate from the layered transition-metal oxide, vacancies become available for interlayer atom migration, and atoms that leave the transition-metal layer may become stuck in the Li layer resulting in irreversible structural changes. Therefore, it is important to understand how the presence of Li, Mg, or Al in the transition-metal layer could affect (potentially irreversible) interlayer atom migration.

Using first-principles methods, we have discovered that the composition of the transition-metal host plays a very important role in facilitating interlayer atom migration. The pathway for migration includes a dumbbell with a Li ion. We analyze the energetics and electronic structure of dumbbell formation to explain why certain systems may experience more interlayer migration than others. Furthermore, we propose design principles for mitigating interlayer migration, with specific implications for NCA, Mg-doped, and Li-excess cathodes but also general insight that can be applied towards a wide range of new cathode materials. This new understanding can help in designing high energy batteries without compromising on material stability.

Layered lithium transition metal oxides cathodes materials (LiNi_1-x-yMn_xCo_yO₂) have become the cathode of choice for state-of-the art lithium-ion batteries, which now dominate the portable energy storage market and even become the leading choice for grid storage. However, these NMC materials still achieve less than 25% of their theoretical energy density, so there is much space for improvement. Increasing the nickel content both increases the energy density and decreases the materials cost. The high nickel 622 composition is in commercial use today. The 811 composition has even higher capacity and tolerates higher materials loading and higher rates better than 622, so will become the material of choice if its increased instability can be controlled. One option to increase the capacity is to increase the charging voltage, but that leads to even higher instability. An alternative option is to reduce the 1^st cycle loss, which approaches 15% of the initial capacity. We will discuss our approaches to reducing this loss, more than half of which can be recovered by reducing the discharge rate, that is the capacity is kinetically limited and by materials modification should be recovered. Part of the remainder is lost to CEI formation. This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy, through the Advanced Battery Materials Research Program (Battery500 Consortium).

Hazen Research, Inc., in collaboration with the National Renewable Energy Laboratory (NREL), has demonstrated a new method, in laboratory-scale experiments, of producing nickel-rich NMC cathodes. These NMC cathodes of the type Li(Ni_1-x-yMn_xCo_y)O₂ (Ni >0.6) were produced using single-source inorganic–organic precursors and a process involving vertical spray pyrolysis and fluidized-bed reactions in a semicontinuous mode. The method produced amorphous phases during spray pyrolysis, which after undergoing a fluidized-bed reaction, resulted in spherical, free-flowing and nondusting soft agglomerates of well-crystallized layered nickel-rich NMC particles. The cathodes showed electrochemical performance with an initial capacity of >200 mAh/g, a voltage stability in the range of 3.0–4.5 V, an energy density of >800 Wh/kg, a coulombic efficiency of >90%, and a capacity retention of approximately 70% over 100 cycles. The presentation will discuss the process, characterization, and electrochemical properties of the NMC-622 cathode powders.

The world’s growing production of carbone dioxide and the desire to use more sustainable energy sources are motivating the minimization of fossil fuels employment. In this context, the development of technologies in advanced energy storage devices appears as a possibility to turn feasible the transition of engine vehicles from internal combustion to full electric motion. The lithium-ion battery which is the technology used in portable electronics has already achieved its theoretical limit (around 200 Wh/kg) [1]. Therefore, the development of novel high energy density batteries is essential to overcome limitation of the actual system. In recent years the lithium-oxygen/air battery has received great attention because of its high theoretical energy density (11686 Wh/kg), which is comparable to oil combustibles [2]. A typical lithium-oxygen battery consists of a metallic lithium anode, a porous diffusion cathode, which is commonly made of carbon compound and a catalyst, and an aprotic electrolyte which acts as a media to lithium ions transport. A full understanding of electrolyte battery element is important to find a proper composition to achieve reversibility and good capacities on charge and discharge processes. So, we are proposing in this paper the evaluation of lithium perchlorate/dimethyl sulfoxide electrolyte by molecular dynamic simulation and the calculation of transport properties such as conductivity, diffusivity and viscosity to a homogeneous media. The methodology is based on building the electrolyte molecular structure and on simulating the system to evaluate transport properties. Molecular Dynamics simulation was performed with LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator. PACKMOL [3] package was used to generate the ionic-electrolyte configuration. The interaction potential parameters (both intermolecular and intramolecular) were set according to the chosen force field [4, 5]. Crossed intermolecular parameters were calculated by Lorentz-Berthelot combining rules. The electrostatic interaction was computed using the Particle-Mesh scheme. For Lennard-Jones interactions, a long-range correction beyond the cutoff was applied. A step for equilibration in the canonical ensemble was carried out followed by a production stage in isothermal-isobaric ensemble. The properties were analyzed at three different temperatures: 273 K, 298 K, and 313 K. The post-processing step consists on calculating transport coefficients. These coefficients will be calculated by Green-Kubo relations. The dimethyl sulfoxide density and diffusivity calculated by simulation were compared with experimental data and good agreement was observed. The electrolyte system analyses show that this approach can be used as electrolyte scanning method to evaluated feasible compounds to lithium-air batteries applications.

[1] Balaish, M.; Kraytsbergb, A.; Ein-Eli, Y. A critical review on lithium–air battery electrolytes. Phys. Chem. Chem. Phys., 16, 2801-2822, 2014.

[2] Girishkumar, G.; Mccloskey, B.; Luntz, A.C.; Swanson, S.; Wilcke, W. Lithium-Air Battery: Promise and Challenges. The Journal of Physical Chemistry Letters, p. 2193-2203, 2010.

[3] Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. Packmol: A package for building initial configurations for molecular dynamics simulations. Journal of Computational Chemistry, 30(13):2157-2164, 2009.

[4] Bordat, P.; Sacristan, J; Reith, D.;Girard, S.; Glättli, A; Müller-Plathe,F. An improved dimethyl sulfoxide force field for molecular dynamics simulations Chemical Physics Letters, p. 201–205, 2003.

[5] Li, T.; Balbuena, P. Theoretical Studies of Lithium Perchlorate in Ethylene Carbonate, Propylene Cabonate, and Their Mixtures. The Journal of The Electrochemical Society, p. 3613-3622, 1999.

Binary transition metal chalcogenides have recently drawn boosted attraction as anodes for sodium ion batteries (SIBs) owing to their greatly enhanced electrochemical performances. Superior intrinsic conductivity and richer redox reactions, as compared to monometal chalcogenides, are the main origins of improved performances of these binary transition metal chalcogenides. In pursuit of employing various binary transition metal selenides (B-TMSs) for energy storage, a simplistic and universal synthesis approach is highly desirable. Here, we present a facile and comprehensive strategy to produce various combinations of nanostructured B-TMSs by the use of either nitrates or sulfates of corresponding metals in a high yield room temperature solution reaction. Furthermore, the structure evolution mechanism of nano rods from precursor nanosheets was investigated through the study of products obtained after various reaction durations. As proof of concept, high surface area and hierarchical nanosheets of Fe₂NiSe₄, Fe₂CoSe₄ and NiCoSe₄ (termed as FNSe, FCSe and NCSe respectively), were manufactured and employed as anodes for sodium ion batteries. These as prepared anodes of B-TMSs exhibited adequately high energy capacities (e.g. 755, 660 and 397 mA h g^-1 after 100 cycles at 1 A g^-1 for FNSe, FCSe and NCSe, respectively) and excellent rate capabilities (e.g. 776 mA h g^-1 at 0.5 A g^-1 and 432 mA h g^-1 at 20 A g^-1 for FNSe, 655 mA h g^-1 at 0.5 A g^-1 and 466 mA h g^-1 at 20 A g^-1 for FCSe and then 660 mA h g^-1 at 0.5 A g^-1 and 366 mA h g^-1 at 20 A g^-1 for NCSe). In addition to this, FNSe and FCSe also presented extraordinary stable life of 2500 cycles (with reversible capacities of 554.2 and 554.6 mA h g^-1 at 4 A g^-1, respectively). In situ X-ray diffraction analysis combined with ex situ X-ray and selected area diffraction analysis revealed that the electrodes of FNSe, reversibly transform into the discharge product (Na₂Se) through multistep reactions with sodium ions. When employed in sodium full batteries with lab-made Na₃V₂(PO₄)₃/C cathode, as prepared B-TMSs anodes presented reasonably high reversible specific capacities (228.5, 216.5 and 100.2 mA h g^-1 at 0.1 A g^-1 after 100 cycles for FNSe, FCSe and NCSe, respectively). Overall, the presented strategy will pave the way for the development of numerous binary transition metal chalcogenides which are the potential materials for energy storage and conversion systems.

The development of Li-ion cathode materials is moving forward to a new era. Lithium iron phosphate (LiFePO4, LFP) is widely used by the battery industry and it has the potential to continue playing an important role in the future. Mainly driving by a stable operating voltage (3.5 V vs. Li/Li⁺) and a high theoretical capacity (170 mAh/g). Additionally, LFP has excellent cycling performance, high safety, environmental friendliness, and low raw material cost To further increase the competitiveness of LFP by decreasing production cost, an approach to synthesize LFP with coated carbon (LFP/C), via lithiation in a quasi-open air environment is investigated in this work. In contrast to the generic solid state synthesis for LFP, our approach is more time efficient with the lithiation process completing under 30 minutes. Furthermore, our approach is a cost-effective process since the reaction is conducted under atmospheric pressure in a quasi-open environment. Starch is used as a carbon source during synthesis, providing a reducing environment to protect the synthesized LFP from oxidation, while forming a carbon coating on the surface of LFP. The carbon coating enhances the electrochemical performance of the assembled Li-ion batteries by improving electric conductivity of the LFP cathode. The combination of LFP synthesis process and LFP carbon coating process in one step further improve the efficiency of our quasi-open synthesis approach.

Starch is selected as carbon source because it has a high carbon content that produces LFP/C with a high efficient coating, at a low fabrication cost. This work investigates the effect of the amount of starch added, reaction temperature, and reaction time for LFP synthesis. Several techniques are used to characterize the properties of synthesized LFP/C. The crystal structure and chemical composition of the synthesized material are characterized by X-ray Diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The grain size of resulted materials is determined by Scanning Electron Microscopy (SEM). The degree of carbonization is characterized by Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The electrochemical performance of synthesized cathode is investigated by Cyclic Voltammetry (CV) and the performance of assembled batteries is tested via an Arbin Tester. The mechanism of starch assisted LFP synthesis and the decomposition process of starch are investigated. These reaction processes are studied by Differential Scanning Calorimetry and Differential Thermal Analysis (DSC and TGA, respectively).

Rechargeable aluminum-ion batteries (AIBs) are attractive new generation energy storage devices due to its
low cost, high specific capacities, and good safety.[1] However, the lack of suitable electrode materials with high
capacity and enhanced rate performance makes it difficult for real applications. Herein, we report the preparation of
three dimensional (3D) reduced graphene oxide (RGO)-supported SnS2 nanosheets hybrid and its enhanced
electrochemical performance as a novel electrode for AIBs. A to-date one of the highest capacities of 392 mAh g-1 at
100 mA g-1 and good cycling stability is achieved in the resultant new material. The 3D reduced graphene oxidebased
network and nano-size active material endow the composite high electronic conductivity and fast kinetic
diffusion, which contributes to the high rate performance (112 mAh g-1 at 1000 mA g-1). Furthermore, our detailed
characterization also verifies the intercalation and de-intercalation of the aluminum anions into the layered SnS2
nanosheets during the charge-discharge process.[2]
References
1. M. C. Lin, M. Gong, B. Lu, Y. Wu, D. Y. Wang, M. Guan, M. Angell, C. Chen, J. Yang, B. J. Hwang, H. Dai, Nature
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2. Y. X. Hu, B. Luo, D. Ye, X. Zhu, M. Lyu, L. Wang, Adv. Mater. 2017, 29, 1606132.

Lithium ion batteries (LIBs) are currently one of the preferred technologies to store electrical energy. However, the worldwide availability of Li is limited, and it is questionable whether the rising energy storage demands can be fulfilled by LIBs in the future [1]. Sodium ion battery (SIB) technology could provide an alternative, as Na is readily available, cheap, and environmentally friendly. SIBs additionally allow Al to be used as current collector instead of Cu, which is heavier and more expensive. Recently, the excellent performance of a SIB anode based on TiO₂ nanoparticles has been demonstrated [2,3]. In contrast to the well-optimized electrochemical behavior of state-of-the-art anodes, however, there is still a lack of understanding of the mechanisms involved in the performance of these nanoparticle anodes. In particular, it is challenging to unravel the structural and electronic changes of the anatase TiO₂ nanoparticles and the loss of crystallinity of the anodic nanomaterial upon Na uptake. For instance, it has been shown that initial sodiation leads to an irreversible capacity loss of nearly 40% [3].
X-ray absorption near edge spectroscopy (XANES) of the Ti K-edge is ideal for studying the electronic and geometrical structure around the probed Ti atom and its nearest neighbors and, hence, can reveal effects of sodium intercalation. However, ex situ experiments performed on disassembled electrodes can raise questions about the relevance of the results due to the expected changes of the electrode material due to air exposure, sample transport, and preparation. To gain direct, relevant insights into the intercalation process of Na into the TiO₂ nanoparticle anode material, we designed an operando XANES experiment using a modified coin cell equipped with an x-ray transparent Kapton® window and a 6 μm-thick Al foil current collector. This setup allowed the measurement of the Ti K-edge (at the HZB BESSY II synchrotron source in the HiKE endstation [4] at the KMC-1 beamline) of the TiO₂ electrode while cycling the coin cell battery, i.e. de/sodiating the electrode.
The Ti K absorption edge reveals the average Ti oxidation state of the TiO₂ anode material, which changes during the sodiation from the expected +4 (Ti⁴⁺) oxidation state in the original anatase structure to values below +3. The operando results of the Ti K-edge show, that the oxidation state increases during the desodiation but does not reach Ti⁴⁺ again due to the presence of irreversibly intercalated sodium. The study is also focused on the evolution of the pre-edge structure of the Ti K-edge during the dis/charging. In the initial sodiation cycle, the pre-peak changes from the characteristic four-peak feature related to anatase [5] to a structure dominated by a single peak, hinting at a change in the number of nearest neighbors around the probed Ti atoms (from six to five or even four neighbors) [6]. The single peak intensity increases during the first desodiation, suggesting that the anode material does not completely recover the original anatase structure after the first full dis/charging cycle. The subsequent cycling shows the same spectroscopic trend on the pre-edge feature, revealing a stable intercalation process after the irreversible structural rearrangement of the TiO₂ during the first sodiation process.

[1] C. Wadia et al., Journal of Power Sources 196 1593-1598 (2011).
[2] L. Wu et al., Journal of Power Sources 251, 379-385 (2014).
[3] L. Wu et al., Adv. Energy Mater. 5, 1401142, 1-11 (2015).
[4] F. Schaefers et al., Rev. Sci. Instrum. 78, 123102 (2007).
[5] R. Brydson et al., J. Phys.: Condens. Matter 797-812 (1989).
[6] F. Farges et al., Physical Review B, Vol. 56, No. 4, 1809-1819 (1997).

Stable intercalation anodes for Li-ion batteries are generally comprised of two groups: graphite and Ti/Nb oxides. Graphite electrodes are reversible but can’t be operated at high current rates because of particle fracture and dendrite formation as a result of volume expansion and a low reaction potential (0.1 V). Li₄Ti₅O₁₂ is a popular insertion anode that demonstrates good stability (zero strain) and fast charging but its high reaction voltage (1.55 V) lowers the energy density of the full cell. A crystal framework that allows low strain Li-insertion at a lower potential (i.e. 0.5 -1.0 V) could allow for fast charging with less propensity for dendrite formation while also increasing the energy density of the cell. Intermetallic compounds tend to have reaction voltages with Li between 0.2 V – 1.0 V, which makes them interesting candidates for insertion anodes. However, Li alloying accompanied by a large volume expansion is common in these materials (e.g. Si, Ge, Sn) so host frameworks that allow bulk Li insertion into the structure before a Li alloying reaction are needed.

Intermetallic clathrates are crystal structures that are comprised of a group IV framework of cages which host alkali guest atoms in the center of the cages (e.g. Ba₈Si₄₆, K₈Ge₄₄). This structure type has led to many interesting materials properties such as thermoelectricity, superconductivity, hydrogen storage, and tunable optical properties. Recently, our group has been studying the electrochemical reactions of clathrates with Li to understand how the defects of clathrates affect the electrochemical properties and if reversible Li insertion into the crystal structure is possible.

Our recent results investigating the Ba₈Al_ySi_46-y and Ba₈Al_yGe_46-y Type I clathrate systems will be summarized^1,2. Interestingly, the Si clathrates do not under go amorphization reactions typical of intermetallic compounds, indicating that the structure is stable in the potential range for anodes. The Ge clathrates, however, do undergo amorphization reactions to form Li-rich amorphous phases. From density functional theory calculations, X-ray diffraction, and electrochemical impedance spectroscopy, Li insertion into the cage structure seems unlikely. We find that the Ba inside the cage frustrates Li mobility and that bulk diffusion in the cage structure would require guest atom vacancies.

Next, the synthesis and electrochemical characterization of empty type II Si clathrates (Si₁₃₆) would be reported. Previous work has proven with nuclear magnetic resonance (NMR) that Li can be inserted into the empty clathrate cages of Si₁₃₆. This manifests as a potential plateau at (300 mV) prior to the amorphization of the Si lattice. This is distinctly different from diamond structured Si which only shows the typical two-phase amorphization reaction plateau. Initial results suggest that reversible Li insertion in Si₁₃₆ is possible by applying a voltage cutoff before the amorphization reaction. We aim to understand the pathways, reversibility, and kinetics of Li insertion in the empty type II Si framework and evaluate them as Li insertion anodes. Since the Li insertion voltage is 300 mV for Si_136, the risk of dendrite formation at higher current rates could be reduced while maintaining higher energy density cell due to a low reaction voltage. These results would be interesting for the future design of anodes that have a lower reaction voltage than the Ti/Nb oxides but still demonstrate rapid, reversible Li insertion.

1. Zhao, R. et al. Anodes for Lithium-Ion Batteries Based on Type I Silicon Clathrate Ba ₈ Al ₁₆ Si ₃₀ - Role of Processing on Surface Properties and Electrochemical Behavior. ACS Appl. Mater. Interfaces 9, 41246–41257 (2017).
2. Dopilka, A. et al. Experimental and Computational Study of the Lithiation of Ba ₈ Al _y Ge _{46– y} Based Type I Germanium Clathrates. ACS Appl. Mater. Interfaces acsami.8b11509 (2018). doi:10.1021/acsami.8b11509

Magnesium-ion batteries (MIBs) or magnesium batteries (MBs) have the potential benefits of using Mg element, such as natural abundance in the earth’s crust, its low redox potential (−2.37 V vs SHE), and the divalency enabling a higher capacity of the cathode material. In addition, magnesium has a higher melting point than lithium and dendrite-free deposition/stripping properties, making the operation of MIBs or MBs safer than LIBs.
However, MIBs suffer from a low energy density of cathode materials in a conventional nonaqueous electrolyte, contrary to the expectation due to the divalent Mg ion. In this talk, we report H₂V₃O₈ (= V₃O₇H₂O) as a high-energy cathode material for MIBs. It exhibits reversible magnesium intercalation behavior with an initial discharge capacity of 231 mAh g⁻¹ at 60 °C, and an average discharge voltage of ∼1.9 V vs Mg/Mg²⁺ in an electrolyte of 0.5 M Mg(ClO₄)₂ in acetonitrile, resulting in a high energy density of 440 Wh kg⁻¹. It was confirmed that the structural water remains stable during cycling. The crystal structure for the magnesium-intercalated phase Mg_0.97H₂V₃O₈ was determined for the first time. Bond valence sum difference mapping showed facile conduction pathways for Mg ions in the structure. The high performance of this material with its distinct crystal structure employing water−metal bonding and hydrogen bonding provides insights to search for new oxide-based stable and high-energy materials for MIBs. In this presentation, some results of electrochemical intercalation chemistry of magnesium ions into other layered vanadium oxides will be also presented.

We have shown that it is possible to reversibly intercalate two lithium or sodium ions into phosphate host lattices without any degradation of the host lattice. Such reactions are highly dependent on the host lattice. We have shown that two lithium ions can intercalate into the e- and b-VOPO₄ phases. In these lattices, the first lithium ion intercalates by a slow two-phase reaction VOPO₄+LiVOPO₄, whereas the second ion intercalates by a fast single-phase reaction, Li_1+xVOPO₄. However, for the larger sodium ion, the expanded lattice formed by the de-intercalation of K from KVOPO₄ is required for the intercalation of two sodium ions. The synthesis and characterization of these materials will be described, together with future opportunities. This work was supported by the DOE-EFRC-NECCES.

One key strategy to overcome the current limitations on energy density of Li-ion batteries is to explore novel cathode chemistries capable of multi-electron transfer per transition-metal redox center. To that end, vanadyl phosphates (VOPO4) seem attractive, since V can assume a variety of oxidation states ranging from 2+ to 5+. Among various VOPO4 polymorphs studied, epsilon-VOPO4 with triclinic structure (often referred to as alpha-polymorph) has emerged as the most promising candidate [1]. It demonstrates the theoretical capacity of 305 mAh/g for 2Li intercalation as a result of multiple redox reactions V5+/V4+ (~4.0V) and V4+/V3+ (~2.5V), which corresponds to the theoretical energy density of 900 Wh/kg. Despite this, the true multi-electron capabilities of VOPO4 has never been realized until very recently [2]. This is due to kinetically sluggish V5+/V4+ redox in the high-voltage region, for which, smaller particles and better particles-carbon contacts are essential [3].
Herein, we report that high-energy ball milling (HEBM), which is widely employed in battery research to optimize electrochemical performance of active materials through particle size reduction, is not suitable for VOPO4. Our results reveal that structural distortions and defects introduced in the material during HEBM impede the kinetics of high-voltage region rather more severely than the low-voltage region, promoting significant side reaction contributions in the high-voltage region, irrespective of cycling conditions [4]. The present work emphasizes the need for nanoengineering of active materials without compromising their bulk structural integrity in order to fully utilize the high-energy density of multi-electron cathode materials, as recently demonstrated for hydrothermally-synthesized epsilon-VOPO4 [2].

References:
[1] B.M. Azmi, T. Ishihara, H. Nishiguchi, Y. Takita, Vanadyl phosphates of VOPO4 as a cathode of Li-ion rechargeable batteries, J. Power Sources. 119–121 (2003) 273–277. doi:10.1016/S0378-7753(03)00148-4.
[2] C. Siu, I.D. Seymour, S. Britto, H. Zhang, J. Rana, J. Feng, F.O. Omenya, H. Zhou, N.A. Chernova, G. Zhou, C.P. Grey, L.F.J. Piper, M.S. Whittingham, Enabling multi-electron reaction of ε-VOPO 4 to reach theoretical capacity for lithium-ion batteries, Chem. Commun. 54 (2018) 7802–7805. doi:10.1039/C8CC02386G.
[3] Y.-C. Lin, B. Wen, K.M. Wiaderek, S. Sallis, H. Liu, S.H. Lapidus, O.J. Borkiewicz, N.F. Quackenbush, N.A. Chernova, K. Karki, F. Omenya, P.J. Chupas, L.F.J. Piper, M.S. Whittingham, K.W. Chapman, S.P. Ong, Thermodynamics, Kinetics and Structural Evolution of ε-LiVOPO 4 over Multiple Lithium Intercalation, Chem. Mater. 28 (2016) 1794–1805. doi:10.1021/acs.chemmater.5b04880.
[4] J. Rana, Y. Shi, M.J. Zuba, K.M. Wiaderek, J. Feng, H. Zhou, J. Ding, T. Wu, G. Cibin, M. Balasubramanian, F.O. Omenya, N. Chernova, K. Chapman, M.S. Whittingham, L.F.J. Piper, Role of disorder in limiting the true multi-electron redox in ε-LiVOPO 4, J. Mater. Chem. A. (2018). doi:10.1039/C8TA06469E.

Tetrahedral polyoxyanion compounds make up a large family of intercalation oxides for lithium cathode and solid electrolytes pursuing in next generation lithium batteries. While cathode and solid electrolytes have different requirements on electronic conductivity, high lithium ionic conductivity is of common significance to both electrode materials for high capacity and solid state electrolytes for better safety. However, designing a fast ionic transport in tetrahedral polyoxyanion oxides requires detailed understanding of the interplay of geometric coordination with electronic band structures. This work presents a rational design of high capacity cathode, which utilizes V-doping to simultaneously engineer electronic conductivity and lithium transport in tetrahedral framework silicate, Li₂CoSiO₄ (LCSO). First-principles modeling, in coupling with electrostatic field analysis by Madelung matrix, reveals a profound structural characters of delithiated Li_xCoSiO₄ (x = 2, 1, 0). However, the calculation indicates that V-doping not only introduces significant gap states that modify both electronic conductivity and redox activity of Li₂CoSiO₄, but also significantly alter the lithium transportation, which predicts a 0.20 eV lower of the transition barrier. The calculated diffusion coefficient confirms that V-doping enhances on transport by three orders of magnitude. A series of Li₂CoV_xSi_1-xO₄/C (x = 0.00, 0.05, 0.10, 0.12, 0.16, 0.20) samples have been synthesized to verify and validate the first-principles prediction, combining with characterization by XRD, SEM and Raman spectrometry studies. Electrochemical testing shows that carbon coating LCSO with 10% V substituted Si delivers an initial discharge capacity of 220.1 mAh g^-1 with 74% coulombic efficiency and impressive cycling stability. This study may provide a new venue for high performance design of all-solid-state lithium secondary batteries from tetrahedral oxide materials.

Earth-abundant spinel ferrites are potential low-cost negative electrodes for rechargeable Li-ion batteries because they can undergo reversible conversion reactions upon deep discharge. But iron oxides can also function as positive electrodes provided that a sufficient number of cation lattice vacancies and proton-stabilized oxygen sites are present and accessible for reversible Li⁺ or Na⁺ insertion. The challenge is to create these lattice sites in an otherwise vacancy-deficient oxide. We address this challenge and achieve technology-relevant cation–insertion capacities by generating cation lattice vacancies using design strategies that significantly increase the defect nature of the spinel. First, we produce spinel ferrites in aerogel forms (materials with high surface area and through-connected mesoporosity), thus amplifying the surface-to-bulk ratio and providing facile access to vacancies at the inherently disordered surfaces. We then make the oxide highly defective with mild heat treatment in an O₂-rich atmosphere that removes synthesis precursors while avoiding crystallization. The defect nature of the spinel is further amplified by substituting a third of the iron sites with high-valent vanadium (V⁵⁺ and V⁴⁺). We find that choosing thermal treatments that predominantly result in ferrites substituted with V⁵⁺ delivers the largest Li⁺ and Na⁺-insertion capacities, approaching 130 mA h g^-1 and 70 mA h g^-1, respectively. We also report on doping the vanadium sites with aluminum (~5%) to improve cycling stability when these spinel ferrites are used as positive electrodes for Li⁺ insertion.

The success of lithium-ion cells is due in part to their versatility and ability to be customized using different electroactive materials to fill numerous consumer niches. Systems have been commercialized with numerous classes of cathodes, types of electrolytes, and various anodes. As one moves beyond lithium-ion systems to alternatives such as Mg-ion, Na-ion, or even organic systems, the number of options remains significant. Two of the most often mentioned advantages of these alternative systems are, depending on configuration, (1) materials costs as Mg or Na-ion materials costs tend to be lower, and (2) the opportunity to use Mg cations to extract more electrons from the cathode materials to yield higher capacities in the same crystallographic space. In this talk I will be discussing some of the recent advances we have made in discovering and characterizing the next generation of Mg-ion systems. In particular we have been interested in the role of Mg(salt)⁺ ion pairs, desolvation issues, and Mg⁺² transport in the solid state as they relate to electrochemical performance. Cathodes across structural families have been used and include Prussian Blues, layered oxides, and mixed anion oxyfluorides.

We study the electrochemical and chemo-mechanical responses of nanoparticles of different sizes as cathode materials in magnesium ion batteries. Distinctive structural phase transition pathways are observed in nanoparticles of different sizes in the Mg ion intercalation as characterized by X-ray diffraction analysis. Small nanoparticles exhibit a single-phase transition while heterogeneous phase evolution is observed in big nanoparticles. Further examination at nanoscale by nano-beam diffraction and energy-dispersive X-ray spectroscopy reveals that the difference in the phase evolutions could be attributed to the short diffusion length and single crystallinity in the small nanoparticles. Our work shows that engineering the nanoparticle size and crystallinity has a direct influence on Mg ion intercalation processes, which rationally guides the design of electrodes with high capacity and high stability.

Rechargeable Mg-ion batteries (MIBs) are rapidly emerging as a plausible alternative to Li-ion batteries (LIBs) in terms of energy density, scalability and operation safety. However, key challenges in addressing the electrochemical stability and cyclability of potential electrodes, such as Mg metal as anode and layered transition-metal dichalcogenides (TMD) as cathode, pose questions regarding their feasibility as electrodes in MIBs. The inadequate understanding of reaction mechanisms during (de)magnesiation process hinders the practical application and optimal design of electrode materials. Here we carry out first-principles calculations to investigate the electrochemical behavior of Mg metal and Group 14 as anodes, as well as layered TMD as cathodes. We find that unexpected self-catalytic behavior of Mg induced by surface hydroxylation can be effectively controlled by impurity enrichment (alloying with Group 14 and 15 elements) and surface oxidation. To replace Mg by Group 14 elements as MIB anodes, comprehensive kinetic and thermodynamic investigations reveal that amorphous Ge and crystalline Sn can work as potentially effective anode for Mg-ion batteries. We also find out that a small overpotential is necessary for avoiding aggregation of Mg at anode/electrolyte interfaces during Mg-X reactions. In addition to anodes, our studies demonstrate that the expansion of layer spacing in TMDs (i.e. TiS₂ and MoS₂) enables faster intercalation and diffusion of MgCl⁺ species in TMD interlayers owing to the low energy barrier, and further enhancing the theoretical energy capacity of Mg_xTMD.

Multi-electron cathodes, which utilizes more than one redox couple per transition metal, are a path towards higher capacities and energy densities. In this talk, I will discuss the efforts of the NorthEast Center for Chemical Energy Storage, a DOE Energy Frontier Research Center, in developing multi-electron cathodes for both Li and Na intercalation utilizing the V^3+/4+ and V^4+/5+ redox couples. I will demonstrate how the integrated application of density functional theory calculations, in operando characterization and electrochemical measurements has provided deep insights into the relative phase stability and electrochemical performance of the ε, β and α-I polymorphs of VOPO₄, and led to optimized cathodes achieving close to full two electron cycling. Furthermore, we have synthesized all three polymorphs starting from a single precursor – LiVOPO₄.H₂O – through careful control of the O₂ environment and temperature. Finally, I will also highlight our exploration of other VOPO₄ phases with larger alkali ions, e.g., KVOPO₄, for improved Na cycling.

The emerging market of electric vehicles (EVs) has raised the bar for development of safer rechargeable batteries with significantly higher energy densities. Even though currently lithium-ion (Li-ion) batteries are the dominant battery technology for portable electronics and EVs; the scarcity of lithium resources, its high cost of extraction from brines, and safety issues arising from their flammability have further increased the need for batteries beyond Li-ion technologies. Among various battery technologies, multivalent-ion batteries that store and deliver charge by intercalation of divalent (Mg²⁺_, Ca²⁺_, Zn²⁺) or trivalent (Al³⁺) ions into layered host materials are of interest as they can potentially deliver higher energy densities at a reduced cost compared to Li-ion batteries. Particularly, rechargeable aluminum batteries that utilize aluminum metal as the anode are considered as one of the most promising alternative energy storage systems for current battery technologies because aluminum is the most abundant metal in Earth’s crust, offers three-electron redox reactions in electrochemical systems, and can be handled in the open air leading to higher safety and facile cell fabrication. Also, it has the highest theoretical volumetric capacity of 8040 mAh cm^-3 among all metals and a reasonably high theoretical gravimetric capacity of 2980 mAh g^-1. Recently, we reported on rechargeable aluminum batteries utilizing two-dimensional (2D) V₂C MXenes as intercalation-type cathodes delivering exceptional capacities and rate-capability.¹ MXenes are a family of 2D transition metal carbides and nitrides with a general formula of M_n+1X_nT_x (M is a transition metal, X is carbon and/or nitrogen, n=1,2, and 3, and T_x represents different surface functional groups) that are produced by selective etching of the A layer atoms (i.e., Al) from MAX phases (i.e., V₂AlC), a large group of layered ternary carbides and nitrides.² Despite their high capacities, the MXene cathodes reported in our previous work showed severe capacity decay in over hundreds of cycles. Here, we report the results of our recent research on addressing this problem through a combination of designing new electrode architectures and modifying electrolyte composition. This approach results in achieving highly stable performance for MXene cathodes over hundreds of cycles. We demonstrate the fabrication of freestanding and binder-free hybrid electrode materials based on different 2D MXenes compositions with exceptional volumetric capacities and long cycle life. We also present the results of our investigation of thermodynamics and kinetics of Al³⁺ intercalation into MXene cathodes through various structural characterizations (XRD, SEM, HRTEM, and XPS) and electroanalytical techniques such as Galvanostatic Intermittent Titration Technique (GITT) and Electrochemical Impedance spectroscopy (EIS). Our research results provide invaluable insights into the mechanism of multivalent-ion intercalation into MXenes. In addition, considering that family of the MXenes now include 20 different compositions, our research guides preparation of an entire group of cathode materials for rechargeable aluminum batteries based on these 2D materials.

References
1. Vahidmohammadi, A., Hadjikhani, A., Shahbazmohamadi, S. & Beidaghi, M. Two-Dimensional Vanadium Carbide (MXene) as a High-Capacity Cathode Material for Rechargeable Aluminum Batteries. ACS Nano 11, 11135–11144 (2017).
2. Anasori, B., Lukatskaya, M. R. & Gogotsi, Y. 2D metal carbides and nitrides (MXenes) for energy storage. Nat. Rev. Mater. 2, 16098 (2017).

Multivalent ion rechargeable batteries are promising candidates as the next generation of rechargeable batteries. The advantages of metal anode such as magnesium or calcium are their high theoretical capacity, available stocks and the absence of dendrite formation. However, multivalent ions exhibit slow diffusion kinetics in solid electrodes and intercalation hosts that can operate at room temperature are currently limited to Chevrel structure type compounds [1].
In this study, we investigated the use of FePO₄-carbon composite prepared by ultracentrifugation (UC) method as multivalent-ion host structure. For nano-hybrid capacitor electrode, the electrode materials prepared by the UC method reported so far are capable of ultrafast lithium-ion insertion and extraction reaction [2]. Therefore, we used the UC method to prepare materials as host electrode for multivalent-ions intercalation. Since the irreversible magnesium ion insertion-extraction reaction in olivine type FePO₄ crystal has been reported [3], in this study, non-crystalline FePO₄ with carbon composite was used.
FePO₄-carbon composite was prepared by UC process [2]. The composite was mixed with PTFE. with a weight ratio of 9:1. The electrodes were firstly charged in 1M LiPF₆ in a 3:7 volume ratio of ethylene carbonate and diethyl carbonate. Then, a three-electrode cell with an Ag⁺/Ag double junction reference electrode was assembled using the previously charged FePO₄-carbon composite electrode. The counter electrode was an active carbon, and the electrolyte was 0.5 M magnesium bis(trifluoromethanesulfonyl)amide in acetonitrile. Charge-discharge measurements were performed at 25^oC. The valence state of Fe and local stracture in the charged / discharged electrodes were tracked by X-ray absorption spectroscopy, which was measured in a transmission mode.
Reversible charge / discharge capacity was observed after the first discharge. At 25^oC, a capacity of approximately 150 mAh per weight of active material at a rate of 1/20 C was achieved at a potential of about 2 V versus Mg⁺⁺/Mg. From TEM-EDX measurements, FePO₄ particles were found to be embedded in carbon and the intensity of magnesium was observed only in regions where FePO₄ particles exist. X-ray absorption edge at Fe K-edge shifts downward in energy during discharge, reflecting the reduction of iron ions to maintain electrical neutrality upon magnesium-ion insertion in the discharge reaction. In the presentation, we will also report about the charge-discharge characterizations of FePO₄-carbon composite in calcium and zinc ions based electrolytes.

References:
[1] D. Aurbach, Z. Lu, A. Schechter, Y. Gofer, H. Gizbar, R. Turgeman, Y. Cohen, M. Moshkovich, E. Levi, Nature 407, 724-727 (2000).
[2] K. Naoi, K. Kisu, E. Iwama, S. Nakashima, Y. Sakai, Y. Orikasa, P. Leone, N. Dupre, T. Brousse, P. Rozier, W. Naoi, P. Simon, Energy Environ. Sci. 9, 2143-2151 (2016).
[3] R. Zhang, C. Ling, ACS Appl. Mater. Interfaces 8, 18018-18026 (2016).

Rechargeable zinc electrodes are attractive forms of energy storage for commercial applications because of their high theoretical energy capacity and their use of inherently cheap, safe, and environmentally friendly materials. The zinc rechargeable electrode in commercial batteries is historically limited in depth of discharge and cycle life, which has prevented them from being widely used on a commercial scale. This limitation is partly due to irreversible reactions occurring during discharge, which lead to passivation of the zinc electrode. Despite extensive study, the properties of these oxide species and the conditions leading to their formation are not fully understood. In this work, using in-operando optical microscopy, we have identified various zinc oxide morphologies in alkaline electrodes classified as Type I and Type II in the literature as well as other species that have not yet been identified. We will further analyze these zinc compounds using in-operando x-ray diffraction and scanning electron microscopy, and chemically identify these species and the conditions at which they form using in-operando confocal Raman spectroscopy. Identification of zinc species that detrimentally affect zinc reachargeability and understanding their formation will promote the design of better electrodes and electrolytes, leading to improved cycle life and depth of discharge in zinc batteries.

Electrochemical energy storage was an important enabler of the wireless revolution and it is touted as a key component of a society that shifts away from its dependence on fossil fuels. Li-ion batteries are the primary technology when high energy devices are required. However, despite their improved functionality over older systems (e.g. lead-acid car batteries), they do not quite yet meet the emerging energy demands in transportation and grid markets. This roadblock sparked interest in the development of batteries that utilize Mg²⁺ as ionic carrier. Theoretical predictions indicate that couples exist between a Mg metal negative electrode and oxide positive electrodes that could surpass the current practical limits of current devices. Among the candidate oxides, those showing a spinel structure have been predicted as the most suitable for the reversible intercalation of ions such as Mg²⁺ or even Ca²⁺ [1], the critical reaction in the positive electrode. However, experimental validation, while incipient [2], has not been fully achieved. In this talk, we will present the most up-to-date insight into the ability of spinel oxides to diffuse and reversibly intercalate Mg²⁺. In this task, the ability to synthesize particles at small dimensions is vital, as is the characterization of chemical and physical phenomena using a combination of tools providing information at different scales. We will rely on data from X-ray diffraction, spectroscopy and scattering, electron microscopy and nuclear magnetic resonance to probe the reactions that occur when spinel oxides are used as working electrodes in cells with electrolytes containing Mg²⁺. The rationale for the choice of techniques and the key pieces they provided to complete the picture will be discussed. Our ultimate aim in the talk will be to establish relationships between crystal-chemistry, charge carrier and outcomes of the electrochemical reaction.
1. Liu, M., et al., Energy Environ. Sci. 2015, 8, 964.
2. Kim, C. et al., Adv. Mater. 2015, 27, 3377.

One-dimensional nanomaterials can offer large surface area, facile strain relaxation upon cycling and efficient electron transport pathway to achieve high electrochemical performance. Hence, nanowires have attracted increasing interest in energy related fields. We designed the single nanowire electrochemical device for in situ probing the direct relationship between electrical transport, structure, and electrochemical properties of the single nanowire electrode to understand intrinsic reason of capacity fading. The results show that during the electrochemical reaction, conductivity of the nanowire electrode decreased, which limits the cycle life of the devices. We have developed a facile and high-yield strategy for the oriented formation of CNTs from metal−organic frameworks (MOFs). The appropriate graphitic N doping and the confined metal nanoparticles in CNTs both increase the densities of states near the Fermi level and reduce the work function, hence efficiently enhancing its oxygen reduction activity. Then, we fabricated a field-tuned hydrogen evolution reaction (HER) device with an individual MoS₂ nanosheet to explore the impact of field effect on catalysis. In addition, we demonstrated the critical role of structural H₂O. The results suggest that the H₂O-solvated Zn²⁺ possesses largely reduced effective charge and thus reduced electrostatic interactions with the V₂O₅ framework, effectively promoting its diffusion. We also identified the exciting electrochemical properties (including high electric conductivity, small volume change and self-preserving effect) and superior sodium storage performance of alkaline earth metal vanadates through preparing CaV₄O₉ nanowires. Our work presented here can inspire new thought in constructing novel one-dimensional structures and accelerate the development of energy storage applications.

Lithium-ion batteries (LIBs) are attractive for electrochemical energy storage due to their high energy density and efficiency. The continuous research and development on LIBs not only consider the storage capacity and the ability to handle high charge/discharge rates but also safety, cost, and cycle life.

Crystalline V₂O₃ is a promising candidate for LIBs on account of its natural abundance, and low toxicity; however, there have been only a few studies exploring so far.[1-3] The current state of the art assumes that V₂O₃ undergoes a significant volume change during galvanostatic charge/discharge cycling; this effect and the low electronic conductivity explain the poor cycling stability. To overcome the limited performance of V₂O₃ mixed with a conductive additive, recent studies have explored hybrid electrodes, which provides a nanoscopic chemical blending of the metal oxide phase with carbon, thus, improved electrochemical performance.[4]

In this work, we have developed a simplified synthesis of V₂O₃/carbon core/shell hybrid materials by thermal treatment of VC and NiCl₂.6H₂O in an inert gas atmosphere, followed by washing of the produced material with aqueous HCl to be freed from by-products. Varying the NiCl₂.6H₂O to VC ratio, the material for optimum conditions yielded a capacity of 110 mAh/g at 2.5 A/g which increased to 225 mAh/g at 0.1 A/g after 500 cycles in the potential range of 0.01-3.00 V vs. Li/Li⁺. This enhanced performance is in stark contrast to the loss of lithium uptake capacity when using commercially available V₂O₃ physically mixed with carbon black, where 90 % of the initial capacity was lost after 50 cycles. Post-mortem analysis of the hybrid materials indicates no significant structural changes of the rhombohedral V₂O₃ but a decreased degree of graphitic ordering of the carbon phase.
[1] Jiang L, Qu Y, Ren Z, Yu P, Zhao D, Zhou W, et al. In Situ Carbon-Coated Yolk–Shell V₂O₃ Microspheres for Lithium-Ion Batteries. ACS Appl Mater Interfaces. 2015;7(3):1595-601.
[2] Shi Y, Zhang ZJ, Wexler D, Chou SL, Gao J, Abruna HD, et al. Facile Synthesis of Porous V₂O₃/C Composites as Lithium Storage Material With Enhanced Capacity and Good Rate Capability. J Power Sources. 2015;275:392-8.
[3] Sun YF, Jiang SS, Bi WT, Wu CZ, Xie Y. Highly Ordered Lamellar V₂O₃-Based Hybrid Nanorods Towards Superior Aqueous Lithium-Ion Battery Performance. J Power Sources. 2011;196(20):8644-50.
[4] Fleischmann S, Tolosa A, Presser V. Design of Carbon/Metal Oxide Hybrids for Electrochemical Energy Storage. Chem - Eur J. 2018;24:12143-53.

There is an increasing demand of high safety, high energy density and lowcost energy storage device for flexible electronics. In this aspect, zinc ion batteries (ZIBs) have received incremental attention because of their high safety, abundance of Zn source, and environmental friendliness. Considering the practical applications of ZIBs for flexible electronics, flexible electrodes are highly demanded. For current electrode fabrication, a current collector is usually used to ensure the integrity and conductivity of the electrode. However, the thickness and weight of the electrode is greatly increased when using a current collector, resulting in low energy density. In the present work, we demonstrate a polyacrylic acid (PAA)-assisted assembling strategy to fabricate freestanding and flexible MnO₂/carbon nanotubes (MnO₂/CNT) cathodes for ZIBs. PAA plays an important role in serving as an agent to provide good mechanical properties. The interaction between carboxylic (–COOH) groups in PAA and hydroxyl (–OH) groups on the surfaces of MnO₂ is utilized to assemble MnO₂ nanosheets around the CNTs scaffolds, which provides good mechanical integrity of the electrodes. When using Polyacrylamide/ZnSO₄/MnSO₄ gel as electrolyte, the as-fabricated quasi-solid-state Zn-MnO₂/CNT batteries show high capacity (330 mAh g^-1 at 0.15 A g^-1), and excellent cycling stability. The capacity retention remains 80% after 1000 charge/discharge cycles at 1.5 A g^-1. Our work demonstrates that the flexible MnO₂/CNT composite electrodes are one of the most attractive cathodes in zinc storage applications.