Symposium CM03 : In Situ/Operando Analysis of Electrochemical Materials and Interfaces

Lithium ion batteries have demonstrated their importance in portable electronics and as alternative to fossil based portable energy in automotive applications.[1] This importance is expected to continue at least in the near and intermediated future. However, in the longer term electrode materials will need improved capacity and charge/discharge rates. As new anode and cathode materials are developed[2] they are typically screened for advantageous properties by assembly into a working battery. This typically involves film fabrication from a mixture of conductive material (e.g.carbon), a binder (e.g. polyvinylidene fluoride), and the active material of interest. How this film is cast onto the current collector, the ratio of the individual components of the film, the drying procedure for the film and the final assembly of the cell can significantly alter the performance of the battery.[3,4] In order to avoid misleading information about the effectiveness of a novel active material many cells are required to validate findings.

Here we present micro-pipet measurements[5,6] which demonstrate the suitability of the technique for probing lithium ion battery materials. Specifically, we probed dispersions of active materials to determine the oxidation and reduction potentials, and the charge capacity of the material. Data obtained on candidate materials by the micro-pipet method was compared to coin cell measurements, to critically assess this technique for characterization of active battery materials.

[1] F. T. Wagner, B. Lakshmanan, M. F. Mathias, J. Phys. Chem. Lett., 1 (2010) 2204–2219 [2] M.S. Whittingham; Chem. Rev., 104 (2004) 4271–4302
[3] P. G. Bruce, B. Scrosati, J.-M. Tarascon, Angew. Chem.-Int. Ed., 47 (2008) 2930-2946
[4] C. Ban, Z. Wu, D. T. Gillaspie, L. Chen, Y. Yan, J. L. Blackburn, A. C. Dillon,Adv. Mater., 22 (2010) E145–E149
[5] Williams, C. G.; Edwards, M. A; Colley, A. L.; Macpherson, J. V; Unwin, P. R.Anal. Chem., 81 (2009) 2486–2495
[6] Y. Takahashi, A. Kumatani, H. Munakata, H. Inomata, K. Ito, K. Ino, H. Shiku, P. R. Unwin, Y. E. Korchev, K. Kanamura, T. Matsue, Nature Com. 5 (2014) Article no.: 5450

The energy materials and devices have been largely limited in a framework of thermodynamic and kinetic concepts or atomic and nanoscale. Synchrotron radiation based x-ray spectroscopic techniques, especially in-situ/operando capabilities, offer unique characterization in many important energy materials of energy conversion, energy storage and catalysis in regards to the functionality, complexity of material architecture, chemistry and interactions among constituents within.
It has been found that the microstructure and composition of materials as well as the microstructure evolution process have a great influence on performances in a variety of fields, e.g., energy conversion and energy storage materials, chemical and catalytic processes. In-situ/operando x-ray spectra characterization technique offers an opportunity to uncover the phase conversion, chemical environment change of elements and other very important information of solid/gas and solid/liquid interfaces in real time. We will present soft x-ray spectroscopy characterization techniques, e.g. soft x-ray absorption spectroscopy (XAS) and resonant inelastic soft x-ray scattering (RIXS), and the development of in situ/operando soft x-ray spectroscopy characterization of interfacial phenomena in energy materials and devices.
We will present a number of the experimental studies that successfully revealed the catalytic and electrochemical reactions in real time, e.g. solid (Au film)/liquid (water) electrochemical interface, Mg-ion and Li-S batteries, and solid-state hydrogen storage materials [1-5]. The experimental results demonstrate that in-situ/operando soft x-ray spectra characterization techniques provide the unique information for understanding the real reaction mechanism.
References:
1. "Mg deposition observed by in situ electrochemical Mg K-edge X-ray absorption spectroscopy", T. S. Arthur, P.-A. Glans, M. Matsui, R. Zhang, B. Ma, J.-H. Guo, Electrochem. Commun. 24, 43 (2012).
2. "The structure of interfacial water on gold electrodes studied by x-ray absorption spectroscopy", J. J. Velasco-Velez, C. H. Wu, T. A. Pascal, L. F. Wan, J.-H. Guo, D. Prendergast and M. Salmeron, Science 346, 831 (2014).
3. "Nucleophilic substitution between polysulfides and binders unexpectedly stabilizing lithium sulfur battery", M. Ling, L. Zhang, T. Zheng, J. Feng, J.-H. Guo, L. Mai, G. Liu, Nano Energy 38, 82 (2017).
4. "Interfacial insights from operando sXAS/TEM for magnesium metal deposition with borohydride electrolytes", T. Arthur, P.-A. Glans, N. Singh, O. Tutusaus, K. Nie, Y.-S. Liu, F. Mizuno, J.-H. Guo, D. H. Alsem, N. Salmon, R. Mohtadi, Chem. Mater. 29, 7183 (2017).
5. "Revealing the Electrochemical Charging Mechanism of Nanosized Li2S by in Situ and Operando X-ray Absorption Spectroscopy", L. Zhang, D. Sun, J. Feng, E. Cairns, J.-H. Guo, Nano Lett. 17, 5084 (2017).

In order to design electrode materials for optimal combinations of energy and power, it is essential to understand kinetic barriers at all applicable length scales and over a wide range of state-of-charge. Here, using a recently developed single-electrode-particle characterization method,¹ we investigate the rate-limiting transport mechanisms in NMC and NCA cathodes. EIS and PITT measurements have been performed on single secondary particles of ~25 µm size, as a function of charge voltage and liquid electrolyte composition. We find that with increasing charge voltage, transport is increasingly limited by surface reaction kinetics; thus increasing the exchange current density is critical to obtaining high capacity utilization at high voltage. Upon performing the single-particle measurements in electrolytes containing salts with different anion groups, we find that electrolytes containing LiTFSI salt have, surprisingly, an order of magnitude higher exchange current density compared to electrolytes containing LiPF₆ salt, and that this improvement is retained to high charge voltages. The improved interfacial kinetics lead to a significantly higher materials utilization during fast charge/discharge, in both the single-particle measurements and in experiments on macroscopic composite electrodes. Possible origins of the strong anion species dependence of interfacial kinetics, and interfacial characterization in these systems, will be presented.
This work was supported as part of the NorthEast Center for Chemical Energy Storage (NECCES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0012583. P.-C. Tsai thanks the Ministry of Science and Technology, Taiwan (MOST 104-2917-I-006-006), for financial support.
Reference:
[1] P.-C. Tsai, B. Wen, M. Wolfman, M.-J. Choe, M. S. Pan, L. Su, K. Thornton, J. Cabana, Y.-M. Chiang, Energy Environ. Sci., 11 (4), 860-871.

The solid electrolyte interphase (SEI) is an interfacial layer formed on lithium ion battery (LiB) anode surfaces due to electrolyte decomposition at low potentials outside the electrolyte’s electrochemical stability window, and is a major source for capacity losses. Due to its electrically insulating and solvent diffusion prohibiting nature, its growth is in principle self-limiting. The ideal SEI can thus prevent further decomposition once formed, while allowing for ion conduction. However, in real systems, where electrodes experience volume and morphological changes, continued SEI growth renders LIB cyclability issues. Despite extensive research efforts to investigate the SEI, open questions still remain. These include the SEI formation processes, the SEI composition and thickness, as well as the structure-function relationship to the electrochemical cycling performance.

In a reductionist approach, we utilized simple and well-defined model systems to study SEI formation, growth, and evolution, in order to obtain an atomic scale fundamental understanding of the occurring processes. We have combined in situ x-ray reflectivity (XRR) and ex situ x-ray photoelectron spectroscopy (XPS) to probe the structure and chemistry of the SEI on two different substrates, i.e. oxide terminated crystalline silicon (Si) and pristine silicon carbide (SiC). We used various electrochemical cycling conditions, including galvanostatic, cyclic voltammetry and potential holds, for different electrolytes, such as lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC)/dimethyl carbonate (DMC). Our results of the thickness, density, roughness, porosity, and composition of the SEI show significant differences between Si and SiC. Specifically, the formation of lithium fluoride (LiF) is significantly suppressed by the presence of a surface oxide, which we attribute to its electrically insulating nature. We compare and contrast our results with recent studies of the electrocatalytic formation of LiF on metal surfaces [1]. Through combining these observations with our findings that the SEI on silicon contains low ion-conductivity lithium silicates, we hypothesize the native oxide is beneficial if a thin and smooth SEI layer is desired, but may be counterproductive if a fast ion-conduction SEI is desired.

Furthermore, we compared our XRR and XPS results with electrochemical data using a cone-cell, which eliminates parasitic currents, and were able to “count” each electron/Li-ion passed into the Si and SEI. Thus, we uniquely disentangled the Si lithiation and SEI contributions to electrochemical current measurements, yielding ultra-sensitive insights into SEI properties. This approach is even more sensitive when a non-active material such as SiC is utilized.

[1] Strmcnik et al., Nature Catalysis 2018, 1, 255.

Understanding the aprotic solution structures at the immediate vicinity of solid/liquid interface (SLI) is critically important for next generation lithium ion battery development. Yet, it is still challenging to investigate the carbonate profiles close to the diffuse layer (about 10 nm) at SLI due to the lack of a highly surface sensitive tool. In this work, we demonstrate the structures of commonly used carbonate solvents (ethylene carbonate (EC) and diethyl carbonate (DEC)) and an carbonate additive (fluoroethylene carbonate (FEC)) in a Li-ion battery electrolyte can be determined at ~17 nm above the electrode surface. This is only enabled by a nanogap surface-enhanced Raman spectroscopy (SERS) technique. SERS stems from the amplification of local electromagnetic (EM) field generated by localized surface plasmons. The local EM-field is extremely intense within metallic nanogap (<10 nm) due to the coupling effect among adjacent nanoparticles. We have developed methods to assemble gold nanoparticles (Au NPs) into large area (cm²) monolayers, which ensures the formation of long-range ordered nanogap arrays. The interparticle gap can be tuned between 1 and 4 nm by surface ligands of different sizes. The SERS enhancement factor (EF) of the carbonates in this study was found to depend on the molecular polarizability, with the maximum EF at ~10⁵ found for EC and FEC. Compared to EC, several vibration modes in FEC, such as C-C skeletal deformation, ring breathing band and C=O stretching band, shift to higher frequencies because of the displacement of a hydrogen atom by a much heavier fluorine atom in a methylene bridge. This counterintuitive observation against the commonly used “ball and spring” model in vibrational spectroscopy is mostly due to the increased bond strength in the FEC ring versus that of EC. A second order empirical polynomial of a single indeterminate best describes the correlation between the SERS band integration of and EC molar fraction, which allows for quantifying the electrolyte species in the carbonate mixture at SLI using SERS.
Our findings open up new opportunities for in-depth understanding of the electrolyte molecular structures at direct solid/liquid interface, which is closely related to the Li-ion battery performance such as energy density, life time and safety of the lithium rechargeable batteries.

Acknowledgment

This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO). SERS measurements were performed at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Near the atomic limit, layered materials such as graphite, transition metal dichalcogenides, and black phosphorus demonstrate enhanced transparency, conductivity, and storage capacity, making them attractive electrode materials for battery and optoelectronic applications. To develop sustainable energy storage devices, however, it is critical to understand electrode-electrolyte interactions. In particular, elucidating the mechanisms of intercalation, SEI formation, and ion transport are areas of active research. To explore these processes in-situ, we have developed a planar battery cell that enables us to visualize intercalation events during charge and discharge cycles. In this work, we focus on the electrochemical intercalation of bisulfate into ultrathin graphite as a model for aqueous intercalation. This compound is of interest, since it exhibits well-defined staging and can be reversibly cycled, paralleling battery technologies, but in an aqueous environment. Our work highlights the important factors to consider when designing an electrochemical cell for aqueous intercalation, specifically in a strongly oxidizing electrolyte, and describes a cell design that enables in-situ optical imaging of both bulk and ultrathin graphite. Qualitatively, we image the intercalation process under an optical microscope, and observe intercalation, deformation, and degradation of the material during cyclic voltammetry and galvanostatic cycling. By visualizing the process of intercalation in-situ, we can acquire and analyze complex image sequences to extract information about the rate of ion transport and diffusion in bulk and ultrathin graphite. To correlate optical images with charge transfer, we perform in-situ reflectance measurements as well as Raman spectroscopy at distinct locations in the bulk crystal and on individual graphite flakes. By coupling optical images with in-situ spectroscopic techniques, we gain insight into how intercalation events and charge transfer occur in a bulk crystal compared to a few-layer electrode. In addition, our observation of differences in color and charge transfer between bulk and ultrathin flakes of varying morphologies highlights the importance of understanding the factors that affect intercalation. The presence of edge sites and grain boundaries are of particular interest since they present a likely pathway to initiate intercalation. Defects within the layers can also lead to degradation and non-uniform charging of the material. However, the role that edges, grain boundaries, and defects play in the ion transport mechanism between the layers of ultrathin materials is not well understood. Thus, we can utilize transmission electron microscopy to study these features in individual graphite flakes prior to intercalation. Combining highly-resolved information about intrinsic defects with spatially-resolved dynamics through optical imaging would provide critical insight into the mechanism of intercalation in 2D electrodes.

Conceptually, there are two related electrochemical storage mechanisms for electrochemical energy storage materials: insertion where an ion inserts into a structure on reduction and then is removed from the structural lattice upon oxidation, and conversion where there is a chemical reaction leading to a new material or phase. For some materials, each of these mechanisms may participate at different stages of the electrochemical redox process, where the kinetics for ion and electron transport can play a determinstic role regarding which process dominates at a particular state of (dis)charge. Complementary insights gained from ex situ, in situ, and operando spectroscopy, diffraction and electrochemistry studies will be highlighted in this presentation, emphasizing materials which undergo both insertion and conversion processes.

Materials adopting two-dimensional (2D) layered structure have been actively explored as electrode for lithium ion batteries since their unique crystal structures is beneficial for lithium ions to be intercalated between layers[1]. Metal chalcogenides which have 2D layered structure have demonstrated intriguing multi-step reaction with lithium ions; for example, it is known that lithiation of tin disulfide (SnS₂) takes place via intercalation, conversion, and alloying[2]. As electrochemical properties are highly dependent on how these complicated reactions proceed, investigation of the reaction pathways with in situ analysis is of importance to improve the electrochemical properties of electrode materials. However, thorough understanding of each reaction mechanism of SnS₂ is still missing and full scenario of lithiation dynamics remains elusive.

In this work, we examine the dynamic lithiation process of tin disulfide using in situ transmission electron microscopy (TEM) and first-principles calculations[3]. Structural evolutions induced by lithium insertion are reflected in diffraction peak shift, appearance and disappearance of peaks; thus, we could distinguish reaction steps by the modifications in diffraction profiles. We find 4 sequential steps of lithiation reaction: intercalation, disordering, conversion and alloying, which is different from well-known three stages. Disordering step is suggested for the first time. After Li ions are intercalated between S-S layer, rock-salt phase is formed by the disordering of Sn and Li cations. As all the octahedral sites are filled with cations in rock-salt phase, intercalation channel can be restricted. In order for further lithiation, decomposition of rock-salt phase may be inevitable, resulting in a conversion reaction. First principles calculations are conducted not only to elucidate the ground state reaction pathways but to validate the founding from experiments. Due to discrepancies between lithiation reactions at equilibrium state and empirical results, we simulate non-equilibrium reaction pathways using non-equilibrium phase searching method [4]. Calculation results corroborate that rearrangement of cations would not increase the energy of whole system and the formation of rock-salt phase is energetically more favorable than other LiSnS₂ polymorphs, which is well-matched with real-time TEM observation.
References:
[1] J.-M. Tarascon, M. Armand, Nature 414 (2001), P. 359.
[2] T.-J. Kim et al. J. Power Sources 167 (2007) p. 529.
[3] S. Hwang et al. ACS Nano, 12, (2018) p. 3638–3645.
[4] Z. Yao et al. Chem. Mater. 29 (2017) p. 9011.
[5] This work is supported by the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy (DOE), Office of Basic Energy Science, under Contract No. DE- SC0012704.

Nanoparticles for Li-ion battery anodes provide high surfaces areas to mediate fast ion transport between the electrode and electrolyte. HRTEM has been used to show that porous nanoparticles can be produced by wet chemistry. The porous morphology of these nanoparticles is then advantageous in mitigating electrode degradation of high theoretical capacity materials such as Si and Sn that are known to undergo significant volume changes and pulverization when the particle sizes are above ~ 150 nm. Therefore, tuning the nanoparticle geometries can be exploited to increase the rate of Li-ion insertion and abstraction, increase the amount of Li-ion storage, and obtain the ideal nanoparticle volume fraction to mitigate material degradation at the individual nanoparticle level and for the anode composite. In this work, we have tested SnO₂ nanocrystalline hollow nanoparticle clusters, where the nanocrystalline particle sizes are tuned separately from the hollow nanoparticle cluster size. In this geometry, the nanocrystallites were tested in a range from 6 – 30 nm, which composed larger hollow nanoparticle structures of 70 – 100 nm in diameter. SnO₂ nanocrystalline cluster nanoparticles were tested in an open-cell configuration using the Nanofactory STM-TEM holder at 300 kV in a TEM. A direct electron detection camera (Gatan K2-IS) attached to a Titan 300kV TEM can be used to monitor the volume changes in the anode hollow nanoparticles during charge cycling. Small nanocrystalline clusters observed even volume expansion and contraction during charge cycling, with no fracture observed within the nanoparticle structure. This nano-anode morphology is promising for the fast Li-ion transport required for fast charging of portable electronic devices.
NR and ST acknowledge the Department of Science and Technology (DST), India, for support, and the Advance Facility for Microscopy & Microanalysis (AFMM) in IISc, Bangalore, for TEM facilities. ST is now at UConn. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

The maximum power output and minimum charging time of a lithium-ion battery – key parameters for its use in, for example, transportation applications – depend on mixed ionic– electronic diffusion. While the discharge/charge rate and capacity can be tuned by varying the composite electrode structure, ionic transport within the active particles represents a fundamental limitation. Thus, to achieve high rates, particles are frequently reduced to nanosize dimensions despite this being disadvantageous in terms of volumetric packing density as well as cost, stability, and sustainability considerations. As an alternative to nanoscaling, we show that complex niobium tungsten oxides with topologically frustrated polyhedral arrangements and dense μm-scale particle morphologies can rapidly and reversibly intercalate large quantities of lithium. Multielectron redox, buffered volume expansion, and extremely fast lithium transport approaching that of a liquid lead to extremely high volumetric capacities and rate performance as very recently reported in both crystallographic shear structure and bronze-like niobium tungsten oxides[1]. The active materials Nb₁₆W₅O₅₅and Nb₁₈W₁₆O₉₃ offer new strategies toward designing electrodes with advantages in energy density, scalability, electrode architecture/complexity and cost as alternatives to the state-of-the-art high-rate anode material Li₄Ti₅O₁₂.

Characterisation of these phenomena and complex material evolution will be presented with structural and chemical insights from operando X-ray diffraction and multi-edge X-ray absorption spectroscopy as well as neutron diffraction and nuclear magnetic resonance spectroscopy. The direct measurement of solid-state lithium diffusion coefficients (D_Li) with pulsed field gradient NMR demonstrates room temperature D_Li values of 10^–12–10^–13m²×s^–1 in the niobium tungsten oxides, which is several orders-of-magnitude faster than typical electrode materials and corresponds to a characteristic diffusion length of ~10 μm for a 1 minute discharge. Materials and mechanisms that enable lithiation of μm particles in minutes have implications for high power applications, fast charging devices, all-solid-state batteries, and general approaches to electrode design and material discovery.

[1] Griffith, Kent J.; Wiaderek, Kamila M.; Cibin, Giannantonio; Marbella, Lauren E.; Grey, Clare P. Niobium Tungsten Oxides for High-Rate Lithium-ion Energy Storage. Nature, 2018, 559, 556–563.

Next generation of energy storage and sensors may largely benefit from fast Li⁺ ceramic electrolyte conductors to allow for safe and efficient batteries and real-time monitoring anthropogenic CO₂. Recently, Li-solid state conductors based on Li-garnet structures received attention due to their fast transfer properties and safe operation over a wide temperature range. Through this presentation basic theory and history of Li-garnets will first be introduced and critically reflected towards new device opportunities demonstrating that these electrolytes may be the start of an era to not only store energy or sense the environment but also to emulate data and information based on simple electrochemistry device architecture twists.
In the first part we focus on the fundamental investigation of the electro-chemo-mechanic characteristics and design of disordered to crystallizing Li-garnet structure types and their description. Understanding the fundamental transport in solid state and asking the provocative question: how do Li-amorphous to crystalline structures conduct? As well, as how can we alter their charge-and mass transport properties for solid electrolytes and towards electrodes is discussed. Here, we firstly present new Li-garnet battery architectures for which we discuss lithium titanate and antimony electrodes in their making, electrochemistry and assembly to full battery architectures. Secondly, new insights on degree of glassy to crystalline Li-garnet thin films are presented based on model experiments of the structure types. Here, the thermodynamic stability range of maximum Li-conduction, phase, nucleation and growth of nanostructure is discussed using high resolution TEM studies, near order Raman investigations on the Li-bands and electrochemical transport measurements. The insights provide novel aspects of material structure designs for both the Li-garnet structures (bulk to films) and their interfaces to electrodes, which we either functionalize to store energy for next generation solid state batteries or ... make new applications such as Li-operated CO₂ sensor tracker chips. As a final part we review in a more holistic picture how one can use such materials and change the electrochemistry from energy storage, chemical sensing to data emulation for which we see prospect for electric vehicles, the Internet of Things or hardware in artificial intelligence.

All solid-state batteries are promising solutions for high energy density storage devices. Absence of volatile liquid elements in the system mitigates the safety hazards encountered in conventional batteries. Ceramic electrolytes have showcased outstanding ionic conductivities and high shear modulus, however have stability and processing challenges still exist. While theoretical studies suggested that solid electrolytes with shear moduli greater than 8.5 GPa can mitigate Li dendrite formation, recent experiments have shown contradictory results. So far, the studies to understand the failure mechanism in ceramic electrolytes have been primarily surface based ex-situ characterization/imaging techniques. Toward the goal of understanding processing-structure relationships for practical design of solid electrolytes, the present study tracks structural transformations in solid electrolytes processed at three different temperatures (1050, 1100, and 1150 °C) using synchrotron X-ray tomography. A subvolume of 300 μm3 captures the heterogeneity of the solid electrolyte microstructure while minimizing the computational intensity associated with 3D reconstructions. While the porosity decreases with increasing temperature, the underlying connectivity of the pore region increases. Solid electrolytes with interconnected pores short circuit at lower critical current densities than samples with less connected pores. These insights provide insight into the importance of microstructure on device performance.

Solid-state batteries based on ultra-high ionic conductivity solid electrolytes are a promising technology to increase battery lifetime and capacity, and reduce safety concerns associated with usage of a flammable liquid electrolyte. In recent years, sulfide solid electrolytes such as Li₁₀GeP₂S₁₂ (LGPS) have achieved ionic conductivities comparable to or higher than that of traditional liquid electrolytes. Despite these promising breakthroughs, viable high capacity and high energy density sulfide solid-state batteries have proved elusive. The small electrochemical stability window of sulfide electrolyte materials leads to undesirable reactions at the electrode/electrolyte interface against both high voltage cathode materials and Li metal anodes. This forms a solid electrolyte interphase (SEI), which dramatically degrades battery performance.
To gain a deeper fundamental understanding of the dynamic evolution of the SEI, as well as its spatially varying composition and phase, the LGPS-Li metal interface was characterized by electrochemical measurements, operando X-ray photoelectron spectroscopy (XPS), in-situ auger spectroscopy, scanning electron microscopy (SEM), and optical microscopy. This allowed for quantitative evaluation of time dependent degradation of the interface, which occurs due to the evolution of a variety of decomposition by-products. Operando XPS was used to correlate distinct decomposition products with corresponding increases in overpotential. In-situ Auger, SEM and optical Mapping of the surface shows spatial inhomogeneities leading to preferential lithium plating and corresponding Li₁₀GeP₂S₁₂ breakdown. By performing complementary electrochemical measurements of bulk solid-state batteries employing Li metal, electrochemical signatures associated with interfacial degradation can be identified. Using these techniques and the new mechanistic insights gained, rational interfacial design strategies are identified that can provide a pathway to limit interfacial instability and improve battery performance.

The theoretical capacity of Li metal anode (3860 mAh g^–1) is more than ten times greater than those of other practical anodes such as graphite and Li₄Ti₅O₁₂ for lithium ion battery. However, Li dendrites cause short-circuit in liquid electrolytes during cycles of Li plating/stripping. On the other hand, since inorganic solid electrolytes (e.g. Li₇La₃Zr₂O₁₂) have been expected to prevent dendrite growth, all-solid-state-lithium battery (SSLB) has been regarded to innovate battery technology and enable the use of Li metal anode. Hence, it is important to obtain the fundamental understanding of Li plating/striping processes on solid-state electrolytes.
We have studied the Li plating/stripping reactions with lithium phosphorous oxynitride (LiPON) glass electrolyte and Li_6.6La₃Zr_1.6Ta_0.4O₁₂ [LLZ(Ta0.4)] coated with thin current collector (CC) films of Cu, Ni, W, and Pt [1]. The Li nucleation sites are supposed to exist at solid/solid interfaces in “lithium-free”-thin-film SSLB [2]. Hence, the nuclei must push either electrode or electrolyte to create their own spaces. This process is associated with generation of significant strain energies.
This study applies an in-situ scanning-electron microscope (SEM) observation technique to the investigation on the Li nucleation/growth and dissolution processes with various CC films (Cu, Ni, W, Pt, Au). Cu, Ni, and W are unable to form specific alloy phases with Li. Pt and Au form alloy phases with Li.
The top and bottom surfaces of a Li_1+x+yAl_x(Ti, Ge)_2−xSi_yP_3−yO₁₂(LATP) sheet (Ohara Co.) were coated with 2.5-μm-thick LiPON layers by RF magnetron sputtering. Additionally, a LLZ(Ta0.4) plate (Toshima Manufacturing Co. Ltd.) with a thickness of 0.5 mm was used as an electrolyte [3].
A current collector film (i.e. Cu, Ni, W, Pt, Au) was deposited on the top LiPON surface and LLZ(Ta0.4) surface by pulsed laser deposition (PLD). A Li film with a thickness of 2 to 3 μm was deposited within an area of 9.0 mm in diameter on the other side of a cell as the counter electrode by vacuum evaporation.
The results of dynamic observations of Li plating/stripping and Li alloying/dealloying on LiPON and LLZ(Ta0.4) coated with thin metal CC films via an in-situ SEM technique will be discussed.

The authors gratefully acknowledge JSPS 17H04894 and ALCA-SPRING for the financial supports.

[1] M. Motoyama et al., Electrochemistry, 82, 364 (2014); J. Electrochem. Soc., 162, A7067 (2015); J. Electrochem. Soc.,165,A1338 (2018).
[2] B. J. Neudecker et al., J. Electrochem. Soc., 147,517 (2000).
[3] F. Yonemoto et al., J. Power Sources, 343,207 (2017).