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).

The shift from fossil fuels toward clean, renewable energy will require significant improvements in rechargeable battery technology for electric vehicles. Current battery technology limits electric vehicles to a short travel range, slow recharge, and costly price tag. Li-ion batteries promise the high specific capacity required to replace the internal combustion engine with a number of possible earth abundant electrode materials; however, setbacks such as capacity fading hinder the full capability of these rechargeable batteries. In the search for better electrode materials, multimodal X-ray characterization spanning many relevant length scales during typical battery operation is vital in understand and overcoming the failure mechanisms of these materials. We will discuss our multimodal approach combining information from high resolution X-ray microscopy and spectra-microscopy, micro- and nano-tomography, X-ray diffraction, and X-ray absorption spectroscopy to track electrochemical, morphological, and structural changes in the electrode material in real time during typical battery operation.

Lithium (Li) metal is a high-capacity anode material (3,860 mAh g^-1) that can enable battery chemistries beyond Li-ion. However, Li metal is highly reactive and repeatedly consumed when exposed to liquid electrolyte (during battery operation) or the ambient environment (throughout battery manufacturing). Studying these corrosion reactions on the nanoscale is especially difficult due to the high chemical reactivity of both Li metal and its surface corrosion films. Here, we directly generate pure Li metal inside an environmental transmission electron microscope (TEM), revealing the nanoscale passivation and corrosion process of Li metal in oxygen (O₂), nitrogen (N₂), and water vapor (H₂O). We find that while dry O₂ and N₂ (99.9999 vol%) form uniform passivation layers on Li, trace water vapor (~1 vol%) disrupts this passivation and forms a porous film on Li metal that allows gas to penetrate and continuously react with Li. To exploit the self-passivating behavior of Li in dry conditions, we introduce a simple dry-N₂ pretreatment of Li metal to form a protective layer of Li nitride prior to battery assembly. The fast ionic conductivity and stable interface of Li nitride results in improved battery performance with dendrite-free cycling and low voltage hysteresis. Our work reveals the detailed process of Li metal passivation/corrosion and demonstrates how this mechanistic insight can guide engineering solutions for Li metal batteries.

[1]: Y. Li, et al. “Revealing nanoscale passivation and corrosion mechanisms of reactive battery materials in gas environments,” Nano Letters 17.8 (2017): 5171-5178.

Emerging technologies such as electric vehicles, which rely on high energy density batteries, necessitate the development of advanced materials to replace those currently being used in traditional lithium ion devices. Anodes employing lithium metal are the most promising given their high specific capacity (3,860 mAh/g) and low electrochemical potential (-3.04 V vs SHE). However, lithium metal is highly unstable with large irreversible capacity losses occurring during plating and stripping due to electrolyte decomposition and formation of electrochemically inactive ‘dead’ lithium. To allow for the intelligent design of lithium protection strategies, a fundamental understanding of the plating and stripping behavior is needed, particularly during the first cycle.
These dynamic processes occurring during battery operation require operando characterization. Thus, we have performed grazing incidence small angle X-ray scattering (GISAXS) during the lithium plating and stripping process in order to probe the morphology of plated lithium. Investigating the surface of a copper film in an operating lithium-copper cell, we are able to determine the lithium metal growth mechanism and particle growth rates as a function of relevant battery parameters including electrolyte composition and cycling protocol. The nucleation and growth mechanism is very sensitive to electrolyte impurities and cycling protocol, due to the different solid electrolyte interphase (SEI) formed on the copper surface under different conditions. Fluorine-containing SEI components formed at high potentials due to impurity decomposition create a more uniform current distribution leading to monodisperse columnar growth of Li metal, compared to nonuniform deposition in a baseline electrolyte. A fundamental understanding of the effect of such parameters on lithium plating can be used to engineer high energy density batteries with enhanced safety and cycle life.

Electrochemical cells based on alkali metal (Li, Na) anodes have attracted significant recent attention because of their promise for producing large increases in gravimetric energy density for energy storage in batteries. To facilitate stable, long-term operation of such cells a variety of structured electrolytes have been designed in different physical forms, ranging from soft polymer gels to hard ceramics, including nanoporous versions of these ceramics that host a liquid or molten polymer in their pores. In almost every case, the electrolytes are reported to be substantially more effective than anticipated by early theories in improving uniformity of deposition and lifetime of the metal anode. These observations have been speculated to reflect the effect of electrolyte structure in regulating ion transport to the metal electrolyte interface, thereby stabilizing metal electrodeposition processes at the anode. In this work, we create and study model structured electrolytes composed of covalently linked polymer grafted nanoparticles that host a liquid electrolyte in the pores. The electrolytes exist as freestanding membranes with effective pore size that can be systematically manipulated through straightforward control of the volume fraction of the nanoparticles. By means of physical analysis and direct visualization experiments using advanced optical microscopy, we report that at current densities approaching the diffusion limit, there is a clear transition from unstable to stable electrodeposition at Li metal electrodes in membranes with average pore sizes below 500 nm. We show that this transition is consistent with expectations from a recent theoretical analysis that takes into account local coupling between stress and ion transport at metal–electrolyte interfaces.

References:
1. Choudhury, S., Mangal, R., Agrawal, A. & Archer, L. A. A highly reversible room-temperature lithium metal battery based on crosslinked hairy nanoparticles. Nat. Commun. 6, 10101 (2015).
2. Choudhury, S. et al. Confining electrodeposition of metals in structured electrolytes. Proc. Natl. Acad. Sci. (2018).

Comprehension of redox processes and their influence on structural properties of electrode materials is a key point to improve Li-ion batteries efficiency. The rise of operando techniques leads to a better insight into these reactions due to the real-time observation of the process. In this work, we will mainly develop two examples of lithiated transition metal nitrides studies where different operando techniques were combined.
Our group has clearly demonstrated layered lithiated transition metal nitrides can be considered as genuine Li intercalation compounds and possible negative electrode materials for Li-ion batteries [1-2]. In particular attractive properties for optimized Co and Ni contents in the Li-M-N system were proved with for instance Li₂Ni_0.67N exhibiting a specific capacity of 200 mAh g^-1 and an excellent cycle life. After a precise determination of the chemical composition using NRA technique, its structural mechanism under operation was studied and solved using operando XRD upon successive discharge-charge cycles [2]. A solid-solution behaviour is shown with a very limited volume variation, less than 1% which well explains electrochemical features. A superstructure is mandatory to clearly describe the operando XRD data set recorded here. Indeed, the presence of vacancies in the Li₂N^- planes allows a slight displacement of interlayered nickel ions cell inducing a negligible swelling process of the host lattice in line with a remarkable cycle life.
With a specific capacity of 280 mAh g^-1 at C rate available around 1.2 V vs Li⁺/Li, a 3D Li₇MnN₄ has been proved to be a credible alternative to Li₄Ti₅O₁₂ for moderate to high power applications. The redox-mechanism and structural response upon a full electrochemical oxidation/reduction cycle have been studied using XAS and XRD in-situ operando techniques. The charge curve composed of two successive plateaus and a continuous potential increase, is well explained by two diphasic domains and a solid-solution behaviour [3]. After a MCR-ALS analysis of the Mn K-edge absorption spectra data set, three different environments related to three different Mn oxidation states involved in redox processes were isolated [4]. Using the simple concept of coordination charges and comparison with available data for appropriate reference oxides, the oxidation states of each environment were ascribed and fully explained the achieved specific capacity. By combining these two methods, a comprehensive scenario has been proposed to explain the attractive electrochemical performances of Li₇MnN₄ with the role of Mn 5+, Mn 6+ and Mn7+ ions here stabilized by the nitride framework.
[1] J.B. Ducros, et al, Electrochem. Com. 9 (2007) 2496-2500
[2] T. Cavoué, N. Emery, et al, Inorg. Chem. 56 (2017) 13815-13821
[3] N. Emery et al, J. Power Sources, 247 (2014) 402-405
[4] D. Muller-Bouver, N. Emery et al, PCCP 19 (2017) 27204-27211

Electrochemical device is a term used to describe a group of technologies including fuel cells, batteries, electrolysers and super-capacitors. Whilst many of these technologies are already in common daily usage, for example Li-ion batteries that power our mobile phones, in the future electrochemical devices will play an increasing role in our lives – from fuel cells that can power our homes to high performance batteries for our cars.

At a microscopic length scale, these devices can be considered as one of a general class of porous materials, whereby the physical microstructure will influence a range of phenomena, including diffusion, catalysis and conductivity – our ability to engineer these microscopic features to maximize performance can be translated to substantial improvements in macroscopic device design. At macroscopic length scales the robustness of device design will influence the system energy and power density and its ability to safely store/convert energy over extended periods of time.

As these materials are likely to evolve over time, in response to range of processing and environmental conditions (sintering, corrosion, failure etc); understanding how these changes in microstructure can be linked to understanding of degradation and failure is pivotal to improving device lifetime and safety.

Over the past 10 years the increasingly widespread use of X-ray imaging and tomography has revolutionized our understanding of these materials; with increasing sophistication researchers have been able to characterize samples over multiple time and length scales from nm to mm and from ms to days. Here we consider examples of our work to explore these materials in three and “four” dimensions, to examine materials evolution with time. We will explore case studies that utilize both laboratory and synchrotron X-ray sources across a range of spatial and temporal domains, and examine how improvements in these imaging techniques, alongside correlative spectroscopy, is providing unprecedented insight into these materials and devices.

Li-metal is a promising, high-capacity anode that can be incorporated into Li-ion, Li-S, and Li-air battery systems to meet the need for energy-dense batteries. Coulombic efficiency losses remain a major challenge to adoption and commercialization. These losses are driven by loss of lithium to the solid electrolyte interphase (SEI) and into metallic lithium deposits that become electrically disconnected (‘dead’ Li) due to complex morphological changes. While protection methods, such as coatings and electrolyte additives, have yielded improved efficiencies, fundamentally understanding how these improvements are realized will allow for optimization of their design.

Here, we have developed a method of monitoring lithium metal in the anode through plating and stripping cycles via operando X-ray diffraction (XRD). We obtain quantitative results on the reversible lithium metal, dead lithium development and evolution with cycling, and lithium corrosion. Our results show the lithium metal efficiency, measured through XRD, is higher than the electrochemical Coulombic efficiency measurements. Additionally, the contribution of ‘dead’ lithium to the overall Coulombic efficiency varies much more across electrolytes and cycling conditions than the SEI does. This new experimental methodology and the understanding on the origins of capacity loss in lithium metal anodes which it enables will lead to better-designed protection layers and electrolytes which improve anode performance.

Improving the safety and performance of Li metal anodes is key to enabling next generation batteries such as Li-Air and Li-Sulfur. However, the mechanisms governing poor performance of Li metal anodes are not fully understood, which hinders our ability to characterize, diagnose, and rationally design solutions to problems including dendrite growth and low Coulombic efficiency. In situ and operando analyses are well suited to study the mechanisms of battery degradation, as they avoid any sample alteration due to the disassembly of a cell.

Herein, we present a multifunctional operando visualization platform that synchronizes optical recordings with cycling electrochemistry. This platform was initially used to understand the origins of the “peaking” behavior exhibited in the voltage traces of Li symmetric cells [1]. However, by limiting the viewing angle to a cross sectional perspective, the impact of electrode surface variations, including microstructure, flaws, and chemical inhomogeneities cannot be fully accounted for. To address this, in this study, a plan view optical visualization cell was designed that maintains a highly uniform current distribution along the electrode surface and avoids mechanical deformation of the electrode. This enables correlation of dendrite nucleation location to surface features. Dendrite nucleation and pitting nucleation density are quantified as a function of current density on a Li metal surface. Additionally, the impact of pitting on the performance of a Li metal anode is directly observed. The impact of surface modifications on Li plating, including protective coatings and mechanical deformation, are also quantified. Finally, the knowledge generated on the coupled morphological and electrochemical behavior of Li metal deposition will be discussed as a potential pathway to develop battery failure diagnostics.

1. K. N. Wood, E. Kazyak, A. F. Chadwick, K. H. Chen, J. G. Zhang, K. Thornton, N. P. Dasgupta, ACS Central Science 2, 790 (2016)

Conversion reactions in Li ion batteries, such as the electrochemically-driven phase separation of a transition metal oxide into metal nanoparticles and lithium oxide species, are well-known to have specific capacities far beyond typical intercalation materials. However, these types of reactions invariably suffer from irreversibility and hysteresis due to their substantial volume change and kinetic barriers. Oxides also tend to have substantially lower redox potentials than thermodynamically expected values, limiting their practical use. Given the complex network of metal-rich and lithia domains evolved during lithiation, interfacial processes must play a critical role in the nucleation of the overall conversion reaction and charge/mass transport.
In contrast to the complex network of nanoparticles that form during lithiation, we have studied electrode surfaces with periodic tungsten oxide nanostructures, whose dimensions can be tuned to test the mechanical and kinetic properties of conversion reactions. Our fabrication approach uses the selective growth of ALD on block copolymers, in this case producing arrays of oxide nano-cylinders or their inverse, a film with periodic hole patterns. These electrodes are well-suited for grazing incidence small angle X-ray scattering (GISAXS), which can be used to assess the size, density, and spacing of the electrodes during the reaction. Using operando GISAXS, we find that nanoscale (50 – 80 nm) oxides undergo conversion reactions at 1.4 – 1.7 V, which is close to the theoretical value (1.65 V) and well above the discharge plateau of 0.8 – 0.9 V for micron-sized bulk powders. Insights from this study can bring a new perspective on enhancing the energy density and reversibility of conversion reactions and provide strategies for improving their overall performance.

Demand for cheap energy storage systems has led to growing interest in the development of sodium- and potassium-ion battery systems. Conversion and alloying-type electrode materials with high specific capacity are attractive options for these batteries, but the larger volumetric expansion during reaction with sodium and potassium compared to lithium is thought to limit cycle life. The nanoscale reaction mechanisms of many electrode materials with Na⁺ and K⁺ ions are unknown, however, and they must be investigated to understand how to engineer Na and K-battery materials to undergo minimal mechanical damage due to volume changes. In this study, in situ transmission electron microscopy (TEM) experiments are used to examine the nanoscale transformations of cube-shaped FeS₂ nanocrystal electrode materials as they undergo reaction with Li⁺, Na⁺, and K⁺. Although the FeS₂ nanocrystals underwent a conversion-type reaction via a two-phase mechanism with a sharp reaction front in all cases, fracture was only observed to occur during lithiation, despite the larger volumetric changes associated with sodiation and potassiation. This difference was ascribed to dissimilar shapes of the two-phase reaction fronts during the reaction processes. Specifically, reaction fronts during lithiation were found to retain rectangular shapes with sharp corners, while sodiation and potassiation caused the reaction front to take an oval shape with blunted corners. Chemomechanical modeling of stress evolution during reaction showed that the differences in the evolution of the shape of the two-phase reaction front led to higher tensile stress concentrations and particle fracture during lithiation. The results indicate that even though larger volumetric expansions take place in Na- and K-ion battery materials, these volume changes may be managed through understanding and control of nanoscale reaction mechanisms and do not necessarily cause failure of individual particles.

Designing advanced battery interfaces requires visualizing ionic, electronic, and redox pathways that indicate how local surface activation and site-specific differential reactivity impact ion-intercalation and interfacial evolution with cycling. This presentation will discuss new in-situ analytical tools based on the scanning electrochemical microscope (SECM) that incorporate functions for imaging surface redox reactivity, spectroelectrochemistry, and ionic transfer reactions for species such as Li⁺, Na⁺ and K⁺ at interfacial and bulk nanostructures in non-aqueous electrolytes.

In ionic measurements, the principle is based on SECM probes that integrate mercury micro- and nano- cap electrodes on which alkaline ions can be detected by means of fast-scan anodic stripping voltammetry. The probe potential provided chemical specificity while the probe current enables the measurement of ionic fluxes with excellent stability and linearity. We will demonstrate how these probes were used for the measurement of ion insertion sites on graphene, graphite, and silicon nanostructures. When combined with redox modes using the SECM (e.g. feedback), these probes provide a comprehensive view of how interfacial processes impact charge transfer across operating interfaces.

Complementing these powerful measurements, we will discuss how the combination of SECM with Raman spectroscopy via colocalized and simultaneous measurements of operating electrodes enables real-time correlation of electrochemical and structural information. We will show how our system is used for simultaneous imaging and time-resolved experiments of interfaces of interest for energy storage, such as redox polymers and graphene with a resolution of ~1 micron, sub-second resolution, and high signal to noise ratio. We hope our techniques will contribute to a new understanding of interfacial processes on battery structures, allowing the measurement of ionic reactivity, and elucidating the impact of interfacial processes on single reacting sites. SECM mapping reveals aspects of surface reactivity that are lost during averaging in conventional battery testing.

References:
Barton, Z. J.; Rodríguez-López, J. Fabrication and Demonstration of Mercury Disc-Well Probes for Stripping-Based Cyclic Voltammetry Scanning Electrochemical Microscopy (CV-SECM). Anal. Chem., 2017, 89, 2716-2723.

Barton, Z. J.; Hui, J.; Schorr, N. B.; Rodríguez-López, J. Detecting Potassium Ion Gradients at a Model Graphitic Interface. Electrochim. Acta, 2017, 241, 98-105.

Hernandez-Burgos, K.; Barton, Z. J.; Rodríguez-López, J. Finding Harmony between Ions and Electrons: New Tools and Concepts for Emerging Energy Storage Materials. Chem. Mater. 2017, 29(21), 8918-8931.

Schorr, N.B.; Jiang, A.G.; Rodríguez-López, J. Probing Graphene Interfacial Reactivity via Simultaneous and Co-Localized Raman-SECM Imaging and Interrogation Anal. Chem. 2018, In Press

Cryo-electron microscopy (cryo-EM) received the 2017 Nobel Prize in Chemistry for its ability to elucidate the nanostructure of biomolecules in their native state, revolutionizing the field of structural biology. Here, we pioneer an approach to utilize this powerful technique to enable new discoveries for batteries¹ (Y. Li*, Y. Li*, Y. Cui, et al. Science 2017, DOI: 10.1126/science.aam6014) and show that cryo-EM can potentially have a similar impact in materials science.

Whereas conventional transmission electron microscopy (TEM) studies are unable to preserve the native state of chemically-reactive and beam-sensitive battery materials (e.g. Li metal) after operation, such materials remain pristine at cryogenic conditions. It is then possible to atomically resolve individual Li metal atoms and their interface with the solid electrolyte interphase (SEI). We observe that dendrites in carbonate-based electrolytes grow along the <111> (preferred), <110>, or <211> directions as faceted, single-crystalline nanowires. These growth directions can change at kinks with no observable crystallographic defect. Furthermore, we reveal distinct SEI nanostructures formed in different electrolytes that explain why certain additives lead to better performance. With cryo-EM, we open up exciting new opportunities for scientific discovery, which will be critical for providing fundamental insight to battery materials design.

Yuzhang Li*, Yanbin Li*, A. Pei, K. Yan, Y. Sun, C-L Wu, L-M, Joubert, R. Chin, A.L. Koh, Y. Yu, J. Perrino, B. Butz, S. Chu, Y. Cui. “Atomic structure of sensitive battery materials and interfaces revealed by cryo-electron microscopy,” Science (2017) DOI: 10.1126/science.aam6014

*Denotes equal contribution

Interfaces play an essential role in nearly all aspects of life and are critical for electrochemistry. Electrochemical systems ranging from high-temperature solid oxide fuel cells (SOFC) to batteries to capacitors have a wide range of important interfaces between solids, liquids, and gases which play a pivotal role in how energy is stored, transferred, and/or converted. This talk will focus on our use of ambient pressure XPS (APXPS) to directly probe the solid/gas and solid/liquid electrochemical interface. In particular, I will discuss how we synergistically combine this powerful experimental technique with DFT theoretical insight to reveal a mechanistic understanding of electrochemical interfaces. I will highlight some of our recent investigations into CO₂ adsorption phenomena on various metals. In situ APXPS and DFT together provide a comprehensive understanding of the initial adsorption processes and how oxygen and water transform CO₂’s adsorption behavior. Additionally, I will highlight how theory/modeling has enhanced our knowledge of in situ/operando solid/liquid APXPS investigations including electrochemically promoted dissolution and the interaction of a magnesium electrode in a non-aqueous electrolyte. Information gained from these studies will aid in the guided design and control of future electrochemical interfaces.

Atomic force microscopy (AFM) has been adopted for in situ and in operando electrical and electrochemical (EC) studies with nanometer resolution at electrified solid-electrolyte interfaces. Recent developments include PeakForce EC-AFM for topographic and nanomechanical imaging during EC reactions on the surfaces of Li ion battery anode,s [1] and PeakForce scanning electrochemical microscopy (SECM) for the simultaneous acquisition of local EC and conductivity information at electrified solid/liquid interfaces of nanoparticle-catalysed photoelectrodes for water splitting[2 - 3]. In this work, we introduce these recently-developed techniques with a variety of examples in energy, biological, and semiconductor applications.

We have also developed DataCube SECM to provide highly-dimensional, big-data results allowing us to perform in-depth data mining for improved electrochemical kinetic quantification, 3-D nano-EC and nanomechanical characterization. This mode is based on the fast force volume mapping method, providing a force-distance spectrum in each pixel. With each probe ramping cycle, an SECM probe approach curve (PAC) is acquired. This method overcomes the limitation of conventional SECM methods where only few PACs are inefficiently captured from a single or very few point locations. By direct analysis of the experimentally achieved data cube, one can greatly improve the quantification accuracy. It also allows plotting the SECM current images at different tip-sample distances with nm step accuracy. In addition, during each force-distance cycle, the probe can be held on the surface for a user-defined dwell time. This enables nanocontact EC studies such as local surface EC potential measurement. [4-5]

In addition, many applications require performing nanoelectrical studies in electrolyte solutions, e.g. battery, bioelectricity, and bioelectromechanics. However, these are technically challenging. The general implementation requires avoiding electrical shorting, liquid spilling and chemical corrosion. Minimal parasitic electrochemistry and stray capacitance are also needed. With the use of nanoelectrode AFM tips (~25 nm active tip apex), we have developed a suite of solutions to conduct multi-dimensional nanoelectrical measurements in liquid, including piezoelectric response, conductivity, and Kelvin Probe mapping. The DataCube method has been also integrated with these implementations. For example, DataCube-Piezo Force Microscopy characterizes piezoelectric materials, where pixelwise frequency-based spectra allow studying the contact resonance properties of bio-mimic materials. [6]

[1] Kuma et al., ACS Appl. Mater. Interfaces, 2017, 9, 28406
[2] Nellist et al., Nanotechnology, 2017, 28, 095711
[3] Jiang et al., ChemSusChem 2017, 10, 4657
[4] Nellist et al., Nat. Energy, 2018, 3, 46
[5] Toma, Nat. Energy, 2018, 3, 6
[6] Cui et al., Phys. Rev. Lett, 2018 submitted

Solar-to-fuel technology promises to play a key role in realizing a carbon-neutral future by enabling renewable fuel processing for capacity-independent storage beyond current battery technologies [1]. Metal oxide catalysts enable the two-step thermochemical cycle by catalyzing the reduction of H₂O and CO₂ to produce H₂ and CO. Currently, ceria is the reference material due to its rapid kinetics, crystallographic stability, and abundance [1]. To address the low fuel production of ceria, doping with tetravalent cations and co-doping with 3+/5+ combinations are used to enhance oxygen release and thus increase maximum fuel yield [2]. However, the reaction mechanisms and kinetics of oxygen vacancy formation in doped-ceria are still unclear, and this understanding will be crucial to developing improved redox materials that will make solar-to-fuel technology economically viable.
This novel investigation utilizes in situ Raman spectroscopy under redox cycling schemes to illuminate the reaction mechanisms of oxygen vacancy formation and annihilation in 10mol% doped ceria solid solutions of the composition Ce_0.9(Me)_0.1O₂ with Me= La, Zr, Hf, Nb and in co-doped Ce_0.9La_0.05Nb_0.05O₂. In-situ Raman spectroscopy was utilized to study the chemical reduction of doped-ceria materials under H₂ atmosphere up to 900 °C and the redox cycling under alternating reducing H₂ and oxidizing CO₂ atmospheres at a constant temperature of 800 °C.
Results revealed that the peak corresponding to the triply degenerate F_2g stretching mode of the cation and oxygen ions in the cubic fluorite structure of ceria shifts to lower frequencies and decreases in intensity during reduction; these changes correspond to chemical expansion with the reduction of the cation and an increase in the oxygen vacancy concentration. Additional vacancy bands also appear at different positions depending on the dopant choice and indicate how each dopant cation facilitates the formation of oxygen vacancies
Raman spectroscopy confirms that catalytic oxygen vacancy concentration determines the fuel yield of this process: samples with the highest fuel yield also had the largest shift of the peak corresponding to the triply degenerate F_2g when heated, suggesting the highest oxygen vacancy concentration and thus the highest oxygen storage capacity. This model experiment not only provides crucial preliminary results for understanding the mechanisms of oxygen vacancy formation in doped ceria catalysts but also provides an experimental template for future investigation of reaction mechanisms of other electrochemical materials. By utilizing Raman spectroscopy to understand the redox mechanisms of various materials, as was done in this experiment, materials scientists can optimize doping to engineer the redox performance of materials for a variety of applications.

[1] M. Kubicek, et al., Journal of Materials Chemistry A, vol. 5, no. 24, 2017.
[2] C. Muhich, et al. Acta Materialia, pp. 728-737, 2018.

Detailed understanding of the opto-electronic properties of semiconductors and the driving forces and loss mechanisms that limit device performance is essential to the development of high efficiency solar energy conversion and storage systems. However, many photovoltaic and photoelectrochemical systems are difficult to model and only few experimental methods are available for direct characterization of dominant loss processes under relevant operating conditions. To this end, empirical extraction of the spatial collection efficiency (SCE) is an operando, analytical tool to study new materials, and devices. Defined as the fraction of charge carriers that are photogenerated at a given location that contribute to the measured current, the SCE provides a functional depth profile of the active regions in the device. By combining incident photons to current efficiency (IPCE) measurements with optical modeling, we have extracted SCE cross-sectional profiles of several materials and photoelectrochemical cells. Thus, SCE extraction provides an in depth understanding of the driving forces and limiting mechanisms in new materials with relatively simple apparatus. For example, analyzing the SCE at different operating potentials while performing different chemical reactions allows distinguishing between bulk and surface losses. By focusing on the SCE at the surface, we were able to discern between surface losses attributed to slow reaction kinetics and fast surface recombination processes through charged band states. In this contribution, we analyze the transport properties of four different phases of copper vanadate photoanodes with a wide range of copper vanadium ratios. The spatial collection efficiency analysis is used to extract the potential dependent surface reactivity and collection length. Phases with a high copper content show a relatively high collection length yet suffer from high losses induced by losses induced by D-D transitions.

The dynamical behavior of product bubbles can limit the energy-conversion efficiency for electrodes performing gas-evolving reactions, such as hydrogen or oxygen evolution during water splitting. Bubbles can block or restrict the activity of catalytically-active areas and reduce light absorption due to undesired light reflection. While this problem is ubiquitous for applications like water splitting, the understanding of the collective dynamics of bubbles evolving at electrodes is lacking. Here, we explore these issues by measuring the high-speed dynamics of bubbles evolved from electrode interfaces. We describe the use of high-speed x-ray and optical imaging to study in operando gas-evolving electrodes under conditions relevant solar fuels applications. Imaging the high-speed dynamics of electrolytic bubble evolution allows us to measure the effects of electrostatic surface interactions, spatial organization of catalysts, and the mechanical interactions between evolving bubbles. Through high-speed imaging, we are able to map the local reaction kinetics by tracking bubble transport at the interface to better understand the collective behavior of evolving bubbles. Finally, we will discuss strategies to use rational hierarchical structuring of the electrocatalysts to improve catalytic performance.

Photoelectrochemical (PEC) water splitting is a promising means for converting solar energy into chemical fuels. Decomposition of water into both H2 and O2 by a photocatalysts could be the simplest and efficient approach to scaling up fuel production from water. For decomposition of water, standard Gibbs free energy change of 1.23 eV is required. Therefore, a semiconductor with an energy band gap exceeding 1.23 eV,corresponding to a wavelength of 1008 nm, has the potential to decompose water by light absorption. Though semiconductors with smaller band gaps are suited for efficient use of solar energy, there are still limited number of visible-light responsive photocatalysts capable of splitting water into hydrogen and oxygen. Recently, Ta3N5 nanorods single crystals combined with Rh/Cr2O3 cocatalyst have been shown to split water into hydrogen and oxygen under visible light and simulated sunlight by Domen group. It was a remarkable achievement considering that the Ta3N5 photocatalyst had not yet accomplished overall water splitting despite its superior visible-light absorption.
In this paper, we used transient absorption spectroscopy (TAS) to study the origin of the overall water splitting activity of Ta3N5 nanorod single crystals. We show the correlation between the density of mid-gap defect states and the time of NH3 nitridation process. The results were further investigated theoretically to estimate various parameters controlling the overall water splitting reaction by the one-step excitation.The effect of cocatalyst on the carrier dynamics in Ta3N5 nanorods single crystals was also studied for the Ta3N5 nanorod with different nitridation times. Our findings suggest the correlation between the overall water splitting activity of the Ta3N5 nanorod single crystals and the change in defect states by different nitridation times.

Photoelectrochemical (PEC) water splitting is a promising approach to provide carbon neutral power and renewable fuels. The chemical transformation of water into oxygen and hydrogen takes place at the surface of catalysts and photoactive materials. Consequently, the activity, efficiency, and reaction pathway are critically controlled by the material surface properties. Under PEC operating conditions, surface properties strongly depend on the surrounding environment, and they may be altered in the course of the reaction. Thereby, absorption of molecules can modify the chemistry at the surface, for example by influencing the kinetics of reactants, products, or reaction intermediates, but they can also directly impact the electronic transport properties of the photoactive material by effectively acting as surface trap states. In this context, improved understanding of these complex surface interactions will aid the development of highly efficient light absorbers as well as the integration of effective passivation and catalyst layers for these materials.

In this work, we elucidate the influence of chemical interactions of adsorbed oxygen and water on charge transport and interfacial charge transfer of photogenerated charge carriers in polycrystalline BiVO₄ thin films – a promising material for solar water splitting. The charge transfer between adsorbates and BiVO₄ is monitored by in situ Kelvin probe measurements under dry nitrogen, humid nitrogen, and oxygen environments at atmospheric pressure. To gain complementary insight into the relationship between surface interactions and interfacial charge transport characteristics, we employ photoconductive AFM under the respective in situ conditions. By combining these complementary techniques, we demonstrate that adsorbed oxygen acts as a surface trap state for electrons, which enhances the built-in potential and depletes the BiVO₄ layer, resulting in an increase of the measured surface photovoltage. Furthermore, we have recently demonstrated that the low intrinsic bulk conductivity of BiVO₄ limits the electron transport through the film, and that the transport mechanism can be attributed to space charge limited current (SCLC) in the presence of trap states.[1] By analyzing the SCLC, we estimate that the contribution of surface adsorbed oxygen to the total number of shallow traps is as large as 40%. For humid environments, our results are consistent with the adsorption of water as an oriented dipole layer, which does not induce a surface charge transfer but partially inhibits the adsorption of oxygen at the surface. Disentangling the individual effects of oxygen and water on surface band alignment and charge carrier trapping underpins the importance of trap state passivation for efficient transport of photogenerated charge carriers in BiVO₄.

[1] Eichhorn et al. Nanoscale imaging of charge carrier transport in water splitting photoanodes, Nat. Commun. (2018).

A highly versatile method is introduced that combines microelectrochemistry, photoexcitation and downstream electrolyte analysis to a single method. The simultaneous in-situ use of all methods provides a comprehensive set of information for photoelectrochemical reactions. A 3D printed measuring head with flow guide is equipped with reference and counter electrodes and the electrolyte is continuously pumped down to an inductively coupled plasma optical emission spectrometer. Illumination of the wetted electrode spot is realised through a multi-mode optical fibre connected to a set of laser diodes being chopped for dark and illumination conditions. A state of the art physical vapour deposition system with thermal coevaporator, multi gun cosputtering system, electron deposition system and large travel range energy dispersive x-ray system is employed to fabricate lateral composition gradients of tungsten oxide with iron group metals iron, nickel and cobalt. Subsequent oxidation yields mixed oxide libraries which are characterised by scanning electron microscopy and scanning x-ray diffraction. In an attempt to quantify the performance of various compositions of mixed oxides the photoelectrochemical behaviour of each composition was studied using various illumination wavelengths and polarisation potentials. Wavelengths corresponding to energies lower than the bandgap resulted in very minor effects. Illumination with sufficient photon energy on the other showed clear increase in the photocurrent as the inherent comparison with dark current showed. Variation of the electrochemical potential compensated for the energy difference between bandgap and photostimulation. Electrochemical impedance spectroscopy under illumination was used to provide Mott Schottky plots for both, the determination of the flat band potential and the charge carrier concentration. The highest photocurrent was found fo distinguished concentrations of the iron group metal, demonstrating that this combinatorial approach is very useful in the search for high performance water splitting photoelectrodes.

Semiconductor-based nanomaterials have been widely studied as promising electrocatalysts for energy conversion and storage. Understanding the reaction pathways and active sites on electrocatalysts are of critical importance for the development of highly active electrocatalysts made of earth abundant elements. In this talk, I will give a detailed summary about our approach using various in-situ and ex-situ techniques including X-ray absorption, X-ray diffraction, X-ray photoemission spectroscopy, electrochemical impedance spectroscopy, Rama spectroscopy, etc to study electrochemical reactions.

One of the grand challenges in electrocatalysis is to understand how the properties of a given electrochemical interface give rise to its catalytic reactivity. These interfacial properties are jointly engendered by solid electrode and liquid electrolyte and evolve under operating conditions. The chemical complexity and dynamic nature of the electrocatalytic interface pose formidable experimental challenges for the establishment of property-activity relationships. Herein, we aim to establish such relationships by simultaneously probing the electrocatalytic interface with surface-selective infrared (IR) spectroscopy and online electrochemical mass-spectrometry (OLEMS) under operating conditions.

Specifically, we focus on the electroreduction of surface-adsorbed CO to hydrocarbons on Cu electrodes. While it has previously been shown that cations of the supporting electrolyte can profoundly affect the catalytic selectivity and activity of this electrochemical interface, the physical origins of these cation effects are still poorly understood. In this talk, we will discuss the relative influence of purely electrostatic and specific chemical effects on the promotion of CO reduction.

For the first time, we have found that the CO coverage on the electrode is profoundly dependent on the identity of alkali cations in the supporting electrolyte. Specifically, in the presence of Cs⁺, a dramatic drop of the CO coverage to almost zero is observed at reduction potentials around -1 V vs. RHE, while only a small dip is observed in Li⁺-containing electrolyte. Time-resolved vibrational spectroscopy of the C≡O stretch band reveals that the reduction kinetics of CO is faster by approximately one order of magnitude in the presence of Cs⁺ compared to Li⁺-containing electrolyte. We have further found marked red-shifts of the peak frequencies of the C≡O stretch band of atop-bound CO in the presence of either of the two larger cations (K⁺ and Cs⁺) relative to the peak frequency observed in Li⁺-containing electrolyte, evidence of the electrostatic stabilization of CO on the surface by the highly localized electric fields generated by the larger cations, which have a relatively higher tendency to reside in the electrochemical double layer.

While recent theoretical work disagrees on the existence of specifically adsorbed cations on the copper electrode at potentials relevant to CO₂ reduction, our spectroscopic data suggest that the larger cations specifically adsorb on the polycrystalline copper electrode and induce the reconstruction of the surface.

We will further discuss how CO coverage and interfacial electric field strength correlate with the onset-potentials for ethylene and methane production as concurrently monitored by OLEMS.

A national priority is to convert CO₂ into high-value chemical products such as liquid fuels and synthesis gas. Because current electrocatalysts are not adequate, we aim to develop improved catalysts by obtaining a detailed understanding of the initial steps of CO₂ electroreduction on catalyst surfaces. Thus we exposed Ag surfaces to CO₂ adsorption both alone and in the presence of H₂O at 298 K and modest gas pressure while we probed the electronic structures of the catalyst surfaces and reaction products. We monitored the surface adsorption and reaction using ambient pressure X-ray photoelectron spectroscopy (APXPS), the results of which we compared with density functional theory (DFT) to provide definitive interpretations. We find that l- and b- CO₂ are not stable on the pure Ag (111) surface, but rather g-CO₂ reacts with O on Ag surface to form a carbonic acid like O=CO₂^δ--like species. Adding H₂O and CO₂ then leads to [H₂O--O=CO₂^δ- -- H₂O] and [H₂O -- b-CO₂ -- H₂O] clusters on the surface. The [H₂O -- b-CO₂ -- H₂O] in bulk water leads to CO production. [H₂O--O=CO₂^δ- -- H₂O] requires a surface O_ads so that it may not be stable under negative potentials, but maybe it could play a role under modified conditions. Our studies establish a comprehensive picture of how the interaction between adsorbate and catalyst is altered by tuning the charge transfer between them through changing the adsorption sites and configuration and by introducing surface co-dosing adsorbates. This behavior of CO₂ and H₂O on Ag contrasts dramatically from the results on Cu providing a comprehensive understanding behind the remarkable differences in the catalytic performance of Cu vs. Ag that be useful for tuning CO₂ adsorption behaviors in designing advanced electro-catalysts to facilitate selective product formations.