Lithiation and Delithiation Mechanisms of Gold Thin Film Model Anodes for Lithium Ion Batteries: Electrochemical Characterization

Characterizing Electrode Materials and Interfaces in Solid-State Batteries

Solid-state batteries (SSBs) could offer improved energy density and safety, but the evolution and degradation of electrode materials and interfaces within SSBs are distinct from conventional batteries with liquid electrolytes and represent a barrier to performance improvement. Over the past decade, a variety of imaging, scattering, and spectroscopic characterization methods has been developed or used for characterizing the unique aspects of materials in SSBs. These characterization efforts have yielded new understanding of the behavior of lithium metal anodes, alloy anodes, composite cathodes, and the interfaces of these various electrode materials with solid-state electrolytes (SSEs). This review provides a comprehensive overview of the characterization methods and strategies applied to SSBs, and it presents the mechanistic understanding of SSB materials and interfaces that has been derived from these methods. This knowledge has been critical for advancing SSB technology and will continue to guide the engineering of materials and interfaces toward practical performance.

High-Throughput Combinatorial Analysis of the Spatiotemporal Dynamics of Nanoscale Lithium Metal Plating

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Electrochemical behavior of elemental alloy anodes in solid-state batteries

Lithium alloy anodes in the form of dense foils offer significant potential advantages over lithium metal and particulate alloy anodes for solid-state batteries (SSBs). However, the reaction and degradation mechanisms of dense alloy anodes remain largely unexplored. Here, we investigate the electrochemical lithiation/delithiation behavior of 12 elemental alloy anodes in SSBs with Li6PS5Cl solid-state electrolyte (SSE), enabling direct behavioral comparisons. The materials show highly divergent first-cycle Coulombic efficiency, ranging from 99.3% for indium to ∼20% for antimony. Through microstructural imaging and electrochemical testing, we identify lithium trapping within the foil during delithiation as the principal reason for low Coulombic efficiency in most materials. The exceptional Coulombic efficiency of indium is found to be due to unique delithiation reaction front morphology evolution in which the high-diffusivity LiIn phase remains at the SSE interface. This study links composition to reaction behavior for alloy anodes and thus provides guidance toward better SSBs.

Exploring Reproducible Nonaqueous Scanning Droplet Cell Electrochemistry in Model Battery Chemistries

The discovery and optimization of new materials for energy storage are essential for a sustainable future. High-throughput experimentation (HTE) using a scanning droplet cell (SDC) is suitable for the rapid screening of prospective material candidates and effective variation of investigated parameters over a millimeter-scale area. Herein, we explore the transition and challenges for SDC electrochemistry from aqueous toward aprotic electrolytes and address pitfalls related to reproducibility in such high-throughput systems. Specifically, we explore whether reproducibilities comparable to those for millimeter half-cells are achievable on the millimeter half-cell level than for full cells. To study reproducibility in half-cells as a first screening step, this study explores the selection of appropriate cell components, such as reference electrodes (REs) and the use of masking techniques for working electrodes (WEs) to achieve consistent electrochemically active areas. Experimental results on a Li-Au model anode system show that SDC, coupled with a masking approach and subsequent optical microscopy, can mitigate issues related to electrolyte leakage and yield good reproducibility. The proposed methodologies and insights contribute to the advancement of high-throughput battery research, enabling the discovery and optimization of future battery materials with improved efficiency and efficacy.

Direct imaging of dynamic heterogeneous lithium-gold interaction at the electrochemical interface during the charging/discharging processes

Lithium can smoothly plate on certain lithium alloys in theory, such as the Li-Au alloy, making the alloy/metal films promising current collectors for high energy density anode-free batteries. However, the actual performance of the batteries with alloy film electrodes often rapidly deteriorates. It remains challenging for current imaging approaches to provide sufficient details for fully understanding the process. Here, a "see-through" operando optical microscopic approach that allows direct imaging of Li-Au interaction with high spatiotemporal and chemical resolution has been developed. Through this approach, a two-step Li-Au alloying process that exhibits interesting complementary spatiotemporal evolution paths has been discovered. The alloying process regulates the nucleation of further Li deposition, while the Li nucleation sites generate pores on the electrode film. After several cycles, film rupture occurs due to the generation of an increased number of pores, thus explaining the previously unclear mechanism of poor cycling stability. We have also elucidated the deterioration mechanism of silver electrodes: the growth of defect pores in size, independent of the alloying process. Overall, this new imaging approach opens up an effective and simple way to monitor the dynamic heterogeneity of metal-metal interaction at the electrochemical interface, which could provide helpful insight for designing high-performance batteries.

Formation of lithiated gold and its use for the preparation of reference electrodes — an EQCM study

Lithiated gold wires can be used to build reference electrodes with outstanding potential stabilities over several days and even over the course of one year. These electrodes are well suited for investigations in the context of lithium-ion batteries (LIBs). In this work, a detailed procedure for the preparation of such electrodes with tailored mechanical properties, which can be fitted gastight into electrochemical cells using commercially available fittings, is given. The electrochemical lithiation process is studied using the electrochemical quartz crystal microbalance (EQCM) technique, and the differences in lithiation of wire type and thin film type gold electrodes are discussed. All experiments were carried out with two different electrolytes, namely, a LiPF6 and a lithium bis(trifluoromethane sulfonyl) imide (LiTFSI)-based electrolyte, and we conclude that for a higher lithiation rate and long-term stability, the use of LiTFSI-based electrolyte in the preparation phase is beneficial. The EQCM data provides a better insight in the analysis of film formation processes, like the buildup of the solid electrolyte interphase (SEI) during the lithiation, the rate of deposition of metallic lithium, or additional information on the kinetics of Li-Au alloy formation.

Color Tuning of Electrochromic TiO2 Nanofibrous Layers Loaded with Metal and Metal Oxide Nanoparticles for Smart Colored Windows

Co-axial electrospinning was applied for the structuring of non-woven webs of TiO2 nanofibers loaded with Ag, Au, and CuO nanoparticles. The composite layers were tested in an electrochromic half-cell assembly. A clear correlation between the nanoparticle composition and electrochromic effect in the nanofibrous composite is observed: TiO2 loaded with Ag reveals a black-brown color, Au shows a dark-blue color, and CuO shows a dark-green color. For electrochromic applications, the Au/TiO2 layer is the most promising choice, with a color modulation time of 6 s, transmittance modulation of 40%, coloration efficiency of 20 cm2/C, areal capacitance of 300 F/cm2, and cyclic stability of over 1000 cycles in an 18 h period. In this study, an unexplored path for the rational design of TiO2-based electrochromic device is offered with unique color-switching and optical efficiency gained by the fibrous layer. It is also foreseen that co-axial electrospinning can be an alternative nanofabrication technique for smart colored windows.

Probing Electrochemical Potential Differences over the Solid/Liquid Interface in Li-Ion Battery Model Systems

The electrochemical potential difference (Δμ̅) is the driving force for the transfer of a charged species from one phase to another in a redox reaction. In Li-ion batteries (LIBs), Δμ̅ values for both electrons and Li-ions play an important role in the charge-transfer kinetics at the electrode/electrolyte interfaces. Because of the lack of suitable measurement techniques, little is known about how Δμ̅ affects the redox reactions occurring at the solid/liquid interfaces during LIB operation. Herein, we outline the relations between different potentials and show how ambient pressure photoelectron spectroscopy (APPES) can be used to follow changes in Δμ̅_e over the solid/liquid interfaces operando by measuring the kinetic energy (KE) shifts of the electrolyte core levels. The KE shift versus applied voltage shows a linear dependence of ∼1 eV/V during charging of the electrical double layer and during solid electrolyte interphase formation. This agrees with the expected results for an ideally polarizable interface. During lithiation, the slope changes drastically. We propose a model to explain this based on charge transfer over the solid/liquid interface.

Zr- and Ce-doped Li6Y(BO3)3 electrolyte for all-solid-state lithium-ion battery

The ionic conductivity of Li₆Y(BO₃)₃ (LYBO) was enhanced by the substitution of tetravalent ions (Zr⁴⁺ and Ce⁴⁺) for Y³⁺ sites through the formation of vacancies at the Li sites, an increase in compact densification, and an increase in the Li⁺-ion conduction pathways in the LYBO phase. As a result, the ionic conductivity of Li_5.875Y_0.875Zr_0.1Ce_0.025(BO₃)₃ (ZC-LYBO) reached 1.7 × 10⁻⁵ S cm⁻¹ at 27 °C, which was about 5 orders of magnitude higher than that of undoped Li₆Y(BO₃)₃. ZC-LYBO possessed a large electrochemical window and was thermally stable after cosintering with a LiNi_1/3Mn_1/3Co_1/3O₂ (NMC) positive electrode. These characteristics facilitated good reversible capacities in all-solid-state batteries for both NMC positive electrodes and graphite negative electrodes via a simple cosintering process.

Ionic conductivity of Li₆Y(BO₃)₃ (LYBO) is enhanced by the substitution of tetravalent ions through an increase in the conduction pathways etc. Zr,Ce-doped LYBO can be used as an electrolyte for all-solid-state batteries via a cosintering process.

Mirror-Like Electrodeposition of Lithium Metal under a Low-Resistance Artificial Solid Electrolyte Interphase Layer

Nonuniform electrodeposition and dendritic growth of lithium metal coupled to its chemical incompatibility with liquid electrolytes are largely responsible for poor Coulombic efficiency and safety hazards preventing the successful implementation of energy-dense Li metal anodes. Artificial solid electrolyte interface (ASEI) layers have been proposed to address the morphological evolution and chemical reactions in Li metal anodes. In this study, an ASEI layer consisting of a lithium phosphorus oxynitride (LiPON) thin film electrolyte and gold-alloying interlayer was developed and shown to promote the electrodeposition of smooth, homogeneous, mirror-like Li metal morphologies. The Au layer alloyed with Li, reducing the nucleation overpotential and resulting in a more spatially uniform metal deposit, while the LiPON layer provided a physical barrier between the Li metal and aprotic liquid electrolyte. The effectiveness and integrity of the LiPON protective layer was assessed using in operando impedance spectroscopy and ex situ SEM/EDS characterization. Smooth, homogeneous Li morphologies were realized in capacities up to 3 mAh cm^-2 plated at 0.1 mA cm^-2. At higher current densities up to 1 mA cm^-2 or increased deposition capacities of 6 mAh cm^-2, the LiPON coating fractured due to the localized, nonuniform lithium deposits and rough, dendritic Li morphologies were observed. This approach represents a new strategy in the design of artificial SEIs to enable Li metal anodes with practical areal capacities.

High Areal Capacity Porous Sn-Au Alloys with Long Cycle Life for Li-ion Microbatteries

Long-term stability is one of the most desired functionalities of energy storage microdevices for wearable electronics, wireless sensor networks and the upcoming Internet of Things. Although Li-ion microbatteries have become the dominant energy-storage technology for on-chip electronics, the extension of lifetime of these components remains a fundamental hurdle to overcome. Here, we develop an ultra-stable porous anode based on SnAu alloys able to withstand a high specific capacity exceeding 100 µAh cm^-2 at 3 C rate for more than 6000 cycles of charge/discharge. Also, this new anode material exhibits low potential (0.2 V versus lithium) and one of the highest specific capacity ever reported at low C-rates (7.3 mAh cm^-2 at 0.1 C). We show that the outstanding cyclability is the result of a combination of many factors, including limited volume expansion, as supported by density functional theory calculations. This finding opens new opportunities in design of long-lasting integrated energy storage for self-powered microsystems.

Multishelled Si@Cu Microparticles Supported on 3D Cu Current Collectors for Stable and Binder-free Anodes of Lithium-Ion Batteries

Silicon has proved to be a promising anode material of high-specific capacity for the next-generation lithium ion batteries (LIBs). However, during repeated discharge/charge cycles, Si-based electrodes, especially those in microscale size, pulverize and lose electrical contact with the current collectors due to large volume expansion. Here, we introduce a general method to synthesize Cu@M (M = Si, Al, C, SiO₂, Si₃N₄, Ag, Ti, Ta, SnIn₂O₅, Au, V, Nb, W, Mg, Fe, Ni, Sn, ZnO, TiN, Al₂O₃, HfO₂, and TiO₂) core-shell nanowire arrays on Cu substrates. The resulting Cu@Si nanowire arrays were employed as LIB anodes that can be reused via HCl etching and H₂-reduction. Multishelled Cu@Si@Cu microparticles supported on 3D Cu current collectors were further prepared as stable and binder-free LIB anodes. This 3D Cu@Si@Cu structure allows the interior conductive Cu network to effectively accommodate the volume expansion of the electrode and facilitates the contact between the Cu@Si@Cu particles and the current collectors during the repeated insertion/extraction of lithium ions. As a result, the 3D Cu@Si@Cu microparticles at a high Si-loading of 1.08 mg/cm² showed a capacity retention of 81% after 200 cycles. In addition, charging tests of 3D Cu@Si@Cu-LiFePO₄ full cells by a triboelectric nanogenerator with a pulsed current demonstrated that LIBs with silicon anodes can effectively store energy delivered by mechanical energy harvesters.

Investigating the Dendritic Growth during Full Cell Cycling of Garnet Electrolyte in Direct Contact with Li Metal

All-solid-state batteries including a garnet ceramic as electrolyte are potential candidates to replace the currently used Li-ion technology, as they offer safer operation and higher energy storage performances. However, the development of ceramic electrolyte batteries faces several challenges at the electrode/electrolyte interfaces, which need to withstand high current densities to enable competing C-rates. In this work, we investigate the limits of the anode/electrolyte interface in a full cell that includes a Li-metal anode, LiFePO₄ cathode, and garnet ceramic electrolyte. The addition of a liquid interfacial layer between the cathode and the ceramic electrolyte is found to be a prerequisite to achieve low interfacial resistance and to enable full use of the active material contained in the porous electrode. Reproducible and constant discharge capacities are extracted from the cathode active material during the first 20 cycles, revealing high efficiency of the garnet as electrolyte and the interfaces, but prolonged cycling leads to abrupt cell failure. By using a combination of structural and chemical characterization techniques, such as SEM and solid-state NMR, as well as electrochemical and impedance spectroscopy, it is demonstrated that a sudden impedance drop occurs in the cell due to the formation of metallic Li and its propagation within the ceramic electrolyte. This degradation process is originated at the interface between the Li-metal anode and the ceramic electrolyte layer and leads to electromechanical failure and cell short-circuit. Improvement of the performances is observed when cycling the full cell at 55 °C, as the Li-metal softening favors the interfacial contact. Various degradation mechanisms are proposed to explain this behavior.

Phase Boundary Propagation in Li-Alloying Battery Electrodes Revealed by Liquid-Cell Transmission Electron Microscopy

Battery cycle life is directly influenced by the microstructural changes occurring in the electrodes during charge and discharge cycles. Here, we image in situ the nanoscale phase evolution in negative electrode materials for Li-ion batteries using a fully enclosed liquid cell in a transmission electron microscope (TEM) to reveal early degradation that is not evident in the charge-discharge curves. To compare the electrochemical phase transformation behavior between three model materials, thin films of amorphous Si, crystalline Al, and crystalline Au were lithiated and delithiated at controlled rates while immersed in a commercial liquid electrolyte. This method allowed for the direct observation of lithiation mechanisms in nanoscale negative electrodes, revealing that a simplistic model of a surface-to-interior lithiation front is insufficient. For the crystalline films, a lithiation front spread laterally from a few initial nucleation points, with continued grain nucleation along the growing interface. The intermediate lithiated phases were identified using electron diffraction, and high-resolution postmortem imaging revealed the details of the final microstructure. Our results show that electrochemically induced solid-solid phase transformations can lead to highly concentrated stresses at the laterally propagating phase boundary which should be considered for future designs of nanostructured electrodes for Li-ion batteries.