Dual Protection of a Li-Ag Alloy Anode for All-Solid-State Lithium Metal Batteries with the Argyrodite Li6PS5Cl Solid Electrolyte

Lithium-silver alloys in anode-less batteries: comparison in liquid- and solid-electrolytes

This study comprehensively investigates the phase evolution of silver-carbon composite (Ag/C) layers in anode-less batteries with both liquid and solid electrolytes. The results of in situ X-ray diffraction and cross-sectional electron microscopy analyses reveal that the alloying reaction of Ag and Li is more homogeneous in solid-electrolyte-based cells compared to liquid-electrolyte-based cells. This homogeneity is attributed to diffusional Coble creep across the heterogeneous interfaces of Ag/C layers and solid electrolytes.

Rationally Designed Li-Ag Alloy with In-Situ-Formed Solid Electrolyte Interphase for All-Solid-State Lithium Batteries

All-solid-state lithium batteries (ASSLBs) with sulfide-based solid electrolytes have attracted significant attention as promising energy storage devices, owing to their high energy density and enhanced safety. However, the combination of a lithium metal anode and a sulfide solid electrolyte results in performance degradation, owing to lithium dendrite growth and the side reactions of lithium metal with the solid electrolyte. To address these issues, a Ag-based Li alloy with a favorable solid electrolyte interphase (SEI) was prepared using electrodeposition and applied to the ASSLB as an anode. The electrochemically formed SEI layer on the Li-Ag alloy primarily comprised LiF and Li2O with high mechanical strength and Li3N with high ionic conductivity, which suppressed the formation of lithium dendrites and short-circuiting of the cell. The symmetric cell with the Li-Ag alloy achieved a critical current density of 1.6 mA cm-2 and maintained stable cycling for over 2000 h at a current density of 0.6 mA cm-2. Consequently, the all-solid-state lithium cell assembled with the Li-Ag alloy anode with SEI, Li6PS5Cl solid electrolyte, and LiNi0.78Co0.10Mn0.12O2 cathode delivered a high discharge capacity of 185 mAh g-1 and exhibited good cycling performance in terms of cycling stability and rate capability at 25 °C.

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.

Combining Solid Solution Strengthening and Second Phase Strengthening for Thinning Li Metal Foils

Thin lithium (Li) metal foils have been proved to be indispensable yet elusive for practical high-energy-density lithium batteries. Currently, the realization of such thin foils (<50 μm) is impeded by the inferior mechanical processability of metallic Li. In this work, we demonstrate that the combination of solid solution strengthening and second phase strengthening, achieved by the addition of silver fluoride (AgF) to Li metal, can substantially enhance both the strength and ductility of metallic Li. Benefiting from the improved machinability, we succeed in fabricating an ultrathin (down to 5 μm), freestanding, and mechanically robust Li-AgF composite foil. More interestingly, the in situ-formed LixAg-LiF skeleton in the composite facilitates Li diffusion kinetics and uniform Li deposition, where the thin Li-AgF electrode displays a prolonged cycle life over 500 h at 1 mA cm-2 and 1 mAh cm-2 in a carbonate electrolyte. Coupled with a commercial LiCoO2 cathode (3.4 mAh cm-2), the LiCoO2||Li-AgF cell delivers a notable capacity retention of ∼90% over 100 cycles at 0.5 C with a low negative/positive ratio of 2.5.

Understanding and Engineering Interfacial Adhesion in Solid-State Batteries with Metallic Anodes

High performance alkali metal anode solid-state batteries require solid/solid interfaces with fast ion transfer that are morphologically and chemically stable upon electrochemical cycling. Void formation at the alkali metal/solid-state electrolyte interface during alkali metal stripping is responsible for constriction resistances and hotspots that can facilitate dendrite propagation and failure. Both externally applied pressures (35-400 MPa) and temperatures above the melting point of the alkali metal have been shown to improve the interfacial contact with the solid electrolyte, preventing the formation of voids. However, the extreme pressure and temperature conditions required can be difficult to meet for commercial solid-state battery applications. In this review, we highlight the importance of interfacial adhesion or 'wetting' at alkali metal/solid electrolyte interfaces for achieving solid-state batteries that can withstand high current densities without cell failure. The intrinsically poor adhesion at metal/ceramic interfaces poses fundamental limitations on many inorganics solid-state electrolyte systems in the absence of applied pressure. Suppression of alkali metal voids can only be achieved for systems with high interfacial adhesion (i. e. 'perfect wetting') where the contact angle between the alkali metal and the solid-state electrolyte surface goes to θ=0°. We identify key strategies to improve interfacial adhesion and suppress void formation including the adoption of interlayers, alloy anodes and 3D scaffolds. Computational modeling techniques have been invaluable for understanding the structure, stability and adhesion of solid-state battery interfaces and we provide an overview of key techniques. Although focused on alkali metal solid-state batteries, the fundamental understanding of interfacial adhesion discussed in this review has broader applications across the field of chemistry and material science from corrosion to biomaterials development.

Interface Engineering of All-Solid-State Batteries Based on Inorganic Solid Electrolytes

All-solid-state batteries (ASSBs) based on inorganic solid electrolytes (SEs) are one of the most promising strategies for next-generation energy storage systems and electronic devices due to the higher energy density and intrinsic safety. However, the poor solid-solid contact and restricted chemical/electrochemical stability of inorganic SEs both in cathode and anode SE interfaces cause contact failure and the degeneration of SEs during prolonged charge-discharge processes. As a result, the increasing interface resistance significantly affects the coulombic efficiency and cycling performance of ASSBs. Herein, we present a fundamental understanding of physical contact and chemical/electrochemical features of ASSB interfaces based on mainstream inorganic SEs and summarize the recent work on interface modification. SE doping, optimizing morphology, introducing interlayer/coating layer, and utilizing compatible electrode materials are the key methods to prevent side reactions, which are discussed separately in cathode/anode-SE interface. We also highlight the constant extra stack pressure applied during ASSB cycling, which is important to the electrochemical performance. Finally, our perspectives on interface modification for practical high-performance ASSBs are put forward.

Suppressing Unfavorable Interfacial Reactions Using Polyanionic Oxides as Efficient Buffer Layers: Low-Cost Li3PO4 Coatings for Sulfide-Electrolyte-Based All-Solid-State Batteries

The poor electrochemical performance of all solid-state batteries (ASSBs) that use sulfide electrolytes can be attributed to undesirable side reactions at the cathode/sulfide-electrolyte interface; this issue can be addressed via surface coating. Ternary oxides such as LiNbO₃ and Li₂ZrO₃ are generally used as coating materials because of their high chemical stabilities and ionic conductivities. However, their relative high cost discourages their use in mass production. In this study, Li₃PO₄ was introduced as a coating material for ASSBs, because phosphates possess good chemical stabilities and ionic conductivities. Phosphates also prevent the exchange of S^2- and O^2- in the electrolyte and cathode and, thus, inhibit interfacial side reactions caused by ionic exchange, because they contain the same anion (O^2-) and cation (P⁵⁺) species as those present in the cathode and sulfide electrolyte, respectively. Furthermore, the Li₃PO₄ coatings can be prepared using low-cost source materials such as polyphosphoric acid and lithium acetate. We investigated the electrochemical performance of the Li₃PO₄-coated cathodes and found that the Li₃PO₄ coating significantly improved the discharge capacities, rate capabilities, and cyclic performances of the all-solid-state cell. While the discharge capacity of the pristine cathode was ∼181 mAh·g^-1, that of 0.15 wt % Li₃PO₄-coated cathode was ∼194-195 mAh·g^-1. And the capacity retention of the Li₃PO₄-coated cathode over 50 cycles was much superior (∼84-85%) to that of the pristine sample (∼72%). Simultaneously, the Li₃PO₄ coating reduced the side reactions and interdiffusion at the cathode/sulfide-electrolyte interfaces. The results of this study demonstrate the potential of low-cost polyanionic oxides, such as Li₃PO₄, as commercial coating materials for ASSBs.

Li3TiCl6 as ionic conductive and compressible positive electrode active material for all-solid-state lithium-based batteries

The development of energy-dense all-solid-state Li-based batteries requires positive electrode active materials that are ionic conductive and compressible at room temperature. Indeed, these material properties could contribute to a sensible reduction of the amount of the solid-state electrolyte in the composite electrode, thus, enabling higher mass loading of active materials. Here, we propose the synthesis and use of lithium titanium chloride (Li₃TiCl₆) as room-temperature ionic conductive (i.e., 1.04 mS cm^-1 at 25 °C) and compressible active materials for all-solid-state Li-based batteries. When a composite positive electrode comprising 95 wt.% of Li₃TiCl₆ is tested in combination with a Li-In alloy negative electrode and Li₆PS₅Cl/Li₂ZrCl₆ solid-state electrolytes, an initial discharge capacity of about 90 mAh g^-1 and an average cell discharge voltage of about 2.53 V are obtained. Furthermore, a capacity retention of more than 62% is attainable after 2500 cycles at 92.5 mA g^-1 and 25 °C with an applied external pressure of 1.5 tons. We also report the assembly and testing of a "single Li₃TiCl₆" cell where this chloride material is used as the solid electrolyte, negative electrode and positive electrode.

Enhancing the Interfacial Stability of the Li2S-SiS2-P2S5 Solid Electrolyte toward Metallic Lithium Anode by LiI Incorporation

Chalcogenide solid-state electrolytes (SEs) have been regarded as promising candidates for lithium dendrite suppression due to their high ionic conductivity, suitable mechanical strength, and large Li⁺ ion transference number. However, the wide applications of SEs in pragmatic all-solid-state batteries are still retarded by their limited interface stability, which leads to lithium dendrite growth and formation of interphase with high resistance. In addition, the interphase evolution mechanism between SEs and metallic Li anodes remains unclear. Herein, this work demonstrates that the interfacial stability of Li₂S-SiS₂-P₂S₅ SEs can be effectively enhanced by tuning the interphase through LiI incorporation. This strategy contributes to a high ionic conductivity of the SEs and electronic insulation interphase containing LiI. Thus, the 70(60Li₂S-28SiS₂-12P₂S₅)-30 LiI SEs prepared by melt-quenching exhibit a high ionic conductivity of 1.74 mS cm^-1 at room temperature and a larger critical current density of 1.65 mA cm^-2 at 65 °C. The cycling life of the symmetric Li|SEs|Li cell is up to 200 h without significant resistance growth at 0.1 mA cm^-2 at room temperature. This enhanced interface stability is revealed to originate from the in situ-formed LiI within the interphase, which prevents continual SEs degradation and suppresses lithium dendrite growth. This work provides a vital understanding of interphase evolution, which is valuable for designing SEs with long cycling stability.