Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells

Polymer design for solid-state batteries and wearable electronics

Solid-state batteries are increasingly centre-stage for delivering more energy-dense, safer batteries to follow current lithium-ion rechargeable technologies. At the same time, wearable electronics powered by flexible batteries have experienced rapid technological growth. This perspective discusses the role that polymer design plays in their use as solid polymer electrolytes (SPEs) and as binders, coatings and interlayers to address issues in solid-state batteries with inorganic solid electrolytes (ISEs). We also consider the value of tunable polymer flexibility, added capacity, skin compatibility and end-of-use degradability of polymeric materials in wearable technologies such as smartwatches and health monitoring devices. While many years have been spent on SPE development for batteries, delivering competitive performances to liquid and ISEs requires a deeper understanding of the fundamentals of ion transport in solid polymers. Advanced polymer design, including controlled (de)polymerisation strategies, precision dynamic chemistry and digital learning tools, might help identify these missing fundamental gaps towards faster, more selective ion transport. Regardless of the intended use as an electrolyte, composite electrode binder or bulk component in flexible electrodes, many parallels can be drawn between the various intrinsic polymer properties. These include mechanical performances, namely elasticity and flexibility; electrochemical stability, particularly against higher-voltage electrode materials; durable adhesive/cohesive properties; ionic and/or electronic conductivity; and ultimately, processability and fabrication into the battery. With this, we assess the latest developments, providing our views on the prospects of polymers in batteries and wearables, the challenges they might address, and emerging polymer chemistries that are still relatively under-utilised in this area.

Interlayer Engineering and Prelithiation: Empowering Si Anodes for Low-Pressure-Operating All-Solid-State Batteries

Silicon (Si) anodes, free from the dendritic growth concerns found in lithium (Li) metal anodes, offer a promising alternative for high-energy all-solid-state batteries (ASSBs). However, most advancements in Si anodes have been achieved under impractical high operating pressures, which can mask detrimental electrochemo-mechanical issues. Herein, we effectively address the challenges related to the low-pressure operation of Si anodes in ASSBs by introducing an silver (Ag) interlayer between the solid electrolyte layer (Li6PS5Cl) and anode and prelithiating the anodes. The Si composite electrodes, consisting of Si/polyvinylidene fluoride/carbon nanotubes, are optimized for suitable mechanical properties and electrical connectivity. Although the impact of the Ag interlayer is insignificant at an exceedingly high operating pressure of 70 MPa, it substantially enhances the interfacial contacts under a practical low operating pressure of 15 MPa. Thus, Ag-coated Si anodes outperform bare Si anodes (discharge capacity: 2430 vs 1560 mA h g-1). The robust interfacial contact is attributed to the deformable, adhesive properties and protective role of the in situ lithiated Ag interlayer, as evidenced by comprehensive ex situ analyses. Operando electrochemical pressiometry is used effectively to probe the strong interface for Ag-coated Si anodes. Furthermore, prelithiation through the thermal evaporation deposition of Li metal significantly improves the cycling performance.

Incorporating Sodium-Conductive Polymeric Interfacial Adhesive with Inorganic Solid-State Electrolytes for Quasi-Solid-State Sodium Metal Batteries

Inorganic solid-state electrolytes have attracted enormous attention due to their potential safety, increased energy density, and long cycle-life benefits. However, their application in solid-state batteries is limited by unstable electrode-electrolyte interface, poor point-to-point physical contact, and low utilization of metallic anodes. Herein, interfacial engineering based on sodium (Na)-conductive polymeric solid-state interfacial adhesive is studied to improve interface stability and optimize physical contacts, constructing a robust organic-rich solid electrolyte interphase layer to prevent dendrite-induced crack propagation and security issues. The interfacial adhesive strategy significantly increases the room-temperature critical current density of inorganic Na-ion conductors from 0.8 to 3.2 mA cm-2 and markedly enhances the cycling performance of solid-state batteries up to 500 cycles, respectively. Particularly, the Na3V2(PO4)3-based full solid-state batteries with high cathode loading of 10.16 mg cm-2 also deliver an excellent cycling performance, further realizing the stable operation of solid-state laminated pouch cells. The research provides fundamental perspectives into the role of interfacial chemistry and takes the field a step closer to realizing practical solid-state batteries.

Dendrite-Free Li5.5PS4.5Cl1.5-Based All-Solid-State Lithium Battery Enabled by Grain Boundary Electronic Insulation Strategy through In Situ Polymer Encapsulation

Sulfide-based all-solid-state lithium batteries (ASSLBs) have attracted unprecedented attention in the past decade due to their excellent safety performance and high energy storage density. However, the sulfide solid-state electrolytes (SSEs) as the core component of ASSLBs have a certain stiffness, which inevitably leads to the formation of pores and cracks during the production process. In addition, although sulfide SSEs have high ionic conductivity, the electrolytes are unstable to lithium metal and have non-negligible electronic conductivity, which severely limits their practical applications. Herein, a grain boundary electronic insulation strategy through in situ polymer encapsulation is proposed for this purpose. A polymer layer with insulating properties is applied to the surface of the Li5.5PS4.5Cl1.5 (LPSC) electrolyte particles by simple ball milling. In this way, we can not only achieve a dense electrolyte pellet but also improve the stability of the Li metal anode and reduce the electronic conductivity of LPSC. This strategy of electronic isolation of the grain boundaries enables stable deposition/stripping of the modified electrolyte for more than 2000 h at a current density of 0.5 mA cm-1 in a symmetrical Li/Li cell. With this strategy, a full cell with Li(Ni0.8Co0.1Mn0.1)O2 (NCM811) as the cathode shows high performance including high specific capacity, improved high-rate capability, and long-term stability. Therefore, this study presents a new strategy to achieve high-performance sulfide SSEs.

Bridging the gap between academic research and industrial development in advanced all-solid-state lithium-sulfur batteries

The energy storage and vehicle industries are heavily investing in advancing all-solid-state batteries to overcome critical limitations in existing liquid electrolyte-based lithium-ion batteries, specifically focusing on mitigating fire hazards and improving energy density. All-solid-state lithium-sulfur batteries (ASSLSBs), featuring earth-abundant sulfur cathodes, high-capacity metallic lithium anodes, and non-flammable solid electrolytes, hold significant promise. Despite these appealing advantages, persistent challenges like sluggish sulfur redox kinetics, lithium metal failure, solid electrolyte degradation, and manufacturing complexities hinder their practical use. To facilitate the transition of these technologies to an industrial scale, bridging the gap between fundamental scientific research and applied R&D activities is crucial. Our review will address the inherent challenges in cell chemistries within ASSLSBs, explore advanced characterization techniques, and delve into innovative cell structure designs. Furthermore, we will provide an overview of the recent trends in R&D and investment activities from both academia and industry. Building on the fundamental understandings and significant progress that has been made thus far, our objective is to motivate the battery community to advance ASSLSBs in a practical direction and propel the industrialized process.

Regenerative Solid Interfaces Enhance High-Performance All-Solid-State Lithium Batteries

The performance of all-solid-state lithium batteries (ASSLBs) is significantly impacted by lithium interfacial instability, which originates from the dynamic chemical, morphological, and mechanical changes during deep Li plating and stripping. In this study, we introduce a facile approach to generate a conductive and regenerative solid interface, enhancing both the Li interfacial stability and overall cell performance. The regenerative interface is primarily composed of nanosized lithium iodide (nano-LiI), which originates in situ from the adopted solid-state electrolyte (SSE). During cell operation, the nano-LiI interfacial layer can reversibly diffuse back and forth in synchronization with Li plating and stripping. The interface and dynamic process improve the adhesion and Li+ transport between the Li anode and SSE, facilitating uniform Li plating and stripping. As a result, the metallic Li anode operates stably for over 1000 h at high current densities and even under elevated temperatures. By using metallic Li as the anode directly, we demonstrate stable cycling of all-solid-state Li-sulfur batteries for over 250 cycles at an areal capacity of >2 mA h cm-2 and room temperature. This study offers insights into the design of regenerative and Li+-conductive interfaces to tackle solid interfacial challenges for high-performance ASSLBs.

Accelerated Li Penetration and Crack Propagation Due to Mechanical Degradation of Sulfide-Based Solid Electrolyte

This work presents quantitative investigations into the relationships between lithium dendrite growth in the defects of Li6PS5Cl (LPSCl) solid electrolyte (SE), crack nucleation and propagation in the SE, and the associated mechanical forces driving these dendrites and cracks. Two different growth modes for lithium dendrites are identified by ex situ scanning electron microscopy (SEM) observation: longitudinal cracking inside pores in the SE and lateral penetration along boundaries of the SE particles. These in situ TEM tests reveal that concentrated Li plating in a nano-sized defect on the LPSCl surface will lead to the nucleation and propagation of cracks into the LPSCl under a stress much smaller than the expected mechanical strength of the LPSCl material. This unexpected mechanical degradation is caused by a reduction in the mechanical strength of LPSCl during electrochemical charge/discharge cycling, resulting from a disorder in the crystal structure of LPSCl as revealed by DFT simulations. Due to this mechanical degradation of LPSCl, the threshold force necessary to initiate crack growth is much lower than the previously expected force to drive dendrite growth.

Solid Interfaces for the Garnet Electrolytes

Solid-state electrolytes (SSEs) have attracted extensive interests due to the advantages in developing secondary batteries with high energy density and outstanding safety. Possessing high ionic conductivity and the lowest reduction potential among the state-of-the-art SSEs, the garnet type SSE is one of the most promising candidates to achieve high performance solid-state lithium batteries (SSLBs). However, the elastic modulus of the garnet electrolyte leads to deteriorated interfacial contacts, and the increasing in electronic conduction at either anode/garnet interface or grain boundary results in Li dendrite growth. Here, recent developments of the solid interfaces for the garnet electrolytes, including the strategies of Li dendrite suppression and interfacial chemical/electrochemical/mechanical stabilizations are presented. A new viewpoint of the double edges of interfacial lithiophobicity is proposed, and the rational design of the interphases, as well as effective stacking methods of the garnet-based SSLBs are summarized. Moreover, practical roles of the garnet electrolyte in SSLB industry are also discussed. This work delivers insights into the solid interfaces for the garnet electrolytes, which provides not only the promotion of the garnet-based SSLBs, but also a comprehensive understanding of the interfacial stabilization for the whole SSE family.

Deciphering the critical role of interstitial volume in glassy sulfide superionic conductors

Sulfide electrolytes represent a crucial category of superionic conductors for all-solid-state lithium metal batteries. Among sulfide electrolytes, glassy sulfide is highly promising due to its long-range disorder and grain-boundary-free nature. However, the lack of comprehension regarding glass formation chemistry has hindered their progress. Herein, we propose interstitial volume as the decisive factor influencing halogen dopant solubility within a glass matrix. We engineer a Li3PS4-Li4SiS4 complex structure within the sulfide glassy network to facilitate the release of interstitial volume. Consequently, we increase the dissolution capacity of LiI to 40 mol% in 75Li2S-25P2S5 glass. The synthesized glass exhibits one of the highest ionic conductivities among reported glass sulfides. Furthermore, we develop a glassy/crystalline composite electrolyte to mitigate the shortcomings of argyrodite-type sulfides by utilizing our synthesized glass as the filler. The composite electrolytes effectively mitigate Li intrusion. This work unveils a protocol for the dissolution of halogen dopants in glass electrolytes.

Nanoscale Visualization of Lithium Plating/Stripping Tuned by On-site Formed Solid Electrolyte Interphase in All-Solid-State Lithium-Metal Batteries

The interfacial processes, mainly the lithium (Li) plating/stripping and the evolution of the solid electrolyte interphase (SEI), are directly related to the performance of all-solid-state Li-metal batteries (ASSLBs). However, the complex processes at solid-solid interfaces are embedded under the solid-state electrolyte, making it challenging to analyze the dynamic processes in real time. Here, using in situ electrochemical atomic force microscopy and optical microscopy, we directly visualized the Li plating/stripping/replating behavior, and measured the morphological and mechanical properties of the on-site formed SEI at nanoscale. Li spheres plating/stripping/replating at the argyrodite solid electrolyte (Li6 PS5 Cl)/Li electrode interface is coupled with the formation/wrinkling/inflating of the SEI on its surface. Combined with in situ X-ray photoelectron spectroscopy, details of the stepwise formation and physicochemical properties of SEI on the Li spheres are obtained. It is shown that higher operation rates can decrease the uniformity of the Li+ -conducting networks in the SEI and worsen Li plating/stripping reversibility. By regulating the applied current rates, uniform nucleation and reversible plating/stripping processes can be achieved, leading to the extension of the cycling life. The in situ analysis of the on-site formed SEI at solid-solid interfaces provides the correlation between the interfacial evolution and the electrochemical performance in ASSLBs.

Superionic lithium transport via multiple coordination environments defined by two-anion packing

Fast cation transport in solids underpins energy storage. Materials design has focused on structures that can define transport pathways with minimal cation coordination change, restricting attention to a small part of chemical space. Motivated by the greater structural diversity of binary intermetallics than that of the metallic elements, we used two anions to build a pathway for three-dimensional superionic lithium ion conductivity that exploits multiple cation coordination environments. Li7Si2S7I is a pure lithium ion conductor created by an ordering of sulphide and iodide that combines elements of hexagonal and cubic close-packing analogously to the structure of NiZr. The resulting diverse network of lithium positions with distinct geometries and anion coordination chemistries affords low barriers to transport, opening a large structural space for high cation conductivity.

Chemomechanical Origins of the Dynamic Evolution of Isolated Li Filaments in Inorganic Solid-State Electrolytes

The penetrating growth of Li into the inorganic solid-state electrolyte (SSE) is one key factor limiting its practical application. Research to understand the underlying mechanism of Li penetration has been ongoing for years and is continuing. Here, we report an in situ scanning electron microscopy methodology to investigate the dynamic behaviors of isolated Li filaments in the garnet SSE under practical cycling conditions. We find that the filaments tend to grow in the SSE, while surprisingly, those filaments can self-dissolve with a decrease in the current density without a reversal of the current direction. We further build a coupled electro-chemo-mechanical model to assess the interplay between electrochemistry and mechanics during the dynamic evolution of filaments. We reveal that filament growth is strongly regulated by the competition between the electrochemical driving force and mechanical resistive force. The numerical results provide rational guidance for the design of solid-state batteries with excellent properties.

Effect of solid-electrolyte pellet density on failure of solid-state batteries

Despite the potentially higher energy density and improved safety of solid-state batteries (SSBs) relative to Li-ion batteries, failure due to Li-filament penetration of the solid electrolyte and subsequent short circuit remains a critical issue. Herein, we show that Li-filament growth is suppressed in solid-electrolyte pellets with a relative density beyond ~95%. Below this threshold value, however, the battery shorts more easily as the density increases due to faster Li-filament growth within the percolating pores in the pellet. The microstructural properties (e.g., pore size, connectivity, porosity, and tortuosity) of [Formula: see text] with various relative densities are quantified using focused ion beam-scanning electron microscopy tomography and permeability tests. Furthermore, modeling results provide details on the Li-filament growth inside pores ranging from 0.2 to 2 μm in size. Our findings improve the understanding of the failure modes of SSBs and provide guidelines for the design of dendrite-free SSBs.

Interfacial self-healing polymer electrolytes for long-cycle solid-state lithium-sulfur batteries

Coupling high-capacity cathode and Li-anode with solid-state electrolyte has been demonstrated as an effective strategy for increasing the energy densities and safety of rechargeable batteries. However, the limited ion conductivity, the large interfacial resistance, and unconstrained Li-dendrite growth hinder the application of solid-state Li-metal batteries. Here, a poly(ether-urethane)-based solid-state polymer electrolyte with self-healing capability is designed to reduce the interfacial resistance and provides a high-performance solid-state Li-metal battery. With its dynamic covalent disulfide bonds and hydrogen bonds, the proposed solid-state polymer electrolyte exhibits excellent interfacial self-healing ability and maintains good interfacial contact. Full cells are assembled with the two integrated electrodes/electrolytes. As a result, the Li||Li symmetric cells exhibit stable long-term cycling for more than 6000 h, and the solid-state Li-S battery shows a prolonged cycling life of 700 cycles at 0.3 C. The use of ultrasound imaging technology shows that the interfacial contact of the integrated structure is much better than those of traditional laminated structure. This work provides an interesting interfacial dual-integrated strategy for designing high-performance solid-state Li-metal batteries.

Designing electrolytes and interphases for high-energy lithium batteries

High-energy and stable lithium-ion batteries are desired for next-generation electric devices and vehicles. To achieve their development, the formation of stable interfaces on high-capacity anodes and high-voltage cathodes is crucial. However, such interphases in certain commercialized Li-ion batteries are not stable. Due to internal stresses during operation, cracks are formed in the interphase and electrodes; the presence of such cracks allows for the formation of Li dendrites and new interphases, resulting in a decay of the energy capacity. In this Review, we highlight electrolyte design strategies to form LiF-rich interphases in different battery systems. In aqueous electrolytes, the hydrophobic LiF can extend the electrochemical stability window of aqueous electrolytes. In organic liquid electrolytes, the highly lithiophobic LiF can suppress Li dendrite formation and growth. Electrolyte design aimed at forming LiF-rich interphases has substantially advanced high-energy aqueous and non-aqueous Li-ion batteries. The electrolyte and interphase design principles discussed here are also applicable to solid-state batteries, as a strategy to achieve long cycle life under low stack pressure, as well as to construct other metal batteries.

A Dynamically Stable Mixed Conducting Interphase for All-Solid-State Lithium Metal Batteries

All-solid-state lithium (Li) metal batteries (ASSLMBs) employing sulfide solid electrolytes have attracted increasing attention owing to superior safety and high energy density. However, the instability of sulfide electrolytes against Li metal induces the formation of two types of incompetent interphases, solid electrolyte interphase (SEI) and mixed conducting interphase (MCI), which significantly blocks rapid Li-ion transport and induces uneven Li deposition and continuous interface degradation. In this contribution, a dynamically stable mixed conducting interphase (S-MCI) is proposed by in situ stress self-limiting reaction to achieve the compatibility of Li metal with composite sulfide electrolytes (Li6 PS5 Cl (LPSCl) and Li10 GeP2 S12 (LGPS)). The rational design of composite electrolytes utilizes the expansion stress induced by the electrolyte decomposition to in turn constrain the further decomposition of LGPS. Consequently, the S-MCI inherits the high dynamical stability of LPSCl-derived SEI and the lithiophilic affinity of Li-Ge alloy in LGPS-derived MCI. The Li||Li symmetric cells with the protection of S-MCI can operate stably for 1500 h at 0.5 mA cm-2 and 0.5 mAh cm-2 . The Li||NCM622 full cells present stable cycling for 100 cycles at 0.1 C with a high-capacity retention of 93.7%. This work sheds fresh insight into constructing electrochemically stable interphase for high-performance ASSLMBs.

Insight into Dendrites Issue in All Solid-State Batteries with Inorganic Electrolyte: Mechanism, Detection and Suppression Strategies

All solid-state batteries (ASSBs) are regarded as one of the promising next-generation energy storage devices due to their expected high energy density and capacity. However, failures due to unrestricted growth of lithium dendrites (LDs) have been a critical problem. Moreover, the understanding of dendrite growth inside solid-state electrolytes is limited. Since the dendrite process is a multi-physical field coupled process, including electrical, chemical, and mechanical factors, no definitive conclusion can summarize the root cause of LDs growth in ASSBs till now. Herein, the existing works on mechanism, identification, and solution strategies of LD in ASSBs with inorganic electrolyte are reviewed in detail. The primary triggers are thought to originate mainly at the interface and within the electrolyte, involving mechanical imperfections, inhomogeneous ion transport, inhomogeneous electronic structure, and poor interfacial contact. Finally, some of the representative works and present an outlook are comprehensively summarized, providing a basis and guidance for further research to realize efficient ASSBs for practical applications.

Precise Tailoring of Lithium-Ion Transport for Ultralong-Cycling Dendrite-Free All-Solid-State Lithium Metal Batteries

All-solid-state lithium metal batteries can address crucial challenges regarding insufficient battery cycling life and energy density. The demonstration of long-cycling dendrite-free all-solid-state lithium metal batteries requires precise tailoring of lithium-ion transport of solid-state electrolytes (SSEs). In this work, a proof of concept is reported for precise tailoring of lithium-ion transport of a halide SSE, Li3 InCl6 , including intragranular (within grains) but also intergranular (between grains) lithium-ion transport. Lithium-ion migration tailoring mechanism in crystals is developed by unexpected enhanced Li, In, and Cl vacancy populations and lower energy barrier for hopping. The lithium-ion transport tailoring mechanism between the grains is determined by the elimination of voids between grains and the formation of unexpected supersonic conducting grain boundaries, boosting the lithium dendrite suppression ability of SSE. Due to boosted lithium-ion conduction and dendrite-suppression ability, the all-solid-state lithium metal batteries coupled with Ni-rich LiNi0.83 Co0.12 Mn0.05 O2 cathodes and lithium metal anodes demonstrate breakthroughs in electrochemical performance by achieving extremely long cycling life at a high current density of 0.5 C (2000 cycles, 93.7% capacity retention). This concept of precise tailoring of lithium-ion transport provides a cost, time, and energy efficient solution to conquer the remaining challenges in all-solid-state lithium-metal batteries for fast developing electric vehicle markets.

From Liquid to Solid-State Lithium Metal Batteries: Fundamental Issues and Recent Developments

The widespread adoption of lithium-ion batteries has been driven by the proliferation of portable electronic devices and electric vehicles, which have increasingly stringent energy density requirements. Lithium metal batteries (LMBs), with their ultralow reduction potential and high theoretical capacity, are widely regarded as the most promising technical pathway for achieving high energy density batteries. In this review, we provide a comprehensive overview of fundamental issues related to high reactivity and migrated interfaces in LMBs. Furthermore, we propose improved strategies involving interface engineering, 3D current collector design, electrolyte optimization, separator modification, application of alloyed anodes, and external field regulation to address these challenges. The utilization of solid-state electrolytes can significantly enhance the safety of LMBs and represents the only viable approach for advancing them. This review also encompasses the variation in fundamental issues and design strategies for the transition from liquid to solid electrolytes. Particularly noteworthy is that the introduction of SSEs will exacerbate differences in electrochemical and mechanical properties at the interface, leading to increased interface inhomogeneity-a critical factor contributing to failure in all-solid-state lithium metal batteries. Based on recent research works, this perspective highlights the current status of research on developing high-performance LMBs.

Interface design for all-solid-state lithium batteries

The operation of high-energy all-solid-state lithium-metal batteries at low stack pressure is challenging owing to the Li dendrite growth at the Li anodes and the high interfacial resistance at the cathodes1-4. Here we design a Mg16Bi84 interlayer at the Li/Li6PS5Cl interface to suppress the Li dendrite growth, and a F-rich interlayer on LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes to reduce the interfacial resistance. During Li plating-stripping cycles, Mg migrates from the Mg16Bi84 interlayer to the Li anode converting Mg16Bi84 into a multifunctional LiMgSx-Li3Bi-LiMg structure with the layers functioning as a solid electrolyte interphase, a porous Li3Bi sublayer and a solid binder (welding porous Li3Bi onto the Li anode), respectively. The Li3Bi sublayer with its high ionic/electronic conductivity ratio allows Li to deposit only on the Li anode surface and grow into the porous Li3Bi sublayer, which ameliorates pressure (stress) changes. The NMC811 with the F-rich interlayer converts into F-doped NMC811 cathodes owing to the electrochemical migration of the F anion into the NMC811 at a high potential of 4.3 V stabilizing the cathodes. The anode and cathode interlayer designs enable the NMC811/Li6PS5Cl/Li cell to achieve a capacity of 7.2 mAh cm-2 at 2.55 mA cm-2, and the LiNiO2/Li6PS5Cl/Li cell to achieve a capacity of 11.1 mAh cm-2 with a cell-level energy density of 310 Wh kg-1 at a low stack pressure of 2.5 MPa. The Mg16Bi84 anode interlayer and F-rich cathode interlayer provide a general solution for all-solid-state lithium-metal batteries to achieve high energy and fast charging capability at low stack pressure.

Monodispersed Sub-1 nm Inorganic Cluster Chains in Polymers for Solid Electrolytes with Enhanced Li-Ion Transport

The organic-inorganic interfaces can enhance Li+ transport in composite solid-state electrolytes (CSEs) due to the strong interface interactions. However, Li+ non-conductive areas in CSEs with inert fillers will hinder the construction of efficient Li+ transport channels. Herein, CSEs with fully active Li+ conductive networks are proposed to improve Li+ transport by composing sub-1 nm inorganic cluster chains and organic polymer chains. The inorganic cluster chains are monodispersed in polymer matrix by a brief mixed-solvent strategy, their sub-1 nm diameter and ultrafine dispersion state eliminate Li+ non-conductive areas in the interior of inert fillers and filler-agglomeration, respectively, providing rich surface areas for interface interactions. Therefore, the 3D networks connected by the monodispersed cluster chains finally construct homogeneous, large-scale, continuous Li+ fast transport channels. Furthermore, a conjecture about 1D oriented distribution of organic polymer chains along the inorganic cluster chains is proposed to optimize Li+ pathways. Consequently, the as-obtained CSEs possess high ionic conductivity at room temperature (0.52 mS cm-1 ), high Li+ transference number (0.62), and more mobile Li+ (50.7%). The assembled LiFePO4 /Li cell delivers excellent stability of 1000 cycles at 0.5 C and 700 cycles at 1 C. This research provides a new strategy for enhancing Li+ transport by efficient interfaces.

SEI growth on Lithium metal anodes in solid-state batteries quantified with coulometric titration time analysis

Lithium-metal batteries with a solid electrolyte separator are promising for advanced battery applications, however, most electrolytes show parasitic side reactions at the low potential of lithium metal. Therefore, it is essential to understand how much (and how fast) charge is consumed in these parasitic reactions. In this study, a new electrochemical method is presented for the characterization of electrolyte side reactions occurring on active metal electrode surfaces. The viability of this new method is demonstrated in a so-called anode-free stainless steel ∣ Li6PS5Cl ∣ Li cell. The method also holds promise for investigating dendritic lithium growth (and dead lithium formation), as well as for analyzing various electrolytes and current collectors. The experimental setup allows easy electrode removal for post-mortem analysis, and the SEI's heterogeneous/layered microstructure is revealed through complementary analytical techniques. We expect this method to become a valuable tool in the future for solid-state lithium metal batteries and potentially other cell chemistries.

Correlating the Microstructure and Current Density of the Li/Garnet Interface

Solid-state lithium batteries hold great promise for next-generation energy storage systems. However, the formation of lithium filaments within the solid electrolyte remains a critical challenge. In this study, we investigate the crucial role of morphology in determining the resistance of garnet-type electrolytes to lithium filaments. By proposing a new test method, namely, cyclic linear sweep voltammetry, we can effectively evaluate the electrolyte resistance against lithium filaments. Our findings reveal a strong correlation between the microscopic morphology of the solid electrolyte and its resistance to lithium filaments. Samples with reduced pores and multiple grain boundaries demonstrate remarkable performance, achieving a critical current density of up to 3.2 mA cm-2 and excellent long-term cycling stability. Kelvin probe force microscopy and finite element method simulation results shed light on the impact of grain boundaries and electrolyte pores on lithium-ion transport and filament propagation. To inhibit lithium penetration, minimizing pores and achieving a uniform morphology with small grains and plenty of grain boundaries are essential.

Enhancing Lithium Stripping Efficiency in Anode-Free Solid-State Batteries through Self-Regulated Internal Pressure

Anode-free all-solid-state lithium metal batteries (ASLMBs) promise high energy density and safety but suffer from a low initial Coulombic efficiency and rapid capacity decay, especially at high cathode loadings. Using operando techniques, we concluded these issues were related to interfacial contact loss during lithium stripping. To address this, we introduce a conductive carbon felt elastic layer that self-adjusts the pressure at the anode side, ensuring consistent lithium-solid electrolyte contact. This layer simultaneously provides electronic conduction and releases the plating pressure. Consequently, the first Coulombic efficiency dramatically increases from 58.4% to 83.7% along with a >10-fold improvement in cycling stability. Overall, this study reveals an approach for enhancing anode-free ASLMB performance and longevity by mitigating lithium stripping inefficiency through self-adjusting interfacial pressure enabled by a conductive elastic interlayer.

NaBr-Assisted Sintering of Na3Zr2Si2PO12 Ceramic Electrolyte Stabilizes a Rechargeable Solid-state Sodium Metal Battery

Solid-state metal batteries with nonflammable solid-state electrolytes are regarded as the next generation of energy storage technology on account of their high safety and energy density. However, as for most solid electrolytes, low room temperature ionic conductivity and interfacial issues hinder their practical application. In this work, Na super ionic conductor (NASICON)-type Na3Zr2Si2PO12 (NZSP) electrolytes with improved ionic conductivity are synthesized by the NaBr-assisted sintering method. The effects of the NaBr sintering aid on the crystalline phase, microstructure, densification degree, and electrical performance as well as the electrochemical performances of the NZSP ceramic electrolyte are investigated in detail. Specifically, the NZSP-7%NaBr-1150 ceramic electrolyte has an ionic conductivity of 1.2 × 10-3 S cm-1 (at 25 °C) together with an activation energy of 0.28 eV. A low interfacial resistance of 35 Ω cm2 is achieved with the Na/NZSP-7%NaBr-1150 interface. Furthermore, the Na/NZSP-7%NaBr-1150/Na3V2(PO4)3 battery manifests excellent cycling stability with a capacity retention of 98% after 400 cycles at 1 C and 25 °C.

Solid-state batteries: The critical role of mechanics

Solid-state batteries with lithium metal anodes have the potential for higher energy density, longer lifetime, wider operating temperature, and increased safety. Although the bulk of the research has focused on improving transport kinetics and electrochemical stability of the materials and interfaces, there are also critical challenges that require investigation of the mechanics of materials. In batteries with solid-solid interfaces, mechanical contacts, and the development of stresses during operation of the solid-state batteries, become as critical as the electrochemical stability to keep steady charge transfer at these interfaces. This review will focus on stress and strain that result from normal and extended battery cycling and the associated mechanisms for stress relief, some of which lead to failure of these batteries.

Design Strategies for Anodes and Interfaces Toward Practical Solid-State Li-Metal Batteries

Solid-state Li-metal batteries (based on solid-state electrolytes) offer excellent safety and exhibit high potential to overcome the energy-density limitations of current Li-ion batteries, making them suitable candidates for the rapidly developing fields of electric vehicles and energy-storage systems. However, establishing close solid-solid contact is challenging, and Li-dendrite formation in solid-state electrolytes at high current densities causes fatal technical problems (due to high interfacial resistance and short-circuit failure). The Li metal/solid electrolyte interfacial properties significantly influence the kinetics of Li-metal batteries and short-circuit formation. This review discusses various strategies for introducing anode interlayers, from the perspective of reducing the interfacial resistance and preventing short-circuit formation. In addition, 3D anode structural-design strategies are discussed to alleviate the stress caused by volume changes during charging and discharging. This review highlights the importance of comprehensive anode/electrolyte interface control and anode design strategies that reduce the interfacial resistance, hinder short-circuit formation, and facilitate stress relief for developing Li-metal batteries with commercial-level performance.

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.

Role of Interfacial Defects on Electro-Chemo-Mechanical Failure of Solid-State Electrolyte

High-stress field generated by electroplating of lithium (Li) in pre-existing defects is the main reason for mechanical failure of solid-state electrolyte because it drives crack propagation in electrolyte, followed by Li filament growth inside and even internal short-circuit if the filament reaches another electrode. To understand the role of interfacial defects on mechanical failure of solid-state electrolyte, an electro-chemo-mechanical model is built to visualize distribution of stress, relative damage, and crack formation during electrochemical plating of Li in defects. Geometry of interfacial defect is found as dominating factor for concentration of local stress field while semi-sphere defect delivers less accumulation of damage at initial stage and the longest failure time for disintegration of electrolyte. Aspect ratio, as a key geometric parameter of defect, is investigated to reveal its impact on failure of electrolyte. Pyramidic defect with low aspect ratio of 0.2-0.5 shows branched region of damage near interface, probably causing surface pulverization of solid-state electrolyte, whereas high aspect ratio over 3.0 will trigger accumulation of damage in bulk electrolyte. The correction between interfacial defect and electro-chemo-mechanical failure of solid-state electrolyte is expected to provide insightful guidelines for interface design in high-power-density solid-state Li metal batteries.

Dendrite initiation and propagation in lithium metal solid-state batteries

All-solid-state batteries with a Li anode and ceramic electrolyte have the potential to deliver a step change in performance compared with today's Li-ion batteries^1,2. However, Li dendrites (filaments) form on charging at practical rates and penetrate the ceramic electrolyte, leading to short circuit and cell failure^3,4. Previous models of dendrite penetration have generally focused on a single process for dendrite initiation and propagation, with Li driving the crack at its tip^5-9. Here we show that initiation and propagation are separate processes. Initiation arises from Li deposition into subsurface pores, by means of microcracks that connect the pores to the surface. Once filled, further charging builds pressure in the pores owing to the slow extrusion of Li (viscoplastic flow) back to the surface, leading to cracking. By contrast, dendrite propagation occurs by wedge opening, with Li driving the dry crack from the rear, not the tip. Whereas initiation is determined by the local (microscopic) fracture strength at the grain boundaries, the pore size, pore population density and current density, propagation depends on the (macroscopic) fracture toughness of the ceramic, the length of the Li dendrite (filament) that partially occupies the dry crack, current density, stack pressure and the charge capacity accessed during each cycle. Lower stack pressures suppress propagation, markedly extending the number of cycles before short circuit in cells in which dendrites have initiated.

Lithium crystallization at solid interfaces

Understanding the electrochemical deposition of metal anodes is critical for high-energy rechargeable batteries, among which solid-state lithium metal batteries have attracted extensive interest. A long-standing open question is how electrochemically deposited lithium-ions at the interfaces with the solid-electrolytes crystalize into lithium metal. Here, using large-scale molecular dynamics simulations, we study and reveal the atomistic pathways and energy barriers of lithium crystallization at the solid interfaces. In contrast to the conventional understanding, lithium crystallization takes multi-step pathways mediated by interfacial lithium atoms with disordered and random-closed-packed configurations as intermediate steps, which give rise to the energy barrier of crystallization. This understanding of multi-step crystallization pathways extends the applicability of Ostwald's step rule to interfacial atom states, and enables a rational strategy for lower-barrier crystallization by promoting favorable interfacial atom states as intermediate steps through interfacial engineering. Our findings open rationally guided avenues of interfacial engineering for facilitating the crystallization in metal electrodes for solid-state batteries and can be generally applicable for fast crystal growth.

Understanding the Chemomechanical Function of the Silver-Carbon Interlayer in Sheet-type All-Solid-State Lithium-Metal Batteries

All-solid-state batteries with lithium metal anodes hold great potential for high-energy battery applications. However, forming and maintaining stable solid-solid contact between the lithium anode and solid electrolyte remains a major challenge. One promising solution is the use of a silver-carbon (Ag-C) interlayer, but its chemomechanical properties and impact on interface stabilities need to be comprehensively explored. Here, we examine the function of Ag-C interlayers in addressing interfacial challenges using various cell configurations. Experiments show that the interlayer improves interfacial mechanical contact, leading to a uniform current distribution and suppressing lithium dendrite growth. Furthermore, the interlayer regulates lithium deposition in the presence of Ag particles via improved Li diffusivity. The sheet-type cells with the interlayer achieve a high energy density of 514.3 Wh L^-1 and an average Coulombic efficiency of 99.97% over 500 cycles. This work provides insights into the benefits of using Ag-C interlayers for enhancing the performance of all-solid-state batteries.

Boosting the interfacial superionic conduction of halide solid electrolytes for all-solid-state batteries

Designing highly conductive and (electro)chemical stable inorganic solid electrolytes using cost-effective materials is crucial for developing all-solid-state batteries. Here, we report halide nanocomposite solid electrolytes (HNSEs) ZrO₂(-ACl)-A₂ZrCl₆ (A = Li or Na) that demonstrate improved ionic conductivities at 30 °C, from 0.40 to 1.3 mS cm^-1 and from 0.011 to 0.11 mS cm^-1 for Li⁺ and Na⁺, respectively, compared to A₂ZrCl₆, and improved compatibility with sulfide solid electrolytes. The mechanochemical method employing Li₂O for the HNSEs synthesis enables the formation of nanostructured networks that promote interfacial superionic conduction. Via density functional theory calculations combined with synchrotron X-ray and ⁶Li nuclear magnetic resonance measurements and analyses, we demonstrate that interfacial oxygen-substituted compounds are responsible for the boosted interfacial conduction mechanism. Compared to state-of-the-art Li₂ZrCl₆, the fluorinated ZrO₂-2Li₂ZrCl₅F HNSE shows improved high-voltage stability and interfacial compatibility with Li₆PS₅Cl and layered lithium transition metal oxide-based positive electrodes without detrimentally affecting Li⁺ conductivity. We also report the assembly and testing of a Li-In||LiNi_0.88Co_0.11Mn_0.01O₂ all-solid-state lab-scale cell operating at 30 °C and 70 MPa and capable of delivering a specific discharge of 115 mAh g^-1 after almost 2000 cycles at 400 mA g^-1.

Tailoring Conversion-Reaction-Induced Alloy Interlayer for Dendrite-Free Sulfide-Based All-Solid-State Lithium-Metal Battery

Utilization of lithium (Li) metal anodes in all-solid-state batteries employing sulfide solid electrolytes is hindered by diffusion-related dendrite growth at high rates of charge. Engineering ex-situ Li-intermetallic interlayers derived from a facile solution-based conversion-alloy reaction is attractive for bypassing the Li0 self-diffusion restriction. However, no correlation is established between the properties of conversion-reaction-induced (CRI) interlayers and the deposition behavior of Li0 in all-solid-state lithium-metal batteries (ASSLBs). Herein, using a control set of electrochemical characterization experiments with LixAgy as the interlayer in different battery chemistries, this work identifies that dendritic tolerance in ASSLBs is susceptible to the surface roughness and electronic conductivity of the CRI-alloy interlayer. This work thereby tailors the CRI-alloy interlayer from the typical mosaic structure to a hierarchical gradient structure by adjusting the pit corrosion kinetics from the (de)solvation mechanism to an adsorption model, yielding a smooth organic-rich outer layer and a composition-regulated inorganic-rich inner layer composed mainly of lithiophilic LixAgy and electron-insulating LiF. Ultimately, desirable roughness, conductivity, and diffusivity are integrated simultaneously into the tailored CRI-alloy interlayer, resulting in dendrite-free and dense Li deposition beneath the interlayer capable of improving battery cycling stability. This work provides a rational protocol for the CRI-alloy interlayer specialized for ASSLBs.

Interface Design Enabling Stable Polymer/Thiophosphate Electrolyte Separators for Dendrite-Free Lithium Metal Batteries

Organic/inorganic interfaces greatly affect Li⁺ transport in composite solid electrolytes (SEs), while SE/electrode interfacial stability plays a critical role in the cycling performance of solid-state batteries (SSBs). However, incomplete understanding of interfacial (in)stability hinders the practical application of composite SEs in SSBs. Herein, chemical degradation between Li₆ PS₅ Cl (LPSCl) and poly(ethylene glycol) (PEG) is revealed. The high polarity of PEG changes the electronic state and structural bonding of the PS₄ ^3- tetrahedra, thus triggering a series of side reactions. A substituted terminal group of PEG not only stabilizes the inner interfaces but also extends the electrochemical window of the composite SE. Moreover, a LiF-rich layer can effectively prevent side reactions at the Li/SE interface. The results provide insights into the chemical stability of polymer/sulfide composites and demonstrate an interface design to achieve dendrite-free lithium metal batteries.

Electrochemical Atomic Force Microscopy Study on the Dynamic Evolution of Lithium Deposition

Lithium metal is one of the most promising anode materials for lithium-ion batteries; however, lithium dendrite growth hinders its large-scale development. So far, the dendrite formation mechanism is unclear. Herein, the dynamic evolution of lithium deposition in etheryl-based and ethylene carbonate (EC)-based electrolytes was obtained by combining an in situ electrochemical atomic force microscope (EC-AFM) with an electrochemical workstation. Three growth modes of lithium particles are proposed: preferential, merged, and independent growth. In addition, a lithium deposition schematic is proposed to clearly describe the morphological changes in lithium deposition. This schematic shows the process of lithium deposition, thus providing a theoretical basis for solving the problem of lithium dendrite growth.

Codependent failure mechanisms between cathode and anode in solid state lithium metal batteries: mediated by uneven ion flux

An in-depth understanding of the degradation mechanisms is a prerequisite for developing the next-generation all solid-state lithium metal battery (ASSLMB) technology. Herein, synchrotron X-ray computed tomography (SXCT) together with other probing tools and simulation method were employed to rediscover the decaying mechanisms of LiNi_0.8Co_0.1Mn_0.1O₂ (NCM)|Li₆PS₅Cl (LPSCl)|Li ASSLMB. It reveals that the detachment and isolation of NCM particles cause the current focusing on the remaining active regions of cathode. The extent of Li stripping and the likelihood of Li⁺ plating into LPSCl facing the active NCM particles becomes higher. Besides, the homogeneity of Li stripping/plating is improved by homogenizing the electrochemical reactions at the cathode side by LiZr₂(PO₄)₃ (LZP) coating. These results suggest a codependent failure mechanism between cathode and anode that is mediated by uneven Li ion flux. This work contributes to a holistic understanding of the degradation mechanisms in ASSLMBs and opens new opportunities for their further optimization and development.

Investigating microstructure evolution of lithium metal during plating and stripping via operando X-ray tomographic microscopy

Efficient lithium metal stripping and plating operation capable of maintaining electronic and ionic conductivity is crucial to develop safe lithium metal batteries. However, monitoring lithium metal microstructure evolution during cell cycling is challenging. Here, we report the development of an operando synchrotron X-ray tomographic microscopy method capable of probing in real-time the formation, growth, and dissolution of Li microstructures during the cycling of a Li||Cu cell containing a standard non-aqueous liquid electrolyte solution. The analyses of the operando X-ray tomographic microscopy measurements enable tracking the evolution of deposited Li metal as a function of time and applied current density and distinguishing the formation of electrochemically inactive Li from the active bulk of Li microstructures. Furthermore, in-depth analyses of the Li microstructures shed some light on the structural connectivity of deposited Li at different current densities as well as the formation mechanism of fast-growing fractal Li microstructures, which are ultimately responsible for cell failure.

Surface engineering of inorganic solid-state electrolytes via interlayers strategy for developing long-cycling quasi-all-solid-state lithium batteries

Lithium metal batteries (LMBs) with inorganic solid-state electrolytes are considered promising secondary battery systems because of their higher energy content than their Li-ion counterpart. However, the LMB performance remains unsatisfactory for commercialization, primarily owing to the inability of the inorganic solid-state electrolytes to hinder lithium dendrite propagation. Here, using an Ag-coated Li_6.4La₃Zr_1.7Ta_0.3O₁₂ (LLZTO) inorganic solid electrolyte in combination with a silver-carbon interlayer, we demonstrate the production of stable interfacially engineered lab-scale LMBs. Via experimental measurements and computational modelling, we prove that the interlayers strategy effectively regulates lithium stripping/plating and prevents dendrite penetration in the solid-state electrolyte pellet. By coupling the surface-engineered LLZTO with a lithium metal negative electrode, a high-voltage positive electrode with an ionic liquid-based liquid electrolyte solution in pouch cell configuration, we report 800 cycles at 1.6 mA/cm² and 25 °C without applying external pressure. This cell enables an initial discharge capacity of about 3 mAh/cm² and a discharge capacity retention of about 85%.

Electrolytes in Organic Batteries

Organic batteries using redox-active polymers and small organic compounds have become promising candidates for next-generation energy storage devices due to the abundance, environmental benignity, and diverse nature of organic resources. To date, tremendous research efforts have been devoted to developing advanced organic electrode materials and understanding the material structure-performance correlation in organic batteries. In contrast, less attention was paid to the correlation between electrolyte structure and battery performance, despite the critical roles of electrolytes for the dissolution of organic electrode materials, the formation of the electrode-electrolyte interphase, and the solvation/desolvation of charge carriers. In this review, we discuss the prospects and challenges of organic batteries with an emphasis on electrolytes. The differences between organic and inorganic batteries in terms of electrolyte property requirements and charge storage mechanisms are elucidated. To provide a comprehensive and thorough overview of the electrolyte development in organic batteries, the electrolytes are divided into four categories including organic liquid electrolytes, aqueous electrolytes, inorganic solid electrolytes, and polymer-based electrolytes, to introduce different components, concentrations, additives, and applications in various organic batteries with different charge carriers, interphases, and separators. The perspectives and outlook for the future development of advanced electrolytes are also discussed to provide a guidance for the electrolyte design and optimization in organic batteries. We believe that this review will stimulate an in-depth study of electrolytes and accelerate the commercialization of organic batteries.

Grain Boundary Electronic Insulation for High-Performance All-Solid-State Lithium Batteries

Sulfide electrolytes with high ionic conductivities are one of the most highly sought for all-solid-state lithium batteries (ASSLBs). However, the non-negligible electronic conductivities of sulfide electrolytes (≈10^-8  S cm^-1 ) lead to electron smooth transport through the sulfide electrolyte pellets, resulting in Li dendrite directly depositing at the grain boundaries (GBs) and serious self-discharge. Here, a grain-boundary electronic insulation (GBEI) strategy is proposed to block electron transport across the GBs, enabling Li-Li symmetric cells with 30 times longer cycling life and Li-LiCoO₂ full cells with three times lower self-discharging rate than pristine sulfide electrolytes. The Li-LiCoO₂ ASSLBs deliver high capacity retention of 80 % at 650 cycles and stable cycling performance for over 2600 cycles at 0.5 mA cm^-2 . The innovation of the GBEI strategy provides a new direction to pursue high-performance ASSLBs via tailoring the electronic conductivity.

Understanding the failure process of sulfide-based all-solid-state lithium batteries via operando nuclear magnetic resonance spectroscopy

The performance of all-solid-state lithium metal batteries (SSLMBs) is affected by the presence of electrochemically inactive (i.e., electronically and/or ionically disconnected) lithium metal and solid electrolyte interphase (SEI), which are jointly termed inactive lithium. However, the differentiation and quantification of inactive lithium during cycling are challenging, and their lack limits the fundamental understanding of SSLMBs failure mechanisms. To shed some light on these crucial aspects, here, we propose operando nuclear magnetic resonance (NMR) spectroscopy measurements for real-time quantification and evolution-tracking of inactive lithium formed in SSLMBs. In particular, we examine four different sulfide-based solid electrolytes, namely, Li₁₀GeP₂S₁₂, Li_9.54Si_1.74P_1.44S_11.7Cl_0.3, Li₆PS₅Cl and Li₇P₃S₁₁. We found that the chemistry of the solid electrolyte influences the activity of lithium. Furthermore, we demonstrate that electronically disconnected lithium metal is mainly found in the interior of solid electrolytes, and ionically disconnected lithium metal is found at the negative electrode surface. Moreover, by monitoring the Li NMR signal during cell calendar ageing, we prove the faster corrosion rate of mossy/dendritic lithium than flat/homogeneous lithium in SSLMBs.

Decoupling Ion Transport and Matrix Dynamics to Make High Performance Solid Polymer Electrolytes

Transport of ions through solid polymeric electrolytes (SPEs) involves a complicated interplay of ion solvation, ion-ion interactions, ion-polymer interactions, and free volume. Nonetheless, prevailing viewpoints on the subject promote a significantly simplified picture, likening ion transport in a polymer to that in an unstructured fluid at low solute concentrations. Although this idealized liquid transport model has been successful in guiding the design of homogeneous electrolytes, structured electrolytes provide a promising alternate route to achieve high ionic conductivity and selectivity. In this perspective, we begin by describing the physical origins of the idealized liquid transport mechanism and then proceed to examine known cases of decoupling between the matrix dynamics and ionic transport in SPEs. Specifically we discuss conditions for "decoupled" mobility that include a highly polar electrolyte environment, a percolated path of free volume elements (either through structured or unstructured channels), high ion concentrations, and labile ion-electrolyte interactions. Finally, we proceed to reflect on the potential of these mechanisms to promote multivalent ion conductivity and the need for research into the interfacial properties of solid polymer electrolytes as well as their performance at elevated potentials.

Electro-Chemo-Mechanical Failure of Solid Electrolytes Induced by Growth of Internal Lithium Filaments

Growth of lithium (Li) filaments within solid electrolytes, leading to mechanical degradation of the electrolyte and even short circuit of the cell under high current density, is a great barrier to commercialization of solid-state Li-metal batteries. Understanding of this electro-chemo-mechanical phenomenon is hindered by the challenge of tracking local fields inside the solid electrolyte. Here, a multiphysics simulation aiming to investigate evolution of the mechanical failure of the solid electrolyte induced by the internal growth of Li is reported. Visualization of local stress, damage, and crack propagation within the solid electrolyte enables examination of factors dominating the degradation process, including the geometry, number, and size of Li filaments and voids in the electrolyte. Relative damage induced by locally high stress is found to preferentially occur in the region of the electrolyte/Li interface having great fluctuations. A high number density of Li filaments or voids triggers integration of damage and crack networks by enhanced propagation. This model is built on coupling of mechanical and electrochemical processes for internal plating of Li, revealing evolution of multiphysical fields that can barely be captured by the state-of-the-art experimental techniques. Understanding mechanical degradation of solid electrolytes with the presence of Li filaments paves the way to design advanced solid electrolytes for future solid-state Li-metal batteries.

Mapping and Modeling Physicochemical Fields in Solid-State Batteries

The safety and energy density of solid-state batteries can be, in principle, substantially increased compared with that of conventional lithium-ion batteries. However, the use of solid-state electrolytes instead of liquid electrolytes introduces pronounced complexities to the solid-state system because of the strong coupling between different physicochemical fields. Understanding the evolution of these fields is critical to unlocking the potential of solid-state batteries. This necessitates the development of experimental and theoretical methods to track electrochemical, stress, crack, and thermal fields upon battery cycling. In this Perspective, we survey existing characterization techniques and the current understanding of multiphysics coupling in solid-state batteries. We propose that the development of experimental tools that can map multiple fields concurrently and systematic consideration of material plasticity in theoretical modeling are important for the advancement of this emerging battery technology. This Perspective provides introductory material on solid-state batteries to scientists from a broad physical chemistry community, motivating innovative and interdisciplinary studies in the future.

Long-Cycling All-Solid-State Batteries Achieved by 2D Interface between Prelithiated Aluminum Foil Anode and Sulfide Electrolyte

All-solid-state batteries (ASSBs) with alloy anodes are expected to achieve high energy density and safety. However, the stability of alloy anodes is largely impeded by their large volume changes during cycling and poor interfacial stability against solid-state electrolytes. Here, a mechanically prelithiation aluminum foil (MP-Al-H) is used as an anode to construct high-performance ASSBs with sulfide electrolyte. The dense Li-Al layer of the MP-Al-H foil acts as a prelithiated anode and forms a 2D interface with sulfide electrolyte, while the unlithiated Al layer acts as a tightly bound current collector and ensures the structural integrity of the electrode. Remarkably, the MP-Al-H anode exhibits superior lithium conduction kinetics and stable interfacial compatibility with Li₆ PS₅ Cl (LPSCl) and Li₁₀ GeP₂ S₁₂ electrolytes. Consequently, the symmetrical cells using LPSCl electrolyte can work at a high current density of 7.5 mA cm^-2 and endure for over 1500 h at 1 mA cm^-2 . Notably, ≈100% capacity is retained for the MP-Al-H||LPSCl||LiCoO₂ full cell with high area loadings of 18 mg cm^-2 after 300 cycles. This work offers a pathway to improve the interfacial and performance issues for the application of ASSBs.

Unlocking the in situ Li plating dynamics and evolution mediated by diverse metallic substrates in all-solid-state batteries

The mechanisms of Li deposition behaviors, which overwhelmingly affect battery performances and safety, are far to be understood in solid-state batteries. Here, using in situ micro-nano electrochemical scanning electron microscopy (SEM) manipulation platform, dynamic Li plating behaviors on 10 metallic substrates have been tracked, and the underlying mechanisms for dendrite-free Li plating are elucidated. Distinct Li deposition behaviors on Cu, Ti, Ni, Bi, Cr, In, Ag, Au, Pd, and Al are revealed quantitatively in nucleation densities, growth rates, and anisotropic ratios. For Li alloyable metals, the dynamic Li alloying process before Li growth is visually captured. It is concluded that a good affinity for Li and appropriate lattice compatibility between the substrate and Li are needed to facilitate homogeneous Li plating. Our work not only uncovers the Li plating dynamics, shedding light on the design of solid-state batteries, but also provides a powerful integrated SEM platform for future in-depth investigation of solid-state batteries.

Stabilization of garnet/Li interphase by diluting the electronic conductor

The high interfacial resistance and lithium (Li) dendrite growth are two major challenges for solid-state Li batteries (SSLBs). The lack of understanding on the correlations between electronic conductivity and Li dendrite formation limits the success of SSLBs. Here, by diluting the electronic conductor from the interphase to bulk Li during annealing of the aluminium nitride (AlN) interlayer, we changed the interphase from mixed ionic/electronic conductive to solely ionic conductive, and from lithiophilic to lithiophobic to fundamentally understand the correlation among electronic conductivity, Li dendrite, and interfacial resistance. During the conversion-alloy reaction between AlN and Li, the lithiophilic and electronic conductive Li_xAl diffused into Li, forming a compact lithiophobic and ionic conductive Li₃N, which achieved an ultrahigh critical current density of 2.6/14.0 mA/cm² in the time/capacity-constant mode, respectively. The fundamental understanding on the effect of interphase nature on interfacial resistance and Li dendrite suppression will provide guidelines for designing high-performance SSLBs.