Building Lithiophilic Ion‐Conduction Highways on Garnet‐Type Solid‐State Li+ Conductors

The integration of highly conductive solid‐state electrolytes (SSEs) into solid‐state cells is still a challenge mainly due to the high impedance existing at the electrolyte/electrode interface. Although solid‐state garnet‐based batteries have been successfully assembled with the assistance of an intermediate layer between the garnet and the Li metal anode, the slow discharging/charging rates of the batteries inhibits practical applications, which require much higher power densities. Here, a crystalline sulfonated‐covalent organic framework (COF) thin layer is grown on the garnet surface via a simple solution process. It not only significantly improves the lithiophilicity of garnet electrolytes via the lithiation of the COF layer with molten Li, but also creates effective Li⁺ diffusion “highways” between the garnet and the Li metal anode. As a result, the interfacial impedance of symmetric solid‐state Li cells is significantly decreased and the cells can be operated at high current densities up to 3 mA cm^?2, which is difficult to achieve with current interfacial modification technologies for SSEs. The solid‐state Li‐ion batteries using LiFePO₄ cathodes, Li anodes, and COF‐modified garnet electrolytes thus exhibit a significantly improved rate capability.     

Reversible Short‐Circuit Behaviors in Garnet‐Based Solid‐State Batteries

Garnet‐based solid‐state electrolytes (SSEs) are attractive for solid‐state lithium metal batteries due to their wide electrochemical window, high conductivity, and excellent stability against lithium metal. However, the risk of short‐circuit encumbers the cycle life and capacity of garnet‐based solid‐state batteries without clear reason or mechanism. Here, reversible short‐circuit behavior in the garnet‐based solid‐state batteries, which differs from the short‐circuit in liquid cells, is reported for the first time. In situ neutron depth profiling is adopted to quantitatively measure Li transport, which helps forecast and confirm the reversible nature of the short‐circuit in garnet‐based batteries. A real‐time Li accumulation monitoring system of NMC//CNT/garnet/Li cell is designed to reveal the Li dendrite formation mechanism. The voltage drops of the CNT monitoring electrode during the charging process indicate the formation of Li dendrites inside the garnet bulk, while the smooth voltage profile during the discharging process demonstrates the disappearance of the short‐circuit. This is the first confirmation of short‐circuit behavior that provides clarification of the Li dendrite formation mechanism in garnet‐based solid‐state batteries, which is shown to be a reversible process caused by the low ionic conductivity and non‐negligible electronic conductivity of garnet SSEs.     

Understanding the Reactivity of a Thin Li1.5Al0.5Ge1.5(PO4)3 Solid‐State Electrolyte toward Metallic Lithium Anode

The thickness of solid‐state electrolytes (SSEs) significantly affects the energy density and safety performance of all‐solid‐state lithium batteries. However, a sufficient understanding of the reactivity toward lithium metal of ultrathin SSEs (<100 µm) based on NASICON remains lacking. Herein, for the first time, a self‐standing and ultrathin (70 µm) NASICON‐type Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) electrolyte via a scalable solution process is developed, and X‐ray photoelectron spectroscopy reveals that changes in LAGP at the metastable Li–LAGP interface during battery operation is temperature dependent. Severe germanium reduction and decrease in LAGP particle size are detected at the Li–LAGP interface at elevated temperature. Oriented plating of lithium metal on its preferred (110) face occurs during in situ X‐ray diffraction cycling.     

Recent Progress of the Solid‐State Electrolytes for High‐Energy Metal‐Based Batteries

Secondary batteries based on metal anodes (e.g., Li, Na, Mg, Zn, and Al) are among the most sought‐after candidates for next‐generation mobile and stationary storage systems because they are able to store a larger amount of energy per unit mass or volume. However, unstable electrodeposition and uncontrolled interfacial reactions occuring in liquid electrolytes cause unsatisfying cell performance and potential safety concerns for the commercial application of these metal anodes. Solid‐state electrolytes (SSEs) having a higher modulus are considered capable of inhibiting difficulties associated with the anodes and may enable building of safe all‐solid‐state metal batteries, yet several challenges, such as insufficient room‐temperature ionic conductivity and poor interfacial stability between the electrode and the electrolyte, hinder the large‐scale development of such batteries. Here, research and development of SSEs including inorganic ceramics, organic solid polymers, and organic–inorganic hybrid/composite materials for metal‐based batteries are reviewed. The comparison of different types of electrolytes is discussed in detail, in the context of electrochemical energy storage applications. Then, the focus of this study is on recent advances in a range of attractive and innovative battery chemistries and technologies that are enabled by SSEs. Finally, the challenges and future perspectives are outlined to foresee the development of SSEs.     

Revealing the Short‐Circuiting Mechanism of Garnet‐Based Solid‐State Electrolyte

Garnet‐type solid‐state electrolytes (SSEs) have been widely studied as a promising candidate for Li metal batteries. Despite the common belief that inorganic SSEs can prevent dendrite propagation, garnet SSEs suffer from relatively low critical current density (CCD) at which the SSEs are abruptly short‐circuited by Li dendrites. In this study, the short‐circuiting mechanism of garnet Li₇La_2.75Ca_0.25Zr_1.75Nb_0.25O₁₂ (LLCZN) is investigated. It is found that instead of propagating uniaxially from one electrode to other in a dendritic form, metallic lithium is formed within the SSE. This can be attributed to the fact that electrons combine with Li ions at the grain boundary, which exhibits relatively high electronic conductivity, and then reduce Li⁺ to Li⁰ to cause short circuits. In order to reduce the electronic conductivity at the grain boundary, a thin layer of LiAlO₂ is coated on the grain surface of LLCZN, which results in an improved CCD value. It is also found that under higher external voltages, the electronic conductivity of SSE becomes more significant, which is believed to be the origin of CCD. These findings not only shed light on the short‐circuiting mechanism of garnet‐type SSEs but also offer a novel perspective and useful guidance on their designs and modifications.     

A Versatile Sn‐Substituted Argyrodite Sulfide Electrolyte for All‐Solid‐State Li Metal Batteries

Sulfide‐based solid‐state electrolytes (SSEs) for all‐solid‐state Li metal batteries (ASSLMBs) are attracting significant attention due to their high ionic conductivity, inherently soft properties, and decent mechanical strength. However, the poor incompatibility with Li metal and air sensitivity have hindered their application. Herein, the Sn (IV) substitution for P (V) in argyrodite sulfide Li₆PS₅I (LPSI) SSEs is reported, in the preparation of novel LPSI‐xSn SSEs (where x is the Sn substitution percentage). Appropriate aliovalent element substitutions with larger atomic radius (R_<Sn> > R_<P>) provides the optimized LPSI‐20Sn electrolyte with a 125 times higher ionic conductivity compared to that of the LPSI electrolyte. The high ionic conductivity of LPSI‐20Sn enables the rich I‐containing electrolyte to serve as a stabilized interlayer against Li metal in sulfide‐based ASSLMBs with outstanding cycling stability and rate capability. Most importantly, benefiting from the strong Sn–S bonding in Sn‐substituted electrolytes, the LPSI‐20Sn electrolyte shows excellent structural stability and improved air stability after exposure to O₂ and moisture. The versatile Sn substitution in argyrodite LPSI electrolytes is believed to provide a new and effective strategy to achieve Li metal‐compatible and air‐stable sulfide‐based SSEs for large‐scale applications.     

Electrochemical Stability of Li10GeP2S12 and Li7La3Zr2O12 Solid Electrolytes

The electrochemical stability window of solid electrolyte is overestimated by the conventional experimental method using a Li/electrolyte/inert metal semiblocking electrode because of the limited contact area between solid electrolyte and inert metal. Since the battery is cycled in the overestimated stability window, the decomposition of the solid electrolyte at the interfaces occurs but has been ignored as a cause for high interfacial resistances in previous studies, limiting the performance improvement of the bulk‐type solid‐state battery despite the decades of research efforts. Thus, there is an urgent need to identify the intrinsic stability window of the solid electrolyte. The thermodynamic electrochemical stability window of solid electrolytes is calculated using first principles computation methods, and an experimental method is developed to measure the intrinsic electrochemical stability window of solid electrolytes using a Li/electrolyte/electrolyte‐carbon cell. The most promising solid electrolytes, Li₁₀GeP₂S₁₂ and cubic Li‐garnet Li₇La₃Zr₂O₁₂, are chosen as the model materials for sulfide and oxide solid electrolytes, respectively. The results provide valuable insights to address the most challenging problems of the interfacial stability and resistance in high‐performance solid‐state batteries.     

Germanium Thin Film Protected Lithium Aluminum Germanium Phosphate for Solid‐State Li Batteries

Solid‐state Li batteries using Na⁺ superionic conductor type solid electrolyte attracts wide interest because of its safety and high theoretical energy density. The NASCION type solid electrolyte LAGP (Li_1.5Al_0.5Ge_0.5P₃O₁₂) shows favorable conductivity as well as good mechanical strength to prevent Li dendrite penetration. However, the instability of LAGP with Li metal remains a great challenge. In this work, an amorphous Ge thin film is sputtered on an LAGP surface, which can not only suppress the reduction reaction of Ge⁴⁺ and Li, but also produces intimate contact between the Li metal and the LAGP solid electrolyte. The symmetric cell with the Ge‐coated LAGP solid electrolyte shows superior stability and cycle performance for 100 cycles at 0.1 mA cm^?2. A quasi‐solid‐state Li–air battery has also been assembled to further demonstrate this advantage. A stable cycling performance of 30 cycles in ambient air can be obtained. This work helps to achieve a stable and ionic conducting interface in solid‐state Li batteries.     

Exploiting Lithium‐Depleted Cathode Materials for Solid‐State Li Metal Batteries

Solid‐state lithium metal batteries (SSLMBs) may become one of the high‐energy density storage devices for the next generation of electric vehicles. High safety and energy density can be achieved by utilizing solid electrolytes and Li metal anodes. Therefore, developing cathode materials which can match with Li metal anode efficiently is indispensable. In SSLMBs, Li metal anodes can afford the majority of active lithium ions, then lithium‐depleted cathode materials can be a competitive candidate to achieve high gravimetric energy density as well as save lithium resources. Li_0.33MnO₂ lithium‐depleted material is chosen, which also has the advantages of low synthesis temperature and low cost (cobalt‐free). Notably, solid‐state electrolyte can greatly alleviate the problem of manganese dissolution in the electrolyte, which is beneficial to improve the cycling stability of the battery. Thus, SSLMBs enable practical applications of lithium‐depleted cathode materials.     

An Air‐Stable and Dendrite‐Free Li Anode for Highly Stable All‐Solid‐State Sulfide‐Based Li Batteries

Li metal is a promising anode material for all‐solid‐state batteries, owing to its high specific capacity and low electrochemical potential. However, direct contact of Li metal with most solid‐state electrolytes induces severe side reactions that can lead to dendrite formation and short circuits. Moreover, Li metal is unstable when exposed to air, leading to stringent processing requirements. Herein, it is reported that the Li₃PS₄/Li interface in all‐solid‐state batteries can be stabilized by an air‐stable Li_xSiS_y protection layer that is formed in situ on the surface of Li metal through a solution‐based method. Highly stable Li cycling for over 2000 h in symmetrical cells and a lifetime of over 100 cycles can be achieved for an all‐solid‐state LiCoO₂/Li₃PS₄/Li cell. Synchrotron‐based high energy X‐ray photoelectron spectroscopy in‐depth analysis demonstrates the distribution of different components within the protection layer. The in situ formation of an electronically insulating Li_xSiS_y protection layer with highly ionic conductivity provides an effective way to prevent Li dendrite formation in high‐energy all‐solid‐state Li metal batteries.     

Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

Rechargeable Li‐ion batteries (LIBs) are electrochemical storage device widely applied in electric vehicles, mobile electronic devices, etc. However, traditional LIBs containing liquid electrolytes suffer from flammability, poor electrochemical stability, and limited operational temperature range. Replacement of the liquid electrolytes with inorganic solid‐state electrolytes (SSEs) would solve this problem. However, several critical issues, such as poor interfacial compatibility, low ionic conductivity at ambient temperatures, etc., need to be surmounted before the commercialization of all‐solid‐state Li‐ion batteries (ASSLIBs). In this review, a brief historical context for the inorganic SSEs is described first. Then, two critical issues in the ASSLIBs are highlighted: interfacial incompatibility of the electrodes and SSEs and internal stresses. For the interfacial incompatibility, the discussion is focused on the dynamic characterization of the electrode/SSE interfaces, the origin and evolution of the interfacial resistance, and interface engineering to minimize the interfacial resistance. The internal stresses in the ASSLIBs are another major concern because rigid contacts are introduced. Stress generation, stress evolution during battery cycling, stress measurement/simulation, and ways to alleviate the stresses are outlined in detail. Finally, current challenges and perspectives for future development of the inorganic SSEs and ASSLIBs are outlined.     

A 3D Lithium/Carbon Fiber Anode with Sustained Electrolyte Contact for Solid‐State Batteries

To reconcile the energy storage ability and operational safety of lithium metal batteries (LMBs), a transformation from a liquid to a solid‐state system is required. However, Li volume variation, poor interfacial contact, and high operation temperatures hinder its practical applications. To address the above issues, here, an integral structure design for solid‐state LMBs is shown, in which a Li‐preinfused 3D carbon fiber (Li/CF) anode is ionically connected to a cathode via an autopolymerized gel electrolyte. The gel electrolyte helps to encapsulate the liquid electrolyte within the Li/CF anode and the cathode to improve the interfacial contact. The gel also serves as a reservoir that balances the liquid electrolyte supply during repeated Li stripping/plating process. As a result, the symmetrical cells and full cells with Li/CF electrodes exhibit improved cycling stability and effective suppression of dendrites at ambient temperature. This work facilitates the realization of solid‐state LMBs with high energy and high safety.     

Constructing Multifunctional Interphase between Li1.4Al0.4Ti1.6(PO4)3 and Li Metal by Magnetron Sputtering for Highly Stable Solid‐State Lithium Metal Batteries

Due to high ionic conductivity and low cost, Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) has emerged as a promising solid‐state electrolyte for next‐generation lithium (Li) metal solid‐state batterie with high safety performance and energy density. However, the extremely high impedance and surface instability of LATP with Li metal retard its practical application. Herein, a novel method is proposed to construct an ultrathin ZnO layer that is tightly coated on the LATP pellets, surface (ZnO@LATP) via magnetron sputtering, which in situ reacts with Li to form a low electronic conductivity and multifunctional solid electrolyte interphase (SEI). The formed SEI can not only effectively lower the interfacial resistance, but also overcome the side reactions of LATP with the Li metal anode and suppress the Li dendrite growth. Specifically, the interface resistance decreases from 80 554 to 353 Ω and the overpotential reduces from 1 V to 20 mV. As a result, the Li/ZnO@LATP@ZnO/Li symmetric batteries can stably cycle for more than 2000 h without short circuit at 0.05 mA cm^?2 and Li/ZnO@LATP/LiFePO₄ batteries show excellent cycle stability for 200 cycles at 0.1 C. This work highlights the significance of multifunctional interphase between LATP and Li metal for improvement of interfacial impedance and instability.     

Facilitating Interfacial Stability Via Bilayer Heterostructure Solid Electrolyte Toward High‐energy,Safe and Adaptable Lithium Batteries

Solid‐state electrolytes are widely anticipated to enable the revival of high energy density and safe metallic Li batteries, however, their lower ionic conductivity at room temperature, stiff interfacial contact, and severe polarization during cycling continue to pose challenges in practical applications. Herein, a dual‐composite concept is applied to the design of a bilayer heterostructure solid electrolyte composed of Li⁺ conductive garnet nanowires (Li_6.75La₃Zr_1.75Nb_0.25O₁₂)/polyvinylidene fluoride‐co‐hexafluoropropylene (PVDF‐HFP) as a tough matrix and modified metal organic framework particles/polyethylene oxide/PVDF‐HFP as an interfacial gel. The integral ionic conductivity of the solid electrolyte reaches 2.0 × 10^?4 S cm^?1 at room temperature. In addition, a chemically/electrochemically stable interface is rapidly formed, and Li dendrites are well restrained by a robust inorganic shield and matrix. As a result, steady Li plating/stripping for more than 1700 h at 0.25 mA cm^?2 is achieved. Solid‐state batteries using this bilayer heterostructure solid electrolyte deliver promising battery performance (efficient capacity output and cycling stability) at ambient temperature (25 °C). Moreover, the pouch cells exhibit considerable flexibility in service and unexpected endurance under a series of extreme abuse tests including hitting with a nail, burning, immersion under water, and freezing in liquid nitrogen.     

Marginal Magnesium Doping for High‐Performance Lithium Metal Batteries

Due to unparalleled theoretical capacity and operation voltage, metallic Li is considered as the most attractive candidate for lithium‐ion battery anodes. However, Li metal electrodes suffer from uncontrolled dendrite growth and consequent interfacial instability, which result in an unacceptable level of performance in cycling stability and safety. Herein, it is reported that a marginal amount (1.5 at%) of magnesium (Mg) doping alters the surface properties of Li metal foil drastically in such a way that upon Li plating, a highly dense Li whisker layer is induced, instead of sharp dendrites, with enhanced interfacial stability and cycling performance. The effect of Mg doping is explained in terms of increased surface energy, which facilitates plating of Li onto the main surface over the existing whiskers. The present study offers a useful guideline for Li metal batteries, as it largely resolves the longstanding shortcoming of Li metal electrodes without significantly sacrificing their main advantages.     

Highly Stable Operation of Lithium Metal Batteries Enabled by the Formation of a Transient High‐Concentration Electrolyte Layer

Lithium (Li) metal has been extensively investigated as an anode for rechargeable battery applications due to its ultrahigh theoretical specific capacity and the lowest redox potential. However, significant challenges including dendrite growth and low Coulombic efficiency are still hindering the practical applications of rechargeable Li metal batteries. It is demonstrated that long‐term cycling of Li metal batteries can be realized by the formation of a transient high‐concentration electrolyte layer near the surface of Li metal anode during high rate discharge process. The highly concentrated Li⁺ ions in this transient layer will immediately be solvated by the available solvent molecules and facilitate the formation of a stable and flexible solid electrolyte interphase (SEI) layer composed of a poly(ethylene carbonate) framework integrated with other organic/inorganic lithium salts. This SEI layer largely suppresses the corrosion of Li metal anode attacked by free organic solvents and enables the long‐term operation of Li metal batteries. The fundamental findings in this work provide a new direction for the development of Li metal batteries that could be operated at high current densities for a wide range of applications.     

Solvent‐Free Synthesis of Thin,Flexible, Nonflammable Garnet‐Based Composite Solid Electrolyte for All‐Solid‐State Lithium Batteries

Thin solid‐state electrolytes with nonflammability, high ionic conductivity, low interfacial resistance, and good processability are urgently required for next‐generation safe, high energy density lithium metal batteries. Here, a 3D Li_6.75La₃Zr_1.75Ta_0.25O₁₂ (LLZTO) self‐supporting framework interconnected by polytetrafluoroethylene (PTFE) binder is prepared through a simple grinding method without any solvent. Subsequently, a garnet‐based composite electrolyte is achieved through filling the flexible 3D LLZTO framework with a succinonitrile solid electrolyte. Due to the high content of garnet ceramic (80.4 wt%) and high heat‐resistance of the PTFE binder, such a composite electrolyte film with nonflammability and high processability exhibits a wide electrochemical window of 4.8 V versus Li/Li⁺ and high ionic transference number of 0.53. The continuous Li⁺ transfer channels between interconnected LLZTO particles and succinonitrile, and the soft electrolyte/electrode interface jointly contribute to a high ambient‐temperature ionic conductivity of 1.2 × 10^?4 S cm^?1 and excellent long‐term stability of the Li symmetric battery (stable at a current density of 0.1 mA cm^?2 for over 500 h). Furthermore, as‐prepared LiFePO₄|Li and LiNi_0.5Mn_0.3Co_0.2O₂|Li batteries based on the thin composite electrolyte exhibit high discharge specific capacities of 153 and 158 mAh g^?1 respectively, and desirable cyclic stabilities at room temperature.     

Dopant‐Dependent Stability of Garnet Solid Electrolyte Interfaces with Lithium Metal

Li₇La₃Zr₂O₁₂ (LLZO) garnet‐based materials doped with Al, Nb, or Ta to stabilize the Li⁺‐conductive cubic phase are a particularly promising class of solid electrolytes for all‐solid‐state lithium metal batteries. Understanding of the intrinsic reactivity between solid electrolytes and relevant electrode materials is crucial to developing high voltage solid‐state batteries with long lifetimes. Using a novel, surface science‐based approach to characterize the intrinsic reactivity of the Li–solid electrolyte interface, it is determined that, surprisingly, some degree of Zr reduction takes place for all three dopant types, with the extent of reduction increasing as Ta < Nb < Al. Significant reduction of Nb also takes place for Nb‐doped LLZO, with electrochemical impedance spectroscopy (EIS) of Li||Nb–LLZO||Li symmetric cells further revealing significant increases in impedance with time and suggesting that the Nb reduction propagates into the bulk. Density functional theory (DFT) calculations reveal that Nb‐doped material shows a strong preference for Nb dopants toward the interface between LLZO and Li, while Ta does not exhibit a similar preference. EIS and DFT results, coupled with the observed reduction of Zr at the interface, are consistent with the formation of an “oxygen‐deficient interphase” (ODI) layer whose structure determines the stability of the LLZO–Li interface.     

Hybrid and Aqueous Lithium‐Air Batteries

Lithium‐air (Li‐air) batteries have become attractive because of their extremely high theoretical energy density. However, conventional Li‐air cells operating with non‐aqueous electrolytes suffer from poor cycle life and low practical energy density due to the clogging of the porous air cathode by insoluble discharge products, contamination of the organic electrolyte and lithium metal anode by moist air, and decomposition of the electrolyte during cycling. These difficulties may be overcome by adopting a cell configuration that consists of a lithium‐metal anode protected from air by a Li⁺‐ion solid electrolyte and an air electrode in an aqueous catholyte. In this type of configuration, a Li⁺‐ion conducting “buffer” layer between the lithium‐metal anode and the solid electrolyte is often necessary due to the instability of many solid electrolytes in contact with lithium metal. Based on the type of buffer layer, two different battery configurations are possible: “hybrid” Li‐air batteries and “aqueous” Li‐air batteries. The hybrid and aqueous Li‐air batteries utilize the same battery chemistry and face similar challenges that limit the cell performance. Here, an overview of recent developments in hybrid and aqueous Li‐air batteries is provided and the factors that influence their performance and impede their practical applications, followed by future directions are discussed.     

Stable Lithium Electrodeposition at Ultra‐High Current Densities Enabled by 3D PMF/Li Composite Anode

Lithium metal batteries (LMBs) are promising candidates for next‐generation energy storage due to their high energy densities on both weight and volume bases. However, LMBs usually undergo uncontrollable lithium deposition, unstable solid electrolyte interphase, and volume expansion, which easily lead to low Coulombic efficiency, poor cycling performance, and even safety hazards, hindering their practical applications for more than forty years. These issues can be further exacerbated if operated at high current densities. Here a stable lithium metal battery enabled by 3D porous poly‐melamine‐formaldehyde (PMF)/Li composite anode is reported. PMF with a large number of polar groups (amine and triazine) can effectively homogenize Li‐ion concentration when these ions approach to the anode surface and thus achieve uniform Li deposition. Moreover, the 3D structured anode can serve as a Li host to mitigate the volume change during Li stripping and plating process. Galvanostatic measurements demonstrate that the 3D composite electrode can achieve high‐lithium Coulombic efficiency of 94.7% at an ultrahigh current density of 10 mA cm^?2 after 50 cycles with low hysteresis and smooth voltage plateaus. When coupled with Li₄Ti₅O₁₂, half‐cells show enhanced rate capabilities and Coulombic efficiencies, opening great opportunities for high‐energy batteries.