Simultaneously Enhancing the Thermal Stability,Mechanical Modulus,and Electrochemical Performance of Solid Polymer Electrolytes by Incorporating 2D Sheets

Solid state electrolytes are the key components for high energy density lithium ion batteries and especially for lithium metal batteries where lithium dendrite growth is an inevitable obstacle in liquid electrolytes. Solid polymer electrolytes based on a complex of polymers and lithium salts are intrinsically advantageous over inorganic electrolytes in terms of processability and film‐forming properties. But other properties such as ionic conductivity, thermal stability, mechanical modulus, and electrochemical stability need to be improved. Herein, for the first time, 2D additives using few‐layer vermiculite clay sheets as an example to comprehensively upgrade poly(ethylene oxide)‐based solid polymer electrolyte are introduced. With clay sheet additives, the polymer electrolyte exhibits improved thermal stability, mechanical modulus, ionic conductivity, and electrochemical stability along with reduced flammability and interface resistance. The composite polymer electrolyte can suppress the formation and growth of lithium dendrites in lithium metal batteries. It is anticipated that the clay sheets upgraded solid polymer electrolyte can be integrated to construct high performance solid state lithium ion and lithium metal batteries with higher energy and safety.     

Sulfide Solid Electrolytes for Lithium Battery Applications

The use of solid electrolytes is a promising direction to improve the energy density of lithium‐ion batteries. However, the low ionic conductivity of many solid electrolytes currently hinders the performance of solid‐state batteries. Sulfide solid electrolytes can be processed in a number of forms (glass, glass‐ceramic, and crystalline) and have a wide range of available chemistries. Crystalline sulfide materials demonstrate ionic conductivity on par with those of liquid electrolytes through the utilization of near ideal conduction pathways. Low‐temperature processing is also possible for these materials due to their favorable mechanical properties. The main drawback of sulfide solid electrolytes remains their electrochemical stability, but this can be addressed through compositional tuning or the use of artificial solid electrolyte interphase (SEI). Implementation of sulfide solid electrolytes, with proper treatment for stability, can lead to substantial improvements in solid‐state battery performance leading to significant advancement in electric vehicle technology.     

Ionogel Electrolytes for High‐Performance Lithium Batteries: A Review

Ionic liquids (ILs) are important electrolytes for applications in electrochemical devices. An emerging trend in ILs research is their hybridization with solid matrices, named ionogels. These ionogels can not only overcome the fluidity of ILs but also exhibit high mechanical strength of the solid matrix. Therefore, they show promise for applications in building lithium batteries. In this review, various types of solid matrices for confining ILs are summarized, including nonmetallic oxides, metal oxides, IL‐tethered nanoparticles, functionalized SiO₂, metal–organic frameworks, and other structural materials. The synthetic strategies for ionogels are first documented, focusing on physical confinement and covalent grafting. Then, the structure, ionic conductivity, thermal stability, and electrochemical stability of ionogels are addressed in detail. Furthermore, the authors highlight the potential applications of state‐of‐art ionogels in lithium batteries. The authors conclude this review by outlining the remaining challenges as well as personal perspectives on this hot area of research.     

Highly Tough,Li‐Metal Compatible Organic–Inorganic Double‐Network Solvate Ionogel

Ionogels are considered promising electrolytes for safe lithium‐ion batteries (LIBs) because of their low flammability, good thermal stability, and wide electrochemical stability window. Conventional ionic liquid‐based ionogels, however, face two main challenges; poor mechanical property and low Li‐ion transfer number. In this work, a novel solvate ionogel electrolyte (SIGE) based on an organic–inorganic double network (DN) is designed and fabricated through nonhydrolytic sol–gel reaction and in situ polymerization processes. The unprecedented SIGE possesses high toughness (bearing the deformation under the pressure of 80 MPa without damage), high Li‐ion transfer number of 0.43, and excellent Li‐metal compatibility. As expected, the LiFePO₄/Li cell using the newly developed SIGE delivers a high capacity retention of 95.2% over 500 cycles, and the average Coulombic efficiency is as high as 99.8%. Moreover, the Ni‐rich LiNi_0.8Co_0.1Mn_0.1O₂ (NCM811)/Li cell based on the modified SIGE achieves a high Coulombic efficiency of 99.4%, which outperforms previous solid/quasi‐solid‐state NCM811‐based LIBs. Interestingly, the SIGE‐based pouch cells are workable under extreme conditions (e.g., severely deforming or clipping into segments). In terms of those unusual features, the as‐obtained SIGE holds great promise for next‐generation flexible and safe energy‐storage devices.     

Mixed Ion and Electron‐Conducting Scaffolds for High‐Rate Lithium Metal Anodes

Li metal batteries are considered a promising candidate for next‐generation rechargeable batteries. However, the practical application of Li metal batteries has been hindered by many challenges, especially the cycling stability of Li anodes due to their uncontrollable dendrite growth, volume fluctuation, and side reactions. These problems are more severe under high‐rate charge/discharge process. Therefore, the realization of stable cycling of Li anodes under high current density is crucial for the practical application of Li metal batteries. In this Progress Report, the authors focus on the stability of metallic Li through interphase design or microstructure construction. The advantages and drawbacks of the first‐generation 3D scaffolds are summarized, and a review of recent research progress in this area is generated. As high‐rate cycling of metallic Li is a complex dynamic problem, a scaffold with high mixed ionic and electronic conductivity may be a promising approach. The different design strategies of mixed ion and electron‐conductive scaffolds working with liquid and solid electrolytes are discussed, along with their technical challenges. Further directions of mixed ion and electron‐conductive scaffolds are also proposed.     

Operando Visualization of Morphological Dynamics in All‐Solid‐State Batteries

All‐solid‐state batteries (SSBs) are considered as attractive options for next‐generation energy storage owing to the favorable properties (unit transference number and thermal stabilities) of solid electrolytes. However, there are also serious concerns about mechanical deformation of solid electrolytes leading to the degradation of the battery performance. Therefore, understanding the mechanism underlying the electromechanical properties in SSBs is essentially important. Here, 3D and time‐resolved measurements of an all‐solid‐state cell using synchrotron radiation X‐ray tomographic microscopy are shown. The gradient of the electrochemical reaction and the morphological evolution in the composite layer can be clearly observed. Volume expansion/compression of the active material (Sn) is strongly oriented along the thickness of the electrode. While this results in significant deformation (cracking) in the solid electrolyte region, organized cracking patterns depending on the particle size and their arrangements is also found. This study based on operando visualization therefore opens the door toward rational design of particles and electrode morphology for all‐solid‐state batteries.     

Hexagonal Boron Nitride‐Based Electrolyte Composite for Li‐Ion Battery Operation from Room Temperature to 150 °C

Batteries for high temperature applications capable of withstanding over 60 °C are still dominated by primary cells. Conventional rechargeable energy storage technologies which have exceptional performance at ambient temperatures employ volatile electrolytes and soft separators, resulting in catastrophic failure under heat. A composite electrolyte/separator is reported that holds the key to extend the capability of Li‐ion batteries to high temperatures. A stoichiometric mixture of hexagonal boron nitride, piperidinium‐based ionic liquid, and a lithium salt is formulated, with ionic conductivity reaching 3 mS cm^?1, electrochemical stability up to 5 V and extended thermal stability. The composite is used in combination with conventional electrodes and demonstrates to be stable for over 600 cycles at 120 °C, with a total capacity fade of less than 3%. The ease of formulation along with superior thermal and electrochemical stability of this system extends the use of Li‐ion chemistries to applications beyond consumer electronics and electric vehicles.     

Recent Advances in Non‐Aqueous Electrolyte for Rechargeable Li–O2 Batteries

The rechargeable Li–O₂ battery has attracted much attention over the past decades owing to its overwhelming advantage in theoretical specific energy density compared to state‐of‐the‐art Li‐ion batteries. Practical application requires non‐aqueous Li–O₂ batteries to stably obtain high reversible capacity, which highly depends on a suitable electrolyte system. Up to now, some critical challenges remain in developing desirable non‐aqueous electrolytes for Li–O₂ batteries. Herein, we will review the current status and challenges in non‐aqueous liquid electrolytes, ionic liquid electrolytes and solid‐state electrolytes of Li–O₂ batteries, as well as the perspectives on these issues and future development.     

Improvements to the Overpotential of All‐Solid‐State Lithium‐Ion Batteries during the Past Ten Years

After the research that shows that Li₁₀GeP₂S₁₂ (LGPS)‐type sulfide solid electrolytes can reach the high ionic conductivity at the room temperature, sulfide solid electrolytes have been intensively developed with regard to ionic conductivity and mechanical properties. As a result, an increasing volume of research has been conducted to employ all‐solid‐state lithium batteries in electric automobiles within the next five years. To achieve this goal, it is important to review the research over the past decade, and understand the requirements for future research necessary to realize the practical applications of all‐solid‐state lithium batteries. To date, research on all‐solid‐state lithium batteries has focused on achieving overpotential properties similar to those of conventional liquid‐lithium‐ion batteries by increasing the ionic conductivity of the solid electrolytes. However, the increase in the ionic conductivity should be accompanied by improvements of the electronic conductivity within the electrode to enable practical applications. This essay provides a critical overview of the recent progress and future research directions of the all‐solid‐state lithium batteries for practical applications.     

Design Strategies,Practical Considerations,and New Solution Processes of Sulfide Solid Electrolytes for All‐Solid‐State Batteries

Owing to the ever‐increasing safety concerns about conventional lithium‐ion batteries, whose applications have expanded to include electric vehicles and grid‐scale energy storage, batteries with solidified electrolytes that utilize nonflammable inorganic materials are attracting considerable attention. In particular, owing to their superionic conductivities (as high as ≈10^?2 S cm^?1) and deformability, sulfide materials as the solid electrolytes (SEs) are considered the enabling material for high‐energy bulk‐type all‐solid‐state batteries. Herein the authors provide a brief review on recent progress in sulfide Li‐ and Na‐ion SEs for all‐solid‐state batteries. After the basic principles in designing SEs are considered, the experimental exploration of multicomponent systems and ab initio calculations that accelerate the search for stronger candidates are discussed. Next, other issues and challenges that are critical for practical applications, such as instability in air, electrochemical stability, and compatibility with active materials, are discussed. Then, an emerging progress in liquid‐phase synthesis and solution process of SEs and its relevant prospects in ensuring intimate ionic contacts and fabricating sheet‐type electrodes is highlighted. Finally, an outlook on the future research directions for all‐solid‐state batteries employing sulfide superionic conductors is provided.     

Computation‐Guided Design of LiTaSiO5, a New Lithium Ionic Conductor with Sphene Structure

The development of all‐solid‐state Li‐ion batteries requires solid electrolyte materials with many desired properties, such as ionic conductivity, chemical and electrochemical stability, and mechanical durability. Computation‐guided materials design techniques are advantageous in designing and identifying new solid electrolytes that can simultaneously meet these requirements. In this joint computational and experimental study, a new family of fast lithium ion conductors, namely, LiTaSiO₅ with sphene structure, are successfully identified, synthesized, and demonstrated using a novel computational design strategy. First‐principles computation predicts that Zr‐doped LiTaSiO₅ sphene materials have fast Li diffusion, good phase stability, and poor electronic conductivity, which are ideal for solid electrolytes. Experiments confirm that Zr‐doped LiTaSiO₅ sphene structure indeed exhibits encouraging ionic conductivity. The lithium diffusion mechanisms in this material are also investigated, indicating the sphene materials are 3D conductors with facile 1D diffusion along the [101] direction and additional cross‐channel migration. This study demonstrates a novel design strategy of activating fast Li ionic diffusion in lithium sphenes, a new materials family of superionic conductors.     

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.     

MXene‐Based Mesoporous Nanosheets Toward Superior Lithium Ion Conductors

Although solid polymer electrolytes have some intrinsic advantages in synthesis and film processing compared with inorganic solid electrolytes, low ionic conductivities and mechanical moduli hamper their practical applications in lithium‐based batteries. Here, an efficient strategy is developed to produce a unique solid polymer electrolyte containing MXene‐based mesoporous silica nanosheets with a sandwich structure, which are fabricated via controllable hydrolysis of tetraethyl orthosilicate around the surface of MXene‐Ti₃C₂ under the direction of cationic surfactants. Such unique nanosheets not only exhibit individual, thin, and insulated features, but also possess abundant functional groups in mesopores and on the surface, which are favorable for the formation of Lewis acid–base interactions with anions in polymer electrolytes such as poly(propylene oxide) elastomer, enabling the fast Li⁺ transportation at the mesoporous nanosheets/polymer interfaces. As a consequence, a solid polymer electrolyte with high ionic conductivity of 4.6 × 10^?4 S cm^?1, high Young's modulus of 10.5 MPa, and long‐term electrochemical stability is achieved.     

Carbon Anodes for Nonaqueous Alkali Metal‐Ion Batteries and Their Thermal Safety Aspects

Since their commercialization by Sony in 1991, graphite anodes in combination with various cathodes have enabled the widespread success of lithium‐ion batteries (LIBs), providing over 10 billion rechargeable batteries to the global population. Next‐generation nonaqueous alkali metal‐ion batteries, namely sodium‐ion batteries (SIBs) and potassium‐ion batteries (PIBs), are projected to utilize intercalation‐based carbon anodes as well, due to their favorable electrochemical properties. While traditionally graphite anodes have dominated the market share of LIBs, other carbon materials have been investigated, including graphene, carbon nanotubes, and disordered carbons. The relationship between carbon material properties, electrochemical performance, and charge storage mechanisms is clarified for these alkali metal‐ion batteries, elucidating possible strategies for obtaining enhanced cycling stability, specific capacity, rate capability, and safety aspects. As a key component in determining cell performance, the solid electrolyte interphase layer is described in detail, particularly for its dependence on the carbon anode. Finally, battery safety at extreme temperatures is discussed, where carbon anodes are susceptible to dendrite formation, accelerated aging, and eventual thermal runaway. As society pushes toward higher energy density LIBs, this review aims to provide guidance toward the development of sustainable next‐generation SIBs and PIBs.     

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.     

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.     

Unraveling the Intra and Intercycle Interfacial Evolution of Li6PS5Cl‐Based All‐Solid‐State Lithium Batteries

High‐performance rechargeable all‐solid‐state lithium metal batteries with high energy density and enhanced safety are attractive for applications like portable electronic devices and electric vehicles. Among the various solid electrolytes, argyrodite Li₆PS₅Cl with high ionic conductivity and easy processability is of great interest. However, the low interface compatibility between sulfide solid electrolytes and high capacity cathodes like nickel‐rich layered oxides requires many thorny issues to be resolved, such as the space charge layer (SCL) and interfacial reactions. In this work, in situ electrochemical impedance spectroscopy and in situ Raman spectroscopy measurements are performed to monitor the detailed interface evolutions in a LiNi_0.8Co_0.1Mn_0.1O₂ (NCM)/Li₆PS₅Cl/Li cell. Combining with ex situ characterizations including scanning electron microscopy and X‐ray photoelectron spectroscopy, the evolution of the SCL and the chemical bond vibration at NCM/Li₆PS₅Cl interface during the early cycles is elaborated. It is found that the Li⁺ ion migration, which varies with the potential change, is a very significant cause of these interface behaviors. For the long‐term cycling, the SCL, interfacial reactions, lithium dendrites, and chemo‐mechanical failure have an integrated effect on interfaces, further deteriorating the interfacial structure and electrochemical performance. This research provides a new insight on intra and intercycle interfacial evolution of solid‐state batteries.     

Rechargeable Solid‐State Li–Air and Li–S Batteries: Materials,Construction, and Challenges

Nonaqueous Li–air and Li–S batteries are attracting considerable interest because of their outstanding theoretical capacities and energy densities. However, despite the substantial progress in their development, safety remains an issue because of the flammability of their organic electrolytes. Moreover, the electrolyte volatilization of Li–air batteries and “shuttle effect” in Li–S batteries seriously hinder their development. The use of solid‐state Li–air and Li–S batteries is one of the best solutions. Nevertheless, many challenges remain in solid electrolytes, electrodes, and interfaces. In this review, a comprehensive discussion on the development of solid‐state Li–air and Li–S batteries is provided. The discussion begins with introduction of the progress in solid electrolytes, including their ionic conductivities and chemical stabilities. It then moves on to the cathodes of both batteries and the interface between electrolytes and electrodes. The reaction process inside the cathode is also presented. Suggestions for the optimization of composite cathodes and modification of the electrode–electrolyte interface are provided in the end. Intensive effort is required for the development of solid‐state Li–air and Li–S batteries in the future.     

From Ionogels to Biredox Ionic Liquids: Some Emerging Opportunities for Electrochemical Energy Storage and Conversion Devices

Ionic liquids (ILs) continue to receive attention for applications in electrochemistry because of their unique properties as charge carriers (electrolytes) and redox shuttles (solar cells) and their ability to promote energy storage either electrostatically (supercapacitors) or chemically (secondary batteries). More specifically, the confinement of ILs in nanopores or the adsorption at surfaces, are considered a promising strategy for all solid‐state energy storage and conversion devices. Upon such immobilization, one benefits from the specific properties of ILs (large electrochemical window, relatively high ionic conductivity, task‐specific functionalities, etc.) combined with surface and confinement effects that can be tuned by playing with the porosity and chemical nature of the host. Here, some emerging applications of ILs in electrochemistry are first discussed: silica‐based ionogels as solid electrolytes and systems that involve carbon host substrates, as typical electrode materials in electrical double layer capacitors and lithium secondary batteries. Also, a non‐exhaustive, yet a comprehensive picture of the confinement and surface effects at play in such applications is presented. Then, the confinement of task‐specific ILs such as protonic ILs, IL lithium salts, and biredox ILs, is discussed, which paves the way for promising perspectives. Finally, some concluding remarks are reported and directions for future work are suggested.     

Low‐Operating Temperature,High‐Rate and Durable Solid‐State Sodium‐Ion Battery Based on Polymer Electrolyte and Prussian Blue Cathode

A solid‐state polymer electrolyte (PFSA‐Na membrane) for solid‐state sodium‐ion batteries (SSIBs) to overcome severe safety issues caused by traditional liquid electrolytes is explored. The PFSA‐Na membranes, synthesized by an environmentally and economically friendly method, display high ionic conductivity, excellent thermal stability, and outstanding mechanical flexibility in a wide temperature range. SSIBs based on the PFSA‐Na membranes and Prussian blue cathode exhibit a superior rate performance of 87.5 mA h g^?1 at 8 C and a durable cycling life of up to 1100 cycles at 1 C with only a slight capacity decay of ≈0.014% per cycle. Furthermore, due to the intrinsic advantages of the PFSA‐Na membranes, the cyclic performance of the proposed SSIBs is more stable than that of its liquid counterpart even at the rather low temperature of ?35 °C.