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Solid‐state lithium metal batteries (SLMBs) are attracting enormous attention due to their enhanced safety and high theoretical energy density. However, the alkali lithium with high reducibility can react with the solid‐state electrolytes resulting in the inferior cycle lifespan. Herein, inspired by the idea of interface design, the 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imide as an initiator to generate an artificial protective layer in polymer electrolyte is selected. Time‐of‐flight secondary ion mass spectrometry and X‐ray photoelectron spectroscopy reveal the stable solid electrolyte interface (SEI) is in situ formed between the electrolyte/Li interface. Scanning electron microscopy (SEM) images demonstrate that the constructed SEI can promote homogeneous Li deposition. As a result, the Li/Li symmetrical cells enable stable cycle ultralong‐term for over 4500 h. Moreover, the as‐prepared LiFePO4/Li SLMBs exhibit an impressive ultra‐long cycle lifespan over 1300 cycles at 1 C, as well as 1600 cycles at 0.5 C with a capacity retention ratio over 80%. This work offers an effective strategy for the construction of the stable electrolyte/Li interface, paving the way for the rapid development of long lifespan SLMBs. A stable solid electrolyte interface layer with multiple phases of LiF, Li2Sx, and Li3N is successfully in situ formed on the electrolyte/Li surface with the ionic liquid of 1‐butyl‐1‐methylpyrrolidinium bis(trifluoromethanesulfonyl) imide as the initiator. Impressively, the LiFePO4/PIA‐SPE/Li solid‐state batteries exhibit admirable cyclic stabilities, and the current findings pave a new direction for fabricating long lifespan solid‐state lithium metal batteries.

Lithium metal solid‐state batteries (LMSBs) have attracted extensive attention over the past decades, due to their fascinating advantages of safety and potential for high energy density. Solid‐state electrolytes (SEs) with fast ionic transport and excellent stability are indispensable components in LMSBs. Heretofore, a series of inorganic SEs have been extensively explored, such as sulfide‐ and oxide‐based electrolytes. Unfortunately, they both have difficulty in achieving a satisfactory balance of conductivity and stability, and oxides suffer from a high impedance of grain boundaries, while sulfides encounter poor stability. Halide‐based solid electrolytes are gradually emerging as one of the most promising candidates for LMSBs due to their advantages of decent room temperature ionic conductivity (>10−3 S cm−1), good compatibility with oxide cathode materials, good chemical stability, and scalability. Herein, research and development of the widely studied metal halide SEs including fluorides, chlorides, bromides, and iodides are reviewed, mainly focusing on the structures and ionic conductivities as well as preparation methods and electrochemical/chemical stabilities. And then, based on typical metal halide solid electrolytes, we emphasize the interface issues (grain boundaries, cathode−electrolyte and electrolyte–anode interfaces) that exist in the corresponding LMSBs and summarize the related work on understanding and engineering these interfaces. Furthermore, the typical (or in situ) characterization tools widely used for solid‐state interfaces are reviewed. Finally, a perspective on the future direction for developing high‐performance LMSBs based on the halide electrolyte family is put out. The crystal structures and ionic conductivities for widely studied metal halide solid electrolytes, as well as their synthesis methods and electrochemical/chemical stabilities, are systematically summarized, with a special focus on the interface issues that exist in the corresponding lithium metal solid‐state batteries. Furthermore, the typical characterization tools widely used for solid‐state interfaces and some in situ experimental characterizations are reviewed.

Constructing composite solid electrolytes (CSEs) integrating the merits of inorganic and organic components is a promising approach to developing high‐performance all‐solid‐state lithium metal batteries (ASSLMBs). CSEs are now capable of achieving homogeneous and fast Li‐ion flux, but how to escape the trade‐off between mechanical modulus and adhesion is still a challenge. Herein, a strategy to address this issue is proposed, that is, intercalating highly conductive, homogeneous, and viscous‐fluid ionic conductors into robust coordination laminar framework to construct laminar solid electrolyte with homogeneous and fast Li‐ion conduction (LSE‐HFC). A 9 µm‐thick LSH‐HFC, in which poly(ethylene oxide)/succinonitrile is adsorbed by coordination laminar framework with metal–organic framework nanosheets as building blocks, is used here as an example to determine the validity. The Li‐ion transfer mechanism is verified and works across the entire LSE‐HFC, which facilitates homogeneous Li‐ion flux and low migration energy barriers, endowing LSE‐HFC with high ionic conductivity of 5.62 × 10−4 S cm−1 and Li‐ion transference number of 0.78 at 25 °C. Combining the outstanding mechanical strength against punctures and the enhanced adhesion force with electrodes, LSE‐HFC harvests uniform Li plating/stripping behavior. These enable the realization of high‐energy‐density ASSLMBs with excellent cycling stability when being assembled as LiFePO4/Li and LiNi0.6Mn0.2Co0.2O2/Li cells. A thin laminar solid electrolyte can actualize the homogeneous and fast Li‐ion flux while also breaking the trade‐off between mechanical modulus and adhesion. The robust coordination laminar framework allows electrolytes to achieve a high Young's modulus against punctures. Viscous‐fluid ionic conductor confined in coordination laminar framework provides homogeneous and fast Li‐ion transport channels and adhesive contact with electrodes.

Solid polymer electrolytes (SPEs) are pivotal in advancing the practical implementation of all‐solid‐state batteries. Poly(1,3‐dioxane) (PDOL)‐based electrolytes have attracted significant attention due to the pseudo‐high conductivity achieved through sophisticated in situ polymerization methods; however, such PDOL‐based electrolytes present challenges of crystallization over time and monomers residual during processing. In this study, integrating LiTFSI and LiDFOB as a universal copolymerization strategy for developing high‐performance PDOL electrolytes with a wide range of epoxy crosslinkers is proposed. It is discovered that this approach leverages the protective effects of TFSI anions on the boron active center and catalyzes polymer chain growth via crosslinking. The homogenously crosslinked (benzene‐centered) PDOL electrolyte exhibits remarkable thermo‐mechanical stability (up to 100 °C), high ion migration number (tLi+ = 0.42), a wide electrochemical window (≈5.0 V vs Li+/Li), and high ionic conductivity (4.5×10−4 S cm−1). Notably, the crosslinked PDOL electrolyte is in the all‐solid‐state with minimal monomer/oligomer residual, exhibiting no crystallization during relaxation, delivering a robust performance in all‐solid‐state lithium metal batteries. A universal copolymerization strategy of polyether electrolytes is reported using softer anions (TFSI–). This approach enables efficient in situ polymerization without monomers residual, yielding crosslinked polyether electrolytes with superior electrochemical physicochemical characteristics.

Solid‐state polymer electrolytes are an important factor in the deployment of high‐safety and high‐energy‐density solid‐state lithium metal batteries. Nevertheless, use of the traditional polyethylene oxide‐based solid‐state polymer electrolyte is limited due to its inherently low ionic conductivity and narrow electrochemical stability window. Herein, for the first time, we specifically designed a cyanoethyl cellulose‐in‐deep eutectic solvent composite eutectogel as a promising candidate for hybrid solid‐state polymer electrolytes. It is found that the proposed eutectogel electrolyte achieves high ionic conductivity (1.87 × 10−3 S cm−1 at 25°C), superior electrochemical stability (up to 4.8 V), and outstanding lithium plating/striping behavior (low overpotential of 0.04 V at 1 mA cm−2 and 1 mA h cm−2 over 300 h). With the eutectogel‐based solid‐state polymer electrolyte, a 4.45 V LiCoO2/Li metal battery delivers prominent long‐term lifespan (capacity retention of 85% after 200 cycles) and high average Coulombic efficiency (99.5%) under ambient conditions, significantly outperforming the traditional carbonate‐based liquid electrolyte. Our work demonstrates a promising strategy for designing eutectogel‐based solid‐state polymer electrolytes to realize high‐voltage and high‐energy lithium metal batteries. High‐voltage‐tolerant and ion‐conductive cyanoethyl‐based eutectogel is specifically designed and fabricated as promising solid polymer electrolyte for advanced lithium metal batteries. Benefiting from the stable interfacial chemistry, the 4.45 V LiCoO2/Li metal battery delivered excellent cycling stability and high average Coulombic efficiency. In addition, the cell exhibits outstangding safety under some abuse conditions.

Poly(ethylene oxide) (PEO)‐based solid composite electrolytes suffer from poor conductivity and lithium dendrite growth, especially toward the metallic lithium metal anode. In this study, succinonitrile (SN) is incorporated into a PEO composite electrolyte to fabricate an electrode‐compatible electrolyte with good electrochemical performance. The SN‐doped electrolyte successfully inhibits the lithium dendrite growth and facilitates the SEI layer formation, as determined by the operando nanofocus wide‐angle X‐ray scattering (nWAXS), meanwhile, stably cycled over 500 h in Li/SN‐PEO/Li cell. Apart from the observation of lithium dendrite, the robust SEI layer formation mechanism in the first cycle is investigated in the SN‐enhanced composite electrolyte by nWAXS. The inorganic electrochemical reaction products, LiF and Li3N, are found to initially deposit on the electrolyte side, progressively extending toward the lithium metal anode. This growth process effectively protected the metallic lithium, inhibited electron transfer, and facilitated Li⁺ transport. The study not only demonstrates a high‐performance interfacial‐stable lithium metal battery with composite electrolyte but also introduces a novel strategy for real‐time visualizing dendrite formation and SEI growth directing at the interface area of electrolyte and metallic lithium. The Li/electrolyte interface behavior is investigated by synchrotron‐radiation based operando nano‐focus wide‐angle X‐ray scattering on Li||Li cells facilitating a deeper understanding of dendrite growth and SEI formation. The use of simple additives such as succinonitrile is an effective strategy for enhancing interface stability and improving electrochemical performance in all‐solid‐state lithium metal batteries.

Polymer electrolytes (PEs) compatible with NCM cathodes in solid‐state lithium metal batteries (SSLMBs) are gaining recognition as key candidates for advanced electrochemical storage, offering significant safety and stability. Nevertheless, the inherent properties of PEs and interactions at the interface with NCM cathodes are pivotal in influencing SSLMBs' overall performance. This review offers an in‐depth examination of PEs, focusing on design strategies that leverage electron‐group electronegativity for molecular structure adjustments. Furthermore, it delves into the challenges presented by the interface between PEs and NCM cathodes, including issues like poor interface contact, interface reactions, and elevated resistance. The review also discusses a range of strategies aimed at stabilizing these interfaces, such as applying surface coatings to NCM, optimizing the structure of PEs, and employing in situ polymerization techniques to improve compatibility and battery efficiency. The conclusion offers insights into future developments, highlighting the importance of electron‐group optimization and the adoption of effective methods to enhance interface stability and contact, thus advancing the practical implementation of high‐performance SSLMBs. The development of polymer electrolytes with superior properties, combined with stable and compatible interfaces between the polymer electrolyte and the NCM cathode, is crucial for achieving high energy density in solid‐state lithium metal batteries. In this review, we provide an in‐depth overview of various polymer electrolytes, highlighting the interface challenges and strategies to stabilize contact with the NCM cathode. We also offer guidance on enhancing the performance of solid‐state lithium metal batteries by focusing on polymer electrolyte design strategies, particularly those leveraging electron‐group electronegativity for molecular adjustments. Furthermore, we explore methods to improve interface stability, such as surface coatings on the NCM cathode, optimizing polymer electrolyte structures, and employing in situ polymerization techniques.

Solid polymer electrolytes with high interfacial stability are considered among the most promising alternatives for replacing liquid electrolytes in high‐voltage lithium (Li) metal batteries. However, their application faces significant challenges, such as random dendrite deposition, interfacial side reactions, and sluggish ion transport, leading to performance degradation and safety hazards. Herein, an inherently stable difluorinated polyether electrolyte (DPE) is proposed that exhibits superior interfacial stability and ion conductivity, enabling the reliable operation of high‐voltage all‐solid‐state Li metal batteries (ASSLMBs). Due to the synergistic electron‐withdrawing and ion solvation effects of difluorinated functional groups, DPE shows an improved oxidation voltage of 4.9 V and high Li+ conductivity of 2.0 × 10−4 S cm−1. The generated LiF‐rich electrolyte/electrode interphase further improves the stability of DPEs against both Li metal anode and high‐voltage cathode. Consequently, the assembled all‐solid‐state Li||LFP battery retains 73.17% of its capacity after 700 cycles. The high‐voltage all‐solid‐state Li||LiNi0.6Co0.2Mn0.2O2 (NCM622) battery remains stable over 300 cycles with a high capacity retention of 76.02%. Moreover, the high‐voltage ASSLMB shows negligible capacity degradation during 3000 bending cycles at a small radius curvature of 4.0 mm. This work provides a feasible strategy for designing antioxidant polymer electrolytes for the stable operation of high‐voltage Li metal batteries. This work develops an in situ polymerized difluorinated polyether electrolytes (DPEs) for all‐solid‐state lithium metal batteries (ASSLMBs). The DPE exhibits superior interfacial stability and ion conductivity, due to the synergistic electron‐withdrawing and ion solvation effects of difluorinated functional groups. The generated LiF‐rich electrolyte/electrode interphase further enables the ASSLMBs with ultra‐stable cycling performance.

The challenge of safety problems in lithium batteries caused by conventional electrolytes at high temperatures is addressed in this study. A novel solid electrolyte (HKUST-1@IL-Li) was fabricated by immobilizing ionic liquid ([EMIM][TFSI]) in the nanopores of a HKUST-1 metal–organic framework. 3D angstrom-level ionic channels of the metal–organic framework (MOF) host were used to restrict electrolyte anions and acted as “highways” for fast Li+ transport. In addition, lower interfacial resistance between HKUST-1@IL-Li and electrodes was achieved by a wetted contact through open tunnels at the atomic scale. Excellent high thermal stability up to 300 °C and electrochemical properties are observed, including ionic conductivities and Li+ transference numbers of 0.68 × 10−4 S·cm−1 and 0.46, respectively, at 25 °C, and 6.85 × 10−4 S·cm−1 and 0.68, respectively, at 100 °C. A stable Li metal plating/stripping process was observed at 100 °C, suggesting an effectively suppressed growth of Li dendrites. The as-fabricated LiFePO4/HKUST-1@IL-Li/Li solid-state battery exhibits remarkable performance at high temperature with an initial discharge capacity of 144 mAh·g−1 at 0.5 C and a high capacity retention of 92% after 100 cycles. Thus, the solid electrolyte in this study demonstrates promising applicability in lithium metal batteries with high performance under extreme thermal environmental conditions.

Lithium metal solid‐state batteries (LMSSBs) are considered to be one of the ultimate choices for future energy storage systems because of their high theoretical energy density and enhanced safety. However, the development of LMSSBs has been seriously hindered by some practical issues, such as Li dendrite penetration in the solid‐state electrolytes (SSEs) and uncontrolled interphase growth at the Li/SSE interface, which can cause severe battery degradation, failure, and even safety hazards. To construct safe high‐performance LMSSBs, it is crucial to gain an in‐depth understanding of the failure mechanisms induced by these challenges, especially through direct visualization of the failure processes. In this review, the recent progress on the mechanistic study of LMSSBs by in situ electron microscopy is summarized. In situ transmission electron microscopy (TEM) and scanning electron microscopy (SEM) offer an opportunity to probe the battery failure mechanism by observing the associated physical and chemical processes at nano/atomic resolution. The failure causes of Li dendrites growth and interphase formation are classified and discussed, followed by the corresponding solutions to address these issues. Additionally, the emerging perspectives on future research directions in this field are also summarized. This review summarizes failure mechanisms in lithium metal solid‐state batteries (LMSSBs) using in situ electron microscopy. It highlights lithium penetration, solid electrolyte interphase formation, and poor contact as key issues causing degradation of LMSSBs. In situ electron microscopy observations are essential for understanding the failure mechanisms and addressing the challenges of LMSSBs.