Polymer Coating for Li‐Metal Anode in Polyethylene Oxide‐Based Electrolyte Batteries

The development of solid-state batteries (SSBs) is crucial for advancing next-generation battery technology. Compared to conventional lithium-ion batteries using organic liquid electrolytes, SSBs offer higher energy density, improved safety, and wide operating temperature range. Nonetheless, constructing a practical SSB with lithium-metal anode (LMA) and optimal inorganic solid electrolyte (SE) presents significant challenges. In this presentation, we introduce our findings addressing the two primary obstacles impeding the remarkable advancement of SSBs.The electrochemical instability at the interface between LMA and SE is the first persistent issue to overcome. Solid materials constituting SSBs exhibit distinct physical and chemical properties. When these materials, in the form of electrodes or electrolytes, are assembled into an electrochemical cell configuration, they establish solid–solid interfaces within the energy storage system. Achieving electrochemically stable SSBs requires non-reactive interfaces between LMA and SE with conformal contact. Previous studies have predominantly focused on developing interface modification methods to enhance the long-term cycling stability of SSBs. However, such approaches often offer phenomenological solutions without a thorough understanding of the interface dynamics. Additionally, the role of charges within an electric field in the operation of lithium-ion batteries is often overlooked. The LMA-SE interface of SSBs experiences an in situ electric field during the charging process, where the SEs exhibit semiconducting properties distinct from liquid electrolytes. Consequently, identifying the governing mechanism of interface stabilization cannot rely solely on conventional electrochemical techniques or a materials science perspective alone.Interface modeling by tailoring energy band alignment is a fundamental approach to achieving the cycling stability of SSBs. In this study, we used Li1.3Al0.3Ti1.7(PO4)3 (LATP), a representative NASICON-type SE, alongside a Li-metal foil. Although LATP possesses excellent electrochemical stability with a high anodic limit (4.31 V) and small decomposition energy (- 65 meV per atom at 5 V), its direct contact with LMA leads to the formation of a reactive interface having mixed ionic-electronic conducting interphases (MCI). The presence of MCI expedites the reduction of LATP’s metallic constituents by enabling electron transfer across the interface. Given the heightened risk of interfacial instability in the electrochemical cell, controlling interfacial reactions is imperative for constructing SSBs. Herein, we deposited titanium (45 nm) onto the surface of LATP to stabilize the inherently reactive LMA-LATP interface. The pure transition metal interlayer transformed into an ion-conducting crystal structure through electrochemical interactions with Li-metal. This titanium compound self-induced interlayer (TSI) served as a stable Li-ion conductor and electron buffer between LMA and LATP. Lithium symmetric cells having TSI maintained constant overpotential over 1000 cycles and exhibited significantly reduced impedance.Another critical obstacle to realizing practical SSBs is the intrinsic structural instability of SEs. This physical instability shortens the lifespan of SSBs and severely diminishes their electrochemical performance at the typical ambient temperatures of electric vehicle power batteries (25-50 °C). Over the past decade, various solid-state Li-ion conductors have been developed such as Li7La3Zr2O12 (garnet-type oxide SE) and Li6PS5Cl (argyrodite-type sulfide SE). By associating lattice dynamics and ionic transport in these materials, previous studies could improve the ionic conductivity of SE through major substitution with large cations like tantalum or partial substitution with large anions. Recently, antiperovskite-type SEs with the anion-centered unit of Li3AB, where A represents hydroxyl and B represents halogen groups, have attracted attention. The representative composition Li2OHCl is renowned for its facile fabrication process through solid-state reaction, high ionic conductivity (10-3 S cm-1), and excellent compatibility with LMA. Still, significant limitations exist in its use for constructing practical SSBs, as is common with most other types of SEs.We found that substituting trace amounts of aliovalent cations to modify atomic interactions in SEs can significantly enhance the electrochemical reliability of SSBs. This strategy effectively resolves the intrinsically unstable structure of SEs and simultaneously enhances the LMA-SE interfaces. As a proof-of-concept, we substituted lithium in Li2OHCl with metal elements that do not experience crystal field stabilization energy. The resulting modified electrolytes led to prolonged lifespan and superior rate performance in lithium symmetric cells at room temperature. The cells with the modified SEs exhibited a remarkable critical current density exceeding 10.5 mA cm-2 and maintained a stable voltage profile for 3000 hours at 10.0 mA cm-2, whereas the pristine cells achieved a relatively low critical current density of 2.0 mA cm-2 and a short lifespan of 1000 hours at 1.0 mA cm-2. These findings underscore a strong correlation between the physical properties of inorganic materials and the electrochemical stability of SSBs. Finally, we anticipate that the revealing results will contribute to the advancement of high-power SSBs employing solid electrolytes and metallic electrodes.

Solid electrolyte interface formation between lithium and PEO-based electrolyte

Conventional Li-ion batteries containing flammable and volatile liquid electrolytes are challenged to deliver safe and stable performance for practical energy storage applications. Alternatively, solid polymer electrolytes (SPEs), which intrinsically eliminate the use of organic solvents, display good mechanical stability and flexibility against physical deformation of battery pack and volume retention of electrode materials. The use of SPEs with high thermal and electrochemical stability would contribute to the design and manufacture of safe and cost-effective Li-ion batteries, especially for large-scale applications under elevated temperatures [1].Host materials for most SPEs have yet been dominated by polyethers with ethylene oxide (EO) and propylene oxide (PO) as repeating units. As an interesting replacement to the low-molecular-weight cyclic and linear carbonates, a new class of polymer hosts based on polycarbonates has been considered as Li+-conducting electrolytes [2]. The incorporation of polar carbonate units in the polymer host could effectively facilitate the separation of ion clusters and allow high salt doping in SPEs.In this work, we have developed and explored poly(trimethylene carbonate) (PTMC) as a polymer host alternative for all solid-state Li-ion batteries. High-molecular-weight PTMC was synthesized via bulk ring-opening polymerization. Electrochemical and thermal properties of the PTMC/LiTFSI complexes with varied salt concentration were systematically investigated using electrochemical impedance spectroscopy (EIS) and thermal analysis (DSC/TGA). The as-prepared SPEs are amorphous materials and thermally stable up to 200 °C. The best-performing electrolyte could display useful ionic conductivity at elevated temperatures and was also found electrochemically stable towards 5.0 V vs. Li+/Li. The cycling performance of Li | SPE | LiFePO4 half cells has been tested at elevated temperatures and the cells display long-term cycling stability with retained high capacity up to 153 mAh/g. The gradual increase in capacity might be due to improved interfacial contacts during cycling and storage [3]. Acknowledgements This work has been supported by the STandUP for Energy project and KIC InnoEnergy.

Li metal batteries are regarded as a promising system to overcome the energy density limitations of the current graphite anode Li-ion batteries, by taking advantage of the low reduction potential of lithium metal (-3.04 V vs SHE) and extremely high theoretical capacity (3860 mAh g-1). However, its use is still plagued with challenges that must be solved in order to achieve a successful Li-metal battery operation, that primarily relies on the performance, stability, and reversibility of the anodic side. Those challenges are related to safety concerns, mainly caused by the uncontrolled growth of high surface area lithium (HSAL) and high volumetric expansion. In this sense, to enable the safe use of Li metal, the concept of protective coating has been proposed, enabling to tailor of adequate interphases that can thermodynamically stabilize the lithium metal surface, and therefore improve the lifespan and safety of the battery. The ideal coating should possess a balance between mechanical strength, flexibility, and ionic conductivity, at the time it enables a uniform and efficient transport of lithium ions. Additionally, it must be electronically insulating and possess high chemical and electrochemically stability. In this context, polymeric coatings can offer good flexibility and elasticity which makes them suitable to adapt to the volumetric changes during lithium stripping/deposition cycles. Moreover, they can form uniform films, creating an ideal contact with lithium metal surface. An attractive group of materials that can provide numerous advantages such as high chemical, mechanical and thermal stability, are polysaccharides, aside from being the highly abundant and renewable resource on the Earth, which can be obtained in different forms.Several studies have shown that concentration polarization during battery operation is the main responsible for HSAL generation and limited cycle life, due to the coupled anion-cation movement in the electrolyte. To solve this problem the use of single-ion conducting polymers (SICPs) can result beneficial due to their immobilized anions to a polymeric backbone, which eases lithium ion transport, boosting lithium transference number (𝑡𝐿𝑖+). In addition, the selection of an appropriate polymeric backbone that can withstand volumetric changes, and allow a satisfactory ionic conductivity is essential. In this regard, the use of cellulose is preferred due to their film formation and mechanical properties, lightweight, high abundance, and biodegradability. Herein, a polymeric coating based on cellulose and P(LiMTFSI) single-ion conductor polymer is studied as a protective coating for metallic Li, in order to prevent HSAL initiation and propagation, as well as to enhance electrochemical stability in liquid and solid polymer electrolyte. The coating effectiveness is tested by casting the solution on activated lithium surface and performing lithium stripping/deposition in Li/Cu pouch cells. Additionally, the correlation with its electrochemical and physicochemical properties is performance based on microscopy and spectroscopy technics, to study the most relevant parameters influencing Coulombic Efficiency.

Double-network composite solid electrolyte with stable interface for dendrite-free Li metal anode

The practical application of lithium (Li) metal battery is impeded by the Li dendrite growth and unstable solid electrolyte interphase (SEI) layer. Herein, an ultra‐stretchable and ionic conducting chemically crosslinked pressure‐sensitive adhesive (cPSA) synthesized via the copolymerization of 2‐ethylhexyl acrylate and acrylic acid with poly(ethyleneglycol)dimethacrylate as crosslinker (short for 70cPSA), is developed as both artificial SEI layer and solid polymer electrolyte (SPE) for stable Li‐metal electrode, enabling all‐solid‐state Li metal batteries with excellent cycling performance. As an artificial SEI layer, the 70cPSA‐modified electrodes exhibit excellent electrochemical performance in Li|70cPSA@Cu half cells and 70cPSA@Li|70cPSA@Li symmetric cells. In full cells with LiFePO4 (LFP) as cathode, the 70cPSA@Li|LFP cell exhibits stable cycling performance over 250 cycles. Utilized as SPE, the all‐solid‐state Li|SPE|LFP cell delivers excellent cycling stability with a capacity retention of 86% over 500 cycles. With high‐voltage LiNi0.8Mn0.1Co0.1O2 (NMC811) as cathode, the Li|SPE|NMC811 cell exhibits a discharge capacity of 124.3 mAh g−1 with a capacity retention of 71% after 200 cycles. The rational design of PSAs and investigation of their dual role for stable and safe Li‐metal batteries may shed a light on adhesive polymers for battery applications.

Currently, electrolytes used in lithium-ion batteries are flammable and leaky, causing safety problems when used under high temperatures or extreme conditions. Solid electrolytes can fundamentally avoid the occurrence of such issues, at the same time, it can significantly increase the energy density of solid state lithium batteries. Solid electrolytes are classified into inorganic solid electrolytes and solid polymer electrolytes. Although inorganic solid electrolytes have the high mechanical strength and room temperature ionic conductivity, it has poor flexibility and complicated preparation process. In contrast, solid polymer electrolyte has good elasticity and good interface contact, which can overcome many disadvantages of the inorganic solid electrolyte. Among them, polyethylene oxide (PEO) is first proposed as a polymer electrolyte. Due to its excellent mechanical properties, electrochemical stability and thermal stability, it has been a hot topic for polymer solid electrolytes for decades. However, its low room temperature ionic conductivity limits practical application. Given the problems existing in PEO-based polymer electrolytes, several methods for improving ionic conductivity have been proposed. The primary methods include plasticization, the organic-inorganic composite, and polymer blending. Comparing these methods, it can be seen that adding plasticizers to the polymer can effectively increase the conductivity of the electrolyte, but at the same time it will cause a decrease in the mechanical properties, and the safety issues limit its application in lithium-ion batteries. Inorganic fillers can balance the problems of ionic conductivity and mechanical properties, but the construction of a good filler-polymer dispersion system is complicated in preparing such composite electrolytes, design a new type of multi-dimensional the ion transport network also face tremendous challenges. Undoubtedly, in the future research, inorganic fillers will be the dominant means in the modification of composite electrolytes. Compared with the former two modification methods, polymer blending is easy to operate and friendly to industry, but low ionic conductivity at room temperature still exist. To give full play to the advantages of polymer blending, using it as an auxiliary means to improve the performance of composite electrolytes will be the primary development direction of such modification methods. PEO is the most widely studied polymer electrolyte. Even though a large number of modification methods have emerged in its development history, the ionic conductivity at room temperature is still limited to a relatively low level. To meet the needs of the application in lithium-ion batteries, a high operating temperature (50−70℃) is still needed, the biggest problem remains exist. To solve the problems in polymer solid electrolytes, the research directions in the future may mainly focus on: Expanding the modification method of PEO to other polymer electrolyte host materials, broadening the scope of research, and finding the available polymer electrolytes at room temperature. Concerned about the preparation of ultra-thin polymer electrolytes, reducing the ion transmission path to increase the transmission efficiency of lithium ions between positive and negative electrodes. Effectively combines inorganic solid electrolytes and solid polymer electrolytes to give full play to their advantages. In addition, exploring the effect of inorganic filler morphology on the conductivity of the composite electrolyte will also deserve attention. Many one-dimensional, two-dimensional materials can build ion transport networks so that lithium ions can travel rapidly along the path. With the deepening of research, it is expected that PEO-based polymer electrolytes may replace liquid electrolytes and become an essential cornerstone for building the next generation of safe and reliable lithium-ion batteries.

Significant scientific and economic challenges are presented by developing battery systems with increased energy storage capacity. Geopolitical issues might pose supply chain risks for lithium (Li)-ion batteries, which are frequently used in portable electronics. As an alternative, sodium-ion batteries (NIBs), have drawn attention from researchers all over the world due to the perspective of being lower cost systems[1]. In comparison to lithium-ion batteries, solid-state sodium-ion batteries (SSSBs) promise as a safe, scalable, and environmentally friendly alternative. For the production of robust and adaptable SSSBs, the development of highly conductive, electrochemically stable, and chemically stable solid electrolytes (SEs) is essential [2]. With greater durability, adaptability, and safety, solid-state batteries with inorganic solid electrolytes are being heavily investigated as safer and perhaps more energy-dense alternatives to liquid electrolyte cells.Sulfides solid electrolytes have been extensively investigated due to their mechanical softness and high ionic conductivity. However, the electrochemical oxidation stability of those materials are as low as 2 V (vs. Na+/Na)[3]. Alternatively, the excellent ion conductivities and stability at high potentials have made halide compounds, intriguing candidates for solid electrolytes in batteries. These compounds offer fast ion transport, good stability at highly oxidative potentials, and are easily processed. A wide range of chemistries, such as A3MX6 (A = Li, Na; M = Sc, In, Y, Yb, Er, Hf, Ho, Dy, Al, Zr; X = F, Cl, Br, I), have been synthesized with tunable properties and the characterizations of those materials shows relevant electrochemical proprieties [2].In our work, we investigate different Na-based chloride compounds by Density Functional Theory (DFT). We expand beyond the reported compositions to consider possible cation-substituted compounds. Here, we report on the structures, ionic conduction pathways and ionic conductivity (computed by molecular dynamics methods) expected for such compounds. In the next months, we will also evaluate the compounds´ chemical and electrochemical stability. Our work moves us one step closer towards the development of stable and highly conductive solid electrolytes for Na-based solid-state batteries.References :[1] J. Mater. Chem. A, 2021,9, 281-292[2]J. Mater. Chem. A, 2022,10, 21565-21578[3] ACS Energy Lett. 2022, 7, 10, 3293–3301

The practical application of solid polymer electrolytes in high-energy Li metal batteries is hindered by Li dendrites, electrochemical instability and insufficient ion conductance. To address these issues, flexible composite polymer electrolyte (CPE) membranes with three dimensional (3D) aramid nanofiber (ANF) frameworks are facilely fabricated by filling polyethylene oxide (PEO)-lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) electrolyte into 3D ANF scaffolds. Because of the unique composite structure design and the continuous ion conduction at the 3D ANF framework/PEO-LiTFSI interfaces, the CPE membranes show higher mechanical strength (10.0 MPa), thermostability, electrochemical stability (4.6 V at 60°C) and ionic conductivity than the pristine PEO-LiTFSI electrolyte. Thus, the CPEs display greatly improved interfacial stability against Li dendrites (≥1000 h at 30°C under 0.10 mA cm−2 ), compared with the pristine electrolyte (short circuit in 13 h). The CPE-based all-solid-state LiFePO4/Li cells also exhibit superior cycling performance (e.g., 130 mA h g−1 with 93% retention after 100 cycles at 0.4 C) than the ANF-free cells (e.g., 82 mA h g−1 with 66% retention). This work offers a simple and effective way to achieve high-performance composite electrolyte membranes with 3D nanofiller framework for promising solid-state Li metal battery applications.

Despite major progress in developing solid electrolytes (SEs) with high ionic conductivity, the performance of all-solid-state Li-metal batteries (ASSLBs) remains dominated by interfacial impedance that develops at the electrode-electrolyte interface. Sulfide and halide SEs have emerged as leading candidates for high-energy density ASSLBs owing to their exceptional ionic conductivities, low grain-boundary resistance, and favorable mechanical deformability. However, their practical implementation is still constrained by severe interfacial instabilities with both Li-metal anodes and high-voltage layered oxide cathodes. Interlayer engineering, specifically the incorporation of a functional interlayer between the electrode and SE, has become one of the most effective strategies to mitigate these challenges, enabling suppression of electrolyte decomposition, reduction of space-charge effects, homogenization of Li flux, and stabilization of interphases under high current densities. In this review, we aim to recapitulate the recent developments made in the interlayer-engineering approaches that span over a range of sulfide- and halide-based SE systems, which play a central role as fast Li+-conducting media in enabling high-energy-density ASSLB architectures, and distill unified design principles that connect electrochemical stability, ion-transport behavior, and mechanical compliance. Finally, we discuss future directions and research opportunities that define key priorities for scalable interlayer engineering, aimed at accelerating the development of next-generation high-performance ASSLBs.

(Oxalato)borate: The key ingredient for polyethylene oxide based composite electrolyte to achieve ultra-stable performance of high voltage solid-state LiNi0.8Co0.1Mn0.1O2/lithium metal battery

All-solid-state Lithium metal batteries (ASSLMBs) cannot only maximize the energy density, but also show improved safety compared with the conventional liquid electrolyte-based Li-ion batteries. Solid-state electrolytes (SSEs), as the most important component in the ASSLMBs, can determine the electrochemical performance. High ionic conductivity is one of the most basic requirements for an excellent SSE. Sulfide-based SSEs exhibit ultrahigh ionic conductivity (> 10-2 S cm-1 at room temperature) that can be comparable to the liquid electrolytes, making this kind of SSEs popular and promising.1 However, air sensitivity and Li metal incompatibility are two major obstacles, which are together with the problematic cathode interface, hindering the development of sulfide SSEs for practical ASSLMBs.2, 3 In our work, we employed the strategy of element substitution to design F-substituted Li6PS5Cl, Sn-substituted Li6PS5I, and Sn-substituted Li3PS4 as high-performance Li-metal compatible sulfide SSEs.4-6 Using these element doping SSEs can derive functional interphases that contain LiF, LiI, and Li-Sn alloy, respectively. These interfacial contents were demonstrated as important to achieve stable Li plating/stripping. Importantly, we found that replacing P with Sn could significantly improve the air stability and ionic conductivity at the same time. The high reaction energy between the Sn-substituted PS4 tetrahedra and moisture was regarded as the main reason for the improved air stability, while the expanded unit cell and increased Li+ concentration led to superior ionic conductivity. Our modifications on various common-used sulfide SSEs with multi-function elements provide more choices of sulfide SSEs for practical ASSLMBs. At the cathode side,7-10 we used interface modification (coating strategy) to improve the interfacial Li+ transport and chemical inertness. Thus, excellent electrochemical performance was achieved. Re ferences N. Kamaya, R. Kanno*, et al. A lithium superionic conductor, Nat. Mater. 2011, 10, 682-686.C. Yu, F. Zhao, X. Sun*, et al. Recent Development of lithium argyrodite solid-state electrolytes for solid-state batteries: synthesis, structure, stability and dynamics. Nano Energy, 2021, 83, 105858.Y. Zhao, F. Zhao, X. Sun*, et al. Atomic/molecular layer deposition for energy storage and conversion. Chem. Soc. Rev., 2021, 50, 3889-3956.F. Zhao, X. Sun*, et al. Ultrastable anode interface achieved by fluorinating electrolytes for all-solid-state Li metal batteries, ACS Energy Lett. 2020, 5, 1035-1043F. Zhao, X. Sun*, et al. A Versatile Sn-Substituted Argyrodite Sulfide Electrolyte for All-Solid-State Li Metal Batteries, Adv. Energy Mater. 2020, 10, 1903422.F. Zhao, X. Sun*, et al. An Air-Stable and Li-Metal-Compatible Glass-Ceramic Electrolyte enabling High-Performance All-Solid-State Li-Metal Batteries. Adv. Mater. 2021, 33, 2006577.F. Zhao, X. Sun*, et al. Tuning bifunctional interface for advanced sulfide-based all-solid-state batteries. Energy Storage Mater. 2020, 33, 139-146.C. Wang, F. Zhao, X. Sun* et al. Manipulating Interfacial Nanostructure to Achieve High-Performance All-Solid-State Lithium-Ion Batteries, Small Methods, 2019, 3, 1900261.S. Deng, X. Sun*, et al. Dual-functional interfaces for highly stable Ni-rich layered cathodes in sulfide all-solid-state batteries Energy Storage Mater. 2020, 27, 117-123.S. Deng, X. Sun*, et al. Eliminating the Detrimental Effects of Conductive Agents in Sulfide-Based Solid-State Batteries, ACS Energy Lett. 2020, 5, 1243-1251.

High-energy lithium (Li) metal batteries are constrained by safety and lifespan owing to the lack of suitable electrolyte solutions. Here, we report a synergy of molecular-locking and gelation treatment by cations-bridged polyoxometalate subnanometer nanowires (SNW), which facilitates using ether-based electrolytes for high-energy Li metal batteries. The formed SNW-based gel electrolyte (SNWGE) exhibits continuous three-dimensional networks that effectively capture nonpolar ether electrolytes and promote the dissociation of Li salts, eliminating the risks of electrolyte leakage and volatilization. Homogeneous and continuous Li+ fast transport channels were created in the SNWGE through intermolecular interactions, contributing to sufficient ionic conductivity (1.26 mS cm-1), high oxidative stability (up to 5.0V versus Li+/Li), and good solid/cathode electrolyte interphase formation capability. The SNWGE enables the Li||LiNi0.8Co0.1Mn0.1O2 cells to reach good cyclability (over 88% capacity retention after 670 cycles), excellent low-temperature, and abuse-tolerant properties. Notably, the cost of the SNWGE is only 28.5% of the commercial electrolyte (1M lithium hexafluorophosphate in ethylene carbonate/dimethyl carbonate), highlighting its significant potential for industrial application. When the SNWGE is tested in Li-air batteries, a stable cycling of>520 cycles was obtained. The electrolyte design paves a promising path for highly energetic, durable, and safe rechargeable Li metal batteries.

Carbon fiber (CF) electrodes offer a promising route to enhance both mechanical and electrochemical performance in multifunctional structural batteries for future technologies such as electric vehicles, next‐generation aircraft, and autonomous satellites. Recent developments have focused on solid battery electrolytes that deliver both ionic conductivity and mechanical strength. Central to this is the CF interface, which plays a dual role in structural reinforcement and charge storage. While interfacial chemistry is well studied in composites, its impact on the anode performance of CFs tailored for mechanical performance remains unclear. This article explores how CF surface chemistry influences electrochemical battery performance, by grafting functional groups known to improve composite adhesion (NO 2 , NH 2 , SH, SO 3 H). Surface modifications are confirmed using scanning electron microscopy, Raman spectroscopy, and X‐ray photoelectron spectroscopy. Untreated CFs delivered an initial capacity of 189.1 ± 42.6 mAh g −1 , while SH‐ and NO 2 ‐functionalized CFs achieved enhanced capacities of 244.0 ± 12.1 and 243.8 ± 64.1 mAh g −1 , respectively. These results suggest that chemical surface modification—while originally intended for composite performance—may also improve the anode performance in structural battery systems.

<p indent="0mm">Since the commercialization of lithium-ion batteries in the 1990s, lithium-ion batteries have been successfully applied in portable electronics, electric vehicles, and grid energy storage. Although current organic liquid electrolytes have high ionic conductivities, they are inherently flammable, volatile, and prone to leakage. Moreover, severe side reactions and dendrite growth on the surface of the lithium anode during the charge-discharge process can cause safety hazards, which greatly impede their applications in lithium metal batteries. Solid electrolytes, including inorganic solid electrolytes and polymer electrolytes, are regarded as effective alternatives to organic liquid electrolytes for the construction of lithium metal batteries with high energy density and safety. Among them, solid polymer electrolytes offer excellent flexibility, processability, and interfacial compatibility over inorganic solid electrolytes, and they are extraordinarily promising for lithium metal batteries with high energy density and safety. Ideal solid polymer electrolytes should have following features: (1) High ionic conductivity (&gt; <sc>10 ^–4 S cm ^–1) </sc> at room temperature; (2) high lithium ion transference number (~1) to reduce the concentration polarization and improve the rate performance of batteries; (3) intimate contact at the electrode/electrolyte interfaces; (4) wide electrochemical window <sc>(&gt;4.5 V</sc> vs. Li/Li ⁺) to match high-voltage cathodes and improve the energy density of batteries; (5) good mechanical stability to resist processing, buffer electrode volume change and inhibit dendrite growth; (6) good thermal stability to withstand environmental changes. Generally, the ionic conductivity of pure solid polymer electrolytes at room temperature is low <sc>(~10 ^–6 S cm ^–1). </sc> Researchers have tried to improve the ionic conductivities by adjusting the lithium salt concentration, such as developing “polymer-in-salt” solid electrolytes. However, increasing the concentration of lithium salt leads to the deterioration of the mechanical strength. Strategies such as developing novel lithium salts, modifying polymer matrix, and incorporating inorganic fillers into solid polymer electrolytes are proposed to promote ionic conductivities of solid polymer electrolytes. In particular, composite polymer electrolytes, fabricated by dispersing a certain amount of inorganic fillers into solid polymer electrolytes, have improved ionic conductivities without sacrificing their mechanical performances. Poor interfacial property between electrodes and electrolytes is also a critical issue for solid polymer electrolytes. On one hand, poor and uneven solid/solid contacts at the electrode/electrolyte interfaces lead to high resistance and sluggish ionic transport kinetics. Furthermore, the volume change of the positive and negative electrodes in the charge/discharge process deteriorates the interfacial contacts, blocks the ion and electron transport through the interfaces, and greatly reduces the electrochemical reaction kinetics. On the other hand, the electrochemical windows of solid polymer electrolytes are usually narrow <sc>(＜4.5 V).</sc> During cycling, redox reactions are prone to occur at the electrode/electrolyte interfaces, causing battery failure. Solid polymer electrolytes have also poor thermal and mechanical stabilities. Therefore, design and synthesis of polymer-based solid electrolytes with excellent comprehensive performances and construction of fast and stable ion transport channels at the electrolyte/electrode interfaces are of great significance for the successful development of solid-state lithium metal batteries. This paper presents a brief review of the research progress in solid polymer electrolytes from two aspects: Improving the ionic conductivities of solid polymer electrolytes and enhancing the interfacial performance at electrolyte/electrode interfaces. First, targeted optimization strategies on ionic conductivities of solid polymer electrolytes, including constructing continuously aligned ionic transport paths and shortening the ionic transport distance, are summarized. Second, interface optimization strategies, including constructing wetting interfaces and synthesizing asymmetric electrolytes, are presented to reduce the interface resistance and improve the interfacial contact. Finally, perspectives on the development of solid polymer electrolytes and high-performance solid-state lithium metal batteries are discussed, and key research directions and advanced test methods are proposed. This review may provide a comprehensive understanding and further guidance for not only the material design of solid polymer electrolytes, but also the structural design of lithium metal batteries with favorable electrochemical and interfacial performances.