Programming Ionic Landscapes: Ferroelectric Liquid Crystals, Dielectric Fields, and Process‐Programmed Assembly for the Future of Solid‐State Batteries

Introduction To achieve carbon-neutral society, battery technologies have been attracting attention in recent years owing to its potential for storing renewable energy and serving as a load leveling power sources. In particular, all-solid-state batteries (ASSBs) utilizing non-volatile and flame-resistant solid electrolytes are actively investigated worldwide for practical implementation, as they are expected to enhance safety. Among solid electrolytes, solid polymer electrolytes (SPEs), which exhibit high flexibility and formability, are expected for their application in electrochemical devices such as ASSBs. In SPEs, cations are solvated by the host polymer and occurs ionic conducte through cooperative transport associated with segmental motion. The ionic transport properties of SPE, affected by molecular structures of electrolyte salt, and are further influenced by processes solvation/desolvation of cation and decomposition reactions under electrochemical conditions, is considered different from static states. Therefore, we focused on applying [Li | SPE | Li] symmetric cells and evaluated the ionic transport processes within SPE under reaction conditions using concentration changes through operando Raman spectroscopy measurements. In this study, we applied the measurement approach for use on SPEs incorporating LiN(SO2F)2 (LiFSA), LiN(SO2CF3)2 (LiTFSA), and LiN(SO2C2F5)2 (LiBETI) as Li salts to investigate the impact of anion structure/molecular weight on ionic transport mechanisms under actual reaction conditions. Experiments Li-based solid polymer electrolytes were prepared by mixing cross-linked polyether-based macromonomer P(EO/PO) (EO: PO = 8:2), Li salts (LiFSA, LiTFSA, LiBETI), and photopolymerization initiator DMPA in acetonitrile under an Ar atmosphere within a glovebox. The resulting solution was vacuum-dried and filled between glass plates using a spacer (thickness: 0.5 mm). Each SPE film was then fabricated by UV irradiation. To investigate the solvation structure and salt dissociation characteristics of SPE (salt concentration: [Li]/[O] = 0.04-0.16) under static conditions, Raman spectroscopy measurements were performed (NRS-4500: Jasco). A 9 mm × 9 mm rectangular-shaped [Li | SPE([Li]/[O]=0.04, 0.16) | Li] symmetric cell was prepared for in situ monitoring of concentration variations into the SPE during electrochemical reactions. The symmetric cell was introduced into a sealed cell with a quartz glass having monitoring window and connected to a potentiostat (HZ-7000, Hokuto Denko). In o perando Raman spectroscopy, Raman spectra were obtained from the vicinity of the working electrode (W.E.) and the counter electrode (C.E.) within the SPE during voltage application. The W.E. and C.E. regions were measured at points approximately 10-30 µm from the electrode/electrolyte interface. Results & Discussion Raman spectrum of the P(EO/PO)-LiTFSA(1) and LiBETI(2) system exhibited the peak at approximately 740 cm-1 derived from SNS vibration of TFSA and BETI anion. This peak, which increases with salt concentration, can be utilized as an indicator to evaluate concentration changes within the SPE induced by electrode reactions. To evaluate the ionic transport properties, operand o Raman spectroscopy measurements were performed using [Li | SPE([Li]/[O]=0.04, 0.16)|Li] symmetry cells. The peak area change (A/A 0) from the initial state was calculated by using the obtained Raman spectra. Fig. 1 and 2 exhibited the relationship between A/A 0 and applied voltage time calculated from measured points near W.E. and C.E. in symmetrical cells using the LiTFSA and LiBETI systems. In both Fig. 1 and 2, the peak area changes (A/A 0) were observed that concentrations near W.E. and C.E. increased or decreased with voltage application. In particular, the peak area changes (A/A 0) of the LiTFSA system exhibited a rapid increase in W.E. and decrease in C.E. within the initial 1 h of measurement, followed by no significant changes over 5 h (Fig. 1). This result suggests that the SPE has reached a steady state within at the end of the experiment. And in Fig. 1, the area changes (A/A 0) of [Li]/[O]=0.04 was significantly higher than that of [Li]/[O]=0.16. This results suggests that the faster ionic transport characteristics of [Li]/[O]=0.04 compared to [Li]/[O]=0.16 can be attributed to the lower concentration of the former. No significant changes were observed both [Li]/[O]=0.04 and 0.16 owing to changes in A/A 0, similar to LiTFSA system (Fig. 2). The lower ionic transport properties of LiBETI can be attributed to the larger molecular weight of BETI anion. These results indicated that the ionic transport mechanisms under reaction conditions differ depending on the anion structure (molecular weight).(1) I. Rey, et al, J. Electrochem. Soc., 145, 3034 (1998).(2) C. Capiglia, et al., J. Electrochem. Soc., 150, A525 (2003). Figure 1

Introduction Battery technologies have been attracting attention in recent years as a key solution for achieving a carbon-neutral society, owing to their potential for renewable energy storage and use as a power source. In particular, all-solid-state batteries (ASSBs) using non-volatile and flame-resistant solid electrolytes, are being actively investigated worldwide for practical implementation due to their expected safety advantages. Solid polymer electrolytes (SPEs) are of particular interest due to their high flexibility and processability. In polyether-based SPEs, cations are solvated by the host polymer and occur in ionic conduction through cooperative transport associated with segmental motion. In this study, we focused on gel polymer electrolytes (GPEs) and compared their ionic conductivity and transport properties with those of SPEs. These characteristics are influenced by the electrolyte and salt structures, as well as by the solvation and desolvation processes of cations and their decomposition reactions. To investigate these effects, we fabricated [Li | electrolyte | Li] symmetric cells and evaluated the ionic transport processes in SPEs and GPEs of varying concentrations using operando Raman spectroscopy.1) Specifically, we examined SPEs and GPEs incorporating LiN(SO2CF3)2 (LiTFSI) and LiN(SO2C2F5)2 (LiBETI) as Li salts, aiming to investigate the impact of anion structure, molecular weight, and electrolyte composition on ionic transport mechanisms under actual reaction conditions. Experiments Li-based SPEs were prepared by mixing a cross-linked polyether-based macromonomer, P(EO/PO) (EO: PO = 8:2), with Li salts (LiTFSI, LiBETI) and the photopolymerization initiator DMPA in acetonitrile under an Ar atmosphere in a glove box. The resulting solutions were vacuum-dried overnight. Li-based GPEs were prepared using the same materials but with tetraglyme (G4) as a solvent under an Ar atmosphere. Each prepared SPE and GPE material was filled between glass plates with a 0.5 mm thickness spacer and fabricated by UV irradiation. Raman spectroscopy measurements were performed to analyze the solvation structure and salt dissociation characteristics of SPEs and GPEs (salt concentration: [Li]/[O]=0.04-0.16) under static conditions, where no external field was applied. For in-situ monitoring of concentration variations during electrochemical reactions, a 9 mm × 9 mm rectangular-shaped [Li | SPE/GPE([Li]/[O]=0.10) | Li] symmetric cell was prepared. The cell was placed in a sealed cell with a quartz glass observation window and connected to a potentiostat (HZ-7000, Hokuto Denko). During operando Raman spectroscopy, Raman spectrum were recorded from the regions near the working electrode (W.E.) and counter electrode (C.E.) inside the SPE and GPE while applying a voltage of 300 mV. Measurements were taken at approximately 10-30 µm from the electrode/electrolyte interface. The obtained Raman spectra were normalized to the peak position of a standard sample (polypropylene) and the peak area of the CH2 band (1440-1500 cm-1) in the electrolyte. Results & Discussion Raman spectrum of the P(EO/PO)-LiTFSI (SPE) and P(EO/PO)-G4-LiTFSI (GPE) systems were obtained under static conditions (Figs. 1 and 2). Both systems exhibited a peak at approximately 740 cm-1, corresponding to the SNS vibration of the TFSA anion.2 ) The intensity of this peak increased with salt concentration, making it a useful indicator for evaluating concentration changes within SPEs and GPEs during a electrode reactions. To evaluate the ionic transport properties, operando Raman spectroscopy measurements were performed on [Li | SPE/GPE([Li]/[O]=0.10) | Li] symmetric cells. The peak area change (A/A 0 ) from the initial state was calculated from the obtained Raman spectrum. Fig. 3 shows the relationship between A/A 0 and voltage application time at measurement points near the W.E. and C.E. in both SPE and GPE systems. The results show that concentrations levels near the W.E. and C.E. increased or decreased in response to voltage application. Notably, in both systems, A/A 0 exhibited a rapid increase near W.E., suggesting that this concentration increase is associated with Li dissolution. However, the timing of this increase differed between the systems. In the GPE system, the A/A 0 increase was observed after 4 hours, whereas in the SPE system, it occurred immediately upon voltage application. This delay in the GPE system is attributed to its superior high ionic transport properties and cation transference properties, which are influenced by its electrolyte structure. These findings indicate that the ionic transport mechanisms under reaction conditions differ depending on the electrolyte structure. In the presentation, additional operando Raman spectroscopy results for the LiBETI system and the application of GPEs in Li-ion batteries will also be discussed.(1) K. Hiraoka and S. Seki, J. Phys. Chem. C, 127, 11864 (2023).(2) I. Rey, et al, J. Electrochem. Soc., 145, 3034 (1998). Figure 1

Lithium-ion batteries (LIBs) are widely used in various applications, including personal electronic devices, electric vehicles, and energy storage systems. Given the increasing demand for LIBs, there is need to develop rechargeable batteries with high energy densities, long cycle life, and enhanced safety. Lithium all-solid-state batteries (ASSBs) could improve operational safety by eliminating flammable liquid electrolytes and using non-flammable solid electrolytes. Among the various types of solid electrolytes explored, solid polymer electrolytes (SPEs) have garnered significant attention owing to their desirable properties, such as good processability, light weight, flexibility, and favorable interfacial contact with the electrodes. To develop practical battery systems using SPEs, improving the ionic conductivity mechanism of lithium-ion (Li+) in SPEs is crucial. Li+ transport mechanism of SPE is typically known as segmentation motion and ion hopping. In SPE, crystalline and amorphous phases coexist, and movement of Li+ occurs mainly in the amorphous phase. Therefore, the use of SPE at high temperatures improves ionic conductivity, but still that have limitation by low ionic conductivity (10-7~ 10-8 S cm-1) at ambient temperatures, low Li+ transference numbers, a restricted electrochemical window, and low mechanical strength. Hybrid polymer electrolytes (HPEs) combine an inorganic ceramic electrolyte (ICE) and SPE to create a ceramic-in-polymer or a polymer-in-ceramic hybrid and have improved ionic conductivity and mechanical stability compared to SPE. Ceramic materials of HPE can be classified into oxide-based materials and NASICON-type materials. Oxide-based materials such as SiO2, Al2O3, etc. induce an increase in the amorphous area of PEO and improve segmental mobility by preventing recrystallization of the polymer, thereby increasing ionic conductivity. NASICON-type materials, such as Li7La3Zr2O12 (LLZO), Li1+xAlxTi2-x(PO4)3 (LATP), etc. are promising candidates for improving ionic conductivity by increasing the mobility of Li+ because they form ionic conductive channels within the ceramic bulk. However, despite the observed improvements in performance, the Li+ ion transport mechanism in HPEs and its correlation with the electrochemical cell results remain unclear.In this study, we present a new approach to improve the performance of ASSB by establishing the Li+ transport mechanism through a designed HPE system proven through experimental results and density functional theory (DFT) calculations. To achieve high energy density ASSB, LiNi0.8Co0.1Mn0.1O2 (NCM811) Li ASSB cell was applied, and through a combination of experiment and DFT calculation, HPE was manufactured by PEO-based semi-interpenetrating polymer network (semi-IPN) SPE and Li1+xAlxTi2-x(PO4)3 (LATP) of NASICON-type. As a result, we confirmed that Li+ migrates through the interface of SPE and LATP, which has a significant effect on the improved ionic conductivity (7.23 × 10-4 S cm-1 at 45 ℃). In addition, it was confirmed that Li dendrites were suppressed and cycle performance were improved due to the improved physical strength of the electrolyte with the addition of ceramics. This suggests a promising design strategy for high-performance Li ASSB.

All-solid-state batteries (ASSBs) are widely regarded as one of the most promising candidates for next-generation energy storage technologies. Among the various components of ASSBs, solid polymer electrolytes (SPEs) have attracted significant attention due to their excellent mechanical toughness, low densities, ease of processing, and good interfacial contact with electrodes. In recent years, liquid crystal polymer (LCP) electrolytes have emerged as a research hotspot. Unlike traditional classifications of dielectric and non-dielectric phases, the unique ordered self-assembled structures of LCP electrolytes can provide highly efficient ion transport pathways. This perspective presents a systematic perspective on regulating the performance of lithium-ion batteries (LIBs) (especially ASSBs) through the synergistic combination of dielectric and liquid crystal (LC) phases. The aim of this work is to offer detailed and timely insight into the advantages and disadvantages of polymers and their composite electrolytes from the perspectives of dielectric and ferroelectric phases, while also evaluating the potential of LCPs from the viewpoints of LC and non-LC phases. By combining the advantages of dielectric and LC phases, this work envisions a future for SPEs where ferroelectric LCPs and their composites emerge as a new class of SPEs.

With the growing interest in all-solid-state battery (ASSB) technology for high-energy and high-power applications, the electrochemical performance of cell components and production-related characteristics must be improved to achieve reliable and cost-effective scale-up of laboratory cell concepts [1]. Combining inorganic ceramic and polymer solid electrolytes (SEs) serves to tune the ionic transport and the mechanical properties of composite cathodes [2, 3]. A polymer electrolyte share in the composite cathode is expected to improve the mechanical contact between the cathode active material (CAM) and the SE, resulting in facilitated charge transfer and improved cell performance. Furthermore, polymer SEs serve to overcome the challenges of co-sintering dense composites of CAM and inorganic SE [4, 5].In hybrid cell concepts with polymer- and inorganic ceramic SE, the ionic transport path crosses a ceramic-polymer phase boundary, which leads to additional polarization through charge transfer and ohmic resistance. State-of-the-art literature discusses the influence of ceramic particles in polymer electrolytes, but falls short on the impact of ceramic-polymer phase boundaries on the total cell performance of ASSBs [6, 7, 8].To allow for the simulation of hybrid full cells, a pseudo-two-dimensional (p2D) physicochemical model is introduced within this work. The model is based on the Newman approach, modified with a description for the ceramic-polymer phase boundary [9]. In equilibrium state, the potential drop across the ceramic-polymer boundary was modeled with the Donnan-potential condition as recently proposed by Kim et al. [10]. Under current flow, additional polarization occurs at the interface due to electrochemical charge transfer, which was modeled by the Butler-Volmer equation, and ohmic contact resistance. The conservation of mass and charge over the phase boundary was secured with a current, flux, and a potential boundary condition. A model cell was defined (see figure 1) and parametrized using material-specific values for the ionic and electronic transport characteristics as well as the interface and charge transfer properties. Since the focus of this study was on the ceramic-polymer phase boundary, ideal plating and stripping behavior at the Li-metal anode were assumed neglecting irregular lithium deposition, e.g., lithium dendrite formation. As separator material, the well-known ceramic LLZO SE was modeled. A composite cathode with NMC-622 as CAM and PEO/LiTFSI as polymer electrolyte was assumed. The lower part of figure 1 shows the reduced 1D-model geometry and the ion transport mechanisms in each model domain.The established physicochemical model was applied to identify performance-limiting effects in hybrid ASSBs to conclude on cell designs achieving high energy and power densities.To quantify and localize the polarization contributions in each domain, arising from SE ionic conductivity, diffusion or charge-transfer processes, as well as phase boundaries, the method proposed by Nyman et al. was used [11]. Figure 2 a) shows the results of polarization analysis when simulating a 0.1C charge, while figure 2 b) depicts the results for a 1C charge. Diffusion limitation in the polymer electrolyte led to high concentration gradients in the polymer-phase of the composite cathode, resulting in high diffusion polarization at elevated charging rates as shown in figure 2 b). This determined a critical current density at cell level, which was caused by large Li-ion concentration gradients and a possible depletion of Li-ions near the ceramic separator.The overall cell polarization was further enhanced by the ceramic-polymer phase boundary. For the contact case of LLZO versus PEO/LiTFSI considered here, the equilibrium potential between the phases was calculated according to the theory of Donnan to 31 mV.Since a wide range of values for the contact resistance at the ceramic-polymer interface is reported in the literature [6, 7], ohmic polarization could be important and was therefore evaluated as a function of different contact resistances. A critical contact resistance was determined to achieve the requirements for future battery technologies regarding energy and power density. Figure 1

Rational design of hybrid electrolyte for all-solid-state lithium battery based on investigation of lithium-ion transport mechanism

A review on design considerations in polymer and polymer composite solid-state electrolytes for solid Li batteries

Exploring Advanced Polymeric Binders and Solid Electrolytes for Energy Storage Devices

For the growth of electric vehicle market, lithium-ion batteries (LIBS) used in the EVs still requires safety and reliability. Unfortunately, large-scale application of the LIBs is being challenged due to the fact that the use of flammable liquid electrolytes has caused safety issues such as leakage and fire explosion. In this respect, all-solid-state batteries (ASSBs) have been intensively studied to ensure the safety and mileage that are superior to the current LIBs. In terms of solid electrolytes, oxide electrolytes not only shows high ionic conductivity (10-4 ~ 10-3 S/cm) but also high mechanical strength to suppress surface dendrite formation. In addition, the oxide electrolytes possess advantages such as non-flammability, high thermal stability, and excellent electrochemical stability (~ 6 V), enabling high temperature/high voltage operations of oxide-based ASSBs. However, most of oxide materials require a sintering process at high temperatures to form a planar solid electrolyte. And a lack of flexibility results in non-uniform electrolyte/electrode contact in the battery, which makes it difficult to apply the rigid oxide electrolyte directly. On the other hand, solid polymer electrolytes have also been actively investigated due to no leakage, good electrolyte/electrode contact, easy processing, flexibility, and good film formability. However, the solid polymer electrolytes have critical disadvantages such as low ionic conductivity at room temperature and low thermal/mechanical stability, which precludes commercialization of solid polymer-based ASSBs despite their advantages. To overcome each disadvantages of oxide and polymer electrolytes, we developed hybrid electrolytes for improved ionic conductivity, easy processing, and formation of continuous electrolyte/electrode interface. In this presentation, pragmatic approach and current challenges related to solid batteries will be discussed including innovative manufacturing process. Hybrid electrolytes and their synergistic effect on the battery performance as a promissing solution will be presented [Fig. 1]

Two in one: The use of hexagonal copper sulfide (CuS) nanoparticles as a bifunctional high-performance cathode and as a reinforced electrolyte additive for an all-solid-state lithium battery

All Solid State Batteries (ASSBs) with lithium-ion based conducting solid state electrolytes are considered the next generation high performance batteries. They enable high power densities due to their single ion conducting solid electrolyte, eliminating salt concentration gradients and related polarization losses in the cell, and ensuring an unrivalled level of safety due to their non-combustibility. Currently, a variety of ASSBs based on different solid state electrolytes such as polymers, thiophosphates, oxides and combinations thereof are being developed.One general problem with ASSBs is establishing and maintaining contact between the solid electrolyte and the active material phase during production and cycling, respectively. In conventional lithium-ion batteries (LiBs), this contact is ensured by the liquid state of the electrolyte, but in ASSBs, chemical expansion and contraction of the active material during lithiation and delithiation can detach this contact, resulting in decreased capacity due to the loss of active material. As a consequence, ASSBs are often operated under pressurized conditions, applying pressures significantly exceeding those in conventional LiBs. The same holds for the operating temperature window. Especially for polymer electrolyte-based ASSBs, they are often operated at higher temperatures to compensate for the low ionic conductivity of polymers at room temperature. With respect to cell testing, such operating requirements must be considered, and testing protocols are designed according to the individual requirements of the tested cell.This contribution aims to provide an overview of testing protocols for various types of ASSBs applied to different cells with polymer-, thiophosphate-, oxide-, and hybrid-electrolytes. These protocols will be compared with standardized testing routines for conventional LiBs. Based on this compilation, a harmonized testing procedure that covers the special requirements of the individual cell types and enables a fair comparison of different ASSBs is suggested. Additionally, examples of ASSB testing results will be discussed, taking into consideration the harmonization of different testing parameters.

Cubic garnet solid polymer electrolyte for room temperature operable all-solid-state-battery

Objective: Despite recent global efforts to establish all-solid-state batteries (ASSB) as potentially safe and stable high-energy and high-speed storage technology, long-term performance, specific power, and economic viability remain challenging. However, efficient and short ionic and electronic transport pathways and optimized interfacial contacts between the cell materials and compounds are essential. This poses a significant challenge for the microstructure of cathodes and a careful design and tailoring of anodes and separators in ASSB, which rely solely on solid-state contacts. Here, we present our recent work on ASSB with thiophosphate electrolytes conducted within the German Competence Cluster for Solid-State Batteries (FestBatt). Our work aims to investigate relevant parameters in different cell components on material, electrode, and cell level, such as the importance of particle size distributions, microstructures, and kinetic factors to optimize ASSB cathodes, separators, and anodes. Methodology: The work will be presented in four sections, which showcase key challenges as well as optimization strategies mainly focusing on cathode composites, but also discussing selected examples for separators and anodes in ASSB with thiophosphate electrolytes. Further, an outlook for relevant production steps of ASSB electrodes and cells will be given. For these studies, we mainly conducted focused ion beam scanning electron microscopy tomography, electrochemical impedance spectroscopy, cycling experiments on ASSB cells with thiophosphate solid electrolytes. Results: Fast conducting solid electrolytes are necessary to ensure sufficient transport in composite cathodes but does not represent the only key factor for high-quality cathode composites in ASSB. In our studies, we aimed to optimize the microstructure of composite cathodes consisting of intercalation-type active material and a thiophosphate-based solid electrolyte. We varied the particle size distribution of the solid electrolyte to create various cathode microstructures and assessed the effectiveness of electronic and ionic conduction pathways using partial conductivities. We used both electronically and ionically blocking electrodes for electrochemical impedance spectroscopy measurements to quantify the respective partial conductivities. Our analysis revealed that the particle size of the solid electrolyte significantly influences the charge transport and electrochemical performance of the cathodes. Additionally, using FIB-SEM tomography, we created detailed 3D reconstructions of the cathode microstructure and correlated the obtained partial conductivities with microstructural descriptors such as tortuosity factors, identifying possible kinetic bottlenecks. We found that minimizing residual porosity, which blocks ion and electron transport, is necessary to optimize cathode microstructures and enhance ASSB performance. Moreover, high-performance anodes, along with protective concepts, are crucial for establishing dense, high-energy ASSB cells. Lithium metal anodes may not be the ultimate solution, and silicon anodes can pose advantages, e. g. in terms of electrochemical stability for thiophosphate separators. Discussion: Our studies demonstrate the importance of optimizing the microstructure of cathodes and carefully designing Si anodes for improved ASSB cells. By manipulating the particle size distribution of the solid electrolyte, we achieved more efficient electronic and ionic transport pathways, leading to improved battery performance. The presented work is primarily published in references 1-5, detailing key challenges and showcasing optimization strategies for these topics. Beyond that, diversity in materials, research teams, and approaches is crucial for long-term successful development of ASSB. Acknowledgement: Scientific and administrative coordination of the BMBF Competence Cluster for Solid-State Batteries (FestBatt), FB2-Koord (03XP0431), and cell platform Thiophosphates, FB2-Thio (03XP0430A).

<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.

Rationally designed poly(propylene carbonate)-based electrolyte for dendrite-free all solid-state lithium metal batteries