Composition Modulation and Structure Design of Inorganic-in-Polymer Composite Solid Electrolytes for Advanced Lithium Batteries

Synergistic effect of Ti3C2Tx MXene/PAN nanofiber and LLZTO particles on high-performance PEO-based solid electrolyte for lithium metal battery

Solid polymer electrolytes (SPEs) have been considered the most promising separators for all-solid-state lithium metal batteries (ASSLMBs) due to their ease of processing and low cost. However, the practical applications of SPEs in ASSLMBs are limited by their low ionic conductivities and mechanical strength. Herein, we developed a three-dimensional (3D) interconnected MXene (Ti3C2Tx) network and Li6.4La3Zr1.4Ta0.6O12 (LLZTO) particles synergistically reinforced polyethylene oxide (PEO)-based SPE, where the association of Li+ with ether-oxygen in PEO could be significantly weakened through the Lewis acid-base interactions between the electron-absorbing group (Ti-F, -O-) of Ti3C2Tx and Li+. Besides, the TFSI- in lithium salts could be immobilized by hydrogen bonds from the Ti-OH of Ti3C2Tx. The 3D interconnected Ti3C2Tx network not only alleviated the agglomeration of inorganic fillers (LLZTO), but also improved the mechanical strength of composite solid electrolyte (CSE). Consequently, the assembled Li||CSE||Li symmetric battery showed excellent cycling stability at 35 ℃ (stable cycling over 3000 h at 0.1 mA cm-2, 0.1 mAh cm-2) and -2 ℃ (stable cycling over 2500 h at 0.05 mA cm-2, 0.05 mAh cm-2). Impressively, the LiFePO4||CSE||Li battery showed a high discharge capacity of 145.3 mAh/g at 0.3 C after 300 cycles at 35 ℃. This rational structural design provided a new strategy for the preparation of high-performance solid-state electrolytes for lithium metal batteries.

Review: Application of Bionic-Structured Materials in Solid-State Electrolytes for High-Performance Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) have gained significant attention in energy storage research due to their high energy density and significantly improved safety. But there are still certain problems with lithium dendrite growth, interface stability, and room-temperature practicality. Nature continually inspires human development and intricate design strategies to achieve optimal structural applications. Innovative solid-state electrolytes (SSEs), inspired by diverse natural species, have demonstrated exceptional physical, chemical, and mechanical properties. This review provides an overview of typical bionic-structured materials in SSEs, particularly those mimicking plant and animal structures, with a focus on their latest advancements in applications of solid-state lithium metal batteries. Commencing from plant structures encompassing roots, trunks, leaves, flowers, fruits, and cellular levels, the detailed influence of biomimetic strategies on SSE design and electrochemical performance are presented in this review. Subsequently, the recent progress of animal-inspired nanostructures in SSEs is summarized, including layered structures, surface morphologies, and interface compatibility in both two-dimensional (2D) and three-dimensional (3D) aspects. Finally, we also evaluate the current challenges and provide a concise outlook on future research directions. We anticipate that the review will provide useful information for future reference regarding the design of bionic-structured materials in SSEs.

Gradual gradient distribution composite solid electrolyte for solid-state lithium metal batteries with ameliorated electrochemical performance

Composite solid electrolytes (CSEs) have emerged as promising contenders for tackling the safety concerns associated with lithium metal batteries and attaining elevated energy densities. Nonetheless, augmenting ion conductivity and curtailing the growth of lithium dendrites within the electrolyte remain pressing challenges. We have developed CSEs featuring a unique structure, in which Li6.4La3Zr1.4Ta0.6O12 (LLZTO) is distributed in a gradient decline from the center to both sides (GCSE). This distinctive arrangement encompasses heightened polymer content at the edges, thereby enhancing the compatibility between CSEs and electrode materials. Concurrently, the escalated LLZTO content at the center functions to impede the proliferation of lithium dendrites. The uniform gradient distribution state facilitates the consistent and rapid transport of lithium ions. At room temperature, GCSE exhibits an ionic conductivity of 1.5 × 10-4 S cm-1, with stable constant current cycling of lithium for over 1200 h. Furthermore, CR2032 coin batteries with a LiFePO4 (LFP)|GCSE|Li configuration demonstrate excellent rate performance and cycling stability, yielding a discharge capacity of 120 mA h g-1 at 0.5C and retaining 90 % capacity after 200 cycles at 60 °C. Flexible solid electrolytes with gradient structures offer substantial advantages in dealing with ion conductivity and inhibition of lithium dendrites, thereby expected to propel the practical application of lithium metal batteries.

Metal-Organic Framework Glass as a Functional Filler Enables Enhanced Performance of Solid-State Polymer Electrolytes for Lithium Metal Batteries

Polymers are promising candidates as solid-state electrolytes due to their performance and processability, but fillers play a critical role in adjusting the polymer network structure and electrochemical, thermal, and mechanical properties. Most fillers studied so far are anisotropic, limiting the possibility of homogeneous ion transport. Here, applying metal-organic framework (MOF) glass as an isotropic functional filler, solid-state polyethylene oxide (PEO) electrolytes are prepared. Calorimetric and diffusion kinetics tests show that the MOF glass addition reduces the glass transition temperature of the polymer phase, improving the mobility of the polymer chains, and thereby facilitating lithium (Li) ion transport. By also incorporating the lithium salt and ionic liquid (IL), Li-Li symmetric cell tests of the PEO-lithium salt-MOF glass-IL electrolyte reveal low overpotential, indicating low interfacial impedance. Simulations show that the isotropic structure of the MOF glass facilitates the wettability of the IL by enhancing interfacial interactions, leading to a less confined IL structure that promotes Li-ion mobility. Finally, the obtained electrolyte is used to construct Li-lithium iron phosphate full batteries that feature high cycle stability and rate capability. This work therefore demonstrates how an isotropic functional filler can be used to enhance the electrochemical performance of solid-state polymer electrolytes.

Single-Ion Conductive Polymer-Based Composite Electrolytes for High-Performance Solid-State Lithium Metal Batteries

Flexible composite polymer electrolytes (CPEs) with inorganic electrolyte fillers dispersed in polymer electrolytes integrate the merits of the polymer and inorganic electrolytes and have attracted much attention in recent years. In order to increase the electrochemical performance, especially the low lithium (Li)-ion transference number in traditional dual-ion Li salt-containing CPEs, single-ion conductive CPEs are synthesized with a single-ion polymer conductor (SIPC) as the matrix and Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) particles as the active fillers. The single-ion conductive CPEs show a high Li-ion transference number (up to 0.96), high room-temperature (RT) ionic conductivity (>1.0 × 10-4 S cm-1 ), wide electrochemical stability window (>5.0 V, vs Li/Li+ ), and excellent long-term cycling stability with Li metal at RT (3200 h). Based on the SIPC-LLZTO CPE, the solid-state lithium metal batteries with LiFePO4 - and LiCoO2 -based cathodes deliver average discharge capacities of 159 mAh g-1 for 600 cycles and 119 mAh g-1 for 200 cycles at RT, respectively. This study sheds light on the design of high-performance CPEs for next-generation solid-state lithium metal batteries.

Competitive Li+ Coordination in Ionogel Electrolytes for Enhanced Li-Ion Transport Kinetics

Developing ionogel electrolytes based on ionic liquid instead of volatile liquid in gel polymer electrolytes is regarded to be effective to diminish safety concerns in terms of overheating and fire. Herein, a zwitterion-based copolymer matrix based on the copolymerization of trimethylolpropane ethoxylate triacrylate (ETPTA) and 2-methacryloyloxyethylphosphorylcholine (MPC, one typical zwitterion) is developed. It is shown that introducing zwitterions into ionogel electrolytes can effectively optimize local lithium-ion (Li+ ) coordination environment to improve Li+ transport kinetics. The interactions between Li+ and bis(trifluoromethanesulfonyl)imide (TFSI- )/MPC lead to the formation of Li+ coordination shell jointly occupied by MPC and TFSI- . Benefiting from the competitive Li+ attraction of TFSI- and MPC, the energy barrier of Li+ desolvation is sharply decreased and thus the room-temperature ionic conductivity can reach a value of 4.4 × 10-4 S cm-1 . Besides, the coulombic interaction between TFSI- and MPC can greatly decrease the reduction stability of TFSI- , boosting in situ derivation of LiF-enriched solid electrolyte interface  layer on lithium metal surface. As expected, the assembled Li||LiFePO4 cells deliver a high reversible discharge capacity of 139 mAh g-1 at 0.5 C and good cycling stability. Besides, the pouch cells exhibit a steady open-circuit voltage and can operate normally under abuse testing (fold, cut), showing its outstanding safety performance.

Chemically bonding inorganic fillers with polymer to achieve ultra-stable solid-state sodium batteries

Inorganic/organic composite solid electrolytes (CSEs) have attracted ever-increasing attentions due to their outstanding mechanical stability and processibility. However, the inferior inorganic/organic interface compatibility limits their ionic conductivity and electrochemical stability, which hinders their application in solid-state batteries. Herein, we report a homogeneously distributed inorganic fillers in polymer by in-situ anchoring SiO₂ particles in polyethylene oxide (PEO) matrix (I-PEO-SiO₂). Compared with ex-situ CSEs (E-PEO-SiO₂), SiO₂ particles and PEO chains in I-PEO-SiO₂ CSEs are closely welded by strong chemical bonds, thus addressing the issue of interfacial compatibility and realizing excellent dendrite-suppression ability. In addition, the Lewis acid-base interactions between SiO₂ and salts facilitate the dissociation of sodium salts and increase the concentration of free Na⁺. Consequently, the I-PEO-SiO₂ electrolyte demonstrates an improved Na⁺ conductivity (2.3 × 10^-4 S cm^-1 at 60 °C) and Na⁺ transference number (0.46). The as constructed Na₃V₂(PO₄)₃ ‖ I-PEO-SiO₂ ‖ Na full-cell demonstrates a high specific capacity of 90.5 mAh g^-1 at 3C and an ultra-long cycling stability (>4000 cycles at 1C), outperforming the state-of-the-art literatures. This work provides an effective way to solve the issue of interfacial compatibility, which can enlighten other CSEs to overcome their interior compatibility.

Solid-State Electrolytes in Lithium-Sulfur Batteries: Latest Progresses and Prospects

Solid-state lithium-sulfur batteries (SSLSBs) have attracted tremendous research interest due to their large theoretical energy density and high safety, which are highly important indicators for the development of next-generation energy storage devices. Particularly, safety and "shuttle effect" issues originating from volatile and flammable liquid organic electrolytes can be fully mitigated by switching to a solid-state configuration. However, their road to thecommercial application is still plagued with numerous challenges, most notably the intrinsic electrochemical instability of solid-state electrolytes (SSEs) materials and their interfacial compatibility with electrodes and electrolytes. In this review, a critical discussion on the key issues and problems of different types of SSEs as well as the corresponding optimization strategies are first highlighted. Then, the state-of-the-art preparation methods and properties of different kinds of SSE materials, and their manufacture, characterization and performance in SSLSBs are summarized in detail. Finally, a scientific outlook for the future development of SSEs and the avenue to commercial application of SSLSBs is also proposed.

Advanced Composite Solid Electrolytes for Lithium Batteries: Filler Dimensional Design and Ion Path Optimization

Composite solid electrolytes are considered to be the crucial components of all-solid-state lithium batteries, which are viewed as the next-generation energy storage devices for high energy density and long working life. Numerous studies have shown that fillers in composite solid electrolytes can effectively improve the ion-transport behavior, the essence of which lies in the optimization of the ion-transport path in the electrolyte. The performance is closely related to the structure of the fillers and the interaction between fillers and other electrolyte components including polymer matrices and lithium salts. In this review, the dimensional design of fillers in advanced composite solid electrolytes involving 0D-2D nanofillers, and 3D continuous frameworks are focused on. The ion-transport mechanism and the interaction between fillers and other electrolyte components are highlighted. In addition, sandwich-structured composite solid electrolytes with fillers are also discussed. Strategies for the design of composite solid electrolytes with high room temperature ionic conductivity are summarized, aiming to assist target-oriented research for high-performance composite solid electrolytes.

A thin free-standing composite solid electrolyte film for solid-state lithium metal batteries

A thin free-standing composite solid electrolyte film (CSE, ∼15 μm) is prepared by a tape casting and solution infusion method, which owns a high ionic conductivity (2.76 × 10^-4 S cm^-1 at 25 °C) and a wide electrochemical stability window (4.8 V vs. Li⁺/Li). The CSE-based solid-state lithium metal batteries with the LiCoO₂ cathode present good cycling stability and capacity retention with a 4.5 V cut-off voltage.

Organic-Inorganic Composite Electrolytes Optimized with Fluoroethylene Carbonate Additive for Quasi-Solid-State Lithium-Metal Batteries

Composite solid electrolytes (CSEs) are considered crucial materials for next-generation solid-state lithium batteries with high energy density and reliable safety, and they make full use of the advantages of both organic and inorganic solid-state electrolytes. However, few CSEs have sufficiently high ionic conductivity at room temperature for practical applications. Here, a traditional CSE consisting of poly(ethylene oxide) (PEO) matrix and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) fillers was optimized by introducing a fluoroethylene carbonate (FEC) additive, resulting in an improved high ionic conductivity of 1.99 × 10^-4 S cm^-1 at 30 °C. The symmetric Li||Li cell assembled with the optimized CSE exhibited a low overpotential and a good cycling stability of more than 1500 h at room temperature. Moreover, the Li||LiFePO₄ battery with the optimized CSE delivered a discharge capacity of 132 mAh g^-1 at 0.2 C after 300 cycles at room temperature. Comparisons between the LATP-containing CSE and control electrolytes indicated that the enhanced ion conductivity of the former resulted from the synergistic effect of LATP and FEC. Comprehensive characterizations and DFT calculations suggest that with the presence of LATP, FEC additives in the precursor could transform into some other species in the preparation process of CSE. It is believed that these FEC-derived species improve the ion conductivity of the CSEs. The results reported here may open up new approaches to developing composite electrolytes with high ionic conductivity at room temperature by introducing organic additives in the precursor and converting them into species that facilitate ion conduction in the CSE preparation process.

Self-Healing Polymer Electrolyte for Dendrite-Free Li Metal Batteries with Ultra-High-Voltage Ni-Rich Layered Cathodes

Practical applications of polymer electrolytes in lithium (Li) metal batteries with high-voltage Ni-rich cathodes have been hindered by the dendrite growth and poor oxidative stability of electrolytes. Herein, a self-healing polymer electrolyte is developed by in situ copolymerization of 2-(3-(6-methyl4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate (UPyMA) and ethylene glycol methyl ether acrylate (EGMEA) monomers. With the electrolyte, the dendrite growth is inhibited by spontaneously repairing dendrite-induced defects, cracks, and voids at the Li/electrolyte interface; the suppressed dendrite growth and associated electro-chemo behaviors are visualized by the kinetic Mont-Carlo simulation. Benefitting from the high ionic conductivity, wide electrochemical window and good interfacial stability, the self-healing polymer electrolyte enables stable cycling of the LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811) cathode under 4.7 V, achieving a high specific capacity of ≈228.8 mAh g^-1 and capacity retention of 80.4% over 500 cycles. The new electrolyte is very promising for developing highly safe and dendrite-free Li metal batteries with high energy density.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

Recent Advances and Perspectives on the Polymer Electrolytes for Sodium/Potassium-Ion Batteries

Owing to the low cost of sodium/potassium resources and similar electrochemical properties of Na⁺ /K⁺ to Li⁺ , sodium-ion batteries (SIBs) and potassium-ion batteries (KIBs) are regarded as promising alternatives to lithium-ion batteries (LIBs) in large-scale energy storage field. However, traditional organic liquid electrolytes bestow SIBs/KIBs with serious safety concerns. In contrast, quasi-/solid-phase electrolytes including polymer electrolytes (PEs) and inorganic solid electrolytes (ISEs) show great superiority of high safety. However, the poor processibility and relatively low ionic conductivity of Na⁺ and K⁺ ions limit the further practical applications of ISEs. PEs combine some merits of both liquid-phase electrolytes and ISEs, and present great potentials in next-generation energy storage systems. Considerable efforts have been devoted to improving their overall properties. Nevertheless, there is still a lack of an in-depth and comprehensive review to get insights into mechanisms and corresponding design strategies of PEs. Herein, the advantages of different electrolytes, particularly PEs are first minutely reviewed, and the mechanism of PEs for Na⁺ /K⁺ ion transfer is summarized. Then, representative researches and recent progresses of SIBs/KIBs based on PEs are presented. Finally, some suggestions and perspectives are put forward to provide some possible directions for the follow-up researches.

High-Temperature Ultrafast Sintering: Exploiting a New Kinetic Region to Fabricate Porous Solid-State Electrolyte Scaffolds

Solid-state batteries (SSBs) promise better safety and potentially higher energy density than the conventional liquid- or gel-based ones. In practice, the implementation of SSBs often necessitates 3D porous scaffolds made by ceramic solid-state electrolytes (SSEs). Herein, a general and facile method to sinter 3D porous scaffolds with a range of ceramic SSEs on various substrates at high temperature in seconds is reported. The high temperature enables rapid reactive sintering toward the desired crystalline phase and expedites the surface diffusion of grains for neck growth; meanwhile, the short sintering duration limits the coarsening, thus accurately controlling the degree of densification to preserve desired porous structures, as well as reducing the loss of volatile elements. As a proof-of-concept, a composite SSE with a good ionic conductivity (i.e., ≈1.9 × 10^-4 S cm^-1 at room temperature) is demonstrated by integrating poly(ethylene oxide) with the 3D porous Li_6.5 La₃ Zr_1.5 Ta_0.5 O₁₂ scaffold sintered by this method. This method opens a new door toward sintering a variety of ceramic-SSE-based 3D scaffolds for all-solid-state battery applications.

Enhanced ionic conductivity and lithium dendrite suppression of polymer solid electrolytes by alumina nanorods and interfacial graphite modification

Poor room-temperature ionic conductivity and lithium dendrite formation are the main issues of solid electrolytes. In this work, rod-shaped alumina incorporation and graphite coating were simultaneously applied to poly (propylene carbonate) (PPC)-based polymer solid electrolytes (Wang et al., 2018). The obtained alumina modified solid electrolyte membrane (Al-SE) achieves a high ionic conductivity of 3.48 × 10^-4 S/cm at room temperature with a wide electrochemical window of 4.6 V. The assembled NCM622/Al-SE/Li solid-state battery exhibits initial discharge capacities of 198.2 mAh/g and 177.5 mAh/g at the current density of 0.1 C and 0.5 C, with the remaining capacities of 165.8 mAh/g and 161.3 mAh/g after 100 cycles respectively. The rod-shaped structure of Al₂O₃ provides fast transport channels for lithium ions and its Lewis acidity promotes the dissociation of lithium salts and release of free lithium ions. The lithiophilic Al₂O₃ and Graphite form intimate contact with metallic Li and create fast Li⁺ conductive layers of Li-Al-O layer and LiC₆ layer, thus facilitating the uniform deposition of Li and inhibiting Li dendrite formation during long-term cycling. This kind of composite Al-SE is expected to provide a promising alternative for practical application in solid electrolytes.

Macromolecular Design of Lithium Conductive Polymer as Electrolyte for Solid-State Lithium Batteries

In the development of solid-state lithium batteries, solid polymer electrolyte (SPE) has drawn extensive concerns for its thermal and chemical stability, low density, and good processability. Especially SPE efficiently suppresses the formation of lithium dendrite and promotes battery safety. However, most of SPE is derived from the matrix with simple functional group, which suffers from low ionic conductivity, reduced mechanical properties after conductivity modification, bad electrochemical stability, and low lithium-ion transference number. Appling macromolecular design with multiple functional groups to polymer matrix is accepted as a strategy to solve the problems of SPE fundamentally. In this review, macromolecular design based on lithium conducting groups is summarized including copolymerization, network construction, and grafting. Meanwhile, the construction of single-ion conductor polymer is also focused herein. Moreover, synergistic effects between the designed matrix, lithium salt, and fillers are reviewed with the objective to further improve the performance of SPE. At last, future studies on macromolecular design are proposed in the development of SPE for solid-state batteries with high energy density and durability.

Sulfide and Oxide Inorganic Solid Electrolytes for All-Solid-State Li Batteries: A Review

Energy storage materials are finding increasing applications in our daily lives, for devices such as mobile phones and electric vehicles. Current commercial batteries use flammable liquid electrolytes, which are unsafe, toxic, and environmentally unfriendly with low chemical stability. Recently, solid electrolytes have been extensively studied as alternative electrolytes to address these shortcomings. Herein, we report the early history, synthesis and characterization, mechanical properties, and Li+ ion transport mechanisms of inorganic sulfide and oxide electrolytes. Furthermore, we highlight the importance of the fabrication technology and experimental conditions, such as the effects of pressure and operating parameters, on the electrochemical performance of all-solid-state Li batteries. In particular, we emphasize promising electrolyte systems based on sulfides and argyrodites, such as LiPS5Cl and β-Li3PS4, oxide electrolytes, bare and doped Li7La3Zr2O12 garnet, NASICON-type structures, and perovskite electrolyte materials. Moreover, we discuss the present and future challenges that all-solid-state batteries face for large-scale industrial applications.

Polyethylene Oxide-Based Composites as Solid-State Polymer Electrolytes for Lithium Metal Batteries: A Mini Review

Solid-state polymer electrolytes (SPEs) have great processing flexibility and electrode-electrolyte contact and have been employed as the promising electrolytes for lithium metal batteries. Among them, poly(ethylene oxide) (PEO)-based SPEs have attracted widespread attention because of easy synthesis, low mass density, good mechanical stability, low binding energy with lithium salts, and excellent mobility of charge carriers. In order to overcome the low room-temperature ionic conductivity and the poor thermodynamic stability in high-voltage devices (>4.2 V) of the PEO materials, composition modulations by incorporating PEO with inorganic and/or organic components have been designed, which could effectively enable the applications of PEO-based SPEs with widened electro-stable voltage ranges. In this mini review, we describe recent progresses of several kinds of PEO composite structures for SPEs, and we compare the synthesis strategies and properties of these SPEs in lithium batteries. Further developments and improvements of the PEO-based materials for building better rechargeable batteries are also discussed.