Fluorinated Li10 GeP2 S12 Enables Stable All-Solid-State Lithium Batteries

Nanocellulose-reinforced nanofiber composite poly(aryl ether ketone) polymer electrolyte for advanced lithium batteries

Solid polymer batteries (SPEs) are highly desirable for energy storage because of the urgent need for higher energy density and safer lithium ion batteries (LIBs). In this work, the single-ion lithium salt PAEK50-LiCPSI was synthesized by grafting 3-chloropropanesulfonyl trifluoromethanesulimide lithium (LiCPSI) onto poly(aryl ether ketone)50 (PAEK50). Nanocellulose (NCC), PAEK50-LiCPSI, and poly(vinylidene fluoride) (PVDF-HFP) were compounded to obtain NCC reinforced high-performance nanofiber composite polymer electrolytes (NCC/PAEK/PVDF) through electrospinning, which presented tensile strength of 15.35 MPa, ionic conductivity of 1.13 × 10-4 S cm-1, and Li+ transfer number as high as 0.80 at 25 °C. The assembled LIBs with NCC/PAEK/PVDF illustrated an initial discharge specific capacity of 155.2 mAh g-1 at 0.2C, and the capacity retention rate was close to 93 % after cycling 700 cycles at 25 °C. Furthermore, its initial specific discharge capacity at -20 °C was 103.4 mAh g-1, and can cycle over 300 cycles. The NCC with sulfonic acid group reinforced the mechanical performance, promoted the dissociation of Li+, and synergized with PAEK50-LiCPSI and PVDF-HFP to form a 3D nanofiber ionic bridge network through hydrogen bond, which promoted the more stable and faster Li+ transportation. This work suggested that the NCC/PAEK/PVDF can be a good choice of solid polymer electrolytes (SPE) for the next generation of LIBs, even working at low-temperatures.

High-Performance Alginate-Poly(ethylene oxide)-Based Solid Polymer Electrolyte

Solid polymer electrolytes (SPEs) have gained tremendous attention because they are expected to solve the safety problems caused by liquid electrolytes. However, the low ion-transport capacity, insufficient mechanical strength, and unsatisfying flame-retardant properties greatly limit their further application. Here, we designed a poly(ethylene oxide) (PEO)-based SPE by introducing a calcium alginate (CA) nanofiber membrane obtained by electrospinning as a framework. The abundant C═O and -OH groups in the CA macromolecules not only effectively weakened the coordination environment of lithium ions (Li+) but also promoted the dissociation of LiTFSI, assisting in the transfer of Li+ along PEO polymer chains and providing an effective pathway for Li+ transfer. The introduction of calcium ions (Ca+) during the cross-linking process improved the flame-retardant property of the SPE. The obtained SPE exhibited a high ion conductivity (3.86 × 10-4 S cm-1, 30 °C), excellent mechanical strength (2.01 MPa), and a wide electrochemical window (5.32 V). The assembled lithium-symmetric battery could undergo stable lithium plating/stripping for 3000 h at 30 °C. Meanwhile, LiFePO4 (LFP)/Li all-solid-state lithium metal battery showed excellent cycle stability over 300 cycles with a high discharge capacity (141.2 mAh g-1) and retention rate (92.5%) at 0.3 and 30 °C.

Ductile Inorganic Solid Electrolytes for All-Solid-State Lithium Batteries

Solid electrolytes, as the core of all-solid-state batteries (ASSBs), play a crucial role in determining the kinetics of ion transport and the interface compatibility with cathodes and anodes, which can be subdivided into catholytes, bulk electrolytes, and anolytes based on their functional characteristics. Among various inorganic solid electrolytes, ductile solid electrolytes, distinguished from rigid oxide electrolytes, exhibit excellent ion transport properties even under cold pressing, thus holding greater promise for industrialization. However, the challenge lies in finding a ductile solid electrolyte that can simultaneously serve as catholyte, bulk electrolyte, and anolyte. Fortunately, due to the immobility of solid electrolytes, combining multiple types of solid electrolytes allows for leveraging their respective advantages. In this review, we discuss five types of solid electrolytes, sulfides, halides, nitrides, antiperovskite-type, and complex hydrides, and the challenges and superiorities for these electrolytes are also addressed. The impact of pressure on ASSBs has been systematically discussed. Furthermore, the suitability of electrolytes as the catholyte, bulk electrolyte, and anolyte is discussed based on their functional characteristics and physicochemical properties. This discussion aims to deepen our understanding of solid electrolytes, enabling us to harness the advantages of various types of solid electrolytes and develop practical, high-performance ASSBs.

LiF Artifacts in XPS Analysis of the SEI for Lithium Metal Batteries

The solid electrolyte interphase (SEI) is considered to be the key to the performance of lithium metal batteries (LMBs). The analysis of the SEI and cathode electrolyte interphase (CEI) composition (especially F 1s spectra) by X-ray photoelectron spectroscopy (XPS) has become a consensus among researchers. However, the surface-sensitive XPS characterization is susceptible to LiF artifacts due to several factors, leading to the overexaggerated role of LiF in the analysis of the SEI and CEI. In this paper, we conduct a systematic study on the reasons for the LiF artifacts in the XPS characterization of LMBs. The decomposition of the SEI and CEI components under argon ion sputtering, the reaction between Li2CO3 and LiPF6 in the electrolyte, influence of different sample pretreatments, the selection of the XPS measurement region, and the measurement time on the resulting spectra are investigated. The results indicate that the high content of LiF in the SEI and CEI may be attributed to the LiF artifacts, and the role of LiF in the SEI may be overexaggerated as a consequence. This work sounds an alarm about the potential misuse of argon ion sputtering and the lack of rigorous XPS characterization in SEI studies. This work also helps to set up standardized XPS characterization to provide a more accurate understanding of the role of SEI components.

Building a Better All-Solid-State Lithium-Ion Battery with Halide Solid-State Electrolyte

Since the electrochemical potential of lithium metal was systematically elaborated and measured in the early 19th century, lithium-ion batteries with liquid organic electrolyte have been a key energy storage device and successfully commercialized at the end of the 20th century. Although lithium-ion battery technology has progressed enormously in recent years, it still suffers from two core issues, intrinsic safety hazard and low energy density. Within approaches to address the core challenges, the development of all-solid-state lithium-ion batteries (ASSLBs) based on halide solid-state electrolytes (SSEs) has displayed potential for application in stationary energy storage devices and may eventually become an essential component of a future smart grid. In this Review, we categorize and summarize the current research status of halide SSEs based on different halogen anions from the perspective of halogen chemistry, upon which we summarize the different synthetic routes of halide SSEs possessing high room-temperature ionic conductivity, and compare in detail the performance of halide SSEs based on different halogen anions in terms of ionic conductivity, activation energy, electronic conductivity, interfacial contact stability, and electrochemical window and summarize the corresponding optimization strategies for each of the above-mentioned electrochemical indicators. Finally, we provide an outlook on the unresolved challenges and future opportunities of ASSLBs.

Design strategies and performance enhancements of PVDF-based flexible electrolytes for high-performance all-solid-state lithium metal batteries

Lithium metal is considered one of the most promising anode materials for lithium batteries due to its high theoretical specific capacity (3860 mA h g-1) and low redox potential (-3.04 V). However, uncontrolled lithium dendrite growth and severe interfacial side reactions during cycling result in poor performance and safety risks, significantly limiting its practical applications. Replacing liquid electrolytes with solid polymer electrolytes (SPEs) offers a solution, as SPEs provide flexibility and good electrode compatibility, effectively inhibiting dendrite growth and reducing interfacial reactions. Among SPEs, poly(vinylidene fluoride) (PVDF)-based solid electrolytes offer excellent thermal stability and mechanical strength, making them highly suitable for high-energy-density flexible batteries. This review presents recent advances in PVDF-based solid-state electrolytes (SSEs) for stable, high-performance lithium metal batteries (LMBs). We focus on modification strategies that enhance the performance of PVDF-based SSEs in solid-state LMBs and highlight how synthesis methods, nano/microstructural design, and electrochemical properties are interrelated. Lastly, we discuss the challenges and prospects for PVDF-based SSEs in next-generation high-performance LMBs.

Controllable reconstruction of lignified biomass with molecular scissors to form carbon frameworks for highly stable Li metal batteries

Lithium metal batteries (LMBs) promise high-energy-density storage but face safety issues due to dendrite-induced lithium deposition, irreversible electrolyte consumption, and large volume changes, which hinder their practical applications. To address these issues, tuning lithium deposition by structuring a host for the lithium metal anode has been recognized as an efficient method. Herein, we report a supercritical water molecular scissor-controlled strategy to form a carbon framework derived from biomass wood. Proximate-supercritical water treatment is used to selectively cleave the β-O-4 bonds in lignin, with the extent of degradation controlled by adjusting the treatment environment's acidity. The enhanced thermal power of supercritical water molecules significantly accelerates the etching rate of lignin, increasing the porosity and permeability of the transformed carbon framework. Experimental results and multi-physics simulations show that the interconnected carbon-based pores and inner skeletal multilevel hierarchical structure facilitate rapid electron and ion transfer during battery operation and enhance electrolyte infiltration. Impressively, the as-obtained lithium metal anode exhibits long-term cycling stability for over 2000 hours at 0.5 mA cm-2 with low voltage overpotential. The water-treated Pinus (WTP)-Li//LiCoO2 full cells maintain a high capacity retention rate of 93.3% and a specific capacity of 142 mA h g-1 at 0.5C for 100 cycles.

Surface molecular engineering to enable processing of sulfide solid electrolytes in humid ambient air

Sulfide solid-state electrolytes (SSEs) are promising candidates to realize all solid-state batteries (ASSBs) due to their superior ionic conductivity and excellent ductility. However, their hypersensitivity to moisture requires processing environments that are not compatible with today's lithium-ion battery manufacturing infrastructure. Herein, we present a reversible surface modification strategy that enables the processability of sulfide SSEs (e. g., Li6PS5Cl) under humid ambient air. We demonstrate that a long chain alkyl thiol, 1-undecanethiol, is chemically compatible with the electrolyte with negligible impact on its ion conductivity. Importantly, the thiol modification extends the amount of time that the sulfide SSE can be exposed to air with 33% relative humidity (33% RH) with limited degradation of its structure while retaining a conductivity of above 1 mS cm-1 for up to 2 days, a more than 100-fold improvement in protection time over competing approaches. Experimental and computational results reveal that the thiol group anchors to the SSE surface, while the hydrophobic hydrocarbon tail provides protection by repelling water. The modified Li6PS5Cl SSE maintains its function after exposure to ambient humidity when implemented in a Li0.5In | |LiNi0.8Co0.1Mn0.1O2 ASSB. The proposed protection strategy based on surface molecular interactions represents a major step forward towards cost-competitive and energy-efficient sulfide SSE manufacturing for ASSB applications.

Fast-Charging Solid-State Li Batteries: Materials, Strategies, and Prospects

The ability to rapidly charge batteries is crucial for widespread electrification across a number of key sectors, including transportation, grid storage, and portable electronics. Nevertheless, conventional Li-ion batteries with organic liquid electrolytes face significant technical challenges in achieving rapid charging rates without sacrificing electrochemical efficiency and safety. Solid-state batteries (SSBs) offer intrinsic stability and safety over their liquid counterparts, which can potentially bring exciting opportunities for fast charging applications. Yet realizing fast-charging SSBs remains challenging due to several fundamental obstacles, including slow Li+ transport within solid electrolytes, sluggish kinetics with the electrodes, poor electrode/electrolyte interfacial contact, as well as the growth of Li dendrites. This article examines fast-charging SSB challenges through a comprehensive review of materials and strategies for solid electrolytes (ceramics, polymers, and composites), electrodes, and their composites. In particular, methods to enhance ion transport through crystal structure engineering, compositional control, and microstructure optimization are analyzed. The review also addresses interface/interphase chemistry and Li+ transport mechanisms, providing insights to guide material design and interface optimization for next-generation fast-charging SSBs.

Achieving Higher Critical Current Density in LGPS-Based Lithium Metal Batteries via a Synergistic Interlayer for Physical Inhibition and Chemical Scavenging of Lithium Dendrites

Li10.35Ge1.35P1.65S12 (LGPS) electrolyte has garnered attention due to its high ionic conductivity and processability. However, its strong incompatibility with lithium metal hinders its practical application. Conventional interlayer strategy isolates Li from LGPS, avoiding the detrimental side reactions, but lithium dendrite penetration is still a problem. To address the aforementioned challenges, we develop a PVDF-HFP-supported PDOL-based interlayer (PDOL/PVDF-HFP), which stabilizes the LGPS/Li interface by synergistically physically inhibiting and chemically scavenging lithium dendrites. The multifunctional feature of the interlayer comes from the use of a bifunctional initiator, InCl3. On the one hand, InCl3 induces the polymerization of DOL, forming a physical separator and protecting lithium from LGPS; on the other hand, in situ reactions between In3+/Cl- and Li form a LiCl/LiF/LiIn hybrid SEI, homogenizing the surface Li+ flux and suppressing lithium dendrite formation and penetration. In addition, an unexpected dynamic microdendrite scavenging is realized by virtue of the side reactions of LGPS/Li, which converts the undesirable reaction to be an advantage in our design. Benefiting from the comprehensive advantages of such design, the constructed sulfide-based solid-state batteries achieve a super low interfacial impedance of 5.1 Ω, a high critical current density (CCD) value over 5 mA/cm2, and a super long cycling stability over 8000 h. Our synergistic interlayer strategy would open an effective avenue for solving interfacial challenges for practical sulfide-based solid-state batteries.

Intercepting Dendrite Growth With a Heterogeneous Solid Electrolyte for Long-Life All-Solid-State Lithium Metal Batteries

The application of lithium metal anode in all-solid-state batteries has the potential to achieve both high energy density and safety performance. However, the presence of serious dendrite issues hinders this potential. Here, the ion transport pathways and orientation of dendrite growth are regulated by utilizing the differences of ionic conductivity in heterogeneous electrolytes. The in situ formed Li-Ge alloy phases from the spontaneous reaction between Li10GeP2S12 and the attracted dendrites greatly enhance the ability to resist dendrite growth. As an outcome, the heterogeneous electrolyte achieves a high critical current density of 2.1 mA cm-2 and long-term stable symmetrical battery operation (0.3 mA cm-2 for 17 000 h and 1.0 mA cm-2 for 2000 h). Besides, due to the superior interfacial stability and low interface impedance between the heterogeneous electrolyte and lithium anode, the Li||LiNi0.8Co0.1Mn0.1O2 full battery exhibits great cycling stability (80.5% after 500 cycles at 1.0 mA cm-2) and rate performance (125.4 mAh g at 2.0 mA cm-2). This work provides a unique strategy of interface regulation via heterogeneous electrolytes design, offering insights into the development of state-of the-art all-solid-state batteries.

Homogeneous Fluorine Doping toward Highly Conductive and Stable Li10GeP2S12 Solid Electrolyte for All-Solid-State Lithium Batteries

The unique structure and exceptionally high lithium ion conductivity over 10 mS cm-1 of Li10GeP2S12 have gained extensive attention in all-solid-state lithium batteries. However, its poor resistivity to moisture and chemical/electrochemical incompatibility with lithium metal severely impede its practical application. Herein, a fluorine functionalized Li10GeP2S12 is synthesized by stannous fluoride doping and employed as a monolayer solid electrolyte to realize stable all-solid-state lithium batteries. The atomic-scale mechanism underlying the impact of fluorine doping on both moisture and electrochemical stability of Li10GeP2S12 is revealed by density functional theory calculations. Fluorine surface doping significantly reduces surface hydrophilicity by electronic regulation, thereby retarding the hydrolysis reaction of Li10GeP2S12. After exposed to a relative humidity of 35%-40% for 20 min, the ionic conductivity of Li9.98Ge0.99Sn0.01P2S11.98F0.02 maintains as high as 2.21 mS cm-1, nearly one order of magnitude higher than that of Li10GeP2S12 with 0.31 mS cm-1. Meanwhile, bulk doping of highly electronegative fluorine promotes the formation of lithium vacancies in the Li10GeP2S12 system, thus allowing stable lithium plating/stripping in Li | Li symmetric batteries, boosting a critical current density reaching 2.1 mA cm-2. The LiCoO2 | lithium all-solid-state batteries display improved cycling stability and rate capability, showing 80.1% retention after 600 cycles at 1C.

Ni-Rich Layered Oxide Cathodes/Sulfide Electrolyte Interface in Solid-State Lithium Battery

Because of the high specific capacity and low cost, Ni-rich layered oxide (NRLO) cathodes are one of the most promising cathode candidates for the next high-energy-density lithium-ion batteries. However, they face structure and interface instability challenges, especially the battery safety risk caused by using an intrinsic flammable organic liquid electrolyte. In this regard, a solid electrolyte with high safety is of great significance to promote the development of energy storage. Among them, sulfide electrolytes are considered to be the most potential substitutes for liquid electrolytes because of their high ionic conductivity and good processing properties. Nevertheless, the interfacial incompatibility between the sulfide electrolyte and NRLO cathode is the critical challenge for high-performance sulfide all-solid-state lithium batteries (ASSLBs). In this review, we summarize the problems of the Ni-rich cathode/sulfide solid electrolyte interface and the strategies to improve the interface stability. On the basis of these insights, we highlight the scientific problems and technological challenges that need to be resolved urgently and propose several potential directions to further improve the interface stability. The objective of this study is to provide a comprehensive understanding and insightful recommendations for the enhancement of the sulfide ASSLBs with NRLO cathode.

Lignin Derived Ultrathin All-Solid Polymer Electrolytes with 3D Single-Ion Nanofiber Ionic Bridge Framework for High Performance Lithium Batteries

The lignin derived ultrathin all-solid composite polymer electrolyte (CPE) with a thickness of only 13.2 µm, which possess 3D nanofiber ionic bridge networks composed of single-ion lignin-based lithium salt (L-Li) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) as the framework, and poly(ethylene oxide)/lithium bis(trifluoromethanesulfonyl)imide (PEO/LiTFSI) as the filler, is obtained through electrospinning/spraying and hot-pressing. t. The Li-symmetric cell assembled with the CPE can stably cycle more than 6000 h under 0.5 mA cm-2 with little Li dendrites growth. Moreover, the assembled Li||CPE||LiFePO4 cells can stably cycle over 700 cycles at 0.2 C with a super high initial discharge capacity of 158.5 mAh g-1 at room temperature, and a favorable capacity of 123 mAh g-1 at -20 °C for 250 cycles. The excellent electrochemical performance is mainly attributed to the reason that the nanofiber ionic bridge network can afford uniformly dispersed single-ion L-Li through electrospinning, which synergizes with the LiTFSI well dispersed in PEO to form abundant and efficient 3D Li+ transfer channels. The ultrathin CPE induces uniform deposition of Li+ at the interface, and effectively inhibit the lithium dendrites. This work provides a promising strategy to achieve ultrathin biobased electrolytes for solid-state lithium ion batteries.

Alkalized MXene/carbon nanotube composite for stable Na metal anodes

Ti3C2 MXenes are emerging 2D materials and have attracted increasing attention in sodium metal anode fabrication because of their high conductivity, multifunctional groups and excellent mechanical performances. However, the severe self-restacking of Ti3C2 MXenes is not conducive to dispersing Na+ and limits the function of regulating sodium deposition. Herein, an alkalized MXene/carbon nanotube (CNT) composite (named A-M-C) is introduced to regulate Na deposition behavior, which consists of Na3Ti5O12 microspheres, Ti3C2 MXene nanosheets and CNTs. Ti3C2 MXene nanosheets with large interlayer spaces and "sodiophilic" functional groups can provide abundant active sites for uniform nucleation and deposition of Na. Plenty of nanosheets are grown on the surface of the microsphere, thereby reducing the local current density, which can guide initial Na nucleation and promote Na dendrite-free growth. Furthermore, CNTs increase the electrical conductivity of the composite and achieve fast Na+ transport, improving the cycling stability of Na metal batteries. As a result, at a capacity of 1 mA h cm-2, the A-M-C electrode achieves a high average coulombic efficiency (CE) of 99.9% after 300 cycles at 2 mA cm-2. The symmetric cells of A-M-C/Na provide a long cycling life of more than 1400 h at 1 mA cm-2 with a minimal overpotential of 19 mV at an areal capacity of 1 mA h cm-2. The A-M-C/Na//NVP@C full cell presents a high coulombic efficiency of 98% with 100 mA g-1 in the first cycle. The strategy in this work provides new insights into fabricating novel MXene-based anode materials for dendrite-free sodium deposition.

Deciphering the critical role of interstitial volume in glassy sulfide superionic conductors

Sulfide electrolytes represent a crucial category of superionic conductors for all-solid-state lithium metal batteries. Among sulfide electrolytes, glassy sulfide is highly promising due to its long-range disorder and grain-boundary-free nature. However, the lack of comprehension regarding glass formation chemistry has hindered their progress. Herein, we propose interstitial volume as the decisive factor influencing halogen dopant solubility within a glass matrix. We engineer a Li3PS4-Li4SiS4 complex structure within the sulfide glassy network to facilitate the release of interstitial volume. Consequently, we increase the dissolution capacity of LiI to 40 mol% in 75Li2S-25P2S5 glass. The synthesized glass exhibits one of the highest ionic conductivities among reported glass sulfides. Furthermore, we develop a glassy/crystalline composite electrolyte to mitigate the shortcomings of argyrodite-type sulfides by utilizing our synthesized glass as the filler. The composite electrolytes effectively mitigate Li intrusion. This work unveils a protocol for the dissolution of halogen dopants in glass electrolytes.

Rational Design of F-Modified Polyester Electrolytes for Sustainable All-Solid-State Lithium Metal Batteries

Solid polymer electrolytes (SPEs) are one of the most practical candidates for solid-state batteries owing to their high flexibility and low production cost, but their application is limited by low Li+ conductivity and a narrow electrochemical window. To improve performance, it is necessary to reveal the structure-property relationship of SPEs. Here, 23 fluorinated linear polyesters were prepared by editing the coordination units, flexible linkage segments, and interface passivating groups. Besides the traditionally demonstrated coordinating capability and flexibility of polymer chains, the molecular asymmetry and resulting interchain aggregation are observed critical for Li+ conductivity. By tailoring the molecular asymmetry and coordination ability of polyesters, the Li+ conductivity can be raised by 10 times. Among these polyesters, solvent-free poly(pentanediol adipate) delivers the highest room-temperature Li+ conductivity of 0.59 × 10-4 S cm-1. The chelating coordination of oxalate and Li+ leads to an electron delocalization of alkoxy oxygen, enhancing the antioxidation capability of SPEs. To lower the cost, high-value LiTFSI in SPEs is recycled at 90%, and polyesters can be regenerated at 86%. This work elucidates the structure-property relationship of polyester-based SPEs, displays the design principles of SPEs, and provides a way for the development of sustainable solid-state batteries.

Boosting the Interfacial Stability of the Li6PS5Cl Electrolyte with a Li Anode via In Situ Formation of a LiF-Rich SEI Layer and a Ductile Sulfide Composite Solid Electrolyte

Due to its good mechanical properties and high ionic conductivity, the sulfide-type solid electrolyte (SE) can potentially realize all-solid-state batteries (ASSBs). Nevertheless, challenges, including limited electrochemical stability, insufficient solid-solid contact with the electrode, and reactivity with lithium, must be addressed. These challenges contribute to dendrite growth and electrolyte reduction. Herein, a straightforward and solvent-free method was devised to generate a robust artificial interphase between lithium metal and a SE. It is achieved through the incorporation of a composite electrolyte composed of Li6PS5Cl (LPSC), polyethylene glycol (PEG), and lithium bis(fluorosulfonyl)imide (LiFSI), resulting in the in situ creation of a LiF-rich interfacial layer. This interphase effectively mitigates electrolyte reduction and promotes lithium-ion diffusion. Interestingly, including PEG as an additive increases mechanical strength by enhancing adhesion between sulfide particles and improves the physical contact between the LPSC SE and the lithium anode by enhancing the ductility of the LPSC SE. Moreover, it acts as a protective barrier, preventing direct contact between the SE and the Li anode, thereby inhibiting electrolyte decomposition and reducing the electronic conductivity of the composite SE, thus mitigating the dendrite growth. The Li|Li symmetric cells demonstrated remarkable cycling stability, maintaining consistent performance for over 3000 h at a current density of 0.1 mA cm-2, and the critical current density of the composite solid electrolyte (CSE) reaches 4.75 mA cm-2. Moreover, the all-solid-state lithium metal battery (ASSLMB) cell with the CSEs exhibits remarkable cycling stability and rate performance. This study highlights the synergistic combination of the in-situ-generated artificial SE interphase layer and CSEs, enabling high-performance ASSLMBs.

Recent Progress on the Air-Stable Battery Materials for Solid-State Lithium Metal Batteries

Solid-state lithium metal batteries (SSLMBs) offer numerous advantages in terms of safety and theoretical specific energy density. However, their main components namely lithium metal anode, solid-state electrolyte, and cathode, show chemical instability when exposed to humid air, which results in low capacities and poor cycling stability. Recent studies have shown that bioinspired hydrophobic materials with low specific surface energies can protect battery components from corrosion caused by humid air. Air-stable inorganic materials that densely cover the surface of battery components can also provide protection, which improves the storage stability of the battery components, broadens their processing conditions, and ultimately decreases their processing costs while enhancing their safety. In this review, the mechanism behind the surface structural degradation of battery components and the resulting consequences are discussed. Subsequently, recent strategies are reviewed to address this issue from the perspectives of lithium metal anodes, solid-state electrolytes, and cathodes. Finally, a brief conclusion is provided on the current strategies and fabrication suggestions for future safe air-stable SSLMBs.

A Dynamically Stable Mixed Conducting Interphase for All-Solid-State Lithium Metal Batteries

All-solid-state lithium (Li) metal batteries (ASSLMBs) employing sulfide solid electrolytes have attracted increasing attention owing to superior safety and high energy density. However, the instability of sulfide electrolytes against Li metal induces the formation of two types of incompetent interphases, solid electrolyte interphase (SEI) and mixed conducting interphase (MCI), which significantly blocks rapid Li-ion transport and induces uneven Li deposition and continuous interface degradation. In this contribution, a dynamically stable mixed conducting interphase (S-MCI) is proposed by in situ stress self-limiting reaction to achieve the compatibility of Li metal with composite sulfide electrolytes (Li6 PS5 Cl (LPSCl) and Li10 GeP2 S12 (LGPS)). The rational design of composite electrolytes utilizes the expansion stress induced by the electrolyte decomposition to in turn constrain the further decomposition of LGPS. Consequently, the S-MCI inherits the high dynamical stability of LPSCl-derived SEI and the lithiophilic affinity of Li-Ge alloy in LGPS-derived MCI. The Li||Li symmetric cells with the protection of S-MCI can operate stably for 1500 h at 0.5 mA cm-2 and 0.5 mAh cm-2 . The Li||NCM622 full cells present stable cycling for 100 cycles at 0.1 C with a high-capacity retention of 93.7%. This work sheds fresh insight into constructing electrochemically stable interphase for high-performance ASSLMBs.

Additive-Derived Surface Modification of Cathodes in All-Solid-State Batteries: The Effect of Lithium Difluorophosphate- and Lithium Difluoro(oxalato)borate-Derived Coating Layers

Sulfide-based electrolytes, with their high conductivity and formability, enable the construction of high-performance, all-solid-state batteries (ASSBs). However, the instability of the cathode-sulfide electrolyte interface limits the commercialization of these ASSBs. Surface modification of cathodes using the coating technique has been explored as an efficient approach to stabilize these interfaces. In this study, the additives lithium difluorophosphate (LiDFP) and lithium difluoro(oxalato)borate (LiDFOB) are used to fabricate stable cathode coatings via heat treatment. The low melting points of LiDFP and LiDFOB enable the formation of thin and uniform coating layers by a low-temperature heat treatment. All-solid-state cells containing LiDFP- and LiDFOB-coated cathodes show electrochemical performances significantly better than those comprising uncoated cathodes. Among all of the as-prepared coated cathodes, LiDFP-coated cathodes fabricated using a slightly lower temperature than the phase-transition temperature of LiDFP (320 °C) show the best discharge capacity, rate capability, and cyclic performance. Furthermore, cells comprising LiDFP-coated cathodes showed significantly low impedance. X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy confirm the effectiveness of the LiDFP coating. LiDFP-coated cathodes minimized side-reactions during cycling, resulting in a significantly low cathode-surface degradation. Hence, this study highlights the efficiency of the proposed coating method and its potential to facilitate the commercialization of ASSBs. Overall, this study reports an effective technique to stabilize the cathode-electrolyte interface in sulfide-based ASSBs, which could expedite the practical implementation of these advanced energy-storage devices.

Toward Ultrastable Metal Anode/Li6PS5Cl Interface via an Interlayer as Li Reservoir

All-solid-state sulfide-based Li metal batteries are promising candidates for energy storage systems. However, thorny issues associated with undesired reactions and contact failure at the anode interface hinder their commercialization. Herein, an indium foil was endowed with a formed interlayer whose surface film is enriched with LiF and LiIn phases via a feasible prelithiation route. The lithiated alloy of the interlayer can regulate Li+ flux and charge distribution as a Li reservoir, benefiting uniform Li deposition. Meanwhile, it can suppress the reductive decomposition of the Li6PS5Cl electrolyte and maintain sufficient solid-solid contact. In situ impedance spectra reveal that constant interface impedance and fast charge transfer are realized by the interlayer. Further, long-term Li stripping/plating over 2000 h at 2.55 mA cm-2 is demonstrated by this anode. All-solid-state cells employing a LiCoO2 cathode and the Pre In anode can work for over 700 cycles with a capacity retention of 96.15% at 0.5 C.

Re-sintering induced ionic conductivity recovery for air-exposed Li5.4PS4.4Cl1.6 argyrodite sulfide electrolyte

One of the most common problems with sulfide solid-state electrolytes is weak water stability. We report a re-sintering method to recover the ionic conductivity of argyrodite Li5.4PS4.4Cl1.6 solid-state electrolyte, which has been exposed to moisture for 10 h, from 1.06 to 6.97 mS cm-1.

Fluorine-Substituted Halide Solid Electrolytes with Enhanced Stability toward the Lithium Metal

The high ionic conductivity and good oxidation stability of halide-based solid electrolytes evoke strong interest in this class of materials. Nonetheless, the superior oxidative stability compared to sulfides comes at the expense of limited stability toward reduction and instability against metallic lithium anodes, which hinders their practical use. In this context, the gradual fluorination of Li2ZrCl6-xFx (0 ≤ x ≤ 1.2) is proposed to enhance the stability toward lithium-metal anodes. The mechanochemically synthesized fluorine-substituted compounds show the expected distorted local structure (M2-M3 site disorder) and significant change in the overall Li-ion migration barrier. Theoretical calculations reveal an approximate minimum energy path for Li2ZrCl6-xFx (x = 0 and 0.5) with an increase in the Li+ migration energy barrier for Li2ZrCl5.5F0.5 in comparison to Li2ZrCl6. However, it is found that the fluorine-substituted compound exhibits substantially lower polarization after 800 h of lithium stripping and plating owing to enhanced interfacial stability against the lithium metal, as revealed by density functional theory and ex situ X-ray photoelectron spectroscopy, thanks to the formation of a fluorine-rich passivating interphase.

Elucidating Ion Transport Phenomena in Sulfide/Polymer Composite Electrolytes for Practical Solid-State Batteries

Despite the enormous interest in inorganic/polymer composite solid-state electrolytes (CSEs) for solid-state batteries (SSBs), the underlying ion transport phenomena in CSEs have not yet been elucidated. Here, we address this issue by formulating a mechanistic understanding of bi-percolating ion channels formation and ion conduction across inorganic-polymer electrolyte interfaces in CSEs. A model CSE is composed of argyrodite-type Li6PS5Cl (LPSCl) and gel polymer electrolyte (GPE, including Li+-glyme complex as an ion-conducting medium). The percolation threshold of the LPSCl phase in the CSE strongly depends on the elasticity of the GPE phase. Additionally, manipulating the solvation/desolvation behavior of the Li+-glyme complex in the GPE facilitates ion conduction across the LPSCl-GPE interface. The resulting scalable CSE (area = 8 × 6 (cm × cm), thickness ~ 40 μm) can be assembled with a high-mass-loading LiNi0.7Co0.15Mn0.15O2 cathode (areal-mass-loading = 39 mg cm-2) and a graphite anode (negative (N)/positive (P) capacity ratio = 1.1) in order to fabricate an SSB full cell with bi-cell configuration. Under this constrained cell condition, the SSB full cell exhibits high volumetric energy density (480 Wh Lcell-1) and stable cyclability at 25 °C, far exceeding the values reported by previous CSE-based SSBs.

Recent progress and strategic perspectives of inorganic solid electrolytes: fundamentals, modifications, and applications in sodium metal batteries

Solid-state electrolytes (SEs) have attracted overwhelming attention as a promising alternative to traditional organic liquid electrolytes (OLEs) for high-energy-density sodium-metal batteries (SMBs), owing to their intrinsic incombustibility, wider electrochemical stability window (ESW), and better thermal stability. Among various kinds of SEs, inorganic solid-state electrolytes (ISEs) stand out because of their high ionic conductivity, excellent oxidative stability, and good mechanical strength, rendering potential utilization in safe and dendrite-free SMBs at room temperature. However, the development of Na-ion ISEs still remains challenging, that a perfect solution has yet to be achieved. Herein, we provide a comprehensive and in-depth inspection of the state-of-the-art ISEs, aiming at revealing the underlying Na⁺ conduction mechanisms at different length scales, and interpreting their compatibility with the Na metal anode from multiple aspects. A thorough material screening will include nearly all ISEs developed to date, i.e., oxides, chalcogenides, halides, antiperovskites, and borohydrides, followed by an overview of the modification strategies for enhancing their ionic conductivity and interfacial compatibility with Na metal, including synthesis, doping and interfacial engineering. By discussing the remaining challenges in ISE research, we propose rational and strategic perspectives that can serve as guidelines for future development of desirable ISEs and practical implementation of high-performance SMBs.