Tuning the Anode-Electrolyte Interface Chemistry for Garnet-Based Solid-State Li Metal Batteries

Energy Band Specific to Injected Li-Ion-Induced Formation of Lithium Dendrites in Garnet Solid Electrolytes

As one of the most significant challenges in solid-state batteries, thorough investigation is necessary on the formation process of lithium dendrites in solid-state electrolytes. Here, we reveal that the growth of lithium dendrites in solid electrolytes is a physical-electrochemical reaction process caused by injected lithium ions and electron carriers, which requires a low electrochemical potential. A unique energy band specific to injected Li ions is identified at the bottom of the conduction band, which can be occupied by electron carriers from low-potential electrodes, leading to dendrite formation. In this case, it is quantitatively determined that the employed anodes with higher working voltages (>0.2 V versus Li/Li+) can effectively prevent dendrite formation. Moreover, lithium dendrite formation exclusively occurs during the charging process (i.e., lithium plating), where lithium ions meet electrons at mixed conductive grain boundaries under highly reductive potentials. The proposed model has significant scientific significance and application value.

Small Additives Make Big Differences: A Review on Advanced Additives for High-Performance Solid-State Li Metal Batteries

Solid-state lithium (Li) metal batteries, represent a significant advancement in energy storage technology, offering higher energy densities and enhanced safety over traditional Li-ion batteries. However, solid-state electrolytes (SSEs) face critical challenges such as lower ionic conductivity, poor stability at the electrode-electrolyte interface, and dendrite formation, potentially leading to short circuits and battery failure. The introduction of additives into SSEs has emerged as a transformative approach to address these challenges. A small amount of additives, encompassing a range from inorganic and organic materials to nanostructures, effectively improve ionic conductivity, drawing it nearer to that of their liquid counterparts, and strengthen mechanical properties to prevent cracking of SSEs and maintain stable interfaces. Importantly, they also play a critical role in inhibiting the growth of dendritic Li, thereby enhancing the safety and extending the lifespan of the batteries. In this review, the wide variety of additives that have been investigated, is comprehensively explored, emphasizing how they can be effectively incorporated into SSEs. By dissecting the operational mechanisms of these additives, the review hopes to provide valuable insights that can help researchers in developing more effective SSEs, leading to the creation of more efficient and reliable solid-state Li metal batteries.

Tailoring Practical Solid Electrolyte Composites Containing Ferroelectric Ceramic Nanofibers and All-Trans Block Copolymers for All-Solid-State Lithium Metal Batteries

Ion transport efficiency, the key to determining the cycling stability and rate capability of all-solid-state lithium metal batteries (ASSLMBs), is constrained by ionic conductivity and Li+-migration ability across the multicomponent phases and interfaces in ASSLMBs. Here, we report a robust strategy for the large-scale fabrication of a practical solid electrolyte composite with high-throughput linear Li+-transport channels by compositing an all-trans block copolymer PVDF-b-PTFE matrix with ferroelectric BaTiO3-TiO2 nanofiber films. The electrolyte shows a sustainable electromechanical-coupled deformability that enables the rapid dissociation of anions with Li+ to create more movable Li+ ions and spontaneously transform the battery internal strain into Li+-ion migration kinetic energy. The ceramic framework homogenizes the interfacial potential with electrodes, endowing the electrolyte with a high conductivity of 0.782 mS·cm-1 and stable ion transport ability in ASSLMBs at room temperature. The batteries of LiFePO4/Li can stably cycle 1000 times at 0.5 C with a high capacity retention of 96.1%, and Ah-grade pouch or high-voltage Li(Ni0.8Mn0.1Co0.1)O2/Li batteries also exhibit excellent rate capability and cycling performance.

Understanding the Coupling Mechanism of Intercalation and Conversion Hybrid Storage in Lithium-Graphite Anode

Anodes with high capacity and long lifespan play an important role in the advanced batteries. However, none of the existing anodes can meet these two requirements simultaneously. Lithium (Li)-graphite composite anode presents great potential in balancing these two requirements. Herein, the working mechanism of Li-graphite composite anode is comprehensively investigated. The capacity decay features of the composite anode are different from those of Li ion intercalation in Li ion batteries and Li metal deposition in Li metal batteries. An intercalation and conversion hybrid storage mechanism are proposed by analyzing the capacity decay ratios in the composite anode with different initial specific capacities. The capacity decay models can be divided into four stages including Capacity Retention Stage, Relatively Independent Operation Stage, Intercalation & Conversion Coupling Stage, Pure Li Intercalation Stage. When the specific capacity is between 340 and 450 mAh g-1, its capacity decay ratio is between that of pure intercalation and conversion model. These results intensify the comprehensive understandings on the working principles in Li-graphite composite anode and present novel insights in the design of high-capacity and long-lifespan anode materials for the next-generation batteries.

In-Situ Construction of Electronically Insulating and Air-Stable Ionic Conductor Layer on Electrolyte Surface and Grain Boundary to Enable High-Performance Garnet-Type Solid-State Batteries

Lithophobic Li2CO3/LiOH contaminants and high-resistance lithium-deficient phases produced from the exposure of garnet electrolyte to air leads to a decrease in electrolyte ion transfer ability. Additionally, garnet electrolyte grain boundaries (GBs) with narrow bandgap and high electron conductivity are potential channels for current leakage, which accelerate Li dendrites generation, ultimately leading to short-circuiting of all-solid-state batteries (ASSBs). Herein, a stably lithiophilic Li2ZO3 is in situ constructed at garnet electrolyte surface and GBs by interfacial modification with ZrO2 and Li2CO3 (Z+C) co-sintering to eliminate the detrimental contaminants and lithium-deficient phases. The Li2ZO3 formed on the modified electrolyte (LLZTO-(Z+C)) surface effectively improves the interfacial compatibility and air stability of the electrolyte. Li2ZO3 formed at GBs broadens the energy bandgaps of LLZTO-(Z+C) and significantly inhibits lithium dendrite generation. More Li+ transport paths found in LLZTO-Z+C by first-principles calculations increase Li+ conductivity from 1.04×10-4 to 7.45×10-4 S cm-1. Eventually, the Li|LLZTO-(Z+C)|Li symmetric cell maintains stable cycling for over 2000 h at 0.8 mA cm-2. The capacity retention of LiFePO4|LLZTO-(Z+C)|Li battery retains 70.5% after 5800 ultralong cycles at 4 C. This work provides a potential solution to simultaneously enhance the air stability and modulate chemical characteristics of the garnet electrolyte surface and GBs for ASSBs.

Solid Interfaces for the Garnet Electrolytes

Solid-state electrolytes (SSEs) have attracted extensive interests due to the advantages in developing secondary batteries with high energy density and outstanding safety. Possessing high ionic conductivity and the lowest reduction potential among the state-of-the-art SSEs, the garnet type SSE is one of the most promising candidates to achieve high performance solid-state lithium batteries (SSLBs). However, the elastic modulus of the garnet electrolyte leads to deteriorated interfacial contacts, and the increasing in electronic conduction at either anode/garnet interface or grain boundary results in Li dendrite growth. Here, recent developments of the solid interfaces for the garnet electrolytes, including the strategies of Li dendrite suppression and interfacial chemical/electrochemical/mechanical stabilizations are presented. A new viewpoint of the double edges of interfacial lithiophobicity is proposed, and the rational design of the interphases, as well as effective stacking methods of the garnet-based SSLBs are summarized. Moreover, practical roles of the garnet electrolyte in SSLB industry are also discussed. This work delivers insights into the solid interfaces for the garnet electrolytes, which provides not only the promotion of the garnet-based SSLBs, but also a comprehensive understanding of the interfacial stabilization for the whole SSE family.

LiPF6 Induces Phosphorization of Garnet-Type Solid-State Electrolyte for Stable Lithium Metal Batteries

Garnet solid electrolyte Li6.4 La3 Zr1.4 Ta0.6 O12 (LLZTO) is an excellent inorganic ceramic-type solid electrolyte; however, the presence of Li2 CO3 impurities on its surface hinders Li-ion transport and increases the interface impedance. In contrast to traditional methods of mechanical polishing, acid corrosion, and high-temperature reduction for removing Li2 CO3 , herein, a straightforward "waste-to-treasure" strategy is proposed to transform Li2 CO3 into Li3 PO4 and LiF in LiPF6 solution under 60 °C. It is found that the formation of Li3 PO4 during LLZTO pretreatment facilitates rapid Li-ion transport and enhances ionic conductivity, and the LLZTO/PAN composite polymer electrolyte shows the highest Li-ion transference number of 0.63. Additionally, the dense LiF layer serves to safeguard the internal garnet solid electrolyte against solvent decomposition-induced chemical adsorption. Symmetric Li/Li cells assembled with treated LLZTO/PAN composite electrolyte exhibit a critical current density of 1.1 mA cm-2 and a long lifespan of up to 700 h at a current density of 0.2 mA cm-2 . The Li/LiFePO4 solid-state cells demonstrate stable cycling performances for 141 mAh g-1 at 0.5 C, with capacity retention of 93.6% after 190 cycles. This work presents a novel approach to converting waste into valuable resources, offering the advantages of simple processes, and minimal side reactions.

Insight into Dendrites Issue in All Solid-State Batteries with Inorganic Electrolyte: Mechanism, Detection and Suppression Strategies

All solid-state batteries (ASSBs) are regarded as one of the promising next-generation energy storage devices due to their expected high energy density and capacity. However, failures due to unrestricted growth of lithium dendrites (LDs) have been a critical problem. Moreover, the understanding of dendrite growth inside solid-state electrolytes is limited. Since the dendrite process is a multi-physical field coupled process, including electrical, chemical, and mechanical factors, no definitive conclusion can summarize the root cause of LDs growth in ASSBs till now. Herein, the existing works on mechanism, identification, and solution strategies of LD in ASSBs with inorganic electrolyte are reviewed in detail. The primary triggers are thought to originate mainly at the interface and within the electrolyte, involving mechanical imperfections, inhomogeneous ion transport, inhomogeneous electronic structure, and poor interfacial contact. Finally, some of the representative works and present an outlook are comprehensively summarized, providing a basis and guidance for further research to realize efficient ASSBs for practical applications.

Li, Na, K, Mg, Zn, Al, and Ca Anode Interface Chemistries Developed by Solid-State Electrolytes

Solid-state batteries (SSBs) have received significant attention due to their high energy density, reversible cycle life, and safe operations relative to commercial Li-ion batteries using flammable liquid electrolytes. This review presents the fundamentals, structures, thermodynamics, chemistries, and electrochemical kinetics of desirable solid electrolyte interphase (SEI) required to meet the practical requirements of reversible anodes. Theoretical and experimental insights for metal nucleation, deposition, and stripping for the reversible cycling of metal anodes are provided. Ion transport mechanisms and state-of-the-art solid-state electrolytes (SEs) are discussed for realizing high-performance cells. The interface challenges and strategies are also concerned with the integration of SEs, anodes, and cathodes for large-scale SSBs in terms of physical/chemical contacts, space-charge layer, interdiffusion, lattice-mismatch, dendritic growth, chemical reactivity of SEI, current collectors, and thermal instability. The recent innovations for anode interface chemistries developed by SEs are highlighted with monovalent (lithium (Li+ ), sodium (Na+ ), potassium (K+ )) and multivalent (magnesium (Mg2+ ), zinc (Zn2+ ), aluminum (Al3+ ), calcium (Ca2+ )) cation carriers (i.e., lithium-metal, lithium-sulfur, sodium-metal, potassium-ion, magnesium-ion, zinc-metal, aluminum-ion, and calcium-ion batteries) compared to those of liquid counterparts.

Reference Electrode Reveals Insights on Sodium Metal/Solid Electrolyte Interface Cycling and Voiding Behaviors at High Current Densities and Areal Capacities

Plating and stripping processes at solid metal electrode/solid electrolyte interfaces are of great significance for high-energy, solid-state batteries. Here, we introduce a Na metal reference electrode to a symmetric Na metal/sodium β″ alumina/Na metal cell and study both cycling and unidirectional protocols with a focus on high current density and areal capacity. For example, in a current ramp test at 5 mAh cm-2 we find a shift from stable to unstable interfacial polarization during stripping at ≳3 mA cm-2, and at 7.5 mA cm-2 we measure 100s of mV of voltage magnitude rise at the stripping electrode and 10s of mV of voltage changes at the plating electrode. In unidirectional testing (i.e., passing current in a single direction until cell failure), at 1.2 mA cm-2 we find only ∼40% of the initial Na foil could be transferred through the solid electrolyte and again observe 100s of mV (and larger) voltage magnitude rise at the stripping electrode and 10s of mV of voltage change at the plating electrode. This test also shows that the 100s of mV of interfacial polarization can be sustained for hours (at 1.2 mA cm-2) to tens of hours (in a test at 0.3 mA cm-2). Hence, across several test protocols we find a Na metal reference electrode provides quantitative insights on electrochemical interfacial behavior that are not revealed in two-electrode testing. We also built a two-dimensional model of our three-electrode symmetric cell to quantify the link between the measured interfacial potentials in our testing and changes in electrochemically active interfacial contact and find that 100s of mV of interfacial potential rise indicates loss of electrochemically active contact area of >80%. Our work provides a promising approach to clarify the coupled interfacial electrochemical and contact mechanics processes at solid metal electrode/solid electrolyte interfaces.

Liquid-Like Li-Ion Conduction in Oxides Enabling Anomalously Stable Charge Transport across the Li/Electrolyte Interface in All-Solid-State Batteries

The softness of sulfur sublattice and rotational PS4 tetrahedra in thiophosphates result in liquid-like ionic conduction, leading to enhanced ionic conductivities and stable electrode/thiophosphate interfacial ionic transport. However, the existence of liquid-like ionic conduction in rigid oxides remains unclear, and modifications are deemed necessary to achieve stable Li/oxide solid electrolyte interfacial charge transport. In this study, by combining the neutron diffraction survey, geometrical analysis, bond valence site energy analysis, and ab initio molecular dynamics simulation, 1D liquid-like Li-ion conduction is discovered in LiTa2 PO8 and its derivatives, wherein Li-ion migration channels are connected by four- or five-fold oxygen-coordinated interstitial sites. This conduction features a low activation energy (0.2 eV) and short mean residence time (<1 ps) of Li ions on the interstitial sites, originating from the Li-O polyhedral distortion and Li-ion correlation, which are controlled by doping strategies. The liquid-like conduction enables a high ionic conductivity (1.2 mS cm-1 at 30 °C), and a 700 h anomalously stable cycling under 0.2 mA cm-2 for Li/LiTa2 PO8 /Li cells without interfacial modifications. These findings provide principles for the future discovery and design of improved solid electrolytes that do not require modifications to the Li/solid electrolyte interface to achieve stable ionic transport.

Tuning the Covalent Coupling Degree between the Cathode and Electrolyte for Optimized Interfacial Resistance in Solid-State Lithium Batteries

The development of promising solid-state lithium batteries has been a challenging task mainly due to the poor interfacial contact and high interfacial resistance at the electrode/solid-state electrolyte (SSE) interface. Herein, we propose a strategy for introducing a class of covalent interactions with varying covalent coupling degrees at the cathode/SSE interface. This method significantly reduces interfacial impedances by strengthening the interactions between the cathode and SSE. By adjusting the covalent coupling degree from low to high, an optimal interfacial impedance of 33 Ω cm^-2 was achieved, which is even lower than the interfacial impedance using liquid electrolytes (39 Ω cm^-2). This work offers a fresh perspective on solving the interfacial contact problem in solid-state lithium batteries.

The Vital Role of Electrolyte Reduction Potential in Forming a Stable SEI in Phosphorus-Based Anodes

In view of their high lithium storage capability, phosphorus-based anodes are promising for lithium-ion batteries. However, the low reduction potential (0.74 V versus Li⁺ /Li) of the commonly used ethylene carbonate-based electrolyte does not allow the early formation of a solid electrolyte interphase (SEI) prior to the initial phosphorus alloying reaction (1.5 V versus Li⁺ /Li). In the absence of a protective SEI, the phosphorus anode develops cracks, creating additional P/electrolyte interfaces. This results in the loss of P and the formation of a discontinuous SEI, all of which greatly reduce the electrochemical performance of the anode. Here, the effect of solvent reduction potential on the structure of the SEI is investigated. It is found that solvents with a high reduction potential, such as fluoroethylene carbonate, decompose to form an SEI concomitantly with the P alloying reaction. This results in a continuous, mechanically robust, and Li₃ PO₄ -rich SEI with improved Li-ion conductivity. These attributes significantly improve the cyclic stability and rate performance of the phosphorus-based anode.

Phase-Changeable Dynamic Conformal Electrode/electrolyte Interlayer enabling Pressure-Independent Solid-State Lithium Metal Batteries

Lithium-metal-based solid-state batteries (Li-SSBs) are one of the most promising energy storage devices due to their high energy densities. However, under insufficient pressure constraints (<MPa-level), Li-SSBs usually exhibit poor electrochemical performances owing to the continuous interfacial degradation between the solid-state electrolyte (SSE) and electrodes. Herein, a phase-changeable interlayer is developed to construct the self-adhesive and dynamic conformal electrode/SSE contact in Li-SSBs. The strong adhesive and cohesive strengths of the phase-changeable interlayer enable Li-SSBs to resist up to 250 N pulling force (=1.9 MPa), affording Li-SSBs ideal interfacial integrality even without extra stack pressure. Remarkably, this interlayer exhibits a high ionic conductivity of 1.3 × 10^-3 S cm^-1 , attributed to the shortened steric solvation hindrance and optimized Li⁺ coordination structure. Furthermore, the changeable phase property of the interlayer endows Li-SSBs with a healable Li/SSE interface, accommodating the stress-strain evolution of the lithium metal and constructing the dynamic conformal interface. Consequently, the contact impedance of the modified solid symmetric cell exhibits a pressure-independent manner and does not increase over 700 h (0.2 MPa). The LiFePO₄ pouch cell with the phase-changeable interlayer shows 85% capacity retention after 400 cycles at a low pressure of 0.1 MPa.

Building K-C Anode with Ultrahigh Self-Diffusion Coefficient for Solid State Potassium Metal Batteries Operating at -20 to 120 °C

Solid state potassium (K) metal batteries are intriguing in grid-scale energy storage, benefiting from the low cost, safety, and high energy density. However, their practical applications are impeded by poor K/solid electrolyte (SE) interfacial contact and limited capacity caused by the low K self-diffusion coefficient, dendrite growth, and intrinsically low melting point/soft features of metallic K. Herein, a fused-modeling strategy using potassiophilic carbon allotropes molted with K is demonstrated that can enhance the electrochemical performance/stability of the system via promoting K diffusion kinetics (2.37 × 10^-8 cm² s^-1 ), creating a low interfacial resistance (≈1.3 Ω cm² ), suppressing dendrite growth, and maintaining mechanical/thermal stability at 200 °C. A homogeneous/stable K stripping/plating is consequently implemented with a high current density of 2.8 mA cm^-2 (at 25 °C) and a record-high areal capacity of 11.86 mAh cm^-2 (at 0.2 mA cm^-2 ). The enhanced K diffusion kinetics contribute to sustaining intimate interfacial contact, stabilizing the stripping/plating at high current densities. Full cells coupling ultrathin K-C composite anodes (≈50 µm) with Prussian blue cathodes and β/β″-Al₂ O₃ SEs deliver a high energy density of 389 Wh kg^-1 with a retention of 94.4% after 150 cycles and fantastic performances at -20 to 120 °C.

Constructing a Superlithiophilic 3D Burr-Microsphere Interface on Garnet for High-Rate and Ultra-Stable Solid-State Li Batteries

Garnet-type solid-state electrolyte (SSE) Li_6.5 La₃ Zr_1.5 Ta_0.5 O₁₂ attracts great interest due to its high ion conductivity and wide electrochemical window. But the huge interfacial resistance, Li dendrite growth, and low critical current density (CCD) block the practical applications. Herein, a superlithiophilic 3D burr-microsphere (BM) interface layer composed of ionic conductor LiF-LaF₃ is constructed in situ to achieve a high-rate and ultra-stable solid-state lithium metal battery. The 3D-BM interface layer with a large specific surface area shows a superlithiophilicity and its contact angle with molten Li is only 7° enabling the facile infiltration of molten Li. The assembled symmetrical cell reaches one of the highest CCD (2.7 mA cm^-2 ) at room temperature, an ultra-low interface impedance of 3 Ω cm² , and a super-long cycling stability of 12 000 h at 0.1-1.5 mA cm^-2 without Li dendrite growth. The solid-state full cells with 3D-BM interface show outstanding cycling stability (LiFePO₄ : 85.4%@900 cycles@1 C; LiNi_0.8 Co_0.1 Mn_0.1 O₂ :89%@200 cycles@0.5 C) and a high rate capacity (LiFePO₄ :135.5mAh g^-1 at 2 C). Moreover, the designed 3D-BM interface is quite stable after 90 days of storage in the air. This study offers a facile strategy to address the critical interface issues and accelerate the practical application of garnet-type SSE in high performance solid-state lithium metal batteries.

Engineering Li Metal Anode for Garnet-Based Solid-State Batteries

ConspectusThe past 30 years have witnessed the great achievements of Li-ion batteries (LIBs) based on a graphite anode and liquid organic electrolytes. Yet the limited energy density of a graphite anode and unavoidable safety risks caused by flammable liquid organic electrolytes hinder further developments of LIBs. To reach higher energy density, Li metal anodes (LMAs) with high capacity and low electrode potential are a promising choice. However, LMAs suffer from more serious safety concerns than the graphite anode in liquid LIBs. The dilemma of safety and energy density remains an inevitable obstacle in the way of LIBs.Solid-state batteries (SSBs) offer new opportunities to simultaneously achieve intrinsic safety and high energy density. Among all types of SSBs that are based on oxides, polymers, sulfides, or halides, garnet-type SSBs seem to be one of the most attractive choices due to garnet's merits in high ionic conductivities (10^-4-10^-3 S/cm at room temperature), wide electrochemical windows (0-6 V), and intrinsically high safety. However, garnet-type SSBs are faced with large interfacial impedance and short-circuit problems caused by Li dendrites. Recently, engineered Li metal anodes (ELMAs) have shown unique advantages in tackling interface issues and attracted extensive research interest.In this Account, we focus on fundamental understandings and provide an in-depth review of ELMAs in garnet-based SSBs. Considering the limited space, we mainly discuss the recent progress made in our groups. First, we introduce the design guidelines for ELMAs and emphasize the unique role of theoretical calculation in predicting and optimizing ELMAs. Then we discuss the interface compatibility of ELMAs with garnet SSEs in details. Specifically, we have demonstrated the advantages of ELMAs in enhancing interface contact and suppressing Li dendrite growth. Next, we attentively analyze the gaps between laboratory and practical applications. We strongly recommend establishing a unified testing standard, with a practically desired areal capacity per cycle (>3.0 mAh/cm²) and a precisely controlled Li capacity excess. Finally, novel chances to enhance ELMAs' processability and fabricate thin Li foils are highlighted. We believe that this Account will offer an insightful analysis of ELMAs' recent advancements and push forward their practical applications.

Advanced Nonflammable Organic Electrolyte Promises Safer Li-Metal Batteries: From Solvation Structure Perspectives

Batteries with a Li-metal anode have recently attracted extensive attention from the battery communities owing to their high energy density. However, severe dendrite growth hinders their practical applications. More seriously, when Li dendrites pierce the separators and trigger short circuit in a highly flammable organic electrolyte, the results would be catastrophic. Although the issues of growth of Li dendrites have been almost addressed by various methods, the highly flammable nature of conventional organic liquid electrolytes is still a lingering fear facing high-energy-density Li-metal batteries given the possibility of thermal runaway of the high-voltage cathode. Recently, various kinds of nonflammable liquid- or solid-state electrolytes have shown great potential toward safer Li-metal batteries with minimal detrimental effect on the battery performance or even enhanced electrochemical performance. In this review, recent advances in developing nonflammable electrolyte for high-energy-density Li-metal batteries including high-concentration electrolyte, localized high-concentration electrolyte, fluorinated electrolyte, ionic liquid electrolyte, and polymer electrolyte are summarized. Then, the solvation structure of different kinds of nonflammable liquid and polymer electrolytes are analyzed to provide insight into the mechanism for dendrite suppression and fire extinguishing. Finally, guidelines for future design of nonflammable electrolyte for safer Li-metal batteries are provided.

Bilayer Dense-Porous Li7 La3 Zr2 O12 Membranes for High-Performance Li-Garnet Solid-State Batteries

Li dendrites form in Li₇ La₃ Zr₂ O₁₂ (LLZO) solid electrolytes due to intrinsic volume changes of Li and the appearance of voids at the Li metal/LLZO interface. Bilayer dense-porous LLZO membranes make for a compelling solution of this pertinent challenge in the field of Li-garnet solid-state batteries (SSB). Lithium is thus stored in the pores of the LLZO, thereby avoiding i) dynamic changes of the anode volume and ii) the formation of voids during Li stripping due to increased surface area of the Li/LLZO interface. The dense layer then additionally reduces the probability of short circuits during cell charging. In this work, a method for producing such bilayer membranes utilizing sequential tape-casting of porous and dense layers is reported. The minimum attainable thicknesses are 8-10 µm for dense and 32-35 µm for porous layers, enabling gravimetric and volumetric energy densities of Li-garnet SSBs of 279 Wh kg^-1 and 1003 Wh L^-1 , respectively. Bilayer LLZO membranes in symmetrical cell configuration exhibit high critical current density up to 6 mA cm^-2 and cycling stability of over 160 cycles at a current density of 0.5 mA cm^-2 at an areal capacity limitation of 0.25 mAh cm^-2 .

H3PO4-Induced Nano-Li3PO4 Pre-reduction Layer to Address Instability between the Nb-Doped Li7La3Zr2O12 Electrolyte and Metallic Li Anode

Solid-state batteries based on a metallic Li anode and nonflammable solid electrolytes (SEs) are anticipated to achieve high energy and power densities with absolute safety. In particular, cubic garnet-type Nb-doped Li₇La₃Zr₂O₁₂ (Nb-LLZO) SEs possess superior ionic conductivity, are feasible to prepare under ambient conditions, have strong thermal stability, and are of low cost. However, the interfacial compatibility with Li metal and Li dendrite hazards still hinder the applications of Nb-LLZO. Herein, a quick and efficient solution was applied to address this issue, generating a nano-Li₃PO₄ pre-reduction layer from the reaction of H₃PO₄ with the ion-exchanged passivation layer (Li₂CO₃/LiOH) on the surface of Nb-LLZO. A lithiophilic, electrically insulating interlayer is in situ created when the Li₃PO₄ modified layer interacts with molten Li, successfully preventing the reduction of Nb⁵⁺. The interlayer, which mostly consists of Li₃P and Li₃PO₄, also has a high shear modulus and relatively high Li⁺ conductivity, which effectively inhibit the growth of Li dendrites. The Li|Li₃PO₄|Nb-LLZO|Li₃PO₄|Li symmetric cells stably cycled for over 5000 h at 0.05 mA cm^-2 and over 1000 h at a high rate of 0.15 mA cm^-2 without any short circuits. The LiFePO₄ and S/C hybrid solid-state batteries using the modified Nb-LLZO electrolyte also demonstrated good electrochemical performances, confirming the practical application of this interfacial engineering in various solid-state battery systems. This work offers an efficient solution to the instability issue between the Nb-LLZO SE and metallic Li anode.

All-Solid-State Garnet-Based Lithium Batteries at Work-In Operando TEM Investigations of Delithiation/Lithiation Process and Capacity Degradation Mechanism

Li₇ La₃ Zr₂ O₁₂ (LLZO)-based all-solid-state Li batteries (SSLBs) are very attractive next-generation energy storage devices owing to their potential for achieving enhanced safety and improved energy density. However, the rigid nature of the ceramics challenges the SSLB fabrication and the afterward interfacial stability during electrochemical cycling. Here, a promising LLZO-based SSLB with a high areal capacity and stable cycle performance over 100 cycles is demonstrated. In operando transmission electron microscopy (TEM) is used for successfully demonstrating and investigating the delithiation/lithiation process and understanding the capacity degradation mechanism of the SSLB on an atomic scale. Other than the interfacial delamination between LLZO and LiCoO₂ (LCO) owing to the stress evolvement during electrochemical cycling, oxygen deficiency of LCO not only causes microcrack formation in LCO but also partially decomposes LCO into metallic Co and is suggested to contribute to the capacity degradation based on the atomic-scale insights. When discharging the SSLB to a voltage of ≈1.2 versus Li/Li⁺ , severe capacity fading from the irreversible decomposition of LCO into metallic Co and Li₂ O is observed under in operando TEM. These observations reveal the capacity degradation mechanisms of the LLZO-based SSLB, which provides important information for future LLZO-based SSLB developments.

Opportunities of Flexible and Portable Electrochemical Devices for Energy Storage: Expanding the Spotlight onto Semi-solid/Solid Electrolytes

The ever-increasing demand for flexible and portable electronics has stimulated research and development in building advanced electrochemical energy devices which are lightweight, ultrathin, small in size, bendable, foldable, knittable, wearable, and/or stretchable. In such flexible and portable devices, semi-solid/solid electrolytes besides anodes and cathodes are the necessary components determining the energy/power performances. By serving as the ion transport channels, such semi-solid/solid electrolytes may be beneficial to resolving the issues of leakage, electrode corrosion, and metal electrode dendrite growth. In this paper, the fundamentals of semi-solid/solid electrolytes (e.g., chemical composition, ionic conductivity, electrochemical window, mechanical strength, thermal stability, and other attractive features), the electrode-electrolyte interfacial properties, and their relationships with the performance of various energy devices (e.g., supercapacitors, secondary ion batteries, metal-sulfur batteries, and metal-air batteries) are comprehensively reviewed in terms of materials synthesis and/or characterization, functional mechanisms, and device assembling for performance validation. The most recent advancements in improving the performance of electrochemical energy devices are summarized with focuses on analyzing the existing technical challenges (e.g., solid electrolyte interphase formation, metal electrode dendrite growth, polysulfide shuttle issue, electrolyte instability in half-open battery structure) and the strategies for overcoming these challenges through modification of semi-solid/solid electrolyte materials. Several possible directions for future research and development are proposed for going beyond existing technological bottlenecks and achieving desirable flexible and portable electrochemical energy devices to fulfill their practical applications. It is expected that this review may provide the readers with a comprehensive cross-technology understanding of the semi-solid/solid electrolytes for facilitating their current and future researches on the flexible and portable electrochemical energy devices.

Surficial Oxidation of Phosphorus for Strengthening Interface Interaction and Enhancing Lithium-Storage Performance

By virtue of high theoretical capacity and appropriate lithiation potential, phosphorus is considered as a prospective next-generation anode material for lithium-ion batteries. However, there are some problems hampering its practical application, such as low ionic conductivity and serious volume expansion. Herein, we demonstrated an in situ preoxidation strategy to build a oxidation function layer at phosphorus particle. The oxide layer not only acted as a protective layer to prolong the storage time of phosphorus anode in air but also carbonized N-methyl pyrrolidone and poly (vinylidene fluoride), strengthening the interfacial interaction between phosphorus particles and binder. The oxide layer further induced the formation of a stable solid electrolyte interface with high lithium-ion conductivity. The oxidized P-CNT maintained high specific capacity of 1306 mAh g^-1 and 89% capacity after 100 cycles, much higher than that of pristine P-CNT (17.1%). The strategy of in situ oxidation is facile and conducive to the practical application of phosphorus-based anodes.

Li2Se: A High Ionic Conductivity Interface to Inhibit the Growth of Lithium Dendrites in Garnet Solid Electrolytes

All-solid-state Li metal batteries (ASSLBs) are currently regarded as one of the most promising next-generation energy storage technologies because of their great potential in realizing both high energy density and safety. However, the development of high performance ASSLBs is still restricted by the large interfacial resistance and Li dendrite propagation within solid electrolytes. Herein, a simple and efficient interfacial modification strategy is proposed to improve the interfacial contact between Li and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) by introducing a uniform and thin Li₂Se buffer layer. The Li₂Se buffer layer formed by an in situ conversion reaction can not only enhance the wettability of lithium metal toward LLZTO electrolyte but also facilitate uniform lithium plating/stripping. As a result, the interfacial resistance of Li/LLZTO decreased from 270.5 to 5.1 Ω cm², and the lithium symmetric cell can cycle stably for 350 h at a current density of 0.5 mA cm^-2. Meanwhile, the Li|LLZTO-Li₂Se|LiNi_0.8Co_0.1Mn_0.1O₂ full cells exhibit a high initial capacity of 162.3 mAh g^-1 and good cycling stability with a capacity retention of 84.3% after 100 cycles at 0.2 C. These results prove the effectiveness of this modification method and provide new design strategies for the development of high performance ASSLBs.

Uniform Na Metal Plating/Stripping Design for Highly Reversible Solid-State Na Metal Batteries at Room Temperature

Solid-state alkaline metal batteries are highly sought out for their improved energy density and security over the current lithium-ion batteries. However, their practical application is heavily hindered by the interfacial issues originating from the solid electrolyte/electrode mismatch. This work demonstrates that a CuO coating layer as an active interphase can thoroughly promote the intimate contact between a Na₃ Zr₂ Si₂ PO₁₂ solid electrolyte and a Na metal anode through an in situ conversion reaction. The resultant Cu/Na₂ O matrix forms a mixed electron/ion conducting scaffold, which facilitates stable and homogeneous Na metal plating without dendrite formation. Moreover, the symmetric Na metal cell realizes impressively steady plating/stripping cycles for 5000 h even under a high current density of 0.3 mA cm^-2 . The novelty is further manifested as a room-temperature solid-state Na metal full battery of Na₃ V_1.5 Al_0.5 (PO₄ )₃ |CuO@NZSPO|Na is assembled and exhibits a highly reversible cyclability (99.85% coulombic efficiency and 99.0% capacity retention) under a charge/discharge rate of 5 C for 2250 cycles. This work effectively solves the interfacial issues at the Na metal/solid electrolyte interface and provides a convenient way toward high-performance solid-state Na metal batteries operated at room temperature.

Stabilization of garnet/Li interphase by diluting the electronic conductor

The high interfacial resistance and lithium (Li) dendrite growth are two major challenges for solid-state Li batteries (SSLBs). The lack of understanding on the correlations between electronic conductivity and Li dendrite formation limits the success of SSLBs. Here, by diluting the electronic conductor from the interphase to bulk Li during annealing of the aluminium nitride (AlN) interlayer, we changed the interphase from mixed ionic/electronic conductive to solely ionic conductive, and from lithiophilic to lithiophobic to fundamentally understand the correlation among electronic conductivity, Li dendrite, and interfacial resistance. During the conversion-alloy reaction between AlN and Li, the lithiophilic and electronic conductive Li_xAl diffused into Li, forming a compact lithiophobic and ionic conductive Li₃N, which achieved an ultrahigh critical current density of 2.6/14.0 mA/cm² in the time/capacity-constant mode, respectively. The fundamental understanding on the effect of interphase nature on interfacial resistance and Li dendrite suppression will provide guidelines for designing high-performance SSLBs.

Garnet Li7La3Zr2O12-Based Solid-State Lithium Batteries Achieved by In Situ Thermally Polymerized Gel Polymer Electrolyte

Garnet Li₇La₃Zr₂O₁₂ (LLZO) is a potential solid electrolyte for solid-state batteries (SSBs) because of its high ionic conductivity, electrochemical stability, and mechanical strength. However, large interface resistances arising from deserted cathodes and rigid garnet/electrode interfaces block its application. In order to deal with this issue, a gel polymer electrolyte (GPE) was introduced into the cathode and both sides of LLZO to achieve a solid-state battery. Especially, the provided GPE could be thermally polymerized and solidified in situ, which would integrate LLZO with both anode and cathode and dramatically simplify the battery manufacturing process. Since the interface from rigid LLZO is improved by the flexible GPE buffer, the inability of flexible GPE to inhibit lithium dendrites is compensated by the rigid LLZO in return. As a result, the interface resistances are reduced from 6880 to 473 Ω, the Li symmetric cell exhibits a flat galvanostatic charge/discharge for 400 h without lithium dendrites, and the solid-state Li|GPE@LLZO|LiCoO₂ battery exerts a capacity retention of 82.6% after 100 cycles at 0.5 C at room temperature. Such an interfacial engineering approach represents a promising strategy to address solid-solid interface issues and provides a new design for SSBs with high performance.

Grain Boundary Engineering in Ta-Doped Garnet-Type Electrolyte for Lithium Dendrite Suppression

Solid-state lithium batteries (SSLBs) based on Ta-doped Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO) suffer from lithium dendrite growth, which hinders their practical application. Herein, first principles simulations indicate that the Ta element prefers to segregate along grain boundaries in the form of Ta₂O₅ precipitates due to a high energy difference induced by Ta doping. Grain boundary engineering is employed to regulate the distribution of the Ta element and enhance the density of LLZTO by introducing the La₂O₃ additive. The sufficient La₂O₃ additive reacts with the Ta₂O₅ precipitates, while the residual La₂O₃ nanoparticles fill up void defects, promoting the homogeneous distribution of the Ta element and improving the relative density to ∼98%. Critical current density of the symmetric Li battery reaches 2.12 mA·cm^-2 at room temperature with the solid-state electrolyte (LLZTO + 5 wt % La₂O₃), which increases by 41% compared to pure LLZTO. SSLBs with the LiFePO₄ cathode achieve a stable cycling performance with a discharge capacity of 138.6 mA·h·g^-1 after 400 cycles at 0.2 C. This work provides theoretical insights into the distribution of Ta-doped LLZTO and inhibits lithium dendrite growth through grain boundary engineering.

High-Performance Composite Lithium Anodes Enabled by Electronic/Ionic Dual-Conductive Paths for Solid-State Li Metal Batteries

Solid-state lithium metal batteries (SSLMBs) promise high energy density and high safety by employing high-capacity Li metal anode and solid-state electrolytes. However, the construction of the composite Li metal electrode is a neglected but important subject when the extensive research focuses on the interface between the solid electrolyte Li_6.4 La₃ Zr_1.4 Ta_0.6 O₁₂ and Li metal anode. Here, an electronic-ionic conducting composite Li metal anode consisting of Li-Al alloy and LiF is constructed to achieve the stable electronic-ionic transport channel and the intimate interface contact, which can realize the uniform Li deposition and the efficiency utilization of lithium in composite Li metal electrode. Therefore, the symmetric battery with composite Li metal electrode exhibits the high critical current density with 1.2 mA cm^-2 and stable cycle for 1500 h at 0.3 mA cm^-2 , 25 °C. Moreover, the SSLMBs matched with LiFePO₄ and LiNi_0.8 Co_0.1 Mn_0.1 O₂ achieve the outstanding electrochemical performance, verifying the feasibility of composite Li metal electrode in various SSLMBs systems.

Design of a lithiophilic and electron-blocking interlayer for dendrite-free lithium-metal solid-state batteries

All-solid-state batteries are a potential game changer in the energy storage market; however, their practical employment has been hampered by premature short circuits caused by the lithium dendritic growth through the solid electrolyte. Here, we demonstrate that a rational layer-by-layer strategy using a lithiophilic and electron-blocking multilayer can substantially enhance the performance/stability of the system by effectively blocking the electron leakage and maintaining low electronic conductivity even at high temperature (60°C) or under high electric field (3 V) while sustaining low interfacial resistance (13.4 ohm cm²). It subsequently results in a homogeneous lithium plating/stripping, thereby aiding in achieving one of the highest critical current densities (~3.1 mA cm^-2) at 60°C in a symmetric cell. A full cell paired with a commercial-level cathode exhibits exceptionally long durability (>3000 cycles) and coulombic efficiency (99.96%) at a high current density (2 C; ~1.0 mA cm^-2), which records the highest performance among all-solid-state lithium metal batteries reported to date.

Facile Construction of Nanofilms from a Dip-Coating Process to Enable High-Performance Solid-State Batteries

The use of solid-state electrolytes (SSEs) instead of those liquid ones has found promising potential to achieve both high energy density and high safety for their applications in the next-generation energy storage devices. Unfortunately, SSEs also bring forth challenges related to solid-to-solid contact, making the stability of the electrode/electrolyte interface a formidable concern. Herein, using a garnet-type Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT) electrolyte as an example, we demonstrated a facile treatment based on the dip-coating technique, which is highly efficient in modifying the LLZT/Li interface by forming a MgO interlayer. Using polyvinyl pyrrolidone (PVP) as a coordination polymer, uniform and crack-free nanofilms are fabricated on the LLZT pellet with good control of the morphological parameters. We found that the MgO interlayer was highly effective to reduce the interfacial resistance to 6 Ω cm² as compared to 1652 Ω cm² of the unmodified interface. The assembled Li symmetrical cell was able to achieve a high critical current density of 1.2 mA cm^-2 at room temperature, and it has a long cycling capability for over 4000 h. Using the commercialized materials of LiFePO₄ and LiNi_0.83Co_0.07Mn_0.1O₂ as the cathode materials, the full cells based on the LLZT@MgO electrolyte showed excellent cyclability and high rate performance at 25 °C. Our study shows the feasibility of precise and controllable surface modification based on a simple liquid phase method and highlights the essential importance of interface control for the future application of high-performance solid-state batteries.

Hierarchical Sulfide-Rich Modification Layer on SiO/C Anode for Low-Temperature Li-Ion Batteries

The silicon oxide/graphite (SiO/C) composite anode represents one of the promising candidates for next generation Li-ion batteries over 400 Wh kg^-1 . However, the rapid capacity decay and potential safety risks at low temperature restrict their widely practical applications. Herein, the fabrication of sulfide-rich solid electrolyte interface (SEI) layer on surface of SiO/C anode to boost the reversible Li-storage performance at low temperature is reported. Different from the traditional SEI layer, the present modification layer is composed of inorganic-organic hybrid components with three continuous layers as disclosed by time-of-flight secondary ion mass spectrometry (TOF-SIMS). The result shows that ROSO₂ Li, ROCO₂ Li, and LiF uniformly distribute over different layers. When coupled with LiNi_0.8 Co_0.1 Mn_0.1 O₂ cathode, the capacity retention achieves 73% at -20 °C. The first principle calculations demonstrate that the gradient adsorption of sulfide-rich surface layer and traditional intermediate layer can promote the desolvation of Li⁺ at low temperature. Meanwhile, the inner LiF-rich layer with rapid ionic diffusion capability can inhibit dendrite growth. These results offer new perspective of developing advanced SiO/C anode and low-temperature Li-ion batteries.

Plastic Monolithic Mixed-Conducting Interlayer for Dendrite-Free Solid-State Batteries

Solid-state electrolytes (SSEs) hold a critical role in enabling high-energy-density and safe rechargeable batteries with Li metal anode. Unfortunately, nonuniform lithium deposition and dendrite penetration due to poor interfacial solid-solid contact are hindering their practical applications. Here, solid-state lithium naphthalenide (Li-Naph(s)) is introduced as a plastic monolithic mixed-conducting interlayer (PMMCI) between the garnet electrolyte and the Li anode via a facile cold process. The thin PMMCI shows a well-ordered layered crystalline structure with excellent mixed-conducting capability for both Li⁺ (4.38 × 10^-3  S cm^-1 ) and delocalized electrons (1.01 × 10^-3  S cm^-1 ). In contrast to previous composite interlayers, this monolithic material enables an intrinsically homogenous electric field and Li⁺ transport at the Li/garnet interface, thus significantly reducing the interfacial resistance and achieving uniform and dendrite-free Li anode plating/stripping. As a result, Li symmetric cells with the PMMCI-modified garnet electrolyte show highly stable cycling for 1200 h at 0.2 mA cm^-2 and 500 h at a high current density of 1 mA cm^-2 . The findings provide a new interface design strategy for solid-state batteries using monolithic mixed-conducting interlayers.

ZIF-8@MXene-reinforced flame-retardant and highly conductive polymer composite electrolyte for dendrite-free lithium metal batteries

Though polymer electrolytes have been regarded as promising separators for solid-state lithium metal batteries, their low ionic conductivity, poor thermostability and inflammability limit their practical applications. Herein, a polymer composite electrolyte consisting of metal-organic frameworks modified Ti₃C₂-MXene nanosheets (ZIF-8@MXene) and polymer mixture (PE-ZIF-8@MXene) was fabricated. The fabricated nonflammable ZIF-8@MXene nanosheets have abundant functional groups and Lewis acid sites as well as high specific surface area. In the composite electrolyte, ZIF-8@MXene nanosheets increased the dissociation of lithium salts and provided channels for transporting ions, accelerating the Li ion transportation. They also enhanced the tensile strength, thermostability and flame resistance of PE-ZIF-8@MXene. Consequently, the fabricated flame-retardant PE-ZIF-8@MXene presented high ionic conductivity (4.4 mS cm^-1), impressive Li⁺ transference number (0.76) and enhanced tensile strength (3.77 MPa). In addition, the assembled Li|PE-ZIF-8@MXene|Li had a long cycle life of 2000 h, and Li|PE-ZIF-8@MXene|LiFePO₄ batteries displayed a capacity retention of 89.6% after 500 cycles.

Continuous Carbon Channels Enable Full Na-Ion Accessibility for Superior Room-Temperature Na-S Batteries

Porous carbon has been widely used as an efficient host to encapsulate highly active molecular sulfur (S) in Li-S and Na-S batteries. However, for these sub-nanosized pores, it is a challenge to provide fully accessible sodium ions with unobstructed channels during cycling, particularly for high sulfur content. It is well recognized that solid interphase with full coverage over the designed architectures plays critical roles in promoting rapid charge transfer and stable conversion reactions in batteries, whereas constructing a high-ionic-conductivity solid interphase in the pores is very difficult. Herein, unique continuous carbonaceous pores are tailored, which can serve as multifunctional channels to encapsulate highly active S and provide fully accessible pathways for sodium ions. Solid sodium sulfide interphase layers are also realized in the channels, showing high Na-ion conductivity toward stabilizing the redox kinetics of the S cathode during charge/discharge processes. This systematically designed carbon-hosted sulfur cathode delivers superior cycling performance (420 mAh g^-1 at 2 A g^-1 after 2000 cycles), high capacity retention of ≈90% over 500 cycles at current density of 0.5 A g^-1 , and outstanding rate capability (470 mAh g^-1 at 5 A g^-1 ) for room-temperature sodium-sulfur batteries.

Coordination-Assisted Precise Construction of Metal Oxide Nanofilms for High-Performance Solid-State Batteries

The application of solid-state batteries (SSBs) is challenged by the inherently poor interfacial contact between the solid-state electrolyte (SSE) and the electrodes, typically a metallic lithium anode. Building artificial intermediate nanofilms is effective in tackling this roadblock, but their implementation largely relies on vapor-based techniques such as atomic layer deposition, which are expensive, energy-intensive, and time-consuming due to the monolayer deposited per cycle. Herein, an easy and low-cost wet-chemistry fabrication process is used to engineer the anode/solid electrolyte interface in SSBs with nanoscale precision. This coordination-assisted deposition is initiated with polyacrylate acid as a functional polymer to control the surface reaction, which modulates the distribution and decomposition of metal precursors to reliably form a uniform crack-free and flexible nanofilm of a large variety of metal oxides. For demonstration, artificial Al₂O₃ interfacial nanofilms were deposited on a ceramic SSE, typically garnet-structured Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZT), that led to a significant decrease in the Li/LLZT interfacial resistance (from 2079.5 to 8.4 Ω cm²) as well as extraordinarily long cycle life of the assembled SSBs. This strategy enables the use of a nickel-rich LiNi_0.83Co_0.07Mn_0.1O₂ cathode to deliver a reversible capacity of 201.5 mAh g^-1 at a considerable loading of 4.8 mg cm^-2, featuring performance metrics for an SSB that is competitive with those of traditional Li-ion systems. Our study demonstrates the potential of solution-based routes as an affordable and scalable manufacturing alternative to vapor-based deposition techniques that can accelerate the development of SSBs for practical applications.

Improving the alkali metal electrode/inorganic solid electrolyte contact via room-temperature ultrasound solid welding

The combination of alkali metal electrodes and solid-state electrolytes is considered a promising strategy to develop high-energy rechargeable batteries. However, the practical applications of these two components are hindered by the large interfacial resistance and growth of detrimental alkali metal depositions (e.g., dendrites) during cycling originated by the unsatisfactory electrode/solid electrolyte contact. To tackle these issues, we propose a room temperature ultrasound solid welding strategy to improve the contact between Na metal and Na₃Zr₂Si₂PO₁₂ (NZSP) inorganic solid electrolyte. Symmetrical Na|NZSP | Na cells assembled via ultrasonic welding show stable Na plating/stripping behavior at a current density of 0.2 mA cm^-2 and a higher critical current density (i.e., 0.6 mA cm^-2) and lower interfacial impedance than the symmetric cells assembled without the ultrasonic welding strategy. The beneficial effect of the ultrasound welding is also demonstrated in Na|NZSP | Na₃V₂(PO₄)₃ full coin cell configuration where 900 cycles at 0.1 mA cm^-2 with a capacity retention of almost 90% can be achieved at room temperature.

Grain Boundary Design of Solid Electrolyte Actualizing Stable All-Solid-State Sodium Batteries

Advanced inorganic solid electrolytes (SEs) are critical for all-solid-state alkaline metal batteries with high safety and high energy densities. A new interphase design to address the urgent interfacial stability issues against all-solid-state sodium metal batteries (ASSMBs) is proposed. The grain boundary phase of a Mg²⁺ -doped Na₃ Zr₂ Si₂ PO₁₂ conductor (denoted as NZSP-xMg) is manipulated to introduce a favorable Na_{3-2 δ} Mg_δ PO₄ -dominant interphase which facilitates its intimate contact with Na metal and works as an electron barrier to suppress Na metal dendrite penetration into the electrolyte bulk. The optimal NZSP-0.2Mg electrolyte endows a low interfacial resistance of 93 Ω cm² at room temperature, over 16 times smaller than that of Na₃ Zr₂ Si₂ PO₁₂ . The Na plating/stripping with small polarization is retained under 0.3 mA cm^-2 for more than 290 days (7000 h), representing a record high cycling stability of SEs for ASSMBs. An all-solid-state NaCrO₂ //Na battery is accordingly assembled manifesting a high capacity of 110 mA h g^-1 at 1 C for 1755 cycles with almost no capacity decay. Excellent rate capability at 5 C is realized with a high Coulombic efficiency of 99.8%, signifying promising application in solid-state electrochemical energy storage systems.

Universal lithiophilic interfacial layers towards dendrite-free lithium anodes for solid-state lithium-metal batteries

Solid-state lithium-metal batteries (SSLMBs) using garnet Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) as the solid electrolyte are expected to conquer the safety concerns of high energy Li batteries with organic liquid electrolytes owing to its nonflammable nature and good mechanical strength. However, the poor interfacial contact between the Li anode and LLZTO greatly restrains the practical applications of the electrolyte, because large polarization, dendritic Li formation and penetration can occur at the interfaces. Here, an effective method is proposed to improve the wettability of the LLZTO toward lithium and reduce the interfacial resistance by engineering universal lithiophilic interfacial layers. Thanks to the in-situ formed lithiophilic and ionic conductive Co/Li₂O interlayers, the symmetric Li/CoO-LLZTO/Li batteries present much smaller overpotential, ultra-low areal specific resistance (ASR, 12.3 Ω cm²), high critical current density (CCD, 1.1 mA cm^-2), and outstanding cycling performance (1696 h at a current density of 0.3 mA cm^-2) at 25 °C. Besides, the solid-state Li/CoO-LLZTO/LFP cells deliver an excellent electrochemical performance with a high coulombic efficiency of ~100% and a long cycling time over 185 times. Surprisingly, the high-voltage (4.6 V) solid state Li/CoO-LLZTO/Li_1.4Mn_0.6Ni_0.2Co_0.2O_2.4 (LMNC622) batteries can also realize an ultra-high specific capacity (232.5 mAh g^-1) under 0.1 C at 25 °C. This work paves an effective way for practical applications of the dendrite-free SSLMBs.

Amorphous-Carbon-Coated 3D Solid Electrolyte for an Electro-Chemomechanically Stable Lithium Metal Anode in Solid-State Batteries

The use of solid-state electrolyte may be necessary to enable safe, high-energy-density Li metal anodes for next-generation energy storage systems. However, the inhomogeneous local current densities during long-term cycling result in instability and detachment of the Li anode from the electrolyte, which greatly hinders practical application. In this study, we report a new approach to maintain a stable Li metal | electrolyte interface by depositing an amorphous carbon nanocoating on garnet-type solid-state electrolyte. The carbon nanocoating provides both electron and ion conducting capability, which helps to homogenize the lithium metal stripping and plating processes. After coating, we find the Li metal/garnet interface displays stable cycling at 3 mA/cm² for more than 500 h, demonstrating the interface's outstanding electro-chemomechanical stability. This work suggests amorphous carbon coatings may be a promising strategy for achieving stable Li metal | electrolyte interfaces and reliable Li metal batteries.

Surface Potential Regulation Realizing Stable Sodium/Na3 Zr2 Si2 PO12 Interface for Room-Temperature Sodium Metal Batteries

Inorganic Na₃ Zr₂ Si₂ PO₁₂ is prospective with a high ionic conductivity but suffers large interfacial resistance and stability issues against sodium metal, hindering its practical application in all-solid-state sodium batteries. A surface potential regulation strategy is adopted to address these issues. Na₃ Zr₂ Si₂ PO₁₂ (NZSP) ceramic with homogeneously-sintered surface is synthesized by a simple two-step sintering method to promote its uniform surface potential, which is favorable for mitigating the potential fluctuations at the interface against Na metal and enhancing interfacial compatibility. The Na/NZSP interface can be stabilized for over 4 months with a low interfacial resistance of 129 Ω cm² at 25 °C. The symmetrical Na/NZSP/Na cell exhibits ultra-stable sodium platting/stripping cycling for over 1000 cycles under 0.1 mA cm^-2 . Superior interfacial performance is well retained even under 0.2 mA cm^-2 at room temperature. The robust interface is further signified by its excellence under higher current densities of up to 0.85 mA cm^-2 at 60 °C. A 4 V all-solid-state Na₃ V_1.5 Cr_0.5 (PO₄ )₃ /NZSP/Na metal battery is demonstrated at ambient conditions, which exhibits superior rate capability and delivers a high reversible capacity of 103 mA h g^-1 under 100 mA g^-1 for over 400 cycles with a Coulombic efficiency of over 99%.

Iodine-Substituted Lithium/Sodium closo-Decaborates: Syntheses, Characterization, and Solid-State Ionic Conductivity

Solid-state electrolytes based on closo-decaborates have caught increasing interest owing to the impressive room-temperature ionic conductivity, remarkable thermal/chemical stability, and excellent deformability. In order to develop new solid-state ion conductors, we investigated the influence of iodine substitution on the thermal, structural, and ionic conduction properties of closo-decaborates. A series of iodinated closo-decaborates, M₂[B₁₀H_10-nI_n] (M = Li, Na; n = 1, 2, 10), were synthesized and characterized by thermal analysis, powder X-ray diffraction, and electrochemical impedance spectroscopy; the stability and ionic conductivity of these compounds were studied. It was found that with the increase of iodine substitution on the closo-decaborate anion cage, the thermal decomposition temperature increases. All M₂[B₁₀H_10-nI_n] exhibit an amorphous structure. The ionic conductivity of Li₂[B₁₀H_10-nI_n] is higher than that of the Li₂[B₁₀H₁₀] parent compound. An ionic conductivity of 2.96 × 10^-2 S cm^-1 with an activation energy of 0.23 eV was observed for Li₂[B₁₀I₁₀] at 300 °C, implying that iodine substitution can improve the ionic conductivity. However, the ionic conductivity of Na₂[B₁₀H_10-nI_n] is lower than that of Na₂[B₁₀H₁₀] and increases with the increase of iodine substitution, which could be associated with the increase of the electrostatic potential, mass, and volume of the iodinated anions. Moreover, Li₂[B₁₀I₁₀] offers a Li-ion transference number of 0.999, an electrochemical stability window of 3.3 V and good compatibility with the Li anode, demonstrating its potential for application in high-temperature batteries.

Impact of Protonation on the Electrochemical Performance of Li7La3Zr2O12 Garnets

Li₇La₃Zr ₂O₁₂ (LLZO) garnet ceramics are promising electrolytes for all-solid-state lithium-metal batteries with high energy density. However, these electrolytes are prone to Li⁺/H⁺ exchange, that is, protonation, resulting in degradation of their Li-ion conductivity. Here, we identify how common processing steps, such as surface cleaning in alcohol or acetone, trigger LLZO partial protonation. We deconvolute the contributions to the electrochemical impedance spectra of both the protonated LLZO phase (HLLZO) and its decomposition products forming upon annealing. While the mixed conduction of H⁺/Li⁺ ions in HLLZO decreases the contribution of the electrolyte to the overall impedance, it deteriorates the transport of Li⁺ ions across the LLZO/Li interface. This is also the case after thermal decomposition of HLLZO, which occurs at significantly lower temperature than that for pristine LLZO. As a result, symmetric Li/LLZO/Li cells suffer from inhomogeneous lithium electrodeposition within the first three cycles when stripping and plating a capacity of 1 mA·h/cm² per half-cycle at 0.1 mA/cm². We demonstrate that LLZO protonation is avoided when applying solvents with very low acidity, such as hexane. Such Li/LLZO/Li cells provide stable cycling over more than 300 h, demonstrating the importance of selecting an appropriate solvent for LLZO processing to prevent dendrites formation.

Water based synthesis of highly conductive GaxLi7-3xLa3Hf2O12 garnets with comparable critical current density to analogous GaxLi7-3xLa3Zr2O12 systems

Next generation lithium ion batteries are envisaged as those which feature an all solid-state architecture. This will enable the higher energy density storage required to meet the demands of modern society, especially for the growing electric vehicle market. Solid state batteries have, however, proved troublesome to implement commercially due to the lack of a suitable solid-state electrolyte, which needs to be highly conductive, have a low interfacial resistance and a suitably wide electrochemical stability window. Garnet materials are potential contenders for these batteries, demonstrating many of the desired properties, although there remain challenges to overcome. Here we report a facile synthesis of Li₇La₃Hf₂O₁₂ and Ga/Al_xLi_7-3xLa₃Hf₂O₁₂ garnets, with the synthesis of Ga_0.2Li_6.4La₃Hf₂O₁₂ requiring only dissolution of precursors in water and heating to 700 °C. Ga_0.2Li_6.4La₃Hf₂O₁₂ was shown to display a high room temperature conductivity (0.373 mS cm^-1 at 28 °C). Moreover, in Li|garnet|Li cells, we observed a comparable critical current density compared to Ga_0.2Lai_6.4La₃Zr₂O₁₂, despite a lower density and higher area specific resistance compared to literature values, suggesting Hf systems may be further engineered to deliver additional improvements for use in future solid state batteries.