Toward garnet electrolyte-based Li metal batteries: An ultrathin, highly effective, artificial solid-state electrolyte/metallic Li interface

Interphases, Interfaces, and Surfaces of Active Materials in Rechargeable Batteries and Perovskite Solar Cells

The ever-increasing demand for clean sustainable energy has driven tremendous worldwide investment in the design and exploration of new active materials for energy conversion and energy-storage devices. Tailoring the surfaces of and interfaces between different materials is one of the surest and best studied paths to enable high-energy-density batteries and high-efficiency solar cells. Metal-halide perovskite solar cells (PSCs) are one of the most promising photovoltaic materials due to their unprecedented development, with their record power conversion efficiency (PCE) rocketing beyond 25% in less than 10 years. Such progress is achieved largely through the control of crystallinity and surface/interface defects. Rechargeable batteries (RBs) reversibly convert electrical and chemical potential energy through redox reactions at the interfaces between the electrodes and electrolyte. The (electro)chemical and optoelectronic compatibility between active components are essential design considerations to optimize power conversion and energy storage performance. A focused discussion and critical analysis on the formation and functions of the interfaces and interphases of the active materials in these devices is provided, and prospective strategies used to overcome current challenges are described. These strategies revolve around manipulating the chemical compositions, defects, stability, and passivation of the various interfaces of RBs and PSCs.

Ensemble Design of Electrode-Electrolyte Interfaces: Toward High-Performance Thin-Film All-Solid-State Li-Metal Batteries

In accordance with the fourth industrial revolution (4IR), thin-film all-solid-state batteries (TF-ASSBs) are being revived as the most promising energy source to power small electronic devices. However, current TF-ASSBs still suffer from the perpetual necessity of high-performance battery components. While every component, a series of a TF solid electrolyte (i.e., lithium phosphorus oxynitride (LiPON)) and electrodes (cathode and Li metal anode), has been considered vital, the lack of understanding of and ability to ameliorate the cathode (or anode)-electrolyte interface (CEI) (or AEI) has impeded the development of TF-ASSBs. In this work, we suggest an ensemble design of TF-ASSBs using LiPON (500 nm), an amorphous TF-V₂O_5-x cathode with oxygen vacancies (O_vacancy), a thin evaporated Li anode (evp-Li) with a thickness of 1 μm, and an artificial ultrathin Al₂O₃ layer between evp-Li and LiPON. Well-defined O_vacancy sites, such as O(II)_vacancy and O(III)_vacancy, in amorphous TF-V₂O_5-x not only allow isotropic Li⁺ diffusion at the CEI but also enhance both the ionic and electronic conductivities. For the AEI, we employed protective Al₂O₃, which was specially sputtered using the facing target sputtering (FTS) method to form a homogeneous layer without damage from plasma. In regard to the contact with evp-Li, interfacial stability, electrochemical impedance, and battery performance, the nanometric Al₂O₃ layers (1 nm) were optimized at different temperatures (40, 60, and 80 °C). The TF-ASSB cell containing Al₂O₃ (1 nm) delivers a high specific capacity of 474.01 mAh cm^-3 under 60 °C at 2 C for the 400th cycle, and it achieves a long lifespan as well as ultrafast rate capability levels, even at 100 C; these results were comparable to those of TF Li-ion battery cells using a liquid electrolyte. We demonstrated the reaction mechanism at the AEI utilizing time-of-flight secondary ion mass spectrometry (TOF-SIMS) and molecular dynamics (MD) simulations for a better understanding. Our design provides a signpost for future research on the rational structure of TF-LIBs.

Composite Lithium Protective Layer Formed In Situ for Stable Lithium Metal Batteries

Lithium metal is considered as the ideal anode for next-generation rechargeable batteries due to its highest theoretical specific capacity and lowest electrochemical potential. However, lithium dendrite growth during lithium deposition could lead to a short circuit and even cause severe safety issues. Here, we use solid-state electrolyte Li₃InCl₆ as an additive in nonaqueous electrolytes because of its high ionic conductivity (10^-3 to 10^-4 S cm^-1) and good electrochemical stability. It is found that Li₃InCl₆ can in situ react with metallic lithium to form a ternary composite solid electrolyte interphase (SEI) consisting of a Li-In alloy, LiCl, and codeposited Li₃InCl₆. The composite SEI can effectively suppress Li dendrite growth and thereby maintain stable long-term cycling performance in lithium metal batteries. The protected lithium electrode exhibits stable cycling performance in a symmetric Li|Li battery for nearly 1000 h at a current density of 1 mA cm^-2. Besides, the full battery with a LiFePO₄ cathode and a metallic lithium anode delivers a stable capacity of 140.6 mA h g^-1 for 500 cycles with a capacity retention of 95%. The Li|S battery with Li₃InCl₆-added LiTFSI in 1,3-dioxolane/1,2-dimethoxyethane electrolyte also shows significant improvement in capacity retention at 0.5 C. This work demonstrates an effective approach to design dendrite-free metal anodes.

In Situ Diffusion Measurements of a NASICON-Structured All-Solid-State Battery Using Muon Spin Relaxation

In situ muon spin relaxation is demonstrated as an emerging technique that can provide a volume-averaged local probe of the ionic diffusion processes occurring within electrochemical energy storage devices as a function of state of charge. Herein, we present work on the conceptually interesting NASICON-type all-solid-state battery LiM2(PO4)3, using M = Ti in the cathode, M = Zr in the electrolyte, and a Li metal anode. The pristine materials are studied individually and found to possess low ionic hopping activation energies of ∼50-60 meV and competitive Li+ self-diffusion coefficients of ∼10-10-10-9 cm2 s-1 at 336 K. Lattice matching of the cathode and electrolyte crystal structures is employed for the all-solid-state battery to enhance Li+ diffusion between the components in an attempt to minimize interfacial resistance. The cell is examined by in situ muon spin relaxation, providing the first example of such ionic diffusion measurements. This technique presents an opportunity to the materials community to observe intrinsic ionic dynamics and electrochemical behavior simultaneously in a nondestructive manner.

Applications of Low-Melting-Point Metals in Rechargeable Metal Batteries

Low-melting-point (LMP) metals represent an interesting family of electrode materials owing to their high ionic conductivity, good ductility or fluidity, low hardness and/or superior alloying capability, all of which are crucial characteristics to address battery challenges such as interfacial incompatibility, electrode pulverization, and dendrite growth. This minireview summarizes recent research progress of typical LMP metals including In, Ga, Hg, and their alloys in rechargeable metal batteries. Emphasis is placed on mainstream electrochemical storage devices of Li, Na, and K batteries as well as the representative multi-valent metal batteries. The fundamental correlations between unique physiochemical properties of LMP metals and the battery performance are highlighted. In addition, this article also provides insights into future development and potential directions of LMP metals/alloys for practical applications.

Li7La3Zr2O12 Garnet Solid Polymer Electrolyte for Highly Stable All-Solid-State Batteries

All-solid-state batteries have gained significant attention as promising candidates to replace liquid electrolytes in lithium-ion batteries for high safety, energy storage performance, and stability under elevated temperature conditions. However, the low ionic conductivity and unsuitability of lithium metal in solid polymer electrolytes is a critical problem. To resolve this, we used a cubic garnet oxide electrolyte (Li₇La₃Zr₂O₁₂ - LLZO) and ionic liquid in combination with a polymer electrolyte to produce a composite electrolyte membrane. By applying a solid polymer electrolyte on symmetric stainless steel, the composite electrolyte membrane shows high ionic conductivity at elevated temperatures. The effect of LLZO in suppressing lithium dendrite growth within the composite electrolyte was confirmed through symmetric lithium stripping/plating tests under various current densities showing small polarization voltages. The full cell with lithium iron phosphate as the cathode active material achieved a highest specific capacity of 137.4 mAh g^-1 and a high capacity retention of 98.47% after 100 cycles at a current density of 50 mA g^-1 and a temperature of 60°C. Moreover, the specific discharge capacities were 137 and 100.8 mAh g^-1 at current densities of 100 and 200 mA g^-1, respectively. This research highlights the capability of solid polymer electrolytes to suppress the evolution of lithium dendrites and enhance the performance of all-solid-state batteries.

Interface Issues and Challenges in All-Solid-State Batteries: Lithium, Sodium, and Beyond

Owing to the promise of high safety and energy density, all-solid-state batteries are attracting incremental interest as one of the most promising next-generation energy storage systems. However, their widespread applications are inhibited by many technical challenges, including low-conductivity electrolytes, dendrite growth, and poor cycle/rate properties. Particularly, the interfacial dynamics between the solid electrolyte and the electrode is considered as a crucial factor in determining solid-state battery performance. In recent years, intensive research efforts have been devoted to understanding the interfacial behavior and strategies to overcome these challenges for all-solid-state batteries. Here, the interfacial principle and engineering in a variety of solid-state batteries, including solid-state lithium/sodium batteries and emerging batteries (lithium-sulfur, lithium-air, etc.), are discussed. Specific attention is paid to interface physics (contact and wettability) and interface chemistry (passivation layer, ionic transport, dendrite growth), as well as the strategies to address the above concerns. The purpose here is to outline the current interface issues and challenges, allowing for target-oriented research for solid-state electrochemical energy storage. Current trends and future perspectives in interfacial engineering are also presented.

Review on Li Deposition in Working Batteries: From Nucleation to Early Growth

Lithium (Li) metal is one of the most promising alternative anode materials of next-generation high-energy-density batteries demanded for advanced energy storage in the coming fourth industrial revolution. Nevertheless, disordered Li deposition easily causes short lifespan and safety concerns and thus severely hinders the practical applications of Li metal batteries. Tremendous efforts are devoted to understanding the mechanism for Li deposition, while the final deposition morphology tightly relies on the Li nucleation and early growth. Here, the recent progress in insightful and influential models proposed to understand the process of Li deposition from nucleation to early growth, including the heterogeneous model, surface diffusion model, crystallography model, space charge model, and Li-SEI model, are highlighted. Inspired by the abovementioned understanding on Li nucleation and early growth, diverse anode-design strategies, which contribute to better batteries with superior electrochemical performance and dendrite-free deposition behavior, are also summarized. This work broadens the horizon for practical Li metal batteries and also sheds light on more understanding of other important metal-based batteries involving the metal deposition process.

Elucidating Interfacial Stability between Lithium Metal Anode and Li Phosphorus Oxynitride via In Situ Electron Microscopy

Li phosphorus oxynitride (LiPON) is one of a very few solid electrolytes that have demonstrated high stability against Li metal and extended cyclability with high Coulombic efficiency for all solid-state batteries (ASSBs). However, theoretical calculations show that LiPON reacts with Li metal. Here, we utilize in situ electron microscopy to observe the dynamic evolutions at the LiPON-Li interface upon contacting and under biasing. We reveal that a thin interface layer (∼60 nm) develops at the LiPON-Li interface upon contact. This layer is composed of conductive binary compounds that show a unique spatial distribution that warrants an electrochemical stability of the interface, serving as an effective passivation layer. Our results explicate the excellent cyclability of LiPON and reconcile the existing debates regarding the stability of the LiPON-Li interface, demonstrating that, though glassy solid electrolytes may not have a perfect initial electrochemical window with Li metal, they may excel in future applications for ASSBs.

Promises and Challenges of Next-Generation "Beyond Li-ion" Batteries for Electric Vehicles and Grid Decarbonization

The tremendous improvement in performance and cost of lithium-ion batteries (LIBs) have made them the technology of choice for electrical energy storage. While established battery chemistries and cell architectures for Li-ion batteries achieve good power and energy density, LIBs are unlikely to meet all the performance, cost, and scaling targets required for energy storage, in particular, in large-scale applications such as electrified transportation and grids. The demand to further reduce cost and/or increase energy density, as well as the growing concern related to natural resource needs for Li-ion have accelerated the investigation of so-called "beyond Li-ion" technologies. In this review, we will discuss the recent achievements, challenges, and opportunities of four important "beyond Li-ion" technologies: Na-ion batteries, K-ion batteries, all-solid-state batteries, and multivalent batteries. The fundamental science behind the challenges, and potential solutions toward the goals of a low-cost and/or high-energy-density future, are discussed in detail for each technology. While it is unlikely that any given new technology will fully replace Li-ion in the near future, "beyond Li-ion" technologies should be thought of as opportunities for energy storage to grow into mid/large-scale applications.

Li/Garnet Interface Optimization: An Overview

Solid-state lithium batteries can improve the safety and energy density of the present liquid-electrolyte-based lithium-ion batteries. To achieve this goal, both solid electrolyte and lithium anode technology are the keys. Lithium garnet is a promising electrolyte to enable the next generation solid-state lithium batteries due to its high ionic conductivity, good chemical, and electrochemical stability, and easiness to scale up. It is relatively stable against Li metal but the poor contact area and the presence of resistive impurity or decomposition layers at the interface interfere with fast charge transfer, thereby, spiking the interfacial resistance, overpotential, local current density, and the propensity for dendrite growth. In this Review, we first summarize the recent understanding of the interfacial problems at the Li/garnet interface from both computational and experimental viewpoints while seizing the opportunity to shed light on the chemical/electrochemical stability of garnet against Li metal anode. Also, we highlight various interface optimization strategies that have been demonstrated to be effective in improving the interface performance. We conclude this Review with a few suggestions as guides for future work.

Recent Progress in Designing Stable Composite Lithium Anodes with Improved Wettability

Lithium (Li) is a promising battery anode because of its high theoretical capacity and low reduction potential, but safety hazards that arise from its continuous dendrite growth and huge volume changes limit its practical applications. Li can be hosted in a framework material to address these key issues, but methods to encage Li inside scaffolds remain challenging. The melt infusion of molten Li into substrates has attracted enormous attention in both academia and industry because it provides an industrially adoptable technology capable of fabricating composite Li anodes. In this review, the wetting mechanism driving the spread of liquefied Li toward a substrate is discussed. Following this, various strategies are proposed to engineer stable Li metal composite anodes that are suitable for liquid and solid-state electrolytes. A general conclusion and a perspective on the current limitations and possible future research directions for constructing composite Li anodes for high-energy lithium metal batteries are presented.

Efficient Construction of a C60 Interlayer for Mechanically Robust, Dendrite-free, and Ultrastable Solid-State Batteries

Interfacial instability between solid electrolytes (SEs) and lithium metal remains a daunting challenge for solid-sate batteries. Here, a conformal C₆₀ interlayer is efficiently constructed on Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) SEs by physical vapor deposition, and an ideal interfacial contact is achieved via forming an ionically conducting matrix of Li_xC₆₀ with lithium metal. The obtained Li_xC₆₀ is beneficial to hinder the growth of lithium dendrites at interface and release the local stress during the lithiation and delithiation. As a result, the Li/LAGP-C₆₀/Li symmetric cells demonstrate ultra-stable cycling performance for more than 4,500 h at a current density of 0.034 mA cm⁻². The Li/LAGP-C₆₀/LiFePO₄ full cells deliver a reversible capacity of 152.4 mAh g⁻¹ at room temperature, and the capacity retention rate is 85% after more than 100 cycles. This work provides a feasible and scalable strategy to improve the SEs/Li interface for high-performance solid-state batteries.

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Ionically conducting Li_xC₆₀ matrix is formed at Li-LAGP interface

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Li/LAGP-C₆₀/Li cells display ultra-long cycle life of 6 months

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Li/LAGP-C₆₀/LFP cells exhibit high capacity with good cycle performance

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Mechanical integrity of cycled LAGP-C₆₀ is validated by X-ray CT

Interface Between Solid-State Electrolytes and Li-Metal Anodes: Issues, Materials, and Processing Routes

Li metal, which has a high theoretical capacity and negative electrochemical potential, is regarded as the "holy grail" in Li-ion batteries. However, the flammable nature of liquid electrolyte leads to safety issues. Hence, the cooperation of solid-state electrolyte and Li-metal anode is demanded. However, the short cycle life induced by interfacial issues is the main challenge faced by their cooperation. In this review, dendrite and interfacial side reactions are comprehensively analyzed as the main interfacial problems. Meanwhile, the "state-of-the-art" interphase materials are summarized. The challenges faced by each kind of material are underscored. Moreover, different processing routes to fabricate artificial interphase are also investigated from an engineering perspective. The processing routes suitable for mass production are also underscored.

LiAlO2/LiAl5O8 Membranes Derived from Flame-Synthesized Nanopowders as a Potential Electrolyte and Coating Material for All-Solid-State Batteries

Recently, γ-LiAlO₂ has attracted considerable attention as a coating in Li-ion battery electrodes. However, its potential as a Li⁺ ceramic electrolyte is limited due to its poor ionic conductivity (<10^-10 S cm^-1). Here, we demonstrate an effective method of processing LiAlO₂ membranes (<50 μm) using nanopowders (NPs) produced via liquid-feed flame spray pyrolysis (LF-FSP). Membranes consisting of selected mixtures of lithium aluminate polymorphs and Li contents were processed by conventional tape casting of NPs followed by thermocompression of the green films (100 °C/10 kpsi/10 min). The sintered green films (1100 °C/2 h/air) present a mixture of LiAlO₂ (∼72 wt %) and LiAl₅O₈ (∼27 wt %) phases, offering ionic conductivities (>10^-6 S cm^-1) at ambient with an activation energy of 0.5 eV. This greatly increases their potential utility as ceramic electrolytes for all-solid-state batteries, which could simplify battery designs, significantly reduce costs, and increase their safety. Furthermore, a solid-state Li/Li_3.1AlO₂/Li symmetric cell was assembled and galvanostatically cycled at 0.375 mA cm^-2 current density, exhibiting a transference number ≈ 1.

Matchmaker of Marriage between a Li Metal Anode and NASICON-Structured Solid-State Electrolyte: Plastic Crystal Electrolyte and Three-Dimensional Host Structure

The marriage between a Li metal anode and the solid-state electrolyte is expected to limit the safety risk of secondary batteries. However, dendrites and interfacial stability hinder the combination of Li metal anode and solid-state electrolyte. Herein, a plastic crystal electrolyte (PCE) and three-dimensional (3D) host structure played the role of a matchmaker in combining the solid-state electrolyte and Li metal anode. Succinonitrile cooperated with Li salt and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ nanosize powder and built a PCE interphase, which enhanced the interfacial stability between Li_1.5Al_0.5Ge_1.5(PO₄)₃ and Li metal anode. To protect the soft PCE from the dendrite penetration, commercially sold Super P, carbon nanotube, KS6, and Ketjen black were co-heated with the melted Li metal. However, only KS6 built a 3D host in Li metal successfully because of its high graphitization and layered structure. Benefitting from the matchmakers, the solid-state batteries exhibited enhanced cycling stability.

Wetting Phenomena and their Effect on the Electrochemical Performance of Surface-Tailored Lithium Metal Electrodes in Contact with Cross-linked Polymeric Electrolytes

Li metal batteries (LMBs) containing cross-linked polymer electrolytes (PEs) are auspicious candidates for next-generation batteries. However, the wetting behavior of PEs on uneven Li metal surfaces has been neglected in most studies. Herein, it is shown that microscale defect sites with curved edges play an important role in a wettability-dependent electrodeposition. The wettability and the viscoelastic properties of PEs are correlated, and the impact of wettability on the nucleation and diffusion near the Li|PE interface is distinguished. It is found that the curvature of the edges is a key factor for the investigation of wetting phenomena. The appearance of microscale defects and phase separation are identified as main causes for erratic nucleation. It is emphasized that the implementation of stable and consistent long-term cycling performance of LMBs using PEs requires a deeper understanding of the "soft-solid"-solid contact between PEs and inherently rough Li metal surfaces.

Mirror-Like Electrodeposition of Lithium Metal under a Low-Resistance Artificial Solid Electrolyte Interphase Layer

Nonuniform electrodeposition and dendritic growth of lithium metal coupled to its chemical incompatibility with liquid electrolytes are largely responsible for poor Coulombic efficiency and safety hazards preventing the successful implementation of energy-dense Li metal anodes. Artificial solid electrolyte interface (ASEI) layers have been proposed to address the morphological evolution and chemical reactions in Li metal anodes. In this study, an ASEI layer consisting of a lithium phosphorus oxynitride (LiPON) thin film electrolyte and gold-alloying interlayer was developed and shown to promote the electrodeposition of smooth, homogeneous, mirror-like Li metal morphologies. The Au layer alloyed with Li, reducing the nucleation overpotential and resulting in a more spatially uniform metal deposit, while the LiPON layer provided a physical barrier between the Li metal and aprotic liquid electrolyte. The effectiveness and integrity of the LiPON protective layer was assessed using in operando impedance spectroscopy and ex situ SEM/EDS characterization. Smooth, homogeneous Li morphologies were realized in capacities up to 3 mAh cm^-2 plated at 0.1 mA cm^-2. At higher current densities up to 1 mA cm^-2 or increased deposition capacities of 6 mAh cm^-2, the LiPON coating fractured due to the localized, nonuniform lithium deposits and rough, dendritic Li morphologies were observed. This approach represents a new strategy in the design of artificial SEIs to enable Li metal anodes with practical areal capacities.

Stable Electrochemical Li Plating/Stripping Behavior by Anchoring MXene Layers on Three-Dimensional Conductive Skeletons

The ultrahigh specific capacity of lithium (Li) metal makes it possible to serve as the ultimate candidate for an anode in high-energy density secondary batteries, whereas the safety hazards caused by Li dendrite growth severely hamper the commercialization process of a lithium metal anode. Here, we propose a 3D conductive skeleton by anchoring MXene on Cu foam (MXene@CF) to significantly improve the electrochemical Li plating/stripping behavior. Li metal tends to nucleate uniformly and grow horizontally along the MXene nanosheets under the strong Coulomb interaction between adsorbed Li and MXene. Moreover, the abundant fluorine termination groups in MXene contribute to forming a stable fluorinated solid electrolyte interphase (SEI) and thus effectively regulating the Li deposition behaviors and prolonging the stability of the Li metal anode. Therefore, the MXene@CF skeleton maintains a high Coulombic efficiency (CE) of 98.5% after 200 cycles at 1 mA cm^-2. The MXene@CF-based symmetric cells can run for more than 1000 h without intense voltage fluctuation and demonstrates remarkable deep charge/discharge abilities. The MXene@CF-Li|LiFePO₄ full cell exhibits outstanding long-term cycling stability (95% capacity retention after 300 cycles). Our research suggests that MXene could effectively regulate the Li plating behavior that might provide a feasible solution for a dendrite-free Li anode.

Li2CO3-affiliative mechanism for air-accessible interface engineering of garnet electrolyte via facile liquid metal painting

Garnet based solid-state batteries have the advantages of wide electrochemical window and good chemical stability. However, at Li-garnet interface, the poor interfacial wettability due to Li₂CO₃ passivation usually causes large resistance and unstable contact. Here, a Li₂CO₃-affiliative mechanism is proposed for air-accessible interface engineering of garnet electrolyte via facile liquid metal (LM) painting. The natural LM oxide skin enables a superior wettability of LM interlayer towards ceramic electrolyte and Li anode. Therein the removal of Li₂CO₃ passivation network is not necessary, in view of its delamination and fragmentation by LM penetration. This dissipation effect allows the lithiated LM nanodomains to serve as alternative Li-ion flux carriers at Li-garnet interface. This mechanism leads to an interfacial resistance as small as 5 Ω cm² even after exposing garnet in air for several days. The ultrastable Li plating and stripping across LM painted garnet can last for 9930 h with a small overpotential.

At Li-garnet interface, the poor interfacial wettability due to Li₂CO₃ passivation causes large resistance and unstable contact. Here the authors propose a Li₂CO₃-affiliative mechanism for air-accessible interface engineering of garnet electrolyte with superior wettability via facile liquid metal painting.

Designing composite solid-state electrolytes for high performance lithium ion or lithium metal batteries

Solid-state electrolytes (SSEs) are capable of inhibiting the growth of lithium dendrites, demonstrating great potential in next-generation lithium-ion batteries (LIBs). However, poor room temperature ionic conductivity and the unstable interface between SSEs and the electrode block their large-scale applications in LIBs. Composite solid-state electrolytes (CSSEs) formed by mixing different ionic conductors lead to better performance than single SSEs, especially in terms of ionic conductivity and interfacial stability. Herein, we have systematically reviewed recent developments and investigations of CSSEs including inorganic composite and organic-inorganic composite materials, in order to provide a better understanding of designing CSSEs. The comparison of different types of CSSEs relative to their parental materials is deeply discussed in the context of ionic conductivity and interfacial design. Then, the proposed ion transfer pathways and models of lithium dendrite growth in composites are outlined to inspire future development of CSSEs.

Polymer Template Synthesis of Flexible SiO2 Nanofibers to Upgrade Composite Electrolytes

Flexible oxide ceramic films offer prospects for revolutionizing diverse fields such as energy and electronics, but their fabrication methods are typically elaborate and cannot be expanded. Here, we report a scalable strategy to fabricate flexible and robust SiO₂ nanofiber films with controllable morphology using a sol-gel electrospinning method followed by low-temperature calcination. When applied to composite polymer electrolytes (CPEs) for solid-state batteries by filling polyethylene oxide into porous ceramic films, SiO₂ nanofibers with large surface areas (51 m²·g^-1) demonstrate strong Lewis interfacial interactions and isotropic ionic transfer channels that mitigate polymer crystallinity and Li⁺-concentration polarization, imparting high conductivity (1.3 × 10^-4 S·cm^-1 at 30 °C) and structural stability to the electrolytes. As a result, all-solid-state LiFePO₄||Li shows great rate capability and long cycling stability with high discharge capacities of 159 and 132 mA·h·g^-1 at 0.5C under 60 and 45 °C, respectively, demonstrating broad commercial prospects for the scale production of efficient solid electrolytes.

Advances in Materials Design for All-Solid-state Batteries: From Bulk to Thin Films

All-solid-state batteries (SSBs) are one of the most fascinating next-generation energy storage systems that can provide improved energy density and safety for a wide range of applications from portable electronics to electric vehicles. The development of SSBs was accelerated by the discovery of new materials and the design of nanostructures. In particular, advances in the growth of thin-film battery materials facilitated the development of all solid-state thin-film batteries (SSTFBs)—expanding their applications to microelectronics such as flexible devices and implantable medical devices. However, critical challenges still remain, such as low ionic conductivity of solid electrolytes, interfacial instability and difficulty in controlling thin-film growth. In this review, we discuss the evolution of electrode and electrolyte materials for lithium-based batteries and their adoption in SSBs and SSTFBs. We highlight novel design strategies of bulk and thin-film materials to solve the issues in lithium-based batteries. We also focus on the important advances in thin-film electrodes, electrolytes and interfacial layers with the aim of providing insight into the future design of batteries. Furthermore, various thin-film fabrication techniques are also covered in this review.

Electrochemical and Structural Analysis in All-Solid-State Lithium Batteries by Analytical Electron Microscopy: Progress and Perspectives

Advanced scanning transmission electron microscopy (STEM) and its associated instruments have made significant contributions to the characterization of all-solid-state (ASS) Li batteries, as these tools provide localized information on the structure, morphology, chemistry, and electronic state of electrodes, electrolytes, and their interfaces at the nano- and atomic scale. Furthermore, the rapid development of in situ techniques has enabled a deep understanding of interfacial dynamic behavior and heterogeneous characteristics during the cycling process. However, due to the beam-sensitive nature of light elements in the interphases, e.g., Li and O, thorough and reliable studies of the interfacial structure and chemistry at an ultrahigh spatial resolution without beam damage is still a formidable challenge. Herein, the following points are discussed: (1) the recent contributions of advanced STEM to the study of ASS Li batteries; (2) current challenges associated with using this method; and (3) potential opportunities for combining cryo-electron microscopy and the STEM phase contrast imaging techniques.

A High-Performance Carbonate-Free Lithium|Garnet Interface Enabled by a Trace Amount of Sodium

Garnet-type solid-state electrolytes (SSEs) are promising for the realization of next-generation high-energy-density Li metal batteries. However, a critical issue associated with the garnet electrolytes is the poor physical contact between the Li anode and the garnet SSE and the resultant high interfacial resistance. Here, it is reported that the Li|garnet interface challenge can be addressed by using Li metal doped with 0.5 wt% Na (denoted as Li*) and melt-casting the Li* onto the garnet SSE surface. A mechanistic study, using Li_6.4 La₃ Zr_1.4 Ta_0.6 O₁₂ (LLZTO) as a model SSE, reveals that Li₂ CO₃ resides within the grain boundaries of newly polished LLZTO pellet, which is difficult to remove and hinders the wetting process. The Li* melt can phase-transfer the Li₂ CO₃ from the LLZTO grain boundary to the Li*'s top surface, and therefore facilitates the wetting process. The obtained Li*|LLZTO demonstrates a low interfacial resistance, high rate capability, and long cycle life, and can find applications in future all-solid-state batteries (e.g., Li*|LLZTO|LiFePO₄ ).

Progress on Lithium Dendrite Suppression Strategies from the Interior to Exterior by Hierarchical Structure Designs

Lithium (Li) metal is promising for high energy density batteries due to its low electrochemical potential (-3.04 V) and high specific capacity (3860 mAh g^-1 ). However, the safety issues impede the commercialization of Li anode batteries. In this work, research of hierarchical structure designs for Li anodes to suppress Li dendrite growth and alleviate volume expansion from the interior (by the 3D current collector and host matrix) to the exterior (by the artificial solid electrolyte interphase (SEI), protective layer, separator, and solid state electrolyte) is concluded. The basic principles for achieving Li dendrite and volume expansion free Li anode are summarized. Following these principles, 3D porous current collector and host matrix are designed to suppress the Li dendrite growth from the interior. Second, artificial SEI, the protective layer, and separator as well as solid-state electrolyte are constructed to regulate the distribution of current and control the Li nucleation and deposition homogeneously for suppressing the Li dendrite growth from exterior of Li anode. Ultimately, this work puts forward that it is significant to combine the Li dendrite suppression strategies from the interior to exterior by 3D hierarchical structure designs and Li metal modification to achieve excellent cycling and safety performance of Li metal batteries.

Double-Shelled C@MoS2 Structures Preloaded with Sulfur: An Additive Reservoir for Stable Lithium Metal Anodes

The growth of Li dendrites hinders the practical application of lithium metal anodes (LMAs). In this work, a hollow nanostructure, based on hierarchical MoS₂ coated hollow carbon particles preloaded with sulfur (C@MoS₂ /S), was designed to modify the LMA. The C@MoS₂ hollow nanostructures serve as a good scaffold for repeated Li plating/stripping. More importantly, the encapsulated sulfur could gradually release lithium polysulfides during the Li plating/stripping, acting as an effective additive to promote the formation of a mosaic solid electrolyte interphase layer embedded with crystalline hybrid lithium-based components. These two factors together effectively suppress the growth of Li dendrites. The as-modified LMA shows a high Coulombic efficiency of 98 % over 500 cycles at the current density of 1 mA cm^-2 . When matched with a LiFePO₄ cathode, the assembled full cell displays a highly improved cycle life of 300 cycles, implying the feasibility of the proposed LMA.

Research Progresses of Garnet-Type Solid Electrolytes for Developing All-Solid-State Li Batteries

All-Solid-State Batteries (ASSBs) that use oxide-based solid electrolytes (SEs) have been considered as a promising energy-storage platform to meet an increasing demand for Li-ion batteries (LIBs) with improved energy density and superior safety. However, high interfacial resistance between particles in the composite electrode and between electrodes and the use of Li metal in the ASBS hinder their practical utilization. Here, we review recent research progress on oxide-based SEs for the ASSBs with respect to the use of Li metal. We especially focus on research progress on garnet-type solid electrolytes (Li₇La₃Zr₂O₁₂) because they have high ionic conductivity, good chemical stability with Li metal, and a wide electrochemical potential window. This review will also discuss Li dendritic behavior in the oxide-based SEs and its relationship with critical current density (CCD). We close with remarks on prospects of ASSB.

Multiscale Lithium-Battery Modeling from Materials to Cells

New experimental technology and theoretical approaches have advanced battery research across length scales ranging from the molecular to the macroscopic. Direct observations of nanoscale phenomena and atomistic simulations have enhanced the understanding of the fundamental electrochemical processes that occur in battery materials. This vast and ever-growing pool of microscopic data brings with it the challenge of isolating crucial performance-decisive physical parameters, an effort that often requires the consideration of intricate interactions across very different length scales and timescales. Effective physics-based battery modeling emphasizes the cross-scale perspective, with the aim of showing how nanoscale physicochemical phenomena affect device performance. This review surveys the methods researchers have used to bridge the gap between the nanoscale and the macroscale. We highlight the modeling of properties or phenomena that have direct and considerable impact on battery performance metrics, such as open-circuit voltage and charge/discharge overpotentials. Particular emphasis is given to thermodynamically rigorous multiphysics models that incorporate coupling between materials' mechanical and electrochemical states.

Probing the Mechanical Properties of a Doped Li7La3Zr2O12 Garnet Thin Electrolyte for Solid-State Batteries

Using the nanoindentation technique, we probed the mechanical properties of tape cast and sintered thin doped Li₇La₃Zr₂O₁₂ garnet electrolytes. For comparison, a bulk garnet sample fabricated by die pressing and sintering was also studied. The results indicate that the thin sample has a significantly higher elastic modulus (∼155 GPa), hardness (∼11 GPa), and indentation fracture toughness (∼1.12 ± 0.12 MPa·m^1/2) than the bulk sample (∼142 GPa, ∼10 GPa, and ∼0.97 ± 0.10 MPa·m^1/2, respectively). The above results demonstrate that the thin sample can more effectively prevent lithium dendrite penetration due to its better mechanical properties. Deformation and creep behavior analysis further indicates that the thin sample (1) has a higher resistance to withhold the charge/discharge stress and consequently deformation and (2) a lower creep exponent and likely high resistance to brittle failure.

Building an Air Stable and Lithium Deposition Regulable Garnet Interface from Moderate-Temperature Conversion Chemistry

Garnet-type electrolytes suffer from unstable chemistry against air exposure, which generates contaminants on electrolyte surface and accounts for poor interfacial contact with the Li metal. Thermal treatment of the garnet at >700 °C could remove the surface contaminants, yet it regenerates the contaminants in the air, and aggravates the Li dendrite issue as more electron-conducting defective sites are exposed. In a departure from the removal approach, here we report a new surface chemistry that converts the contaminants into a fluorinated interface at moderate temperature <180 °C. The modified interface shows a high electron tunneling barrier and a low energy barrier for Li⁺ surface diffusion, so that it enables dendrite-proof Li plating/stripping at a high critical current density of 1.4 mA cm^-2 . Moreover, the modified interface exhibits high chemical and electrochemical stability against air exposure, which prevents regeneration of contaminants and keeps high critical current density of 1.1 mA cm^-2 . The new chemistry presents a practical solution for realization of high-energy solid-state Li metal batteries.