Interfacial Reactivity Benchmarking of the Sodium Ion Conductors Na3PS4 and Sodium β-Alumina for Protected Sodium Metal Anodes and Sodium All-Solid-State Batteries

Electrolytes for Sodium Ion Batteries: The Current Transition from Liquid to Solid and Hybrid systems

Sodium-ion batteries (NIBs) have recently garnered significant interest in being employed alongside conventional lithium-ion batteries, particularly in applications where cost and sustainability are particularly relevant. The rapid progress in NIBs will undoubtedly expedite the commercialization process. In this regard, tailoring and designing electrolyte formulation is a top priority, as they profoundly influence the overall electrochemical performance and thermal, mechanical, and dimensional stability. Moreover, electrolytes play a critical role in determining the system's safety level and overall lifespan. This review delves into recent electrolyte advancements from liquid (organic and ionic liquid) to solid and quasi-solid electrolyte (dry, hybrid, and single ion conducting electrolyte) for NIBs, encompassing comprehensive strategies for electrolyte design across various materials, systems, and their functional applications. The objective is to offer strategic direction for the systematic production of safe electrolytes and to investigate the potential applications of these designs in real-world scenarios while thoroughly assessing the current obstacles and forthcoming prospects within this rapidly evolving field.

Reinvestigation of Na5GdSi4O12: A Potentially Better Solid Electrolyte than Sodium β Alumina for Solid-State Sodium Batteries

Developing high-performing solid electrolytes that could replace flammable organic liquid electrolytes is vital in designing safer solid-state batteries. Among the sodium-ion (Na+) conducting solid electrolytes, Na-β″-alumina (BASE) is highly regarded for its employment in solid-state battery applications due to its high ionic conductivity and electrochemical stability. BASE has long been employed in commercial Na-NiCl2 and Na-S batteries. However, the synthesis of highly conductive BASE is energy-intensive, involving elevated temperatures for sintering and the incorporation of stabilizing additives. Additionally, BASE is highly sensitive to humidity, which limits its applications. Hence, there is an intense search to identify suitable high-performing solid electrolytes that could replace BASE. In this context, we reinvestigated Na5GdSi4O12 (NGS) and demonstrated that phase pure NGS could be synthesized by a simple solid-state reaction. Beyond a high ionic conductivity of 1.9 × 10-3 S cm-1 at 30 °C (1.5 × 10-3 S cm-1 for BASE), NGS exhibited high chemical as well as electrochemical stability, lower interfacial resistance, lower deposition and stripping potential, and higher short-circuiting current, designating NGS as a better solid electrolyte than BASE.

Bio-based ether solvent and ionic liquid electrolyte for sustainable sodium-air batteries

Sodium-air batteries (SABs) are receiving considerable attention for the development of next generation battery alternatives due to their high theoretical energy density (up to 1105 W h kg-1). However, most of the studies on this technology are still based on organic solvents; in particular, diglyme, which is highly flammable and toxic for the unborn child. To overcome these safety issues, this research investigates the first use of a branched ether solvent 1,2,3-trimethoxypropane (TMP) as an alternative electrolyte to diglyme for SABs. Through this work, the reactivity of the central tertiary carbon in TMP towards bare sodium metal was identified, while the addition of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyr][TFSI]) as a co-solvent proved to be an effective strategy to limit the reactivity. Moreover, a Na-β-alumina disk was employed for anode protection, to separate the TMP-based electrolyte from the sodium metal. The new cell design resulted in improved cell performance: discharge capacities of up to 1.92 and 2.31 mA h cm-2 were achieved for the 16.6 mol% NaTFSI in TMP and 16.6 mol% NaTFSI in TMP/[C4mpyr][TFSI] compositions, respectively. By means of SEM, Raman and 23Na NMR techniques, NaO2 cubes were identified to be the major discharge product for both electrolyte compositions. Moreover, the hybrid electrolyte was shown to hinder the formation of side-products during discharge - the ratio of NaO2 to side-products in the hybrid electrolyte was 2.4 compared with 0.8 for the TMP-based electrolyte - and a different charge mechanism for the dissolution of NaO2 cubes for each electrolyte was observed. The findings of this work demonstrate the high potential of TMP as a base solvent for SABs, and the importance of careful electrolyte composition design in order to step towards greener and less toxic batteries.

Inorganic Sodium Solid Electrolytes: Structure Design, Interface Engineering and Application

All-solid-state sodium batteries (ASSSBs) are particularly attractive for large-scale energy storage and electric vehicles due to their exceptional safety, abundant resource availability, and cost-effectiveness. The growing demand for ASSSBs underscores the significance of sodium solid electrolytes; However, the existed challenges of sodium solid electrolytes hinder their practical application despite continuous research efforts. Herein, recent advancements and the challenges for sodium solid electrolytes from material to battery level are reviewed. The in-depth understanding of their fundamental properties, synthesis techniques, crystal structures and recent breakthroughs is presented. Moreover, critical challenges on inorganic sodium solid electrolytes are emphasized, including the imperative need to enhance ionic conductivity, fortifying interfacial compatibility with anode/cathode materials, and addressing dendrite formation issues. Finally, potential applications of these inorganic sodium solid electrolytes are explored in ASSSBs and emerging battery systems, offering insights into future research directions.

Enhancing interface stability and ionic conductivity in the designed Na3SbP0.4xS4-xOx sulfide solid electrolyte through bridging oxygen

The all-solid-state sodium battery has emerged as a promising candidate for energy storage. However, the limited electrochemical stability of the solid electrolyte, particularly in the presence of Na metal at the anode, along with low ionic conductivity, hinders its widespread application. In this work, the design of P and O elements in Na3SbS4 solid electrolyte was investigated through a series of structural tests and characterizations. The electrochemical stability was remarkably improved in the Na/Na3SbP0.16S3.6O0.4/Na battery, exhibiting a stability of 260 h under a current of 0.1 mA cm-2. Additionally, the room temperature conductivity of Na3SbP0.16S3.6O0.4 was enhanced to 3.82 mS cm-1, maintaining a value comparable to commercial standards. The proposed design strategy provides an approach for developing sodium ion solid-state batteries with high energy density and long lifespan. The stability of the solid electrolyte interface at the Na | solid electrolyte interface proves critical for the successful assembly of all-solid-state sodium ion batteries.

Protective NaSICON Interlayer between a Sodium-Tin Alloy Anode and Sulfide-Based Solid Electrolytes for All-Solid-State Sodium Batteries

This paper presents a suitable combination of different sodium solid electrolytes to surpass the challenge of highly reactive cell components in sodium batteries. The focus is laid on the introduction of ceramic Na3.4Zr2Si2.4P0.6O12 serving as a protective layer for sulfide-based separator electrolytes to avoid the high reactivity with the sodium metal anode. The chemical instability of the anode|sulfide solid electrolyte interface is demonstrated by impedance spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The Na3.4Zr2Si2.4P0.6O12 disk shows chemical stability with the sodium metal anode as well as the sulfide solid electrolyte. Impedance analysis suggests an electrochemically stable interface. Electron microscopy points to a reaction at the Na3.4Zr2Si2.4P0.6O12 surface toward the sulfide solid electrolyte, which does not seem to affect the performance negatively. The results presented prove the chemical stabilization of the anode-separator interface using a Na3.4Zr2Si2.4P0.6O12 interlayer, which is an important step toward a sodium all-solid-state battery. Due to the applied pressure that is mandatory for battery cells with sulfide-based cathode composite, the use of a brittle ceramic in such cells remains challenging.

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.

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

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

Scalable Glass-Fiber-Polymer Composite Solid Electrolytes for Solid-State Sodium-Metal Batteries

In this work, we report a method for producing a thin (<50 μm), mechanically robust, sodium-ion conducting composite solid electrolyte (CSE) by infiltrating the monomers of polyethylene glycol diacrylate (PEGDA) and polyethylene glycol (PEG) and either NaClO₄ or NaFSI salt into a silica-based glass-fiber matrix, followed by an UV-initiated in situ polymerization. The glass fiber matrix provided mechanical strength to the CSE and enabled a robust, self-supporting separator. This strategy enabled the development of CSEs with high loadings of PEG as a plasticizer to enhance the ionic conductivity. The fabrication of these CSEs was done under ambient conditions, which was highly scalable and can be easily implemented in roll-to-roll processing. While NaClO₄ was found to be unstable with the sodium-metal anode, the use of a NaFSI salt was found to promote stable stripping and plating in a symmetric cell, reaching current densities of as high as 0.67 mA cm^-2 at 60 °C. The PEGDA + PEG + NaFSI separators were then used to form solid-state full cells with a cobalt-free, low-nickel layered Na_2/3Ni_1/3Mn_2/3O₂ cathode and a sodium-metal anode, achieving a full capacity utilization exhibiting 70% capacity retention after 50 cycles at a cycling rate of C/5 at 60 °C.

Operando Characterization and Theoretical Modeling of Metal|Electrolyte Interphase Growth Kinetics in Solid-State Batteries. Part I: Experiments

To harness all of the benefits of solid-state battery (SSB) architectures in terms of energy density, their negative electrode should be an alkali metal. However, the high chemical potential of alkali metals makes them prone to reduce most solid electrolytes (SE), resulting in a decomposition layer called an interphase at the metal|SE interface. Quantitative information about the interphase chemical composition and rate of formation is challenging to obtain because the reaction occurs at a buried interface. In this study, a thin layer of Na metal (Na⁰) is plated on the surface of an SE of the NaSICON family (Na_3.4Zr₂Si_2.4P_0.6O₁₂ or NZSP) inside a commercial X-ray photoelectron spectroscopy (XPS) system while continuously analyzing the composition of the interphase operando. We identify the existence of a solid electrolyte interphase at the Na⁰|NZSP interface, and more importantly, we demonstrate for the first time that this protocol can be used to study the kinetics of interphase formation. A second important outcome of this article is that the surface chemistry of NZSP samples can be tuned to improve their stability against Na⁰. It is demonstrated by XPS and time-resolved electrochemical impedance spectroscopy (EIS) that a native Na _x PO _y layer present on the surface of as-sintered NZSP samples protects their surface against decomposition.

Operando Characterization and Theoretical Modeling of Metal|Electrolyte Interphase Growth Kinetics in Solid-State Batteries. Part II: Modeling

Understanding the interfacial dynamics of batteries is crucial to control degradation and increase electrochemical performance and cycling life. If the chemical potential of a negative electrode material lies outside of the stability window of an electrolyte (either solid or liquid), a decomposition layer (interphase) will form at the interface. To better understand and control degradation at interfaces in batteries, theoretical models describing the rate of formation of these interphases are required. This study focuses on the growth kinetics of the interphase forming between solid electrolytes and metallic negative electrodes in solid-state batteries. More specifically, we demonstrate that the rate of interphase formation and metal plating during charge can be accurately described by adapting the theory of coupled ion-electron transfer (CIET). The model is validated by fitting experimental data presented in the first part of this study. The data was collected operando as a Na metal layer was plated on top of a NaSICON solid electrolyte (Na_3.4Zr₂Si_2.4P_0.6O₁₂ or NZSP) inside an XPS chamber. This study highlights the depth of information which can be extracted from this single operando experiment and is widely applicable to other solid-state electrolyte systems.

Electrolytes in Organic Batteries

Organic batteries using redox-active polymers and small organic compounds have become promising candidates for next-generation energy storage devices due to the abundance, environmental benignity, and diverse nature of organic resources. To date, tremendous research efforts have been devoted to developing advanced organic electrode materials and understanding the material structure-performance correlation in organic batteries. In contrast, less attention was paid to the correlation between electrolyte structure and battery performance, despite the critical roles of electrolytes for the dissolution of organic electrode materials, the formation of the electrode-electrolyte interphase, and the solvation/desolvation of charge carriers. In this review, we discuss the prospects and challenges of organic batteries with an emphasis on electrolytes. The differences between organic and inorganic batteries in terms of electrolyte property requirements and charge storage mechanisms are elucidated. To provide a comprehensive and thorough overview of the electrolyte development in organic batteries, the electrolytes are divided into four categories including organic liquid electrolytes, aqueous electrolytes, inorganic solid electrolytes, and polymer-based electrolytes, to introduce different components, concentrations, additives, and applications in various organic batteries with different charge carriers, interphases, and separators. The perspectives and outlook for the future development of advanced electrolytes are also discussed to provide a guidance for the electrolyte design and optimization in organic batteries. We believe that this review will stimulate an in-depth study of electrolytes and accelerate the commercialization of organic batteries.

Degradation at the Na3SbS4/Anode Interface in an Operating All-Solid-State Sodium Battery

All-solid-state sodium batteries utilize earth-abundant elements and are sustainable systems for large-scale energy storage and electric transportation. Replacing flammable carbonate-based electrolytes with solid-state ionic conductors promotes battery safety. Using solid-state electrolytes (SEs) also eliminates the need for packing when fabricating tandem cells, potentially enabling further enhanced energy density. Na₃SbS₄, a Na⁺ conductor, remains stable in dry air and shows high Na⁺ conductivity (σ ≈ 1.0 × 10^-3 S/cm) and is thus a promising SE for applications in sodium batteries. However, upon repeated electrochemical cycling, Na₃SbS₄-containing Na batteries exhibit decaying capacity and limited cycle life, which is likely associated with the decomposition of Na₃SbS₄ at the electrode/electrolyte interface. This work presents an in-depth analysis of the decomposition chemistry occurring at the Na₃SbS₄/anode interface using combined in situ Raman and post-mortem characterization. The results indicate that the SbS₄^3- counterion is electrochemically reduced when experiencing Na⁺ reduction potentials, and this reduction chemistry likely follows multiple pathways. The observed reduction products include SbS₃^3-, the Sb₂S₇^4- dimer, the NaSb binary phase, and Na₂S. We also observed the irreversibility of the decomposition and, as a consequence, the accumulation of the degradation products over cycles. Also notable is the heterogeneity of this degradation chemistry across the interface. Through the spectroelectrochemical characterizations, we reveal the possible mechanisms of the Na₃SbS₄ decomposition at the solid electrolyte/anode interface in an operating device.

Evaluating Electrolyte-Anode Interface Stability in Sodium All-Solid-State Batteries

All-solid-state batteries have recently gained considerable attention due to their potential improvements in safety, energy density, and cycle-life compared to conventional liquid electrolyte batteries. Sodium all-solid-state batteries also offer the potential to eliminate costly materials containing lithium, nickel, and cobalt, making them ideal for emerging grid energy storage applications. However, significant work is required to understand the persisting limitations and long-term cyclability of Na all-solid-state-based batteries. In this work, we demonstrate the importance of careful solid electrolyte selection for use against an alloy anode in Na all-solid-state batteries. Three emerging solid electrolyte material classes were chosen for this study: the chloride Na_2.25Y_0.25Zr_0.75Cl₆, sulfide Na₃PS₄, and borohydride Na₂(B₁₀H₁₀)_0.5(B₁₂H₁₂)_0.5. Focused ion beam scanning electron microscopy (FIB-SEM) imaging, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS) were utilized to characterize the evolution of the anode-electrolyte interface upon electrochemical cycling. The obtained results revealed that the interface stability is determined by both the intrinsic electrochemical stability of the solid electrolyte and the passivating properties of the formed interfacial products. With appropriate material selection for stability at the respective anode and cathode interfaces, stable cycling performance can be achieved for Na all-solid-state batteries.

Metallic Sodium Anodes for Advanced Sodium Metal Batteries: Progress, Challenges and Perspective

Sodium (Na)-based batteries, as the ideal choice of large-scale and low-cost energy storage, have attracted much attention. Na metal anodes with high theoretical specific capacity and low potential are considered to be one of the most promising anodes for next-generation Na-based batteries. However, the high reactivity of Na metal anodes makes the electrode/electrolyte phase unstable, resulting in formation of Na dendrites, short cycle life and safety problems. Herein, the contribution outlines the latest development of Na metal anodes for Na metal batteries. The design strategies for high efficiency utilization of Na metal anodes are elucidated, including sophisticated electrode construction, liquid electrolyte optimization, electrode/electrolyte interface stabilization, and solid electrolyte adaptation. Finally, the future research direction and existing problems are proposed.

Formation of Passivate Interphases by Na3BS3-Glass Solid Electrolytes in All-Solid-State Sodium-Metal Batteries

Interphase formation at the interface between a solid electrolyte and negative electrode is one of the main factors limiting the practical use of all-solid-state sodium batteries. Sulfide-type solid electrolytes with group 15 elements (P and Sb) exhibit high ductility and ionic conductivity, comparable to those of organic liquid electrolytes. However, the electronically conductive interphase formed at the interface between Na₃PS₄ and sodium metal increases the cell resistance and deteriorates its electrochemical properties. Contrarily, Na₃BS₃, containing boron as an electrochemically inert element, forms an electronically insulating thin passivate interphase, facilitating reversible sodium plating and stripping. Sodium-metal symmetric cells with Na₃BS₃ exhibit steady operation over 1000 cycles. Thus, reduction-stable solid electrolytes can be developed by substitution with an electrochemically inert element versus sodium.

An electrochemically stable homogeneous glassy electrolyte formed at room temperature for all-solid-state sodium batteries

All-solid-state sodium batteries (ASSSBs) are promising candidates for grid-scale energy storage. However, there are no commercialized ASSSBs yet, in part due to the lack of a low-cost, simple-to-fabricate solid electrolyte (SE) with electrochemical stability towards Na metal. In this work, we report a family of oxysulfide glass SEs (Na₃PS_4−xO_x, where 0 < x ≤ 0.60) that not only exhibit the highest critical current density among all Na-ion conducting sulfide-based SEs, but also enable high-performance ambient-temperature sodium-sulfur batteries. By forming bridging oxygen units, the Na₃PS_4−xO_x SEs undergo pressure-induced sintering at room temperature, resulting in a fully homogeneous glass structure with robust mechanical properties. Furthermore, the self-passivating solid electrolyte interphase at the Na|SE interface is critical for interface stabilization and reversible Na plating and stripping. The new structural and compositional design strategies presented here provide a new paradigm in the development of safe, low-cost, energy-dense, and long-lifetime ASSSBs.

Single sodium-ion solid electrolyte that meets the requirements of practical applications is difficult to design. Here, the authors show how kinetic stability via the creation of a self-passivating solid electrolyte interphase allows a homogenous glass solid electrolyte to exhibit remarkable electrochemical stability with sodium metal.

Influence of Porosity of Sulfide-Based Artificial Solid Electrolyte Interphases on Their Performance with Liquid and Solid Electrolytes in Li and Na Metal Batteries

Realization of all-solid-state batteries combined with metallic Li/Na is still hindered due to the unstable interface between the alkali metal and solid electrolytes, especially for highly promising thiophosphate materials. Artificial and uniform solid-electrolyte interphases (SEIs), serving as thin ion-conducting films, have been considered as a strategy to overcome the issues of such reactive interfaces. Here, we synthesized sulfide-based artificial SEIs (Li_xS_y and Na_xS_y) on Li and Na by solid/gas reaction between the alkali metal and S vapor. The synthesized films are carefully characterized with various chemical/electrochemical techniques. We show that these artificial SEIs are not beneficial from an application point of view since they either contribute to additional resistances (Li) or do not prevent reactions at the alkali metal/electrolyte interface (Na). We show that Na_xS_y is more porous than Li_xS_y, supported by (i) its rough morphology observed by focused ion beam-scanning electron microscopy, (ii) the rapid decrease of R_interface (interfacial resistance) in Na_xS_y-covered-Na symmetric cells with liquid electrolyte upon aging under open-circuit potential, and (iii) the increase of R_interface in Na_xS_y-covered-Na solid-state symmetric cells with Na₃PS₄ electrolyte. The porous SEI allows the penetration of liquid electrolyte or alkali metal creep through its pores, resulting in a continuous chemical reaction. Hence, porosity of SEIs in general should be carefully taken into account in the application of batteries containing both liquid electrolyte and solid electrolyte.

Machine Learning enabled High-Throughput Screening of Inorganic Solid Electrolytes for Regulating Dendritic Growth in Lithium Metal Anodes

The Li-S secondary battery system has gained popularity owing to their advantage of higher specific energy compared to the Li ion battery. However, it suffers majorly due to the Li...

Computational Auxiliary for the Progress of Sodium-Ion Solid-State Electrolytes

All-solid-state sodium batteries (ASSBs) have attracted ever-increasing attention due to their enhanced safety, high energy density, and the abundance of raw materials. One of the remaining key issues for the practical ASSB is the lack of good superionic and electrochemical stable solid-state electrolytes (SEs). Design and manufacturing specific functional materials used as high-performance SEs require an in-depth understanding of the transport mechanisms and electrochemical properties of fast sodium-ion conductors on an atomic level. On account of the continuous progress and development of computing and programming techniques, the advanced computational tools provide a powerful and convenient approach to exploit particular functional materials to achieve that aim. Herein, this review primarily focuses on the advanced computational methods and ion migration mechanisms of SEs. Second, we overview the recent progress on state-of-the-art solid sodium-ion conductors, including Na-β-alumina, sulfide-type, NASICON-type, and antiperovskite-type sodium-ion SEs. Finally, we outline the current challenges and future opportunities. Particularly, this review highlights the contributions of the computational studies and their complementarity with experiments in accelerating the study progress of high-performance sodium-ion SEs for ASSBs.

The rise of X-ray spectroscopies for unveiling the functional mechanisms in batteries

Synchrotron-based techniques have been key tools in the discovery, understanding, and development of battery materials. In this review, some of the most suitable X-ray spectroscopy related techniques employed for addressing diverse scientific cases connected to battery science are highlighted. Furthermore, current shortcomings, intrinsic limitations, and ongoing challenges of individual techniques are pointed out, providing an outlook of future trends that are relevant to the battery research community. In particular, the ongoing development of next generation synchrotrons, machine learning algorithms for data analysis and combined theoretical/experimental approaches will enhance the already powerful assets of these advanced spectroscopic methods.

Review on Interface and Interphase Issues in Sulfide Solid-State Electrolytes for All-Solid-State Li-Metal Batteries

All-solid-state batteries have emerged as promising alternatives to conventional Li-ion batteries owing to their higher energy density and safety, which stem from their use of inorganic solid-state electrolytes instead of flammable organic liquid electrolytes. Among various candidates, sulfide solid-state electrolytes are particularly promising for the development of high-energy all-solid-state Li metal batteries because of their high ionic conductivity and deformability. However, a significant challenge remains as their inherent instability in contact with electrodes forms unstable interfaces and interphases, leading to degradation of the battery performance. In this review article, we provide an overview of the key issues for the interfaces and interphases of sulfide solid-state electrolyte systems as well as recent progress in understanding such interface and interphase formation and potential solutions to stabilize them. In addition, we provide perspectives on future research directions in this field.

Visualizing plating-induced cracking in lithium-anode solid-electrolyte cells

Lithium dendrite (filament) propagation through ceramic electrolytes, leading to short circuits at high rates of charge, is one of the greatest barriers to realizing high-energy-density all-solid-state lithium-anode batteries. Utilizing in situ X-ray computed tomography coupled with spatially mapped X-ray diffraction, the propagation of cracks and the propagation of lithium dendrites through the solid electrolyte have been tracked in a Li/Li₆PS₅Cl/Li cell as a function of the charge passed. On plating, cracking initiates with spallation, conical 'pothole'-like cracks that form in the ceramic electrolyte near the surface with the plated electrode. The spallations form predominantly at the lithium electrode edges where local fields are high. Transverse cracks then propagate from the spallations across the electrolyte from the plated to the stripped electrode. Lithium ingress drives the propagation of the spallation and transverse cracks by widening the crack from the rear; that is, the crack front propagates ahead of the Li. As a result, cracks traverse the entire electrolyte before the Li arrives at the other electrode, and therefore before a short circuit occurs.

Novel Solid-State Sodium-Ion Battery with Wide Band Gap NaTi2(PO4)3 Nanocrystal Electrolyte

NaTi2(PO4)3 (NTP), a well-known anode material, could be used as a solid wide-band gap electrolyte. Herein, a novel solid-state sodium-ion battery (SSIB) with the thickness of electrolyte up to the millimeter level is proposed. The results of the difference in charge density investigated by the first-principles calculations imply that using the NTP nanocrystals as electrolytes to transport sodium ions is feasible. Moreover, the SSIB exhibits a high initial discharge capacity of 3250 mAh g-1 at the current density of 50 mA g-1. As compared with other previously reported SSIBs, our results are better than those reported and suggest that the NTP nanocrystals have potential application in SSIBs as solid electrolytes.

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.

Lithium-Metal Anode Instability of the Superionic Halide Solid Electrolytes and the Implications for Solid-State Batteries

Owing to high ionic conductivity and good oxidation stability, halide‐based solid electrolytes regain interest for application in solid‐state batteries. While stability at the cathode interface seems to be given, the stability against the lithium metal anode has not been explored yet. Herein, the formation of a reaction layer between Li₃InCl₆ (Li₃YCl₆) and lithium is studied by sputter deposition of lithium metal and subsequent in situ X‐ray photoelectron spectroscopy as well as by impedance spectroscopy. The interface is thermodynamically unstable and results in a continuously growing interphase resistance. Additionally, the interface between Li₃InCl₆ and Li₆PS₅Cl is characterized by impedance spectroscopy to discern whether a combined use as cathode electrolyte and separator electrolyte, respectively, might enable long‐term stable and low impedance operation. In fact, oxidation stable halide‐based lithium superionic conductors cannot be used against Li, but may be promising candidates as cathode electrolytes.

Theoretical study of Na+ transport in the solid-state electrolyte Na3OBr based on deep potential molecular dynamics

Deep potential molecular dynamics is used to study Na⁺ transport in Na₃OBr.

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.

Printable, high-performance solid-state electrolyte films

Current ceramic solid-state electrolyte (SSE) films have low ionic conductivities (10^-8 to 10^-5 S/cm ), attributed to the amorphous structure or volatile Li loss. Herein, we report a solution-based printing process followed by rapid (~3 s) high-temperature (~1500°C) reactive sintering for the fabrication of high-performance ceramic SSE films. The SSEs exhibit a dense, uniform structure and a superior ionic conductivity of up to 1 mS/cm. Furthermore, the fabrication time from precursor to final product is typically ~5 min, 10 to 100 times faster than conventional SSE syntheses. This printing and rapid sintering process also allows the layer-by-layer fabrication of multilayer structures without cross-contamination. As a proof of concept, we demonstrate a printed solid-state battery with conformal interfaces and excellent cycling stability. Our technique can be readily extended to other thin-film SSEs, which open previously unexplores opportunities in developing safe, high-performance solid-state batteries and other thin-film devices.

Tuning Rate-Limiting Factors to Achieve Ultrahigh-Rate Solid-State Sodium-Ion Batteries

Exhibiting superior safety features and low costs, solid-state sodium (Na)-ion batteries have been proposed as an attractive candidate for energy storage. However, the poor rate capability of solid-state batteries has limited their applications. In this work, an all-solid-state Na-ion battery is fabricated, delivering an unprecedented rate capability (60% capacity retention at a C-rate of 100 C with an areal loading of 1.5 mg cm^-2), which far exceeds other reports so far. More importantly, it is further demonstrated that instead of the Na-ion conductivity of the solid electrolyte, the rate-limiting factors are determined to be charge-transfer resistance at electrode/solid electrolyte interfaces and lack of percolation pathways in the electrode, which can be optimized by tuning the electrode design and testing protocols.

Material and Interfacial Modification toward a Stable Room-Temperature Solid-State Na-S Battery

Room-temperature solid-state sodium batteries have the remarkable potential to simultaneously achieve high safety, high energy density, and low cost. However, their current performance is far below expectations. Through material and interfacial modification based on Na₃PS₄ solid electrolytes, progress is made toward stable room-temperature solid-state sodium-sulfur (Na-S) batteries. First, the ionic liquid N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₄FSI) is employed to modify the anode/electrolyte interface. An overpotential of 0.55 V after 900 h of a symmetrical battery indicates enhanced interfacial stability. A stable in situ solid electrolyte interphase layer is formed at the interface of NaSn alloy and Na₃PS₄, proved by X-ray photoelectron spectroscopy measurements. Furthermore, selenium-doped sulfurized polyacrylonitrile (Se_0.05S_0.95@pPAN) is used to boost the ionic and electronic conductivities of the sulfur cathode. As a result, the Na-S battery using a Se_0.05S_0.95@pPAN cathode and the interfacial modification delivers stable cycle performance and enhanced rate capability.

Design Strategies to Enable the Efficient Use of Sodium Metal Anodes in High-Energy Batteries

Sodium-based batteries have attracted considerable attention and are recognized as ideal candidates for large-scale and low-cost energy storage. Sodium (Na) metal anodes are considered as one of the most promising anodes for next-generation, high-energy, Na-based batteries owing to their high theoretical specific capacity (1166 mA h g^-1 ) and low standard electrode potential. Herein, an overview of the recent developments in Na metal anodes for high-energy batteries is provided. The high reactivity and large volume expansion of Na metal anodes during charge and discharge make the electrode/electrolyte interphase unstable, leading to the formation of Na dendrites, short cycle life, and safety issues. Design strategies to enable the efficient use of Na metal anodes are elucidated, including liquid electrolyte engineering, electrode/electrolyte interface optimization, sophisticated electrode construction, and solid electrolyte engineering. Finally, the remaining challenges and future research directions are identified. It is hoped that this progress report will shape a consistent view of this field and provide inspiration for future research to improve Na metal anodes and enable the development of high-energy sodium batteries.

Structural Polymorphism in Na4Zn(PO4)2 Driven by Rotational Order-Disorder Transitions and the Impact of Heterovalent Substitutions on Na-Ion Conductivity

Solid electrolytes have regained tremendous interest recently in light of the exposed vulnerability of current rechargeable battery technologies. While designing solid electrolytes, most efforts concentrated on creating structural disorder (vacancies, interstitials, etc.) in a cationic Li/Na sublattice to increase ionic conductivity. In phosphates, the ionic conductivity can also be increased by rotational disorder in the anionic sublattice, via a paddle-wheel mechanism. Herein, we report on Na₄Zn(PO₄)₂ which is designed from Na₃PO₄, replacing Na⁺ with Zn²⁺ and introducing a vacancy for charge balance. We show that Na₄Zn(PO₄)₂ undergoes a series of structural transitions under temperature, which are associated with an increase in ionic conductivity by several orders of magnitude. Our detailed crystallographic study, combining electron, neutron, and X-ray powder diffraction, reveals that the room-temperature form, α-Na₄Zn(PO₄)₂, contains orientationally ordered PO₄ groups, which undergo partial and full rotational disorder in the high-temperature β- and γ-polymorphs, respectively. We furthermore showed that the highly conducting γ-polymorph could be stabilized at room temperature by ball-milling, whereas the β-polymorph can be stabilized by partial substitution of Zn²⁺ with Ga³⁺ and Al³⁺. These findings emphasize the role of rotational disorder as an extra parameter to design new solid electrolytes.

New Concepts in Electrolytes

Over the past decades, Li-ion battery (LIB) has turned into one of the most important advances in the history of technology due to its extensive and in-depth impact on our life. Its omnipresence in all electric vehicles, consumer electronics and electric grids relies on the precisely tuned electrochemical dynamics and interactions among the electrolytes and the diversified anode and cathode chemistries therein. With consumers' demand for battery performance ever increasing, more and more stringent requirements are being imposed upon the established equilibria among these LIB components, and it became clear that the state-of-the-art electrolyte systems could no longer sustain the desired technological trajectory. Driven by such gap, researchers started to explore more unconventional electrolyte systems. From superconcentrated solvent-in-salt electrolytes to solid-state electrolytes, the current research realm of novel electrolyte systems has grown to unprecedented levels. In this review, we will avoid discussions on current state-of-the-art electrolytes but instead focus exclusively on unconventional electrolyte systems that represent new concepts.

Sodium/Na β″ Alumina Interface: Effect of Pressure on Voids

Three-electrode studies coupled with tomographic imaging of the Na/Na-β″-alumina interface reveal that voids form in the Na metal at the interface on stripping and they accumulate on cycling, leading to increasing interfacial current density, dendrite formation on plating, short circuit, and cell failure. The process occurs above a critical current for stripping (CCS) for a given stack pressure, which sets the upper limit on current density that avoids cell failure, in line with results for the Li/solid-electrolyte interface. The pressure required to avoid cell failure varies linearly with current density, indicating that Na creep rather than diffusion per se dominates Na transport to the interface and that significant pressures are required to prevent cell death, >9 MPa at 2.5 mA·cm^-2.

Fundamentals of inorganic solid-state electrolytes for batteries

In the critical area of sustainable energy storage, solid-state batteries have attracted considerable attention due to their potential safety, energy-density and cycle-life benefits. This Review describes recent progress in the fundamental understanding of inorganic solid electrolytes, which lie at the heart of the solid-state battery concept, by addressing key issues in the areas of multiscale ion transport, electrochemical and mechanical properties, and current processing routes. The main electrolyte-related challenges for practical solid-state devices include utilization of metal anodes, stabilization of interfaces and the maintenance of physical contact, the solutions to which hinge on gaining greater knowledge of the underlying properties of solid electrolyte materials.

Building Better Batteries in the Solid State: A Review

Most of the current commercialized lithium batteries employ liquid electrolytes, despite their vulnerability to battery fire hazards, because they avoid the formation of dendrites on the anode side, which is commonly encountered in solid-state batteries. In a review two years ago, we focused on the challenges and issues facing lithium metal for solid-state rechargeable batteries, pointed to the progress made in addressing this drawback, and concluded that a situation could be envisioned where solid-state batteries would again win over liquid batteries for different applications in the near future. However, an additional drawback of solid-state batteries is the lower ionic conductivity of the electrolyte. Therefore, extensive research efforts have been invested in the last few years to overcome this problem, the reward of which has been significant progress. It is the purpose of this review to report these recent works and the state of the art on solid electrolytes. In addition to solid electrolytes stricto sensu, there are other electrolytes that are mainly solids, but with some added liquid. In some cases, the amount of liquid added is only on the microliter scale; the addition of liquid is aimed at only improving the contact between a solid-state electrolyte and an electrode, for instance. In some other cases, the amount of liquid is larger, as in the case of gel polymers. It is also an acceptable solution if the amount of liquid is small enough to maintain the safety of the cell; such cases are also considered in this review. Different chemistries are examined, including not only Li-air, Li-O2, and Li-S, but also sodium-ion batteries, which are also subject to intensive research. The challenges toward commercialization are also considered.

Stabilizing the Interface between Sodium Metal Anode and Sulfide-Based Solid-State Electrolyte with an Electron-Blocking Interlayer

Sulfide-based Na-ion conductors are promising electrolytes for all-solid-state sodium batteries (ASSSBs) because of high ionic conductivity and favorable formability. However, no effective strategy has been reported for long-duration Na cycling with sulfide-based electrolytes because of interfacial challenges. Here we demonstrate that a cellulose-poly(ethylene oxide) (CPEO) interlayer can stabilize the interface between sulfide electrolyte (Na₃SbS₄) and Na by shutting off the electron pathway of the electrolyte decomposition reaction. As a result, we achieved stable Na plating/stripping for 800 cycles at 0.1 mA cm^-2 in all-solid-state devices at 60 °C.

Sodium Superionic Conductors Based on Clusters

Owing to the high abundance and low cost of sodium (Na), Na-based rechargeable batteries hold great potential for large-scale applications in the future energy industry. However, as key component of the battery electrolyte, only a few Na-based superionic conductors can reach the ionic conductivity comparable to that of liquid or gel electrolytes. Here, we provide a guideline for the development of cluster-based Na-rich antiperovskite superionic conductors using computational studies. With a selected cluster ion BCl₄^-, we are able to achieve high-room-temperature Na-ionic conductivity over 10^-3 S/cm and low activation energies below 0.2 eV in the antiperovskite crystals Na₃S(BCl₄) and Na₃S(BCl₄)_0.5I_0.5. In addition, these materials have large bandgaps and favorable mechanical properties. A comprehensive study of the stability and formation energy of these materials further illustrates possible routes for their synthesis. New insights into the conduction mechanism of these cluster-based superionic conductors are provided, including the cooperative motion of Na ions and the significant reduction of the migration barrier due to the changing orientation of the cluster ion.

Long Cycle Life All-Solid-State Sodium Ion Battery

All-solid-state sodium ion batteries (ASIBs) based on sulfide electrolytes are considered a promising candidate for large-scale energy storage. However, the limited cycle life of ASIBs largely restricts their practical application. Cycling-stable ASIBs can be achieved only if the designed cathode can simultaneously address challenges including insufficient interfacial contact, electrochemical and chemical instability between the electrode and electrolyte, and strain/stress during operation , rather than just addressing one or part of these challenges. Chevrel phase Mo₆S₈ has inherent high electronic conductivity and small volume change during sodiation/desodiation, and is chemically and electrochemically stable with the sulfide electrolyte, and therefore the only challenge of using Mo₆S₈ as the cathode for ASIBs is the insufficient contact between Mo₆S₈ and the solid electrolyte (SE). Herein, a thin layer of SE is coated on Mo₆S₈ using a solution method to achieve an intimate contact between Mo₆S₈ and the SE. Such a SE-coated Mo₆S₈ cathode enabled an ASIB with a high cycling performance (500 cycles), even much better than that of the liquid-electrolyte batteries with the Mo₆S₈ cathode. This work provides valuable insights for developing long-cycle life ASIBs.