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Aqueous sodium-ion batteries (ASIBs) represent a promising battery technology for stationary energy storage, due to their attractive merits of low cost, high abundance, and inherent safety. Recently, a variety of advanced cathode, anode, and electrolyte materials have been developed for ASIBs, which not only enhance our fundamental understanding of the Na insertion mechanism, but also facilitate the research and development of practical ASIB systems. Among these electrode materials, iron-based materials are of particular importance because of the high abundance, low price, and low toxicity of Fe elements. However, to our knowledge, there are no review papers that specifically discuss the properties of Fe-based materials for ASIBs yet. In this review, we present the recent research progress on Fe-based cathode/anode materials, which include polyanionic compounds, Prussian blue, oxides, carbides, and selenides. We also discuss the research efforts to build Fe-based ASIB full cells. Lastly, we share our perspectives on the key challenges that need to be addressed and suggest alternative directions for aqueous Na-ion batteries. We hope this review paper can promote more research efforts on the development of low-cost and low-toxicity materials for aqueous battery applications. 

Abstract High‐voltage lithium metal batteries with nickel‐rich oxide cathodes (LiNi_0.8Co_0.1Mn_0.1O₂, NCM811) represent one of the most promising approaches to achieve high energy density up to 500 Wh kg⁻¹. However, severe interfacial side reactions occur at both NCM811 cathode and lithium anode at ultrahigh voltages (>4.6 V). To address these issues, various electrolytes have been developed, but they still suffer from electrolyte decomposition, leading to moderate voltages and insufficient cycling. Herein, we introduce (3,3,3‐trifluoropropyl)trimethoxy silane (TTMS) as an asymmetrically fluorinated single solvent, which incorporates both strongly solvating (─OCH₃) and weakly solvating (─CF₃) groups. The designed 2.1 mol L⁻¹(M) LiFSI/TTMS electrolyte achieves excellent compatibility with both NCM811 cathode and Li metal anode due to its unique anion‐dominating solvation structures and inorganic‐rich interphase formation. Consequently, it enables stable cycling in the Li||NCM811 battery at an ultrahigh voltage of 4.8 V, with 84.5% capacity retention after 300 cycles. Even under more aggressive conditions, including high temperature (60 °C) and anode‐less configuration (N/P ratio = 1.76), the Li||NCM811 battery exhibits remarkable capacity retention (>80%) over 300 cycles. This work underscores the effectiveness of electrolyte engineering for developing ultrahigh‐voltage and long‐cycling battery systems. 

Abstract Solid‐state batteries (SSBs) are competitive contenders for energy storage due to their inherent safety and high energy. However, the lack of an appropriate anode has hindered their development. Graphite and lithium metal are widely used anode materials, but graphite suffers from a low capacity, whereas lithium metal presents severe dendrite and reactivity challenges. Herein, the promising performance of micro‐sized alloys is demonstrated as high‐capacity and long‐cycling anodes for SSBs. Using antimony as a model anode, its full theoretical capacity (660 mAh g⁻¹), high‐rate capability (3 A g⁻¹), and long cycling life (1000–2000 cycles) is achieved at room temperature. Comparative studies further reveal an overlooked “micro‐size effect”, where micro‐sized alloys establish more efficient electron/ion conduction pathways, significantly exceeding their nano‐sized counterparts. This micro‐size effect challenges the conventional belief that nano‐sized alloys always outperform micro‐sized ones. Based on this discovery, similarly high performance of other micro‐alloys (lead and bismuth) in SSBs is further demonstrated. Given the additional benefits of easy synthesis, low cost, high tap density, and high stability, micro‐sized alloys hold great promise as excellent anode candidates for SSBs. 

Abstract The solid–solid electrode–electrolyte interface represents an important component in solid‐state batteries (SSBs), as ionic diffusion, reaction, transformation, and restructuring could all take place. As these processes strongly influence the battery performance, studying the evolution of the solid–solid interfaces, particularly in situ during battery operation, can provide insights to establish the structure–property relationship for SSBs. Synchrotron X‐ray techniques, owing to their unique penetration power and diverse approaches, are suitable to investigate the buried interfaces and examine structural, compositional, and morphological changes. In this review, we will discuss various surface‐sensitive synchrotron‐based scattering, spectroscopy, and imaging methods for the in situ characterization of solid–solid interfaces and how this information can be correlated to the electrochemical properties of SSBs. The goal is to overview the advantages and disadvantages of each technique by highlighting representative examples, so that similar strategies can be applied by battery researchers and beyond to study similar solid‐solid interface systems.