Making Na-Ion Batteries Solid

Abstract

Along with the rapid increase of market penetration rate of electric vehicles (EVs) and the continuous increase in the capacity of installed energy storage systems (ESSs), problems associated with limited and unevenly distributed Li resources are becoming prominent with Li-ion batteries (LIBs) serving as the supporting technology. As an alternative, Na-ion batteries (NIBs) have been widely accepted as an effective new route to supplement the market, especially in the field of energy storage. (1−4) Owing to the great efforts and contributions from various groups over the world, NIBs are now stepping into commercialization with a bright future. In 2024, the first NIB energy storage systems, one with a capacity of 10 MWh (5) in Guangxi province and another with 100 MWh (6) in Hubei province, China, were successfully launched. (Figure 1). Figure 1. (a) 10 MWh and (b) 100 MWh Na-ion battery energy storage systems. Although NIBs are developing steadily and rapidly, thanks to the analogies in their principles and fabrication with LIBs, achieving even higher energy density, longer cycle life, and better safety is critical for the ESS applications. Therefore, a transition from liquid-state to solid-state NIBs is significant and necessary. Solid-state NIBs have some unique advantages compared to liquid-state batteries: 1) inorganic solid electrolytes ensure inherent nonflammability, which highly enhances the safety; 2) solid electrolytes show higher oxidation potential than many organic liquid electrolytes, promising a higher working voltage and energy density; and 3) due to the fluidity of liquid electrolytes, some side reactions continuously occur at the electrode–electrolyte interface during cycling, but when using solid electrolytes, interfacial side reactions can be impeded, and much longer lifespan is expected; and 4) again due to the fluidity of liquid electrolytes, it is easy for short-circuits to occur in the bipolar configuration, however because only aluminum foils are used as current collectors at both the cathode and anode sides, NIBs can be assembled as bipolar cells for higher voltage and energy density. Solid electrolytes make the fabrication of bipolar cells feasible and deliver better performance and lower cost. The key for the development of solid-state NIBs is the solid electrolyte material, which should possess high enough ionic conductivity and flexibility with proper contact with the electrodes to adapt to the strain and guarantee fast Na⁺ diffusion in the bulk and at the interface. Currently, similar to the case with solid-state LIBs, organic solid electrolytes, represented by polymers, and inorganic electrolytes, including oxides, sulfides, and halides, are the most studied types in NIB research. Polymer electrolytes usually have pliable properties with a deformable interface that can keep excellent contact between the electrode and electrolyte, but their room-temperature ionic conductivities require further increase. Oxide electrolytes exhibit wide electrochemical stability and are compatible with both cathode and anode materials, yet the biggest challenge with them is how to construct a proper electrode–electrolyte interface and reduce the grain boundary, owing to their mechanical rigidity. Sulfide electrolytes display superior room-temperature ionic conductivity of over 10 mS cm^–1; however, a limited electrochemical stability window and high air-sensitivity still restrict their commercialization. (7) In recent years, halide electrolytes have gained increasing attention because of their comprehensive properties including high ionic conductivity, a wide electrochemical window, and good deformability, offering a balance between oxide and sulfide electrolytes. These advantages are based on the anion chemistry of monovalent halogens. (8,9) However, although various halides have been investigated with superior ionic conductivity for Li systems, the analogues for Na systems usually show unsatisfactory performance. One efficient strategy to solve this problem is to design amorphous structures. The introduction of O into LiAlCl₄/NaAlCl₄ (called VIGLAS: Viscoelastic Inorganic GLASs) was reported to create amorphous structures with Al-O-Al chains and deliver a high ionic conductivity of over 1 mS/cm. More importantly, the added O can lower the glass transition temperature below room temperature, making the material viscoelastic, like polymers, and allowing better electrode–electrolyte interfacial contact (Figure 2). (10) Although compositing inorganic electrolytes with polymers is also a widely studied approach to simultaneously achieve high room-temperature ionic conductivity and flexibility to improve the interface, the incompatibility and the grain boundary between the inorganic and organic materials cause many problems in the composite design and manufacture. Therefore, the proposal of the use of VIGLAS is recognized as a milestone in that it not only merges the merits of inorganic and organic electrolytes but also has the lowest cost among the reported solid electrolytes. (11−13) Following high-energy mechanochemical reactions with long enough ball-milling time, NaTaCl₆ can deliver an ionic conductivity as high as 4 mS/cm due to the formation of reconstructed amorphous poly(TaCl₆). (14) A dual-anion sublattice of Na superionic glass, Na-Ta-Cl-O can exhibit higher ionic conductivity up to 4.62 mS/cm. (15,16) Figure 2. (a) VIGLAS solid electrolytes for Li and Na systems showing viscoelastic properties. (b) Ionic conductivity at different oxygen contents. Reproduced or adapted with permission from ref (10). Copyright 2023, Springer Nature. Another class of clay-like electrolytes for LIBs was also reported to have such amorphous structures, high ionic conductivity, and polymer-like pliability, (17−19) but an analogous Na⁺ conductor has not been obtained yet. However, it is worth noting that, because the two most urgent requirements of solid-state NIBs are fast Na diffusion and an excellent electrode–electrolyte interface, the design of such inorganic electrolyte materials with the synergy between high ionic conductivity and polymer-like viscoelasticity is a significant direction for future study. Based on the development of solid electrolytes, researchers are realizing that there are still bottlenecks in the state-of-art routines to obtain further higher ionic conductivity, especially for Na systems. For example, similar to the Li systems, halide electrolytes with close-packed configurations have limited ion transport in Na systems, so non-close-packed structures need to be explored, such as UCl₃-type materials. (8,20) A single system of polymers, sulfides, oxides, or halides may not fulfill all the requirements of the solid-state NIBs, and multisystem materials could be a future solution, such as the above-mentioned oxychlorides and even Li_9.54[Si_1−δM_δ]_1.74P_1.44S_11.1Br_0.3O_0.6 (M = Ge, Sn; 0 ≤ δ ≤ 1), which contains all the necessary S, O, and Br elements and shows the highest reported ionic conductivity of 32 mS/cm for solid-state batteries. (21) In addition to the solid electrolytes, the electrode materials also need to be carefully designed to adapt different types of solid electrolytes to ensure low interfacial resistance and fast ionic transport. In particular, metallic Na has a low melting point of ∼98 °C and is unstable even in dry air, (22) which may make it unsuitable due to safety issues. Therefore, more stable anode materials, such as carbons or carbon-alloy composites, should be considered. This also requires updating current solid anolytes to construct thermally and dynamically stable anode–electrolyte interfaces. (23,24) Overall, the study of solid-state NIBs is still in its starting stages, but their potential to meet the future demand for long-duration energy storage is clear. This makes them a promising area of research, deserving great efforts from the research and industry communities. This article references 24 other publications. This article has not yet been cited by other publications.