Research Advances in Polyanion-Type Cathodes for Sodium-Ion Batteries

Because of their high energy density and long cycle life, lithium-ion batteries (LIBs) have dominated the portable electronics market for over 20 years. However, with the increasing demand for large-scale energy storage systems for grid applications, the price of Li resources has increased owing to the low abundance of Li in Earth's crust and non-uniform distribution on the planet. Because Na has similar physical and chemical properties as Li and is an abundant natural resource, room-temperature sodium-ion batteries (SIBs) are expected to be among the most promising next-generation large grid energy storage devices. It is known that the cathode, anode, separator and electrolyte materials are the main components of batteries. Among these, Na-containing cathode materials are of critical importance. As a cathode material for SIBs, polyanion-type compounds have become a hot research topic owing to their versatile structural frameworks, high thermal stabilities, high ambient stabilities even in the charging state, small volume changes, tunable operating voltage by tuning the chemical environment of the polyanions, and high operating voltages owing to the inductive effects of the polyanionic groups (PO43-,SO42-, SiO44- etc.). In particular, for Earth's abundant resources and inherent stability, polyanion-based compounds are suitable for large-scale stationary energy storage. Taking grid balancing into account, batteries with fast charge rates are in demand, which requires cathodes having high rate capability. However, despite the presence of ion diffusion channels in polyanion compounds, the electronic transport channels are blocked owing to the separation of the metal polyhedral and the strong electronegativity of the anions, leading to poor electron conductivity, which largely limits the rate capability of polyanion compounds. Therefore, it is crucial to understand the inherent limitation of the kinetics in terms of the structural aspects and to determine strategies for improving the rate capability. This review discusses the intrinsic reasons for the factors impacting ion diffusion based on the different structures of polyanion-type cathodes. From the perspectives of surface modification and morphology, strategies for enhancing the transport of sodium ions and electrons at the surface and interface are summarized and discussed. Then, from the standpoint of the hierarchical structures of materials to the design of a structural framework, which have been rarely reported, this review proposes schemes that intrinsically enhance the rate capability of polyanion compounds and provides a perspective on developments that can further improve the rate capability of cathode materials. This review provides suggestions for designing and optimizing high-rate polyanion-type and other kinds of cathodes from both academic and practical viewpoints.