Recent Progress in Polyanionic Anode Materials for Li (Na)-Ion Batteries

In recent years, rechargeable lithium-ion batteries (LIBs) have become widely used in everyday applications such as portable electronic devices, electric vehicles and energy storage systems. Despite this, the electrochemical performance of LIBs cannot meet the energy demands of rapidly growing technological evolutions. And although significant progress has been made in the development of corresponding anodes based primarily on carbon, oxide and silicon materials, these materials still possess shortcomings in current LIB applications. For example, graphite exhibits safety concerns due to an operating potential close to that of lithium (Li) metal plating whereas Li4Ti5O12 possesses low energy density for high operation potential and silicon experiences limited cyclability for large volume expansion during charging/discharging. Alternatively, polyanionic compounds such as (PO4)(3-), (SiO4)(4-), (SO4)(2-) and (BO3)(3-) as electrode materials have gained increasing attention in recent years due to their ability to stabilize structures, adjust redox couples and provide migration channels for "guest" ions, resulting in corresponding electrode materials with long-term cycling, high energy density and outstanding rate capability. Based on these advantages and combined with recent findings in terms of silicate anodes, this review will summarize the recent progress in the development of polyanion-based anode materials for LIBs and sodium-ion batteries. Furthermore, this review will present our latest research based on polyanion groups such as (GeO4)(4-) to compensate for the lack of available studies and to provide our perspective on these materials.