Sodium Superionic Conductors (NASICONs) as Cathode Materials for Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have developed rapidly owing to the high natural abundance, wide distribution, and low cost of sodium. Among the various materials used in SIBs, sodium superion conductor (NASICON)-based electrode materials with remarkable structural stability and high ionic conductivity are one of the most promising candidates for sodium storage electrodes. Nevertheless, the relatively low electronic conductivity of these materials makes them display poor electrochemical performance, significantly limiting their practical application. In recent years, the strategies of enhancing the inherent conductivity of NASICON-based cathode materials have been extensively studied through coating the active material with a conductive carbon layer, reducing the size of the cathode material, combining the cathode material with various carbon materials, and doping elements in the bulk phase. In this paper, we review the recent progress in the development of NASICON-based cathode materials for SIBs in terms of their synthesis, characterization, functional mechanisms, and performance validation/optimization. The advantages and disadvantages of such SIB cathode materials are analyzed, and the relationship between electrode structures and electrochemical performance as well as the strategies for enhancing their electrical conductivity and structural stability is highlighted. Some technical challenges of NASICON-based cathode materials with respect to SIB performance are analyzed, and several future research directions are also proposed for overcoming the challenges toward practical applications.