A High-Energy NASICON-Type Na 3.2 MnTi 0.8 V 0.2 (PO 4 ) 3 Cathode Material with Reversible 3.2-Electron Redox Reaction for Sodium-Ion Batteries.

Na superionic conductor (NASICON) structured cathode materials with robust structural stability and large Na + diffusion channels have aroused great interest in sodium-ion batteries (SIBs). However, most of NASICON-type cathode materials exhibit redox reaction of no more than three electrons per formula, which strictly limits capacity and energy density. Herein, a series of NASICON-type Na 3+x MnTi 1-x V x (PO 4 ) 3 cathode materials are designed, which demonstrate not only a multi-electron reaction but also high voltage platform. With five redox couples from V 5+/4+ (≈4.1 V), Mn 4+/3+ (≈4.0 V), Mn 3+/2+ (≈3.6 V), V 4+/3+ (≈3.4 V), and Ti 4+/3+ (≈2.1 V), the optimized material, Na 3.2 MnTi 0.8 V 0.2 (PO 4 ) 3 , realizes a reversible 3.2-electron redox reaction, enabling a high discharge capacity (172.5 mAh g -1 ) and an ultrahigh energy density (527.2 Wh kg -1 ). This work sheds light on the rational construction of NASICON-type cathode materials with multi-electron redox reaction for high-energy SIBs.