Exceeding Three-Electron Reactions in Polyanionic Cathode To Achieve High-Energy Density for Sodium-Ion Batteries

Activating multielectron reactions of sodium superionic conductor (NASICON)-type cathodes toward higher energy density remains imperative to boost their application feasibility. However, multisodium storage with high stability is difficult to achieve due to the sluggish reaction kinetics, irreversible phase transitions, and negative structural degradation. Herein, a kind of NASICON-type Na_2.5V_1.5Ti_0.5(PO₄)₃/C (NVTP-0.5) hierarchical microsphere consisting of abundant primary nanoparticles is designed, realizing a reversible 3.2-electron reaction with high stability. The optimized NVTP-0.5 cathode demonstrates an ultrahigh discharge capacity of 192.42 mAh g^-1, energy density of up to 497.3 Wh kg^-1 at 20 mA g^-1, and capacity retention ratio of 94.1% after 1000 cycles at 1 A g^-1. Additionally, the NVTP-0.5 cathode delivers excellent tolerance to extreme temperatures while also achieving a high-energy density of 400 Wh kg^-1 (based on the cathode mass) in a full-cell configuration. Systematic in situ/ex situ analysis results confirm the multisodium storage processes of NVTP-0.5 involving successive redox reactions (V²⁺/V³⁺, Ti³⁺/Ti⁴⁺, and V³⁺/V⁴⁺ redox couples) and reversible structure evolution (solid-solution and biphasic mechanisms), which contribute to the high capacity and excellent cycling stability. This work indicates that the rational regulation of components with different functions can unlock more possibilities for the development of NASICON-type cathodes.