A Medium-Entropy NASICON Cathode for Sodium-Ion Batteries Achieving High Energy Density Through Dual Enhancement of Voltage and Capacity

Abstract

Na₃V₂(PO₄)₃ (NVP) is recognized for its promising commercialization potential as a sodium-ion battery (SIB) cathode, due to its thermodynamic stability and open structure. However, the limited energy density remains a major obstacle to further advancement of NVP. Herein, a medium-entropy NASICON Na_3.3V_1.4Al_0.3(MgCoNiCuZn)_0.06(PO₄)₃ (NVAMP-0.3) is designed by introducing Al³⁺, Mg²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ to regulate configurational entropy. These NVAMP-0.3 achieve an elevated average operating voltage (3.33 V) and high capacity (138.1 mAh g⁻¹, based on 2.3 Na⁺) through V³⁺/V⁴⁺/V⁵⁺ multi-electron reactions. By simultaneously enhancing capacity and voltage, NVAMP-0.3 exhibits an impressive energy density of 460 Wh kg⁻¹. Furthermore, NVAMP-0.3 demonstrates excellent low-temperature tolerance with a capacity retention rate of 94.6% after 300 cycles at −40 °C. In situ XRD unveils the underlying cause of the unique phenomenon where the solid-solution reaction accounts for the faster electrochemical reaction kinetics of the V⁴⁺/V⁵⁺ compared to the V³⁺/V⁴⁺ redox. DFT calculations indicate that NVAMP-0.3 possesses superior electronic conductivity and reduced Na⁺ migration energy barriers. A pouch cell assembled with the NVAMP-0.3 cathode and hard carbon anode exhibits highly stable cycling (89.3% after 200 cycles at 1 C). This study provides valuable insights into developing NASICON-type cathodes with high energy densities for SIBs.