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 Na3.3V1.4Al0.3(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.