High-Entropy and Na-Rich-Designed High-Energy-Density Na₃V₂(PO₄)₃/C Cathode

Abstract

The Na₃V₂(PO₄)₃ (NVP) cathode holds the merit of a stable 3D NASICON structure for ultrafast Na⁺ diffusion, yet it is still confronted with poor electronic conductivity (10^-9 S cm^-1) and insufficient energy density (∼370 W h kg^-1). Herein, a series of high-entropy-doped Na_3+<i>x</i>V_1.76-xZn_<i>x</i>(GaCrAlIn)_0.06(PO₄)₃ (<i>x</i> = 0, 0.2, 0.35, and 0.5) cathodes are systematically prepared with an activated V⁵⁺⇌V⁴⁺ high-voltage plateau (4.0 V) and elevated discharge capacity, which is derived from the charge compensation of divalent Zn substituting for trivalent V accompanied by extra Na⁺ input to create an Na-rich phase. A range of in situ/ex situ characterization studies and DFT calculations radically verify the charge conservation mechanism, enhanced bulk conductivity, and robust structural stability. Accordingly, in half-cells, the optimized cathode (<i>x</i> = 0.35) is capable of giving a much-improved discharge capacity (126.8 mA h g^-1), reliable cycling stability (97.4%@5000 cycles@40 C), and a competitive energy density (426.1 W h kg^-1) at 2.0-4.3 V. Upon reducing the discharge cutoff voltage to 1.4 V, the three-electron reaction (V⁵⁺⇌V²⁺) is entirely activated with superior stability, delivering an unparalleled capacity of 193.4 mA h g^-1 with higher energy density (544.3 W h kg^-1). Besides, it displays high capacity (126.1 mA h g^-1) and energy density (417.2 W h kg^-1) in NVPZGCAI-35//hard carbon full-cells at 1.6-4.1 V. Hence, this pioneering high-entropy and Na-rich strategy is above rubies for developing high-energy-density and high-stability sodium-ion batteries.