High-Entropy and Na-Rich-Designed High-Energy-Density Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>/C Cathode
Xiang Ding, Xiaofen Yang, Jie Li, Y. A. Yang, Liangwei Liu, Yi Xiao, Lili Han
Abstract
The Na 3 V 2 (PO 4 ) 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+ x V 1.76–x Zn x (GaCrAlIn) 0.06 (PO 4 ) 3 ( x = 0, 0.2, 0.35, and 0.5) cathodes are systematically prepared with an activated V 5+ ⇌V 4+ 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 ( x = 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 (V 5+ ⇌V 2+ ) 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.