A high-entropy-designed cathode with V5+-V2+ multi-redox for high energy density sodium-ion batteries

Na3V2(PO4)3 (NVP) is gifted with fast Na+ conductive NASICON structure. But it still suffers from low electronic conductivity and inadequate energy density. Herein, a high-entropy modification strategy is realized by doping V3+ site with Ga3+/Cr3+/Al3+/Fe3+/In3+ simultaneously (i.e. Na3V2−x(GaCrAlFeIn)x(PO4)3; x = 0, 0.04, 0.06, and 0.08) to stimulate the V5+⇋V2+ reversible multi-electron redox. Such configuration high-entropy can effectively suppress the structural collapse, enhance the redox reversibility in high working voltage (4.0 V), and optimize the electronic induced effect. The in-situ X-ray powder diffraction and in-situ electrochemical impedance spectroscopy tests efficaciously confirm the robust structural recovery and far lower polarization throughout an entire charge-discharge cycle during 1.6–4.3 V, respectively. Moreover, the density functional theory calculations clarify the stronger metallicity of high-entropy electrode than the bare that is derived from the more mobile free electrons surrounding the vicinity of Fermi level. By grace of high-entropy design and multi-electron transfer reactions, the optimal Na3V1.7(GaCrAlFeIn)0.06(PO4)3 can exhibit perfect cycling/rate performances (90.97%@5000 cycles@30 C; 112 mA h g−1@10 C and 109 mA h g−1@30 C, 2.0–4.3 V). Furthermore, it can supply ultra-high 185 mA h g−1 capacity with fantastic energy density (522 W h kg−1) in half-cells (1.4–4.3 V), and competitive capacity (121 mA h g−1) as well as energy density (402 W h kg−1) in full-cells (1.6–4.1 V), demonstrating enormous application potential for sodium-ion batteries.