Delicate lattice modulation enables superior Na storage performance of Na3V2(PO4)3 as both an anode and cathode material for sodium-ion batteries: understanding the role of calcium substitution for vanadium

Abstract

Na₃V₂(PO₄)₃ with a 3D open NASICON framework can accommodate a wide range of Na contents, which makes it capable of working as both a cathode and anode material. However, severe capacity degradation and inferior rate capability resulting from low electronic/ionic conductivities and poor structural stability have hindered its practical implementation. Herein, excellent sodium storage performance of Na₃V₂(PO₄)₃ is realized by delicate lattice modulation. Aliovalent Ca²⁺ substitution for V³⁺ increases both the electronic and ionic conductivities by producing electronic defects and enlarging the sodium ion migration channels. DFT calculations reveal that the fifth Na ion intercalation/deintercalation produces a large lattice volume change, which is possibly the origin of the poor redox reaction reversibility of Na₃V₂(PO₄)₃ at low potential (∼0.3 V vs. Na⁺/Na). The Ca²⁺ doping enhances significantly the structural stability to suppress the large crystal lattice distortion during the anode reaction process. The multiple effects enable superior rate-capability and ultralong cycle-life of Ca-doped Na₃V₂(PO₄)₃ as both a cathode and anode material. The symmetric full cell constructed with the optimized Na₃V_1.95Ca_0.05(PO₄)₃@C electrode delivers a very high energy density of 166 W h kg⁻¹ and an exceptional cycling stability (0.02% capacity decay per cycle over 2000 cycles at 10C rate). This study provides a feasible strategy for obtaining high-rate and long cycle-life electrode materials for high-efficiency energy storage.