High-rate Na0.7Li2.3V2(PO4)2F3 hollow sphere cathode prepared via a solvothermal and electrochemical ion exchange approach for lithium ion batteries

Abstract

Na₃V₂(PO₄)₂F₃ (NVPF) has been extensively studied, and has demonstrated excellent electrochemical activity in Na-ion batteries owing to its high reversible specific capacity and stability. The direct chemical synthesis of a Li analogue of NVPF (LVPF) is aided by the high thermodynamic stability of intermediate products. Even more challenging is the synthesis of LVPF with a well-controlled uniform morphology and a stable crystal structure. Herein, an electrochemical ion exchange approach was used to synthesize Na_0.7Li_2.3V₂(PO₄)F₃ (N_0.7L_2.3VPF), an isostructural composition of Li₃V₂(PO₄)F₃. This compound was prepared via lithiation of Na_0.7V₂(PO₄)F₃ with a hierarchical morphology prepared by desodiation of NVPF. We track the phase formation, reversible structural transformation from Pnnm to Cmc2₁ and back to Pnnm. An initial specific discharge capacity of 185 mA h g⁻¹ and two distinct voltage plateaus visualize the prominence of N_0.7L_2.3VPF as a cathode material for LIBs. It exhibits a specific discharge capacity of 173, 159, 154, 134 and 114 mA h g⁻¹ at 45, 105, 135, 265, and 535 mA g⁻¹ respectively along with >98% coulombic efficiency, which indicates pronounced electrochemical activity at high current rates due to better diffusivity of smaller Li⁺ ions than Na⁺ ions through the partially occupied alkali metal sites in the lattice. Long-term cycling at 45 mA g⁻¹ exhibits 173 mA h g⁻¹ with 96% of capacity retention for 200 cycles. This stable performance further indicates the prominence of N_0.7L_2.3VPF HMS as a cathode for LIBs. Our findings provide a strategic pathway towards controlling the morphology and crystal structure and shed light on its importance in realization as a cathode material for LIBs.