Effect of Crystal Structure and Morphology on Na₃V₂(PO₄)₂F₃ Performances for Na-Ion Batteries

Na-ion batteries (SIB) are considered promising systems for energy storage devices, however diversity of available cathode materials is lower compared to lithium ion batteries. Recently, Na₃V₂(PO₄)₂F₃ (NVPF) has been demonstrated as promising cathode material for SIB owing to high specific capacity and electrochemical reversibility. However, most of reports demonstrates capacities lower than theoretical value and optimization of electrochemical performances by controlled morphology and crystal structure was not demonstrated yet. Here, we demonstrate a scalable synthesis strategy to tailor the crystal structure and morphology of NVPF and showed that our approach enables to optimize the Na⁺ ion accommodation, diffusion and stability. A flower morphology (NVPF-F) crystalizes in tetragonal structure, demonstrates discharge capacity of 109.5 mA.h.g⁻¹ and 98.1 % columbic efficiency whereas a hollow spherical morphology (NVPF-S) with orthorhombic structure exhibits discharge capacity of 124.8 mA.h.g⁻¹ (very close to theoretical value) and 99.5 % columbic efficiency. The observed discharge capacity for NVPF-S is highest reported value which is ascribed due to stable crystal structure and monodispersed morphology. Long term stability with negligible capacity loss is demonstrated over 550 cycles. Our findings shed light on importance of crystal structure and morphology of NVPF on electrochemical response, and realization as cathode material for SIB.