Exposing the (002) active facet by reducing surface energy for a high-performance Na3V2(PO4)2F3 cathode

Abstract

Increasing the exposure of active crystal facets is an effective strategy to promote the transmission of Na⁺ through the electrolyte/cathode interface and is key to developing high-performance sodium-ion battery (SIB) cathodes. Here, Na₃V₂(PO₄)₂F₃ (NVPFs) were synthesized with diverse (002) active facet exposure rates, in which the growth rate of the different crystal facets was regulated by adjusting the surface energy. The growth mechanism of the NVPF crystals was proposed using density functional theory calculations. The NVPFs with 82% (002) active facet exposure delivered a remarkable reversible capacity of 123 mA h g⁻¹ and energy density of 431 W h kg⁻¹ in a full cell. Theoretical investigations of the Na⁺ adsorption, diffusion, and storage clarified that the exposed (002) crystal facet had more stable Na⁺-storage sites, which resulted in a lower energy barrier during the Na⁺-diffusion process. The promoted Na⁺-transportation kinetics and related enhanced stability were further elucidated based on the combined analyses of synchrotron X-ray absorption spectroscopy and electrochemical measurements. This research aimed to develop a way to promote the transmission process of Na⁺ through the electrolyte/cathode interface from the perspective of crystal facet regulation, which is expected to promote the commercial application of SIB cathodes.