Nano self-assembly of fluorophosphate cathode induced by surface energy evolution towards high-rate and stable sodium-ion batteries

In the field of materials science and engineering, controlling over shape and crystal orientation remains a tremendous challenge. Herein, we realize a nano self-assembly morphology adjustment of Na₃V₂(PO₄)₂F₃ (NVPF) material, based on surface energy evolution by partially replacing V³⁺ with aliovalent Mn²⁺. Crystal growth direction and surface energy evolution, main factors in inducing the nano self-assembly of NVPF with different shapes and sizes, are revealed by high-resolution transmission electron microscope combined with density functional theory. Furthermore, NVPF with a two-dimensional nanosheet structure (NVPF-NS) exhibits the best rate capability with 68 mAh·g⁻¹ of specific capacity at an ultrahigh rate of 20 C and cycle stability with 80.7% of capacity retention over 1,000 cycles at 1 C. More significantly, when matched with Se@reduced graphene oxide (rGO) anode, NVPF-NS//Se@rGO sodium-ion full cells display a remarkable long-term stability with a high capacity retention of 93.8% after 500 cycles at 0.5 C and −25 °C. Consequently, experimental and theoretical calculation results manifest that NVPF-NS demonstrates such superior performances, which can be mainly due to its inherent crystal structure and preferential orientation growth of {001} facets. This work will promise insights into developing novel architectural design strategies for high-performance cathode materials in advanced sodium-ion batteries.