Topotactic Syntopogenous Sodium Vanadium Fluoride for High-Performance Sodium-Ion Batteries: Electron and Sodium-Ion Reservoirs in Perovskite/Diperovskite Superlattice

In order to simultaneously accelerate ion and electron transfer in sodium-ion battery (SIB) cathodes, a topotactic superlattice was utilized, in which the atomically intrinsic lattice-matching effect from inner to external surface can boost the charge transfer due to the disappearance of the heterojunction interface. Herein, a topotactic syntopogenous Na₃VF₆/NaVF₃ superlattice formulated as Na_2.9V_1.1F₆ (NVF) was synthesized by a facile one-step low-temperature hydrothermal reaction. NVF nanoparticles show an excellent Na⁺ storage capacity (∼205 mAh g^-1) in a high voltage window up to 4.2 V with ultralong cycling stability. That is associated with the mixed occupancy of V and Na in NVF. The multivalent V centers serve as electron reservoirs to inhibit phase transformation, and the Na-enriched Na₃VF₆ with better electron conductivity acts as a Na⁺ reservoir for effective electron transfer. Highly reversible (de)intercalation of Na⁺ is achieved in the channel of perovskite-type NaVF₃ with structural integrity.