N-Containing Na₂VTi(PO₄)₃/C for Aqueous Sodium-Ion Batteries

Phosphates featuring a 3D framework offer a promising alternative to aqueous sodium-ion batteries, known for their safety, cost-effectiveness, scalability, high power density, and tolerance to mishandling. Nevertheless, they often suffer from poor reversible capacity stemming from limited redox couples. Herein, N-containing Na₂VTi(PO₄)₃ is synthesized for aqueous sodium-ion storage through multi-electron redox reactions. It demonstrates a capacity of 155.2 mAh g^-1 at 1 A g^-1 (≈ 5.3 C) and delivers an ultrahigh specific energy of 55.9 Wh kg^-1 in a symmetric aqueous sodium-ion battery. The results from in situ X-ray diffraction analysis, ex situ X-ray photoelectron spectroscopy analysis, and first-principle calculations provide insights into the local chemical environment of sodium ions, the mechanisms underlying capacity decay during cycling, and the dynamics of ion and electron transfer at various states of charge. This understanding will contribute to the advancement of electrode materials for aqueous sodium-ion batteries.