Fast-charging 2D phosphate cathodes via green exfoliation: low steric hindrance and efficient Na+ transport

Abstract

The realization of high energy density and fast-charging capability is severely limited by the low intrinsic electronic conductivity and slow ion diffusion rates for the Na₃V₂(PO₄)₂O₂F (NVPOF) cathode in sodium-ion batteries (SIBs). Inspired by the rapid transport of carriers in two-dimensional (2D) frames, we designed a carbonless fast-charging 2D-NVPOF cathode material using H₂O molecules as the initial green exfoliant for the first time, which achieves the breakage of strong interlayer ionic bonds under mild and safe conditions. After exfoliation operation via mechanical expansion with the assistance of a thermal field, the H₂O molecules can enter into interlayers of 2D-NVPOF and further coordinate with the defective V atoms, thus enhancing the electronic conductivity, structural robustness and Na⁺ diffusion kinetics, which can be verified from the enhanced (002) lattice plane exposure, reduced band gap and lower Na⁺ migration energy barrier of 2D-NVPOF. In concert, these merits contribute to achieving the excellent fast-charging properties (80% of total battery capacity in 120 s of charging), higher energy density (up to 465 W h kg⁻¹), and long-term cycling stability of 2D-NVPOF, highlighting the great potential for practical application in SIBs. This strategy implies that the enhancement of electronic/ionic conductivity in the NASICON structure is achievable without introducing carbon and altering the active center, thus sparking new ideas for improving the fast-charging characteristics of cathodes for SIBs.