Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

Investigation of the electronic structure of contending battery electrode materials is an essential step for developing a detailed mechanistic understanding of charge-discharge properties. Herein, we use synchrotron soft X-ray absorption spectroscopy (XAS) in combination with complementary experiments and density functional theory calculations to map the electronic structure, band positioning, and band gap of prototype vanadium-(III) phosphate cathode materials, Na3V2(PO4)(3), Li3V2(PO4)(3), and K3V3(PO4)(4)center dot H2O, for alkali-ion rechargeable batteries. XAS fluorescence yield and electron yield measurements reveal substantial variation in surface-to-bulk atomic structure, vanadium oxidation states, and density of oxygen hole states across all samples. We attribute this variation to an intrinsic alkali metal surface depletion identified across these alkali metal vanadium(III) phosphates. We propose that an alkali-depleted surface provides a beneficial interface with the bulk structure(s) that raises the Fermi level and improves surface charge transfer kinetics. Furthermore, we discuss how this effect can play a significant role in reducing the electronic and ionic diffusion limitations of alkali vanadium phosphates in alkali-ion rechargeable batteries. These findings clarify the electronic structure and properties of alkali metal vanadium phosphates and offer guidance on future strategies to improve vanadium phosphate battery performance.

Automated summary

The investigation conducted on the electronic structure of vanadium phosphate cathode materials for alkali-ion rechargeable batteries using synchrotron soft X-ray absorption spectroscopy (XAS) revealed substantial variation in surface-to-bulk atomic structure, vanadium oxidation states, and density of oxygen hole states. This variation was attributed to an intrinsic alkali metal surface depletion identified across the samples, which provides a beneficial interface with the bulk structure(s) and improves surface charge transfer kinetics. The findings clarify the electronic structure and properties of alkali metal vanadium phosphates, offering guidance on future strategies to enhance vanadium phosphate battery performance.