Validating the Electronic Structure of Vanadium Phosphate Cathode Materials

Abstract

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·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.

Regulation of surface oxygen activity in Li-rich layered cathodes using band alignment of vanadium phosphate surface coatings

“…This pre-edge peak was observed previously during an sXAS study of Na 3 V 2 (PO 4 ) 3 raw material and is assigned to the Na 2p–O 2p hybridized region. 25 This extra peak region disappears during charging to 4.5 V, whereas a reduction of P 1 and P 2 intensity is observed for NVP–LLO. The relative intensity of surface O K pre-edge regions is stable with further charging, albeit with a slight reduction in P 1 during discharge to 2.0 V. Overall at the O–K edge, LVP–LLO and NVP–LLO show improved reversibility and a lower normalized integrated peak intensity of TM 3d–O 2p pre-edge regions during initial charge–discharge (Fig.…”

“…For energy calculations, the sX-LDA non-local hybrid functional was used for improved accuracy of electronic structure contribution in highly correlated 3d transition metal phosphate materials as previously reported. 25 An energy cut-off of 720 eV and k -point separation of 0.07 Å −1 was used with convergence tolerance of 1 × 10 −5 eV per atom. For LVP and NVP materials, antiferromagnetic ordering was imposed on each unit cell structure prior to calculation.…”

“…This difference in valence is in excellent agreement with our previous studies on surface chemistry of Li 3 V 2 (PO 4 ) 3 and Na 3 V 2 (PO 4 ) 3 and indicates a degree of alkali depletion in the VP coating layers. 25 P 2p spectra tted at $133 eV and 134 eV correspond to orbital splitting peaks for PO 4 . 47 Spectral tting for O 1s reveals the presence of lattice O 2− species across all 3 samples at $529.8 eV for PR-LLO and $529.5 eV for LVP-LLO and NVP-LLO.…”

“…However, our recent electronic structure studies of these materials have revealed that despite a wide bandgap, LVP and NVP both show relatively low ionization potentials with surface impurity states at similar energies to the valence band maximum (VBM) and conduction band minimum (CBM) of LiTMO 2 and Li 2 MnO 3 materials. [23][24][25] This similarity offers potential for valence and/or conduction band overlap between NVP/LVP and LLO materials, which in theory should allow for Fermi level alignment, and good conduction at an interface with Li-rich layered oxides.…”

“…The spectral shape of the DFT calculated pDOS agrees with experimental measurements. In the DFT model, strong contributions from the TM 3d-O 2p antibonding interactions (region A) overlap and reduce to below the bonding TM 3d and TM 3d-O 2p valence band interactions (region B) introduced by LVP and NVP coating layers 25. This surface electronic structure in VP-coated LLO materials is illustrated as an energy diagram given in Fig.6d.…”