First principles prediction on the formation and properties of polyanion deficient iron phosphate

We evaluate the possible formation of recently proposed PO ₄-deficient FePO₄ by calculating its structure and stability at various charge states using the DFT+U theory. Unpaired electrons resulting from PO4 deficiency tend to localize on undercoordinated Fe neighbors. The absence of a PO₄ unit causes local lattice distortions which are found to be sensitive to the charge state. Our calculations show that neutral and negatively charged PO₄ vacancies may coexist under intrinsic conditions. The PO₄-deficient FePO₄ matrix turns out to be substantially softened, which may contribute to enhanced Li diffusion and provide clues toward the design of high performance LiFePO₄ cathode.