“Hydrotriphylites” Li1-xFe1+x(PO4)1-y(OH)4y as cathode materials for Li-ion batteries

Lithium iron phosphate LiFePO4 triphylite is now one of the core positive electrode (cathode) materials enabling the Li-ion battery technology for stationary energy storage applications, which are important for broad implementation of the renewable energy sources. Despite the apparent simplicity of its crystal structure and chemical composition, LiFePO4 is prone to off-stoichiometry and demonstrates rich defect chemistry owing to variations in the cation content and iron oxidation state, and to the redistribution of the cations and vacancies over two crystallographically distinct octahedral sites. The importance of the defects stems from their impact on the electrochemical performance, particularly on limiting the capacity and rate capability through blocking the Li-ion diffusion along the channels of the olivine-type LiFePO4 structure. Up to now, the polyanionic (i.e., phosphate) sublattice has been considered idle in this case. Here, we demonstrate that under hydrothermal conditions up to 16% of the pho...