Lithium iron phosphate LiFePO<sub>4</sub>
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,
LiFePO<sub>4</sub> 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 LiFePO<sub>4</sub> structure. Up to now
the polyanionic (i.e. phosphate) sublattice has been considered idle on this
playground. Here, we demonstrate that under hydrothermal conditions up to 16%
of the phosphate groups can be replaced with hydroxyl groups yielding the Li<sub>1-x</sub>Fe<sub>1+x</sub>(PO<sub>4</sub>)<sub>1-y</sub>(OH)<sub>4y</sub>
solid solutions, which we term “hydrotriphylites”. This substitution has
tremendous effect on the chemical composition and crystal structure of the
lithium iron phosphate causing abundant population of the Li-ion diffusion
channels with the iron cations and off-center Li displacements due to their
tighter bonding to oxygens. These perturbations trigger the formation of an
acentric structure and increase the activation barriers for the Li-ion
diffusion. The “hydrotriphylite”-type substitution also affects the magnetic
properties by progressively lowering the Néel temperature. The
off-stoichiometry caused by this substitution critically depends on the overall
concentration of the precursors and reducing agent in the hydrothermal
solutions, placing it among the most important parameters to control the
chemical composition and defect concentration of the LiFePO<sub>4</sub>-based
cathodes.
