Phase Transformation Mechanism of Li-Ion Storage in Iron(III) Hydroxide Phosphates

Many ion storage compounds used for
electrodes in Li-ion batteries
undergo a first order phase transformation between the Li-rich and
Li-poor end-members during battery charge and discharge. This often
entails large transformation strains due to lattice misfits, which
may hamper charge and discharge kinetics. Iron­(III) hydroxide phosphate,
Fe2–y(PO4)­(OH)3–3y(H2O)3y−2 is a promising new cathode material with high Li-ion storage capacity,
low production costs and low toxicity. Previous reports on this material
indicate that the Li-ion intercalation and extraction in this material
is accompanied by a second-order solid solution transformation. However,
direct information about the transformation mechanism in Fe2–y(PO4)­(OH)3–3y(H2O)3y−2 is
lacking, and several details remain unclear. In this work, Fe2–y(PO4)­(OH)3–3y(H2O)3y−2 is prepared by hydrothermal synthesis and characterized structurally,
morphologically and by electrochemical analysis (galvostatic cycling
and cyclic voltammetry). A wide range of synthesis conditions is screened,
which provides information about their correlation with chemical composition,
crystallite size, particle morphology and electrochemical performance.
The phase transformation mechanism of selected materials is investigated
through synchrotron radiation powder X-ray diffraction collected during
galvanostatic discharge–charge cycling. This confirms a complete
solid solution transformation both during Li-insertion (discharge)
and -extraction (charge), but also reveals a highly anisotropic evolution
in lattice dimensions, which is linked to an irreversible reaction
step and the high vacancy concentration in Fe2–y(PO4)­(OH)3–3y(H2O)3y−2.