Energetics of LiFePO₄ and polymorphs of its delithiated form, FePO₄

High-temperature oxide melt solution calorimetry was performed on the battery material LiFePO₄ and the orthorhombic and trigonal polymorphs of its delithiated form, FePO₄. The enthalpies of formation from oxides and from elements at 25°C were determined. The phase transition of FePO₄ from orthorhombic symmetry to trigonal symmetry was investigated using differential scanning calorimetry. The enthalpies of formation from oxides at 25°C for LiFePO₄, o-FePO₄, and t-FePO₄ are -151.52±1.68, -113.68±1.26, and -102.01±1.26 kJ/mol, respectively. The enthalpy of transition from o-FePO₄ to t-FePO₄ is 11.67±1.56 kJ/mol. Thus the orthorhombic form of FePO₄ is energetically more stable than the trigonal phase and is therefore stable at low temperatures. The equilibrium temperature of the o-t transition is probably sufficiently above room temperature, so that under the operating conditions of a battery, it is themodynamically impossible that o-FePO₄ will decompose to t-FePO₄. The t-o transition on cooling is kinetically hindered.