Energetics of LiFePO4 and Polymorphs of Its Delithiated Form, FePO4

High-temperature oxide melt solution calorimetry was performed on the battery material Li Fe P O 4 and the orthorhombic and trigonal polymorphs of its delithiated form, Fe P O 4 . The enthalpies of formation from oxides and from elements at 25 ° C were determined. The phase transition of Fe P O 4 from orthorhombic symmetry to trigonal symmetry was investigated using differential scanning calorimetry. The enthalpies of formation from oxides at 25 ° C for Li Fe P O 4 , o - Fe P O 4 , and t - Fe P O 4 are − 151.52 ± 1.68 , − 113.68 ± 1.26 , and − 102.01 ± 1.26 kJ ∕ mol , respectively. The enthalpy of transition from o - Fe P O 4 to t - Fe P O 4 is 11.67 ± 1.56 kJ ∕ mol . Thus the orthorhombic form of Fe P O 4 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 - Fe P O 4 will decompose to t - Fe P O 4 . The t - o transition on cooling is kinetically hindered.