Energetics of LiFePO4 and Polymorphs of Its Delithiated Form, FePO4

Abstract

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