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.LiFePO 4 has shown considerable promise as a cathode material in Li-ion batteries due to its high cyclability, stability, low toxicity, and low cost. Since the insulating LiFePO 4 phase was first proposed as a cathode material, 1 research has been mostly devoted to designing LiFePO 4 materials with better conductivities. The use of chimie douce methods such as hydrothermal synthesis 2-4 or lowtemperature ceramic routes 5 consisting of the annealing of intimately mixed precursor powders has enabled the preparation of submicrometric/nanometric particles, for which Li + ions diffusion lengths are shortened and ohmic drops reduced, thus giving rise to LiFePO 4 powders with enhanced electrochemical properties. Another approach consisted of making a coating of the LiFePO 4 particles with conductive carbon, formed by thermal decomposition of carbon-containing precursors, which provides a good electric contact between the current collector and the particles. 5-7 Delithiation of the olivine LiFePO 4 leads to the formation of the isostructural FePO 4 ͑hereby denoted as o-FePO 4 ͒. Structurally, Fe exists in an octahedral environment in LiFePO 4 and o-FePO 4 . In addition to this olivine form, FePO 4 also exists as monoclinic ͑m-FePO 4 ͒, orthorhombic FePO 4 ͑in which Fe has tetrahedral coordination͒, and trigonal ͑t-FePO 4 ͒ phases. 8 Both the orthorhombic and monoclinic forms can be converted to the trigonal form on heating. 8 t-FePO 4 crystallizes in the berlinite ͑AlPO 4 ͒ structure-type similar to ␣-quartz with both Fe and P atoms in a tetrahedral environment of oxygen atoms. Reversal from t-FePO 4 to the other structures is not possible. 4 The trigonal phase undergoes a reversible ␣- transition, from a P6 4 22 to P3 1 21 phase, similar to that of quartz. 9,10 The orthorhombic form was shown to exhibit the best properties, compared to the other polymorphs.Even though considerable study has been done to improve the performance of the LiFePO 4 -FePO 4 system in batteries, there have bee...
