Determination of the High Pressure-Temperature Phase of LiMnPO₄ Energised by Battery Applications

Olivine structured lithium-metal-phosphates (α-LiMPO₄, where M = Fe, Ni, Co, Mn) are established or prospective cathode materials for lithium-ion batteries. At high pressures (P, in giga pascal; GPa) and temperatures (T, in Celsius; °C), the electrochemically active α-LiFePO₄, α-LiNiPO₄, and α-LiCoPO₄ undergo a solid-state phase transition to an electrochemically less active or inactive Cmcm (β) orthorhombic structure. This phase delineation in P-T space is significant because it differentiates between α and β phases and constrains boundary conditions for synthesis of these cathode materials. To date, pressure-temperature phase diagrams showing this delineation in P-T space, either via theoretical investigations or experimental results, have not been reported for any of the LiMPO₄ cathode materials. In this work, we conducted experiments on α-LiMnPO₄ up to 8 GPa and 1113°C in a large volume hydraulic press. Using ex-situ X-ray diffraction and Raman spectroscopy, we determined the high P-T phase to be a β structure (a = 5.5465(3) Å; b = 8.4110(6) Å; c = 6.2990(4) Å) and report the first phase diagram up to 9 GPa and 1200°C. A linear Clapeyron relation was determined to be -3.15 MPa/°C and predicts a room temperature α→β transition at ~8.2 GPa.