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 gigapascal; 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 the 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 reported the first phase diagram up to 9 GPa and 1200 °C. A linear Clapeyron relation was determined to be − 3.15 MPa/°C and predicted a room-temperature α → β transition at ~ 8.2 GPa.