Thermal Behavior of Delithiated L i _1−x M n PO ₄ (0 ≤ x <1) Structure for Lithium‐Ion Batteries

High‐voltage and high‐capacity cathode‐active materials are required to increase the energy density of rechargeable lithium‐ion batteries for hybrid vehicles. The olivine‐type L i M n P O ₄ is considered as a good candidate for the next‐generation lithium‐ion battery due to its high voltage (4.1 V vs L i ⁺ / L i), low cost, and lower toxicity compared with the currently used layered materials. However, recent research has demonstrated that the thermal stability of delithiated phase of Li _{1− x} M n PO ₄ (0 ≤ x <1) was less than that of Li _{1− x} FePO ₄ . These reports verified that the delithiated M n PO ₄ decomposed and changed into M n ₂ P ₂ O ₇ with O ₂ release at high temperature. In this study, we focused on the particle and crystal changes in L i M n P O ₄ / M n PO ₄ at high temperature on a nanoscale. As a result, we have succeeded to observe directly the particle and crystal changes using transmission electron microscope ( TEM ) with heating. It revealed that M n PO ₄ was a thermally unstable phase because dendrites of M n ₂ P ₂ O ₇ began to generate around 200°C.