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.