On the thermal stability of olivine NaMnPO4

Abstract

Olivine NaMnPO₄ is inherently metastable, but the specific factors that determine its stability remain uncertain. This study investigates the thermal behavior of synthesized olivine NaMnPO₄ and reveals that the thermal stability is determined not only by crystallinity but also by antisite defects in the bulk and the grain size of the samples. Better thermal stability is observed for the hydrothermal samples due to their higher crystallinity, and impurity phases are yielded upon heating because of the precipitation of antisite defects from the bulk at high temperatures. More impurity phases are produced by the same thermal treatment in the hydrothermal sample with more antisite defects. In addition, the phase transformation of olivine NaMnPO₄ appears to be size-dependent, and smaller grains are more prone to phase transformation. These findings on the thermal stability provide useful references for developing olivine NaMnPO₄ as a cathode material for sodium-ion batteries.