Understanding the stability of MnPO4

Abstract

We have revealed the critical role of carbon coating in the stability and thermal behaviour of olivine MnPO₄ obtained by chemical delithiation of LiMnPO₄. (Li)MnPO₄ samples with various particle sizes and carbon contents were studied. Carbon-free LiMnPO₄ obtained by solid state synthesis in O₂ becomes amorphous upon delithiation. Small amounts of carbon (0.3 wt%) help to stabilize the olivine structure, so that completely delithiated crystalline olivine MnPO₄ can be obtained. Larger amount of carbon (2 wt%) prevents full delithiation. Heating in air, O₂, or N₂ results in structural disorder (<300 °C), formation of an intermediate sarcopside Mn₃(PO₄)₂ phase (350–450 °C), and complete decomposition to Mn₂P₂O₇ on extended heating at 400 °C. Carbon coating protects MnPO₄ from reacting with environmental water, which is detrimental to its structural stability.