M. Stanley Whittingham

Understanding the stability of MnPO4

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.

Structure, defects and thermal stability of delithiated olivine phosphates

Studies of thermal decomposition mechanism of olivine Fe1−yMnyPO4 are reported here for inert (He), oxidizing (O2) and oxidizing and moist (air) atmospheres using in situ X-ray diffraction and thermal gravimetric analysis with mass spectroscopy. The results indicate that the olivine structure is inherently stable up to at least 400 °C and y = 0.9 for particle size down to 50 nm. However, structural disorder and oxygen loss in the presence of reductive impurities, e.g. carbon and hydrogen, can occur as low as 250 °C for particles larger than 100 nm and at 150 °C for 50 nm particles. Fe1−yMnyPO4 is hygroscopic at high Mn contents, y ≥ 0.6, and moisture exposure is more detrimental to its thermal stability than carbon or small particle size. Nano-Fe1−yMnyPO4 (y > 0.7) with particle size about 50 nm, when exposed to moisture, disorders at 150 °C and transforms to sarcopside phase by 300 °C, no matter whether the delithiation was done electrochemically or chemically. Contrary, under inert atmosphere the sample produced by chemical delithiation is stable up to 400 °C.