Cycling stability and degradation mechanism of LiMnPO4 based electrodes

• Undoped LiMnPO4 was cycled more than 500 times at 55 °C.
• Average capacity loss was less than 0.06% cycle.
• After ca 550 cycles very fast capacity degradation was observed.
• Surface film formation, Mn dissolution and occurrence of Li4P2O7 was observed.
• Decomposition starts at local defects in the microstructure of carbon coating.

Long term stability of LiMnPO4 particles with a crystallite size between ∼20 and 50 nm covered with a dense native carbon coating (14 wt.%) is demonstrated. More than 500 cycles at a rate of C/20, in the potential window of 2.7–4.5 V and a temperature of 55 °C were achieved. During most of the cycling the average capacity decay was less than 0.06% per cycle. After about 500 cycles a sudden capacity drop was observed. Degradation processes in various stages of cycling were thoroughly examined using a range of techniques. Severe surface film formation, manganese dissolution and degradation of LixMnPO4 accompanied by formation of Li4P2O7 were clearly identified. The good long term stability seems to be due to dense, protective carbon coating. Decomposition is most likely initiated at local defects in the microstructure of pyrolytic carbon coating around LiMnPO4 particles. In addition to known degradation mechanisms of LiMnPO4 we observed pronounced gradual amorphization of the olivine crystallites during long-term cycling at 55 °C. Finally, changes in morphology of the carbon black additive after prolonged cycling are reported and commented.

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