Novel LiMnPO4 nanocomposite material for high-power lithium battery cathodes

Abstract

Lithium ion batteries are emerging as one of the most promising technology for high powered energy storage applications. LiMnPO4, is currently one of the most promising cathode materials for lithium batteries considering its low cost, environmental safety, high theoretical capacity and operating voltage (4.1 V vs Li/Li+), achievable within the stability window of conventional carbonate ester-based electrolytes. The practical use of LiMnPO4 is however limited by several intrinsic obstacles: (1) low electrical and ionic conductivity; (2) kinetic limits of Li+ diffusion and; (3) large volume change between LiMnPO4 and MnPO4 phases during charge/discharge cycles. In this dissertation, a novel sol-gel procedure has been developed to produce LiMnPO4 and carbon coating derived from the in-situ addition of sucrose. This work has shown how the purity of the prepared materials can be modulated by the temperature and atmosphere used. Focusing on how the inclusion of a two-step heating regime can help produce phase pure LiMnPO4 at a lower temperature. Results have shown that variations in the thermal treatment of the dried gel precursor can produce pure LiMnPO4 or produce an impure phase such as Mn2P2O7. It also shows that the temperature at which pure, stable and highly crystalline LiMnPO4 is produced can be reduced significantly (400°C instead of 900ºC). The electrochemical performance was best at when tested under higher temperatures of 40°C and for products synthesised with the final heating step at 700°C. Further, when analysing LiMnPO4/carbon composite, the G band peak shifted to a lower wavenumber with increased temperature due to the increase in delocalised π electrons caused by the transformation of alkenic C=C chains to aromatic hexagonal rings. It was found that the ID/IG ratios of the carbon increased with temperature, confirming an increase in D band peak intensity, attributed to the material being in a pyrolysis state. This is critical information as it indicates that the material is not yet in a graphitised stage and that the ID/IG ratio does not imply the extent of graphitisation but confirms that the carbon is approaching the state of graphitisation. Because of the close link between the electric conductivity and delocalisation of the π electrons, this study shows that the optimisation of the heat-treatment of the LiMnPO4/carbon composite is critical to the rate capability of the cathode material.

Date of Award	2016
Original language	English