Preparation of LiFe1-xMnxPO4/C cathode materials at a pH of 6.5 using a hydrothermal process with high-temperature calcination

Abstract

In this study, manganese ion-doped lithium iron phosphate/carbon (LiFe1-xMnxPO4/C, LF1-xMxP/C) composite cathode materials with different Mn2+ doping levels (x = 0,0.03,0.05,0.07, and 0.10) were synthesized using a hydrothermal process combined with high-temperature calcination. Loss during the calcination was compensated for by using 3 at.% of excess lithium. Acetic acid or ammonium hydroxide was used to adjust the initial pH value of the hydrothermal solution to 6.5. The results revealed that the LF0.93M0.07P/C composite cathode material had the most favorable electrochemical performance and delivered the highest discharge capacities at different C-rates: 140.8 mAh/g at 0.2 C, 135.8 mAh/g at 0.5 C, 123.6 mAh/g at 1 C, 101.2 mAh/g at 3 C, 88.1 mAh/g at 5 C, and 69.5 mAh/g at 10 C. In addition, the capacity retention rate of 96.5% for LF0.93M0.07P/C after 100 cycles at 1 C–1 C was higher than the 86.5% for the pristine LiFePO4/C under the same C-rate and cycle number conditions. Moreover, the electrochemical performance of LF0.93M0.07P/C was noticeably improved by adding 1 wt% of graphene. Therefore, an appropriate amount of Mn2+ doping could effectively improve the electrochemical performance of the pristine LiFePO4/C cathode materials, particularly the electrochemical performance at a high C-rate, due to the increase in electronic and ionic transportation attributed to the addition of graphene and substitution of Mn2+ for Fe2+ in LiFePO4/C cathode materials.