Preparation And Electrochemical Properties Of Nanosized LiMn₂O₄ And LiMn₂O₄/Graphene Composite

The lithium ion battery(LIB) is one of the most widely-used new energy resources due to its high safety and portability. Compared with the previous dry battery, lead storage battery, NI-MH battery, etc., the LIB has obvious advantages in working voltage, specific capacity, service life and safety, so almost all electric cars use the LIB as the power source. However, the capacity and power of the LIB still needs to be further improved. In this dissertation, the preparation of Li Mn2O4(LMO) nanoparticles and LMO/graphene composite were investigated and their electrochemical properties were also studied.Firstly, KMn O4, Li OH and ascorbic acid were used as the Mn source, Li source and the reductant, respectively. The LMO were synthesized by hydrothermal method. The hydrothermal temperatures were set to 140-200 论 and the reaction time was 1-7 hours. The effects of hydrothermal temperature, reaction time and the raw material ratio on the final product were studied. The microstructures of the as-prepared products were analyzed by scanning electron microscopy(SEM) and transmission electron microscopy(TEM), and the electrochemical properties were tested by galvanostatic charge-discharge(GCD). It was revealed that the best synthesis parameter was 180 论, 5 hours and 3:5 molar ratio of KMn O4 and ascorbic acid. SEM and TEM results indicated that the as-prepared LMO had uniform particle size distribution with the particle size of 50 nm, and a single-crystal structure. The as-prepared LMO exhibited a specific capacity of 95 m A h g-1 at 5 C charge-discharge rate without calcination and, after 100 charge-discharge cycles, the specific capacity could still remain at 82 m A h g-1.Secondly, KMn O4 and Mn SO4 were used as the Mn source and Li OH was used as the Li source during the hydrothermal process. The hydrothermal temperatures were set to 80-150 论 and the reaction time was 3-12 hours. The effects of hydrothermal temperature, reaction time and the raw material ratio onthe final product were investigated. The microstructures of the as-prepared products were analyzed by SEM. It was revealed that the best synthesis parameter was 90 论, 6 hours and 3:7 molar ratio of KMn O4 and Mn SO4. The phase analysis results indicated that the as-prepared product contained a small amount of Mn3O4, which could be removed by hydrothermal treatment in water at 180 论.Then, LMO/reduced graphene oxide(RGO) composite was synthesized through physical mixture approach. SEM results revealed the LMO and RGO agglomerated seriously. The as-prepared composite exhibited a specific capacity of 109 m A h g-1 at 0.1 C and 80 m A h g-1 at 5 C. In order to improve the uniformity of LMO and RGO, LMO/RGO composite was synthesized through in-situ growth approach. The characterizaion results revealed LMO particles distributed uniformly on the surface of RGO, and the composite exhibited a capacity of 100 m A h g-1 at 1 C charge-discharge rate.