Investigation On Fluorine-doping And Properties Of The Spinel Lithium Nickel Mangenese Oxide

A detailed investigation of the structural and electrochemical impact of fluorine substitutions on the 4.7V LiNi0.5Mn1.5O4 cathode material for Li-ion batteries was accomplished. With different fluoride source of LiF and NH4F as precursor, fluorine substituted LiNi0.5Mn1.5O4 spinel powders have been synthesized by using the solid-state method. The effects of fluorine content and calcination temperature on structural, electrochemical, thermal properties and interface are extensively investigated. The main contents are as follows:First, the effects of LiF content on structural, electrochemical, thermal properties and interface are extensively investigated. FTIR results indicate that the cation disordering degree increases with an increasing in content of the co-doping with Li and F. The LiF concentration is optimized in the range of 0.06≤x≤0.2 in which the cyclic stability and rate capability of the Li1+xNi0.5Mn1.5O4-xFx spinels are significantly improved. Moreover, TEM images reveal that co-doped (Li+ and F-) particles form a smooth and uniform surface film, which mitigate the interaction of active materials and electrolyte solution as well as improve cycling stability. The graphite coated aluminum foil was used to substitute conventional aluminum foil as current collectors,5C discharge cycle performance of Li1.2Ni0.5Mn1.5O3.8F0.2 at room temperature and 55 ℃ is significantly increased. At room temperature, initial discharge capacity is 128 mAh/g. The capacity retention was 96.4% after 50 cycles. At 55 ℃, the initial discharge capacity can be reached 120.1 mAh/g, the capacity retention was 98.6% after 50 cycles.Second, the effects of calcination temperature on structural, electrochemical, thermal properties and interface are extensively investigated. When the temperature was higher than 700 ℃, the material structure studied by Raman and FTIR was transformated from the structure P4332 to the structure Fd3m. When calcination temperature is 400 ℃,500 ℃ and 900 ℃, the capacity retention of LiNi0.5Mn1.5O3.8F0.2 was 92.4%,92.4% and 92.4% respectively at 1C after 300 cycles at 25℃. The capacity retention was 92.1%,92.8%,96.1% respectively at 1C after 100 cycles at 55 ℃. The normalized discharge capacity was 83.7% at 10C after the calcination at 400 ℃ or 500 ℃. The graphite coated aluminum foil was used to substitute conventional aluminum foil as current collectors, the discharge capacity of LiNi0.5Mn1.5O3.8F0.2 after the calcination at 400℃ was 116mAh/g at IOC, and the capacity retention was 100% after 150 cycles. From EIS analysis concluded that Rsf of LiNi0.5Mn1.5O3.8F0.2 decreased. When using carbon coating aluminum foil, Rsf and Rct has significantly decreased; TEM images reveal that LiNi0.5Mn1.5O3.8F0.2 particles form a smooth and uniform surface film, which mitigate the interaction of active materials and electrolyte solution as well as improve cycling stability.Besides, the thermal property of the fluorine substitutions on LiNi0.5Mn1.5O4 can effectively suppress the exothermic reaction according to the DSC results, which improve the thermal stability of the spinel material.