Fluorine gradient-doped LiNi_0.5Mn_1.5O₄ spinel with improved high voltage stability for Li-ion batteries

The fluorine gradient-doped LiNi_0.5Mn_1.5O₄ spinels are synthesized by a facile one-step method and the effect of heat treatment on their structure, morphology, and electrochemical performance are investigated. The results show that introduction of fluorine leads to a larger lattice parameter and particle size, and the formation of F-enriched surface. Whereas at 400 °C, the fluorine gradient-doped LiNi_0.5Mn_1.5O₄ sample exhibits an improved long-term cycling stability and high rate performance, due to the suppression of the reaction between electrolyte and cathode, resulting in a decrease in the total resistance and the formation of a thin, uniform and smooth film on the surface. As a result of in situ XRD with charged pristine and the fluorine gradient-doped samples, the similar thermal-decomposition pathways from the charged spinel to the final NiMn₂O₄-type spinel structure with a small amount of NiMnO₃ and α-Mn₂O₃ are observed. In addition, the disappearance temperature of the charged spinel structures is at about 280 °C for the fluorine gradient-doped sample, exhibiting an improved thermal stability of high voltage cathode material. These results show that fluorine gradient-doped LiNi_0.5Mn_1.5O₄ sample is a promising positive electrode material for high performance lithium ion batteries.