Influence of annealing temperature on the electrochemical and surface properties of the 5-V spinel cathode material LiCr_0.2Ni_0.4Mn_1.4O₄ synthesized by a sol–gel technique

LiCr_0.2Ni_0.4Mn_1.4O₄ was synthesized by a sol–gel technique in which tartaric acid was used as oxide precursor. The synthesized powder was annealed at five different temperatures from 600 to 1,000 °C and tested as a 5-V cathode
material in Li-ion batteries. The study shows that annealing at higher temperatures resulted in improved electrochemical performance, increased particle size, and a differentiated surface composition. Spinel powders synthesized at 900 °C had initial discharge capacities close to 130 mAh g⁻¹ at C and C/2 discharge rates. Powders synthesized at 1,000 °C showed capacity retention values higher than 85 % at C/2, C, and 2C rates at 25 °C after 50 cycles. Annealing at 600–800 °C resulted in formation of spinel particles smaller than 200 nm, while almost micron-sized particles were obtained at 900–
1,000 °C. Chromium deficiency was detected at the surface of the active materials annealed at low temperatures. The XPS results indicate presence of Cr⁶⁺ impurity when the annealing temperature was not high enough. The study revealed that increased annealing temperature is beneficial for both improved
electrochemical performance of LiCr_0.2Ni_0.4Mn_1.4O₄ and for avoiding formation of Cr⁶⁺ impurity on its surface.