Influence of annealing temperature on the electrochemical and surface properties of the 5-V spinel cathode material LiCr0.2Ni0.4Mn1.4O4 synthesized by a sol-gel technique

LiCr0.2Ni0.4Mn1.4O4 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 A degrees 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 A degrees C had initial discharge capacities close to 130 mAh g(-1) at C and C/2 discharge rates. Powders synthesized at 1,000 A degrees C showed capacity retention values higher than 85 % at C/2, C, and 2C rates at 25 A degrees C after 50 cycles. Annealing at 600-800 A degrees C resulted in formation of spinel particles smaller than 200 nm, while almost micron-sized particles were obtained at 900-1,000 A degrees 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 LiCr0.2Ni0.4Mn1.4O4 and for avoiding formation of Cr6+ impurity on its surface.