Research On The Modification Of Layered LiNiO₂ Cathode Materials

The cathode materials have a strong influence over the properties of the lithium-ion battery. Compounds such as LiCoO₂, LiMn₂O₄ and LiNiO₂ are attractive cathode materials for lithium-ion battery. Now the commercialized lithium-ion battery mostly uses LiCoO₂ as cathode material, but cobalt suffers the disadvantage of limited resource, high price and heavy pollution to environment. Though LiMn₂O₄ enjoys the advantage of low-price, easy preparation and non-toxicity to environment, yet its cycleability not good and specific capacity low. In this paper, both LiNi_1-xCo_xO₂ and LiNi_0.8-xCo_0.2Al_xO₂ materials synthesized by sol-gel method and LiNi_0.8Co_0.2O₂ coated Al₂O₃ by precipitation method have the same layered structure (α-NaFeO₂) with LiNiO₂. The ratio of doping element and coating amount and the relation between the ratio and the electrochemical performance are studied detailed.In the LiNi_1-xCo_xO₂ system, the research of electrochemical performance tests shows that LiNi_0.8Co_0.2O₂ have the best electrochemical properties, which had an initial discharge specific capacity of 170.40 mAh·g^-1 and retaining about 89.95% after 30 cycles.In the second part, the electrochemical performance of the LiNi_0.8Co_0.2O₂ cathode is modified by multiple substitutions with Al element by sol-gel method. Single-phase solid solution of LiNi_0.8-xCo_0.2Al_xO₂ has been synthesized and characterized by X-ray diffraction (XRD) and examined by charge-discharge cycling. The LiNi_0.77Co_0.2Al_0.03O₂ compounds show a discharge capacity of 149.51 mAh·g^-1 after 50 cycles that corresponds to 90.66% capacity retention. The results of XRD, cyclic voltammograms (CV), and electrochemical impedance spectrum (EIS) show that co-doping with Al stabilizes the layered structure, the phase transitions that occur during extraction or intercalation of Li⁺ ions are significantly suppressed, and the cyclic capability is improved.LiNi_0.8Co_0.2O₂ coated with Al₂O₃ was synthesized by precipitation method. The initial discharge specific capacity reduce somewhat, but the cycle performance had been improved. The initial discharge specific capacity is 164.84 mAh·g^-1 and retaining about 94.41% (155.63 mAh·g^-1) after 30 cycles, while the coating amount is 0.7%.