Study On Preparation And Interface Modification Of LiNi_0.6Co_0.2Mn_0.2O₂ Cathode Materials Of Lithium Ion Battery

Lithium-ion batteries?LIBs?have been extensively applied as power sources for portable electronic devices since their first commercialization by Sony in 1990s.In recent years,the huge interests in electric vehicles,including pure electric vehicles?PEVs?and plug-in hybrid electric vehicles?PHEVs?have stimulated more intensive research to develop high-energy-density and low-cost LIBs.Compared to traditional LiCoO₂ cathode,Ni-rich ternary layered oxides(LiNi_xCo_yMn_1-x-yO₂,NCM)are supposed to be more applicable in future LIBs because of their lower costs?less Co?and comparable or even higher energy density.High Ni content offers high capacity of the material due to the two-electron Ni⁴⁺/Ni²⁺redox reaction,but over high Ni also leads to significant Li⁺/Ni²⁺cation mixing because the ionic radius of Li⁺and Ni²⁺are very close to each other.Among many NCM candidates,the 622-type LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?has become a global mainstream cathode material since it compromises well between the multiple criteria of LIBs including electrochemical performance,safety and cost.However,the co-precipitation methods generally require rigorous control of multiple parameters such as reaction temperature,concentration of ammonia and solution pH value,which makes the production process time-and energy-consuming.On the other hands,NCM622 still have room to grow with regard to improving capacity density,cycle stability and safety performances.Thus,it is urgent to find an effective method to improve the electrochemical performance of NCM622.Therefore,we investigate these two issues in this paper.?1?We report the use of oxalic acid to precipitate Ni-Co-Mn acetates for the synthesis of the NCM-622 precursor and studied the electrochemical performances of the materials after solid-state reaction with LiOH.The as-obtained precursor show excellent homogeneity in morphology.The sample sintered at 850°C shows the best cycling stability and rate capability due to high quality of its layered structure exhibiting the lowest level of cation mixing,which is evidenced by XRD and XPS.The as-synthesized NCM-622 delivers a capacity of 152 mAh g^-11 after 100 cycles at 1 C?93.2%retention?,and shows excellent rate capability with 76.8 mAh g^-1capacity at10 C.In addition,the effect of the sintering temperature on the material electrochemical performance was systematically studied.The results show that the intermediate temperature of 850°C is found more suitable for the synthesis of NCM-622,at which the solid state reaction and Li loss can reach a better balance.?2?Based on the synthesis of the NCM-622 by oxalic acid,the oxide precursor is modified with Al₂O₃ by solution mixing with Al₂?SO₄?₃ followed by post-calcination in order to improve the electrochemical performance of NCM622.The Al₂O₃ modified method is not only served as an effective physiochemical protective layer to mitigate surface reactions,but also as dopants to stabilized layered structure.After a facial wet-chemical Al₂O₃ coating of the precursor,the cycling stability at elevated temperature,storage behavior and high-voltage performances of the NCM-622 electrode are significantly enhanced.The oxalate co-precipitation method shows the advantages of low-cost and easy control in comparison with many other methods,which opens up another opportunity to synthesize cathode materials for high energy-density LIBs.