Improvement of Structural Stability of LiCoO_2 Cathode during Electrochemical Cycling by Sol-Gel Coating of SnO_2

摘要

We report that sol-gel coating of SnO_2 and subsequent heat-treatment at relatively low temperature greatly improve the structural stability of LiCoO_2 cathodes. Coated LiCoO_2 materials heat-treated at 400 and 500 ℃ exhibit excellent structural stability, retaining 86 and 84%, respectively, of their initial capacities after 47 cycles between 4.4 and 2.75 V at the 0.5 C rate. This is attributed to the disappearance of the transition to a hexagonal phase from a monoclinic phase at 4.15-4.2 V during cycling, a result of the formation of nonuniform Sn distribution throughout the particle, i.e., higher Sn concentration near the particle surface. In contrast, coated LiCoO_2 heat-treated at 600 ℃ that shows uniform Sn distribution throughout the particles exhibits this phase transition during cycling, as does uncoated LiCoO_2, leading to 51% capacity loss after 47 cycles. This phase transition also increases cation disorder upon cycling.