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We report that sol-gel coating of SnO2 and subsequent heat-treatment at relatively low temperature greatly improve the structural stability of LiCoO2 cathodes. Coated LiCoO2 materials heat-treated at 400 and 500 degrees C 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 LiCoO2 heat-treated at 600 degrees C that shows uniform Sn distribution throughout the particles exhibits this phase transition during cycling, as does uncoated LiCoO2, leading to 51% capacity loss after 47 cycles. This phase transition also increases cation disorder upon cycling.
Jaephil Cho (Fri,) studied this question.