关键词: 锂离子电池, 硅氧材料, 石墨-硅氧混合材料, 膨胀性能, 膨胀改善

Abstract:

The silicon-based anode materials have the potential to meet the ever-increasing demand for energy density in lithium-ion batteries market owing to their high theoretical specific capacity. Unfortunately, their commercialization was hindered by the continuous volume expansion. Herein, the expansion characteristics and corresponding mechanism of the silicon oxide and graphite-silicon oxide composites were investigated by in-situ displacement detection systematically. The results showed that the expansion property was improved by material process modifications. During the de/lithiation processes of graphite, the expansion ratio in 30% ~ 50% SOC changed little because of the small interlayer spacing variation of the intercalated graphite. Unlike the graphite anode, there was no obvious platform in the expansion ratio curve of silicon oxide except for the first lithiation process. As for the graphite-silicon oxide composite, the expansion ratio was influenced by two-component materials. In order to figure out how the expansion ratio of the composite changed, the capacity contributions of graphite and silicon oxide at various states of charge were calculated. It was found that the graphite dominated the initial stage of the first and second delithiation processes, while delithiation of silicon oxide started from 36% SOC, leading to the steep decline of the expansion ratio curves. During the second lithiation process, the capacity of the first 20% SOC mainly came from silicon oxide, after which the capacity proportion of graphite increased gradually. In 40% ~ 50% SOC region, the capacity contribution of silicon oxide was negligible, resulting in the reduction of expansion increase rate. The calculated capacity contribution of the component materials corresponded to the evaluation of expansion ratio, indicating the reliability of the calculation method, which could be applied in other graphite-silicon oxide composites with different proportions. The irreversible expansion of graphite mainly occurred at the first three charges processes, while the irreversible expansion of silicon oxide increased significantly over all cycling processes. The reversible expansion of silicon oxide decreased gradually as the capacity fading. And the total expansion of silicon oxide tended to be decreased from the third cycle because the decrement of reversible expansion surpassed the increment of irreversible expansion. Finally, the expansion ratio especially the irreversible expansion of silicon oxide was effectively reduced by optimizing the surface coating, prelithiation and particle size. These results could provide favorable guidance for developing high-performance silicon-based anode materials with stable structure and low expansion ratio.

Key words: Li-ion battery, silicon oxide material, graphite-silicon oxide composite, expansion property, expansion improvement