Degradation of Cobalt Oxide Anode for Lithium Batteries at High Conducting Material Content

Cobalt oxide is considered as promising option for the anode material in next generation rechargeable lithium battery thanks to its large specific capacity (~900 mAh/g). However, its low electrical conductivity causes poor energy efficiency and decreased power density. In order to lower the electrode resistance, many researchers have performed their experiments using the cobalt oxide anode containing large amount of conducting material. On the other hand, the overgrowth of the solid electrolyte interphase (SEI) layer on the cobalt oxide surface and the resulting decrease in specific capacity and energy density are another key issue for its practical use. Specifically, it has been known that the active materials move away from its original position to the overgrown SEI layer during the repetitive charge-discharge cycling and is gradually lost. Accordingly, the larger is the quantity of the SEI layer, the greater is the decrease in the specific capacity.

In this work, we report that the larger amount of conducting material in the cobalt oxide anode leads to its faster degradation during the operation. From the microscopic observation and porosity measurement of the pre- and post-cycled samples with different conducting material contents, it is proved that the free volume in the electrode decreases as the content of conducting material is raised. Furthermore, the analysis of the change in cumulative irreversible capacity loss during charge-discharge cycles shows that the high content of the conducting material promotes the growth of the SEI layer.

In order to further investigate the character of the SEI layers formed at different conducting material contents, the SEI layer formation is mathematically modeled on the basis of the Butler-Volmer relation. By fitting the model to the experimental data, a few physical parameters of the SEI layer can be successfully determined. In particular, from the calculation it is confirmed that the resistivity of SEI layer is considerably changed with the conducting material content. This implies that the incorporation rate of metallic cobalt in the SEI layer during cycling might be critically dependent on the conducting material content.

In this presentation, the optimal content of conducting material is discussed from the viewpoint of cycling stability and rate capability. Furthermore, the validity of the model for the systems used in this work is addressed by an analysis of fitting accuracy.