关键词: 电极形状, 锂化过程, 原位观测, 曲率, 电场分布

Abstract:

This paper studies the influence of electrode shape on the lithiation process of lithium ion batteries. Both experimental observation and numerical simulation are employed to investigate the competitive interaction between the diffusion of lithium ions in both solid and liquid phases and the lithium intercalation reaction at the electrode surface. Experimental cells were prepared with the anode and cathode being placed parallel, leaving the latter embracing the former. An experimental device based on CCD camera was set up for in situ observation of electrode lithiation. The lithiation levels of the graphite anodes were estimated according to the observed color profile. Three shapes of electrodes, namely, circular, square and triangular shapes, were investigated. It is found that the lithiation levels were not uniform for all cases. The edge areas of all anodes were lithiated to approach saturation quickly, meanwhile the core areas of the anodes remained in very low lithiation level. The sharp tips of the electrode with high curvature were more likely to have more lithium ions intercalated, leading to quick saturation and even lithium dendrite deposition. Compared with the electrode voltage-capacity curves, although the recorded reacted capacity was equal to the theoretical capacity of the anode at the end of charge operation, the color profiles show that the anodes were far from full saturation. In addition, large clusters of lithium dendrite depositions were found at the electrode edges. It indicates that quite a portion of lithium ions were consumed in side reactions instead of being intercalated into the anode. Numerical simulations reveal that the non-uniform lithiation is induced due to the combination effects of the electric field distribution, the lithium flow in the electrolyte and the lithium concentration distribution in the active material. The electrode edges lead to a singular distribution of the electric field in the electrolyte, resulting in a concentrated flow of lithium ions in an electrolyte. Therefore, the electrode edges are subjected to excessive supply of lithium ions in an electrolyte, leading to quick saturation and even dendrite deposition in the edge areas. At the same time, the core area of electrode cannot capture enough lithium ions from an electrolyte and, therefore, remain in low lithiation level. This effect is more significant in the electrodes with sharper tips. For example, the square and triangular anodes show more heterogeneous distribution of lithiation level than the circular anodes. In addition, more lithium dendrites are found around the electrode tips. Therefore, electrode designs with irregular shapes should be avoided to minimize the edge effects. The electrode surface should also be prepared smoothly to reduce the edge effects due to rough surfaces. This work sheds some lights on the understanding the lithiation process of lithium-ion batteries. It would be helpful in the design of lithium-ion batteries.

Key words: electrode shape, lithiation process, in-situ observation, curvature, electric field distribution