Failure mode of thick cathodes for Li-ion batteries: Variation of state-of-charge along the electrode thickness direction

The construction of thick electrodes with high loading level is considered one of the promising approaches to improving the energy density of Li ion batteries because it can reduce the amounts of separators and current collectors in a cell. Thick electrodes, however, show poor rate capability and cycle performance compared to thin electrodes. Although the correlation between electrode thickness and electrochemical performance has been extensively investigated, the failure mechanism of thick electrodes has not yet been fully demonstrated. In this paper, a direct spatial analysis using the configuration of multi-stacked electrodes is introduced to investigate the failure mode of thick Li[Ni0.6Co0.2Mn0.2]O2 electrodes. Each layer of the multi-stacked electrodes is facilely separated after cycling, followed by performing various ex situ analyses on each layer, independently. Eventually, the degradation behavior of thick electrodes, such as the uneven state-of-charge (SOC) distribution along the electrode thickness direction during cycling, is demonstrated in terms of the deterioration of Li+ ion conduction in electrolytes. Thicker CEI layers are accumulated on the cathode surface closer to the outermost electrode during cycling, leading to pore clogging initiated at the outermost electrode. This results in a greater increase in ionic resistance closer to the outermost electrode during cycling. Changes in ionic resistance along the electrode thickness direction during cycling give rise to the uneven SOC distribution, thus resulting in the poor cycle performance of thick electrodes. Therefore, a particle size gradient electrode, in which the particle size gradually increases along the electrode thickness direction from the innermost to the outermost electrode, is introduced to mitigate the SOC inhomogeneity of thick electrodes during cycling. The optimized pore and particle size distributions along the electrode thickness direction suppress the uneven cell resistance during cycling, leading to the improved cycle performance of thick electrodes. These findings provide fundamental insights into strategies to design thick electrodes for high energy density Li ion batteries.