Analyzing the capacity fading mechanism in high-capacity Cr8O21 as a cathode material for Li-ion batteries

The capacity fading mechanism of Cr8O21 remains unclear owing to the lack of reliable evidence. To elucidate the source of capacity loss, we investigated the structures, the structural evolution process, and phase transitions of lithiated Cr8O21 (LixCr8O21) using first-principles calculations. On the one hand, we found that LixCr8O21 adopts a sandwich structure at low Li content but exhibits a rock salt-like structure at high Li content. Compared to the sandwich-like LixCr8O21, the rock salt-like LixCr8O21 possesses a reduced volume and, therefore, decreased interstitial Li sites. The rock salt-like LixCr8O21 structure can maximally store 14 Li, less than 4 Li compared to the sandwich structure. Therefore, this structural change from sandwich-like into rock salt-like is a reason for the capacity loss in Cr8O21 in the discharge process. On the other hand, the convex hull and phase diagram demonstrate that the rock salt-like LixCr8O21 is thermodynamically unstable, and a portion of it transforms into active LiCrO2 during the first discharge. However, LiCrO2 is then converted into CrO2, rather than Cr8O21 in the subsequent charge process. This irreversible phase transition (Cr8O21 → LiCrO2 in the discharge process and LiCrO2 → CrO2 in the charge process) is another significant cause for the capacity loss in Cr8O21. This study provides key theoretical evidence for the drastic capacity fading in Cr8O21 and thereby facilitates the future development of high-capacity cathodes for lithium-ion batteries.