Exploring the structure transformation mechanism of the spent cathodes during their regeneration process is the key to optimizing the processing protocol. The spent cathode materials are in different state of health (SOH) due to their different operation history, which leads to the challenge of restoring them in one-pot. Herein, to address the effect of SOH on the direct regeneration protocol, we systematically investigated the spent LiCoO2 (LCO) cathode with low and high SOH. We find that lithium-deficiency and Al impurity are the two important factors in affecting the regeneration quality of the spent LCO. Lithium-deficiency can cause void defects and disordered lattice structure. Al impurity is brought to the LCO surface during lithium replenishment process, which further diffuses into the LCO subsurface layer during the following high temperature sintering. Based on our understanding of the regeneration process, the regeneration protocol is optimized accordingly, which can successfully restore the spent LCO with different SOH.

中文翻译：

探索废旧正极在再生过程中的结构转变机制是优化处理方案的关键。废正极材料由于其不同的运行历史而处于不同的健康状态（SOH），这导致了对它们进行一锅式修复的挑战。在此，为了解决 SOH 对直接再生方案的影响，我们系统地研究了具有低和高 SOH 的废 LiCoO 2 (LCO) 正极。我们发现锂缺乏和Al杂质是影响废LCO再生质量的两个重要因素。锂缺乏会导致空隙缺陷和无序的晶格结构。补锂时Al杂质被带到LCO表面过程，在随后的高温烧结过程中进一步扩散到 LCO 亚表面层中。基于我们对再生过程的理解，对再生方案进行了相应的优化，可以成功地恢复具有不同SOH的废LCO。