Understanding fundamental effects of Cu impurity in different forms for recovered LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials

When identifying and developing lithium ion battery recycling technologies, understanding the role of impurities is always a concern as they may have an impact on electrochemical performance, material stability, or lifetime for recovered active materials. Here we show that Cu impurity in different forms could play very different roles on the recovered LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622). Cu metal impurity could easily short the cell, while Cu ion impurity could play a positive role due to partially substitution of the Ni²⁺ sites to lower the cation mixing degree. In specific, NCM622 material with certain amount (0.34 at%) of Cu ion impurity delivers high initial specific charge/discharge capacity of 211.8 and 186.0 mAh g^-1 at 0.05 C, which are 12.2 and 14.1 mAh g^-1 higher than those of virgin NCM622 cathode. Moreover, a superior retention capacity of 171.3 mAh g^-1 after 90 cycles at 0.33 C, and striking rate capability are achieved in this NCM622 cathode sample. These results clearly indicate that Cu metal impurity should be avoided and the ionic Cu impurity should be controlled with optimal concentration during recycling process for spent LIBs.