Optimizing pH conditions for impurity removal in closed-loop Li-ion battery recycling

Abstract

The closed-loop recycling or resynthesis of spent cathode active materials is an emerging hydrometallurgy-based Li-ion battery (LIB) recycling approach. The conventional acidic leaching process generates various impurities in addition to the constituent elements of Ni, Co, Mn, and Li from spent Li[Ni1-x-yCoxMny]O2 (NCM) cathode active materials. It is well known that even a trace amount of these impurities in resynthesized cathode active materials can affect the LIB performance significantly. Therefore, this study aims at investigating the effects of impurities on the physicochemical and electrochemical characteristics of resynthesized NCM by varying pH conditions during the purification of the LIB leachate. After the purification process where the pH is raised from 1.9 to 6.5, Al, Cu, and Zn are completely removed and other impurities including Fe, Mg, Ca, Li, and Na mostly or partially remain in the leachate. The physicochemical properties and LIB performance of resynthesized NCM at different pH conditions are obtained by various analytical techniques. The composition of leachate and cathode active materials is correlated with the initial discharge capacity, rate capability, and cyclability. Resynthesized NCM from the purified leachate shows a similar initial discharge capacity to pristine NCM. While the purification condition at a high pH aggravates the rate capability, the cyclability of resynthesized NCM is enhanced. This established correlation between the pH condition for the impurity removal in the LIB leachate and the LIB performance of resynthesized NCM would trigger the successful implementation of the closed-loop recycling of spent cathode active materials.