Quantitative Electrochemical Analysis Method for Cu Impurities in Nickel-Cobalt-Manganese Cathode Materials

Lithium-ion batteries are among the most important energy-storage devices. In this regard, nickel-cobalt-manganese (NCM) cathodes are widely used because of their high energy density and stability. Cu on NCM can enhance the overall performance by aiding lithium-ion transport through cation mixing; however, it leads to issues, such as internal short circuits. The precipitation pH of Cu is high, making its chemical separation from the NCM challenging. Given the impacts and the challenge of separation, an accurate quantification of the residual Cu content in the NCM cathode is essential. Inductively coupled plasma methods struggle with the accurate quantification of trace impurities in NCM owing to the high contents of material elements, leading to instrument malfunction and time-consuming labor. In this study, the introduction of electrochemical methods significantly weakened the matrix effect and facilitated the pretreatment of the solution. In particular, a thin-film electrode (TFE) made of Rh allowed quantification of the Cu present in commercial NCM powder. Cyclic voltammetry and an electrochemical quartz crystal microbalance were used to confirm the formation of two types of underpotential deposition (UPD) Cu on the Rh TFE. Square-wave voltammetry was used to analyze the kinetic differences in Cu_upd and quantify trace amounts of Cu with high sensitivity. The results included a relative standard deviation of 2.54%, linear range of 13-450 ppb, and limit of detection of 3.9 ppb. The method was successfully applied to commercial NCM products, where the standard addition method determined Cu content in the range 40-60 ppb. This method provides standardized guidelines for both laboratory and industry for evaluating the effects of impurities across various NCM cathodes.