Accessing copper oxidation states of dissolved negative electrode current collectors in lithium ion batteries

A novel capillary electrophoresis (CE) method with ultraviolet-visible spectroscopy (UV-Vis) detection for the investigation of dissolved Cu⁺ and Cu²⁺ in lithium ion battery (LIB) electrolytes was developed. This method is of relevance, as the current collector at the anode of LIBs may dissolve under certain operation conditions. In order to preserve the actual oxidation states of dissolved copper in the electrolytes and to prevent any precipitation during sample preparation and CE measurements, neocuproine (NC) and ethylenediamine tetraacetic (EDTA) were effectively applied as complexing agents for both ionic copper species. However, precipitation and loss of the Cu⁺ -NC-complex for quantification occurred in presence of the commonly applied conducting salt lithium hexafluorophosphate (LiPF₆ ). Therefore, acetonitrile (ACN) was added to the sample in order to suppress this precipitation. Dissolution experiments with copper-based negative electrode current collectors in a LIB electrolyte were conducted at 60°C under non-oxidizing atmosphere. First findings regarding the copper species via CE revealed dissolved Cu⁺ and mainly Cu²⁺ . However, primarily Cu⁺ dissolved from the passivating oxide layer (Cu₂ O and CuO) of the current collector due to the formation of acidic electrolyte decomposition products. Due to the instability of Cu⁺ in the electrolyte a further disproportionation reaction to Cu⁰ and Cu²⁺ occurred. The results show the high potential of this CE method for prospective investigations regarding the current collector stability in new battery electrode formulations and correlations of dissolution events with dissolution mechanisms and battery cell operation conditions.