A novel capillary electrophoresis (CE) method with ultraviolet-visible spectroscopy (UV-Vis) detection for the investigation of dissolved Cu + and Cu 2+ 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 (LiPF 6). 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 Cu 2+. However, primarily Cu + dissolved from the passivating oxide layer (Cu 2 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 Cu 0 and Cu 2+ 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.

A capillary electrophoresis (CE) method with ultraviolet/visible (UV-Vis) spectroscopy for iron speciation in lithium ion battery (LIB) electrolytes was developed. The complexation of Fe 2+ with 1,10-phenantroline (o-phen) and of Fe 3+ with ethylenediamine tetraacetic acid (EDTA) revealed effective stabilization of both iron species during sample preparation and CE measurements. For the investigation of small electrolyte volumes from LIB cells, a sample buffer with optimal sample pH was developed to inhibit precipitation of Fe 3+ during complexation of Fe 2+ with o-phen. However, the presence of the conducting salt lithium hexafluorophosphate (LiPF 6) in the electrolyte led to the precipitation of the complex [Fe(o-phen) 3 ](PF 6) 2. Addition of acetonitrile (ACN) to the sample successfully redissolved this Fe 2+-complex to retain the quantification of both species. Further optimization of the method successfully prevented the oxidation of dissolved Fe 2+ with ambient oxygen during sample preparation, by previously stabilizing the sample with HCl or by working under counterflow of argon. Following dissolution experiments with the positive electrode material LiFePO 4 (LFP) in LIB electrolytes under dry room conditions at 20°C and 60°C mainly revealed iron dissolution at elevated temperatures due to the formation of acidic electrolyte decomposition products. Despite the primary oxidation state of iron in LFP of +2, both iron species were detected in the electrolytes that derive from oxidation of dissolved Fe 2+ by remaining molecular oxygen in the sample vials during the dissolution experiments.

DOI: https://doi.org/10.1002/elps.202000097 and https://doi.org/10.1002/elps.202000155 The back cover picture shows the dissolution of iron from the LiFePO4‐based positive electrode and copper from the current collector of the negative electrode into a lithium ion battery (LIB) electrolyte (center of picture). The transition‐metal dissolution plays a fundamental role in the progressive loss of capacity of the LIBs. Therefore, knowledge of the dissolved transition‐metal species is the key to understanding the dissolution mechanisms, as well as the capacity loss. However, dissolved iron and copper species in LIBs have not yet been investigated. Therefore, novel CE methods were developed to investigate Fe2+/Fe3+ and Cu+/Cu2+ in LIB electrolytes (electropherograms at the top and bottom). [The cover was updated October 22, 2020, after publication of the issue.]