Investigating the oxidation state of Fe from LiFePO₄‐based lithium ion battery cathodes via capillary electrophoresis

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 re‐dissolved 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.