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

• Published in: Electrophoresis 2020-10, Vol.41 (18-19), p.1549-1556
• Main Authors: Hanf, Lenard, Diehl, Marcel, Kemper, Lea‐Sophie, Winter, Martin, Nowak, Sascha
• Format: Article
• Language: English
• Subjects: Acetonitrile; Acidic oxides; Argon; Capillary electrophoresis; Cathode; Cathodes; Complexation; Counterflow; Dissolution; Electrode materials; Electrolytes; Electrolytic cells; Electrophoresis; Ethylenediamine; Ethylenediaminetetraacetic acids; Ferric ions; Ferrous ions; High temperature; Iron; Iron speciation; Lithium; Lithium ion battery; Optimization; Oxidation; Oxygen; Rechargeable batteries; Speciation; Transition metal dissolution; Valence
• ISSN: 0173-0835; 1522-2683
• DOI: 10.1002/elps.202000097

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 Fe2+ with 1,10‐phenantroline (o‐phen) and of Fe3+ 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 Fe3+ during complexation of Fe2+ with o‐phen. However, the presence of the conducting salt lithium hexafluorophosphate (LiPF6) in the electrolyte led to the precipitation of the complex [Fe(o‐phen)3](PF6)2. Addition of acetonitrile (ACN) to the sample successfully re‐dissolved this Fe2+‐complex to retain the quantification of both species. Further optimization of the method successfully prevented the oxidation of dissolved Fe2+ 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 LiFePO4 (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 Fe2+ by remaining molecular oxygen in the sample vials during the dissolution experiments.