Investigation of Sodium Leaching into Electrolytes during Electrochemical Testing

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Many mechanisms and degradation pathways have been identified in the literature that result in leaching of transition metals from the cathode, resulting in loss of capacity during cycle life testing due to fewer sites for lithium intercalation into the cathode during discharge1. But electrode formulations contain more components than just active material- there are also binders, conductive carbon, other additives, and coatings. The extent to which these other components can break down and cause electrode degradation and any subsequent effects on cell performance is not well known. This poster explores the chemical and electrochemical influences on extraction of materials from electrodes, such as sodium, that may be harmful to battery performance. Post mortem analyses of electrolytes after electrochemical testing in multilayer pouch cells have found sodium content in the range of 200-300 ppm, well above typical specifications for battery-grade products (1-10 ppm). The most probable source of sodium in the battery is sodium carboxymethyl cellulose (CMC), often used in graphite anodes as a rheology modifier during production. In this study, the leachability of sodium from CMC in various extraction solutions was investigated through model systems. Model system experiments have shown that while the sodium in CMC was insoluble in organic solvents and electrolyte, sodium can be extracted by contact between salt-containing electrolytes and CMC-containing anodes. The amount of sodium recovered in the model system closely corresponds with the amount that was recovered from cells after electrochemical operation. Other anode and cell components were also analyzed as potential sources of sodium, and the prospect of each of these as a source of sodium was explored. Finally, the effect of different levels of sodium content on cell performance was rigorously tested by doping lithium-ion electrolytes with NaPF6 and testing these electrolytes in pouch cells. Overall, we show that sodium can be a major contaminant extracted into the electrolyte during cycling in pouch cells, and investigate the impact of sodium content on critical performance metrics.1 Ochida, M.; Domi, Y.; Doi, T.; Tsubouchi, S.; Nakagawa, H.; Yamanaka, T.; Abe, T.; Ogumi, Z. Influence of Manganese Dissolution on the Degradation of Surface Films on Edge Plane Graphite Negative-Electrodes in Lithium-Ion Batteries. Journal of the Electrochemical Society 2012, 159(7), A961-A966.