Solid Electrolyte Interphase (SEI) of Water-Processed Graphite Electrodes Examined in a 65 mAh Full Cell Configuration

Electrode binders, such as sodium carboxymethyl cellulose (CMC-Na), styrene–butadiene rubber (SBR) and poly­(sodium acrylate) (PAA-Na) are commonly applied binder materials for the manufacture of electrodes from aqueous slurries. Their processability in water has considerable advantages over slurries based on N-methylpyrrolidone (NMP) considering toxicity, environment and production costs. In this study, water-processed graphite electrodes containing either CMC-Na:SBR, PAA-Na, or CMC-Na:PAA-Na as binders have been prepared on a pilot scale, cycled in graphite||LiFePO₄ Li-ion battery cells and analyzed post-mortem with respect to the binder impact on the SEI composition, using in-house (1486.6 eV) and synchrotron-based (2300 eV) photoelectron spectroscopy (PES). The estimated SEI layer thickness was smaller than 11 nm for all samples and decreased in the order: PAA-Na > CMC-Na:SBR > CMC-Na:PAA-Na. The SEI thickness correlates with the surface concentration of CMC-Na, for example, the CMC-Na:PAA-Na mixture showed signs of polymer depletion of the PAA-Na component. The SEI layer components are largely comparable to those formed on a conventional graphite:poly­(vinylidene difluoride) (PVdF) electrode. However, the SEI is complemented, by notable amounts of carboxylates and alkoxides, whose formation is favored in water-based negative electrodes. Additionally, more electrolyte salt degradation is observed in formulations comprising PAA-Na. The choice of the binder for the negative electrode had little impact on the surface layer formed on the LiFePO₄ positive electrode, except for different contents of sodium salt deposits, as a result of ion migration from the counter electrode.