Fluorine-Free Poly(ionic Liquid)s Binders for the Aqueous Processing of High-Voltage NMC811 Cathodes

Abstract

The use of water-soluble binders enables the transition to more sustainable batteries by the replacement of toxic N-methyl-2-pyrrolidone (NMP) by water. Herein, two new fluorine-free poly(ionic liquid)s are proposed as binders for LiNi0.8Mn0.1Co0.1O2 (NMC811) cathodes, based on poly(diallyldimethylammonium) (PDADMA) and two-phosphate counter anions, which are recognized as effective corrosion inhibitors and electrolyte additives. Due to their high ionic conductivity (10−6 S cm−1 at 25 °C) and ability to prevent degradation of NMC811 particles, the PDADMA phosphate cells are able to achieve a 91% of capacity retention after 90 cycles at 0.5C, similar to the organic fluorinated polyvinylidene fluoride (PVDF) (96%) under the same conditions. However, aqueous sodium carboxymethyl cellulose (Na-CMC) only provides 81% of capacity retention. Among the PDADMA-based binders under study, PDADMA- diethyl phosphate (PDADMA-DEP) delivers the highest discharge capacity (101.1 mAh g−1) at high C-rate (5C). Degradation of Na-CMC electrodes is observed in postmortem analysis and a notable increase in the charge transfer-resistance. However, the NMC811 particles preserve their spherical shape when PDADMA-phosphates are used as binders, also leading to lower polarization resistances and improved lithium diffusion. In conclusion, PDADMA-phosphates manifest high performance as binders for sustainable NMC811 cathodes, while disposing of fluoropolymers and toxic solvents.