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.