Due to regulatory pressure, fluorine-free alternatives to polyvinylidene fluoride (PVDF) as cathode binders have become potentially important for the industrial production of batteries. However, the majority of research into fluorine-free binders has focused on aqueous processing, which is not a viable short-term alternative for existing gigafactories that rely on the solvent N-methyl-2-pyrrolidone (NMP).
Within this study, a bio-based polysaccharide binder X that is soluble in NMP was used and compared with PVDF. For both binders, electrode pastes were optimised to achieve the same viscosity during electrode coating process within standard deviations. Interestingly, the Binder X electrode pastes exhibit thixotropic behaviour, which is why the binder content was reduced to 1.5 wt.% of the solid content. Despite the lower binder content compared to PVDF electrodes, a relatively high adhesion strength of (120 ± 20) N cm⁻² was measured ((232 ± 2) N cm⁻² for PVDF electrodes).
The carbon-binder domain has an altered structure in the case of Binder X electrodes, as agglomerates can be seen in SEM images. This is due to the binder having a stronger affinity for the active material. This accounts for the higher through-plane resistance (3.7 ± 0.8 Ω vs. 0.80 ± 0.04 Ω). Modifying the side groups of binder X would improve the structure of the carbon-binder domain; however, this is beyond the scope of this study and will be addressed in a follow-up study.
Electrochemically, Binder X is stable up to 5.0 V vs. Li|Li⁺. Excellent long-term stability is also observed, although the initial capacity is slightly lower. Thus, the study provides proof of concept for processing polysaccharides in an NMP solvent. Following further optimisation, binder X could be used as a drop-in solution in current cathode production.