Carrageenans as Sustainable Water-Processable Binders for High-Voltage NMC811 Cathodes

Poly(vinylidene fluoride) (PVDF) is the most common binder for cathode electrodes in lithium-ion batteries. However, PVDF is a fluorinated compound and requires toxic N-methyl-2-pyrrolidone (NMP) as a solvent during the slurry preparation, making the electrode fabrication process environmentally unfriendly. In this study, we propose the use of carrageenan biopolymers as a sustainable source of water-processable binders for high-voltage NMC811 cathodes. Three types of carrageenan (Carr) biopolymers were investigated, with one, two, or three sulfonate groups (SO₃^-), namely, kappa, iota, and lambda carrageenans, respectively. In addition to the nature of carrageenans, this article also reports the optimization of the cathode formulations, which were prepared by using between 5 wt % of the binder to a lower amount of 2 wt %. Processing of the aqueous slurries and the nature of the binder, in terms of the morphology and electrochemical performance of the electrodes, were also investigated. The Carr binder with 3SO₃^- groups (3SO₃^-Carr) exhibited the highest discharge capacities, delivering 133.1 mAh g^-1 at 3C and 105.0 mAh g^-1 at 5C, which was similar to the organic-based PVDF electrode (136.1 and 108.7 mAh g^-1, respectively). Furthermore, 3SO₃^-Carr reached an outstanding capacity retention of 91% after 90 cycles at 0.5C, which was attributed to a homogeneous NMC811 and a conductive carbon particle dispersion, superior adhesion strength to the current collector (17.3 ± 0.7 N m^-1 vs 0.3 ± 0.1 N m^-1 for PVDF), and reduced charge-transfer resistance. Postmortem analysis unveiled good preservation of the NMC811 particles, while the 1SO₃^-Carr and 2SO₃^-Carr electrodes showed damaged morphologies.