Optimizing the Performance of NMC 622 Battery Half Coin Cells by Using Amphiphilic Aqueous Binders to Modulate the Percolation of Carbon Black
Authors: Patel Chirag , Poizer William , Omole Ayo , Middlemiss Laurence A. , Efimov Igor , Dunbar Alan , Hart Abarasi , Pérez Gabriel , Nedoma Alisyn 
Published: December 2024

N-methyl-2-pyrrolidone (NMP) is the conventional solvent for processing high-nickel cathodes as it dissolves the polymer binder polyvinylidene fluoride (PVDF) and does not leach nickel from the cathode material. NMP is restricted under REACH1 regulations as a ‘substance of very high concern’. Aqueous processing with hydrophilic polymer binders is an emerging alternative to the NMP/PVDF solvent/binder system. Aqueous processed cathodes offer advantages over conventional processing with NMP, namely nontoxic, unrestricted use and cost-effectiveness being 100 times less expensive than NMP/PVDF. However, carbon black is hydrophobic so it does not disperse in water. One way around this problem is to substitute the binder for one that is water-soluble so it can bind to and disperse the carbon black.

Aqueous binders have generally yielded unimpressive battery performance, therefore a novel binder system has been studied. In this work, an amphiphilic polymer (pluronic F68) was used to control the dispersion of carbon black in the slurry, and a high molecular weight polyethylene oxide (PEO) was used as a viscosity modifier. Measuring steady-state viscosity flow curves and oscillatory strain measurements, a narrow viscosity range in which carbon black percolates in the cathode slurry was identified, and validated by conductivity data. Working within this range of viscosities, a series of formulations were developed that maximize the mass fraction of active material whilst ensuring enough binder is present to disperse the carbon black in water. The PEO to pluronic ratio was varied for each formulation. Slurries are optically opaque and difficult to characterize, so zeta potential measurements were taken to assess the degree of carbon-black dispersion in the slurry and QCMD to quantify the density. The ratio of active material (NMC 622) to carbon black to polymer was 95:3:2 respectively. Ultimately, the formulations presented were of a constant viscosity, attained by controlling the solids loading of the slurries. Half-coin cells were cycled at charge and discharge rates of 1C, using a voltage range of 3.0 to 4.3 V.

An optimal ratio of 25% PEO to 75% pluronic-F68 was identified; these half-coin cells achieved a mean 1st specific discharge capacity of 147 mAhg-1 with capacity retention averaging 86%. Through adsorption measurements, it was found that the affinity between the binder and carbon black seems to correlate with the size of carbon black flocs that form, enabling the tuning of the percolation of carbon black in the slurry. By carefully controlling the viscosity, it was shown that electrochemical cell performance correlates very strongly with both the PEO to pluronic ratio and the slurry conductivity for this system, as seen in Fig.1. This data, along with adsorption and zeta-potential measurements indicate that the affinity of the polymer binder to carbon black can affect the electrochemical performance of the cell.

References

Regulation (EC) No 1907/2006 of the European Parliament and of the Council concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) 2019, Article 57.

Figure 1