Enabling Aqueous Processing for Low/No-Cobalt Lithium-Ion Battery Cathode Active Materials

Abstract

Industrially, lithium-ion battery cathodes are manufactured through a wet slurry mixing and coating process. There is great interest in replacing the presently-used N-methyl-2-pyrrolidone (NMP) solvent with water due to the cost and toxicity associated with NMP. In this study, the modifications made to the active materials as a result of water exposure were studied and two materials, LiMn2O4 (LMO) and LiNi0.8Co0.15Al0.05O2 (NCA), were assembled in high-performing aqueous-processed cathodes for the first time.Five cathode active materials were the subject of this study: LiCoO2 (LCO), LiFePO4 (LFP), LMO, NCA, and LiNi0.5Mn0.3Co0.2O2 (NMC532). All materials exposed to DI water exhibited rapid increases in pH over the first 10 min, with stable pH values ranging from 9.0 (LFP and LMO) to 12.5 (NCA). Given that the protective aluminum oxide layer on the current collector breaks down at a pH of 8.5, corrosion is predicted for all materials, with NCA being the most extreme case. Pristine samples of the same five materials were submerged in water for 24 h, then the moisture was dried in a vacuum oven. The crystal structures of these materials (henceforth referred to as the wetted samples) were evaluated using X-ray diffraction and compared with pristine samples. No changes were observed in the crystal structure of any material as confirmed by Rietveld refinement. X-ray photoelectron spectroscopy was used to evaluate the changes of the surface composition of each wetted and pristine material.Based on inductively coupled plasma-mass spectrometry results, the amount of Li and transition metal leaching in water was found to be independent of time in excess of 1 h, and the amount of Li leaching in water for NCA was found to be five times that of NMC532 and more than 30 times that of the other materials. As a result, using a traditional aqueous slurry formulation (active material, carbon black, emulsion binder, carboxymethylcellulose thickener) resulted in a severely cracked and unusable NCA cathode. By including high-molecular weight polyacrylic acid (PAA), the adhesion of the cathode was restored. Half coin cells of this PAA-processed and an NMP-processed NCA cathode were constructed using punches with 1.7 mAh·cm-2 areal capacity and a charge and discharge rate of C/3 between 3.0-4.3 V. The initial capacity of the PAA-processed NCA cathode was slightly lower (176.3 mAh·g-1 to 185.2 mAh·g-1), though it did exhibit better capacity retention through 100 cycles (84.2% to 78.1%).