Gelation mechanism of organic additives with LiFePO4 in the water-based cathode slurries

Carboxymethyl cellulose (CMC) is a commonly used thickening agent for water-based lithium (Li) ion batteries. Addition of CMC improves the electrochemical properties of most electrodes but excessive amounts may cause the corresponding electrode slurries to gel, making it unfavorable for casting. This study investigates the gelation mechanism of CMC with the popular active cathode material lithium iron phosphate (LiFePO4) in a water-based slurry. Since there are two main types of functional groups, carboxyl (-COOH) and hydroxyl (-OH), in the chemical structure of CMC, it is essential to clarify which functional group determines the gelation of the slurry. On comparing the experimental results obtained by substituting CMC with poly(vinyl alcohol) that carries merely the -OH group and poly(acrylic acid) that carries merely the -COOH group, it is evident that the -OH rather than the -COOH group causes the cathode slurry to gel. In addition, the gelation between CMC and LiFePO4 is more significant when the surface carbon on the LiFePO4 particles has more chemical defects. Contrary to the thickening agent containing the -OH group, that containing the -COOH group helps in the dispersion of the cathode materials in the aqueous slurry. The electrochemical results show that the improved dispersion of electrode slurries leads to better cell performance of Li-ion batteries.