Interactions Between LiFePO4 and Binders in Water-Based Electrode Slurries

Since 1997, lithium iron phosphate (LiFePO₄) has been well-known as a potential cathode-active material for a lithium-ion battery because of its advantages of having large theoretical capacity, good cycle-life performance and better safety. Nevertheless, LiFePO₄ has other disadvantages, such as its poor electrical conductivity and low diffusion for lithium ions (Li⁺), and thus it is common for the commercial LiFePO₄ to have surface coating of conductive materials, which carbon is the most popular one, to enhance the electrical conductivity and the Li⁺ diffusivity.

On the other hand, for considerations of environmental consistency and low cost, the technique of fabricating electrodes based on a water-based process by using water as the mixing solvent and water compatible polymer as the binder is gaining favor and has attracted significant interest. In this investigation, the interactions between the cathode active powder, LiFePO₄, and the water soluble binder, SBR and CMC binder blend, in the water-based electrode slurries were discussed. The experimental results showed that the factor of the quality of the surface carbon coating on LiFePO₄ dominated the interactions between LiFePO₄ and the binder. When the surface carbon coating on the active powder was less completed and showing more exposure of the inner LiFePO₄, the active powder gelled with the binder significantly and caused the as-prepared water-based electrode slurry to exhibit poorer dispersion property and higher elastic flow behavior. The poorer dispersion of slurry resulted in a less-uniform distribution of compositions in the dried electrode sheets and harmed the electrochemical performance. In addition, the initial mechanism for the gelation of LiFePO₄ with the binder blend had also been proposed, which was highly probable due to the chemical chelation between the exposed iron from the inner LiFePO₄ and the hydroxyl group of CMC.