In the present investigation, a stable slurry was prepared with a combination of two organic additives, namely CMC and polyacrylic acid (PAA). CMC was used primarily as a thickening agent to prevent the nano-sized LiFePO4 particles from settling and segregation during processing. PAA was introduced as a dispersant for LiFePO4. The effect of introducing PAA to the suspension was evaluated by measuring the electrokinetic response and shear rheology, as well as by conducting scanning electron microscopy observations for the formulated suspension. Finally, the electrochemical characteristic of electrodes by our formulated suspension was evaluated using a coin-type cell.
For electrokinetic measurements, suspensions containing a LiFePO4 mass fraction of 5% were prepared and equilibrated at room temperature for 12h, with magnetic stirring. The required amount of CMC was then added and the suspensions were equilibrated for an additional 12h. Electrokinetic curves were determined using an electroacoustic analyzer (Model ESA-9800, Matec Applied Sciences, Hopkinton, MA, USA) operating nominally at 1MHz. The measured electrokinetic sonic amplitude (ESA)is the pressure amplitude that is generated by the colloid per unit electric field strength and is directly proportional to the dynamic electrophoretic mobility, which, in turn, is a function of the particle zeta potential.
The rheological behavior of LiFePO4paste containing LiFePO4 and carbon black was determined at 25711C using a controlled stress rheometer (MCR501, Paar Physica, Stuttgart, Germany) with a concentric cylinder geometer (DG 27). The mass fraction ofLiFePO4and carbon black was 40% and 0.83%, respectively. Sample preparation followed the same procedure previously described forESA measurements. The apparent viscosity of LiFePO4suspensionswas measured as a function of ascending shear rate.
It is reported that PAA was given its higher adsorption affinity for the hydrophobic particles than CMC. Moreover, under basic conditions, PAA has a stretched confirmation due to electrostatic repulsion between neighboring carboxylgroups. Therefore, it is concluded that the addition of PAA enhances the dispersion properties of LiFePO4 through a steric interparticle repulsive force. The decrease of viscosity of LiFePO4suspensions by the addition of PAA results in increasing the concentration of solid phase, which is directly related to the electrochemical properties.
The dispersion properties of aqueous-based LiFePO4 particulates for the application of lithium-ion battery cathodes were investigated. It was found that the addition of PAA decreases the viscosity of LiFePO4 suspension, which indicated that the dispersion properties of LiFePO4 particles are enhanced through a steric interparticle force. An aqueous-based LiFePO4 cathode for a lithium-ion battery can be successfully fabricated with a homogeneous and dense microstructure. From the analysis of the electrochemical performance of Li-ion batteries, considering the addition of PAA dramatically decreases the viscosity of the LiFePO4 suspensions, the coin cell fabricated using CMC and PAA can potentially have much enhanced specific discharge capacity.