Evaluation of Surface Acid and Base Properties of LiFePO4 in Aqueous Medium with pH and Its Electrochemical Properties

Abstract

Changes in the surface chemistry of LiFePO4 cathodes in an aqueous medium for lithium ion battery are investigated based on a quantitative evaluation of surface functional groups of LiFePO4 as a function of pH. It is found that, while the dissolution of Li ions from LiFePO4 increases the number of acidic surface groups, the dissolution of Fe ions reduces the number of basic surface groups. The pK(a) values for various processing conditions are calculated. Interestingly, none of the investigated processing conditions changes the characteristic acid moieties on the particles (pK(a) similar to 2.8). However, the processing condition strongly impacts the chemistry of the basic surface groups showing three distinct basic surface groups with pK(b) values of 0.1, 0.9, and 1.5, respectively. Correlation is made between dissolution behavior, surface groups, dispersion properties, and electrochemical properties. It is found that Li-and Fe-ion depleted LiFePO4 powder increases the viscosity in N-methylpyrrolidone pastes 5-fold and results in a looser and less homogeneous microstructure. This results From a lower zeta-potential of LiFePO4 paste at pH 7-8 caused by all increased number of acid groups and decreased number of basic groups. The changes in surface acid and base properties of LiFePO4 caused by significant dissolution and readsorption of Li and Fe ions in aqueous processing lead to a degraded rate capability for LiFePO4 cathodes, especially at 5 and 6C.