Evaluation of Surface Acid and Base Properties of LiFePO₄in 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 ∼ 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 an 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.

Morphology and Agglomeration Control of LiMnPO4 Micro- and Nanocrystals

“…In iron phosphate, acidic groups resulting from unsaturated metallic cations are supposed to dominate the surface properties in aqueous solution leading to a negative surface charge for pH-values even below 3. 36 This coincides with the isoelectric point at acidic pH-values. In contrast, our data presented above show that the dominant surface groups of LiMnPO 4 are less acidic.…”

“…Acidic conditions, however, were never reached in our experiments so that there is a clear contrast to LiFePO 4 . In iron phosphate, acidic groups resulting from unsaturated metallic cations are supposed to dominate the surface properties in aqueous solution leading to a negative surface charge for pH-values even below 3 . This coincides with the isoelectric point at acidic pH-values.…”

New eco-friendly low-cost binders for Li-ion anodes

“…For all these reasons, one of the goals for future research on battery technologies is the development of greener, cheaper, and electrochemically stable electrode binders [16]. Several of these problems could be solved or reduced by using water-soluble binders [15,[19][20][21][22]38]. Typically, aqueous binders have some advantages, for example, their cost is generally low, they are eco-friendly, and can be easily disposed at battery end of life.…”

Enhanced Electrochemical Performance of Graphite Anodes for Lithium-Ion Batteries by Dry Coating with Hydrophobic Fumed Silica

“…More recently, CMC has been used as a binder for lithium iron phosphate/carbon composite electrodes, incorporating carbon-coated LiFePO 4 and, even, for high voltage cathode electrodes based on Li-rich NMC. [55][56][57][58][59] Experimental Nano-coating of graphite.-Synthetic graphite (TIMREX SLP30, TIMCAL, Switzerland, d 50 = 16.0 μm/d 90 = 32.0 μm) and different hydrophobic fumed silica (Evonik-Degussa, Germany) were used as received. Graphite was mixed with 3 wt% of different hydrophobic fumed silica by a turbulent mixing process for 150 minutes to obtain the core-shell particles (Figure 1a).…”