Charge inversion and colloidal stability of carbon black in battery electrolyte solutions

• Specific adsorption of cations on carbon black in carbonate solvent.
• Charge inversion at 2 mM for Li+, 20 mM for Na+.
• First flocculation regime at 0.1–0.5 mM.
• Re-entrant colloidal stability for Li+ around 10 mM.
• All systems flocculated at high salt concentration.

We studied the influence of salt on a commercially available carbon black (Ketjenblack 600, KB) in carbonate solvents commonly applied in rechargeable batteries. Adopting the typically used salts: lithium hexa-fluorophosphate (LiPF6), lithium bis(trifluoromethane sulfonyl) imide (LiTFSI), as well as sodium hexafluorophosphate (NaPF6) dissolved in mixtures of ethylene carbonate and propylene carbonate, we investigated both the zeta potential and the flocculation kinetics of the KB particles as a function of salt concentration between 0.01 mM and 1.0 M. Clear evidence was found for the preferential adsorption of cations. In the absence of salt, KB was found to carry a negative surface charge, but this gets neutralized by Li+ at very low concentrations (∼1 mM), and by Na+ at intermediate concentrations (∼30 mM). In the case of lithium ions, the increased adsorption at higher concentration led to a recovery of the colloidal stability around 3–30 mM, depending on the anion. At high concentrations exceeding 30–100 mM, all salts cause flocculation of the KB particles, due to a reduction of the electric double layer thickness. Since the charge neutralization of the KB by Na+ takes place in the same concentration regime, no re-entrant stability is found for Na+. These findings could have implications in formulation protocols for semi-solid flow batteries, or other systems where an intermediate stable regime could assist mixing and/or structure formation at small length scales.

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