Comparative studies of three redox shuttle molecule classes for overcharge protection of LiFePO4-based Li-ion cells

Three classes of molecules have been shown to be successful for protecting lithium-ion cells during overcharge. These three classes are, the molecule 2,5-di-tert-butyl-1,4-dimethoxybenzene (DDB), molecules based on a phenothiazine core, such as 10-methylphenothiazine (MPT) and molecules based on a 2,2,6,6-tetramethylpiperidinyl-oxide (TEMPO) core, such as TEMPO and 4-cyano-TEMPO. These molecules were examined using Li-ion coin cells, three-electrode cyclic voltammetry and four-electrode cyclic voltammetry. Three-electrode cyclic voltammetry was used to measure the diffusion coefficients and the stability of the shuttle molecules at high and low potentials. The transport of electrons through the solid-electrolyte interface on the negative electrode to the oxidized shuttle molecule was studied using the four-electrode cell for electrolytes containing both LiPF₆ and lithium bis(oxalato)borate salts. The rate of charge transfer to the oxidized TEMPO and MPT molecules is significantly reduced on glassy carbon below 1.7 V (versus Li/Li⁺) in electrolytes containing LiBOB, but not in electrolytes containing LiPF₆. Charge transfer to oxidized DDB seems facile at all potentials above 0.2 V in both LiPF₆ and LiBOB electrolytes.