Redox Ionic Liquid and Electroactive Lithium Salt As Redox Shuttle for Overcharge of Lithium-Ion Batteries
Authors: Gélinas Bruno , Bibienne Thomas , Dollé Mickael , Rochefort Dominic 
Published: February 2020

Lithium-ion technology currently answers the need for energy storage in many electronic devices due to their good cycle life and high energy density. Larger lithium-ion batteries (LIB) are also developed to meet the requirements for the transportation sector, but the performance and security still need to be improved. For instance, secondary reactions can occur during an overcharge which initiate when the positive electrode is over-oxidized. The over-oxidation of the positive electrode leads to an irreversible degradation of electrode material, generates radical and other reactive species, and oxygen evolution which can cause a fast thermal runaway into the LIB leading to its possible explosion. Several measures can be taken to avoid thermal runaway of the battery when an overcharge event occur. Amongst them, the use of a redox shuttle is particularly efficient since it will prevent the electrode from reaching an unstable oxidation state above the one obtained during normal charging.
Redox shuttles (R-S) operate by carrying the excess charge from the positive to the negative electrode to prevent reaching higher oxidation states. To be used efficiently, R-S must be stable in both oxidized and reduced states to maintain protection for a large number of cycles and should have an oxidation potential of at least 300 mV above that of the electrode to avoid self-discharge during normal cell operation. In addition, to achieve protection at high charging rates, it should diffuse rapidly and be solubilized at a high concentration in the electrolyte. We demonstrated recently that these conditions can be met by modifying the structure of ionic liquids with an electroactive group and that these R-S can be applied to prevent the overcharge of LiFePO4 electrodes for more than 200 cycles without affecting its charge storage properties. The modification of ionic liquids with redox moieties offers numerous possibilities to modulate the redox potential and the solubility of redox shuttles.
In this study, we propose to use redox ionic liquids (RILs) or electroactive lithium salts (ELi) as R-S. We present the physicochemical and electrochemical characterizations of RIL or ELi electrolytes such as viscosity and ionic conductivity measurements of LIB electrolyte and diffusion coefficient of electroactive species using cyclic voltammetry. Ionic liquid redox shuttles based on ferrocene and 2,5-di-tert-butyl-1,4-dimethoxybenzene have been tested in LIB, using different positive electrodes (e.g. LiTi2(PO4)3, and LiFePO4). We present charge–discharge curves with a 100% overcharge and long term overcharge curves for each R-S with the appropriate electrode at C/10. Charge-discharge curves with overcharge at different C rates were also done to obtain the Ragone plot and the results suggest a better protection above C/4 for the anion RIL and ELi than cation RIL because of migration effect favoring the modification of anion. The importance of the structure of the ionic liquid redox shuttles on the protection and stability upon cycling will be discussed.

Figure 1