The Effect of 2,2,6,6-Tetramethylpiperidinyl-Oxide (TEMPO) As an Electrolyte Additive and Its SEI Formation on Mcmb-Electrode

Abstract

The stable radical molecule, 2,2,6,6-tetramethylpiperidinyl-oxide which is commonly known as TEMPO, is well known for its unusual stability as a neutral radical. (1, 2) The use of TEMPO and its derivatives have been examined in secondary batteries, since the year 2000. It is mainly used to enhance cycling performance, as a fire retardant, or as an additive in lithium sulfur batteries. (3) The TEMPO molecules are also used as liquid electrolyte. (4) It can also be used as redox shuttles in lithium-ion batteries. (5) In terms of the protection offered for a length of overcharge, TEMPO molecules are among the best shuttle molecules. (3-5) Recently, Dagger et al. (6) investigated an electrolyte combinations including both flame retardant (FRs) and film forming additives (FFAs) in order to generate synergetic effects and create electrolytes with enhancement in safety and electrochemical performance. The synergy of the FR and FFAs gives an effective SEI, diminishing the electrolyte ageing effect and also elevation of the life-time of the cycled cells at elevated C-rates. Incorporation of 2% TEMPO as an additive into STD + 5% FEC electrolyte results in a modification of the MCMB-electrode SEI. The direct in-situ DRIFTS analysis of the electrolyte additive; TEMPO, give a substantial insight into the nature SEI-formation of the negative electrode. The beneficial effect from the TEMPO additive might be the suppression of the solvent/additive decomposition by quenching the radicals formed. The positive effect of TEMPO was explained by the formation of polymeric species like, poly (VC), polycarbonate and also ROCOOLi, Li2CO3 and alkyl phosphorous fluorides. The studies described herein demonstrate that MCMB-electrodes in STD + 5% FEC solution containing TEMPO additive can be utilized together with high voltage cathode. The CV and in-situ DRIFTS for the electrolyte contribution with TEMPO additive is shown in Fig. 1. The possible reaction mechanism with the presence of TEMPO additive is discussed.Fig. 1. CV (left) and in-situ DRIFTS (right) recorded on MCMB-electrode of the electrolyte containing STD+5%FEC+2%TEMPO.