Contribution of I−/I3
− and I3
−/I2 Redox Couples to Charge and Discharge Reactions of Li-Oxygen Battery with Solvate Ionic Liquid Electrolytes
Authors: Nakanishi Azusa , Kobayashi Yuki , Tatara Ryoichi , Kwon Hoi-Min , Thomas Morgan L. , Dokko Kaoru , Watanabe Masayoshi 
Published: February 2020

Introduction

Rechargeable Li-Air batteries have recently attracted great attention due to their much higher theoretical energy density than widespread Li-ion batteries. However, to realize reversible and stable operation, many serious problems must be overcome such as volatility of electrolyte, electrolyte decomposition by O2
−
radicals, and high overpotential especially during charging[1]. We recently reported that certain equimolar mixtures of glymes and Li salts behave like ionic liquids (ILs), consisting of solvate cations (Li[glyme]+) and the anions, and thus they are categorized as ‘‘solvate’’ ILs[2]. They have desirable properties as electrolytes for Li batteries, such as low volatility, thermal and electrochemical stability, and high Li+ transference number. In our recent study, we applied solvate ILs as electrolytes for Li-Air batteries[3].
However, as mentioned above, extremely high overpotential during charging is a serious problem. A way to reduce the charge overpotential and improve cycle capability is the use of soluble redox mediators such as TTF/TTF+, I−/I3
−, and TEMPO/TEMPO+ redox couples for the facile oxidation of lithium peroxide[4][5][6][7]. In this study, we adopted [Li(G4)]I as a mediator for Li-Air batteries and elucidated the mediation mechanism. Iodide (I−) can be electrochemically oxidized to triiodide (I3
−) and iodine (I2) according to the following equations.
3 I− ⇆ I3
− + 2 e− (1)
I3
− ⇆ 3/2 I2 + e− (2)
However, it is unclear which of these redox couples (I−/I3
− and I3
−/I2) is active as a mediator in the Li-air cells. Possible mediation mechanisms during charging of the cell　are shown below.
Li2O2 + I3
− → 2 Li+ + O2 + 3 I− (3)
Li2O2 + I2 → 2 Li+ + O2 + 2/3 I3
− (4)

Experimental

The solvate IL ([Li(G4)][TFSA]; Fig.1) and a mediator ([Li(G4)]I) were prepared by simply mixing purified tetraglyme (G4) and lithium bis(trifluoromethanesulfonyl)amide (Li[TFSA]) or lithium iodide (LiI) in a 1:1 molar ratio in an Ar-filled glove box. [Li(G4)]I was mixed with [Li(G4)][TFSA] to obtain several electrolytes containing different concentrations of the mediator. In order to prepare the carbon cathode, 75 wt% Ketjen black (KB) and 25 wt% polytetrafluoroethylene (PTFE) were mixed in N-methyl-2-pyrrolidone (NMP) and then spread onto carbon paper and dried. Galvanostatic discharge-charge tests were performed under dry O2 atmosphere using a coin cell (CR2032-type) consisting of Li metal, a glass separator, the electrolyte, and carbon cathode.

Results and Discussion

Fig. 2 shows the discharge-charge curves of [Li | 100 or 200 mM [Li(G4)]I in [Li(G4)][TFSA] | KB/O2 ] cells. During charging at the 1st cycle in each electrolyte, there were two plateaus around 3.0 V and 3.3 V which were not observed when a mediator was not added in electrolyte. These plateaus were tentatively assigned to eq. (1) and eq. (2). In this figure, vertical red lines indicate the theoretical capacity calculated from the amount of electrolyte and the concentration of iodide when I− completely oxidizes to I3
− unaccompanied with a mediation reaction. Since the length of the first plateau (~3.0 V, eq. (1)) was less than the theoretical value (did not reach the red line), I−/I3
− did not effectively operate as a mediator and eq. (3) did not proceed after the formation of I3
−. On the other hand, without a subsequent mediation reaction, the length of the second plateau (~3.3 V, eq. (2)) should have been half that of the first plateau, considering the stoichiometry of eq. (1) and (2). In fact, the length of the second plateau was much longer than that of the first plateau. This suggests that I3
−/I2 operated as a mediator and eq. (4) proceeded. Assuming that the mediation due to the combination of eq. (2) and eq. (4) proceeded, at the end of the charge (limited capacity), I3
− must be present in the electrolyte. During discharging at the 2nd cycle, there was a new plateau around 2.9 V, notably different from the oxygen reduction reaction. This plateau is tentatively assigned to the reduction of I3
− (formed during charging) to I− (reverse of eq.(1)).
In conclusion, these results suggest that the mediation reaction using LiI in [Li(G4)][TFSA] (SIL) occurred not via the I−/I3
− redox couple, but via the I3
−/I2 redox couple.

Acknowledgement

This study was partly supported by the RISING program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan, for which the authors are grateful.

Figure 1