Breathing of Water during Charge/Discharge Cycling in a Nonaqueous Li/O2 Cell with Tetraglyme-Based Electrolytes
Authors: Ue Makoto , Asahina Hitoshi , Matsuda Shoichi , Uosaki Kohei 
Published: December 2020

Nonaqueous Li-oxygen batteries have garnered considerable research interest over the past decade due to their extremely high theoretical energy densities. For instance, the theoretical energy density based on the reaction, 2Li + O2 = Li2O2, is calculated to be 3154 Whkg-1 by multiplying its specific capacity, 1168 Ahkg-1, by an assumed voltage of 2.7 V.1) However, its practical energy density is at most about 500 Whkg-1 with very limited cycle life at present technology level.2) The major reasons to limit the energy density and cycle life are too much excess electrolyte weight and its consumption by parasitic reactions, respectively.
Tetraethylene glycol dimethyl ether (TEGDME) and LiN(CF3SO2)2 (LiTFSI) are the most popular solvent and Li salt, respectively, for Li/O2 cells due to their relatively high stability.1,3) Although there are many papers to indicate that TEGDME decomposes during discharge and charge, it is only reported that the formation of Li2O2 on the first discharge (ORR) was accompanied by the formation of Li2CO3, HCO2Li, CH3CO2Li, polyethers/esters, CO2, H2O4) and CH3OH, CH3OCH2CH2OH, CH3O(CH2CH2O)2CH3,5) and the electrochemical decomposition of Li2O2 to O2 on recharge (OER) was accompanied by more sever decomposition with the similar by-products5) (OER/ORR < 0.9).
We have examined decomposition products in O2 cathode by using a 2-compartment cell design (Fig. 1), where anode and cathode compartments were separated by a lithium ion solid electrolyte to eliminate possible interference from the reactions at anode.6) The used carbon electrode was a self-standing sheet consisting of a Ketjenblack (6.5 mgcm-2-carbon), and the electrolyte (30 μlcm-2 in the cathode) was selected from (a) 1 M LiTFSI in TEGDME and (b) 0.5 M LiTFSI + 0.5 M LiNO3 + 0.2 M LiBr in TEGDME. The discharge/charge was carried out at 0.4 mAcm-2 (cut-off: 4 mAhcm-2, 615 mAhg-1-carbon).
We have observed negligible H2O formation during discharge under O2 atmosphere and gradual increase of H2O during recharge under He atmosphere detected by on-line mass spectrometry (Fig. 2). This H2O behavior during cycling was monitored by on-line gas chromatography (Fig. 3), and it was found that H2O is consumed during discharge. Some papers insist that H2O can be an advantageous additive to increase discharge capacity and decrease the charge overpotential,7) however, our observation on the H2O consumption might indicate the parasitic reactions with LiO2/Li2O2. H2O is reproduced by oxidative decomposition of TEGDME during charge, followed by the oxidation of CH3OH/other organic fragments with the accumulation of HCHO/HCOOH, which are eventually converted to CO2 and H2O at the final stage of charge.
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Figure 1