The Lithium–Oxygen Battery with Ether-Based Electrolytes

TLDR: It is demonstrated that ether-based electrolytes are not suitable for rechargeable Li–O2 cells, although the ethers are more stable than the organic carbonates, the Li2O2 that forms on the first discharge is accompanied by electrolyte decomposition, to give a mixture of Li2CO3, HCO2 Li, CH3CO2Li, polyethers/ esters, CO2, and H2O.

Abstract: The rechargeable Li–air (O2) battery is receiving a great deal of interest because theoretically it can store significantly more energy than lithium ion batteries, thus potentially transforming energy storage. Since it was first described, a number of aspects of the Li–O2 battery with a non-aqueous electrolyte have been investigated. The electrolyte is recognized as one of the greatest challenges. To date, organic carbonate-based electrolytes (e.g. LiPF6 in propylene carbonate) have been widely used. However, recently, it has been shown that instead of O2 being reduced in the porous cathode to form Li2O2, as desired, discharge in organic carbonate electrolytes is associated with severe electrolyte decomposition. As a result it is very important to investigate other solvents in the search for a suitable electrolyte. In this regard much attention is now focused on electrolytes based on ethers (e.g. tetraglyme (tetraethylene glycol dimethyl ether)). Ethers are attractive for the Li–O2 battery because they are one of the few solvents that combine the following attributes: capable of operating with a lithium metal anode, stable to oxidation potentials in excess of 4.5 V versus Li/Li, safe, of low cost and, in the case of higher molecular weights, such as tetraglyme, they are of low volatility. Crucially, they are also anticipated to show greater stability towards reduced O2 species compared with organic carbonates. Herein we show that although the ethers are more stable than the organic carbonates, the Li2O2 that forms on the first discharge is accompanied by electrolyte decomposition, to give a mixture of Li2CO3, HCO2Li, CH3CO2Li, polyethers/ esters, CO2, and H2O. The extent of electrolyte degradation compared with Li2O2 formation on discharge appears to increase rapidly with cycling (that is, charging and discharging), such that after only 5 cycles there is little or no evidence of Li2O2 from powder X-ray diffraction. We show that the same decomposition products occur for linear chain lengths other than tetraglyme. In the case of cyclic ethers, such as 1,3dioxolane and 2-methyltetrahydrofuran (2-Me-THF), decomposition also occurs. For 1,3-dioxolane, decomposition forms polyethers/esters, Li2CO3, HCO2Li, and C2H4(OCO2Li)2, and for 2-Me-THF the main products are HCO2Li, CH3CO2Li; in both cases CO2 and H2O evolve. The results presented herein demonstrate that ether-based electrolytes are not suitable for rechargeable Li–O2 cells. A Li–O2 cell consisting of a lithium metal anode, an electrolyte, comprising 1m LiPF6 in tetraglyme, and a porous cathode (Super P/Kynar) was constructed as described in the Experimental Section. The cell was discharged in 1 atm O2 to 2 V. The porous cathode was then removed, washed with CH3CN, and examined by powder X-ray diffraction (PXRD) and FTIR. The results are presented in Figure 1 and Figure 2. The PXRD data demonstrate the presence of Li2O2, consistent with previous PXRD data for a Li–O2 cell with a tetraglyme electrolyte at the end of the first discharge. However, examination of the FTIR spectra, Figure 2, reveals that, in addition to Li2O2, other products form. Although the FTIR spectra provide clear evidence of electrolyte decom-