Behind Extended Cycling of Li-O2 Battery

New battery technologies are demanded for a growing market of electric and hybrid vehicles. Rechargeable Li-O₂ batteries are promising candidates for this application because of their high theoretical energy density. Although high capacity values are achievable for Li-O₂batteries, their cycling stability is typically limited by only a few full discharge-charge cycles. On the other hand, good cycling behavior under limited capacity conditions has been reported by several groups.

Rechargeability of Li-O₂ batteries is based on the reversible formation/oxidation of Li₂O₂. Unfortunately, the electrochemical processes of oxygen reduction and evolution are harmful for commonly used carbon-based electrodes and organic electrolytes causing severe decompositions. Generation and continuous accumulation of insulating side products on the electrode/electrolyte interface results in increasing polarization and fast capacity fading with cycling.

In the present report we demonstrate that long cycle life of Li-O₂ batteries is in principle achievable upon selection of appropriate carbon material and catalyst for air cathode fabrication in addition to a proper cycling protocol (Fig. 1a). The origin of the extended cycling is in efficient decomposition of both Li₂O₂ and side products during the charging step of the battery operation. Extended cycling of a Li-O₂ battery was investigated by quantification of Li₂O₂ yield during discharge and O₂ release during charging. There is a drastic shift from the predominant formation of Li₂O₂ to the predominant formation of carbonate- and carboxylate-containing side products within first few cycles followed by a stabilization of Li₂O₂ yield. Mass spectroscopy of gasses evolved during the corresponding charging steps demonstrate drop in oxygen release and increase in CO₂release in the very beginning of battery cycling (Fig. 1b).

Drastic switch to the formation of side products in the beginning of battery cycling originates from certain changes on the electrode/electrolyte interface as was analyzed with X-ray photoelectron spectroscopy. In addition, morphological considerations and cycling conditions to achieve unrestricted cycling of Li-O₂battery will be discussed in the presentation.

Acknowledgement

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U. S. Department of Energy (DOE). The microscopic analysis in this work was supported by the Joint Center for Energy Storage Research, an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences.

Figure 1. a) Voltage profiles and b) in situ MS analysis of O₂ and CO₂ evolved during charging of the battery with CNTs/Ru electrodes cycled in TEGDME-LiTf electrolyte.