Lithium-air (Li-O₂) batteries, with their superior energy density, are looked to as one of the possible “beyond Li-ion” solutions that could in the future accelerate the transition to full battery electric vehicles, among other applications. The DOE, in its recently released EV Everywhere Blueprint (earlier post) pegs lithium-air as one of the potential long-term (2017-2022) technologies required, but notes major challenges remain.

Demand for better electric vehicles motivates the search for lower cost, higher capacity, rechargeable batteries. Aprotic electrolyte Li−O₂ batteries have received considerable attention due to very high theoretical specific energy, but degradation during cycling of every major component of the O₂ electrode, including solvent, lithium salt, binder and carbon, has plagued efforts to develop this technology for practical purposes. Recognition that the most successful Li-ion battery materials are unsuitable for use in Li−O₂ batteries has grown apace. In particular, solvents such as carbonates, ethers, esters, and lactones decompose in the presence of reactive oxygen species formed in the O₂ electrode.

Recently, the superior stability of N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMA) in the O₂ electrode has been demonstrated by electrochemical mass spectrometry....The effective utilization of straight-chain alkyl amides as electrolyte solvents in Li−O₂ batteries has heretofore been limited due to interfacial instability with Li anodes. The present study demonstrates the successful operation of a rechargeable Li−O₂ battery using a Li metal anode and an electrolyte consisting of DMA and lithium nitrate (LiNO₃).

—Walker et al.

Straight-chain alkyl amides are one of the few classes of polar, aprotic solvents that resist chemical degradation in the O₂ electrode, the researchers note, but these solvents do not form a stable solid-electrolyte interphase (SEI) on the Li anode. Lack of a persistent SEI leads to rapid and sustained solvent decomposition in the presence of Li metal.

In their work, the Liox Power team used the salt, lithium nitrate (LiNO₃) to stabilize the SEI.

In addition to demonstrating the 2000 h of cycling, the LioxPower team also found that O₂ was the primary gaseous product formed during charging. This is important, they noted, because although other work has shown high cycle numbers in Li−O₂ cells using carbonate- and ether-based electrolyte systems, these systems exhibited CO₂ as the main gaseous product evolved during charging—indicating that electrolyte decomposition occurs rapidly in these cells.

The electrolyte composition described in this work demonstrates unprecedented stability toward both the O₂ electrode and Li anode. The stability of this system can be attributed to the inertness of the amide core toward reactive oxygen species combined with the ability of the nitrate anion to contribute to the formation of a protective SEI that inhibits reaction between the solvent and the Li anode.

Heretofore it has been assumed that a ceramic membrane separating the anode and cathode compartments is required in order to enable the use of solvents that are unstable toward Li metal yet advantageous regarding the O₂ electrode. The use of an SEI-forming Li salt instead of a ceramic membrane to prevent reactivity with the Li anode provides a new direction for Li−O₂ battery research and creates opportunities in the identification of stable combinations of cell components.

—Walker et al.

Resources

• Wesley Walker, Vincent Giordani, Jasim Uddin, Vyacheslav S. Bryantsev, Gregory V. Chase, and Dan Addison (2013) A Rechargeable Li-O₂ Battery Using A Lithium Nitrate/N,N-Dimethylactamide Electrolyte. Journal of the American Chemical Society doi: 10.1021/ja311518s