Univ. of St. Andrews researchers show reversible, higher-rate non-aqueous Li-air battery using gold cathode and DMSO electrolyte
Very high energy density rechargeable lithium air (or Li-O2) batteries are of great interest for future electrified transportation because at best their practical energy density could approach that of current gasoline engined vehicles (after factoring in tank-to-wheel efficiencies). However, researchers are still looking at a multi-decade development cycle for the technology, which can be embodied in a several different architectures. (Earlier post.)
Operation of a rechargeable non-aqueous Li-O2 battery depends on repeated and highly reversible formation/decomposition of Li2O2 at the porous cathode on cycling. As described in a paper published in the journal Science, a team at the University of St. Andrews (Scotland) led by Dr. Peter Bruce has now demonstrated this is possible. Using a dimethyl sulfoxide (DMSO) electrolyte and nanoporous porous gold electrode (NPG), they crafted a cell with 95% capacity retention from cycles 1 to 100) and with >99% purity of Li2O2 formation at the cathode, even on the 100th cycle, and its complete oxidation on charge.
A typical rechargeable non-aqueous Li-O2 cell comprises a lithium metal anode; a non-aqueous Li+-conducting electrolyte; and a porous cathode, the authors note. Operation depends on the reduction of O2 at the cathode to O22–, which combines with Li+ from the electrolyte to form Li2O2 on discharge, with the reverse reaction occurring during charging.
An issue of concern with any battery or chemical/electrochemical reaction is side-reactions, particularly on the first cycle. A key question, the authors say, is the extent of such side-reactions—i.e., whether this is sufficiently small compared with the amount of electrolyte used in practical cells, and whether the extent increases on cycling.
Previous efforts have resulted in only partial Li2O2 formation/decomposition and with limited cycling. The St. Andrews team reported no evidence of any decomposition products. Differential electrochemical mass spectrometry (DEMS) also confirmed that discharge was overwhelmingly dominated by Li2O2 formation.
Furthermore, the data indicated that the kinetics of Li2O2 oxidation on charge is approximately 10-fold faster than on carbon electrodes.
The charge to mass ratio on discharge and charge is 2e–/O2, confirming that the reaction is overwhelmingly Li2O2 formation/decomposition. It has also been shown that such electrodes are particularly effective at promoting the decomposition of Li2O2, with all the Li2O2 being decomposed below 4 V and around 50% below 3.3 V and at a rate approximately one order of magnitude higher than on carbon.
Although DMSO is not stable with bare Li anodes, it could be used with protected Li anodes. Nanoporous Au electrodes are not suitable for practical cells, but if the same benefits could be obtained with Au coated carbon then low mass electrodes would be obtained, although cost may still be a problem.
A cathode reaction overwhelmingly dominated by Li2O2 formation on discharge, its complete oxidation on charge and sustainable on cycling, is an essential prerequisite for a rechargeable non-aqueous Li-O2 battery. Therefore, the results presented here encourage further study of the rechargeable non-aqueous Li-O2 cell, although many challenges to practical devices remain.—Peng et al.
Zhangquan Peng, Stefan A. Freunberger, Yuhui Chen, and Peter G. Bruce (2012) A Reversible and Higher-Rate Li-O2 Battery. Science doi: 10.1126/science.1223985