Researchers propose unified mechanism for reduction of O2 at cathode in Li-air batteries; guidance for direction of future research
12 November 2014
Researchers from the UK and France are proposing a unified mechanism for the reduction of O2 at the cathode of a Li-air (Li-O2) battery. The results of their study, published in the journal Nature Chemistry, suggest that the future direction of research for lithium–oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.
The researchers, led by Dr. Peter Bruce at the University of Oxford; Dr. Jean-Marie Tarascon, Collège de France; and Dr. Kishan Dholakia at the University of St. Andrews, investigated O2 reduction across a range of solvents. They showed that O2 reduction can be described by a single unified mechanism that embraces previous models as limiting cases.
The rechargeable Li–O2 battery would transform energy storage if a significant proportion of its theoretical specific energy, which exceeds by some margin that of lithium-ion batteries,could be realized in practice. At the positive electrode, on discharge, O2 enters the pores of the electrode, where it is reduced and combines with the Li+ ions from the electrolyte to form solid Li2O2. The process is reversed on charging. However, realizing these processes rapidly, efficiently and sustainably for many cycles is a formidable challenge. To overcome the challenges at the positive electrode it is essential to understand the electrochemical mechanism of O2 reduction in Li+-containing aprotic electrolytes.
Two different models of O2 reduction have been proposed. One describes O2 reduction to form Li2O2 as a process taking place on the electrode surface, and the other involves Li2O2 formation in solution (electrolyte) and is based on the ‘hard soft acid–base theory’ of Pearson. The two models have very different implications for how fast and reversible formation and decomposition of Li2O2 with low polarization and sustainable cycling may be achieved.
—Johnson et al.
At high voltages (low overpotentials) O2 undergoes a one-electron reduction to LiO2 that is partitioned between LiO2 dissolved in the electrolyte and LiO2 adsorbed on the electrode surface. High-donor-number (DN) solvents result in strong solvation of Li+ or Li+-containing species, yielding mainly soluble LiO2.
In low-DN solvents, solvation is weaker, resulting in surface-adsorbed LiO2 being dominant. In the latter case, LiO2 then disproportionates or undergoes a second reduction to Li2O2 on the electrode surface, whereas in the former pathway, disproportionation of LiO2 in solution dominates, precipitating Li2O2 .
They also demonstrated that the Li2O2 morphologies (large particles or particulate surface films) vary with the solvent, in accord with the unified mechanism.
The mechanism has implications for the performance of Li–O2 cells. The dominance of Li2O2 surface films in low-DN solvents is likely to lead to premature cell death. In contrast, the dominance of solution Li2O2 growth in high-DN solvents can sustain discharge and a capacity more than three times that of low-DN solvents. These results are encouraging efforts to identify new sufficiently stable electrolytes based on high-DN solvents for Li2O2 batteries.
—Johnson et al.
Resources
Lee Johnson, Chunmei Li, Zheng Liu, Yuhui Chen, Stefan A. Freunberger, Jean-Marie Tarascon, Praveen C. Ashok, Bavishna B. Praveen, Kishan Dholakia & Peter G. Bruce (2014) “The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries,” Nature Chemistry doi: 10.1038/nchem.2101
These researchers must be rich working for that long and will still be working for a long time without delivering any meaningful results, LOL. We should replace them be a computer program instead.
Posted by: gorr | 14 November 2014 at 08:22 PM