Capacity Limiting Factors in Ambient Pressure Li‐Air Battery
Authors: Dutta Arghya , Ito Kimihiko , Kubo Yoshimi 
Published: February 2020

Nonaqueous Li‐Air battery is considered to be one of the most promising future energy storage technologies due to its very high theoretical specific energy (~3.5 kWh kg⁻1) based on the reversible electrochemical reaction 2Li+ + O2 + 2e⁻ ↔ Li2O2(s); E0 = 2.96 V vs. Li/Li+. An ideal Li-Air battery breathes oxygen from ambient air continuously through the porous cathode and deposits solid lithium peroxide (Li2O2) as the discharge (DC) product until all Li metal at the anode is consumed. However, Li‐Air batteries manifest discharge capacities significantly lower than that limited by Li metal consumption and undergo a sudden death denoted by a steep decrease in potential. An enhancement in capacity has been observed when the cells are fed with oxygen instead of air but the capacity is still lower than predicted. Hence a precise understanding about the mechanism behind sudden death is a prerequisite in order to develop Li‐Air battery with high DC capacity. Unfortunately, there is lack of proper knowledge about the sudden death of Li‐Air battery under ambient oxygen pressure. So far, majority of the research on this system has been performed in pure oxygen and these studies on sudden death have come up with quite different explanations concluding that either surface passivation1 or pore clogging2 of conducting porous electrodes by insulating Li2O2 discharge product causes premature death of the cell and limits the discharge capacity. The reason behind these different opinions is the difference in Li2O2 deposition-and-growth mechanism that depends on several parameters, such as nature of the electrolytes,3 surface properties of electrodes,4 rate of DC5
etc. Additionally, oxygen partial pressure provided to the cell has also been found to influence the discharge capacity significantly.6 Taken all together, the sudden death of Li‐Air battery is the result of a complex combination of several parameters. Here we have systematically investigated the sudden death of these cells with the help of several analytical techniques under both oxygen and dry air atmospheres. Multiple stacked carbon nanotube (CNT) electrodes are used in our experiments to understand the spatial distribution of DC products in the cathode at different conditions. Scanning electron micrographs (SEM) and quantitative titration show inhomogeneous deposition of discharge products at different layers of cathode and the distribution depends on the DC rate and oxygen partial pressure. At high DC rates the fraction of Li2O2 has been found to be higher on the oxygen side of the electrodes, while at low DC rates the bulk of the electrode contains higher fraction. The DC rate at which this transition occurs, depends on the partial pressure of oxygen and usually the rate is higher for higher oxygen partial pressure. These results indicate that the limitation of oxygen diffusion through the bulk of the electrode under high DC rate and low oxygen partial pressure plays vital role behind sudden death. Based on our observations we propose that in order to achieve high discharge capacity in ambient pressure Li‐Air batteries, the air electrode should be properly designed so that oxygen diffusion is facilitated throughout the electrode.

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