Electrolyte Engineering Toward Increasing Nonaqueous Lithium-Oxygen Battery Capacity

Of the “beyond lithium-ion” battery electrochemistries studied to-date, the lithium-oxygen battery has the highest theoretical specific energy. As such, the lithium-oxygen (Li-O2) battery has great potential for enabling, as listed in NASA’s technology roadmap for high-specific-energy batteries, “the next generation of deep-space EVA suits that require advanced life support, communications, and computing equipment,” and increasing the range and functionality of other electric power space technologies. The lithium-oxygen battery consists of a lithium metal negative electrode, an oxygen-saturated nonaqueous organic solvent-based electrolyte, and a porous carbon scaffolding for the positive electrode. The battery operates via the formation (discharge) and decomposition (charge) of lithium peroxide (Li2O2), and as such has a theoretical specific energy more than eight times that of current lithium-ion materials. Unfortunately, the lithium-oxygen battery suffers from critical challenges, including that Li2O2 is an insulator, so after only a thin film has formed on the carbon surface during discharge, an exponential increase in overpotential occurs that effectively stops the battery much before it attains its full capacity. The goal of this research was to identify electrolyte additives that could, without deleteriously affecting cell rechargeability, increase the attainable lithium-oxygen cell capacity. The ability of electrolytes to induce solubility of the intermediate to Li2O2 formation, lithium superoxide (LiO2), was studied, as this enables a solution mechanism of growth whereby Li2O2 grows in large, aggregated structures, allowing more Li2O2 to form before cathode passivation. Non-Li alkali metal cations and alcohols were studied as methods of inducing the solution mechanism by lowering the free energy of the superoxide anion (O2-) in solution. Galvanostatic cycling of Li-O2 batteries containing non-Li alkali metal salts showed a small increase in the achievable discharge capacity, attributed to the softer acidity of non-Li alkali metal cations more favorably solvating the soft base O2-. However, gas analysis of a sodium-oxygen battery with a small amount of Li+ salt added to the battery electrolyte showed Li+ quickly scavenges any non-Li alkali metal cation-associated O2-, and the resultant LiO2 quickly disproportionates into the insoluble Li2O2. It is therefore anticipated that an increase in Li-O2 battery capacity upon the addition of non-Li alkali metal cations is only expected at high currents, when oxygen reduction rates are sufficiently high to allow some O2- to temporarily avoid Li+ in solution. Ppm quantities of methanol, ethanol, and 1-propanol were added to ether-based Li-O2 battery electrolytes as analogues to water, which has been previously shown to induce the solution mechanism due to its strong Lewis acidity lowering the free energy of O2- in solution. The additives induced a two-fold increase in battery capacity, though with little trend in the capacity as a function of the additive’s Acceptor Number. These results highlight the complexity of interactions between the constituent species in an electrolyte in terms of their Lewis basicities, Lewis acidities, and other physicochemical properties. Toward determining the important parameters of an electrolyte for LiO2 solubility, collaborators at NASA Ames Research Center developed a molecular dynamics model with polarizable force fields for these electrolytes from ab initio quantum chemistry calculations. Experimental measurements of conductivity and ion diffusion were measured to verify the model. This work has been submitted for publication. While the formation of Li2O2 in large aggregated structures increases the achievable discharge capacity, an electrolyte-soluble redox mediator is required to oxidize aggregated Li2O2 on charge and shuttle electrons back to the electrode surface. The rechargeability of Li-O2 batteries containing redox mediators in the presence of water impurities, which are likely difficult to eliminate in practical lithium-air batteries, was studied. Specifically, the effect of water contamination in the electrolyte on the promising redox mediator lithium iodide (LiI) was studied. DEMS and titrations of cathodes extracted from discharged batteries confirmed recent reports that lithium hydroxide (LiOH) – a possible higher solubility, and thus higher capacity product – formed as the dominant discharge product via a 4 e-/O2 process. However, isotopic labeling and DEMS were used to show LiOH is not reversibly oxidized back to its reactants (Li+, O2, H2O). Rather, charge current in batteries containing both LiI and H2O was found to be a complex mixture of side reactions and redox shuttling. This work was published in ACS Energy Letters. With LiOH identified as an undesirable discharge product, the mechanism for Li2O2 degradation to LiOH in the presence of LiI and H2O was studied. Galvanostatic cycling of lab-scale Li-O2 batteries containing LiI and H2O in DME and dimethyl sulfoxide (DMSO) showed that DMSO prevents Li2O2 degradation to LiOH. Cyclic voltammetry of these electrolytes showed DMSO exhibits a higher potential for iodide oxidation than DME, indicating iodide-mediated H2O2 reduction is more difficult in DMSO than DME. The ability of an additive to reduce H2O2 was therefore identified as a key consideration in Li2O2’s stability in the presence of water impurities. A tangential important finding during this study was the difficulty in selection of an appropriate reference electrode for studying redox mediators in Li-O2 batteries, as electrodes with too high of a lithium intercalation voltage will chemically oxidize the redox mediator, while electrodes with too low of a lithium intercalation voltage exhibit spontaneous oxygen consumption in a Li-O2 battery.