On the Origin and Implications of Li2O2 Toroid Formation in Nonaqueous Li-O2 Batteries

The free energy of dissolution for LiO2* into Li+ and O2- in different solvents as a function of the Gutman acceptor and donor numbers (AN and DN). The free energy plot is normalized relative to that of pure dimethoxyethane (DME). Dimethyl formamide (DMF), dimethyl acetamide (DMAc) and dimethyl sulfoxide (DMSO) have high DN and thus are capable of stabilizing Li+. Water and methanol on the other hand, have high acceptor numbers and thus stabilize O2-. This predicts that solvents that fall in the top right quadrant of this plot will favor solution-mediated deposition of Li2O2, which will be essential for high capacity Li-O2 batteries. Note that because this figure and work is part of a submission to Nature Chem., there can be no pre-publication publicity, however remote the possibility.

The recent surge in activity seeking higher energy battery chemistries than are possible from Li-ion intercalation is fueled by the goal of developing mass-market electrification of road transportation, a goal that currently seems difficult with Li-ion batteries. This “beyond Li-ion chemistry” is broadly defined as searching for a chemistry that will allow development of a safe, long lived and cost effective battery with sufficient specific energy (kWh/kg) and energy density (kWh/L) to extend the electrical driving range to cover most daily use. Of all current candidates, the nonaqueous Li-air battery has attracted the most attention to date because of its high theoretical specific energy, one that rivals that of gasoline. In this battery, the net electrochemical reaction is 2Li + O2 -> Li2O2, with the forward direction describing discharge of the battery and the reverse direction describing charge. The very high theoretical specific energy arises because the O2 can in principle come from air-breathing and from the use of Li metal as the anode. However, the maximum discharge capacity in nonaqueous Li-O2 batteries is generally limited to a small fraction of its theoretical value due to the insulating nature of lithium peroxide (Li2O2), the battery’s primary discharge product, because once thin films build up on the cathode surface it electrically passivates the cathode.

In work recently submitted to Nature Chemistry by a collaboration involving scientist at IBM, University of CA (Berkeley), Carnegie Mellon University and SUNCAT, it was shown that the inclusion of trace amounts of electrolyte additives, such as H2O, significantly improve the capacity of the Li-O2 battery. These additives trigger an additional solution-based Li2O2 growth mechanism due to their solvating properties of LiO2, the species that exists at the surface of the dominant Li2O2 surface. This allows a second solution-mediated electrochemical process driven by the LiO2 partial solubility, where O2- acts as a redox mediator thereby circumventing the Li2O2 conductivity limitation inherent in the surface electrochemical path. Experiments and kinetic modeling show that this solution growth results in Li2O2 toroid formation. This resolves the long standing issue of how and why toroids are formed and why some experiments observed them and others not, i. e. it was simply related to the extent of H2O impurity in the Li-O2 batteries. The model also successfully accounts for the dependence of the toroid size and discharge capacity increase on H2O content and discharge current.

Unfortunately, while added H2O increases the discharge capacity, it also increases the parasitic chemistry and causes higher charging potentials, thus reducing rechargeability of the Li-O2 battery. In efforts to identify more beneficial additives than H2O for capacity increases, descriptors for the solubility of LiO2 have been identified; the solvent/additive Gutman acceptor number (AN) and donor number (DN). This allows identification of other solvents/additives that could also induce the solution mechanism. A typical “volcano” plot of the solubility in terms of the AN and DN is given in Figure 1 below. Experiments with added CH3OH or DMSO yield an increase in capacity and/or the formation of toroids, although neither of these avoid increasing parasitic chemistry. However, the rational design principle given in this figure for selecting solvents/additives for solution growth of Li2O2 encourages us that solvents/additives can ultimately be identified that do not also reduce the rechargeability of the Li-O2 battery.