Controlling Solution-Mediated Reaction Mechanisms of Oxygen Reduction Using Potential and Solvent for Aprotic Lithium−Oxygen Batteries David G. Kwabi,* ,† Michal Tulodziecki, ‡ Nir Pour, ‡ Daniil M. Itkis, ∥ Carl V. Thompson, § and Yang Shao-Horn* ,†,‡,§ † Department of Mechanical Engineering, ‡ Research Laboratory of Electronics, § Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ∥ Department of Chemistry and Materials Science, Moscow State University, Leninskie gory 1, Moscow 1199922, Russia * S Supporting Information ABSTRACT: Fundamental understanding of growth mechanisms of Li 2 O 2 in Li−O 2 cells is critical for implementing batteries with high gravimetric energies. Li 2 O 2 growth can occur first by 1e − transfer to O 2 , forming Li + −O 2 − and then either chemical disproportionation of Li + −O 2 − , or a second electron transfer to Li + −O 2 − . We demonstrate that Li 2 O 2 growth is governed primarily by disproportionation of Li + −O 2 − at low overpotential, and surface-mediated electron transfer at high overpotential. We obtain evidence supporting this trend using the rotating ring disk electrode (RRDE) technique, which shows that the fraction of oxygen reduction reaction charge attributable to soluble Li + −O 2 − -based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measure- ments of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li 2 O 2 morphologies observed at different discharge overpotentials: formation of large (∼250 nm−1 μm) toroids, and conformal coatings (<50 nm) at higher overpotentials. These results highlight that RRDE and EQCM can be used as complementary tools to gain new insights into the role of soluble and solid reaction intermediates in the growth of reaction products in metal−O 2 batteries. L ithium−oxygen (Li−O 2 ) batteries are theoretically pro- jected to store up to ∼3500 Wh/kg cell , 1,2 (i.e., considering the weight of active materials only) compared to ∼1000 Wh/ kg cell for current Li-ion systems such as LiCoO 2. 3 Although the practical, system-level energy density advantage might be smaller, 4 the potential for improvements over current systems has spurred increasing research attention on Li−O 2 electro- chemistry. Li−O 2 batteries differ from traditional Li-ion batteries in that rather than intercalating Li + ions into a transition metal oxide-based host lattice, they react directly with reduced oxygen resulting (in nonaqueous media) mainly in the precipitation of Li 2 O 2, 1,5 in addition to other insoluble Li−O 2 compounds. As a result, the practical energy density of an Li− O 2 battery is critically linked to the degree of Li 2 O 2 filling of void spaces in the cathode during discharge, which is highly dependent on the Li −O 2 reaction and Li 2 O 2 growth mechanism in operation. 6−8 Li 2 O 2 growth morphologies that have been observed in lab- based cells can be grouped under two broad categories: toroids, which range from 250 nm to ∼1 μm in size, 9−13 and thin conformal coatings on the electrode surface (<50 nm). 11,14 These morphologies exhibit a discharge rate/overpotential dependence, with toroids forming at low applied oxygen reduction reaction (ORR) overpotential (defined as the difference between the applied potential and reversible potential of 2.96 V vs Li + /Li 15 ) typically above 2.7 V vs Li + / Li 9,11,12,14,16,17 and thin deposits at larger overpotential (<2.6 V vs Li + /Li) 11,14,18 and current densities. There is an apparent discrepancy between large toroidal morphologies and the fact that Li 2 O 2 is a bulk insulator with a band gap between 4−5 eV, 19−21 which grows to only 5−10 nm when electrochemically deposited on planar electrodes. 22 Recent electrochemica- l 23−26 and in situ surface-enhanced Raman spectroscopy (SERS) 8 studies suggest that strongly coordinating electrolyte salt anions, high donor number solvents, or protic additives such as water, methanol, and perchloric acid enhance toroidal Li 2 O 2 growth by solvating and stabilizing the lithium superoxide (Li + −O 2 − ) intermediate and thus promoting Li 2 O 2 via disproportionation of Li + −O 2 − (2Li + −O 2 − → Li 2 O 2 +O 2 ) rather than direct 2e − transfer to the surface (2Li + +O 2 + 2e − → Li 2 O 2 ). Similar reasoning has been proposed to account for the formation of toroids at low overpotentials, 11,27 where the low driving force for electron transfer results in the disproportionation pathway being dominant. It is believed that Received: February 12, 2016 Accepted: March 7, 2016 Letter pubs.acs.org/JPCL © XXXX American Chemical Society 1204 DOI: 10.1021/acs.jpclett.6b00323 J. Phys. Chem. Lett. 2016, 7, 1204−1212