Stronger solvation of I À and Li + ions enables the oxidation of Li 2 O 2 to O 2 and LiOH to LiIO 3 by I 3 À in DMSO, whereas no reaction occurs in DME as a result of the insufficient thermodynamic driving force. Solvation effects can dramatically influence the performance of soluble redox mediators for Li-O 2 batteries by altering thermodynamics and reactivity of the mediators. HIGHLIGHTS Solvation energy of I À and Li + dictate the reactivity between I 3 À and Li 2 O 2 /LiOH I 3 À and I 2 react irreversibly with LiOH to form LiIO 3 at potentials above $3.1 V Li Electrolytes can critically alter the performance of Li-O 2 soluble redox mediators Leverick et al., SUMMARYLi-O 2 batteries offer higher gravimetric energy density than commercial Li-ion batteries. Despite this promise, catalyzing oxidation of discharge products, Li 2 O 2 and LiOH, during charging remains an obstacle to improved cycle life and round-trip efficiency. In this work, reactions between LiI, a soluble redox mediator added to catalyze the charging process, and Li 2 O 2 and LiOH are systematically investigated. We show that stronger solvation of Li + and I À ions led to an increase in the oxidizing power of I 3 À , which allowed I 3 À to oxidize Li 2 O 2 and LiOH in DMA, DMSO, and Me-Im, whereas in weaker solvents (G4, DME), the more oxidizing I 2 was needed before a reaction could occur. We observed that Li 2 O 2 was oxidized to O 2 , whereas LiOH reacts to form IO À , which could either disproportionate to LiIO 3 or attack solvent molecules. This work clarifies significant misconceptions in these reactions and provides a thermodynamic and selectivity framework for understanding the role of LiI in Li-O 2 batteries.I À ions can go through two distinct redox transitions during oxidation in aprotic electrolytes, having first iodide anions (I À ) oxidized to form triiodide (I 3 À ) and I 3 À oxidized À /I À and I 3 À /I 2 . The reduction and oxidation peaks of the I 3 À /I À (centered between 0.02 and 0.23 V Me10Fc ) and I 3 À /I 2 (centered at $0.64 V Me10Fc ) couples were observed in cyclic voltammograms (CVs), from which the redox potentials of I 3 À /I À and I 3 À /I 2

Iodide ions promote deprotonation of water; in consequence LiOH/LiOH·H2O is formed as a final discharge product.

Fundamental understanding of growth mechanisms of Li2O2 in Li-O2 cells is critical for implementing batteries with high gravimetric energies. Li2O2 growth can occur first by 1e(-) transfer to O2, forming Li(+)-O2(-) and then either chemical disproportionation of Li(+)-O2(-), or a second electron transfer to Li(+)-O2(-). We demonstrate that Li2O2 growth is governed primarily by disproportionation of Li(+)-O2(-) 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(+)-O2(-)-based intermediates increases as the discharge overpotential reduces. Electrochemical quartz crystal microbalance (EQCM) measurements of oxygen reduction support this picture, and show that the dependence of the reaction mechanism on the applied potential explains the difference in Li2O2 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-O2 batteries.

The performance of current Li–air batteries is greatly limited by critical obstacles such as electrolyte decomposition, high charging overpotentials, and limited cycle life. Thus, much effort is devoted to fundamental studies to understand the mechanisms of discharge/charge processes and overcome the above-mentioned obstacles. In particular, the search for new stable electrolytes is vital for long-lasting and highly cyclable batteries. The highly reactive lithium superoxide intermediate (LiO2) produced during discharge process can react with the electrolyte and produce a variety of byproducts that will shorten battery life span. To study this degradation mechanism, we investigated oxygen reduction reaction (ORR) in highly concentrated electrolyte solutions of lithium bis­(trifluoromethanesulfonyl)­amide (Li­[TFSA])/dimethyl sulfoxide (DMSO). On the basis of rotating ring disk electrode measurements, we showed that LiO2 dissolution can be limited by increasing lithium salt concentration over 2.3 mol dm–3. Our Raman results suggested that this phenomenon can be related to lack of free DMSO molecules and increasing DMSO–Li+ interactions with higher Li+ concentration. X-ray diffraction measurements for the products of ORR suggested that the side reaction of DMSO with Li2O2 and/or LiO2 could be suppressed by decreasing the solubility of LiO2 in highly concentrated electrolytes.