No abstract

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.

Lithium–oxygen (Li–O2) batteries offer considerably higher gravimetric energy density than commercial Li-ion batteries (up to three times) but suffer from poor power, cycle life, and round-trip efficiency. Tuning the thermodynamics and pathway of the oxygen reduction reaction (ORR) in aprotic electrolytes can be used to enhance the Li–O2 battery rate and discharge capacity. In this work, we present a systematic study on the role of the solvent and anion on the thermodynamics and kinetics of Li+-ORR, from which we propose a unified descriptor for its pathway and kinetics. First, by thoroughly characterizing the solvation environment of Li+ ions using Raman spectroscopy, 7Li NMR, ionic conductivity, and viscosity measurements, we observe increasing Li+–anion interactions with increasing anion DN in low DN solvents such as 1,2-dimethoxyethane and acetonitrile but minimal Li+–anion interactions in the higher DN dimethyl sulfoxide. Next, by determining the electrolyte-dependent Li+/Li, TBA+,O2/TBA+–O2 –, and Li+,O2/Li+–O2 – redox potentials versus the solvent-invariant Me10Fc reference potential, we show that stronger combined solvation of Li+ and O2 – ions leads to weaker Li+–O2 ‑ coupling. Finally, using rotating ring disk electrode measurements, we show that weaker Li+–O2 – coupling in electrolytes with strong combined solvation leads to an increased generation of soluble Li+–O2 –-type species and faster overall kinetics during Li+-ORR.