The role of iodide in the formation of lithium hydroxide in lithium–oxygen batteries

Abstract

Lithium iodide has been studied extensively as a redox-mediator to reduce the charging overpotential of Li–oxygen (Li–O₂) batteries. Ambiguities exist regarding the influence of lithium iodide on the reaction product chemistry and performance of lithium–oxygen batteries. In this work, we examined the role of lithium iodide on the reduction product chemistry under two conditions: (i) mixing KO₂ with lithium salts and (ii) discharging Li–oxygen batteries at high and low overpotentials, in the presence of an ether-based electrolyte with different ratios of H₂O : LiI. The addition of iodide to electrolytes containing water was found to promote the formation of LiOOH·H₂O, LiOH·H₂O and LiOH at the expense of Li₂O₂. At low H₂O : LiI ratios (lower than 5), LiOH instead of Li₂O₂ was formed, which was accompanied by the oxidation of iodide to triodide while at high H₂O : LiI ratios (12, 24, 134), a mixture of Li₂O₂, LiOOH·H₂O and LiOH·H₂O was observed and no triiodide was detected. The reaction between peroxide Li₂O₂ and/or superoxide LiO₂ with H₂O to form LiOH is facilitated by increased water acidity by strong I⁻–H₂O interactions as revealed by ¹H NMR and FT-IR measurements. This mechanism of LiOH formation in the presence of LiI and H₂O was also found upon Li–O₂ cell discharge, which is critical to consider when developing LiI as a redox mediator for Li–O₂ batteries.