First-Principles Study of the Charge Transport Mechanisms in Lithium Superoxide

Abstract

Lithium–air batteries have attracted intense interest due to their high energy density, yet their practical applications are still severely hindered by the low conductivity of lithium peroxide (Li2O2). Here, we perform first-principles calculations on the recently synthesized lithium superoxide (LiO2) which has the potential to replace its peroxide counterpart as the discharge product. Using HSE hybrid functional, we predict an electrical insulating behavior for LiO2. Excess electrons and holes will be localized on the oxygen dimer, thus forming small polarons that can diffuse by hopping between lattice sites. With the calculated concentrations and mobilities of the intrinsic charge carriers, we show that the charge transportation in LiO2 is governed by the migration of hole polarons and positively charged oxygen dimer vacancies. The electronic conductivity associated with polaron hopping (3 × 10–12 S cm–1) exceeds that of Li2O2 by 8 orders of magnitude, while a comparable value (4 × 10–12 S cm–1) is found for the ionic conductivity contributed by superoxide ions. Our calculations provide a detailed understanding of the role of small polarons in describing the charge transport properties of LiO2.