Enhanced Charge Transport in Amorphous Li2O2

Abstract

The properties of the Li 2 O 2 discharge phase are expected to impact strongly the performance of Li–air batteries. Although both crystalline Li 2 O 2 (c-Li 2 O 2 ) and amorphous Li 2 O 2 (a-Li 2 O 2 ) have been reported to form in Li–air cells, little is known regarding possible differences in charge and mass transport within these phases. To reveal these differences, here we predict the properties of a-Li 2 O 2 using first-principles “melt-and-quench” molecular dynamics and percolation theory. We find that the band gaps and equilibrium electrochemical potentials of c-Li 2 O 2 and a-Li 2 O 2 are similar; nevertheless, their transport properties are quite different. Importantly, the ionic conductivity of a-Li 2 O 2 is predicted to be 2 × 10 –7 S/cm, which is 12 orders of magnitude larger than that in the crystalline phase. This enhancement arises from increases in both the concentration and mobility of negative lithium vacancies. The electronic conductivity of a-Li 2 O 2 is also enhanced, but to a much smaller extent (4 orders of magnitude), and remains low overall, 2 × 10 –16 S/cm. These data suggest that the formation of amorphous Li 2 O 2 during discharge may enhance cell performance if charge or mass transport through the discharge product is a rate-limiting process.