Grain Boundary Induced Conductivity in Li2O2

The dominant discharge product in Li–air batteries, lithium peroxide (Li2O2), is intrinsically a wide band gap insulator as a perfect crystal. Recent density functional theory studies have suggested both vacancy- and polaron-mediated electron transportation mechanisms. We here show computational evidence from both semilocal and hybrid density functional calculations that the Σ3(1100)[1120] tilt grain boundaries (GBs) in Li2O2 can produce spin-polarized gap states. For each type of Σ3 GBs, GB1, GB2, and GB2* which has different atomic layer as the mirror plane, we have examined stoichiometric and a number of O-rich chemistry and find that stable geometry can take both forms. We find stoichiometric GBs disturb negligibly the electronic structure of Li2O2, yet the O-rich GBs produce spin-polarized gap states in a similar manner to free surface cases. Lithium deficiency leads to compression of interfacial O–O bonds, enlarges the πp–πp* split, and pushes up the antibonding πp* to (GB2) or beyond (GB2*) the F...