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 Sigma 3(1 (1) over bar 00)[11 (2) over bar0] tilt grain boundaries (GBs) in Li2O2 can produce spin-polarized gap states. For each type of Sigma 3 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 pi(p)-pi(p)* split, and pushes up the antibonding pi(p)* to (GB2) or beyond (GB2*) the Fermi energy. As a result, GB2 becomes half-metallic and GB2* becomes semiconducting with a small band gap of 1.0 eV. In both cases, spin polarization of O ions help to stabilize the GB by leaving the up spin of its gap states shifted down below the Fermi level and the down spin states open. Since Li2O2 is always polycrystalline as a discharge product, the presence of GBs may enhance conductivity.