Structural and electronic properties of small lithium peroxide clusters in view of the charge process in Li-O batteries.

The Li-O2 battery is an ideal energy storage device due to its highest theoretical energy density; however, its high charge overpotential limits its practical application. Herein, through ab initio calculations, we systematically investigated the structural and electronic properties of small (Li2O2)nm+ (n = 1, m = 0, 1 and n = 2, m = 0, 1, and 2) clusters and calculated the reaction energies of various decomposition reactions. Results show that the (Li2O2)1 monomer has a low spin, whereas the (Li2O2)2 dimer has a high spin. The analysis of bond length, molecular orbitals, and projected density of states reveals that the interaction of O-O is stronger in the cationic cluster than in the neutral one, whereas the interaction of O-Li is weaker in the cationic cluster than in the neutral one; this facilitates the decomposition of cationic lithium peroxide cluster. Furthermore, the calculated reaction energies indicate that the peroxide lithium decomposition preferentially favors two-step reaction over one-step reaction. Finally, the lowest-energy reaction pathway for the decomposition of (Li2O2)2 dimer was predicted to be (Li2O2)2 → Li2O2 → (Li2O2)+ → LiO2 → O2, and the rate-determining step was predicted to be the first step.