Invited: Ab Initio Molecular Dynamics Study of Electrochemical Reaction at EC-Li2O2 Interfaces
In this presentation, we demonstrate EC adsorption and ring-opening process of the EC molecules on Li2O2 substrate by ab initio molecular dynamics. We find out that ring opening of EC molecule leads O-O bond breaking in an O22- unit in the Li2O2.
We use openMX code  in order to carry out ab initiomolecular dynamics (AIMD). The exchange-correlation functional is treated within the generalized gradient approximation proposed by Perdew, Burke, and Ernzerhof (PBE). The norm-conserving pseudopotential scheme combined with pseudo-atomic basis sets imposing cutoff energy of 300 eV. The Brillouin-zone summation is evaluated only on Γ point.
In our simulation model, there are thirty-two EC molecules on Li2O2 slab of six atomic layers. We first run classical molecular dynamics with force field named “pcff”  during 100 ps with a time step of 1 fs to relax the interface structure. Bottom layer of the Li2O2slab is fixed through the simulation. To eliminate electrostatic interaction between slab images in the repeated slab model, effective screening medium (ESM) method is adopted. Vacuum(ESM)/unit cell/metal(ESM) boundary conditions are imposed. No extra charge is introduced into the system. Simulation temperature keeps 350 K controlled by Nose-Hoover thermostat scheme. Time step is 1.0 fs. After the initial large fluctuation in the total energy and system temperature were converged (in 100 fs), we analyzed the trajectory.
Figure 1 (a) shows the adsorbed structure of EC molecule onto the Li2O2. In the adsorbed EC molecule, sp2 orbital of carbon changes to sp3 like one with a surface oxygen of Li2O2. Figure 2 shows time evolution of the molecular charge estimated by Bader analysis.. The adsorption is reduction process. There are three other adsorbed EC molecules in the system. They are the same characteristics with each other.
After adsorption, ring opening of one adsorbed EC molecule occurs around 600 fs in simulation time depicted in Fig. 1 (b). The reactant contains a single EC molecules and a O atom that is an adsorption site. 0.8 electron flows into the reactant as is shown in Fig.3. Therefore, the ring opening is one-electron reduction process. Interestingly, O22- bond in Li2O2 is broken and carboxylate structure with the EC molecule is made from O-. Finally, O-(CH2)2OCOO- is produced. The left O- exchange another O22-pair circled by white broken line in Fig.1 (b). It indicates that EC decomposition makes charging process impossible in Li-Air batteries.
 T. Laino andA. Curioni, Chem. Eur. J. 18, (2012) 3510.
 See http://www.openmx-square.org/for the code, OpenMX, pseudo-atomic basis functions and pseudopotentials.