Invited: Ab Initio Molecular Dynamics Study of Electrochemical Reaction at EC-Li2O2 Interfaces

Development of next-generation secondary batteries beyond Li-ion ones has been progressing all over the world. Li-air batteries are one of attracted candidates because of its large energy density. Organic liquids are normally used as a solvent of the electrolyte because of the lithium metal as an anode in the battery. However, it has been well known that some kinds of organic liquids such as ethylene carbonate (EC) are not stable on Li₂O₂ that grows up on the cathode in the discharging process. Previous ab initio simulation based on density functional theory (DFT) showed that a ring in the EC molecule is open by electron reduction [1]. Nevertheless, large dipole moment fluctuation by movement of EC molecule must change the electrostatic potential at the Li₂O₂-EC interface and will affect the chemical reaction process.

     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 O₂^2- unit in the Li₂O₂.

     We use openMX code [2] 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” [3] 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, O₂^2- 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 O₂^2-pair circled by white broken line in Fig.1 (b). It indicates that EC decomposition makes charging process impossible in Li-Air batteries.

 Reference

[1]  T. Laino andA. Curioni, Chem. Eur. J. 18, (2012) 3510.

[2] See http://www.openmx-square.org/for the code, OpenMX, pseudo-atomic basis functions and pseudopotentials. 

[3] H. Sun, S. J. Mumby, J. R. Maple, A. T. J. Hagler, Am. Chem. Soc. 116, (1994), 2978.