First Principles Investigation of the Delithiation Process in Li2s

Lithium ion batteries are among the most widely used energy storage devices in consumer electronics. However, limited capacity is still a major problem that hinders their application in a few key markets including electric vehicles. Many different materials are currently under intensive investigation, and lithium sulfide (Li 2 S) is a promising high capacity cathode material with a theoretical capacity of 1,166 mAh/g that is almost four times higher than what is offered by current commercially available cathodes. The Li 2 S cathode is also attractive since it can be used with a lithium metal free anode. Unfortunately, the delithiation of Li 2 S is usually sluggish, and a high charge voltage is required as a result. To understand the associated microscopic mechanism of the delithiation process in Li 2 S, first principles calculations based on the density function theory are performed using the Vienna Ab initio Simulation Package (VASP). A few low energy Li 2 S surfaces are created, and the delithiation processes on these surfaces are simulated by extracting a lithium ion on the surface. The resulting energy barriers are recorded and compared, and it is found that different surfaces lead to very different diffusion barriers, suggesting a possible route to minimize the diffusion barrier through the control of the equilibrium shape of the Li 2 S particles. The structural evolutions for systems with different amount of lithium vacancies that correspond to different percent of delithiation are simulated using ab initio molecular dynamics. To study the possible effects of the electrolyte, these simulations are also performed in the presence of electrolyte. The equilibrium structure of the electrolyte is first determined by ab initio molecular dynamics using a melt-quench process. The Li 2 S/electrolyte interfaces are then formed and the diffusion barriers are calculated. Three different electrolytes are studied and compared. These calculations provide an atomistic understanding of the delithiation process in Li 2 S, and help to develop new methods that can be used to minimize the activation barrier of Li 2 S particles.