Ab Initio Modeling of Li/Li6PS5Cl Interface Formation in Lithium Solid State Batteries
Authors: Cheng Tao , Merinov Boris V , Kozinsky Boris , Goddard William A 
Published: February 2020

Several sulfide glasses, glass-ceramics or ceramics have been developed as promising electrolytes, mainly based on binary and ternary systems, including Li2S and P2S5 with ternary components such as P2S3, GeS2 and P2O5. Currently, Li10GeP2S12 exhibits one of the highest lithium ionic conductivities, 12 mS/cm at room temperature1 which is even higher then that of liquid electrolytes. However, this material is costly, because Ge is expensive. Recently, a relatively new family of sulfide ceramics, so-called Li-argyrodites, Li6PS5X (X = Cl, Br, I), has been studied and high ionic conductivity (7 mS/cm at 298 K) has been observed.2 These materials can provide a balance of efficiency and price and can be used as solid electrolyte in electrochemical devices, for example, in Li-batteries. However, in this case, one of the questions is how stable Li6PS5X in contact with Li-electrode. To study this problem, we have built the Li6PS5Cl structure and performed ReaxFF reactive force field and ab initio molecular dynamics simulations of this structure, as well as a Li/Li6PS5Cl interface. Figure 1 shows our optimized Li/Li6PS5Cl interface model with eight layers of Li [the BCC 3x3 (100) surface] and six layers of Li6PS5Cl [the (100) surface]. During optimization, PS4 at the interface quickly decomposed, breaking P-S bonds and producing new P-Li bonds and Li-S bonds. The radial distribution function (RDF) also confirms these bonding changes: the P-S peak decreases, while new peaks of P-Li appear in a short range (2-3 Å). Integrating RDF shows that the coordination number of S to P decreases, and in the same time the number of Li coordinated to P increases. Since the decomposition occurs during the optimization, we conclude that these decomposition reactions might be barrierless. This indicates that Li6PS5Cl is unstable in contact with Li metal. The quick decomposition of the Li6PS5Cl electrolyte may attribute to the weak bonding between P and S and, therefore, the chemical instability may be a general problem for P-S based solid electrolytes. The Li-ion diffusion is significantly slower in the interface compared to that in the Li6PS5Cl bulk structure. One of the possible solutions of this problem might be partial oxidation of the interface, using strong P-O bonds to replace weak P-S bonds. This may decrease the ionic conductivity of the interface. Therefore, the oxidized layer should be thin enough in order not to decrease the overall conductivity of the system to the unacceptable value. We built a corresponding model and after the energy minimization of the modified interface, no decomposition was observed. To further investigate stability of the Li/Li6PS5Cl interface, a molecular dynamics (MD) simulation was carried out at 298 K. We find that after the 100 ps MD simulation, the partially oxidized interface still remains very stable. However, to make a firm conclusion about stability of the interface studied, a longer MD simulation is required. Such a simulation is in process.
Authors gratefully acknowledge support from Bosch Energy Research Network Grant No. 13.01.CC11.
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Figure 1