(714e) Lithium Superionic Conductivity and Interfacial Stability in Newly Predicted Li-Sulfide Battery from Reactive Force Field

Several poly-sulfide ceramics based on Li₂S-P₂S₅ systems are studied as solid-state electrolyte and sulfur-based cathode candidate due to high Li-ionic conductivity and higher capacity.¹ Currently Li₁₀GeP₂S₁₂ (LGPS) demonstrates one of the highest Li-ion conductivity, 12 mS/cm at room temperature but it is not stable with Li-metal electrode, and it is expensive due to the presence of Ge. Here, we studied ion diffusion mechanism and ionic conductivities using classical molecular dynamics in a promising alternative Li₆(PS₄)SCl based on Li₂S-P₂S₅ system.² Based on our understanding on the diffusion mechanism we proposed a new Li-superionic conductor solid state electrolyte. We have also studied new polysulfide cathodes, Li_3+yPS_4+n and reported the interfacial stability between newly predicted electrolyte and cathode/ Li-metal anode.

Computational Details

Experimental crystal structures were relaxed with quantum mechanics (PBE-D3). QM calculated results were used to train the MD (LAMMPS+UFF/ReaxFF) force field parameters. MD simulations were run on Li_7-x(PS₄)S_2-xCl_x and Li_3+yPS_4+n single crystal at different temperatures for 20 ns. The MSD of the Li-ions reaches the Fickian regime for Li-diffusion after 1 ns of the simulation. Sulfur based Cathode-electrolyte interfacial stability was studied with ReaxFF.

Results and Discussion

We compared our computed Li-ionic conductivity at different temperatures with previously reported experimental results with EIS and NMR measurements in Figure 1. Our calculated activation energy of 0.24 eV is well within experimentally reported range. We noticed that Li migration occurs via conjugated substitutional type diffusion involving rearrangements of three or more Li-ions along with available Li-vacancy sites. Using this Li-vacancy mediated diffusion concept, we modified the Li₆(PS₄)SCl composition to control the Li-ionic conductivity in it. Calculated Li-ionic conductivities are reported in Table 1.^2-6 Li-ion diffusion was also studied on Li_3+yPS_4+n cathode. Structural stability of the electrolyte-cathode structure was held with PS₄ unit (Figure 2).⁷

Conclusion

Li-ionic diffusivity/ conductivity and activation energy in Li₆(PS₄)SCl were correctly predicted from classical MD. The computationally predicted composition Li₅(PS₄)Cl₂ is not yet experimentally realized and its Li-ionic conductivity is double that in LGPS. Li_3+yPS_4+n - Li_7-x(PS₄)S_2-xCl_x and Li-aonde-Li_7-x(PS₄)S_2-xCl_x interfacial stability and SEI formation were studied with ReaxFF.

Acknowledgement

Financial support of this research from the Centre of Hong Kong Quantum AI Lab Ltd established by the University of Hong Kong (the InnoHK initiative) is gratefully acknowledged.

References:

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