Computationally Predicted New Solid-State Electrolyte (Li5+x
PS4+x
Cl2-x
: 0 ≤ x ≤ 2) and Poly Sulfide Cathodes (Li3+y
PS9 or Li5+y
PS9Cl2: 0 ≤ y ≤ 9) for High Performance Li Metal Anode Batteries
Authors: Das Tridip , Morozov Sergey , Merinov Boris , Zybin Sergey , Yang Moon Young , Goddard William A 
Published: February 2024

We used Molecular Dynamics (MD) simulations to study the structures and conductivity of sulfide-based ceramics from the superionic conductor argyrodite family for applications as solid-state electrolyte and cathode materials. These studies combined quantum mechanics (QM) and ReaxFF reactive force field based MD, where the ReaxFF parameters were developed from QM MD.[1]
Currently Li10GeP2S12 (LGPS) demonstrates one of the highest Li-ion conductivity for solid state electrolyte, 12 mS/cm at 300K, but it is highly reactive with the Li-metal anode, and it is expensive due to the presence of Ge. We studied the Li ion diffusion mechanism and ionic conductivity of the promising Li6(PS4)SCl solid state electrolyte, predicted to have a conductivity of s = 6 mS/cm, close to experiment (4 mS/cm). We find that Li migration in this electrolyte occurs via conjugated substitutional type diffusion involving rearrangements of three (or more) Li-ions and ~ 3 vacant sites in a 3D matrix of anions that are essentially stationary at 298K (over 20 ns of simulation). We carried out 10 ns of MD simulation to predict a Li-ion conductivity for a single phase Li6(PS4)SCl of 5.9 mS/cm in agreement with solid state NMR measurements of 3.9 mS/cm. Our calculated activation energy of 0.24 eV is within experimentally reported range.[2]
We report here the computationally predicted structure of Li5PS4Cl2, a new sulfide Li-superionic conductor with the highest Li-ion conductivity at solid state, which we predict to have Li-ion conductivity of 20 mS/cm at 298 K. We also predicted the directional ionic conductivity and Li-migration barrier in it. This Li5PS4Cl2 has not yet been synthesized and studied experimentally.
We also report our results on QM and ReaxFF MD for new polysulfide cathodes, Li3+y
PS4+n and Li5+y
PS4+nCl2 , both based on Li3PS4. Here, n is the number of excess sulfur atoms in the fully charged polysulfide cathode per formula unit with y=0 and y is the excess Li added to the cathode during discharge processes. We evaluated the performance of these cathode electrolyte systems in terms of interfacial stability and discharge voltages.
Li3PS4+5 is a potential cathode for lithium-sulfur batteries. We predicted the structures of Li3PS4+5 finding extra S3 and S7 chains attached to one S atom of each PS4 group. This leads to a density of 2.2 g/cm3. As the cathode is discharged, the lithium atoms fill up all the gaps between S atoms, leading to Li3+9
PS4+5, with 1.7 g/cm3. We studied the dynamics of the discharge as lithium ions from the metal move into the material and react with the S-S bonds to make Li2S. Our predicted discharge curve agrees well with the experimental data (Figure 1).[3]
We extended these studies to design a new polysulfide cathode Li5PS4+5Cl2 to be used with our newly predicted Li-superionic conductor electrolytes. The interfacial stability of the Solid electrolyte with S-based cathode and with Li-anode were studied (Figure 2).
Our work presents a new strategy for designing materials and processes for solid-state batteries using computational screening and chemical substitution. This demonstrates the potential of using lithium PS4 based superionic conductors with PS4 based cathodes for applications in solid-state batteries. We also addresses some of the key challenges in accelerating solid-state battery development, such as phase stability, transport properties, electrode compatibility etc. This work demonstrates how computational studies can guide new avenues for experimental developments to advance of solid-state battery technology for applications ranging from grid storage to electric vehicles.
Acknowledgements:
We acknowledge the support from Hong Kong Quantum AI Lab, AIR@InnoHK of Hong Kong Government.
References:

Yang et al., Adv. Energy Mater., 2023,13, 2202949

Das et al., J. Mater. Chem. A, 2022, 10, 16319-16327

Morozov et al., Cell Reports Physical Science, 2023, 4(3), 101326

Figure 1