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First principles computational techniques are used to study properties of promising Li ion electrolytes, recently developed at Oak Ridge National Laboratory, based on alloys having the composition Li3 + xAs1 − xGexS4. The crystal structure of pure Li3AsS4 is found to be characterized by the Pmn21 space group. Based on modifications of this structure, reasonable models of the x = 1/4 and x = 1/3 alloys are found to be in good agreement with the experimental X-ray diffraction patterns and to be consistent with the measured trends in Li ion conduction. As a consequence of their Pmn21-based structures, interstitial and interstitialcy mechanisms are found to be important for the Li ion conduction processes in these systems.

Recent experimental literature reports the solid state electrolyte properties of Li 4 SnS 4 and Li 4 SnSe 4 , identifying interesting questions regarding their structural details and motivating our first principles simulations. Together with Li 4 GeS 4 , these materials are all characterized by the orthorhombic space group Pnma and are found to be isostructural. They have a ground state crystal structure (denoted Li 4 SnS 0 4 ) having interstitial sites in void channels along the c-axis. They also have a meta-stable structure (denoted Li 4 SnS * 4 ) which is formed by moving one fourth of the Li ions from their central sites to the interstitial positions, resulting in a 0.5 Å contraction of the a lattice parameter. Relative to their ground states, the meta-stable structures are found to have energies 0.25 eV, 0.02 eV, and 0.07 eV for Li 4 From this literature, some interesting questions arise regarding crystal structures and mechanisms for ion mobility. In order to address these questions, we use first principles methods to examine the ideal crystal forms and defect structures of Li 4 SnS 4 and the structurally and chemically related materials Li 4 GeS 4 and Li 4 SnSe 4 . For each of these materials, we identify two closely related structures -an ideal ground state structure and an ideal meta-stable structure. The simulations show that the meta-stable structural form is most accessible to Li 4 SnS 4 of the three materials studied. The simulations are extended to study mechanisms of Li ion migration in both Li 4 SnS 4 and Li 4 SnSe 4 and are related to the experimental results reported in the literature. Computational MethodsThe computational methods used in this work are based on density functional theory (DFT), 8,9 using the projected augmented wave (PAW) 10 formalism. The PAW basis and projector functions were generated by the ATOMPAW 11 code and the crystalline materials were modeled using the QUANTUM ESPRESSO 12 and ABINIT 13 packages. Visualizations were constructed using the XCrySDEN, 14 The exchange correlation function is approximated using the localdensity approximation (LDA). 17 The choice of LDA functional was made based on previous investigations 18-20 of similar materials which showed that, provided that the lattice constants are scaled by a correction factor of 1.02, the simulations are in good agreement with experiment, especially lattice vibrational frequencies and heats of formation. The partial densities of states were calculated as described in previous work, 20,21 using weighting factors based on the charge within the augmentation spheres of each atom with radii r The calculations were well converged with plane wave expansions of the wave function including |k + G| 2 ≤ 64 bohr −2 . Calculations for the conventional unit cells were performed using a Brillouin-zone sampling grid of 4 × 8 × 8. Simulations of Li ion migration were performed at constant volume in supercells constructed from the optimized conventional cells extended by 1 × 2 × 2 and a Brillouin-zone sampling grid of 2 × 2 × 2. In ...

Two recently synthesized crystalline materials which belong to the LiPON materials family, Li12P3N9 and it’s high–pressure polymorph Li4PN3, have been computationally examined as solid electrolytes for the usage in Li–ion batteries through first–principles simulations. The simulations of the idealized electrolyte properties of both materials suggests that these materials are promising solid electrolytes. The simulated crystal structures of both materials found to be in good agreement with experimental data, the phase transition between Li12P3N9 and Li4PN3 at high pressure has been validated through the simulations and found to be consistent with computational and experimental literature. The Li–ion migration analysis predicts that these materials are a very promising conductors for Li–ions with a calculated value for the activation energy comparable and even smaller to those of good solid electrolytes in the same materials family. The Li–ion migration analysis also suggests that the Li–ion migration is dominated with the vacancy migration mechanism which takes place along the three diffusion axes in Li12P3N4 and along the c–axis only in Li4PN3. The simulations of idealized interfaces with metallic Li anode demonstrate that these materials are likely to form a metastable interfaces with Li metal.

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