Li-Ion Conduction Behaviors of Glass-Ceramic Lithium Thiophosphates: Empirical Force Fields and Molecular Dynamics Simulations

The all-solid-state batteries (ASSBs) have drawn increasing interests and demands as the next-generation rechargeable Li-ion batteries (LIBs) for electric vehicles and energy storage devices since they have much more potentials than conventional LIBs [1,2]. However, all we know any further advances in ASSBs are not possible without solving the current raising issues of ASSBs, such as low Li-ion conductivity (σ                   ion), structural instability, and high resistance of electrode/electrolyte interface [1,2]. Recently, sulfide-based ASSBs are exhibiting the best performance approaching to the commercialization stage with various material advantages (e.g., excellent σ                   ion, room-temperature formability, and so on) [3,4]. Despite various recent studies on the development of advanced ASSBs, fundamental understanding of fast Li-ion conductors are still lacking. Here, we present an analysis of the Li-ion migration behavior of glass-ceramic (Li2S)0.75(P2S5)0.25 (LPS) structure, a topic that has not yet been investigated, by employing molecular dynamics (MD) simulations.                First, we identified unique PS4                   3− anion clusters in the short-range region of the glassy structure and newly developed the empirical pair potential (EPP) of the Li-[PS4                   3−] bonds. To reasonably represent the ionic and covalent bonds that constitute Li-[PS4                   3−] in γ-Li3PS4, EPP was composed of the long-range Coulombic, short-range Morse, and three-body harmonic potentials [5]. The two-body-interaction E                   I of the Coulombic and Morse potential was expressed as                                   E                   I                    = (qiqj                    / εr) e                   2 + D[e                      -α(r - r0                      ) - 2e-2α(r - r0                      )]                , where qi                    and qj                    are the ionic charges of the ion i and j, e is the elementary charge, ε is the dielectric constant, r is the distance between the ion i and j, r0                    is the equilibrium distance, D is the well-depth, and α is the stiffness parameter, respectively. The three-body harmonic potential E                   II was expressed as                                   E                   II                    = K(θ - θ0)2                                                    , where K is the prefactor and θ0                    is the equilibrium angle, respectively. The final converged fitting values are summarized in Table 1, and well reproduce the experimentally observed thermomechanical properties of γ-Li3PS4 [5]. Employing EPP, we constructed the glass−ceramic structure including the crystalline γ-Li3PS4, interfacial, and glassy LPS regions. The density obtained from each part of the glass−ceramic LPS structure agree well with the reported experimental results, and we concluded that our constructed glass−ceramic structure is reasonable. Based on the equilibrated glass-ceramic LPS structure, MD calculation was performed to evaluate the σ                   ion of each region. The glassy LPS has a σ                   ion of 4.08 × 10−                   1 mS cm−                   1, an improvement of ∼100 times relative to that of the γ-phase, which is in agreement with the experiments. By introducing the local S-sublattice analysis, we also found that 40% of the glassy and interfacial region comprised cubic S-sublattice, unlike the crystalline γ-phase, which consisted of only hexagonally closed-packed S-sublattice. These S-sublattice evolutions in the interfacial and glassy domains led to a notable improvement of σ                   ion.                In summary, we determined the EPP of the Li-[PS4                   3-] building blocks in (Li2S)0.75(P2S5)0.25 and predicted the σ                   ion of its glass-ceramic structure. The Li-ion conduction characteristics in the crystalline/interfacial/glassy structure were decomposed by considering the structural ordering differences, The superior σ                   ion of the glassy structure could be attributed to the fact that ~40% of its structure consists of the short-ranged cubic S-sublattice instead of hexagonally close-packed γ-phase. These results provide new directions for understanding glass−ceramic sulfides with the view of achieving superior Li-ion conducting materials as a solid electrolyte for ASSBs.                                   References                                 1. J. Janek, W.G. Zeier, Nat. Energy                    1 (2016) 16141.                2. S. Chen, D. Xie, G. Liu, J.P. Mwizerwa, Q. Zhang, Y. Zhao, X. Xu, X. Yao, Energy Storage Mater.                    14 (2018) 58–74.                3. Y. Kato, S. Hori, T. Saito, K. Suzuki, M. Hirayama, A. Mitsui, M. Yonemura, H. Iba, R. Kanno, Nat. Energy                    1 (2016) 16030.                4. A. Kato, M. Yamamoto, A. Sakuda, A. Hayashi, M. Tatsumisago, ACS Energy Mater.                    1 (2018) 1002–1007.                5. J.-S Kim, W. D. Jung, J.-W. Son, J.-H. Lee, B.-K Kim, K.-Y. Chung, H.-G. Jung, and H. Kim, ACS Appl. Mater. Interfaces                    11(2019) 13−18.                                                                                                            Figure 1