Li-Ion Conduction Behaviors of Glass-Ceramic Lithium Thiophosphates: Empirical Force Fields and Molecular Dynamics Simulations
Authors: Kim Byung-Kook , Kim Ji-Su , Jung Wo Dum , Son Ji-Won , Lee Jong-Ho , Kim Hyoungchul 
Published: May 2020

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.

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Figure 1