Optimization of lithium ion conductivity of Li2S-P2S5 glass ceramics by microstructural control of crystallization kinetics
Introduction
All-solid-state (ASS) lithium-ion batteries (LIBs) have attracted great attention owing to their safety and high energy density [1]. Currently, the major lithium ion- conducting inorganic solid electrolytes can be distinguished as oxide and sulfide electrolytes. An oxide electrolyte has excellent stability and exhibits good ionic conductivity. Several kinds of oxide electrolytes have been investigated previously, such as, perovskite type like Li3xLa2/3−x□1/3−2xTiO3 (LLTO; 0.04 ˂ x ˂0.17), anti-perovskite type like Li3OCl, NASICON type like Li1.5Al0.5Ge(PO4)3 (LAGP) and Li1.3Al0.3Ti1.7(PO4)3 (LATP), and garnet type like Li7La3Zr2O12 (LLZO) [[2], [3], [4], [5], [6]]. Owing to the high resistance at the grain boundary, however, the total ionic conductivity at room temperature, which influences the power density of LIBs, is still far from being useful for practical applications. Among all the types of solid electrolytes for a lithium-ion battery, the sulfide solid electrolyte has particularly attracted much attention. Because of the lower electronegativity and larger ionic radius of sulfur than oxygen, a relatively smaller bonding energy and larger ionic migration channel of sulfur and Li ion pairs can be achieved in comparison with oxygen and Li ion pairs, which can lead to more free-moving lithium cations [7]. Consequently, the sulfide electrolyte has a high ionic conductivity at room temperature, which is comparable to an organic liquid electrolyte [[8], [9], [10]]. The Li10GeP2S12 (LGPS) and Li2S-P2S5 families are the most common sulfide solid electrolytes. Kayama et al. reported that LGPS has a high conductivity of 1.2 × 10−2 S cm−1 [9]. Kato et al. discovered that Li9.54Si1.74P1.44S11.7Cl0.3, which has a similar structure to LGPS, exhibits the world's highest conductivity of 2.5 × 10−2 S cm−1 [10]. Seino et al. determined that 70Li2S∙30P2S5 has a conductivity of 1.7 × 10−2 S cm−1 [8]. However, the chemical stability and charge/discharge efficiency of the abovementioned materials are very low [[7], [8], [9], [10]]. Although Li ion conductivity of 75Li2S∙25P2S5 glass-ceramics is not very high (approximately 2 × 10−4 S cm−1), it shows high chemical stability [11]. Compared with other sulfide materials, the Li3PS4 phase in 75Li2S∙25P2S5 glass-ceramics finds it difficult to release H2S through the reaction with water when exposed to the atmosphere [12]. In addition, during charge/discharge cycles, Li3PS4 is formed as a buffer layer between the electrode and electrolyte, which protects the electrode, and thus significantly improves the charge/discharge efficiency [13]. Such specific properties have made 75Li2S∙25P2S5 one of the most promising Li ion solid electrolytes for ASS LIBs.
So far, the mechanisms of Li ion conduction have been extensively investigated. In previous studies [9,14], the crystalline phases, such as a Li10GeP2S12 analog (Li3.25P0.95S4) and Li7P3S11, which have been classified as superionic conductors, were synthesized through heat treatment in Li2S-P2S5 binary systems. The correlation between Li ion conduction and crystallinity in the glass ceramics have also been discussed. For example, Li ion conductivity in a 70Li2S-30P2S5 glass-ceramics system was promoted with an increase in crystallinity of up to approximately 80%, using magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy for P [15]. Although the crystal (β-Li3PS4) showed relatively high Li conductivity [16,17], 75Li2S-25P2S5 glass heated to 160 °C showed a much higher Li conductivity than β-Li3PS4 in a previous report [18]. Tsukasaki et al. [19] investigated the correlation between the crystallinity and ionic conductivity of a 75Li2S-25P2S5 glass-ceramics electrolyte using transmission electron microscopy (TEM). They showed that the crystallinity of 75Li2S∙25P2S5 glass-ceramics should be controlled under 5% to enhance Li ion conductivity. These phenomena may be explained by the effective medium theory in a composite solid electrolyte system, as is discussed in this paper.
A composite solid electrolyte (CSE), which contains an ionic conducting phase and an insulating phase, exhibits a higher ionic conductivity than a pure ionic conductor [[20], [21], [22]]. The dramatic increase would mainly be caused by a space charge layer at the interface between the insulator and ionic conductor [23,24]. The formation of a continuous ion conducting pathway in an interfacial phase can contribute to a high ionic conductivity at a typical percolation rate mainly from 10% to 40% [25]. Nan et al. reported an optimized effective medium approach (EMA) for AC electrical conductivity at any percolation rate [26]. Also, a porous β-Li3PS4 electrolyte promoted the ionic conductivity by 3 orders of magnitude [16]. The porous β-Li3PS4 electrolyte was used as an ionic conductor-insulating phase system, and it was found that the conductivity reached the maximum at the volume fraction of porosity of approximately 30% based on EMA [27].
In the classical theory, the crystallization process of glass can be divided into a nucleation stage and a crystal growth stage. In a Li2O-SiO2 system, the nucleation rate, I, and crystal growth rate, U, were discussed to control them through differential thermal analysis (DTA) [28,29]. This approach was also applied to the 78Li2S∙22P2S5 glass-ceramics system, and their sample was totally crystallized (thio-LISICON II analogue) [30]. To improve the conductivity, however, more detailed structural control of the glass ceramics, like interfacial conductivity, is required. In this study, I and U were calculated through TEM and used to control the interfacial microstructures of the glass ceramics.
To promote Li ion conductivity by controlling the crystallinity of the 75Li2S-25P2S5 glass-ceramics electrolyte, we quantitatively analyze the kinetics of the crystallization process for 75Li2S∙25P2S5 glass using the DTA method. Furthermore, we discuss a correlation of the size and number of crystalline particles, and crystallinity with Li ion conductivity using EMA, and we improve the Li ion conductivity by up to 80% under optimized heat treatment conditions. The fabrication process of the glass ceramics is important. In this study, after the densification of the samples and appropriate heat treatments, the best Li conductivity for the 75Li2S∙25P2S5 glass-ceramics was achieved. Therefore, this is the first report that controls the interfacial ionic pathway in the microstructure of a lithium sulfide electrolyte, to achieve the highest ionic conductivity for 75Li2S∙25P2S5 reported to date.
Section snippets
Preparation of Li2S·P2S5 glass powder
75Li2S·25P2S5 glass powder was prepared by high-energy ball milling in Ar [7]. Li2S (Idemitsu Kosan Co., purity >99.9%) and P2S5 (Aldrich, purity 99%) crystalline powders were mixed and set in a ZrO2 pot containing ZrO2 balls with 10 mmϕ, and then mechanically milled at room temperature with a planetary ball mill (Fritsch Pulverisette 7, Germany) at 370 rpm for 40 h. During the mechanical milling, the cooling time in the middle was 30 min for every 10 h. The sample treatments were performed in
Results and discussion
Fig. 1 shows the XRD patterns of 75Li2S∙25P2S5 glass at room temperature before and after heat treatment at different temperatures for 1 h. The as-prepared glass and glass-ceramics obtained after annealing at 180 °C showed no particular diffraction peaks, which indicated that the sample was in an amorphous state. Glass ceramics obtained after heat treatment above 200 °C showed the diffraction patterns of a β-Li3PS4 crystalline phase.
Fig. 2 shows a cross-sectional SEM image of an electrolyte
Conclusions
To improve the electrical conductivity of 75Li2S∙25P2S5 glass-ceramics materials, the crystallization kinetics I and U were investigated. Based on the crystallization kinetics and effective medium approach (EMA), control of the microstructure of 75Li2S∙25P2S5 glass-ceramics was successfully performed. After a heat treatment at 190 °C for 42 min, the conductivity was improved by 80% to reach 1.33 × 10−3 S cm−1, which is the highest value ever achieved for 75Li2S∙25P2S5 glass-ceramics materials.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, and The University of Tokyo for use of SEM facility. We also thank Mr. Rui Huang for data arrangements.
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