Lithium conduction and the role of alkaline earth cations in Li₂S–P₂S₅–MS (M = Ca, Sr, Ba) glasses

Highlights

• The glass-forming regions of ternary Li₂S–P₂S₅–MS (M = Ca, Sr, Ba) were identified.

• The alkaline earth cations positions for bridges between the sulfur atoms in PS${}_4^{4-}$.

• Low-mobility Li was observed in Li₂S–P₂S₅–MS glasses.

• The space around the PS${}_4^{4-}$ can serve as an effective conduction path .

Abstract

There has been considerable interest in lithium-sulfide glasses with high lithium ion conductivities because of their potential applications in solid-state electrolytes. Here, new sulfide glasses containing alkaline earth cations, Li₂S–P₂S₅–MS (M = Ca, Sr, Ba), were synthesized by melt quenching. Furthermore, the glass-forming regions in these glasses were identified.

Three samples containing equimolar alkaline earth cations (60Li₂S–30P₂S₅–10MS) were selected and their ${\mathrm{Li}}^{+}$ conductivities were measured. Lower ${\mathrm{Li}}^{+}$ conductivities were observed in 60Li₂S–30P₂S₅–10MS than in Li₂S–P₂S₅. The local structures of P and Li were characterized by ³¹P and ⁶Li solid-state magic angle spinning (MAS) NMR to determine the cause for the observed decrease in the ${\mathrm{Li}}^{+}$ conductivities and understand the role of alkaline earth cations in the Li₂S–P₂S₅ glasses. A higher-field shift of the ${\mathrm{PS}}_4^{3-}$ was observed in the ³¹P MAS NMR spectra of the glasses containing alkaline earth cations, which was attributed to the formation of bridging structures between ${\mathrm{PS}}_4^{3-}$ units by alkaline earth cations. Moreover, more than half of the Li had low mobility, which is consistent with the conductivity. These results highlight the presence of an effective conduction path around the ${\mathrm{PS}}_4^{3-}$ in contrast to ${\mathrm{P}}_2{\mathrm{S}}_7^{4-}$ and ${\mathrm{P}}_2{\mathrm{S}}_6^{4-}$.

Introduction

Lithium-ion batteries are a key technology enabling the effective use of electronic energy because of their power density, safety, and rechargeability [1]. To facilitate practical applications, all-solid-state battery systems with inorganic solid electrolytes have been developed, which have enabled the miniaturization of the battery package and improved the safety compared to current ${\mathrm{Li}}^{+}$ batteries containing flammable organic electrolytes [2], [3].

Main studies of inorganic solid electrolytes have focused on sulfides and oxides, which exhibit performance comparable to those of currently used liquid electrolytes. Crystalline materials, such as Li₇P₃S₁ [4], [5] and Li₁₀GeP₂S₁₂ [6], have been developed to achieve higher ${\mathrm{Li}}^{+}$ conductivity than in glassy amorphous materials. Indeed, crystalline materials were found to exhibit relatively high ${\mathrm{Li}}^{+}$ conductivity [6], [7]. Generally, these materials contain effective ${\mathrm{Li}}^{+}$ conduction paths in their crystalline structures, as observed by spectroscopic and computational analyses of their basic atomic structures [8], [9], [10], [11], [12]. However, the ${\mathrm{Li}}^{+}$ conduction path in glassy materials is relatively complicated because of their complex structural ordering [13]. To achieve higher conductivity, glassy materials are of less interest than crystalline materials; however, the isotropic character of amorphous materials makes them advantageous as a material for battery fabrication. Generally, in practical use, the temperature and voltage vary because of volumetric expansion of the battery components. The anisotropic expansion of crystalline materials leads to the deterioration of the interference between the electrolyte and electrodes. Therefore, the isotropic performance of amorphous materials is attractive for practical applications.

Various glass compositions have been studied in the interest of discovering new types of Li₂S–P₂S₅ glassy materials. For examples, Li₂S–P₂S₅ glass systems such as Li₂S–P₂S₅–LiI with a variety of additives have already been developed [14], [15], [16], [17], [18], [19]. In this study, we focus on Li₂S–P₂S₅ glass systems containing alkaline earth cations (Ca, Sr, and Ba). The purpose of this study is to develop a new group of Li₂S–P₂S₅ glasses with alkaline earth cations as additives. Divalent cations will be incorporated at the stable sites for cations, which will affect the host structure and ${\mathrm{Li}}^{+}$ diffusion path in the glass. An unexpected role for Li conduction may work for these materials. Moreover, the mechanism of ${\mathrm{Li}}^{+}$ conduction in the presence of alkaline earth cations is investigated. Herein, we evaluate the glass-forming regions of various ternary glass systems, Li₂S–P₂S₅–MS (M = Ca, Sr, Ba). Next, the ${\mathrm{Li}}^{+}$ conductivity and its mechanisms in the Li₂S–P₂S₅ glasses containing equimolar alkaline earth are investigated using ³¹P and ⁶Li solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.