Structural change of Li2S–P2S5 sulfide solid electrolytes in the atmosphere

doi:10.1016/j.ssi.2010.10.013

Solid State Ionics

Volume 182, Issue 1, 3 February 2011, Pages 116-119

https://doi.org/10.1016/j.ssi.2010.10.013 Get rights and content

Abstract

The sulfides Li₂S−P₂S₅ are attractive solid electrolytes for all-solid-state lithium batteries since they have high lithium-ion conductivities and wide electrochemical windows. However, chemical stability of Li₂S-P₂S₅ solid electrolytes in the atmosphere is concerned. In the present study, the structure of Li₂S−P₂S₅ glasses and glass-ceramics after exposure to air was investigated for the first time. Raman spectroscopy demonstrated that the 75Li₂S∙25P₂S₅ (mol%) glass and glass-ceramic composed of Li⁺ and PS₄³⁻ ions did not exhibit obvious structural changes on exposure to the atmosphere. The amount of H₂S generated from Li₂S−P₂S₅ glasses exposed to air depended on the glass composition. The composition of 75Li₂S∙25P₂S₅ generated the least amount of H₂S of Li₂S−P₂S₅ binary systems. 75Li₂S∙25P₂S₅ glass and glass-ceramic are promising solid electrolytes for all-solid-state lithium batteries in terms of chemical stability and conductivity.

Research Highlights

► A chemical stability of Li₂S-P₂S₅ solid electrolytes in air was evaluated. ► The 75Li₂S∙25P₂S₅ electrolyte did not exhibited structural changes in air. ► The amount of H₂S generated from Li₂S-P₂S₅ glasses depended on the glass composition. ► The composition of 75Li₂S∙25P₂S₅ generated the least amount of H₂S.

Introduction

Solidification of electrolytes is useful for improving the safety and reliability of lithium secondary batteries. Solid electrolytes with high lithium-ion conductivities are indispensable for realizing all-solid-state lithium batteries. Sulfide-based solid electrolytes exhibit high ion conductivities due to the high polarizabilities of sulfide ions [1], [2], [3], [4], [5], [6]. We prepared sulfide glass electrolytes in the system Li₂S−P₂S₅ by mechanical milling [7]. Li₂S−P₂S₅ glasses containing 75 mol% or more Li₂S had conductivities of over 10^− 4 S cm^− 1 at room temperature.

We also prepared Li₂S−P₂S₅ glass-ceramics by crystallizing the glasses [8], [9], [10]. Superionic crystals such as Li₇P₃S₁₁ crystals or thio-LISICON analogs were precipitated in glass-ceramics containing 70 mol% or more Li₂S. The thio-LISICON III analog (Li_3.2P_0.96S₄) was precipitated by crystallization of the 75Li₂S∙25P₂S₅ (mol%) glass; the conductivity of the obtained glass-ceramic exceeded 10^− 4 S cm^− 1 at room temperature. Furthermore, Li₇P₃S₁₁ crystals and the thio-LISICON II analog (Li_3.25P_0.95S₄) were respectively precipitated in 70Li₂S∙30P₂S₅ and 80Li₂S∙20P₂S₅ glass-ceramics; these glass-ceramics had conductivities of over 10^− 3 S cm^− 1 at room temperature.

Li₂S−P₂S₅ glasses and glass-ceramics have the advantage of high lithium-ion conductivities. The main drawback of these sulfide electrolytes is that they must be handled in an inert gas atmosphere since they have a low chemical stability in air. Hydrolysis of these sulfides by water molecules in air generates harmful H₂S gas. However, no studies have investigated generation of H₂S gas from Li₂S−P₂S₅ solid electrolytes in the atmosphere. The structural changes that these solid electrolytes undergo in air are expected to strongly affect H₂S generation. It is important to examine these electrolytes in air to elucidate key factors for improving their chemical stability and reliability. The structural changes that these sulfide electrolytes undergo in the atmosphere have not been investigated.

Here, we demonstrate that the 75Li₂S∙25P₂S₅ glass and glass-ceramic are attractive sulfide electrolytes with relatively high chemical stabilities and conductivities in the atmosphere. This is the first study to investigate the structure and stability of Li₂S-based sulfide electrolytes in air. The present study examines H₂S generation from Li₂S−P₂S₅ glass and glass-ceramic electrolytes with various compositions in air. Changes to the structures and conductivities of these electrolytes were investigated as a function of exposure time to air.

Section snippets

Experimental

xLi₂S∙(100−x)P₂S₅ (mol%; 67 ≤ x ≤ 80) glasses were prepared by mechanical milling. Reagent-grade Li₂S (Idemitsu Kosan, 99.9%) and P₂S₅ (Aldrich, > 99%) crystalline powders were used as the starting materials. A mixture of these materials was mechanically milled at room temperature by a planetary ball mill (Fritsch, Pulverisette 7) using a zirconia pot (volume: 45 ml) with 500 zirconia balls (diameter: 4 mm). The rotation speed was 510 rpm and the milling times were in the range 8−24 h. 75Li₂S∙25P₂S₅

Results and discussion

The amounts of H₂S generated from Li₂S−P₂S₅ glasses were investigated. Fig. 1 shows the amounts of H₂S generated from the pelletized Li₂S−P₂S₅ glasses containing 67, 70, 75 and 80 mol% Li₂S. Data for the 75Li₂S∙25P₂S₅ glass-ceramic and a Li₂S crystal are also shown. The amount of H₂S at the y-axis indicates the calculated amounts of H₂S generated by these sulfide samples after exposure to air for 1 min. The calculated amount of H₂S shown is the product of the H₂S concentration and the volume of

Conclusions

The amounts of H₂S generated from Li₂S−P₂S₅ solid electrolytes and their structural changes in air were investigated for the first time. In the case of the Li₂S crystal and the 67Li₂S∙33P₂S₅ glass, the structural units of S²⁻ or P₂S₇⁴⁻ ions changed greatly in air and significant amounts of H₂S were generated. In contrast, the 75Li₂S∙25P₂S₅ glass and glass-ceramic composed of the PS₄³⁻ ion did not show obvious structural changes in the atmosphere and generated the least amount of H₂S of the Li₂

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Structural change of Li2S–P2S5 sulfide solid electrolytes in the atmosphere

Abstract

Research Highlights

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Solid State Ionics

Solid State Ionics

J. Non-Cryst. Solids

Solid State Ionics

Solid State Ionics

Electrochem. Commun.

Structural change of Li₂S–P₂S₅ sulfide solid electrolytes in the atmosphere