Lesson Learned from NMR: Characterization and Ionic Conductivity of LGPS-like Li7SiPS8

Sascha Harm,Bettina V Lotsch,Constantin Hoch,Anna-Katharina Hatz,Alexander Kuhn,Roland Eger,Igor Moudrakovski

doi:10.1021/acs.chemmater.8b04051

Abstract

We report on the facile solid-state synthesis and characterization of Li7SiPS8, a new member of the tetragonal Li10GeP2S12 (LGPS)-type family of ultrafast Li+ solid electrolytes. We analyze the structure, phase stability, and Li+ conductivity of tetragonal and orthorhombic LSiPS by pulsed field gradient NMR and impedance spectroscopy, which show conductivities at room temperature of up to 2 mS cm–1. Although ranking tetragonal LiSiPS as an ultrafast solid electrolyte, the observed conductivity is unexpectedly low compared to other members of this solid-solution system. Utilizing solid-state NMR, quantitative phase analysis, and impedance spectroscopy, we identify an amorphous thiophosphate side phase with low Si content, which limits the intergrain conductivity and, hence, a potentially higher total conductivity. This case study thus highlights the need for comprehensive structural analysis of LGPS-type materials beyond the crystalline fractions to fully characterize the structure–property relationships i...

Highlights

All-solid-state lithium-ion batteries (ASSLiBs) have been identified as promising candidates for future mobile battery applications
We report on the facile solid-state synthesis and characterization of Li7SiPS8, a new member of the tetragonal Li10GeP2S12 (LGPS)-type family of ultrafast Li+ solid electrolytes
A glass phase with high impedance can limit the performance of such materials shown recently in NASICON-like Li1−xTi2−xScx(PO4)3.22 In this article, we report on the facile solid-state synthesis of Li7SiPS8, a new LGPS-like superionic conductor

Summary

Introduction

All-solid-state lithium-ion batteries (ASSLiBs) have been identified as promising candidates for future mobile battery applications. High ionic and low electronic conductivity are key prerequisites, along with high electrochemical and structural stability, as well as low production costs.[2] Regarding conductivity, some of the best solid-state electrolytes are sulfides and were discovered just recently.[3−7] Their high conductivities as compared to most oxide SSEs are supposed to be due to the high polarizability of the sulfide ion enabling fast lithium movement and, at least for the thio-LISICON (lithium superionic conductor) family, a structure that is derived from a strongly distorted hexagonal close-packed sulfur sublattice featuring many distorted face and edge sharing tetrahedral and octahedral voids, facilitating lithium hopping.[8−10] Tetragonal Li10GeP2S12 (lithium thiogermanate thiophosphate, LGPS) is one of the most promising thio-LISICON materials It was first introduced by Mitsui et al in 2011 and shown to exceed the conductivity values of the hitherto best crystalline lithium-ion conductors by 1 order of magnitude.[7,11] The LGPS system can be described as a solid-solution Li11−xTt2−xP1+xS12

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Conclusion