Real Time Observation of Lithium Insertion into Pre-Cycled Conversion-Type Materials.

Sooyeon Hwang,Dong Su

doi:10.3390/nano11030728

Abstract

Conversion-type electrode materials for lithium-ion batteries experience significant structural changes during the first discharge–charge cycle, where a single particle is taken apart into a number of nanoparticles. This structural evolution may affect the following lithium insertion reactions; however, how lithiation occurs in pre-cycled electrode materials is elusive. In this work, in situ transmission electron microscopy was employed to see the lithium-induced structural and chemical evolutions in pre-cycled nickel oxide as a model system. The introduction of lithium ions induced the evolution of metallic nickel, with volume expansion as a result of a conversion reaction. After pre-cycling, the phase evolutions occurred in two separate areas almost at the same time. This is different from the first lithiation, where the phase change takes place successively, with a boundary dividing the reacted and unreacted areas. Structural changes were restricted at the areas having large amount of fluorine, implying the residuals from the decomposition of electrolytes may have hindered the electrochemical reactions. This work provides insights into phase and chemical evolutions in pre-cycled conversion-type materials, which govern electrochemical properties during operation.

Highlights

Nanomaterials 2021, 11, 728. https://Conversion-type electrode materials (MX, where M is generally a transition metal ion and X is O, S, F, etc.) have been extensively explored, as they can deliver much higher capacities than intercalation-type materials [1,2,3,4,5,6]
Structural evolutions during the first discharge–charge cycle may change the local environments around the electrode material through volume expansion [7] and fragmentation of the bulk active material; subsequent electrochemical reactions and corresponding phase evolutions may occur in different ways than those at the initial reaction
Despite the fact that the dynamic phase evolutions at the first lithiation have been thoroughly investigated by in situ transmission electron microscopy (TEM), [8,9,10,11,12,13] the knowledge achieved from the first lithium insertion may not be applicable to understanding the structural changes in the second and following cycles

Summary

Introduction

Nanomaterials 2021, 11, 728. https://Conversion-type electrode materials (MX, where M is generally a transition metal ion and X is O, S, F, etc.) have been extensively explored, as they can deliver much higher capacities than intercalation-type materials [1,2,3,4,5,6]. Despite the fact that the dynamic phase evolutions at the first lithiation have been thoroughly investigated by in situ transmission electron microscopy (TEM), [8,9,10,11,12,13] the knowledge achieved from the first lithium insertion may not be applicable to understanding the structural changes in the second and following cycles. It is of importance to observe how the phase evolutions after the first cycle take place in real time since they determine the electrochemical performances during operation. It is still elusive how lithiation occurs in pre-cycled conversion materials

Methods

Results

Conclusion