Visualization and Chemical Characterization of the Cathode Electrolyte Interphase Using He-Ion Microscopy and In Situ Time-of-Flight Secondary Ion Mass Spectrometry.

Laura Wheatcroft,Gregor Hlawacek,Doğan Özkaya,James Cookson,Nico Klingner,Beverley J Inkson,René Heller

doi:10.1021/acsaem.0c01333

Abstract

Unstable cathode electrolyte interphase (CEI) formation increases degradation in high voltage Li-ion battery materials. Few techniques couple characterization of nano-scale CEI layers on the macroscale with in situ chemical characterization, and thus, information on how the underlying microstructure affects CEI formation is lost. Here, the process of CEI formation in a high voltage cathode material, LiCoPO4, has been investigated for the first time using helium ion microscopy (HIM) and in situ time-of-flight (ToF) secondary ion mass spectrometry (SIMS). The combination of HIM and Ne-ion ToF-SIMS has been used to correlate the cycle-dependent morphology of the CEI layer on LiCoPO4 with a local cathode microstructure, including position, thickness, and chemistry. HIM imaging identified partial dissolution of the CEI layer on discharge resulting in in-homogenous CEI coverage on larger LiCoPO4 agglomerates. Ne-ion ToF-SIMS characterization identified oxyfluorophosphates from HF attack by the electrolyte and a Li-rich surface region. Variable thickness of the CEI layer coupled with inactive Li on the surface of LiCoPO4 electrodes contributes to severe degradation over the course of 10 cycles. The HIM–SIMS technique has potential to further investigate the effect of microstructures on CEI formation in cathode materials or solid electrolyte interphase formation in anodes, thus aiding future electrode development.

Highlights

The electrification of transport is being implemented as a method to significantly reduce fossil fuel consumption
cathode electrolyte interphase (CEI) formation on C-LiCoPO4 is similar to the results found with transmission electron microscopy (TEM) imaging in the literature.[11]
He+ ion-induced secondary electron (He-induced secondary electrons (iSEs)) imaging enabled visualization of the CEI at a field of view which allows for comparison of the CEI distribution with the underlying electrode microstructure and imaging of the variation in CEI layer thickness

Summary

INTRODUCTION

The electrification of transport is being implemented as a method to significantly reduce fossil fuel consumption. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) for helium ion microscopy (HIM), using an incident He+ or Ne+ beam, has been developed by Klingner and co-workers.[19,20] The combination of high-resolution surface imaging provided by HIM and ToF SIMS within the helium ion microscope capable of detecting Li+ makes HIM−SIMS a potentially powerful technique to study CEI layers on lithium ion battery cathodes. We correlate the morphology of CEI growth on LiCoPO4 with the chemistry of the electrode surface, that is, the CEI composition and its location on the electrode with respect to the active material or conductive additive, using high spatial resolution helium ion imaging and Ne-ion ToF-SIMS for HIM.[19,20]

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

Chemical Characterization with Secondary Ion

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES