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

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, LiCoPO 4 , 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 LiCoPO 4 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 LiCoPO 4 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 LiCoPO 4 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.

Face to Face at the Cathode Electrolyte Interphase: From Interface Features to Interphase Formation and Dynamics

“…Even though operando analysis tools have to be the preferred strategy for future CEI evolution investigations, great scientific results have been published in this field of research using ex situ/post mortem analysis. XPS, [171,[173][174][175][176]178,179,182,[190][191][192]197] EIS, [174,175,180,182] and time-of-flight secondary ion mass spectrometry (TOF-SIMS) [172,173,181] are widely utilized methods, often in combination or with other complimentary methods.…”

“…They discovered an inhomogeneous coverage and thickness of the CEI on the cathode surface, leading to local differences regarding resistance and CEI dissolution and deposition behavior, ultimately leading to an ongoing electrolyte consumption on freed up cathode material. [181] Other publications reported on the enhancing effect of different cathode coatings on the CEI stability. [174,179] However, almost no research has been conducted on the influence of cathode coatings on the CEI.…”

“…They discovered an inhomogeneous coverage and thickness of the CEI on the cathode surface, leading to local differences regarding resistance and CEI dissolution and deposition behavior, ultimately leading to an ongoing electrolyte consumption on freed up cathode material. [ 181 ]…”

npSCOPE: A New Multimodal Instrument for In Situ Correlative Analysis of Nanoparticles

“…27 It has been demonstrated that SIMS on the HIM can be performed with sub-20 nm lateral resolution, which gives outstanding results in that respect compared to standalone SIMS tools. 24,26−29 The application fields covered by investigations using SIMS on the HIM range from thin-film studies, 30 battery research, and optoelectronic devices characterization, 25,31 to solar cell materials 32 or steel investigations, 33 to geological and biological relevant topics. 25,34,35 A most recent review summarizes the applications of HIM−SIMS.…”

“…In this way, it is possible to perform SIMS analysis of surfaces with excellent detection sensitivity, a high dynamic range (signal variations over several orders of magnitude are detectable), and differentiating the various isotopes . It has been demonstrated that SIMS on the HIM can be performed with sub-20 nm lateral resolution, which gives outstanding results in that respect compared to standalone SIMS tools. ,− The application fields covered by investigations using SIMS on the HIM range from thin-film studies, battery research, and optoelectronic devices characterization, , to solar cell materials or steel investigations, to geological and biological relevant topics. ,, A most recent review summarizes the applications of HIM–SIMS …”