Post Mortem and Operando XPEEM: a Surface-Sensitive Tool for Studying Single Particles in Li-Ion Battery Composite Electrodes

Abstract

X-ray photoemission electron microscopy (XPEEM), with its excellent spatial resolution, is a well-suited technique for elucidating the complex electrode–electrolyte interface reactions in Li-ion batteries. It provides element-specific contrast images that allows the study of the surface morphology and the identification of the various components of the composite electrode. It also enables the acquisition of local X-ray absorption spectra (XAS) on single particles of the electrode, such as the C and O K-edges to track the stability of carbonate-based electrolytes, F K-edge to study the electrolyte salt and binder stability, and the transition metal L-edges to gain insights into the oxidation/reduction processes of positive and negative active materials. Here we discuss the optimal measurement conditions for XPEEM studies of Li-ion battery systems, including (i) electrode preparation through mechanical pressing to reduce surface roughness for improved spatial resolution; (ii) corrections of the XAS spectra at the C K-edge to remove the carbon signal contribution originating from the X-ray optics; and (iii) procedures for minimizing the effect of beam damage. Examples from our recent work are provided to demonstrate the strength of XPEEM to solve challenging interface reaction mechanisms via post mortem measurements. Finally, we present a first XPEEM cell dedicated to operando/in situ experiments in all-solid-state batteries. Representative measurements were carried out on a graphite electrode cycled with LiI-incorporated sulfide-based electrolyte. This measurement demonstrates the strong competitive reactions between the lithiated graphite surface and the Li2O formation caused by the reaction of the intercalated lithium with the residual oxygen in the vacuum chamber. Moreover, we show the versatility of the operando XPEEM cell to investigate other active materials, for example, Li4Ti5O12.

Instability of PVDF Binder in the LiFePO4versus Li4Ti5O12 Li‐Ion Battery Cell

“…Composite electrodes are prepared by casting a mixture of active material Li 4 Ti 5 O 12 (LTO, Clariant ) and LiFePO 4 (LFP, Clariant ), respectively, polyvinylidene fluoride binder ( PVDF Kynar Flex , Solvay ), and Super‐C carbon ( Imerys ), in a ratio of 80 : 10 : 10, all suspended in N ‐methyl‐2‐pyrrolidone (NMP, Fluka ) and then spreading onto aluminum foil serving as a current collector. After drying the cast slurry at 120 °C under dynamic vacuum overnight, circular 13 mm diameter electrodes are punched out and pressed under two tons in order to reduce the surface roughness and porosity, which also helped to improve the quality of the X‐ray photoemission electron microscopy (XPEEM) images [12] . Afterwards, both positive and negative electrodes are reheated at 120 °C in a dynamic vacuum chamber connected to an Ar‐filled glovebox in order to remove any remaining water in the electrode.…”

“…An Ar‐filled transfer chamber is used to transfer the samples to the X‐ray photoelectron spectroscopy (XPS) chamber and to the synchrotron SIM beamline for XPEEM measurements, respectively, to avoid air exposure and surface modification. This procedure is well known to neither affect nor damage the surface layer of organic and inorganic species resulting from the electrolyte reduction or oxidation [12] …”

“…All spectra are normalized by the incoming flux measured from a gold mesh or from the signal from a focusing mirror, except for the C K‐edge where the normalization is done on a signal measured on gold nanoparticles. For these spectra, details on the normalization are described in a separate publication [12] …”

“…The absence of a fluorine containing salt ( e. g ., LiPF 6 ) helps ascribing the presence of LiF and C x F y compounds exclusively to the PVDF origin, being the only source of fluorine in the system. Furthermore, on the characterization level, the use of local, element‐specific spectroscopic tools such as the synchrotron based X‐ray photoemission electron microscopy (XPEEM) has been proven to be an indispensable technique to track the chemical evolution of single components across the complex composite‐electrode surface, without signal overlap between the PVDF binder, conductive carbon additives, and active materials [12,13] . Specifically, the PVDF can be examined with XPEEM through the change in local absorption at the F K‐edge [9] .…”

“…Furthermore, on the characterization level, the use of local, element-specific spectroscopic tools such as the synchrotron based X-ray photoemission electron microscopy (XPEEM) has been proven to be an indispensable technique to track the chemical evolu-tion of single components across the complex composite-electrode surface, without signal overlap between the PVDF binder, conductive carbon additives, and active materials. [12,13] Specifically, the PVDF can be examined with XPEEM through the change in local absorption at the F K-edge. [9] Thanks to the excellent lateral resolution of the XPEEM technique and the background knowledge acquired on the electrolyteelectrode interface reaction mechanism in LFP//LTO cells, we are able to identify other interfacial processes and especially to track the stability of the PVDF binder.…”