Rapid Mapping of Lithiation Dynamics in Transition Metal Oxide Particles with Operando X-ray Absorption Spectroscopy

Since the commercialization of lithium ion batteries (LIBs), layered transition metal oxides (LiMO2, where M = Co, Mn, Ni, or mixtures thereof) have been materials of choice for LIB cathodes. During cycling, the transition metals change their oxidation states, an effect that can be tracked by detecting energy shifts in the X-ray absorption near edge structure (XANES) spectrum. X-ray absorption spectroscopy (XAS) can therefore be used to visualize and quantify lithiation kinetics in transition metal oxide cathodes; however, in-situ measurements are often constrained by temporal resolution and X-ray dose, necessitating compromises in the electrochemistry cycling conditions used or the materials examined. We report a combined approach to reduce measurement time and X-ray exposure for operando XAS studies of lithium ion batteries. A highly discretized energy resolution coupled with advanced post-processing enables rapid yet reliable identification of the oxidation state. A full-field microscopy setup provides sub-particle resolution over a large area of battery electrode, enabling the oxidation state within many transition metal oxide particles to be tracked simultaneously. Here, we apply this approach to gain insights into the lithiation kinetics of a commercial, mixed-metal oxide cathode material, nickel cobalt aluminium oxide (NCA), during (dis)charge and its degradation during overcharge.


Contents Section A. Electrode Preparation
Section B. Calculating X-ray Dosage

A. Electrode Preparation
Electrodes are fabricated by doctor-blading a slurry with 50wt% NCA, 20wt% carbon black, and 30wt% PVDF to a wet thickness of 100 µm on a 10-µm aluminum foil. The slurry is dried overnight under vacuum at 80 °C.

B. Calculating X-ray Dosage
The dose rate D is dependent on the flux and energy absorbed 15 : where N is the number of incident photons per second (here 3x10 12 ph s -1 ), A is the examined area (here 0.5625 mm 2 , µ is the linear attenuation coefficient taken from NIST (here, 6.56 cm -1 for DMC and 9.18 cm -1 for PVDF), ρ is the density of the material (here, 1.78g cm -3 for PVDF and 1 g cm -3 LP30 electrolyte), and E is the energy. For an energy of 8.46 keV, this gives about 560 Gray for PVDF and 710 Gray for DMC per image. The absorbed dose for 20 µm PVDF is 18 Gray and for 70 µm of DMC 33 Gray. In comparison, reducing the beam to 1µm 2 results in a local absorbed dose in the range of 10 7 Gray, which means only one scan is possible. Figure S1 shows an image of a particle taken during exposure to a focused beam. We note that this degradation does not immediately show up on most electrochemical measurements because the degradation due to a focused beam occurs only in a very small volume (~50 µm 3 ) and electrochemical performance is measured over the entire electrode (~50 µm x 2 cm x 2 cm).
Degrading the ~1/1000000000th of the electrode volume will not effect the overall electrochemistry, but it will effect the electrochemistry locally at the particular particle being imaged. This means that what is being visualized is not representative of the overall behavior of the electrode.
Figure S1 Images of a battery particle taken by a focused beam with 1 µm beam size. After 5 scans beam damage is clearly visible in transmission (not in fluorescence) in the system. Slight differences already develop during the 3 rd scan. We therefore assume that the system is influenced by this measurement technique after 2 scans and observed this for several samples.

C. NCA Characterization
The NCA particles are obtained from an industrial partner and contain LiNi 0.8 Co 0.15 Al 0.05 O 2 .
Scanning electron microscopy (SEM) images are taken using a Hitachi S4800 and SEM-focused ion beam (FIB) tomography is performed with FEI Company Helios NanoLab 450s. The particle size analysis is done using a Sympatec Helios particle size analyzer based on volume frequency distribution. Electrochemical characterization is carried out on electrodes of the same batch as the ones used in the experiments at the beamline.

Section D. XAS Experiment
This section provides details of the XAS experimental setup and data processing. A photograph and schematic of the beam line is also shown in Figure S4. The horizontally deflecting collimating torroidal mirror is placed ~18m from the source. Cryo-cooled, vertically deflecting, fixed-exit double crystal monochromator equipped with Si(111) crystals is located at ~24 m from the source. Two two-dimensional collimation guard slit systems are located at 31 and 37 m respectively (or 1 and 7m upstream the sample). The scintillator is located 0.03 m downstream the sample plane.

Figure S4
Sample and beamline setup. Left: Sample holder for pouch cell, which is placed between the spring and the rubber plate to guarantee a uniform pressure distribution. All metallic parts are anodized to avoid short-circuiting of the sample. Right: Photograph of the beamline setup (anodized sample holder is red). Top and side-view schematics of the beamline. NCA particles in a pouch cell. All spectra are collected in fluorescence mode. The reference powders are applied to Scotch® Tape, which is then folded over. Reference powders do not have pure oxidation states (e.g. the "NiO 2 " is actually a mixture of NiO 2 and Ni 2 O 3 and therefore has an average XANES of Ni 3.5+ ) so that they cannot be taken as absolute benchmarks for the oxidation state of nickel in NCA. However, we observe the correct trends. The lithiated NCA (discharged) state shows XANES similar to NiO and LiNiO 2 , while delithiated NCA (charged) has an edge higher than that of the NiO 2 , indicative of an oxidation state closer to 4+.