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Methods.

Transmission electron microscopy. The TEM study was performed using a JOEL-2100 S/TEM microscope operated at 200 kV. The FFT and IFFT images were carried out using Gatan DigitalMicrograph software.
Nano-resolution x-ray spectro-microscopy. We conducted x-ray spectro-microscopic scan of the charged Li0.5Ni0.6Mn0.2Co0.2O2 particles using the transmission x-ray microscopy (TXM) at beamline 6-2C of Stanford Synchrotron Radiation Lightsource of the SLAC National Accelerator Laboratory. The powder sample was loaded into a quartz capillary (100 microns in diameter and 10 microns in wall thickness) for imaging under the inert gas environment (slow and steady helium flow). The typical exposure time for single images is 0.5 second. The nominal spatial resolution of this instrument is ~30 nm. More details of the synchrotron beamline configuration and the concept of x-ray spectro-microscopy and spectro-tomography can be found elsewhere\cite{Meirer_2011},\cite{Liu_2011}. In the 3D spectro-microscopic scan, the energy of the incident x-rays is scanned from 8200 eV to 8630 eV to cover the absorption K-edges of Ni, in which the tomography was performed at 68 different energy points. In the near edge region (8330 eV to 8355 eV), we chose the energy step at 1 eV to ensure sufficient energy resolution. The pre-edge and post-edge regions were scanned with larger energy steps of 10 eV to cover a relatively wide energy window for normalization of the spectra. The TXM data processing was performed using an in-house developed software package known as TXM-Wizard\cite{Liu_2012}. The segmentation and visualization of the 3D data were carried out using a commercial software package known as Avizo.
Soft x-ray absorption spectroscopy (XAS). The soft XAS measurements were carried out at the elliptically polarizing undulator (EPU) beamline 13-3 of the Stanford Synchrotron Radiation Lightsource (SSRL). The charged NMC622 electrodes were mounted in a ultra-high vacuum (UHV) chamber for the measurement. For comparison of the top and bottom of the electrode, the aluminum current collector was carefully peeled off and two pieces of the electrode were mounted facing up and down, respectively. The samples were handled in a Argon-filled glove box to minimize the air exposure. The vertically polarized x-ray (sigma-polarization) was used. The incident beam was monochromatized by a 1100-lines/mm spherical grating monochromator (SGM), and its angle was set at 30 degrees from the sample surface. Both fluorescence yield (FY) and total electron yield (TEY) signals were acquired simultaneously to probe the depth-dependent spectroscopic fingerprints. All the XAS spectra were normalized by the intensity of the incoming x-ray beam that was concurrently measured as a drain current on an electrically isolated gold-coated mesh. A linear background, which was determined by the intensity of the pre-edge region, was subtracted from the data.
X-ray phase contrast tomography. For the morphological study of the micro-scale structures of the Li battery cathode, synchrotron hard x-ray tomography based on phase contrast with high spatial resolution becomes our method of choice. Holotomography measurements of the samples were conducted at the ID16A-NI nano-imaging beamline\cite{Cesar_da_Silva_2017} of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. This beamline offers a unique combination of nanofocus (~20nm) and a very high photon flux (up to 1012 photons/s at ΔE/E~1%). Two pairs of multilayer coated Kirkpatrick-Baez (KB) optics are used to focus the x-rays at 17 keV and 33.6 keV respectively. This experiment were performed at 17 keV. Besides the benefits of the ability to perform nano-tomography with a high energy, the magnifying geometry of the cone beam also allows large field of view (FOV) with 100 nm and 70 nm voxel size. Due to free space propagation of the x-ray beam, the contrast in the images is dominated by phase contrast, related to the real part of the complex refractive index, which is determined by the electron density of the material. By measuring the Fresnel diffraction patterns at different effective propagation distances, the phase maps of the sample can be retrieved via holographic reconstruction, the so called phase retrieval procedure\cite{Cloetens_1999} implemented using GNU Octave software. In our measurements, the sample was placed downstream of the KB focus and magnified radiographs were recorded onto an x-ray detector using a FReLoN charged-coupled device (CCD) with a 2048x2048 binned pixels array. For every tomography scan, 1500 projections were acquired with 0.2s exposure time at 100 nm, or 70nm pixel size. Tomographies at four different focus to sample distances were acquired to complete one holotomography scan, which were subsequently used for phase retrieval. The 2D phase maps retrieved from the angular projections were then used as input for a tomographic reconstruction based on the filtered back projection (FBP) algorithm method (ESRF PyHST software package)\cite{Mirone_2014}. The reconstructed 3D volumes are proportional to the changes in electron density of the sample.
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