**4. Methodology**

#### *4.1. Electrode Preparation*

For the Li-ion cell electrodes, LiNi0.33Mn0.33Co0.33 O2 (NCM111, MTI Corp., St. Louis, MO, USA) or LiMn2O4 (LMO, MTI Corp.), conductive carbon black (Super C65, Imerys, Paris, France), and polyvinylidenefluoride (PVDF) (Solef 5130, Solvay, Brussels, Belgium) in a 90:5:5 mass ratio were homogenized with a planetary centrifugal mixer (ARE-250, Thinky Corporation, Tokyo, Japan) using anyhdrous *N*-methyl-2pyrrolidinone (NMP, Sigma Aldrich, Saint Louis, MO, USA) as solvent. The slurry was cast on a 15 μm thick aluminium foil (MTI Corp.) using a micrometre adjustable film applicator set to a blade gap of 150 μm.

For the Li-S cell, elemental sulfur (325 mesh, Alfa Aesar, Haverhill, MA, USA), conductive carbon black (Super C65, Timcal, Bodio, Switzerland), Ketjenblack (EC600-JD, Akzo Nobel, Amsterdam, The Netherlands), and polyvinylidene fluoride binder (Solef 5130, Solvay) in a 75:12:3:10 mass ratio were homogenized with a high shear laboratory mixer (L5M, Silverson, Buckinghamshire, UK) to form an ink with 20% total solids content with anhydrous NMP as solvent. The ink was cast onto 15 μm thick aluminium foil using a micrometre adjustable film applicator set to a blade gap of 400 μm.

The electrode sheets were initially dried on a hot plate at 80 ◦C and subsequently dried overnight under vacuum at 120 ◦C (for Li-ion electrodes) or 60 ◦C (for S electrode). Additionally, 3.15 mm and 0.77 mm disks were cut from the sheets by using a laser micro-machining instrument (A Series Compact Micromachining System, Oxford Lasers Ltd., Didcot, UK).

#### *4.2. Coin Cell with Kapton Window*

CR2032-type coin cell (CR2032, MTI) were cleaned in isopropanol ( ≥99.5% purity) and dried overnight under vacuum at 60 ◦C prior to use. A 16 mm wide × 3 mm high letterbox-shaped aperture with a 6 mm diameter circular hole in the centre was drilled into the can, cap, and spacer components of the coin cells. Rectangular strips (ca. 19 mm × 5.5 mm) of 50 μm thick adhesive Kapton tape were applied on both the internal and external surfaces of the coin cell cap and can components to create an X-ray transparent window. Epoxy adhesive (Araldite) was then applied over the edges of the external Kapton strips to create a hermetic seal around both windows. The circular hole in the centre of the coin cell was designed to aid in sample alignment during tomography scans and the letterbox shaped portion of the X-ray window was designed to provide a sufficiently large angular range for tomographic acquisition while maintaining the mechanical stability of the coin cell.

This window design meant that tomographic acquisition performed with the in-situ coin cells were limited angle scans with the angular range dependent on the field of view. During coin cell assembly, the X-ray windows on the cell casings and spacer components were carefully aligned in order to avoid beam attenuation by the dense metal components, which ensures a clear 'line-of-sight' for the X-ray beam being transmitted through the electrode material. All coin cells were assembled with the LMO electrode as a working electrode, glass fiber separator soaked in LiPF6-based electrolyte and a metallic lithium counter electrode.

#### *4.3. 1/8" PFA Swagelok*

The 3.15 mm electrode disks were dried in a transferrable vacuum oven (Glass Oven B-585 Drying, Buchi, Flawil, Switzerland) at 120 ◦C (NMC electrode) or 60 ◦C (S electrode) overnight and transferred to an argon filled glovebox (MBraun, LABstar, Garching, Germany) where both O2 and H2O levels were maintained below 0.5 ppm. Customized 1/8" PFA Swagelok unions (PFA-220-6, Swagelok, Soren, OH, USA) were used as cell bodies and these were assembled using 1/8" 316L stainless steel plungers as the current collector. Excess material was removed from the centre of the PFA union to reduce the X-ray attenuation. Lithium metal punched to 1/8" was used as the counter electrode with glass fiber punched to 4 mm (GF/D, Whatman, Maidstone District, UK) as a separator. For Li-ion cells, 1.2 M lithium hexafluorophosphase (LiPF6) in ethylene carbonate and ethyl methyl carbonate

(EC:EMC, 1:2 *<sup>v</sup>*/*<sup>v</sup>*, Soulbrain, Northville Township, MI, USA) was used as an electrolyte. For Li-S cells, 1 M lithium bis(trifluoromethane) sulfonimide (LiTFSI) in 1,3-dioxolane and 1,2-dimethoxyethane (DOL:DME, 1:1 *v*/*v*) with 0.3 M lithium nitrate as an additive (Soulbrain, Northville Township, MI, USA) was used as an electrolyte.

#### *4.4. 1/32" PEEK Union*

The 0.77 mm electrode disks were dried in a transferrable vacuum oven (Glass Oven B-585 Drying, Buchi) at 120 ◦C (NMC electrode) or 60 ◦C (S electrode) overnight and transferred to an argon filled glovebox (MBraun, LABstar, Garching, Germany) where both O2 and H2O levels were maintained below 0.5 ppm. Bespoke 1/32" polyether ether ketone (PEEK) unions were used as cell bodies for the miniature tomography cells with 316L stainless steel plungers as the current collector.

Lithium metal punched to 0.8 mm was used as the counter electrode with glass fiber punched to 1 mm (GF/D, Whatman) as a separator. For Li-ion cells, 1.2 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate and ethyl methyl carbonate (EC:EMC, 1:2 *<sup>v</sup>*/*<sup>v</sup>*, Soulbrain, Northville Township, MI, USA) was used as an electrolyte. For Li-S cells, 1 M lithium bis(trifluoromethane) sulfonimide (LiTFSI) in 1,3-dioxolane and 1,2-dimethoxyethane (DOL:DME, 1:1 *v*/*v*) with 0.3 M lithium nitrate as an additive (Soulbrain, Northville Township, MI, USA) was used as an electrolyte.

#### *4.5. Synchrotron Micro-CT Acquisition and Reconstruction*

Synchrotron micro-CT was performed at the i13-2 beamline at Diamond Light Source (Harwell, UK) in the absorption contrast imaging mode. A parallel beam was used for the interior tomography of an LMO electrode sample assembled within the in-situ coin cell. The incident X-ray beam was monochromatized to 16 keV by a water-cooled double crystal Si <111> monochromator. The sample to detector distance was set to 25 mm and an average useful rotation range of 147◦ was achieved through the Kapton window. Projection images were acquired when the sample was rotated through angular steps of 0.1◦ about its long axis with a 6 s exposure time per projection. A 9.6 μm thick GGG:Eu scintillator was coupled to a 10× objective lens and projections were captured with a 2000 × 2000 pixel pco4000 CCD detector, which resulted in an effective pixel size of 0.365 μm.

#### *4.6. Laboratory Micro-CT Acquisition and Reconstruction*

X-ray micro-CT was performed on the PFA and PEEK in-situ cells with a lab-based micro-CT instrument (Zeiss Xradia Versa 520, Carl Zeiss Inc., Oberkochen, Germany). The instrument consisted of a polychromatic micro-focus sealed source set to an accelerating voltage of 80 kV on a tungsten target at a maximum power of 7 W. The scintillator was coupled to either a 20× or 40× objective lens and 2048 × 2048 pixel CCD detector with a pixel binning of 1, which results in a pixel size of ca. 460 nm and a field of view of ca. 940 μm for the 20× objective and ca. 230 nm and a field of view of ca. 470 μm for the 40× objective. There was no significant geometric magnification since the sample was set close to the detector to reduce the effects of penumbral blurring arising from the cone beam nature of the source. The sample was rotated through 360◦ with radiographs collected at discrete angular intervals amounting to a total of 1601 projections. The radiographic projections were then reconstructed with proprietary reconstruction software (Version 11.1.8043, XMReconstructor, Carl Zeiss Inc.) by using a modified Feldkamp-David-Kress (FDK) algorithm for cone beam geometry.

**Author Contributions:** C.T. and S.R.D. contributed equally in writing the manuscript and all experimental work. O.O.T provided data for the synchrotron study and coin cell design and T.M.M.H. contributed to the theoretical descriptions of X-ray C.T., D.J.L.B. and P.R.S. directed research. All authors discussed the results and contributed to the manuscript.

**Funding:** This research was funded by the EPSRC under grants EP/R020973/1, EP/N032888/1 and through the Faraday Institution, the Royal Academy of Engineering under gran<sup>t</sup> CiET1718/59, and Diamond Light Source for beamtime under MT11539.

**Acknowledgments:** The authors would like to acknowledge the EPSRC for funding under grants EP/R020973/1, EP/N032888/1, and the Faraday Institution. PRS acknowledges funding from the Royal Academy of Engineering for financial support under the Chair in Emerging Technologies scheme. The authors acknowledge Diamond Light Source for synchrotron beam time on the Diamond-Manchester Branchline (I13-2) of the I13 imaging and coherence beamline.

**Conflicts of Interest:** The authors declare no conflict of interest.
