X-RAY NANOPROBE

96 ESRF

form a Solid Electrolyte Interphase (SEI) layer that can prevent extensive lithium reduction during cycling. This layer allows Li+ transport while, in principle, blocks electrons and other electrolyte species. However, it impacts the homogeneity of lithium transport and its electrodeposition [3]. Thus, besides concentration gradient in the electrolyte, obtaining a fully homogeneous plating depends on the mechanical properties of the electrolyte and of the SEI.

In this study, a new lithium metal anode was developed (surface morphology is shown in Figures 80a and 80b), constructed with lithium foil and a conductive carbon-nitrogen modified stainless steel mesh (CNSSM) substrate, and was characterised in LMBs. The stainless steel mesh (SSM) was carbonised to obtain a dense functional coating layer, then pressed onto lithium foil via mechanical pressing. In this anode, the bulk lithium stays in contact with the CNSSM, onto which a coating allows easy lithium nucleation. Thus, a fresh and ionically conductive SEI is expected to form on top of a homogeneous lithium deposit, allowing stable cycling performance and low impedance (versus SEI formed atop of a regular Li foil, covered by a Li2O oxide layer). X-ray holographic nanotomography at beamline ID16A was used

as a non-invasive characterisation technique to probe the bulk of the electrode before and after lithium plating.

Figures 80c and 80e compare virtual cross-sections reconstructed from the nanotomographic measurements for both a pristine CNSSM-Li and a CNSSM-Li after 2h lithium electrodeposition at 1 mA cm-2. The virtual cross-section allows the lithium metal and stainless steel domains to be identified through the grayscale differences and edge contrast. The bright areas in the pristine CNSSM-Li correspond to internal cracks, likely due to the manufacturing process of the CNSSM- Li composite. The stainless steel rod exhibits coaxial layers, in agreement with scanning electron microscopy (SEM) cross-sections. The external carbon-nitrogen coated layer of ~1 µm in thickness can be seen as contrast to the unmodified residual (black) stainless steel at the core of the wire. In between, the dark grey region corresponds to modified stainless steel, where the colour deepens gradually, indicating a decrease in carbon content from surface to core. After the lithium electrodeposition, the coating layer cannot be distinguished anymore, and both Figure 80e and its 3D tomographic reconstructed image (Figure 80f) suggest that

Fig. 80: Digital camera (a) and SEM (b) images of the CNSSM-Li electrode. Virtual cross-sections from X-ray holographic nanotomography on a pristine (c) and a lithium electrodeposited CNSSM-Li electrode (e) with corresponding 3D tomographic reconstructed structures ((d)

and (f), respectively). For (c) and (e), lithium: sandy brown; carbon-nitrogen-rich coating layer: green; CNSSM: grey; SSM: black. The scale bar of electron density applies to all panels. For (d) and (f), lithium: coral, CNSSM: pale blue; SSM: dim grey.