High-Throughput Combinatorial Analysis of the Spatiotemporal Dynamics of Nanoscale Lithium Metal Plating

The development of Li metal batteries requires a detailed understanding of complex nucleation and growth processes during electrodeposition. In situ techniques offer a framework to study these phenomena by visualizing structural dynamics that can inform the design of uniform plating morphologies. Herein, we combine scanning electrochemical cell microscopy (SECCM) with in situ interference reflection microscopy (IRM) for a comprehensive investigation of Li nucleation and growth on lithiophilic thin-film gold electrodes. This multimicroscopy approach enables nanoscale spatiotemporal monitoring of Li plating and stripping, along with high-throughput capabilities for screening experimental conditions. We reveal the accumulation of inactive Li nanoparticles in specific electrode regions, yet these regions remain functional in subsequent plating cycles, suggesting that growth does not preferentially occur from particle tips. Optical-electrochemical correlations enabled nanoscale mapping of Coulombic Efficiency (CE), showing that regions prone to inactive Li accumulation require more cycles to achieve higher CE. We demonstrate that electrochemical nucleation time (tnuc) is a lagging indicator of nucleation and introduce an optical method to determine tnuc at earlier stages with nanoscale resolution. Plating at higher current densities yielded smaller Li nanoparticles and increased areal density, and was not affected by heterogeneous topographical features, being potentially beneficial to achieve a more uniform plating at longer time scales. These results enhance the understanding of Li plating on lithiophilic surfaces and offer promising strategies for uniform nucleation and growth. Our multimicroscopy approach has broad applicability to study nanoscale metal plating and stripping phenomena, with relevance in the battery and electroplating fields.

Figure S1.(a) XPS survey and (b) high-resolution Au 4f spectra obtained for the thin-film Au electrodes.(c) AFM topography of a region of the thin-film Au electrode, and (d) line profiles for specific positions indicated in (c).

Figure S3 .
Figure S3.Cyclic voltammetry (2 cycles) recorded with a cut-off potential of 0 V vs Li/Li + , where lithium plating should be minimal or non-existent.These measurements were obtained with pipettes of diameter ca. 10 µm filled with 50 mM LiPF 6 in PC solution.Scan rate was 25 mV s -1 .

Figure S4 .
Figure S4.A series of frames of the IRM movie corresponding to the SECCM experiment recording 10 voltammetric cycles with a cut-off potential of -0.27 V and a scan rate of 100 mV s -1 .One frame is shown herein for each ten frames of the full movie (ca.4.6 s).Scale bar is 10 µm.The voltammetric profile is represented in Figure 2a (main manuscript).The experimental time increases from left-to-right and from top-to-bottom.

Figure S5 .
Figure S5.(a) Relationship between the coulombic efficiency (CE) and the ratio of the change in IRM intensity after stripping (I a ) and after plating (I c ).A linear relationship was found, with the regression equation given by CE = 0.0726 + 0.9988(Ia/Ic) and an R 2 value of 0.96.Data were acquired from two separate SECCM voltametric experiments (10 cycles and 5 cycles) to improve the accuracy of the linear regression.(b) Schematic demonstrating the calculation of I a and I c values from the IRM data for a specific voltammetric cycle.

Figure S6 .
Figure S6.Histograms of spatially-resolved coulombic efficiency values extracted from IRM, CE (IRM) , for the first, fifth and ninth voltammetric cycles of the SECCM voltammetric experiment.

Figure S7 .
Figure S7.Full sequence of cycles of spatially-resolved maps of coulombic efficiency extracted from IRM, CE (IRM) , for the SECCM voltammetric experiment.

Figure S8 .
Figure S8.Histogram of diameter (d) values for the lithium nanoparticles observed by SEM imaging after the SECCM voltammetric experiment.The fitting line follows a gaussian distribution with mean of 72 nm.

Figure S9 .
Figure S9.Secondary ion mass spectrometry (SIMS) Li + map of the footprint left after the SECCM voltammetric experiment with 10 cycles of plating and stripping.SIMS analysis confirmed the presence of Li across the SECCM footprint.Limitations of SIMS herein include detecting Li originating from various sources such as deposited Li metal, Li from Li x Au y alloys, and rests of electrolyte residue.The residue was not rinsed to preserve the original structure and prevent the removal of Li nanoparticles.

Figure S10 .
Figure S10.All individual galvanostatic (E-t) curves recorded for the SECCM combinatorial experimentfor each current density.63 independent measurements were recorded in total (7 repetitions for 0.22 mA cm -2 and 8 repetitions for each other current density).

Figure S11 .
Figure S11.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 0.22 mA cm -2 for 5 s.No lithium particles were found under these conditions.The crystals observed are from electrolyte residues.

Figure S12 .
Figure S12.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 0.39 mA cm -2 for 5 s, and the corresponding histogram of diameter distribution for the lithium nanoparticles detected.Lithium particles were only found in three repetitions, indicated in the images.The only repetition reaching the nucleation overpotential (E nuc ) is also indicated.

Figure S13 .
Figure S13.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 0.67 mA cm -2 for 5 s, and the corresponding histogram of diameter distribution for the lithium nanoparticles detected.

Figure S15 .
Figure S15.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 1.78 mA cm -2 for 5 s, and the corresponding histogram of diameter distribution for the lithium nanoparticles detected.

Figure S16 .Figure S17 .
Figure S16.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 2.42 mA cm -2 for 5 s, and the corresponding histogram of diameter distribution for the lithium nanoparticles detected.

Figure S18 .
Figure S18.SEM images for each SECCM footprint left after the combinatorial galvanostatic experiment at 2.92 mA cm -2 for 5 s, and the corresponding histogram of diameter distribution for the lithium nanoparticles detected.

Figure S19 .
Figure S19.Selected IRM images for each current density recorded after the combinatorial SECCM experiment.The background intensity of each row was corrected to minimize any influence from sample tilting.

Figure S22 .
Figure S22.Galvanostatic E-t curve (blue line) and IRM intensity (red line) over the time of a SECCM galvanostatic experiment at 3.82 mA cm -2 for 3 s.The inset illustrates the time period during which the potential shifted towards more negative values, while the rate of change in IRM intensity remained constant.

Figure S23 .
Figure S23.A series of frames of the IRM movie corresponding to the SECCM galvanostatic experiment at 0.20 mA cm -2 for 60 s.One frame is shown herein for each three frames of the full movie (ca.1.4 s).The colorbar on the right represents the IRM intensity.

Figure S24 .
Figure S24.SEM image of the area for the SECCM galvanostatic experiment at 0.22 mA cm -2 where a high density of nuclei is deposited.This hot spot is shown as a particularly bright area on the IRM images.

Figure S25 .
Figure S25.Galvanostatic E-t curve (brown line), IRM intensity (red line), and time derivative of the average IRM intensity (dI/dt) (blue line) over the time of SECCM galvanostatic experiments at 0.20 mA cm -2 for 30 s (a) and 0.78 mA cm -2 for 15 s (b).A local maximum in dI/dt is detected when the potential decreases towards the value where Li metal is deposited, which is termed as IRM nucleation time (t nuc (IRM)).This local maximum is attributed to a variation in the rate of change of the IRM intensity, resulting from the onset of the Li plating.

Figure S26 .
Figure S26.Galvanostatic E-t curve (solid brown line), and time derivative of the average IRM intensity (dI/dt) traces obtained from IRM analysis of selected individual LiNPs clusters shown in Figure 5c.The vertical dashed line indicates the average nucleation time from the IRM measurement, t nuc(IRM) .

Figure S27 .
Figure S27.Histogram representing the distribution of all Li nucleation times obtained from IRM analysis (t nuc(IRM) ) for an experiment carried out at 0.22 mA cm -2 .The line fits to a gaussian distribution with mean of 16.4 s.

Figure S28 .
Figure S28.Full sequence of frames of the IRM movie corresponding to the SECCM galvanostatic experiment at 0.78 mA cm -2 for 15 s.The colorbar on the right represents the IRM intensity.

Figure S29 .
Figure S29.Selected frames of the IRM movie corresponding to the SECCM galvanostatic experiment at 0.20 mA cm -2 for 60 s, after the SECCM probe has been retracted from the surface.The initial frame where probe retraction was detected served as the background frame and was subtracted from subsequent frames.This demonstrates that a droplet of LiPF 6 /PC electrolyte can undergo evaporation in a short time.

Figure S30 .
Figure S30.(a) Selected frames (5, 10, 15 s) of an IRM movie for a SECCM galvanostatic experiment at 3.65 mA cm -2 for 30 s.(b) Images representing the difference in IRM intensity between the frames at 10 and 5 s and the frames at 15 and 10 s, which clearly shows the increased Li deposition at the meniscus edge at longer experimental times compared to the initial preferential deposition at the centre.Note that the appearance of a few dark blue spots in the t(15-10 s) frame indicates the growth of very thick Li structures due to different interference phenomena. 1,2