NSRRC Activity Report 2023

Energy Science 067 Transmission X-ray Microscope: A Powerful Tool for the Structural Analysis of Energy Materials Understanding the electrochemical performance of energy materials through micro-structural analysis. S tructural changes in energy materials during charging and discharging significantly affect their coulombic efficiency, cycle life, and overall stability. The synchrotron-based transmission X-ray microscope (TXM) employs a high-energy hard X-ray source, providing spatial resolutions down to 60 nm. Its penetrating capability allows for the observation of sub- micron structural changes of the material within a battery’s complex layers, including those containing electrolytes. TXM’s real-time and synchronized monitoring capabilities grant comprehensive insights into how different materials, electrolytes, and additives influence battery performance. This makes TXM an indispensable instrument in the field of energy materials research. The research group led by Bing Joe Hwang from the National Taiwan University of Science and Technology recently employed a TXM at the beamline TLS 01B1 to investigate the deposition patterns of lithium metal on electrodes during charge– discharge cycles of anode-free lithium-sulfur batteries (AFLSBs). Their study uncovered the fundamental origins of superior electrochemical performance behind these batteries. The efficacy of lithium plating and stripping processes is vital for the performance of anode-free batteries. Notably, redox- active polysulfides (PS) have crucial effects on the development of lithium plating morphologies. In operando TXM, used during live charge and discharge cycles of Li 2 S||Cu and LiFePO 4 ||Cu anode-free batteries, captures the evolving states of lithium plating. 1 Figure 1 features TXM images showcasing the plating morphologies within the Li 2 S||Cu and LiFePO 4 ||Cu cells. Initially, the contrast between copper and electrolyte is evident in the TXM images of both batteries ( Figs. 1(a) and 1(f) ). After the first plating in the Li 2 S||Cu AFLSB cell, a dense lithium layer emerges, as seen in Fig. 1(b) , which is indicative of the PS’s role in stabilizing lithium deposition. Conversely, the lithium in the LiFePO 4 ||Cu cell displays a mossy morphology ( Fig. 1(g) ). Upon the second plating cycle, a compact layer of lithium forms closer to the copper interface in the Li 2 S||Cu AFLSB cell (depicted in yellow in Fig. 1(d) ), a phenomenon not seen in the LiFePO 4 ||Cu cell. These findings affirm the role of PS species in regulating the morphology of lithium plating in all-solid-state lithium-sulfur batteries. The in operando TXM observations highlight the importance of PS species in the stable deposition of lithium. Fig. 1 : In operando TXM lithium morphologies after charge (plating) and discharge (stripping) steps in Li 2 S||Cu (a−e) and LiFePO 4 ||Cu (f−j) at 1.5 mA cm −2 . [Reproduced from Ref. 1] Fig. 2 : Schematic illustration of the working mechanism of electrolyte without and with SnBr 2 . [Reproduced from Ref. 2]