Morphological Heterogeneities of Stripped and Plated Lithium Metal Analyzed By X-Ray Tomography

Abstract

Lithium (Li) metal is the most promising anode battery material to be implemented in stationary and electric vehicle applications [1]. For years, its use and subsequent industrialization was hampered because of the inhomogeneous Li electrodeposits onto Li metal leading to dendrite growth [2]. The use of solid polymer electrolyte is a solution to mitigate dendrite growth but the Li redox processes, stripping and plating, remain heterogeneous. Blue Solutions compagny is already commercializing lithium metal batteries using a solid polymer electrolyte to prevent dendrite growth. A precise characterization of the behaviour of these heterogeneities during cycling is essential to move towards an optimized anode. During our research, we have developed a characterization method based on X-ray tomography applied to model Li symmetric cells toquantify and spatially probe the Li stripping/plating process. Ante- and post-mortem cells are recut in order to allow a 1µm voxel sizein a conventional laboratory scanner. The reconstructed cell volume is post-processed to numerically re-flatten the Li electrodes, allowingus a subsequent precise measurement of the electrode and electrolyte thicknesses and revealing local interface modifications. This in-depthanalysis brings information on the location of heterogeneities and the impact on the electrode microstructure both at the electrode grains andgrain boundaries. This simple methodology permits to finely retrieve and then surface-map the local current density at both electrodes basedon the local thickness change during the redox process (see Figure 1a : 2D map of the cathode local current density variations overlaped withgrain boundaries pattern (image analysis of a cell after moving 22µm of lithium)). Thanks to quantitative data that can be extracted from the2D maps (see Figure 1b: anode and cathode quantitative distribution of local current density variations), we show that the plating process(reduction) induces more pronounced heterogeneous deposits compared to the stripping (oxidation) one. The existence of cross-talkingbetween the electrodes is also highlighted. [1] Wu Xu et al., Energy Environ. Sci. 7, no 2 (2014): 513-37[2] Xin-Bing Cheng et al., Chemical Reviews 117, n°15 (2017): 10403-73 Figure 1