In Situ Imaging Comparison of Lithium Electrodeposits By Neutron and X-Ray (Synchrotron and Laboratory) Tomography

Abstract

Most of the post-Lithium-ion (Li-ion) devices, such as Li–sulfur, Li–O2, or all-solid-state Li batteries, have in common an negative electrode made of metallic Li.1 The main failure mode of Li based batteries is the inhomogeneity of the Li electrodeposits onto the Li metal negative electrode during charge steps, leading to dendrite growth and low Coulombic efficiency.2 Tomography and radiography imaging techniques are now vastly employed for battery materials analysis thanks to the possibility to perform noninvasive in situ and operando experiments.3 X-ray tomography is a powerful tool for studying dendrites as it provides useful information about their locations, dynamics, and microstructures. However, the use of neutron tomography is scarcely reported for Li electrodeposit analysis due to the difficulty to reach sufficient image resolution to capture the deposit microstructure, that is, typically below 10 to 20 µm. Furthermore, the different interactions of X-rays and neutrons with Li, which has significantly different opacity in the two cases, make the two techniques highly complementary.4 Notably, the capacity of neutrons to discern different Li isotopes is pivotal to get insight into the composition of Li deposits, by distinguishing between Li originating from an electrode (6Li in this study) and Li originating from the Li salt electrolyte (mainly in 7Li here). Indeed, the theoretical linear neutron attenuation coefficient of 6Li is about 15 and 2,000 times larger than that of natural Li and 7Li, respectively.In this study, we report as a proof of concept, an in situ neutron tomography imaging of Li electrodeposits in a cycled Li symmetric cell (see Figure 1).5 The electrochemical cell comprises a natural Li electrode, a 6Li electrode, and a deuterated liquid electrolyte. The neutron tomography is compared with X-ray tomography images of the same electrochemical cell acquired both at an X-ray synchrotron beamline and at a laboratory X-ray tomograph. Neutron tomography is shown to be compatible with in situ analysis and capable of capturing the overall morphology of the Li deposits in good accordance with X-ray tomography analyses. References M. S. Whittingham, Lithium batteries and cathode materials. Chem. Rev. 104 (2004) 4271.Z. Li et al., A review of Li deposition in Li-ion and Li metal secondary batteries. J. Power Sources 254 (2014) 168.K. Harry et al., Detection of subsurface structures underneath dendrites formed on cycled Li metal electrodes, Nat. Mater. 13 (2014) 69.M. Strobl et al., Advances in neutron radiography and tomography. J. Phys. D: Appl. Phys. 42 (2009) 243001.L. Magnier et al., Tomography imaging of Li electrodeposits using Neutron, synchrotron X-ray, and laboratory X-ray sources: A comparison. Front. Energy Res., 9 (2021) 657712. Figure 1