Scripta Materialia 188 (2020) 164–168 Contents lists available at ScienceDirect Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat Granular phase transformation of polycrystalline aluminum during electrochemical lithiation Tianye Zheng a , Xiaogang Wang b , Ekta Jain b , Dominik Kramer d,e , Reiner Mӧnig d,e , Matteo Seita b,c , Steven T. Boles a,∗ a Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore c School of Materials Science and Engineering, Nanyang Technological University, 639798, Singapore d Institute for Applied Materials, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen 76344, Germany e Helmholtz Institute Ulm for Electrochemical Energy Storage (HIU), Ulm 89069, Germany a r t i c l e i n f o Article history: Received 5 June 2020 Revised 7 July 2020 Accepted 15 July 2020 Keywords: Phase transformation Operando light microscopy EBSD Mechanical strain Lithium-ion batteries a b s t r a c t Lithium storage in aluminum stems from a phase transformation from lithium-poor α phase (Al, face- centered cubic) to lithium-rich β phase (LiAl, cubic) at room temperature. We investigate the electro- chemical lithiation of polycrystalline Al using operando light microscopy coupled with electron backscat- ter diffraction. The operando videos reveal that the β phase patches expand radially from discrete nu- clei with inhibition observed for specific Al grains at the α/β interface. Statistical analyses indicate that these grains have a preferred out-of-plane <111> orientation. This study suggests that lithiation of Al is a whole-grain process that is influenced by grain textures rather than grain boundaries. © 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Alloy anodes for lithium-ion batteries (LIBs) have been exten- sively investigated over the past few decades due to the increasing demand for mobile energy storage. Among all candidates, Al-based anode materials remain poorly understood compared to C-, Si-, and Sn- counterparts [1–9], even though Al as an element possesses in- trinsic properties that fit the requirements for new anode material in LIBs, such as low cost and relatively high specific capacity which is ~993 mAh g −1 for the formation of the β phase (LiAl) at room temperature [10–12]. Furthermore, it also holds potential of signif- icantly simplifying LIB manufacturing processes since Al foil is al- ready used as cathodic current collector and it can theoretically replace the composite (i.e. slurry-based) electrode that is currently used as the commercial anode. The solid-state first-order phase transformation from α phase (Al structure, fcc) to the β phase (LiAl, NaTl structure, cubic) at room temperature is the origin of lithium storage during the elec- trochemical lithiation of crystalline Al. This process involves break- ing of Al-Al bonds and an atomic rearrangement with the for- mation of Li-Al bonds. Previous studies suggest that the kinetics of this phase transition is limited by the reaction at the phase front, and that the β phase growth is quasi-isotropic (but no- ∗ Corresponding author. E-mail address: steven.t.boles@polyu.edu.hk (S.T. Boles). tably anisotropic at granular level) [13]. This observation infers that there might be (un-)favored crystalline orientations and/or grain boundary (i.e. misorientations) for the α/β interphase to propa- gate. This preference during a lithiation-driven phase transforma- tion also exists in other alloy anode materials. For instance, Lee at al. tried to lithiate single crystalline Si nanopillars with various grain orientations, and found that lithium diffusion along <110> direction is preferred during the electrochemical lithiation [14]. A similar outcome was also reported by Goldman et al. [15]. How- ever, to the best of the authors’ knowledge, no existing information is available regarding anisotropy during the electrochemical lithia- tion of Al. It should be noted that Si single crystals are more com- mercially approachable due to the well-developed semiconductor industry. For Al, on the contrary, almost all commercially avail- able products are polycrystalline. It is, therefore, more relevant to study polycrystalline Al, such that the knowledge can be directly extended to the effort of commercializing LIBs with Al-based an- odes. Compared to single crystalline materials, the influence of grain boundaries and texture may be also expected. In this work, we use the electron backscatter diffraction (EBSD) technique to assess the crystallographic texture and grain bound- ary character of polycrystalline Al thin films prepared by physi- cal vapor deposition (PVD) sputtering. The phase transformation can be tracked during the entire electrochemical lithiation process https://doi.org/10.1016/j.scriptamat.2020.07.029 1359-6462/© 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.