Germanium (Ge) has been studied as an anode active material for high energy density lithium ion batteries (LIBs) (1, 2), due to its high volumetric capacity, low operation voltage, fast bulk Li diffusion, and high electrical conductivity. Similar to other high capacity lithium alloys (Si and Sn), Ge electrode accompanies large volume change of the active material during (de)lithiation processes. The repeated volume change causes fractures, pulverizations, and delamination of the electrode. The mechanical degradation reduces the reversible capacity and shortens the cycle life of the Ge anode LIBs. Klavetter et al. introduced micron sized selenium-doped germanium (Ge_0.9Se_0.1) as an active material, and reported higher reversible capacity and longer cycle life of a Ge_0.9Se_0.1 electrode than a Ge electrode (3). They proposed that the additional superionically conductive inactive phase (Li-Se-Ge) buffered the volumetric change of the active phase (Ge) and increased the rate of Li diffusion during the cell operation and it enhanced mechanical stability of the Ge_0.9Se_0.1 electrode. Thus, it is necessary to investigate mechanical stability of the lithium alloy electrodes for high performance LIBs.

Recently, in-situ transmission X-ray microscopy (TXM) tomography was introduced to investigate 3D volume change of anode electrodes (4-6). The non-invasive X-ray imaging technique provides practical visual electrode information to understand the impact of the electrode’s microstructure change on LIB performance. In this study, a novel approach is used to investigate mechanical stability of Ge and Ge_0.9Se_0.1 electrodes by in-situ and in-operando monitoring the microstructure change. An X-ray transparent LIB cell was designed to capture the microstructure of high capacity anode electrodes with the synchrotron TXM technique at the beamline 32-ID-C of the Advanced Photon Source at the Argonne National Lab. In-operando TXM scan was implemented to monitor structural evolution of the Ge and Ge_0.9Se_0.1 electrodes under galvanostatic cell operation. Moreover, in-situ TXM tomography captured 3D microstrures of the electrodes at pristine, lithiated, and delithiated states. The obtained 2D dynamics and 3D volume changes of the Ge and Ge_0.9Se_0.1 electrodes contribute to understand mechanical stability and degradation mechanism of high capacity lithium alloy anode.

Acknowledgments: This work was supported by US National Science Foundation under Grant No. 1603847.

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