John Mortimer, Fan Xia and Junjie Niu A Novel Two-Dimensional MXene/Sn Composite As Anode In Lithium Ion Batteries With Super-High Energy Density John Mortimer, Fan Xia and Junjie Niu Email: niu@uwm.edu Department of Materials Science and Engineering, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, United States

INTRODUCTION Lithium-ion batteries (LIBs), with their high capacity and cycling stability, dominate the portable electronics market. Herein, we present a novel lithium-based battery system. The anode electrode is comprised of two-dimensional (2D) MXene and Sn hybrid nanocomposite, which was synthesized using a facile hydrothermal-assisted approach. It shows a greatly improved electrochemical property and energy density. 2

APPROACH 3

CHARACTERIZATION — X-Ray Diffraction The composition was characterized by X-ray powder diffraction (XRD). It represents the XRD patterns of pristine MXene powder and the composite of MXene/Sn, respectively. In case of MXene/Sn, most strong peaks are consistent with the XRD pattern of Sn. A uniform distribution of the Sn nanoparticles affects the detection of the MXene. 4

CHARACTERIZATION — Scanning Electron Microscopy Sn nanoparticles MXene layers SEM was used to investigate the morphology of the samples. Layered structure was found after final preparation. Abundant Sn nanoparticles were distributed on the surface and into the interlamination of the 2D MXene matrix. No agglomerated and hardened particles were found. 5

Top View Side View CHARACTERIZATION — Transmission Electron Microscopy Sn nanoparticles MXene layers TEM was used to observe the detailed morphology of the sample. Sn nanoparticles with average size of less than 10 nm were uniformly dispersed on MXenes. The TEM images verify the successful synthesis of MXene/Sn nanocomposite. 6

RESULTS — Battery Performance The assembled battery system exhibits a high specific capacity of 700 mAh/g after 100 cycles at 0.5 C and a capacity of ∼450 mAh/g after ultra-long 1000 cycles at 2 C, respectively. 7

RESULTS — Applications: LED A MXene/Sn anode and a NMC-622 cathode were wound together with a separator to form a coin cell battery. The battery has a capacity of 3 mAh, which can light up multi LEDs with different colors. 8

CONCLUSIONS Sn nanoparticles decorated highly conductive 2D MXene has been fabricated by a facile hydrothermal method. The MXene plays multiple roles in this work: acting as the matrix for providing ion-change sites, the buffer layer for accommodating the volume change during electrochemical reaction process, and the highly conductive matrix for excellent battery performance. The results demonstrate the MXene/Sn composite has a promising potential in next-generation LIBs. It opens an avenue for large-scale stacking batteries in broad applications such as potable electronic devices. 9

BIBLIOGRAPHY ACKNOWLEDGEMENT 10 Michael Ghidiu, Yury Gogotsi, et al. Conductive two-dimensional titanium carbide ‘clay’ with high volumetric capacitance. Nature, 516: 78-81 (2014). MariaR. Lukatskaya, Yury Gogotsi, et al. Cation intercalation and high volumetric capacitance of two-dimensional titanium carbide. Science, 341: 1502-1505 (2013). Jianmin Luo, Wenkui Zhang, et al. Sn4+ ions decorated highly conductive Ti3C2 MXene: promising lithium-ion anodes with enhanced volumetric capacity and cyclic performance. ACS Nano, 10: 2491-2499 (2016). ACKNOWLEDGEMENT This work was supported by the UWM Support for Undergraduate Research Fellows (SURF), Research Growth Initiative (RGI), and UWM start-up. 10