title: Asynchronous Crystal Cell Expansion during Lithiation of K(+)-Stabilized α-MnO2.
abstract: α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO2's tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li(+) occupancy. The sequential Li(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.
authors: †Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.|‡Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors: Yifei
authors: Y
authors: Yuan
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.
authors: Anmin
authors: A
authors: Nie
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.
authors: Gregory M
authors: GM
authors: Odegard
authors: ‡Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors: Rui
authors: R
authors: Xu
authors: ‡Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors: Dehua
authors: D
authors: Zhou
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.|∥Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, 500 West First Street, Arlington, Texas 76019, United States.
authors: Sunand
authors: S
authors: Santhanagopalan
authors: †Department of Materials Science and Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.|⊥Department of Materials Science and Engineering, Shandong University, 17923 Jingshi Road, Jinan 250061, China.
authors: Kun
authors: K
authors: He
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.
authors: Hasti
authors: H
authors: Asayesh-Ardakani
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.|∥Department of Mechanical and Aerospace Engineering, University of Texas at Arlington, 500 West First Street, Arlington, Texas 76019, United States.
authors: Dennis Desheng
authors: DD
authors: Meng
authors: ∇Department of Physics, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States.
authors: Robert F
authors: RF
authors: Klie
authors: ‡Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors: Christopher
authors: C
authors: Johnson
authors: ‡Chemical Science and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States.
authors: Jun
authors: J
authors: Lu
authors: §Department of Mechanical Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, United States.
authors: Reza
authors: R
authors: Shahbazian-Yassar
keywords: Li-ion batteries; MnO2; in-situ TEM; nanowires; tunneled structure