title: Two-phase electrochemical lithiation in amorphous silicon.
abstract: Lithium-ion batteries have revolutionized portable electronics and will be a key to electrifying transport vehicles and delivering renewable electricity. Amorphous silicon (a-Si) is being intensively studied as a high-capacity anode material for next-generation lithium-ion batteries. Its lithiation has been widely thought to occur through a single-phase mechanism with gentle Li profiles, thus offering a significant potential for mitigating pulverization and capacity fade. Here, we discover a surprising two-phase process of electrochemical lithiation in a-Si by using in situ transmission electron microscopy. The lithiation occurs by the movement of a sharp phase boundary between the a-Si reactant and an amorphous Li(x)Si (a-Li(x)Si, x ~ 2.5) product. Such a striking amorphous-amorphous interface exists until the remaining a-Si is consumed. Then a second step of lithiation sets in without a visible interface, resulting in the final product of a-Li(x)Si (x ~ 3.75). We show that the two-phase lithiation can be the fundamental mechanism underpinning the anomalous morphological change of microfabricated a-Si electrodes, i.e., from a disk shape to a dome shape. Our results represent a significant step toward the understanding of the electrochemically driven reaction and degradation in amorphous materials, which is critical to the development of microstructurally stable electrodes for high-performance lithium-ion batteries.
authors: Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA.
authors: Jiang Wei
authors: JW
authors: Wang
authors: Yu
authors: Y
authors: He
authors: Feifei
authors: F
authors: Fan
authors: Xiao Hua
authors: XH
authors: Liu
authors: Shuman
authors: S
authors: Xia
authors: Yang
authors: Y
authors: Liu
authors: C Thomas
authors: CT
authors: Harris
authors: Hong
authors: H
authors: Li
authors: Jian Yu
authors: JY
authors: Huang
authors: Scott X
authors: SX
authors: Mao
authors: Ting
authors: T
authors: Zhu