Atomic-Scale Mechanisms of Sliding along an Interdiffused Li–Si–Cu Interface

We perform ab initio calculations on the shear deformation response of the interdiffused Li–Si–Cu phase structure existing between a lithiated Si electrode and a Cu current collector. We show that the formation of well-delineated and weakly bonded Si–Cu and Li–Cu crystalline atomic layers within this phase structure facilitates interface sliding. However, sliding can be terminated by the formation of LiSi₃ compounds across these atomic layers, which causes the abrupt capacity fade of the electrode after repeated cycling

Two-Phase Electrochemical Lithiation in Amorphous Silicon

Lithium-ion batteries have revolutionized portable electronics and will be a key to electrifying transport vehicles and delivering renewable electricity. Amorphous silicon (<i>a</i>-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 <i>a</i>-Si by using <i>in situ</i> transmission electron microscopy. The lithiation occurs by the movement of a sharp phase boundary between the <i>a</i>-Si reactant and an amorphous Li_<i>x</i>Si (<i>a</i>-Li_<i>x</i>Si, <i>x</i> ∼ 2.5) product. Such a striking amorphous–amorphous interface exists until the remaining <i>a</i>-Si is consumed. Then a second step of lithiation sets in without a visible interface, resulting in the final product of <i>a</i>-Li_<i>x</i>Si (<i>x</i> ∼ 3.75). We show that the two-phase lithiation can be the fundamental mechanism underpinning the anomalous morphological change of microfabricated <i>a</i>-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