Fig. 9
Schematics of the correlation between Li–Si electrochemical/structural characteristics and Coulombic efficiency. a Legend of six different Li–Si local environments upon (de)lithiation, which are used in (b, c). Fully delithiated amorphous (a-)Si and c-Li3.75Si are further subcategorized, according to the electrochemical and various atomistic structural probing results listed in Fig. 1. b Schematics of structural characteristics seen in two different electrochemical Li–Si reaction regimes. The dotted and solid arrowed axes at top show the asymmetric and symmetric regimes over cycle, respectively, the duration of which is significantly altered by the depth of discharge (DOD) control. The crossed empty squares embedded in the lines show the structural probing points inserted every 20 cycles. The shifts of the structural characteristics seen at these points in each regime are synchronized with the electrochemical regime shift. Firstly, the reaction flow of the Li–Si local environments also changes from asymmetric to symmetric, which is overlaid with the electrochemical processes. Secondly, the properties of Si–Si bonding and the morphology in a-Si are altered. Thirdly, the c-Li3.75Si crystal size is saturated at the shift. Finally, the electrode thickness also saturates near the shift. c Profiles of the local Li–Si environments at various potential during (de)lithiation over cycling under different DOD controls. The phases formed on lithiation and delithiation are shown on the left and right sides along blue and red arrows, respectively. The constant voltage domain at 10 mV is illustrated by black arrows. Each component in the pie charts shows a local environment present at the given potential, while their sizes indicate the relative proportion of the phases. The dotted and solid lines in cycle number and CE profiles indicate the asymmetric and the symmetric regimes, respectively. The local environments and their profiles on (de)lithiation significantly depend on the affiliated electrochemical regimes, which consequently governs CE profiles. The affiliation and CE behaviors can be prominently altered by the repeated a–c phase transformations
