Fig. 8

Ex situ XAFS analyses for delithiated amorphous-Si. a Stacked Fourier-transformed EXAFS profiles at Si K-edge for fully delithiated type-A electrodes at 1.5 V over 190 cycles under depth of discharge (DOD)100, 90, and 80%. Black and red dotted lines, and yellow, blue, magenta, purple, and blue solid lines correspond to the as-is electrodes, the electrodes after the amorphization of c-Si, and after the 23^rd, 65^th, 107^th, 149^th, and 191^st cycles, respectively. The profiles for the same DOD control are overlaid, whereas those under different DOD controls are stacked with a constant pitch to show the intensity changes more clearly. b Normalized integration of 2 Å Si–Si correlation peak in (a), named A_{(2Å Si–Si)}, over 190 cycles for different DOD controls. The as-is type-A electrodes and the electrodes after amorphizing pc-SiNP are shown by black and red empty circles, respectively. The dashed and solid lines indicate the asymmetric and symmetric regimes seen in the Li–Si electrochemical processes (Fig. 3h), respectively. Schematized Si–Si tetrahedral boding environments are illustrated in the chart, suggesting the regime shift from asymmetric to symmetric. c Coulombic efficiency (CE) as a function of A_{(2Å Si–Si)} over 190 cycles for different DOD controls