1. Schematics of (a) synthetic process and (b) lithiation-delithiation process for Si@NC

2. FESEM images of (a, b) Si@NC and (c, d) Si@PC

3. (a) TEM, (b) HRTEM, (c) HAADF-STEM images, (d) SAED pattern (rectangular area in (c)), and (e-f) corresponding EDX elemental maps of Si@NC

4. (a) XRD patterns and (b) Raman spectra of Si@NC and Si@PC, (c) nitrogen adsorption-desorption isotherm and (d) pore size distribution of Si@NC

5. (a) CV curves at a scanning rate of 0.2 mV/s and (b) charge-discharge curves for initial 3 cycles at 0.2 A/g of Si@NC, (c) rate performances and (d) cycling performances at 0.5 A/g of Si@NC and Si@PC, and (e) long cycling performances of Si@NC, NC and Si@PC at 1 A/g (black circles showing Coulombic efficiencies of Si@NC)

6. (a) Charge-discharge profiles in the range of 1~4.5 V, and (b) cycle performance at 0.4C of Si@NC//NCM622 full-cell

S1. TG-DSC curves of (a) Si@NC and (b) NC

S2. XPS spectra of Si@NC composite

S3. EIS plots of pure Si, Si@NC and Si@PC, (b) Coulombic efficiencies of Si@NC and pure Si for the first 5 cycles, (c) cycle performance of pure Si at 1 A/g

S4. Cross section and surface SEM images of Si@NC electrodes for half-cell (a) before the cycle, after (b)50 and (c) 200 cycles

Table 1. Comparison of electrochemical performance for Si/C composite materials