S1. XPS survey spectra of GeP₃/KB

1. Characterization of synthetic GeP₃/KB(a) Schematic diagram of GeP₃/KB synthesis; (b) XRD patterns of GeP₃ and GeP₃/KB composites; (c) Raman spectra of GeP₃ and GeP₃/KB composites

2. High resolution XPS spectra of GeP₃/KB nanocomposites(a) C1s XPS spectrum of GeP₃/KB-600-40; (b-d) P2p XPS spectra of (b) GeP₃/KB-600-40, (c) GeP₃/KB-300-40 and (d) GeP₃/KB-500-40. Colorful figures are available on website

S2. CV curves with calculated capacitive contribution of GeP₃/KB-600-40Scan rates of (a) 0.2, (b) 0.4, (c) 0.6 (d) 0.8 and (e) 1.0 mV·s^-1

S3. (a) Relationship between peak current and scan rate in logarithmic format and (b) capacitance contribution at different scan rates of GeP₃/KB-600-40

3. SEM images and EDS mappings of (a-c) GeP₃ and (d-i) GeP₃/KB-600-40(a) SEM image of GeP₃; (b, c) EDS mappings of P and Ge in GeP₃; (d) SEM image of GeP₃/KB-600-40; (e-i) EDS mappings of C, O, P and Ge in GeP₃/KB-600-40

4. (a, b) TEM images and (c, d) SAED patterns of GeP₃/KB-600-40

S4. SEM images of (a, b) bare GeP₃ and (c, d) GeP₃/KB-600-40 (a, c) before and (b, d) after 100 cycles

5. Electrochemical performance of GeP₃ and GeP₃/KB composites(a) Initial and (b) second charge-discharge curves of bare GeP₃ and GeP₃/KB electrodes; (c) Rate performance of bare GeP₃ and GeP₃/KB electrodes at 0.05, 0.1, 0.5, 1 and 2 A·g^-1; (d) Cycling performance of all electrodes at 2 A·g^-1Colorful figures are available on website

6. CV curves of (a) GeP₃ and (b) GeP₃/KB-600-40, and (c) Nyquist plots of all electrodes with inset showing equivalent circuitColorful figures are available on website