Appendix A. Supporting Information

Cobalt selenide decorated carbon spheres for

excellent cycling performance of sodium ion

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Figure S1: (a) XRD, (b) TEM, (c) SEM and (d) CV analysis of the pure cobalt selenide (CoSe-

P) produced as control material without the addition of glycerol.

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Figure S2. XRD patterns of CoSe@CSs annealed at different conditions.

Figure S3. (a) XPS survey spectrum of CoSe@CSs and (b) Peak fitting analysis of C 1s.

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Figure S4. TEM images of CoSe@CSs (a, b, c, d, and e) showing the effect of varying amounts

of glycerol on sphere diameters, (f) non-spherical morphology obtained by glycerol content

higher than 25vol%.

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Figure S5. Electrochemical performance of CoSe@100CSs: (a) CV curves at the rate of 0.1 mV

S-1 between the voltage range of 0.01-3.0 V, (b) corresponding charge-discharge profiles with

500 mA g-1 current density, (c) effect of different charge-discharge voltage windows on cyclic

performance of CoSe@100CSs and (d) Charge-discharge profiles of CoSe@100CSs for first

200cycles (the elaboration of activation process) with 1000 mA g-1 current density.

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Figure S6. Electrochemical performance comparison of different CoSe@CSs composites and

CoSe-P (pure Cobalt selenide). Charge-discharge profiles of (a) CoSe@100CSs, (b)

CoSe@300CSs, (c) CoSe@500CSs, (d) CoSe-P and corresponding (e) cyclic performances at a

scan rate of 100 mA g-1 respectively. (f) long-term cyclic performance of CoSe@100CSs at a

faster scan rate of 1000 mA g-1.

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Figure S7. (a) Electrochemical Impedance Spectrometry (EIS) analysis of all three composites

(CoSe@500CSs, CoSe@300CSs and CoSe@100CSs) inset is the equivalent circuit used to fit

the EIS spectrum. (b) Rate performance of CoSe@100CSs after activation process (200 cycles)

between the voltage range of 0.5-2.8 V and (c) corresponding charge-discharge curves at

different current densities.

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Figure S8. Ex situ XRD analysis of the cells after 10 cycles.

Figure S9. The morphology of CoSe@100CSs (a & b) before activation process and (c & d)

after the activation process. (e) Ex situ XRD analysis of the cells after 10 cycles.

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Figure S10. Thermogravimetric analysis (TGA) of the composite CoSe@100CSs. Inset shows

the elemental composition obtained from EDX detector of the same sample.

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