Tuning electrochemical performance of carbon-sphere-based supercapacitors by compressive stress

The progress in flexible/stretchable electronics has increased the demand to develop highly reliable and efficient devices and systems for energy storage, which likely experience large mechanical stresses/deformation. In this work, we systematically investigate the effects of compressive stress on the electrochemical performance of symmetrical supercapacitor cells with xylose-derived activated-carbon spheres as electrode materials under different current densities. The electrolytes are aqueous solutions with different Na₂SO₄ concentrations; the compressive stress is in a range of 2.55 to 40.75 MPa. Increasing the compressive stress from 2.55 to 40.75 MPa leads to the increase of the specific gravimetric capacitance from 123.6 to 238.1 F g⁻¹ under a current density of 1 A g⁻¹ and the decrease of IR drop from 0.18 to 0.04 V. A power-law relationship between the specific gravimetric capacitance and the compressive stress is derived under the framework of mechanical deformation. This relationship is qualitatively in accord with the experimental results. There exists stress-assisted diffusion of ions in the activated carbon spheres during electrochemical cycling, and the nominal diffusion coefficient of ions in the activated carbon spheres is an exponential function of the compressive stress. The results reveal that increasing the compaction of activated carbon can increase the charge storage in supercapacitors.