Electrode Flexibility Enhances Electrolyte Dynamics during Supercapacitor Charging

Supercapacitors are energy storage devices with high power density and long cycle life. Combined with spectroscopy and electrochemistry, molecular simulations and theory have allowed to characterize their charging mechanisms, and we now have an excellent understanding of the effect of properties such as nanopore size or structural disorder on the supercapacitor performances. However, the influence of electrode flexibility remains to be addressed as state-of-the-art models focused on the polarization effects of the electrode, but enforced its structural rigidity, as an approximation. Here we overcome this limitation by integrating a constant-potential molecular dynamics scheme with a state-of-the-art machine-learning potential for carbon, while controlling the applied potential. Using nanoporous sp²/sp³ carbon electrodes filled with an ionic liquid electrolyte, we compare the behavior of the rigidified and flexible frameworks; the latter allowing for local atomic relaxation, breathing modes, etc. We demonstrate that flexibility significantly enhances in-pore ionic diffusivity, thus shortening the characteristic charging time by a factor of 3 relative to the rigid analogue, while the specific capacitance remains in the experimental range (≈140 F·g^-1) for both cases. More specifically, our analyses demonstrate that flexibility accelerates co-ion expulsion, mitigates pore overcrowding, and promotes a homogeneous induced-charge profile that penetrates deeply from the very start of the applied voltage onset, explaining the improved kinetics.