Charge Me Slowly, I Am in a Hurry: Optimizing Charge–Discharge Cycles in Nanoporous Supercapacitors

Abstract

Nanoporous supercapacitors attract much attention as green energy storage devices with remarkable cyclability and high power and energy densities. However, their use in high-frequency applications is limited by relatively slow charging processes, while accelerating charging without compromising the energy storage still remains a challenging task. Here, we study in detail the charging and discharging behavior of nanoporous supercapacitors with narrow pores, which provide exceptionally high capacitances and stored energy densities. We scrutinize the dynamic modes of charging, revealing, in particular, a transient formation of crowded and dilute ionic-liquid phases inside the pores, which leads to co-ion trapping and correspondingly slow charging. We show how trapping can be circumvented by applying a slow voltage sweep, and we demonstrate that it can accelerate the overall charging process considerably if the sweep rate is chosen appropriately. While one might be tempted to apply a similar strategy to discharging, we find that the best discharge rates are obtained when the voltage is switched off in a step-like fashion, whereby the optimal charge and discharge times differ a few-fold. We unveil the scaling laws for such optimal quantities, which allow one to predict quantitatively the charging behavior for realistically long pores. On the basis of our findings, we propose an optimal charge-discharge cycle and elaborate on optimization strategies.

Superionic liquids in conducting nanoslits: A variety of phase transitions and ensuing charging behavior

“…Our findings may have important practical implications. It is known that ordered phases typically exhibit sluggish dynamics, particularly ordered (or quasi-ordered) ionic liquids in slit nanopores [20,44,45]. The knowledge of the parameter space corresponding to such phases may guide to avoid a potential slowdown of charging.…”

The effects of slit-pore geometry on capacitive properties: a molecular dynamics study

“…It's good to choose the mass value so that it would not be far from the majority of the experiments and ILs' applications. A well known IL is BMIM-PF6 that is used in many simulation and experimental studies 20,21,23,33,40,41 . The anion of this IL, PF6 (or Hexafluorophosphate), is one of the most stable anions of ILs and provide the largest EPWs when they are paired with conventional organic cations 13 .…”

Curvature affects electrolyte relaxation: Studies of spherical and cylindrical electrodes

“…Note that q s eq (r) and q c eq (r) can be derived more easily: At equilibrium, J q (r) = 0 gives q(r) = −2φ(r) thus ∂ r [r 2 ∂ r φ] = r 2 κ 2 φ [Eq. (5)]. The solution to this equation, φ s (r) = c 2 exp [−κr]/r + d 2 exp [κr]/r, contains two constants (c 2 and d 2 ), which are fixed with Eq.…”

“…Since ions forming the electric double layer (EDL) must be partially drawn from a reservoir, τ could depend on "long" length scales like the electrode separation. This makes predicting τ with molecular simulations difficult, as typical simulation domains only capture a small portion of the nanoporous electrode structure [4] or rely on simplified geometries [5].…”