Mass Balancing of Hybrid Ion Capacitor Electrodes: A Simple and Generalised Semi-Empirical Approach

Abstract

Hybrid ion capacitor (HIC), an energy-dense device compared to the conventional supercapacitor, has won immense scientific acclaim due to its long cycle life and high energy density.1 The combination of a high-capacity faradaic electrode and a fast-charging capacitive type electrode enables HICs to surpass the bottlenecks of low energy density (5-8 Wh kg-1) supercapacitors.2 One of the key parameters to manipulate the performance of HICs is mass balancing. However, the difference in the specific capacities and rate capabilities of the electrodes make the mass balancing of a HIC difficult. A device with conventionally capacity matched electrodes, balanced at low current densities, often fail at high power applications due to rapid capacity fading of the sluggish faradaic type electrode at high current densities. In general, researchers overcome this issue by randomly fabricating the devices at different mass ratios and select the best out of them, which is often a time-consuming task. However, without any laborious trial and error experiments, it is mathematically possible to dictate the energy and power density of the devices with different mass ratios by identifying the capacity of the limiting electrode at different current densities. Building upon this understanding, in this work, we propose a method to predict well-performing mass ratios of electrodes for a full-cell by individually analysing the capacities of anode and cathode as a function of current density. Further, we exemplify the validity of our calculations by comparing our results with a sodium-ion capacitor fabricated using homemade human hair derived carbon. Our predictions based on individual electrodes with various mass ratios are in good agreement with experimental performance.Further, the devices are tested in a three-electrode design to find its safe operational window and understand the voltage swings of anode and cathode separately. The device, which is mass balanced to get maximum capacity and power, had a wide operational window and less prone to metal plating compared to others. The way of dictating full cell capacity based on current densities experienced by the individual electrodes present a simple, generalised, yet never reported method which would elicit the interests from the energy storage community.(1) Ding, J.; Hu, W.; Paek, E.; Mitlin, D. Review of Hybrid Ion Capacitors : From Aqueous to Lithium to Sodium. Chem. Rev. 2018, 118, 6457–6498. https://doi.org/10.1021/acs.chemrev.8b00116.(2) Béguin, F.; Presser, V.; Balducci, A.; Frackowiak, E. Carbons and Electrolytes for Advanced Supercapacitors. 2014, 2219–2251. https://doi.org/10.1002/adma.201304137. Figure 1