The flexible and wearable electronics market is expected to increase from €14.5 billion (2016) to around €70 billion by 2023 and to €137 billion by 2027, making developing flexible energy storage essential to market expansion. Currently, flexible energy
storage research is based around battery or supercapacitor designs using two different approaches, flexible electrodes or miniaturization. The problem is that batteries trade off power dentistry for energy density and vice versa
for supercapacitors. For instance, commercial lithium batteries have high energy density, 220 Wh/kg, but low power density <350 W/kg and cyclability (<2000 cycles), while supercapacitors have low energy density ~5 Wh/kg, but high-power density 10 000 W/kg and cyclability (1 000 000+ cycles). This leaves an energy/power gap between batteries and supercapacitors, with the ultimate goal for energy storage research to close the gap and produce a device that has high cyclability, power and energy
density. A device that could achieve this would mean that smart phones could be charged in seconds and last for days.
This project aims to develop a sustainable, environmentally friendly, flexible, Hybrid Sodium Ion Capacitor to support the rapid development of flexible/wearable electronics and sodium ion energy storage technology. This will be achieved by
integrating a flexible hard carbon battery-style anode (higher energy density) and a flexible porous carbon supercapacitor style cathode (higher power density), electrospun from renewable lignin, into one energy storage device. As a result, this
device will deliver higher energy density than flexible supercapacitors, while maintaining high cyclability and power density.
The choice of sodium in this project over lithium is for the following reasons: (i) Na is more abundant and more evenly globally distributed on land (salt, sodium carbonate, and sodium hydroxide) and in salt water, (ii) Al current collectors can be
used, instead of Cu in Li, representing a considerable cost saving, and (iii) metal plating of the carbon electrodes occurs less in Na, suggesting that cyclability will be greater with Na than Li.
The overall objective is to develop the flexible carbon electrodes for this hybrid device, which can also find applications in batteries and supercapacitors.
With this in mind, the project found early on that carbon fibers produced from organosolv and Kraft lignin have poor Na-ion battery performance making them unsuitable for the hybrid ion capacitor. However, combining the lignin with naturally abundant tannins almost doubled its performance. This phenomenon was investigated, finding that the tannin created a different internal porous structure within the fibers that allowed Na-ion to be more efficiently stored within the material.
At this stage the thought was to electrospin pure tannin, but this was not possible as the properties of tannin did not allow it to be effectively spun into fibers. Thus, the combined approach of intermixing tannin with lignin prior to spinning presented the best of both worlds, a strong, conductive, carbon fiber with improved Na-ion storage.
