Flexible Hybrid Sodium Ion Capacitors from Renewable Electrospun Lignin: Device Manufacture, Performance Testing and Solid Electrolyte Layer Investigations

Project Information

HybridFlex

Grant agreement ID: 888124

DOI

10.3030/888124

Project closed

EC signature date 20 April 2020

Start date 1 October 2021

End date 22 November 2023

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 224 933,76

EU contribution

€ 224 933,76

Coordinated by

IMPERIAL COLLEGE OF SCIENCE TECHNOLOGY AND MEDICINE
United Kingdom

Periodic Reporting for period 1 - HybridFlex (Flexible Hybrid Sodium Ion Capacitors from Renewable Electrospun Lignin: Device Manufacture, Performance Testing and Solid Electrolyte Layer Investigations)

Reporting period: 2021-10-01 to 2023-09-30

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The project found within the first few months that pure lignin fibers were not suitable candidates for Na-ion storage. Co-spinning with other biomaterials was investigated as a solution to increase the Na-ion storage performance of the lignin carbon fibers. Cellulose Nano Crystals (CNCs) were investigated first and were successfully incorporated into the fibers. The results of this study was presented at the international conference Carbon 2022. Unfortunately, the CNCs did not significantly improve the Na-ion storage performance over the pure lignin fibers.

Tannin was another biomaterial that showed significant performance in Na-ion storage and was investigated as another way to improve the Na-ion performance of the lignin fibers. Different concentrations of tannin were incorporated into the lignin carbon fibers and they were carbonized at a range of different temperatures to investigate the impact of tannin. At 15% tannin incorporation and carbonization at 800C, the Na-ion storage performance of the carbon fibers was doubled over the pure fibers. Increasing the tannin amount had diminishing returns on performance and on the spinnability of the fibers. Temperature also had diminishing returns, with higher temperatures holding less Na-ions than their lower temperature counterparts. This was mainly attributed to the increase in the specific surface area of the fibers that increased with temperature, creating a larger solid electrolyte layer and impacting on performance. These results are currently being written into a publication.

Additionally, the project also investigated a range of other different fibers. For example, crisscrossed fiber patterns that improved the mechanical strength while maintaining performance, nitrogen doped fibers with improved surface area using ammonia for supercapacitors, and even carbon coated activated carbon to improve its Na-ion storage performance.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Free-standing electrodes represent a significant improvement to current battery designs. A free-standing electrode removes the need for binders, conductive agents, and current collectors, while enabling flexibility. Developing free-standing electrodes with performance that is similar or exceeds their non-flexible, non-freestanding counterparts is seen as one of the next big steps for battery and energy storage technology.

This project demonstrated that two renewable biomaterials, lignin and tannin, can be combined and electrospun into a flexible carbon fiber mat that displays high performance as a anode material for Na-ion batteries. This combination of materials had not been previously investigated and is one of the first steps in realizing the potential to create flexible batteries from renewable resources.

Electrospinning Process for Producing Carbon Fibers