U Alberta team develops hybrid sodium-ion capacitor; intermediate in energy & power between ultracaps and batteries
A team led by researchers from the University of Alberta (Canada) Scientists has developed a hybrid sodium-ion capacitor (NIC) using active materials in both the anode and the cathode derived entirely from peanut shells—a green and highly economical waste globally generated at more than 6 million tons per year. The hybrid NIC stores charge both electrostatically and electrochemically, and sits between ultracapacitors and batteries in terms of power (ultracaps) and energy (batteries) storage capability.
According to their paper, published in the RSC journal Energy & Environmental Science, the electrodes deliver among the most promising sodiation capacity–rate capability–cycling retention combinations reported in the literature. The resultant NIC also offers an advanced cyclically stable combination of energy and power, not only in respect to previously developed sodium-ion capacitors, but also as compared to Li-ion capacitors (LICs).
The NIC features an ion-adsorption cathode and an ion-intercalation anode.
The ion-adsorption cathode, based on Peanut Shell Nanosheet Carbon (PSNC), displays a hierarchically porous architecture; a sheet-like morphology down to 15 nm in thickness; a surface area on par with graphene materials (up to 2396 m2 g−1); and high levels of oxygen doping (up to 13.51 wt%). Scanned from 1.5–4.2 V vs. Na/Na+, PSNC delivers a specific capacity of 161 mAh g−1 at 0.1 A g−1 and 73 mAh g−1 at 25.6 A g−1.
The low-surface-area ion-intercalation Peanut Shell Ordered Carbon (PSOC) anode delivers a total capacity of 315 mAh g−1 with a flat plateau of 181 mAh g−1 occurring below 0.1 V (tested at 0.1 A g−1), and is stable at 10,000 cycles (tested at 3.2 A g−1).
The assembled NIC operates within a wide temperature range (0–65 °C), yielding at room temperature (by active mass) 201, 76 and 50 Wh kg−1 at 285, 8,500 and 16,500 W kg−1, respectively. At 1.5–3.5 V, the hybrid device achieved 72% capacity retention after 10,000 cycles tested at 6.4 A g−1, and 88% after 100,000 cycles at 51.2 A g−1.
In a review of the technology in the RSC’s Chemistry World, Professor David Mitlin, who led the research, explained:
In conventional batteries the cathode often limits performance and so what people are starting to do is swap regular cathodes for supercapacitor cathodes. Ions are adsorbed onto the surface of the cathode in an NIC, which avoids the degradation seen in batteries due to ion absorption into the bulk.
In addition to being easy to source and being inexpensive, peanut shells (both inner and outer) offer structural characteristics desirable for anode and cathode materials, respectively.
Jia Ding, Huanlei Wang, Zhi Li, Kai Cui, Dimitre Karpuzov, Xuehai Tan, Alireza Kohandehghan and David Mitlin (2015) “Peanut shell hybrid sodium ion capacitor with extreme energy–power rivals lithium ion capacitors” Energy Environ. Sci. doi: 10.1039/C4EE02986K