Hybrid supercapacitor offers NiMH energy density, charges much faster

Researchers at the Queensland University of Technology have added another hybrid supercapacitor design to the mix, promising the near-instant charge and discharge of a supercap with vastly improved energy storage on par with NiMH batteries.

The key concepts to understand here are energy density (Wh/kg), referring to the total amount of energy a device can store per weight, and power density (W/kg), referring to how quickly the device can move power in and out while charging and discharging.

Lithium batteries store energy in a chemical form, and are widely used because they offer a relatively high energy density, but as anyone who owns a smartphone or electric car knows, they charge fairly slowly. Supercapacitors, on the other hand, store energy statically rather than in a chemical form, meaning they can charge and discharge much, much faster without degrading their internal structures. Thus, they have a very high power density, but this is offset by the fact that their energy density is much, much lower than chemical batteries.

In recent times we've covered a number of devices that sit somewhere in between the two: hybrid supercapacitors that lean into the middle on both metrics, storing a lot more energy than a regular supercapacitor, while charging almost as quickly. Your car or phone battery won't last as long with one of these on board, but it'll charge so fast that range might cease to be an issue.

In new research published in Advanced Materials in December, the QUT team describes a design that uses a capacitor-style titanium carbide-based negative electrode and a battery-style graphene-hybrid positive electrode. The result, says the team, is a hybrid capacitor with a power density (and thus charging capability) "about 10 times that of lithium batteries", and an energy density "close to that of nickel metal hydride batteries."

The actual figures are a tested energy density up to 73 Wh/kg – thus about 28 percent of what today's state of the art EV batteries offer – and a sky-high power density up to 1,600 W/kg, where lithium batteries offer around 250-340 W/kg. So let's say you pop a pack like this in the Tesla Model S Plaid+. Instead of a 520-mile (837-km) range, you'd be getting more like 145 miles (233 km), but you'd be able to charge at least five times faster if the infrastructure allows. And yes, infrastructure is a bottleneck right now. Super-high-rate charge stations will get into the megawatt range, placing extreme strain on the energy grid unless they have huge energy storage capacity on site.

The more demented acceleration freaks among us would be pleased to note that power density works both ways, meaning the battery pack would provide no impediment to truly monstrous power outputs. Where today's Plaid+ Tesla makes a ridiculous 1,100-plus horsepower, a hybrid supercap-based equivalent would have a battery pack capable of feeding five times that power to the motors. Completely impractical, but that's never stopped performance car lovers in the past.

The QUT team is pleased to note that these hybrid supercaps also last about twice as long as lithium batteries on the test bench, retaining 90 percent of their initial storage capacity after 10,000 full charge/discharge cycles.

These numbers are in the ballpark of what Kurt.Energy is finding with the low-density hybrid powercapacitors it's receiving from Shenzen Toomen New Energy, what the Skeleton SuperBattery is promising, and what Chinese/British researchers found with their design last year. Thus, while not appearing to blow anything out of the water, and without any immediate commercialization plans, this news adds even more academic credibility to the hybrid supercapacitor sector as a whole.

While electric car battery packs are an easy point of comparison to current technology, hybrid supercaps are unlikely to replace lithium batteries in the EV world, where range anxiety is still such an issue for buyers. But as Skeleton points out, there are plenty of other applications where these in-between solutions will find their place. They may replace the lead-acid board net batteries that are still required in today's lithium-powered EVs. They will be excellent for quick-response power-smoothing and peak load management in industrial settings.

The research is available for free access in the peer-reviewed Advanced Materials journal.

Source: QUT