Batteries and supercapacitors are two complementary electrochemical energy-storage technologies. They typically contain positive and negative electrodes with the active electrode materials coated on a metal current collector (normally copper or aluminium foil), a separator between the two electrodes, and an electrolyte that facilitates ion transport. The electrode materials actively participate in charge (energy) storage, whereas the other components are passive but nevertheless compulsory for making the device work.

Batteries offer high energy density but low power density while supercapacitors provide high power density with low energy density. Although lithium-ion batteries are the most widely employed batteries today for powering consumer electronics, there is a growing demand for more rapid energy storage (high power) and higher energy density. Researchers are thus looking to create materials that combine the high-energy density of battery materials with the short charging times and long cycle life of supercapacitors. Such materials need to store a large number of charges (such as Li ions) and have an electrode architecture that can quickly deliver charges (electrons and ions) during a given charge/discharge cycle.

Increasing the mass loading of niobia in electrodes

Nanostructured materials fit the bill here, but unfortunately only for electrodes with low areal mass loading of the active materials (around 1 mg/cm2). “This is much lower than the mass of the passive components (around 10 mg/cm2 or greater),” explains team leader Xiangfeng Duan. “As a result, in spite of the high intrinsic capacity or rate capability of these new nanostructured materials, the scaled area capacity or areal current density of nanostructured electrodes rarely exceeds those of today’s Li-ion batteries. Thus, these electrodes have not been able to deliver their extraordinary promise in practical commercial devices.

“To take full advantage of these new materials, we must increase the mass loading to a level comparable to or higher than the mass of the passive components. To satisfy the energy storage requirement of an electrode with 10 times higher mass loading requires the rapid delivery of 10 times more charge over a distance that is 10 times greater within a given time. This is a rather challenging task and has proven to be a critical roadblock for new electrode materials.

“We have now addressed this very issue of how we can increase the mass loading of niobia (Nb2O5) in electrode structures without compromising its merit for ultrahigh-rate energy storage,” he continues. “Electrodes with intrinsically high capacity or high rate capability in practical devices require a new architecture that can efficiently deliver sufficient electrons or ions. We have designed a 3D holey-graphene-Nb2O5 composite with excellent electron and ion transport properties for ultrahigh-rate energy storage at practical levels of mass loading (greater than 10 mg/cm2).”

Porous structure facilitates rapid ion transport

“The highly interconnected graphene network in the 3D architecture provides excellent electron transport properties and its hierarchical porous structure facilitates rapid ion transport,” he adds. “What is more, by systematically tailoring the porosity in the holey graphene backbone, we optimize charge transport in the composite architecture to simultaneously deliver areal capacity and high-rate capability at practical levels of mass loading – something that is a critical step forward towards commercial applications.”

The researchers made their mechanically strong 3D porous composites in a two-step synthesis technique. “We uniformly decorate Nb2O5

Decreasing the fraction of inactive materials

The in-plane pores in the holey graphene sheet function as ion transport “shortcuts” in the hierarchical porous structure to facilitate rapid ion transport throughout the entire 3D electrode and so greatly improve ion transport kinetics and access to ions on the surface of the electrode, Duan tells

Spurred on by these results, the researchers say they will now try to incorporate high-capacity active materials such as silicon and tin oxide to further increase output energy levels in electrochemical cells. “Extremely high mass-loaded electrodes (for example, five times that of practical mass loading, or 50 mg/cm2) could also help decrease the fraction of inactive materials in a device and so simplify the process to make these cells.”

The research is detailed in Science DOI: 10.1126/science.aam5852.