In the surface-mediated cell (SMC) made by Bor Jang and colleagues, both the cathode and anode are made of porous graphene. The anode also contains particles or foil of lithium metal, which are ionized during the first discharge cycle of the device. The ions then migrate to the nanostructured cathode through the liquid electrolyte, entering the pores and reaching the cathode surface. The porous electrodes mean that a large surface area of graphene is in direct contact with a liquid electrolyte, something that enables fast and direct capture of lots of lithium ions.

When the cell is recharged, a massive flux of lithium ions is quickly released from the cathode and migrates into the anode zone, re-establishing an electrochemical potential difference between the lithium-decorated anode and the cathode.

Jang says that the SMC typically stores an energy density of 80–160 Wh/kg (based on the total cell weight), which is comparable to the values seen in current lithium-ion batteries. The device also has a power density of 100 kW/kg. For comparison, lithium-ion batteries have much lower power densities at just 0.5 kW/kg. And although so-called symmetric supercapacitors can deliver a power density of 1–10 kW/kg, they store a low energy density of only 5–8 Wh/kg.

Struggling supercapacitors
"This technology will have a significant impact on the energy-storage industry," Jang told "The supercapacitor industry has been struggling with the issue of trying to significantly increase the energy density of supercapacitors but even state-of-the-art 'hybrid supercapacitors' – that is, lithium-ion capacitors can only store up to 15 Wh/kg. The SMC is also superior to conventional supercapacitors in terms of power density by a big margin – even though having a high power density is the hallmark of a supercapacitor."

According to the team, the SMCs could be further improved to store even more energy. "Our immediate task is to push the energy density to a level much higher than those seen in lithium-ion batteries," said Jang, "but having said that, we feel that the current SMC technology is already good enough to commercialize."

The researchers revealed that they are now working on several SMC prototypes and will shortly present these to potential customers and investors. Watch this space, they say.

The current work is reported in Nano Letters.