Scanning electron microscope photo of hollow carbon nanofiber-encapsulated sulfur tubes, at the heart of a new battery design. Credit: Wesley Guangyuan Zheng; Stanford
Stanford University associate professor of materials science and engineering, Yi Cui, knows his way inside and out of a carbon nanotube, and he’s using his knowledge of that terrain to design new electrodes for lithium-ion batteries and ultracapacitors. Two papers published in recent weeks in Nano Letters describe the details.
The first paper (published Sept. 14, subscription required) describes an approach to cathode design for Li-Ion batteries. Sulfur is an attractive cathode material because of its high storage capacity at relatively low voltage. It is also inexpensive, abundant and nontoxic. According to a Stanford press release, batteries with sulfur cathodes can store four to five times as much energy as existing Li-ion batteries.
Previous cathode designs coated sulfur onto porous carbon structures. However, they fail quickly during the charge-recharge cycle because intermediate lithium polysulfide compounds are in contact with the electrolyte solution and dissolve into it. Cui’s graduate student, Wesley Guangyuan Zheng, describes the problem: “[W]e don’t want a large surface area contacting the sulfur and the electrolyte, and on the other hand we want a large surface area for electrical and ionic conductivities.”
The Cui team separated the sulfur from the electrolyte by simply moving it inside the cathode.
Adapting a commercially available water filtration process, they coated the interior of CNTs with sulfur. The new design prevents polysulfides from leaking into the electrolyte solution, while still allowing easy transport of Li ion through the CNT wall. Tests showed a high specific capacity after 150 charge-discharge cycles. In the same paper, they reported improved coulumbic efficiency gained by adding LiNO3 to the electrolyte.
Cui’s second paper (published Sept. 26) also investigates electrode efficiency, in this case MnO2, which is a promising material for supercapacitors (also called ultracapacitors) because of its high theoretical specific capacity, low cost and nontoxicity. Although it is blessed with a high charge storage capacity, it has low electrical and ionic conductivity, so getting the charge in or out quickly is a barrier.
To improve the conductivity of the electrode at its surface, two conductive coatings were investigated: carbon nanotubes and a conductive polymer. Coatings were applied by dipping a graphene-MnO2 nanostructured composite electrode into a solution of the coatings.
Both coatings increased electrode conductivity, and therefore capacitance. The specific capacitance of the CNT-coated electrode increased by 25 percent and that of the polymer-coated electrode increased by 45 percent. The paper also reports that the coated electrodes, which the authors describe as ternary composites, delivered superior cycling performance, retaining over 95 percent of their capacitance after more than 3000 cycles.
In a Technology Review story, it was pointed out that the energy density of the electrode has yet to be reported.
We first reported on Cui’s simple approaches to using CNTs to make supercapacitors about two years ago.
Coincidentally, Penn State just announced that is has received $5 million from DOE to develop a battery that can provide 600 watt-hours per hour. Included will be research on developing a “nanocomposite sulfur cathode and lithium-rich composite anode material.”