UC Berkeley

With the rising demands for small, lightweight, and long-lasting portable electronics, the need for energy storage devices with both large power and large energy densities becomes vitally important. From their usage in hybrid electric vehicles to wearable electronics, supercapacitors and rechargeable batteries have been the focus of many previous works. Electrode materials with large specific surface areas can enhance the charging speed and total amount of stored energy. To this end, vertically self-aligned carbon nanotube (CNT) forests are well suited, as they possess outstanding electrical conductivities as well as high mechanical strength and large specific surface areas. In addition, forests of vertically aligned CNTs allow the ions within an electrolyte to pass freely between the individual CNTs from electrode to electrode. In order to minimize the system resistance of the battery or supercapacitor, a thin molybdenum current collector layer is deposited beneath catalyst of the CNT forest, thus ensuring that when the CNT forest grows from its substrate, each CNT has an innate connection to the current collector. This versatile CNT-Mo film architecture is used in this work as both supercapacitor as well as lithium-ion battery electrodes.

It is desirable to have energy storage devices of adjustable shapes, such that they may conform to the shrinking form factors of modern portable electronics and mechanically flexible electrodes are an attractive prospect. The CNT-Mo film is shown here to easily release from its growth substrate, after which it may be placed onto a number of surfaces and topographies and densified. Two polymer films, Kapton® and Thermanox^TM, have been used as substrates for the demonstrations of flexible supercapacitor electrodes. Test results show that the attached active CNT-Mo film can withstand bending to at least as large an angle as 180°. The specific capacitance of a 5 mm by 5 mm area electrode in the K2SO4 aqueous electrolyte with an original CNT height of 40 micrometers is measured to be 7.0 mF/cm2. To further increase the surface area of the energy storage electrode, a thin, conformal coating of amorphous silicon is deposited onto a vertically aligned carbon nanotube forest using low pressure chemical vapor deposition (LPCVD). Various silicon film thickness depositions are tested as supercapacitor electrodes. A coating of 35 nm is shown to improve the specific capacitance by a factor of 2 as compared to a bare CNT electrode.

For applications in which a larger operating voltage is desirable, the electrochemical window of the supercapacitor devices are increased by tailoring the electrolyte used. Using an ionic liquid electrolyte (1-ethyl-3-methylimidazolium tetrafluoroborate, or EMIM-BF4) improves the voltage window from 1 V (in aqueous electrolyte) to 4 V, yielding a power density from the range of 19 to 53 kW/kg. In addition, the CNT-Mo film is shown to outperform an activated carbon (AC) electrode in this ionic liquid in terms of volumetric capacitance by a factor of 12 (388 mF/cm3 versus 31 mF/cm3 for the CNT-Mo film and the AC, respectively). The cycling life of the film in ionic liquid at a number of current densities is also analyzed, and shown to be stable over 7000 charge-discharge cycles. Finally, the CNT-Mo film architecture is further utilized and tested as a lithium ion battery electrode. The high surface area, excellent CNT conductivities, and the extremely high lithium ion intercalation capacity of silicon all promise long-lived and energy-dense lithium ion electrodes. Preliminary results show high energy density of 4000 mAh/g initially. The value quickly drops to 600 mAh/g after 5 charge/discharge cycles and stay the same until failure after 15 cycles. Further studies into thinner silicon coatings and electrolyte selections may result in better performance and longer cycling life.