Electrochemical Performance of Thin-Film Functionalized Carbon Nanotube Electrodes in Nonaqueous Cells

Abstract

Charge storage mechanisms in oxygen-functionalized, multiwall carbon nanotube (MWNT) electrodes fabricated by the Layerby-Layer (LbL) self-assembly process were studied in two-electrode nonaqueous cells. In asymmetric cells with a Li metal or a lithium titanate (Li 4 Ti 5 O 12 ) negative electrode, double-layer charging of MWNT surfaces is supplemented by surface faradaic reactions between Li + and oxygen functional groups. The utilization of faradaic reactions leads to high gravimetric energies of 350 Wh/kg electrode (with Li) and 140 Wh/kg electrode (with LTO) at 5 kW/kg electrode . Self-discharge studies of LbL-MWNT electrodes in asymmetric Li cells revealed that charge leakage comparable to traditional symmetric double-layer capacitors occurs at open circuit, and the presence of surface faradaic reactions accelerates the voltage recovery for electrodes polarized below open circuit voltage. Studies in different electrolytes reveal that both Li + and TEA + can participate in faradaic reactions with surface oxygen. The role of faradaic reactions is further demonstrated by tests in symmetric cells composed of two identical LbL-MWNT electrodes, where charge storage is limited to double layer charging owing to electrolyte charge neutrality requirements. As a result, the electrode energy decreased substantially (40 Wh/kg electrode at 5 kW/kg electrode ).Electrochemical capacitors (ECs) 1-8 play an important role as energy storage devices in applications requiring high power and durability, from rapid-charging power tools and load leveling of the electric grid 3,6,9 to integrated power supplies for on-chip and microscale devices. 7,10,11 ECs, which store charge by physical charge separation and/or surface faradaic reactions (pseudocapacitance) at the electrode surface, exhibit high cell gravimetric powers (10 kW/kg cell ) compared to Li-ion batteries (1 kW/kg cell ) 5 and can undergo hundreds of thousands of cycles with minimal capacity fade. 3 However, a major limitation of ECs is low gravimetric energy density (5 Wh/kg cell ) compared to Li-ion batteries (100 Wh/kg cell ), 5 where Li-ion batteries achieve higher energies through intercalation redox reactions utilizing the entire volume of storage materials. Thus, it is critical to develop electrode materials that can improve the energy density of high-power, long cycle life materials and devices by combining the charge storage elements of batteries and ECs.The gravimetric energy of an EC is proportional to the product of cell capacitance, C, and the square of the cell voltage, V, by E = 1 2 CV 2 . Strategies to increase the gravimetric energy of devices have focused on increasing both of these parameters. High capacitance is achieved using electrode materials with high specific surface areas such as nanoporous (e.g. activated) carbons (1000-2000 m 2 /g), 12 where gravimetric capacitances of 150-200 F/g carbon are commonly reported in aqueous cells. 2 In nonaqueous cells, fine-tuning of the pore size relative to the desolvated ion size 10,11,13,14 yields a severalfo...

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