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

Charge storage mechanisms in oxygen-functionalized, multiwall carbon nanotube (MWNT) electrodes fabricated by the Layer-by-Layer (LbL) self-assembly process were studied in two-electrode nonaqueous cells. In asymmetric cells with a Li metal or a lithium titanate (Li4Ti5O12) 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)). 2014 The Electrochemical Society. All rights reserved.