Layer-by-layer assembled carbon nanotube nanostructures for high-power and high-energy lithium storage

The layer-by-layer (LbL) electrostatic assembly technique is utilized to incorporate positively- and negatively-charged electroactive species in self-assembled electrodes that are binder- and additive- free. This work first studies electrochemical performance of LbLassembled functionalized multiwall carbon nanotube (MWNT) electrodes as positive electrodes in asymmetric cells with a lithium metal or LTO negative electrode. In these cells, lithium ions undergo reversible Faradaic reactions with oxygen-containing functional groups on MWNTs, leading to high electrode energy of 450 Wh/kg (lithium) and 140 Wh/kg (LTO) at 5 kW/kg. In symmetric cells, charge storage occurs predominantly through double layer charging owing to electrolyte charge neutrality requirements, yielding lower electrode energy and power of 30 Wh/kg at 5 kW/kg. LbL-MWNT electrodes exhibit comparable energy and power performance in LiPF 6, LiBF 4, and LiClO4 electrolyte, and also in non-lithium containing TEABF 4 electrolyte, indicating that surface redox is independent of electrolyte solvent or anion, and suggesting that functional groups can be a versatile charge storage mechanism for redox of cationic species. Self-discharge studies reveal that voltage decay is governed by several mechanisms at characteristic timescales, whereas the phenomenon of voltage recovery from potentials below open circuit potential can be explained in terms of competition between thermodynamically allowed oxygen redox and kinetically favorable double layer charging. Finally, a temperature study at 50*C reveals that LbL-MWNT electrodes can be used with comparable performance at high temperature, with some loss of energy density at high powers. In order to demonstrate the versatility of the LbL process, we next prepare titanium oxide (TiO2)-MWNT thin films using electrostatic interactions between positively charged anatase TiO2 nanoparticles and negatively charged functionalized MWNTs. MWNT-TiO2 film growth and quality are investigated in terms of film thickness and roughness as a function of bilayer pairs. Cyclic voltammetry and galvanostatic testing data in lithium cells show that MWNT-TiO2 electrodes can utilize several charge storage mechanisms: 1) intercalation in TiO2 in the voltage range 1.5 - 3.0 V vs. Li; 2) intercalation in MWNTs at voltages < 1 V vs. Li; and 3) double-layer charging of MWNT and TiC 2 nanoparticles at all potentials. The effect of electrode thickness and microstructure on lithium reaction kinetics is discussed.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Mechanical Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2010

Author dc:creator

• Gallant, Betar M. (Betar Maurkah)

Advisor dc:contributor.advisor

• Yang Shao-Horn.

• Mechanical Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/61864

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/61864