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Functionalized Carbon Nanotube Electrodes for Increased Power Density in Lithium Ion Batteries

November 24, 2010
Applications such as electric vehicles and renewable energy sources require the development of materials combining advantages of both battery and electrochemical capacitor device technologies. Carbon nanotubes (CNT) have gained widespread attention for a range of electrochemical energy storage and conversion device applications given  their unique properties, including electrical conductivity, high surface area, and chemical and mechanical stability. Lee and colleagues investigated a new class of electrodes for energy storage devices facilitated by Li+ reactions with functional surface groups on the MWNT electrode structure. Combining this with a high surface area transition metal oxide lithium storage material at the opposing negative electrode, the authors have demonstrated unprecedented performance for energy storage device electrodes potentially impacting a range of hybrid applications.


Reviewed by Jeff Morse, PhD., National Nanomanufacturing Network

Emerging hybrid energy storage applications require characteristics bridging the attributes and performance gaps between batteries and electrochemical capacitors. Applications such as electric vehicles and renewable energy sources require the development of materials combining advantages of both device technologies, including high energy density provided by Faradaic reaction in batteries, and high rate power delivery provided by surface redox reactions in electrochemical capacitors. Thus a power source providing rapid charge and discharge with superior gravimetric energy density remains essential to the long-term success of these applications. Research efforts have considered this challenge from both directions, including increasing the power density of lithium-ion (L+) batteries by reducing the lithium diffusion time through decreased thickness of the lithium storage materials. In this approach, nanostructured electrode materials still exhibit lower power density in comparison to electrochemical capacitors. On the other hand, nanostructured carbon and transition metal oxide materials have been investigated as a means to increase the energy density and charge storage capability of capacitor technologies.

In order to demonstrate the functional design of specific thin film architectures, precise control of CNT film thickness, density, and adhesion properties is necessary. One such approach developed by the Hammond group at MIT, referred to as layer-by-layer (LBL) assembly, effectively assembles thin films of densely woven multiwall nanotube (MWNT) structures via electrostatic attraction of functionalized MWNTs from highly dispersed aqueous solutions. Recently, Lee et.al. extended this technique through investigation of additive-free MWNT assemblies with stable pseudo-capacitive functional groups. Combining this functional LBL-MWNT electrode with a lithiated Li4Ti5O12 (LTO) negative electrode, the authors demonstrated electrode structures having enhanced energy and power density in comparison to batteries and supercapacitors. The authors theorized that the high gravimetric energy density is a result of Faradaic reactions between the lithium ions and the functional groups on the MWNT surface.

Lee, Figure 5
Faradaic reactions between surface oxygen functional species (orange arrows) and Li schematically illustrated on an HRTEM image of the LBL-MWNT electrodes. Intact graphite layers inside the MWNTs (white arrows) are indicated as electron conduction channels.
Experimental results of the LTO/LBL-MWNT electrodes demonstrated gravimetric energy densities up to ~200 Wh/Kg with an associated power density of ~100 KW/Kg. Estimations for the relative performance of these electrode materials at the cell level resulted in competitive energy densities with order of magnitude improvements in power levels. Furthermore, charge/discharge cycling exhibited no degradation charge storage capacity after 1000 cycles. Thus the authors investigated a new class of electrodes for energy storage devices facilitated by Li+ reactions with functional surface groups on the MWNT electrode structure. Combining this with a high surface area transition metal oxide lithium storage material at the opposing negative electrode, the authors have demonstrated unprecedented performance for energy storage device electrodes potentially impacting a range of hybrid applications. While the LBL-MWNT films reported in this investigation were in the <3 m range, future studies will look to increase the thickness of the LBL-MWNT electrode films to tens of microns employing a significantly higher spray-on LBL assembly method. Additional studies will seek to bring the voltages for charge and discharge of the cell structure closer as the charge cycle has a higher voltage resulting in power losses.


Image reproduced by permission from MacMillan Publishers: Lee SW, Yabuuchi N, Gallant BN, Chen S, Kim BS, Hammond PT, and Shao-Horn Y. 2010.High-powered Lithium Batteries from Functionalized Carbon Nanotube Electrodes. Nature Nanotechnology 5:531-537. DOI: 10.1038/nnano.2010.116, copyright 2010. 

Last updated: June 13, 2014
 

DOI: 10.4053/er466-101124

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Tags: Carbon nanotubes, Energy and Utilities, layer-by-layer, Lithium Ion Batteries, Batteries

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