|Reddy et. al. recently report the synthesis and electrochemical testing of hybrid coaxial MnO2/Carbon Nanotube array electrodes for lithium battery applications. With the high aspect ratio of the hybrid nanotube configuration and the ultra-high surface area associated with the entire array, the potential exists to engineer the battery electrodes for long life while significantly enhancing the Li storage capacity.|
Reviewed by Jeff Morse, Ph.D., National Nanomanufacturing Network
- Reddy, A.L.M., Shaijumon, M.M., Gowda, S.R., Ajayan, P.M. 2009. Coaxial MnO2/Carbon Nanotube Array Electrodes for High-Performance Lithium Batteries. Nano Letters Article ASAP. DOI: 10.1021/nl803081j
The synthesis of hybrid nanorods or nanotubes provides a means for the systematic control of the materials properties and composition making up the core-shell structure. Combining the properties of two different materials for specific applications can lead to multiple functionalities for a hybrid nanostructure that can subsequently be used to optimize the properties. In the case of Li-ion batteries, nanostructured electrode materials are promising due to their high surface area. Of particular interest have been transition metal oxide materials because of their high Li storage capacity, low cost, and environmental benignity. The limitations of these materials arelow electrical conductivity and cyclical degradation during the lithium uptake and release processes. Typical solutions to overcome these issues have been the inclusion of electrically conductive additives, which in turn degrade the ionic conductivity, thereby requiring a tradeoff in performance.
Reddy et. al. recently reported the synthesis and electrochemical testing of hybrid coaxial MnO2/Carbon Nanotube array electrodes for lithium battery applications. Such a hybrid nanostructure for Li-ion battery electrodes benefits from the high electrical conductivity of the CNT core, while exploiting the high Li storage capacity of the porous MnO2 shell material. With the high aspect ratio of the hybrid nanotube configuration and the ultra-high surface area associated with the entire array, the potential exists to engineer the battery electrodes for long life while significantly enhancing the Li storage capacity.
To synthesize the hybrid nanotube, the authors used anodized alumina porous membrane templates with a nominal pore diameter of 200nm and length of 50 µm. The MnO2 nanotubes were fabricated by vacuum infiltration of a manganese nitrate solution, coating the sidewalls of the alumina pores with MnO2. After annealing, they then placed the template supporting the MnO2 nanotubes in a furnace, where CNTs are grown by chemical vapor deposition (CVD). At this point, the template is coated with a gold film providing electrical contact across the array. Finally, a sodium hydroxide bath selectively dissolves the template, leaving a free standing array of hybrid nanotubes.
Analysis of the hybrid nanotubes shows very uniform shell coatings with a high porosity surface. Further electrochemical testing of the electrodes demonstrated significant performance improvement of the hybrid nanotube structure in comparison to MnO2 or carbon nanotubes alone. The authors report some degradation in specific capacity after numerous cycles, which is attributed to possible Li2O formation and decomposition accompanying the reduction and oxidation of the metal nanoparticles in the electrodes. The authors note that the specific capacity of the hybrid nanostructured electrode is still superior to bulk materials, nominally as a result of the significantly higher surface to volume ratio.
This work describes a reasonably simple process for design and synthesis of hybrid nanostructures. Further selection and control of materials compositions offers a versatile approach to fabricate components having specific functionalities for a range of applications, including batteries, fuel cells, solar cells, and sensors. Additional control over template structure may broaden the dimensional control of the hybrid structures that can be achieved, along with the uniformity of the array.
Image reproduced with permission from Reddy, A.L.M., Shaijumon, M.M., Gowda, S.R., Ajayan, P.M. 2009. Coaxial MnO2/Carbon Nanotube Array Electrodes for High-Performance Lithium Batteries. Nano Letters Article ASAP. Copyright 2009 American Chemical Society.