New Coaxial Nanocables Show Enhanced Rate, Energy and Cycling Performance as Li-ion Electrode Materials; “Yin-Yang Principle” Extensible to Other Electrochemical Storage Devices
|Comparison of the rate performance of CNT@TiO2, TiO2-free CNT, and CNT-free TiO2 sample between voltage limits of 0.01-3 V. Shaded areas represent the capacity contribution from TiO2 or CNT in the nanocables. Credit: ACS, Cao et al. Click to enlarge.|
Researchers in China and Germany have coated carbon nanotubes (CNT) with a nanoporous layer of TiO2 to create coaxial nanocables for use as electrode materials in Li-ion batteries (LIB). The CNT@TiO2 coaxial nanocables show excellent rate capability, energy and cycling performance compared with both pure CNT and pure TiO2 when used as anode materials for LIBs.
Both the specific capacity in the CNT core and that in the TiO2 sheath are much higher than that of the TiO2-free CNT and that of the CNT-free TiO2 sample, respectively. A paper on the work was published online 22 January in the ACS journal Chemistry of Materials.
While the carbon nanotubes assist the storage in TiO2 by providing electrons, the nanoporous TiO2 sheath assists the storage in the carbon nanotubes by enabling rapid access of Li+ from the liquid electrolyte. As the roles of ions and electrons are very different but complementing (compare acid-base activity with redox activity), the mutually beneficial role of the two intimately connected components TiO2 (providing Li+ for CNT) and CNT (providing electrons for TiO2) finds a picturesque metaphor in the Chinese yin-yang principle.—Cao et al.
|HRTEM images of the nanocable. Credit: ACS, Cao et al. Click to enlarge.|
A key problem in Li-battery research is guaranteeing sufficiently rapid transport of both ions and electrons, the researchers say, noting that only “a few exceptional materials” such as Ag2S provide fast ionic and electronic conduction even at room temperature that is sufficient to enable rapid chemical transport even in big crystals. Carbon provides sufficient electronic conductivity but lacks sufficient ion conductivity.
Among the many different approaches under investigation to addressing this problem is the use of carbon nanotubes (CNT).
CNT is also a fine Li-storage host as well as a fast Li insertion-extraction host at a low voltage, which makes it an attractive anode material for lithium-ion batteries. However, the practical applications suffer from a high level of irreversibility (low columbic efficiency) and poor cycle life because of the pronounced surface reactions between CNTs and electrolyte.
The basic point in our paper is the mutually beneficial, i.e., symbiotic, role of the two intimately connected phases CNT and TiO2. CNT is not just a metallizer for the storage material TiO2, it efficiently stores Li as well. In turn, for the storage of Li in CNT, the TiO2 proves helpful, too. It allows for a rapid access of ions to the CNT.—Cao et al.
Among the results of the testing of the material, the researchers found a total reversible capacity (per total mass) of about 406 mAh g-1 in the voltage range of 0.01-3 V for the CNT@TiO2 nanocables under a current density of 50 mAg-1; acid-treated CNTs showed a total reversible capacity of around 367 mAh g-1 under the same experimental condition.
At a current density of as much as 3,000 mA g-1, CNT@TiO2 can still deliver a specific capacity of 244 mAh g-1 between the voltage limits of 0.01 and 3 V. CNTs without TiO2 coating layers deliver 74 mAh g-1, and the CNT-free TiO2 has nearly no capacity under those conditions.
...our results demonstrate that very effective synergism could be introduced by using two-phase structures such as the coaxial nanocables reported here. They can be used for designing superior electrode materials with improved performance in terms of power (rate), energy, and cycling behavior. The cable morphology also allows for a dense packing of electroactive materials.
...In the specific case of CNT@TiO2 core/porous-sheath coaxial nanocable, on one hand, the benefit of CNT for TiO2 storage consists in the electronic wiring principle (i.e., the CNT core providing sufficient e- for the TiO2 sheath). On the other hand, the benefit of nanoporous TiO2 for CNT is the almost unperturbed Li+ supply for the CNT core, most probably because of the porosity and the small thickness of the passivation layer. It is the synergism of the two parts that leads to a high, fast and stable lithium storage material. The strategy is simple, yet very effective; because of its versatility, it may also be extended to other electrode materials for future electrochemical energy storage devices (LIBs, supercapacitors, or hybrid) combining high-power and high-energy densities.—Cao et al.
Their work was supported by the National Natural Science Foundation of China, National Key Project on Basic Research, the Chinese Academy of Sciences, and the Max Planck Society in Germany.
Fei-Fei Cao, Yu-Guo Guo, Shu-Fa Zheng, Xing-Long Wu, Ling-Yan Jiang, Rong-Rong Bi, Li-Jun Wan and Joachim Maier (2010) Symbiotic Coaxial Nanocables: Facile Synthesis and an Efficient and Elegant Morphological Solution to the Lithium Storage Problem. Chem. Mater., Article ASAP doi: 10.1021/cm9036742