Researchers at Rice University develop vanadium oxide-graphene materials for high power-density Li-ion batteries with ultrafast charging and discharging
|Rate capacities of VO2-graphene architectures with different VO2 contents, measured for 30 cycles at each selected rate from 1C to 190C. Credit: ACS, Yang et al. Click to enlarge.|
A team from Rice University has developed vanadium oxide (VO2)-graphene ribbon materials that, when used as cathode materials in Li-ion batteries, enable ultrafast, “supercapacitor-like” charge and discharge rates with long cycle life while maintaining highly reversible capacity. In a paper published in the ACS journal Nano Letters, the researchers suggested that this “breakthrough in cathode materials with ultrafast charging and discharging capability...can significantly prompt the rapid development and applications of high-power lithium ion batteries.”
With the single crystalline VO2-graphene ribbons as cathodes, a full charge or discharge is capable in 20 seconds. The electrodes retain more than 90% of the initial capacity after cycling more than 1,000 times at an ultrahigh rate of 190C, providing the best reported rate performance for cathodes in lithium ion batteries to date.
The achievement of high-rate capability, most important for high power density applications such as in electric vehicles, is known to be hindered by kinetic problems involving slow ion and electron diffusions in the electrode materials. An effective strategy to enhance current rates is by reducing the characteristic dimensions of the electrochemically active materials, since the diffusion time of lithium ions (t) is proportional to the square of the diffusion length (L) (t ≈ L2/D). In this regard, numerous nanoscale materials including nanowires, nanotubes, nanoparticles, nanosheets, and nanoribbons have been recently synthesized and demonstrated for improving electrochemical performance for lithium storage. However, only modest improvements in rate performances have been observed due to difficulties to simultaneously possess efficient ion and electron pathways in unmodified nanomaterials.
To further circumvent this problem, various three-dimensional (3D) architectures with high electrical conductivity have been employed to serve as current collectors for nanomaterials. Although some improvements in charging and discharging rates have been achieved with minimal capacity loss, these architectures commonly lead to the high-weight fraction of current collectors in electrodes, decreasing the overall energy density of batteries. Moreover, the complicated and limited approaches to fabricate such 3D architectures largely hamper their practical applications in lithium-ion batteries.
Here, we demonstrate a simple bottom-up approach for synthesizing vanadium dioxide (VO2) ribbons with thin, flexible, single-crystalline features simultaneously included with graphene layers as building blocks to construct 3D architectures.—Yang et al.
The architecture of the material provides the following benefits, the researchers said:
Numerous channels for the access of electrolyte, facilitating rapid diffusion of lithium ions within the electrode material;
A short solid-state diffusion length for lithium owing to the thin nature of VO2 ribbons;
A high electrical conductivity of the overall electrode, based on the graphene network; and
The highest content of electrochemically active material within the electrode (up to 84% by weight).
The material enables rapid ion and electron diffusions—the kinetic requirements for ultrafast charging and discharging.
Vanadium oxide is known as a promising electrode material for both organic and aqueous lithium ion batteries owing to its high capacity, structure and electrode potential; however, it suffers poor cyclic performance as a result of high charge-transfer resistance. The use of graphene solves the electrical resistance problem with the VO2 electrodes, the researchers said.
In the study, they synthesized VO2-graphene architectures with various VO2 content (84%, 78%, and 68%). As examples of some of the test results, a very high reversible capacity of 415 mAh g−1 with stable cycle performance was achieved at 1C by VO2-graphene architecture with the VO2 content of 78%.
More remarkably, the VO2-graphene architectures exhibit ultrafast charging and discharging capability. For example, the VO2-graphene architecture containing 78% VO2 demonstrates reversible capacities as high as 222 and 204 mAh g−1 at the extremely high rates of 84C and 190C (corresponding to 43 and 19 s total discharge or charge), respectively. These high discharge−charge rates are 2 orders of magnitude greater than those currently used in lithium ion batteries. Moreover, even after 1000 cycles at the ultrahigh rate of 190C, both discharge and charge capacities are stabilized at about 190 mAh g−1, delivering over 90% capacity retention. To the best of our knowledge, such an excellent high-rate performance is superior to all existing cathode materials reported for lithium ion batteries.—Yang et al.
Shubin Yang, Yongji Gong, Zheng Liu, Liang Zhan, Daniel P. Hashim, Lulu Ma, Robert Vajtai, and Pulickel M. Ajayan (2013) Bottom-up Approach toward Single-Crystalline VO2-Graphene Ribbons as Cathodes for Ultrafast Lithium Storage. Nano Letters doi: 10.1021/nl400001u