Hollow carbon nanowires show high capacity and cycle life as anodes for sodium-ion batteries; insight into Na-ion insertion-extraction mechanism
|Discharge capacity of the HCNW electrode as a function of charge−discharge cycles at different charge−discharge current densities of 50 (0.2 C), 125 (0.5 C), 250 (1 C), and 500 (2 C) mAh g−1, respectively. Credit: ACS, Cao et al. Click to enlarge.|
Researchers at the Pacific Northwest National Laboratory have developed hollow carbon nanowires (HCNWs) for use as anode material for Na-ion batteries. In a paper published in the ACS journal Nano Letters, they report achieving a high reversible capacity of 251 mAh g−1 and 82.2% capacity retention over 400 charge−discharge cycles between 1.2 and 0.01 V (vs Na+/Na) at a constant current of 50 mA g−1 (0.2 C).
This novel carbon nanostructure also displayed reversible capacity of more than 200 mAh g−1 and more than 90% capacity retention at 125 mA g−1 after 200 cycles. Even at 500 mA g−1 (2 C) a high reversible capacity of 149 mAh g−1 was observed.
The researchers attributed the good sodium-ion insertion properties to the short diffusion distance in the HCNWs and the large interlayer distance (0.37 nm) between the graphitic sheets, which agrees with the interlayered distance predicted by theoretical calculations to enable Na-ion insertion in carbon materials.
Sodium-ion batteries are of interest because they could potentially be less expensive, safer, and more environmentally benign; the team at PNNL has been working on different aspects of the chemistry. (Earlier post.) However, Na ions are significantly larger (about 55%) than Li ions, making it difficult to find a suitable host material to accommodate the Na ions and allow reversible and rapid ion insertion and extraction. Most materials do not have an interstitial space sufficiently large within their crystallographic structure to host the Na ions. This greatly limits the range of potential candidate materials.
Nanostructured materials such as nanowires/nanotubes have provided new opportunities to improve the properties of energy storage materials for various batteries because of their structural stability and good conducting connectivity. Herein, we report hollow carbon nanowires (HCNWs) by direct pyrolyzation of a hollow polyaniline nanowire precursor...From theoretical calculations, we showed that a critical minimum spacing of 0.37 nm between the graphitic layers is required to enable good Na-ion insertion properties, which agrees well with experimental observations.—Cao et al.
They investigated sodium ion insertion−extraction behavior in HCNWs by cyclic voltammetry (CV) and galvanostatic charge−discharge cycling.
In addition to capacity and cyclability results, they determined that Na ions experience two types of insertion−extraction mechanisms: the reaction at a higher voltage range of 0.2−1.0 V is characterized by a charge transfer mechanism on the surface of the small graphitic clusters, while that at a lower voltage range of 0.0−0.2 V is related to Na ion insertion−extraction in the graphitic interlayers.
The development of Na-ion batteries is still in early stages. Although we showed that the nanostructure is helpful for Na-ion insertion, the detailed reaction mechanisms need more careful study, in particular on the nanostructured hard carbon which has more complex structures and reactive sites than graphite. Electrolytes also play an important role in the properties observed, but the effects of composition and additives need to be systematically evaluated and optimized.
Furthermore, the full cell structure of Na ion batteries may be similar or different from Li ion batteries, depending on the electrodes and electrolytes used. We are currently conducting full cell studies with different designs using the hard carbon nanowires as the anode and hope to report our progress in the future.—Cao et al.
Yuliang Cao, Lifen Xiao, Maria L. Sushko, Wei Wang, Birgit Schwenzer, Jie Xiao, Zimin Nie, Laxmikant V. Saraf, Zhengguo Yang, and Jun Liu (2012) Sodium Ion Insertion in Hollow Carbon Nanowires for Battery Applications. Nano Letters doi: 10.1021/nl3016957