Sodium-ion energy storage in nanocellular carbon foams shows high capacity and rate retention; not an intercalation battery
|Ragone plot of an NCCF-Acid/Na cathode together with two other representative Na-ion battery cathodes and lithium batteries. Credit: ACS, Shao et al. Click to enlarge.|
Researchers at Pacific Northwest National Laboratory (PNNL) report in a paper in the ACS journal Nano Letters on high-capacity, high-rate sodium-ion (Na-ion) energy storage in functionalized high-surface-area nanocellular carbon foams (NCCF). The NCCF delivers 152 mAh/g capacity at the rate of 0.1 A/g and a capacity retention of 90% for more than 1600 cycles.
Sodium-ion intercalation batteries—i.e., using the same process of ion insertion and removal as in Li-ion batteries—have been discussed in the literature for some time; members of the PNNL team have been active in that work. (e.g., Earlier post.) Using sodium ions instead of lithium ions in a battery is attractive because it could potentially be much less expensive and safer, and it would be more environmentally benign.
However, developing efficient Na+ intercalation compounds is a challenge because sodium ions are much larger than lithium ions—about 70% larger in radius. Thus, insertion/deinsertion of sodium ions in a host material is much more difficult than that of lithium ions, the researchers note.
Large structural change occur during Na+ insertion and de-insertion, leading to low capacity and poor cycling stability. For cathode materials, the reversible, stable capacity of bulk Na+ intercalation is usually limited to ∼120 mAh/g—far below what can be obtained in Li-ion electrode materials.
Since Na+ intercalation is difficult in most host materials, it is important to consider new storage mechanisms that are not dependent on bulk diffusion and intercalation. In this paper, we explore a surface-driven Na+ energy storage mechanism based on the reactions between Na+ and oxygen functional groups of high-surface-area, free-standing, binder-free nanocellular carbon foam (NCCF) papers. A Na+ energy storage device based on this new mechanism and the high-surface-area functionalized NCCF can have significantly enhanced energy storage capacity and rate capability and exceptional cycling stability. (The surface storage mechanism indeed looks like a capacitor. But compared to regular capacitor materials, as we will present in the following sections, the specific energy density is much higher. So, we used the term “Na+ energy storage device”, instead of “Na+ capacitor” or “Na+ battery”.)
In addition, the use of free-standing binder-free NCCF papers as electrodes simplifies the electrode structure and reduces the parasitic weight from binders and other additives.—Shao et al.
Based on the study, the team concluded that the superior performance comes from the new surface-reaction driven charge storage mechanism on the high-surface-area NCCF. The reaction kinetics on the highly conductive carbon substrate are much faster than the Na+ bulk intercalation reaction because there is no bulk diffusion of Na+ as in intercalation compounds; nor is there an electrode structure change during the surface reaction. These lead to high rate performance and cycling stability.
The team expects further improvement could be made by tuning functional groups, morphology, and structure of carbon materials. An anode in the charged state is needed for practical application of this new cathode. Sodium metal may be a candidate, they noted, but there is “probably a safety concern.”
Recent progresses in lithium metal protection might provide guidance for sodium metal anodes. New anode materials with novel electrode design are also under development and will be reported later. As is known, nanomaterials are often penalized by low volumetric energy density. Efforts are also needed to increase the volumetric energy density in future research.—Shao et al.
Yuyan Shao, Jie Xiao, Wei Wang, Mark Engelhard, Xilin Chen, Zimin Nie, Meng Gu, Laxmikant V. Saraf, Gregory Exarhos, Ji-Guang Zhang, and Jun Liu (2013) Surface-Driven Sodium Ion Energy Storage in Nanocellular Carbon Foams. Nano Letters doi: 10.1021/nl401995a