|Left. Systematic diagram for the nanocomposite of SWNT−amorphous porous FePO4 nanoparticles. Right. cycling stability at 50 mA/g. Credit: ACS, Liu et al. Click to enlarge.|
Researchers at the University of Maryland have developed a nanocomposite material of amorphous, porous FePO4 nanoparticles electrically wired by single-wall carbon nanotubes as a potential cathode material for sodium-ion batteries (SIBs). The hydrothermally synthesized nanocomposite shows excellent cell performance with strong cycling stability and reversibility.
The discharge capacity of as high as 120 mAh/g is delivered at a 0.1 C rate (10 mA/g). The capacity retentions are about 70 mAh/g, 60 mAh/g, and 55 mAh/g at higher currents of 20 mA/g, 40 mA/g, and 60 mA/g, respectively. Even at a 1 C rate (100 mA/g), a capacity of about 50 mAh/g is still retained after 300 cycles. With a simple synthetic procedure, cost-effective chemicals, and desirable cell performance, the method offers a promising candidate for commercialized cathode materials of SIBs, the researchers suggest in a paper in the ACS journal Nano Letters.
Recently, sodium ion batteries (SIBs) have drawn increasing attention from researchers, despite the fact that lithium ion batteries (LIBs) are still the predominant power source for home electronics and for future large-scale energy storage devices. Compared with LIBs, SIBs can be manufactured using more abundant resources, far lower prices, and a greener synthesis while maintaining a similarity in ion insertion chemistry.
However, many challenges remain before SIBs can become commercially competitive with LIBs. For instance, compared to lithium, sodium weighs more and has a higher ionization potential and a larger ionic radius. Further, recent computational studies show that the voltages for the intercalation materials are 0.18−0.57 V lower than that of the corresponding Li voltages.
The smaller energy density of SIBs as compared to LIBs can nevertheless be compensated by the earth-abundance and lower cost of Na, thus making SIBs more promising candidates for commercialized large-scale energy devices and stationary storage.
On the cathode side, iron phosphate-based materials show great potential, the researchers noted, due to their thermal stability and higher voltage. However, most previous research regarding FePO4-related SIB cathode materials has demonstrated poor cell performance.
Amorphous FePO4 nanoparticles have shown good discharge performance and a theoretical capacity as high as 178 mAh/g for Li-ion batteries. However, the team said, to the best of their knowledge, similar nanostructures have not previously been applied in the ﬁeld of Na-ion batteries.
Using a simple hydrothermal method, they synthesized a networked nanocomposite of single-wall carbon nanotubes (SWNTs)/amorphous porous FePO4 nanoparticles for SIBs. The sizes of the porous space within the FePO4 NPs range from a few to tens of nanometers.
Electrochemical performance of the FePO4/SWNTs composite was examined in a coin cell using sodium as the counter electrode.
The composite delivered charge and discharge capacities of as high as 98 mAh/g and 66 mAh/g, respectively, in the first cycle, corresponding to a Coulombic efficiency of 67%. The capacity slightly decreased to ∼60 mAh/g and then became stable up to 300 cycles. The Coulombic efficiency approaches 98% after 40 cycles, indicating excellent cycling stability and reversibility, they said.
The results show that the amorphous FePO4 /SWNT composite is a promising cathode material for viable sodium-ion batteries. First, unlike the crystal-insertion reaction in which the structure phase changes upon ion insertion/extraction, the Na-ion insertion/extraction reaction does not change the structure of the amorphous FePO4 materials, improving cycling stability. Second, the large surface area of the porous FePO4 ensures a high electrode−electrolyte contact area, which could significantly improve the charge transfer reaction for sodium ions and thus its kinetic property.—Liu et al.
The team is working to extend its synthesis technique to other commercially available, low-cost carbon materials—such as graphene—for better commercialization.
Yonglin Liu, Yunhua Xu, Xiaogang Han, Chris Pellegrinelli, Yujie Zhu, Hongli Zhu, Jiayu Wan, Alex Chong Chung, Oeyvind Vaaland, Chunsheng Wang, and Liangbing Hu (2012) Porous Amorphous FePO4 Nanoparticles Connected by Single-Wall Carbon Nanotubes for Sodium Ion Battery Cathodes. Nano Letters doi: 10.1021/nl302819f