Comparison of anode materials with the current NiMoO4 electrode and the corresponding electrochemical performance in sodium ion battery. Cycle numbers (1st and 2nd) are indicated in the plots.
Low (a, c, e) and high (b, d, f) magnification secondary electron microscope images of NiMoO4 at varying concentrations of oxidizer: (a–b) 0 g; (c–d) 1 g; and (e–f) 2 g. Images show the formation of needles (0 g) and rod-shaped particles (=1 g) with surface nodules apparent at higher concentration materials (insets in (d) and (f)).The boom in lithium (Li)-ion batteries has led to a rapid rise in in the price of Li. With cost and availability becoming potentially unsustainable, interest is growing in cheaper alternatives such as sodium (Na)-based batteries. Despite advantages in cost, safety, and environmental friendliness, new electrode materials are needed for Na-ion batteries. Like other batteries, Na-based batteries work by shuttling Na+ ions between two electrodes through an electrolyte. During the electrochemical process, Na+ ions are inserted into the anode material to store charge and removed upon discharge. But because Na+ ions are so large, this can represent a problem.
“The large volume changes in alloy and metallic anodes upon repeated cycling results in electrode pulverization and eventual significant capacity fading,” explains Manickam Minakshi of Murdoch University in Australia. “Insertion-type electrodes with a different mechanism chemistry are a significant area of interest.”
Together with researchers from La Trobe University, University of Wollongong, and the Helmholtz Institute Ulm for Electrochemical Energy Storage in Germany, Minakshi and his colleagues have come up with a promising candidate based on nickel molybdate (NiMoO4) [Minakshi et al., Materials Today Energy 10 (2018) 1-14].
The new insertion reaction anode material is produced via combustion synthesis. In this process, an oxidant and fuel in the form of urea are mixed together in the presence of metal ions Ni and Mo. The key to the process is the oxidant, NH4NO3, which can radically change the material characteristics of the NiMoO4 nanoparticles produced. If there is too little, the resulting material is only partly crystalline and contains carbon-based impurities. At the other extreme, too much oxidant produces unwanted, secondary phases and leaves the synthesis reaction incomplete. But with just the right amount, NH4NO3 reacts to form a ring-like complex with metal cations, creating a crystalline phase of nickel oxide particles anchored onto the surface of NiMoO4 nanorods.
“The fuel and oxidizer have a central role in the solution combustion synthesis, allowing manipulation of the material architecture to produce optimized properties,” says Minakshi. “At the optimized fuel-to-oxidizer ratio, the fuel is able to interact with most of the metal cations (Ni and Mo) to form a well-connected gel network of chelated to metal cations.”
When incorporated into a test battery as the anode, the researchers found that the new material has comparable capacity to C-based anodes, with a retention level of more than 80% over 50 cycles, and outperforms in terms of voltage. The cyclability and capacity retention of the material now needs to be tested over many thousands of cycles.
If this, or a similar anode material proves itself, Na-ion batteries could find a niche in large-scale applications such as electrical grid stabilization.