Nanocubes make promising Na-ion battery electrodes

Sodium-ion batteries are a potentially attractive replacement for lithium-ion technology because of the cheapness and abundance of sodium. Despite recent advances, high-performance anode materials are still urgently needed. Researchers from the Hong Kong Polytechnic University have explored a novel option, tiny cubes of a transition metal selenide, with promising results [Qian and Lau, Materials Today Energy 10 (2018) 62-67].

“Existing anode materials for Na-ion batteries are still not as good as the Li-based batteries,” says Shu Ping Lau. “Carbon materials (i.e. graphite) show good cyclic stability but suffer from relatively low reversible capacities, while elementary substances (i.e. Sb and P) and some transition metal oxides (i.e. Fe₂O₃ and CuO) show high theoretical capacities but suffer from poor cyclic stability,” he explains.

Recently, transition metal selenides, such as Cu₂Se, MoSe₂, and FeSe₂, have shown promising behavior for Na-ion batteries. Reducing the size of the transition metal particles from the micro- to the nano-scale has proven particularly fruitful. Lau and his colleague Jiasheng Qian have broadened the search to include MnSe₂ particles as well. Mn-based compounds have the added attraction of high abundance combined with low cost and toxicity. But, until now, MnSe₂ has not been tested as an electrode material for Na-ion batteries.

Lau and Qian synthesized MnSe₂ nanocubes using a simple and scalable hydrothermal process. Precursor materials, MnSO₄, Se, and citric acid, are first dispersed in water, before adding hydrazine hydrate. After stirring for an hour, the mixture is autoclaved for 12 hours at 180°C. The as-prepared black powder comprises nanoparticles <200 nm in diameter with a cubic pyrite structure.

“We demonstrated a simple and scalable approach to synthesize MnSe₂ nanocubes with high yield and high crystallinity,” says Lau.

A test electrode was fabricated from a mixture of the as-prepared MnSe₂ nanocubes with carbon black and a cellulose-based binder coated onto copper foil. The material is promising for Na-ion batteries, believes Lau, because it has a high theoretical storage capacity (of 503 mAh·g^-1), is highly stable in air, and non-toxic. In a test battery, with a Na metal counter electrode in a diglyme-based electrolyte, the MnSe₂ nanocube electrode retains over 90% capacity after 100 cycles.

“The cyclic capacity of MnSe₂ nanocubes already outperforms other metal selenides and graphite anodes,” points out Lau.

And there is still plenty of room for improvement, say the researchers. The electrical conductivity of MnSe₂ nanocubes could be better and complicated redox behaviors could allow unwanted side reactions to take place.

“The rate capability of MnSe₂ nanocubes needs to be further optimized, while the initial coulombic efficiency needs further improvement,” adds Lau.

The researchers are now exploring whether MnSe₂ nanocubes can be grown directly on conductive substrates to circumvent some of these issues.