Mesoporous Cu2-xSe nanocrystals as an ultrahigh-rate and long-lifespan anode material for sodium-ion batteries

Transitional metal chalcogenides, one important family of promising candidates as electrode materials for sodium-ion batteries, gradually convert to Cu2S or Cu2Se in the discharge/charge processes, due to electrochemically driven copper diffusion. Thus, their performances after the electrochemical activation, actually come from copper chalcogenides. In this regard, the thorough investigation on the electrochemical properties of copper chalcogenides is important and necessary for the rational design of transitional metal chalcogenides. Here, mesoporous Cu2-xSe nanocrystals synthesized by a simple solvothermal reaction, are used as a model. They experience the complicated intercalation reaction and the conversion reaction upon cycling, as supported by insitu/ex-situ techniques and first-principle calculations. All these reactions are highly reversible, leading to 96.1% of the theoretical capacity. These nanocrystals preserve 88% of the initial capacity after 3000 cycles at 5 A g(-1). Even at 10 A g(-1), the capacity is still kept as 92.1% of that at 0.1 A g(-1). In full cells, the nanocrystals without any pre-sodiation and pre-activation, present a lifespan up to 2000 cycles at 0.5 A g(-1) and a capacity retention about 74.2%.