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 Cu 2 S or Cu 2 Se 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 Cu 2-x Se 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 in-situ / 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%.