Chemical Synthesis And Sodium-Storage Properties Of Cu_xSe Nanomaterials

Due to social development,the demand for energy storage equipments like lithium/sodium ion batteries with long cycle life,high energy density and power density is increasing.Many transition metal chalcogenides will gradually convert to copper sulfides/selenides in the discharge/charge process due to electrochemically driven diffusion of copper current collector.Thus,it is copper sulfides/selenides cycled for Na storage.In this case,the thorough investigation on the electrochemical properties of copper sulfides/selenides is important and necessary for the rational design of transitional metal chalcogenides.Meanwhile,copper selenides show the promising potential as an anode material for NIBs,due to its simple preparation,low cost,and excellent electrochemical performances.In this thesis,the chemical synthesis and Na-storage properties of mesoporous Cu2-xSe nanocrystals and CuSe nanosheets are investigated in detail as follows.(1)Mesoporous Cu2-xSe nanocrystals are synthesized via a simple solvothermal reaction.They experience the complicated intercalation reaction and the conversion reaction upon cycling,as supported by in-situlex-situ techniques and first-principle calculations.These nanocrystals preserve a capacity of 212.4 mAh g-1after 3000 cycles at 5 A g-1,resulting in a capacity retention of 88%relative to the capacity after the electrochemical activation.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 Ag-1 and a capacity retention about 74.2%.All these results are obtained without carbon coating.(2)CuSe nanosheets are also synthesized by a simple solvothermal reaction and are applied as anode materials for sodium ion batteries.Investigated by ex-situ XRD,the Na-storage reactions in the first cycle are irrevesibile and the electrode can only reform Cu2Se in the initial charge process.CuSe nanoplates deliver a capacity of 169.3 mAh g-1 after 800 cycles at 2 A g-1 with a capacity retention of 97.4%.When the current density increases to 5 A g-1,the reversible capacity is still 183.5 mAh g-1,70.7%of that at 0.1 A g-1.The electrochemical properties of CuSe on different current collectors are tested to investigate the effect of current collector on the electrochemical performance of CuSe.