Synthesis Of Ni/Co Hydroxides (Sulfides) Nano-Materials And Their Energy Storage Performance

The popularity of portable energy storage devices and electric or hybrid power vehicle stimulates the development of higher power and energy density for energy storage devices such as supercapacitors and lithium ion batteries.The electrode is the core component of lithium battery and supercapacitor.Thus,the electrode material is the key factor of overall performance quality.Therefore,exploitation of high performance electrode material has great significance for the research and application for supercapacitor and lithium battery.In this thesis,we focused on the electrochemical energy storage materials such as transition-metal hydroxides and sulfides,and systematically investigated their synthesis methods,formation mechanism,and the applications in lithium-ion batteries and supercapacitors,in order to improve their electrochemical cycle stability.The main points can be summarized as follows:1,3D hierarchical structures composed of 2D nanosheets were more stable than 2D nanosheets,preventing the re-stacking of nanosheets.Furthermore,the stability of layered double hydroxide can be improved by the modification of organic compounds because they can intercalate and/or absorb into the layers to reduce the surface energy.Herein,a facile hydrothermal method was developed to directly prepare the organic molecule modified 3D hierarchical Co-Al-LDHs composed of atomically thin nanosheets.The as-obtained hierarchical Co-Al-LDHs show a high specific capacitance of 801 F g^-1 at 5 A g^-1,excellent rate performance with a retention 677 F g^-1 at 100 A g^-1,and the cycling stability with only5%decline after 20000 cycles(5 A g^-1).Such excellent performances were derived from its atomically thin building unites modified by organic molecule and the unique 3D hierarchical structures.At the same time,this mixed solvent system can also be used with other periodic elements?Ni,Fe?to synthesize 3D ultrathin nano-materials with excellent electrochemical performance.2,Transition-metal sulfides have received increasing research interests in lithium storage owing to their high specific capacities exceeding traditional intercalation materials?e.g.graphite?.However,the conversion of metal and Li₂S to metal sulfides in delithiation process is largely irreversible because of the multiple electron-transfer per metal center,which leads to the oxidation of Li₂S to polysulfide.The in-situ formed polysulfides prefer to dissolve into electrolyte,resulting in uncomplete conversion of metal to metal sulfides because of the loss of Li₂S,which usually leads to the poor cycle stability of sulfides.Herein,Ni₃S₄ nanoparticles coated by a layer of Al₂O₃ were designed and prepared through a one-pot hydrothermal process.The regeneration of nickel sulfides could be promoted by the localized conversion from metal to metal sulfides because Al₂O₃ layer could prevent the diffusion and dissolution of polysulfides.Furthermore,Al₂O₃/AlF₃ layer could also prevent the growth and aggregation of Ni₃S₄ nanoparticles to keep the structure of electrodes during cycling process.The as-prepared Ni₃S₄@Al₂O₃ exhibited a high reversible capacity of 651 mAh g^-1 at 500mA g^-1 even after 400 cycles.Meanwhile,this method could also be extended to synthetic CoS_x@Al₂O₃ with improving electrochemical performances about 42%compared with CoS_x,which indicated this method could also be extended to other metal sulfides with improving electrochemical performances.