Investigation On Design And Preparation Of Cobalt-based Materials And Their Supercapacitor Performance

With the rapid development of new energy technology in the world,the demand for electrochemical energy storage is increasing.Supercapacitors with high power density,long cycle life and wide temperature range have become an electrochemical energy storage device with high research value.With high theoretical capacitance,simple preparation and abundant resources,cobalt-based electrode materials are one of the most promising electrode materials for supercapacitor.However,there are some problems,such as low electrical conductivity and poor cycle life,leading to the limited application.In this thesis,our focus is on the design and preparation of new high-performance cobalt-based electrode materials to further improve their electrochemical performance.The specific scope of this research includes:（1）Ni-Co-O/Ni Co-LDH composite electrode materials were prepared by hydrothermal and annealing processes through a self-templating method.The as-prepared Ni-Co-O/Ni Co-LDH exhibited a chestnut-like nano-needle core-shell structure.Due to its good electrical conductivity and unique core-shell structure,the pseudocapacitive characteristics was significantly improved,resulting in a high specific capacitance of 1434 F·g^-1 at a current density of 1 A·g^-1.When assembled in an asymmetric supercapacitor device with activated carbon（AC）as negative electrode,the device showed an energy density of 26 Wh·kg^-1 at a power density of 807 W·kg^-1.The device also achieved 95%capacity retention after 3600cycles at a current density of 6 A·g^-1.（2）Combining the excellent electrochemical properties of metal sulfide and layered double hydroxide,Co₃S₄@Ni Ga-LDH composites with hierarchically hollow core-shell structure were prepared by a two-step solvothermal method using ZIF-67 as a sacrificial template.The hollow structure is beneficial to shorten the diffusion path of electrolyte,which could increase the close contact between electrolyte ions and electrochemically active sites.Moreover,the structural damage due to volume change during multiple charge and discharge can be prevented becaused of its unique strucure.Therefore,Co₃S₄@Ni Ga-LDH afforded a specific capacitance of 1322 F·g^-1 at a current density of 1 A·g^-1.The assembled Co₃S₄@Ni Ga-LDH//AC asymmetric supercapacitors exhibited high energy density(58.7Wh·kg^-1 at 803.8 W·kg^-1)and power density(39.3 Wh·kg^-1 at 7993 W·kg^-1).After 4,000charge/discharge tests at 6 A·g^-1,84%of the initial capacitance was maintained.（3）K⁺-doped Co Ni-MOFs three-dimensional microspheres were prepared by one-step in situ hydrothermal process.The morphology,crystal and electronic structure of the as-prepared material were adjusted to enhance the electrical conductivity and structural stability.The K⁺-doped Co Ni-MOFs material has a specific capacitance as high as 1453 F·g^-1 at 1A·g^-1.The assembled asymmetric device also has a high energy density(35.88 Wh·kg^-1)at a power density of 797.3 W·kg^-1.The capacitance retention of 86%was obtained after 5000cycles at 6 A·g^-1,showing excellent electrochemical properties of K⁺-doped Co Ni-MOFs.