Preparation Of (Ni,Mn,Co) Metal Sulfides (Oxides) For Electrochemical Energy Storage

Metal sulfides/oxides,especially containing Ni,Mn and Co elements,have emerged as prominent candidates for electrode materials for high-performance supercapacitors because of their multiple valence states and pseudocapacitive characteristics.However,metal sulfides/oxides show unsatisfactory rate performance and cycling stability compared with carbon-based materials.Nanoporous/hollow electrode materials possess the features of abundant active sites,high porosity and large surface area,eventually endowing them with prominent advantages in the specific capacitance,energy density,rate performance and cycle life.Templating methods,which include hard templating methods and sacrificial templating methods,have been considered as the most common synthetic strategy to design porous/hollow nanostructures.Therefore,we develop loose/hollow nanostructured(Ni,Mn,Co)metal sulfides/oxides and explore the preparation technology of controllable microstructures to significantly improve the energy-storage performance of sulfide/oxide-based capacitors.The research is generalized as follows:(1)Natural holloysite nanotube(HNT)possesses excellent cation/anion exchange capacity and abundant hydroxyl groups lining the surface,which is highly possible to improve the ion diffusion rate and the rate capability of metal sulfides.Herein,we prepare core-shell NiS2@NiMn/HNT hollow nanotubes with the sulfidation of carboxylic acid functionalized HNT,as the hard template,thickly wrapped by Ni-Mn oxides nanoflakes(NM/HNT-2).HNT effectively restrains neighboring nanosheets from aggregating,thus enhancing the electrochemical utilization of the active material and ions/electrons transport.Benefiting from its large surface area,good electrochemical activity and convenient ion diffusion channel,hollow NiS2@NiMn/HNT shows much improved electrochemical performance including specific capacitance(2609.98 F/g)and cycling stability(92.6%,2000 cycles)compared with Ni-Mn oxides,NM/HNT-2 and Ni-Mn sulfides.(2)By using the high-performance NiS2@NiMn/HNT respectively as positive and negative material,we further assemble the aqueous and solid-state symmetric supercapacitors,which both deliver excellent energy density and power density at the current density of 0.7 A/g,laying the foundation of hollow NiS2@NiMn/HNT architecture application.Compared with the aqueous symmetric device,the solid-state device using PVA/KOH gel as solid electrolyte exhibits better rate capability,higher specific capacitance(107.5 F/g),energy density(38.2 Wh/kg)and power density(16 kW/kg)at the current density of 10 A/g.(3)The porous/hollow nanostructured metal sulfides(oxides)synthesized using sacrificial templates have versatile features,such as complex interiors,fast ion transport pathways and rich redox reaction.Co(NO3)2·6H2O and 2-methylimidazole are used to synthesize wall-like solid functional complexes[Co(2MI)]as the sacrificial template.After reaction with thiourea solution,Co(2MI)is completely transformed into porous cobalt sulfide composed of loose nanosheets.Such construction could improve the contact between the material and electrolyte,provide effective electrolyte penetration,and reduce ion diffusion path.As-obtained cobalt sulfide reaches a high specific capacitance of 680 F/g at a current density of 1 A/g and keeps approximately 98%capacitance retention after 12000 cycles.At the same time,we also successfully synthesize porous hexagonal NiCo2O4 nanoplates with the remarkable cycle stability through a facile hydrothermal process and subsequent calcination.