Preparation And Mechanism Investigation Of Nickel-based Oxide/Hydroxide Electrode Materials For Supercapacitors

Nickel-based oxides/hydroxides,as hybrid supercapacitor electrode materials,have the advantages of high power density,high energy density,wide temperature range and so on,which play an important role in energy storage and other fields.However,the low electrical conductivity and poor cyclic stability caused by the phase transition from?-Ni(OH)₂/?-Ni OOH to?-Ni(OH)₂/?-Ni OOH severely limit their development.In order to improve the electrical conductivity,suppress the adverse phase transition and improve the cycling performance,the researches on nickel based oxide/hydroxide electrode materials have mainly focused on the regulation of morphology,structure and chemical composition.And there are generally two effective methods,as following:(1)compositing with carbon materials to improve electron conductivity;(2)doping high-valence transition metal to stabilize structure of Ni O_x slabs.However,there still exist the corresponding problems.It is difficult to effectively increase the contact area between active material and carbon as well as maintain the structural integrity of electrode material.Besides,the local structure around doped sites,reaction mechanism,structural evolution process and changes of dynamics properties after doping metal ions during cycles remain unclear.Herein,the specific research works carried out include:Ni²⁺ions react with small ligand-terephthalic acid(PTA),and formed Ni-based metal-organic framework compound(Ni-MOF).Ni-MOF was dispersed on the GO carrier by electrostatic interaction of their superficial functional groups.The particle size of Ni O was reduced to 2-3 nm,which can effectively improve the Faraday capacity(496 C g^-1 at the current density of 1 A g^-1)and the cycling stability(capacity retention of 82%after 3000 cycles at 5 A g^-1)of Ni O/C/r GO.Doping Mn ions(Ni Mn-LDH)at the 3a site of the?-Ni(OH)₂ with low content(about6%)can cause the presence of Mn⁴⁺and increase of Ni²⁺content,which can effectively reduce the Jahn-Teller distortion caused by Ni³⁺and stabilize Ni O₂ slabs.In addition,the modified electronic configuration after Mn-doping induces local contraction of metal-O/metal bond length and increases curve degree within ab planes,which further introduces special structure between layers.The synergistic effect of low distortion within the Ni O₂ slabs and the special layer structure between Ni O₂ layers caused by the suitable doping level and occupied sites of Mn atoms can suppress the transformation from?-Ni(OH)₂/?-Ni OOH to?-Ni(OH)₂/?-Ni OOH and stabilize the structure of Ni O₂slabs.In final,the suitable-dose Mn doped Ni Mn-LDH exhibits high capacity(1498 C g^-1 at 2 A g^-1),excellent rate capability(specific capacity at 50 A g^-1 can retain 61%of that at 2 A g^-1)and superior cycling performance(almost 96.3%capacity retention after30,000 cycles at 50 A g^-1).With a synergistic combination of theoretical calculations and various structural probes including XRD and ²H MAS NMR,we unveil the irreversible oxidization of Mn³⁺at the initial cyclic voltammetry cycles,which will remain as Mn⁴⁺in the Ni O₂slabs after the first oxidization to effectively suppress the phase transformation from?-Ni(OH)₂/?-Ni OOH to?-Ni(OH)₂/?-Ni OOH and further maintain the structural integrity of electrode material.In addition,we also decode the structure evolution and dynamics in the initial redox reaction cycles,and it can be found as following:the nearby anions containing NO₃^-and CO₃^2-as well as water molecules would move into the interlayers within a few cycles;after tens of cycles,NO₃^-and CO₃^2-may move away into the electrolyte,and other anions,such as,OH^-would move into interlayers for charge compensation;during the whole process,the movement of anions and water molecules has a great influence on the layer stacking sequence,resulting in the change of stacking disorders at any time.In addition,the electrical conductivity is enhanced after tens of cycles,because of the increase of Ni³⁺number.The particle size of the Ni Mn-LDH material on the surface of ppy-C material has been significantly reduced,and the structural integrity of the electrode material was increased through in situ growth-reduction-oxidation by adjusting crystal orientation of Ni metal materials.Among them,pyrolysis of Ni Mn-LDH/ppy-C in H₂/Ar atmosphere and electrochemical oxidation reaction limit the height of the Ni Mn-LDH nanosheets and make them contact closely with the conductive network to improve the electrical conductivity.In addition,the(111)crystal plane of the Ni metal can easily lose electrons and be hydroxylated.And the hierarchical structure including Ni Mn/Ni Mn-LDH heterostructure and conductive substrate ppy-C is helpful to disperse active materials,suppress phase change,maintain the integrity of the electrode material,and further greatly improve cyclic stability.Furthermore,the morphology and structural evolution of the electrode materials during the cycles were analyzed by various characterization methods,and the phase transformation process was proposed as following:the surface of Ni Mn particles was gradually oxidized to Ni Mn-LDH,and the coexistence of Ni²⁺,Mn³⁺and Mn⁴⁺in the Ni Mn/Ni Mn-LDH heterostructure appeared in the structural distortion region.