Preparation Of Nickel-cobalt Bimetallic Compounds Applied In Lithium/Sodium Ion Batteries

Attention to the nervous fossil energy and pollution afterwards,secondary batteries have gradually become the main force in the energy field.Among them,a kind of high-energy battery（lithium-ion batteries）with no memory effect comes out.Sodium ion battery develops rapidly with the advantages of abundant element reserves and high economic benefits.However,existing commercial lithium/sodium ion battery anode materials（hard carbon,graphite）are limited by their poor capacity,which is difficult to meet the development,resulting a large number of studies focused on different anode materials.Nickel and cobalt as transition metals,the components and valence of its oxide/sulfide are rich,and the chemical reactions are more complex,which shows apparent benefits on energy storage.In addition,compared with mono-metal compounds,bimetallic compounds have a positive synergistic effect between the two metal ions and have higher redox activity,which has become a research hotspot in anode materials.However,as anode materials,nickel-cobalt bimetallic compounds face a low conductivity and apparent volume changing.Consequently,nickel-cobalt bimetal compounds in different structural morphologies were synthesized by hydrothermal and calcination methods,and to construct graphene composites.（1）The urchin-like precursor basic nickel cobalt carbonate and bulk NCSG composites were prepared by a two-step hydrothermal method,respectively.Among them,the bulk NCS with a diameter of about 500 nm is uniformly coated by rGO,which improves the structural stability of the nickel-cobalt sulfide during cycling.730 m Ah g^-1of NCSG implies a nice performance,while the sodium storage capacity shows a continuous decay.Considering the problems of graphene stacking and dispersion,PDDA was introduced as an active agent in the optimization step,and the intermediate product Ni₃S₄/Co₃S₄-PDDA-GO composite was prepared by electrostatic adsorption.Afterwards,PDDA-NCSG were prepared by hydrothermal reduction at 180℃.The above synthetic route makes the final product maintain a more regular nanostructure with a diameter of about 50-200 nm;high temperature hydrothermal improves the reduction degree of graphene,builds a better dispersed wrinkled graphene conductive network,and further improves the electrochemical performance.In Li/Na-ion half-cells,PDDA-NCSG exhibits a reversible specific capacity of 1026/550 m Ah g^-1after 100 cycles(0.1 A g^-1),with an excellent Li/Na storage performance.（2）Considered to the volume expansion,urchin-like basic nickel-cobalt carbonate precursor with a hollow structure was designed and prepared by a solvothermal method.NCO was prepared by calcination under air atmosphere,which successfully maintained the morphological structure of the precursor while generating a thin-walled porous structure that could participate in the reaction in a larger area and be more resistant to volume changes.At the same time,the electrochemical performance of NCO at different calcination temperatures was explored,and it can be found that when the calcination temperature was 550℃,the best electrochemical performance(1570 m Ah g^-1)can be observed.However,even no capacity at 1 A g^-1shows necessity to introduce thermally expanded graphene.The precursor and graphene were composited by hydrothermal method,and finally calcined at 350℃ to obtain the hollow urchin-like NCOG.When graphene was added at a concentration of 2 mg L^-1,the performance that 1845 and 1248 m Ah g^-1at different conditions(0.1 A g^-1100 cycles;1 A g^-1400 cycles)is the best.（3）To make full use of the special structure of the precursor,the hollow urchin-like precursor（raw material）prepared in the second chapter and Na₂S·9H₂O（sulfur source）are used to hydrothermally prepare s-NCSG.The diameter of s-NCSG is 2-6μm,and the structure consists of the self-assembly of the outer hollow tube and the inner sphere of hexagonal nanosheets.However,due to the micron size of the material,Co Ni₂S₄are not well dispersed in graphene,and the two are prone to agglomeration.The material can achieve a specific charge capacity of 738 m Ah g-1 after 100 cycles at 0.1A g^-1.However,s-NCSG exhibits capacity fading in NIBs,considering that the larger size of Na ions is more likely to lead to pulverization of the material.As a result,it is considered to improve the structural stability of the material by nano-ization and the introduction of oxygen-sulfur dual elements.Hollow urchin-like precursor is sequentially calcined at high temperature and hydrothermally vulcanized to obtain NCOSG.The particle size is about50～100nm,so the dispersion in graphene is more uniform.When graphene was added at a concentration of 2 mg L^-1,the material had the best performance,with a lithium storage capacity of 1117 m Ah g^-1after 100 cycles,respectively.In terms of sodium storage,the capacity after 100 cycles is 470m Ah g^-1,and the previous sharp decrease in capacity did not occur.The nanoscale of the material and the oxides in it as a supporting structure endow the NCOSG with excellent stability.