Research On Controllable Preparation Of Three-Dimensional (3D) Graphene-Based Composites And Their Potassium Storage Performance

Potassium-ion batteries are regarded as a complementary technology for lithium-ion batteries in the field of large-scale energy storage due to their low cost and other advantages and have received close attention from researchers in recent years.Graphene-based materials are regarded as excellent candidates for anode materials of potassium-ion batteries due to their advantages such as high specific surface area and high electronic conductivity.However,the van der Waals forces existing in graphene sheets can cause irreversible agglomeration and stacking of them,resulting in low reversible specific capacity and poor cycling stability.3D graphene-based materials can theoretically not only suppress the agglomeration and stacking of graphene sheets,but also promote the infiltration of electrolytes,ion/electron transfer,and increase the active sites for electrochemical reactions.Nevertheless,the construction of 3D graphene-based materials is still limited by the uncontrollability and complexity of assembling process,which makes the obtained 3D graphene-based materials difficult to meet practical requirements.In response to the above problems,this paper combines the chemical blowing and chemical vapor deposition(CVD)to realize the controllable preparation of 3D graphenebased materials and further utilizes the experiments and theoretical calculations to study the electrochemical potassium storage properties and underlying mechanisms of as-prepared materials.Specific contents are as follows.(1)The controllable 3D assembly of 2D graphene sheets is achieved by adjusting the parameters of chemical blowing(mass ratio of raw materials,annealing temperature,heating rate,etc.)with ferric nitrate nonahydrate(Fe(NO3)3·9H2O)and polyvinylpyrrolidone(PVP,K30)as raw materials,then the honeycomb-like 3D nitrogen-doped few-layer graphene framework decorated with Fe3C@C core-shell nanoparticles is obtained.The catalytic graphitization effect of Fe3C nanoparticles plays an important role in the formation of 3D graphene framework.When the as-prepared material is used as anode for potassium ion battery,it delivers a high reversible capacity of 195 mAh g-1 at 1000 mA g-1 and remains at 155 mAh g-1 after 10000 cycles.The high specific capacity exhibited by the anode material can be attributed to the large specific surface area of 3D graphene framework and the reversible formation of SEI catalyzed by Fe3C nanoparticles.The excellent rate performance is related to the three-dimensional conductive network and pseudocapacitive behavior of nitrogen-doped three-dimensional graphene.The outstanding cycle life is derived from the excellent structural stability of three-dimensional graphene.(2)The 3D sulfur,nitrogen-doped few-layer graphene framework decorated with FeS2@C core-shell nanoparticles is synthesized by CVD sulfidation of the above 3D graphene framework decorated with Fe3C@C.When the as-prepared material is used as anode for potassium ion battery,the experimental results show that the 3D graphene framework can confine FeS2 nanoparticles and buffer their volume expansion during charge/discharge process.Moreover,when using 1 M KFSI in DME as the electrolyte and increasing the discharge cut-off voltage,the asprepared anode material exhibits significantly improved cycling stability.Experimental and theoretical studies show that the FeS2 transforms into conductive phase KxFeS2 after first cycle,which promotes the conductivity of electrons in the bulk phase of nanoparticles,benefiting to the high rate performance.Therefore,the half-cells exhibit a reversible specific capacity as high as 269 mAh g-1 at 1000 mA g-1,the specific capacity remains 175 mAh g-1 after 1000 cycles at 2000 mA g-1.(3)The precursor of 3D nitrogen-doped few-layer graphene framework decorated with Fe3C nanoparticles is prepared firstly by tailoring the parameters of chemical blowing,then the novel hierarchical architecture of confining nano-Fe7Ss in carbon nanotubes covalently bonded onto 3D few-layer graphene framework is obtained via two-step CVD with Fe3C nanoparticles as the catalyst,acetylene as the carbon source for the growth of carbon nanotubes,and sublimation sulfur as the sulfur source.The bifunctional catalytic effect of Fe3C nanoparticles on the formation of 3D graphene framework and carbon nanotubes during synthesis is revealed.Benefiting from the hierarchical confinement effect of 0D nano-Fe7S8 to 1D carbon nanotubes and further 1D carbon nanotubes to 3D graphene framework,the excellent 3D conductive network,and the highly reversible potassium storage mechanism of Fe7S8,the assembled half-cells exhibit a high reversible specific capacity of 277 mAh g-1 at 1000 mA g-1 and maintains 167 mAh g-1 after 1300 cycles at 2000 mA g-1.When matched with the Prussian blue analog cathode,the full-cells exhibit an energy density of 120 Wh kg-1(based on the total mass of the active material)at 50 mA g-1 and the specific capacity seems to be no decay after 65 cycles.