Promoting High Supercapacitor Performance Of MoS₂by Chemical Modification And Graphene Supporting

Exploring supercapacitor with high energy density and long cycle life is of great importance for developing portable electronic devices and electric automobiles.As the critical part of the supercapacitor,the electrode material determines its capacity and cycle performance to a great extent.MoS₂,which has a two-dimensional layered structure,can store and release charges from electric double layer,the redox reaction of Mo⁴⁺and the intercalation pseudocapacitance.Moreover,the larger interlayer distance?0.615 nm?of MoS₂is beneficial to the intercalation and deintercalation of electrolyte ions in comparing with that of graphite.All these lead to a high theoretical capacitance of 1200 F g^-1 for MoS₂.Combined with natural abundance and low cost,MoS₂ is considered to be a promising pseudocapacitor electrode material.However,the poor conductivity between the adjacent layers of MoS₂makes the limited capacitance delivery and poor rate performance,hindering its application as a commercial supercapacitor electrode material.In this research,therefore,we developed a new generation of MoS₂ based nanocomposite electrode material with enlarged interlayer distance and improved conductivity through the structural designing,which leads to large capacity,high rate capability and cycle performance.In this work,sandwiched heteroatom incorporated MoS₂/polyaniline?MoS₂/PANI?nanosheets array were studied by adjusting the preparation process and raw materials based on the principle of organic-inorganic hybrid in order to obtain the modified MoS₂ nanocomposite with high capacitive performance.First,hierarchical MoS₂/PANI sandwiched nanosheets?MoS₂/PANI SNSs?were synthesized based on the principle of organic/inorganic hybrid.PANI intercalated into the MoS₂ layers at the molecular level enlarging the interlayer distance of MoS₂ from 0.61 nm to0.93 nm and forming a self-assembled flower-like structure,which not only benefit for the electrolyte ion diffusion,but also restrain the agglomeration of the MoS₂/PANI nanosheets.The improved conductivity between the adjacent layers of MoS₂ make the electrochemical reaction deep into the depths of the MoS₂ interlayer.In addition,the MoS₂ nanosheets restrict the volume change of the PANI chains in the charge/discharge process,solving the problem of poor cyclic stability for PANI.Assembled as symmetric supercapacitors in the two-electrode configuration,the MoS₂/PANI SNSs show 81.3 F g^-11 at 1 A g^-11 in the first cycle,and have a capacitance retention of 86.2%after 8000 cycles.The energy density is 8.56 Wh kg^-1at the power density of 0.2 kW kg^-1.In order to further promote the electrochemical performance of MoS₂ based material,we used reduced graphene oxide as conducting template to construct a novel hierarchical nanosheets that sandwiched MoS₂/PANI nanosheets arrayed vertically align on reduced graphene oxide?MoS₂/PANI/rGO HNSs?,which significantly enhanced the specific surface area and the conductivity of the nanocomposite,and then improve the contact area between electrolyte and electrode material,thus,gave rise to a large pseudocapacitance.Assembled as symmetric supercapacitor,it delivered an energy density of 8.56 Wh kg^-11 at the power density of 0.2 kW kg^-1 and exhibited a capacitance of 97.8 F g^-1 at a current density of 2 A g^-11 at first cycle.After 20000 cycles,84.2%of initial capacitance was retained.When asymmetric supercapacitor was assembled using MoS₂/PANI/rGO-300 HNSs as anode and active carbon?AC?as cathode,it still shows a high capacitance of 73.3 F g^-1 at current density of 2 A g^-1 at first cycle,retains 87.9%after 20,000 cycles.Meanwhile,the energy density is up to 19.5 Wh kg^-1 at the power density of 0.3 kW kg^-1.The interlayer distance is reduced to 0.70 nm with the introducing of rGO in MoS₂/PANI/rGO HNSs.Therefore,it is necessary to further optimize the structure design to enlarge the interlayer distance and then facilitate the diffusion rate of electrolyte ion in the interlayer of MoS₂.In order to further enlarge the interlayer distance of MoS₂ in the MoS₂/PANI/rGO HNSs,we incorporated part of oxygen into MoS₂ nanosheets based on the method of incomplete sulfuration of MoOx at a relative low temperature and obtained oxygen incorporated MoS₂/PANI/r GO nanocomposites?O-MoS₂/PANI/rGO HNSs?.The interlayer distance was enlarged to 0.88 nm with the oxygen incorporation.And a hybrid structure of 1T and 2H phase was formed in MoS₂ nanosheets.Due to the metallic quality of 1T MoS₂,it significantly improve the conductivity of the whole composite.The symmetric cell with O-MoS₂/PANI/rGO-160 as both cathode and anode,has the capacitance of 79.6 F g^-1,100.1 F g^-1 and 122.0 F g^-1 at 2 A g^-1 at 0 ^oC,room temperature?RT?and 50 ^oC,and maintains about89.9%,86.1%and 73.9%of initial capacitance,respectively,after 30,000 cycles.Moreover,when the power density is 0.25 kW kg^-1,the energy densty is up to 22.5 Wh kg^-1 at 25 ^oC.To further explore the electrochimical performance of heteroatom intercalated MoS₂ based nanocomposites,we studied the synthetic process of phosphorus intercalated MoS₂/PANI/rGO HNSs?P-MoS₂/PANI/rGO HNSs?and prepared the P-MoS₂,P-MoS₂/PANI and P-MoS₂/PANI/rGO HNSs.Through the structure characterization and the energy storage mechanism analysis,we found that a hybrid structure of 1T and 2H phase was formed in P-MoS₂ nanosheets,while the phosphorus intercalation led to a disordered stacking layer distance of MoS₂ in P-MoS₂/PANI/rGO HNSs and brought about an enhanced capacitance and rate capability.PANI stabilize the structure of P-MoS₂ and maintain a high capacity in long charge/discharge cycles.While rGO improve the capacitance,rate capability and cyclic stability for P-MoS₂/PANI/rGO HNSs via increasing the specific surface area of the composites.For the asymmetric supercapacitor assembled using P-MoS₂/PANI/r GO HNSs as anode and rGO as cathode,the energy density is up to 56.0 Wh kg^-1 at the power density is0.45 kW kg^-1.When cycled at 5 A g^-1,about 93.5%of initial capacitance could be maintained after 30000 cycles.