Controllable Synthesis Of Doped Graphene With High Electrochemical Performance

In the new electrochemical energy storage system,lithium ion capacitor?LIC?have been used as a new energy storage device of lithium ion battery and electric double layer supercapacitor"internal hybrid",due to its high energy density,high power density,wide operating voltage range,high safety factor,etc.At the same time,lithium-sulfur battery?LSB?composed of elemental sulfur cathode and lithium anode has obvious advantages of high theoretical specific energy,low price of elemental sulfur,high abundance,etc.In recent year,numerous researchs and developments about LIC and LSB have been employed to meeting the demands for portable electronic equipment,green new energy vehicles,smart grid and other fields.At present,the main problem of LIC is the capacities and dynamic rates mismatch of positive and negative electrode,which seriously affects the rate performance and cycle performance.To solve the main problem,carbon materials used as LIC's negative,and activated carbon as LIC's positive is the most widely studied.Due to the poor conductivity of elemental sulfur and well-kown shuttle effects of polysulfides,LSB suffers polarization and self-discharge phenomenas,which will cause poor rate and cycle performance.To address these issues,a lot of efforts have been devoted to developing sulfur host carbon materials and modified separator with carbon materials,polymers and inorganic.Among them,N-doped carbon materials have been extensively studied,while there are few reports on the coordination effects of intrinsic graphene as a host material and S-doped graphene modified separator.In order to solve the above mentioned problems,we rationally design the porous graphene with high specific surface areas,abundant pore structures,numerous defect active sites and excellent conductivity.N-doped?NMG?and S-doped?SMG?mesoporous graphene with abundant defect sites are prepared by chemical vapor deposition method,using NH₃and Mg SO₄as N and S sources,and applied to LIC and LSB,respectively.?1?NMG is used as the anode and cathode electrodes of LIC.NMG endowing abundant pore structures,high specific surface area and the defect active sites origned from the incorporation of N element into carbon lattice can effectively improve the capacitance energy storage capacity and solve the capacity mismatch of lithium ion capacitors.NMG//NMG LIC exhibits maximum energy density of 128 Wh kg^-1and power density of 10,000 W kg^-1.The capacity retention reachs up to 98.3%after 4000cycles at 3 A g^-1.The regular mesoporous structure combinated with the doped-N are conducive tor the rapid diffusion of electrolyte ions and quick charge transfer,which are significantly contributing to the excellent rate capability,high capacitance,and long cyclability.The cyclability is superior to the previously reported values of AC-based or high-rate porous carbon-based LICs.?2?Severing as the sulfur host material,mesoporous graphene?MG?endows high specific surface area and abundant pore structures,which can effectively suppress the volume expansion effect of LSB.However,the non-polar of MG has poor adsorption effect for polysulfides only via the physical interaction,which leads to poor rate and cycling performances.We adopt a method of functional modified separator with SMG.Severing as the separator material,SMG possess light wheight density,high specifical surface areas,abundant micro-mesorporous volumes and enhanced thiophil polar,which can offer abundant physical-chemical adsorption active sites and reduced interface contact resistance to suppress the shuttle effects of polysulfides and enhance the electrochemical performances.The LSB cell composed of MG host material and SMG modifiled separator exhibits initial discharge specific capacityof 955.64 m Ah g^-1at 1 C,capacity retention of 0.109%per cycle for 300 cycles in the charge and discharge process.These results show that MG and SMG play a very good synergistic effect and effectively suppressing the shuttle effects of lithium polysulfides lead to the enhanced rate and cycling prformances in LSB.