Crystallization-induced Morphology Tuning Of Two-dimensional Carbon Materials And The Electrochemical Energy Storage Performance

Two-dimensional carbon materials(2D CM)have the potential to be used as the anode materials of lithium-ion batteries(LIBs)and sodium-ion batteries(SIBs).The structure and morphology of 2D CM greatly influences the electrochemical performance of electrochemical devices with CM anode.So exploring both available fabrication methods and the way to design the morphology of 2D CM is very important.Salt template method is a green and simple route to prepare 2D CM.But some problems exist in this route.Most of the 2D CM prepared by salt template method possess non-uniform morphologies,and the thickness of these samples is quite large.Moreover,the growth mechanism of salt crystals and the reaction mechanism between crystals and carbon precursor to form 2D CM are still lacking.In this work,2D CM with different morphologies were fabricated based on the design of crystals growth process of neutral salts and ice crystals.The component variation of crystals system,the growth process of crystals,the matching of carbon precursor and crystals template were systematically investigated.The relationship between the structure and the electrochemical performance of samples was studied.The mechanism of capacity enhancement of porous carbon anode in LIBs during cycling was also investigated.(1)Slow growth of salt crystals in F127-sodium chloride solution system was used to prepare carbon nanosheets(CNS).The size of CNS is 1-5 μm and the thickness is 6 nm.Both abundant defects and expanded few-layer-stacked nanocrystallites with larger interlayer distances(0.43 nm)exist in CNS.Sodium chloride(NaCl)crystals can grow as micron-sized crystals with cubic structure by using drying with distillation.These crystals were coated with carbon precursor F127 During carbonization,CNS were fabricated by the salt surface-assisted"bottom-up" route.Apart from NaCl,potassium chloride can also be used as the salt template to prepare CNS.So it is a general strategy to fabricate 2D CM with F127 as the carbon precursor by controlling the slow growth of salt crystals.When CNS were used as anode materials of LIBs,the reversible capacity is 830 mAh g-1 at 1 A g-1 after 500 cycles.When used as anode materials of SIBs,the initial reversible capacity is 264.4 mAh g-1 at 1 A g-1,and the reversible capacity can maintain at 190 mAh g-1 after 500 cycles.The good lithium and sodium storage performance of CNS is ascribed to the synergistic effect of defects and expanded graphene layers of CNS.The defects can offer an amount of reaction sites for ions storage.The expanded graphene layers can greatly improve the ion diffusion kinetics.(2)Rapid growth of NaCl crystals in F127-rhodanine-NaCl solution system was used to fabricate N,S co-doped nanobubble-linked graphene network(NSBGN)and N,S co-doped graphene nanosheets(NSGNS).By controlling the freezing temperature and components of solutes,the growth of NaCl crystals can be well designed to get salt crystals scaffold with various morphologies..NaCl crystals grow as nanoscale spheroidal at-55℃ and polyhedral crystals at-25℃.Then these NaCl crystals work as template to induce the morphology variation of 2D CM.The diameter of nanobubbles of NSBGN is ca.500 nm.NSGNS stack with each other.When used as LIBs anode materials,NSBNG electrode shows the reversible capacities of 1456,784,654 mAh g-1 at 0.05,0.5 and 1 A g-1,respectively.NSGNS electrode shows the reversible capacities of 1439,625,497 mAh g-1 at 0.05,0.5 and 1 A g-1,respectively.NSBGN shows better rate performance than NSGNS,which is ascribed to that the graphene network connected with nanobubbles can benefit the ion transport and the charge transfer.(3)The dual-crystallization mechanism of ice and KCl crystals in F127-KCl solution was systematically investigated.By controlling the dual-crystallization process,the morphology of freeze-dried scaffold can be well designed.Then these scaffolds with various morphologies were used to fabricate graphene nanosheets with different morphologies.The freeze-dried scaffold changes from fractal structure with with little bumps and dips,sheet-like structure with orderly stepped surface,to brick-like structure,with increasing KCl concentration.When the concentration of F127 is increased,the freeze-dried scaffold changes into fractal structure.Denim-like graphene nanosheets with ordered stripes was fabricated by annealing the stepped sheet-like scaffold.The physical enrichment of F127 at the corners of the stepped KCl crystals result in the higher thickness of carbon at the corner than that at the plane of stepped KCl after carbonization.Then the graphene nanosheets with stripes can be produced.When used as anode materials of LIBs,the denim-like graphene nanosheets possess the capacity of 1025 mAh g-1 at 1 A g-1 after 600 cycles.(4)To investigate the mechanism of capacity enhancement of carbon anode(especially porous carbon anode)of LIBs during cycling,we prepared porous carbon materials by template method.The coexistence of blind pores and closed pores in porous carbon is demonstrated.The porous carbon anode shows the reversible capacity of 700 mAh g-1 at 50 mA g-1 after 40 cycles.The reversible capacity increases to 1300 mAh g-1 after 110 cycles.The enhancement of Li+diffusion into the pores and defects of inner part can lead to the capacity enhancement of carbon anode during cycling.