Nanocellulose/Solvent Assisted Liquid Phase Exfoliation Of Graphene-Based Composites And Their Electro-Chemical Performance Study

To facilitate the commercialization of sodium-ion battery(SIB),a key challenge is to explore inexpensive and high-performance electrode materials,especially the development of anode materials with high reversible capacity and good rate performance.Similarly,carbon materials are hopeful anode materials for SIBs,albeit the significant differences in selection criteria between SIB and LIB because of inherent differences in ion charge carriers.Graphene would be an excellent SIB anode candidate due to its success in various kinds of batteries.Currently,a variety of preparation methods have emerged,but fewer methods combine both low cost and high-quality graphene sheets,which affects the development of graphene.In contrast,liquid-phase exfoliation(LPE)is an inexpensive,facile and potentially scalable method to produce less-defected graphene sheets.In the present thesis,we developed an improved,dispersant-assisted LPE method to produce graphene-based composites materials from raw graphite with high yield and better quality for SIB anode,and investigated their electrochemical properties.(1)Bacterial cellulose(BC)was used as a green dispersant/stabilizer and antistacking "spacer" to prepare graphene sheets from pristine graphite powder(G)by liquid phase exfoliation(LPE),and the composite material with BC as carbon precursor was obtained by pyrolysis.The atomic force microscopy and transmission electron microscopy showed that the thickness of the graphene sheet obtained by exfoliation was 3.04 nm and the sheet structure had fewer defects.Further,it was found that the carbonized BC(CBC)/LPE graphene(LEGr)presented improved electrochemical performance compared to composite with graphene prepared by Hummers method.It exhibited a reversible capacity of 233 mAh g-1 at a current density of 20 mA g-1,and 157 mA g-1 after 200 cycles at a high current density of 100 mA g-1 with capacity retention rate of 87.73%,the reversible capacity of CBC/LEGr was consistently higher than that of the graphene-based composites prepared by Hummers method.(2)Graphene sheets were prepared by exfoliating from expanded graphite(EG)with the cellulose nanocrystals(CNC)through the high shear(GS)external force,and then the concentration of graphene dispersion was calculated by UVVis spectroscopy to be 0.1790 mg/mL,which further improved the exfoliation efficiency effectively.Subsequently,the CNC were pyrolyzed at 800℃ and 1000℃with a heating rate of 5 ℃/min as the precursor,and the cCNC1 and cCNC2 samples were obtained after holding for 1 h,respectively;the CNC were pyrolyzed at 1000℃ with a heating rate of 2℃/min,and the cCNC3 samples were obtained after holding for 2 h.Following the above reaction conditions,cCNC/GSGr1,cCNC/GSGr2,cCNC/GSGr3 and cCNC/GSGr-I composites can be obtained respectively.cCNC/GSGr’s unique mesoporous structure is conducive to provide electron transfer paths and electrode conductivity.The electrochemical performance of CNC/GSGr-I is slightly better than the other three samples,and the CNC/GSGr-I electrode has a reversible specific capacity up to 149 mAh g-1 at a current density of 20 mA g-1 and still reach 77 mAh g-1(0.1 A g-1)after 200 cycles,showing a better rate performance and cycling stability.Meanwhile,cCNC/GSGr1 as the negative electrode of sodium-ion capacitor,the current density increased from 50 to 2000 mA g-1(5 cycles respectively),and when it returned to 50 mA g-1 again,its discharge specific capacity recovered to 43 mAh g-1 after gradually decreasing from the original 48 mAh g-1,which shows that cCNC/GSGrl has excellent rate performance.This further indicates that the cCNC/GSGr electrode obtained by CNC-assisted shear exfoliation has better rate performance compared with cCNC.