Design,Preparation And Electrochemical Performance Of Flexible Material With A Wave Structure

The ever-increasing development of flexible electronics,especially those are wearable and adaptive to humans,can bring great changes in our daily lives.And it puts forward greater requirements for power sources,which should have reversible deformation capabilities.In addition to the requirements of high capacity and high stability,the design and preparation of electrodes with high flexibility is the key to the practical use of flexible LIBs,and it is also a major challenge.In this dissertation,we focus on two-dimensional materials with flexibility and diversity,and make effort to introduce elastic structures during the fabrication of graphene-based assembly materials,which can improve the intrinsic flexibility and stability.In this way,the electrodes with high electrochemical performance and high reversible deformability are prepared,and consequently flexible LIBs can be achieved.By forming graphitic carbon nitride(g-C3N4)between graphene sheets,a periodic wave-like structure of graphene assembly was obtained.The wave-like structure of graphene is due to the deformation caused by the formation of g-C3N4,and the use of different ratios of graphene oxide to cyanamide can adjust the in-situ formed wave-like structure.Accordingly,a versatile strategy of forming elastic structures simultaneously during the fabrication process was proposed for 2D materials.A variety of 2D materials,such as graphene oxide,carbon nitride and molybdenum oxide sheets,can be used to form this wave-like structure as designed when they are mixed with cyanamide for the formation of g-C3N4 in combination with liquid-solid assembly methods like vacuumfiltration,freeze-drying or spin coating,revealing the wide applicable range of twodimensional materials.And the wave-like structure endows the 2D materials with unique surface states and stress response characteristics.The 2D material-g-C3N4 materials with a wave-like structure can be used for the preparation of highly flexible materials with different scales and structures.Based on the in-situ formed wave-like structure,a hierarchical reduced graphene oxide(rGO)-carbon nitride foam(HGCNF)was prepared by freeze-drying and heat treatment,which combines the 3D porous structure with the wave-like structure.The wave-like structure with wavelength of 0.08～0.2 μm improves the reversible deformability of walls,and enhances the nonlinear response characteristic of walls,while the connected walls with abundant nodes promote the amplification of deformation.Therefore,the recoverable compression strain of HGCNF reaches>95%,and the compressive strength is as high as 4.6～6.8 MPa at the density of 3.8～5.4 mg/cm3,which is more than 1000 times higher than that of rGO foam,also higher than common foams;and the fatigue resistance is good,with the structures remaining stable after 1000 compression-rebound cycles.In addition,HGCNF maintains its high elastic deformability in a wide temperature range of-100～600℃,and the high conductivity(200 S/m)remains unchanged during the compression process.The HGCNF shows light weight,high elasticity and strength,cyclic stability and wide temperature applicability,suggesting high potential to be used as flexible conductors or highly flexible,stable networks for flexible electrodes.By adding Fe3+,the rGO-g-C3N4-Fe3O4 film with intrinsic wrinkles(wGCNFe3O4)was synthesized,able to improve the deformation capability and alleviate the volume change during charge and discharge of high-capacity Fe3O4.The wGCN-Fe3O4 film was then processed by a pre-strain relaxation method to create a large wave-like pattern(Extended-wGCN-Fe3O4),endowed with a larger deformation ability.Therefore,the wGCN-Fe3O4 film possesses excellent mechanical properties,and its stretchability can reach 12%and its toughness is ten times higher than that of rGO-Fe3O4 film.Moreover,the wGCN-Fe3O4 film has a high specific capacity and stable chargedischarge performance.At a current density of 922 mA/g for 500 cycles,the discharge capacity of the wGCN-Fe3O4 maintains～780 mAh/g with a capacity retention of>99.5%;at 9.22 A/g(fast charge and discharge),it still delivers a high capacity of 560 mAh/g.The extended wave-like structure can accommodate large deformation induced by applied loads,through folding and unfolding of its pattern.The ExtenedwGCN-Fe3O4 film processed by 100%pre-strain has a reversible tensile strain exceeding 105%,with no structure damages after repeated stretches.Using the Extended-wGCN-Fe3O4 as anode and a LiFePO4-carbon nanotube film as cathode,a thin and flexible lithium-ion battery was assembled with PDMS as the packaging material,which can work normally at bent and stretched state.The utilization of deformed structures at different characteristic scales has universality for preparing flexible electrodes,and it can be used to achieve a variety of electrodes with high electrochemical performance and high flexibility.