Efficient And Precise Fabricating By 3D Active Coating Of Graphene-based Fibers For Flexible Energy Storage

Flexible and wearable electronics are expected to be leading the next generation of electronics like electronic textiles,smart garments,bio-signal monitors and implantable medical devices.To meet these applications,flexible energy storage devices are urgently required,with the critical step of fabricating flexible electrodes.Taking the advantages of tiny volume,flexibility and stitchability,fibrous electrode becomes one of the most competitive candidates.Graphene-based fiber（GBF）,which holds the excellent mechanical and electrical properties of graphene and fibrous structure,providing a feasible way to manufacture wearable supercapacitors and flexible batteries.To fulfill the challenges of low efficiency,easy fracture and adhesion,poor continuity and uniformity of wet spinning method,a controllable and efficient strategy based on3D active coating technology is proposed,in which the achieved GBFs show reasonable performances of flexible energy storage.In addition,the concept of graphene“integration”graphene oxide is proposed with expecting optimize the structure and peformances of GBFs.Finally,GBFs composed of graphene and Mn O₂ are fabricated by the universal 3D active coating strategy,enable the fibrous electrodes robust,conductive and high loading,comparing to GBFs with entire of graphene or graphene oxide with low tensile strength and energy density.This work constructs a bridge between materials and fibrous electrodes,expecting to promote the design and manufacture of fiber-shaped energy storage devices for flexible wearable electronics.The main contents and conclusions are as follows:（1）A 3D active coating technology is proposed,boosting the production rate up to 771.4 m h^-1 by separating the process of spinning and solidification,that is the graphene oxide dispersion using as precursor is loaded into a syringe and injected onto a rotating roller while stepping,forming a gelatinous graphene oxide fiber,and then immersed into ethanol with the roller for solvent exchanging to solidify.The unilateral support of the roller during the solvent exchange process was also found help to compact the layer structure of GBFs,making the hollow GBFs with a high tensile strength of 190.5 MPa with 6.1%elongation strain and a good durability for bending over 5000 cycles.More importantly,the achieved GBFs are found to be an Archimedean-type spiral hollow structure,with the formation mechanism of the counteracting force of evaporation with a parabolic cross section,inducing to curl from both ends to the center.The demonstrative solid-state supercapacitor exhibited a high specific capacitance of 170.6 F g^-1.A series of this kind of GBFs with different diameters can be achieved efficiently by adjusting the key process parameters of injecting rate and rotating rate.With the specific structure and reasonable performances,it holds great potential application to flexible and wearable electronics.（2）The ratio of carbon/oxygen within the fabricated GBFs was improved by using the integration of exfoliated graphene in graphene oxide with different mass ratio as precursor.It was found that graphene is a“bulking agent”,decreasing the compactness of GBFs,which leads to a decreasing of conductivity with an additive amount less than 30%till to the prominence of carbon/oxygen at addition of 50%(2.05×10⁴ S m^-1),implying the positively correlated with compactness and carbon/oxygen value to the conductivity of GBFs.Meanwhile,the mechanical strength of GBFs decreased with the decreasing of compactness,showing a tensile strength of 57.6 MPa with 6.6%elongation strain by adding 50%exfoliated graphene.It is hard to achieve hollow structure regularly due to the high speed stirring and high loading of graphene,reducing the specific capacitance of GBFs(66.6 F g^-1@50 m A g^-1 for additing 50%graphene).It may be attributed to the morphology of exfoliated graphene with few defects and low specific surface area.（3）Three types of graphene（exfoliated graphene（EG）,3D graphene（3DG）and porous graphene（PG））were employed to integrate graphene oxide with the mass ratio of 1:1 as precursor to fabricate GBFs via 3D active coating technology repectively.It was found that the structure and performances of GBFs were significantly affected by the morphology of the additive graphene,in which the cross section showed a lamellar structure of GBFs@EG while it seems fluffy of GBFs@3DG and GBFs@PG,leading a more compact for the former than the latter two.The diameters of the GBFs were around 135,186 and 210μm respectively,indicating the positively correlated between the specific surface area of graphene and the diameter of GBFs.It shows high mass specific capacitance of 235.9 and 181.2 F g^-1 at the current density of 25 m A g^-1for GBFs@PG and GBFs@3DG respectively with porous structure,which is 2.9 and 2.3 times higher than that of GBFs@EG with the same condition.The porous structure is found help to energy storage,but reduce the mechanical strength,volumetric energy density and power density simultaneously.（4）Robust,conductive and high loading fibrous electrodes fabricating by 3D active coating strategy were reported,that is the graphene composite electrode paste within a stepping syringe was injected onto a rotating conductive wire synchronously to achieve a fiber-shaped electrode,distinguishing from the conventional direct-write 3D printing technology without current collector.The root cause of the capability of achieving high mass loading by this method is proven to be the active transfer process.The relationship between the three key rate parameters of injecting,stepping and rotating has been clarified with a set of derived equations to efficiently promote and accurately monitor the fabrication.The demonstrated fibrous Zn-Mn O₂ battery with high loading of 14.9 mg cm^-2shows reasonable energy density of 108 m Wh cm^-3,can lighten 34 led lights and a 3 W bulb by two batteries in series,and power a watch by single battery for about 120 days,and also the fabricated porous graphene supercapacitor exhibits high capacitance of 142.9 F g^-1,cyclic stability to retain over 84%capacity after3000 cycles and good durability to retain over 90%capacity for bending 10000cycles.This method constructs a bridge between materials and fiber-shaped devices to facilitate the design and manufacture of flexible power sources for portable and wearable electronics.