3D Printing Of Flexible Electrodes For Lithium-ion Batteries And Their Electrochemical Properties

Recent years have seen a boom in wearable and compositive electrics,so there is growing demand for the quantity and quality of flexible or micro power source devices.In order to adapt to this situation,we need to design and develop flexible energy storage devices with light-weight,small size,deformation,high-efficiency fabrication and low-cost.Three-dimensional(3D)printing has attracted broad interest due to its high accuracy,low cost,high manufacturing efficiency and environmental friendliness.As a cutting edge technology,it has been widely used in energy,biotechnology,electronics and engineering composite materials.Among various kinds of 3D fabrication technologies,extrusion-based 3D printing method is the most facile and efficient one,so it is extraordinarily suitable for fabricating flexible and micro electrodes of lithium-ion batteries.In extrusion-based 3D printing,ink properties are directly related to material preparation,printing strategy,and architectural design.To develop a good printable ink,it is important to endow materials with high viscosity and shear-thinning behavior to facilitate 3D printing.Here we develop a printable ink with high concentrated polyvinylidene fluoride(PVDF)solution mixed with carbon nanotube(CNT),and print highly conductive and strong PVDF/CNT composite fiber via 3D printing method.CNTs were successfully dispersed in PVDF solution resulting in a mixture with an obvious shear-thinning property.Noted that its apparent viscosity reaches over 103 Pa·s and storage modulus reaches over 102 Pa.The well aligned composite fiber can be obtained under shear stress,which help improve the interaction and percolation between these two building blocks,leading to a combination of high mechanical strength(247±5 MPa)and electrical conductivity(216.7±10 S cm-1).Either lithium iron phosphate(LFP)or lithium titanium oxide(LTO)was added as cathode or anode active materials in the PVDF/CNT ink above to form new ink with better printability.The same method was used to fabricate fiber-shaped electrodes with both inks.Both fiber electrodes demonstrate good flexibility and high electrochemical performance in half-cell configurations.All-fiber lithium-ion battery can be successfully assembled by twisting the as-printed LFP and LTO fibers together with gel polymer as the quasi-solid electrolyte and separator.The all-fiber device exhibits a high specific capacity of ~110 m Ah g-1 at a current density of 50 m A g-1 and maintains a good flexibility of the fiber electrodes,which can be potentially integrated into textile fabrics for future wearable electronicapplications.By improving the total solid mass ratio of the PVDF/LFP/CNT(WPVDF: WLFP:WCNT=10:6:3)and PVDF/LTO/CNT(WPVDF: WLFP: WCNT=10:6:3)inks to 300 mg m L-1,inks with better viscoelasticity were obtained.Flexible fabric electrodes for lithium ion batteries can be printed directly with these inks,without coagulation bath.Noted that,these printed fabric electrodes possess high specific capacity(higher than 155 m Ah g-1 at a current density of 50 m A g-1)and cycling stability,which demonstrates this strategy is able to balance the electrochemical performance and flexibility,leading to a promising direction for wearable electronics applications.In order to accept the high mass ratio of electrode active materials,simpler drying processing and lower venomous manufacturing,PVDF in above ink was replaced with the water soluble graphene oxide(GO)as viscosity modifier to prepare GO-based printable ink.This ink(m GO: m LFP=2:8 and m GO: m LTO=2:8)obvious shear-thinning behavior with a high apparent viscosity value over 104 Pa·s order of magnitude and elastic modulus of 104 Pa order of magnitude.These ink enable 3D micro-structures as an entirely 3D-printed full cell.This full cell can delivers relatively high capacities of 117 m Ah g-1 with good cycling stability.In order to improve the ionic conductivity along the electrode thickness,the GO-based ink above was used to print low tortuosity(as low as 1),ultra-thick hierarchically porous electrode for lithium-ion batteries.It is demonstrated via the electrochemical test that,compared with traditional electrodes with uniform active materials loading mass,the initial voltage hysteresis between the charge and discharge plateaus of the 3D printed electrode is only one third of this value of the traditional electrode.Meanwhile,the rate performance of 3D printed electrode has been improved substantially,also its charge transfer resistance was decreased obviously.