| Sodium ion hybrid capacitors(SICs)exhibit the characteristics of high energy/power density,meanwhile,considering the abundant distribution of sodium reserve,SICs have attracted widespread attention and hold a prospect to be an applicable energy storage device.As a new type of negative electrode material,sodium titanate has a relatively low sodium insertion potential,small volume variation after sodium intercalation reactions,and a facile method for preparation at low cost,making sodium titanate a promising negative electrode material for SICs.However,the intrinsic low conductivity of sodium titanate inducing the poor rate performance limits its further development.Therefore,this study aims to improve the sodium storage behavior by tailoring its electronic conductivity,eventurally,boosting the performance of SICs:First,sodium titanate/graphene(Na2Ti7O15@graphene,NTO@graphene)composites were prepared by an atomic layer deposition(ALD)coupled with hydrothermal method.NTO nanofibers and graphene nanosheets interweave together.At current densities of 3.54 A g-1 and 17.7 A g-1,the NTO@graphene can provide specific discharge capacities of 130 and 60 m Ah g-1,with the Coulombic efficiency of?100%.After 10,000 cycles,it still has a capacity retention rate of 90%,which is greatly improved compared with that of the pristine NTO,indicating that the unique structure of the composite,coupled with the lower sodium ion diffusion barrier of NTO,are beneficial for the electron transport and structural stability.At the same time,the high surface capacitive contribution enables its excellent rate performance.The SICs assembled with the activated carbon(AC)positive electrode exhibit a maximum energy density of 82.7 Wh kg-1 since the power density is 97.5 W kg-1.And a maximum power density of 25 k W kg-1 can be reached with an energy density of 16Wh kg-1 remained.Furthermore,the SICs have an excellent cycle stability,when the current density is 3.2 A g-1,after 10,000 cycles,the capacity retention rate is about83%,proving the excellent performance of NTO@graphene.In order to achieve the preparation of high performance flexible SICs,in this study,a graphene fiber/sodium titanate(GF-NTO)non-woven fabrics anode was successfully prepared by the method of wet spinning.In this non-woven fabric structure,NTO nanowires are wrapped in the graphene fibers,and the entire graphene fiber can be woven into a complete non-woven fabric without breakpoints.This fiber interlocking structure reduces the contact resistance between fibers,improving the three dimensional transport of electrons,and also strengthening the mechanical stability of the non-woven fabric.Meanwhile,through the hole-etching treatment of the non-woven fabric,the permeation between NTO and the electrolyte is greatly enhanced,the diffusion of sodium ions into the non-woven fabric is also improved,finally,boosting the electrochemical activity.The hole-etched non-woven electrode can deliver a stable discharged specific capacity of 211 m Ah g-1 at a current density of0.05 A g-1,and still provide a discharged specific capacity of 31 m Ah g-1 at a high current density of 5 A g-1,which is about 410%higher than that of the untreated non-woven fabric electrode.The flexible SICs can be assembled by the non-woven fabrics and a gel polymer electrolyte membrane.When the power density is 612 W kg-1,the maximum energy density can reach up to 89.3 Wh kg-1.Even if the power density is increased to 12.8 k W kg-1,the energy density still remains 41.3 Wh kg-1.The flexible SICs can power the LEDs and digital watches,which demonstrate the practical application prospect of the non-woven fabrics material. |
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