Design And Synthesis Of High Performance Anode Materials For Sodium Ion Capacitors

With the rapid development of clean and sustainable energy sources,high performance energy storage devices have attracted great attention.Among them,sodium ion capacitor is a new type of energy storage device.In a typical sodium ion capacitor,the anode is battery-type and the cathode is a capacitor-type.The battery-type electrode stores energy via sodium ions intercalation in the electrode bulk phase and possesses high energy density.The capacitive electrode stores energy via ion adsorption/desorption at the interface of electrode/electrolyte,and shows high power density and long cycle life.Therefore,sodium ion capacitors are expected to combine the merits of high energy density of sodium ion batteries and high power density of supercapacitors.However,the slow reaction kinetics of sodium ions-insertion in battery-type electrode is hard to match with the capacitive electrode,which limits the advantages of sodium ion capacitors.To solve the issue of the slow reaction kinetics of sodium ions in anode materials,the research contents are as follows:(1)Two-dimensional layered molybdenum disulfide(MoS2)shows the advantages of abundant resource and high theoretical specific capacity.Meanwhile,large interlayer spacing is benificial to the rapid diffusion of sodium ions.MoS2 is an ideal anode materials for sodium ion capacitors to achieve high energy density and high power density.However,the poor conductivity,and serious volume expansion of MoS2 limit its large-scale application.In order to improve the electrochemical performance of the MoS2 anode,coupling with different carbon materials such as graphene,carbon nanotubes,carbon fibers and so on are common methods.But the above carbon materials have the disadvantages of high cost and complex preparation process.Selecting the industrial waste bagasse as biomass carbon source may be more feasibale.After heat treatment,the three-dimensional porous carbon was obtained,connecting molybdenum disulfide with porous carbon,a high temperature solid-state method for preparing molybdenum disulfide was developed:carbon→Mo2C@carbon→MoS2@carbon.The high temperature solid phase process is simple and facile for large-scale production.The electrochemical properties of MoS2 nanosheets prepared by high temperature solid-state method were improved by the close combined of the MoS2 and biomass porous carbon.The MoS2@carbon exhibits obvious pseudocapacitive sodium ions storage behavior and displays a ultralong life up to 5000 cycles.Sodium ion capacitor assembled by MoS2@carbon anode and biomass porous carbon cathode and shows high working voltage up to 4 V as well as the high energy density of 112.2 Wh kg-1.(2)The high-temperature chemical reaction between dicyanamide and ammonium molybdate is utilized to synthesize a Mo2C@N-doped carbon(NC)composite and is further in-situ converted into the MoS2@NC.The effects of reaction temperature on the electrochemical properties of the MoS2@NC anode were investigated.The materials obtained at 800°C(MOS2@NC-800)exhibited the best performance:at 0.1 A g-1,it displays a high specific capacity of 535.3 mAh g-1,when the current density increases to 20 A g-1,and the specific capacity is still as high as 420.2 mAh g-1.The capacity retention rate(relative to 0.1 A g-1)is as high as 78%,showing excellent electrochemical reactivity.MOS2@NC-800 was coated with different thickness of titanium dioxide(TiO2)by atomic layer deposition(ALD),which further improved the cyclic stability of the electrode.When the thickness of TiO2 is 7 nm(TiO2/MoS2@NC-800-7),the electrode shows no capacity decay after 500 cycles at a high current density of 2 A g-1、displaying excellent structural stability.Sodium ion capacitors was assembled by TiO2/MoS2@NC-800-7 as anode and commercial activated carbon(AC)as cathode,which exhibits a high energy density of 148 Wh kg-1 and a high power density of 20 kW kg-1 as well as a long cycle life of 3000 cycles.(3)Use a simple electrospinning method to fabricate free-standing mesoporous T-Nb2O5/carbon nanofiber films(m-Nb2O5/CNF)by employing Nb(C2O4H)5 as a niobium precursor,polyacrylonitrile(PAN)as carbon source and tetraethyl orthosilicate(TEOS)as a template precursor.The nanofibers that make up the m-Nb2O5/CNF film has large number of mesoporous.The unique mesoporous structure can shorten the diffusion path of sodium ions and provide abundant active sites.In addition,the conductivity and mechanical properties of m-Nb2O5/CNF thin film electrodes were improved by the binder-free three-dimensional cross-linking structure.The reversible specific capacity of the m-Nb2O5/CNF thin film electrode is still as high as 171.4 mAh g-1 at high rate of 150 C,with a capacity retention rate of 59.8%(relative to 0.5 C).At a high rate of 100 C,the corresponding capacity decay of the m-Nb2O5/CNF thin film electrode is only 0.6%per 1000 cycles,showing high electrochemical reactivity and ultrahigh cycle stability.Flexible quasi-solid sodium ion capacitor was assembled by m-Nb2O5/CNF film as anode and GF/mCNF flexible film as cathode.The capacitor exhibits a high energy density of 124 Wh kg-1 and ultrahigh power density of 60 kW kg-1.The corresponding maximum volumetric energy/power densities are 11.2 mWh cm-3 and 5.4 W cm-3,respectively.(4)Anatase phase titanium dioxide(TiO2)has many advantages as the anode,especially have obvious pseudocapacitance characteristics,which is expected to show ultrafast sodium ion reaction kinetics.However,poor conductivity limits its performance.A high reaction kinetics TiO2 electrode was obtained based on carbon composite and N-doped as an advanced anode(N-TiO2/C)for Na-ion capacitors through the simple electrospinning approach followed by anneal treatment.The oxygen vacancy was formed in TiO2 and effectively improves its conductivity.Due to the synergistic effects of carbon compound and nitrogen doping,at the current density of 0.1 A g-1,the N-TiO2/C electrode has a high reversible specific capacity of 294.1 mAh g-1.Even at 15 A g-1,the specific capacity is still as high as 177 mAh g-1,showing superhigh reaction kinetics.Sodium ion capacitor were assembled using N-TiO2/C as anode and commercial activated carbon as cathode.The capacitor exhibits a high energy density of 111.4 Wh kg-1 and a high power density of 20 kW kg-1.