A Study On Design And Performance Of Novel Asymmetric Supercapacitors Based On Fe₃O₄ And MXene Nanocomposites

With the increasing attention toA enbvisrotnrmaenctatl issues and the strong demand for clean and sustainable energy,the development of advanced energy storage devices with high energy and power densities,as well as long cycle stability,has become an important worldwide topic.Nowdays,there are two major commercial energy storage devices,namely secondary batteries and supercapacitors.The former one has higher energy density,but lower power density.The latter one can provide higher power density and cycle performance,but that is restricted by the low energy density.Constructing an asymmetric supercapacitor is one of the most effective ways to increase energy density without sacrificing high power density and excellent cycle perdormance.In particular,developing the novel ion hybrid capacitors,with the introduction of high-capacity battery-type materials into supercapacitor,greatly improved energy density.The design and development of electrode materials is the key technology to construct high-performance asymmetric supercapacitors.According to the current characteristics and research status of electrode materials,the nanosized materials and porous structures can effectively shorten ion transmission path and improve the ion transmission dynamics.Although pseudocapacitive and battery-type materials can deliver high capacity at low rate,the capacity decay is severe at high rates due to the low conductivity.The hybridzation of pseudocapacitive and battery-type materials with highly conductive materials at the molecular level not only exposes more active sites,but also improve poor conductivity and general cycle stability.The main research contents are summarized as follows:1.Iron oxide encapsulated in nitrogen-doped carbon as high energy anode material for asymmetric supercapacitorsN-doped carbon-encapsulated Fe₃O₄（Fe₃O₄@NC）is prepared by a novel and simple method.The polyaniline precursor provides a high concentration of N-doping,which improves the electronic conductivity of Fe₃O₄@NC.The pseudocapacitive behavior of N-doping and Fe₃O₄ increases the overall capacity and the carbon coating layer ensures the circle stability.Benefiting from the well-designed heterostructure,the resultant Fe₃O₄@NC shows an ultra-high gravimetric capacitance of 313 F·g^-1 at 1A·g^-1 and still delivers 193 F·g^-1 at a high current density of 15 A·g^-1,demonstrating an excellent rate performance.Subsequently,Fe₃O₄@NC is used as anode and Co Mn-LDH as cathode to construct an asymmetric supercapacitor,widening the voltage window and increasing the energy density of the device.Due to the enlarged voltage window and matched capacity and kinetics,the obtained Co Mn-LDH//Fe₃O₄@NC ASC device shows a remarkable electrochemical performance with a large energy density up to 35 Wh·kg^-1 at a power density of 900 W·kg^-1 and a high energy density of 24 Wh·kg^-1 at a power density of 13.5 k W·kg^-1 as well as an excellent rate capability and a good cycling stability.2.Iron oxide encapsulated in nitrogen-rich carbon enabling high-performance lithium-ion capacitorNovel lithium-ion capacitors（LICs）are designed to combine the virtues of high power capability of conventional supercapacitors and high energy density of lithium-ion batteries.However,the lack of high-performance electrode materials and the kinetic imbalance between positive and negative electrodes have been the major challenges.In this work,Fe₃O₄ nanoparticles encapsulated in nitrogen-rich carbon（Fe₃O₄@NC）are prepared through a self-assembly of the colloidal Fe OOH with polyaniline（PANI）and then a subsequent pyrolysis.Due to the well-designed nanostructure,conductive nitrogen-rich carbon shells,abundant micropores and high specific surface area,Fe₃O₄@NC-700 sample delivers a high capacity,high rate capability and long cycling stability.Kinetic analyses of the redox reactions reveal the pseudocapacitive mechanism of Fe₃O₄@NC-700 and the feasibility as negative material in LIC devices.A novel LIC is constructed with Fe₃O₄@NC-700 as negative electrode and expanded graphene（EGN）as positive electrode.The well-matched two electrodes effectively alleviates the kinetic imbalance between positive and negative electrodes.As a result,Fe₃O₄@NC-700//EGN LIC exhibits a wide operating voltage window,and thus achieves an ultrahigh energy density of 137.5 Wh·kg^-1.These results provide fundamental insights into the design of pseudocapacitive electrode and show future research directions towards the next generation energy storage devices.3.Ultrahigh rate capability of 1D/2D polyaniline/titanium carbide（MXene）nanohybrid for advanced asymmetric supercapacitorsAs one of the most well-recognized pseudocapacitive materials,the excellent chemical and physical characteristics of PANI make them versatile and highly tunable for application in energy storage systems.Unfortunately,the efficient use of PANI is usually limited by the relatively low rate capability and poor stability.Moreover,the electrochemical research of PANI is restricted within single electrode and symmetric supercapacitor systems,rarely used in asymmetric supercapacitors.In this work,PANI/Ti₃C₂T_x（MXene）nanohybrid is synthesized through a facile and cost-effective self-assembly of 1D PANI nanofibers and 2D Ti₃C₂T_x nanosheets.PANI/Ti₃C₂T_xdelivers greatly enhanced specific capacitance,ultrahigh rate capability(67%capacitance retention from 1 to 100 A·g^-1)and good cycling stability.Electrochemical kinetic analysis reveals that PANI/Ti₃C₂T_x is featured with surface capacitance-dominated process and has a quasi-reversible kinetics at high scan rates,giving rise to an ultrahigh rate capability.By using PANI/Ti₃C₂T_x as positive electrode,an aqueous ASC is successfully assembled,showing a maximum energy density of 50.8 Wh·kg^-1(at 0.9 k W·kg^-1)and a power density of 18 k W·kg^-1(at 26 Wh·kg^-1).Moreover,an organic ASC is also elaborately fabricated by using PANI/Ti₃C₂T_x,achieving an ultrahigh energy density of 67.2 Wh·kg^-1 and a power density of 30 k W·kg^-1.4.Fast redox kinetics of two-dimensional Ti3C2T_x（MXene）/Ti O2 hybrid for high-performance lithium-ion capacitorThe hybrization of Ti₃C₂T_x MXene nanosheets and TiO₂ nanoparticles can efficiently improve the electronic conductivity of TiO₂ and the kinetics of the lithium storage reaction.The advanced architecture of Ti₃C₂T_x/TiO₂ hybrid,prepared by simple aqueous self-assembly,demonstrates outstanding rate capability and excellent cyclic stability in half cells.The contribution of extrinsic pseudocapacitance affects the rate capability to a large extent,which is identified by kinetics analysis.Then,novel LICs is constructed utilizing Ti₃C₂T_x/TiO₂ hybrid as an intercalation-type anode and carrot derived porous carbon（CPC）with high surface area as an ion adsorption cathode in an organic electrolyte.Owing to the advantages of structures and excellent performances of both anode and cathode materials,the assembled Ti₃C₂T_x/TiO₂//CPC LIC provide an exceptionally high energy density(129.4 Wh·kg^-1)and high power density(10 k W·kg^-1)within 0-4.0 V as well as excellent long-term cycle stability.