Mn-O-based Electrode Materials And Their Energy Storage Behavior In Hybrid Supercapacitors

In recent years,higher demands have been placed on energy storage and conversion devices with the development of electric vehicles and portable electronic devices.Lithium-ion batteries and supercapacitors have been widely studied in the past decades and been applied in the market.However,single device can't meet the requirements with both high energy density and high power densityowing to their respective disadvantages.Hybrid supercapacitor,the most potential next-generation energy storage devices,is a state-of-the-art energy storage device with high energy density,power density and long cycle life that combines the advantages of both lithium-ion battery and supercapacitor.However,the sluggish kinetics of battery-type electrode materials severely limits the overall power density of the hybrid capacitor,making the hybrid capacitor unable to attain high energy densities at high power densities as expected.Thus,there is an urgent demand to improve the rate performance of the battery type material.This work is to improve electrochemical performance of manganese oxide based cathode materials via several different approaches,including controlling the particle size,conductivity,material composition,morphology,and interplanar spacing of crystals.A series of facile and environmentally friendly methods have be proposed to synthesize manganese oxide cathode materials with optimized specific capacitance,higher rate capability,and excellent cycling life.The manganese oxide based cathode can be also used in the sodium and potassium ion supercapacitors with improved electrochemical performance.Firstly,to synthesize high-performance nanocomposite,an efficient one-step ball milling method was applied to mix commercial micro-sized LiMn2O4 particles and conductive carbon black Super P.After the ball milling process,the micron-sized LiMn2O4 particles are pulverized into nanoparticles while being uniformly distributed into a conductive network composed of Super P,forming LiMn2O4@Super P nanocomposite.This structure can provide a large number of active sites for electrochemical reactions,greatly shorten the ion transport path,and improve the conductivity among active material particles,thus the rate capability has been significantly enhanced.The LiMn2O4@Super P composite delivers a high specific capacitance of 3.5 times that of pristine commercial LiMn2O4 even being charge-discharged 80 times faster.Secondly,carbon nanotubes and graphene with good conductivity and excellent capacitive performance are used instead of the Super P.and a hierarchal structure is formed composed of zero-dimensional LiMn2O4 nanoparticles,one-dimensional carbon nanotubes and two-dimensional graphene.With this hierarchal structure,LiMn2O4 nanoparticles can be connected closely with the conductive carbon nanotubes and graphene in different dimensions,providing a full range of conductive networks.Graphene and carbon nanotubes can not only work as conductive agents,but also excellent capacitive materials which make contributions to the total capacitance of the composite.Therefore,the LiMn2O4@CNTs@graphene composite delivers high specific capacitance and excellent rate capability.The hybrid capacitor with this cathode delivers a maximun energy density of 62.77 Wh kg-1,and a maximum power density of 2.92 kW kg-1,and the capacitence retains 90.8%after 5,000 cycles.Thirdly,a facile hydrothermal method was applied to further control the particle size and morphology of manganese-based electrode materials,and a new method for preparing ultrafine LixMnO2 nanowires without any template or surfactant was invented.The LixMnO2 nanowires show much improved rate performance and cycle life owing to their large surface area and structure stability.Li-ion hybrid capacitors assembled with LixMnO2 nanowires cathode show a capacitance retention of 85.2%after 20,000 cycles.Finally,to further improve the rate performance,potassium ions with larger radius are pre-inserted into the tunnel structure to form KxMnO2 nanowires.Thus,interplanar spacing in the tunnel structure are enlarged for faster lithium insertion and desertion,and better electrochemical performance is achieved.Furthermore,due to the enlarged interplanar space,it allows the easy(de)intercalation of sodium ions and potassium ions.After lattice space control,the KxMnO2 nanowires deliver improved performance in lithium ion hybrid capacitors,and also show excellent performance in sodium ion and potassium ion capacitors.The potassium ion hybrid capacitor assembled with KxMnO2 nanowires as the positive electrode delivers an energy density of 30.2 Wh kg-1 even at a high power density of 4.72 kW kg-1,and nearly no capacitance decay after 7000 cycles.This work provides efficient methods and favorable guidance for the preparation of high-performance electrode materials.In the future,the composition and structure of the electrode materials can be well designed and investigated for the hybrid supercapacitors with low cost and high performance.