Application Of Mn-O-based Electrode Material In Aqueous Hybrid Supercapacitors

With the increasing demands of energy consumption,the researches and development of high-efficient energy storage system is a matter of great concern.It is obviously that lithium-ion batteries and supercapacitors dominate the present energy market.What's more,lithium-ion batteries hold high energy density but limited power density,meanwhile,supercapacitors have high power density but low energy density.Hence,neither lithium-ion batteries or supercapacitors can meet the demands with both high energy density and high power density of market applications,hybrid supercapacitors with high energy density and high power density emerge at the time requires.Hybrid supercapacitors,the exciting prospect for future applications,use battery-type electrode and capacitor-type electrode to combine their advantages.However,the sluggish kinetics of battery-type electrode restricts power density of hybrid device.In addition,nonaqueous electrolyte is unsafe and conventional aqueous electrolyte is unable to attain high overall energy density and power density because of narrow working window.Therefore,we can improve the electrochemical performance of hybrid supercapacitors by improving the performance of battery-type electrode and choosing suitable electrolyte.In this work,we improve the specific capacitance and rate capability of battery-type electrode materials via several methods such as controlling of particle size,material composition and increasing conductivity.Moreover,high-concentrate aqueous electrolyte can not only avoid the unsafety of nonaqueous electrolyte but also refrain from the disadvantage with narrow working windows of conventional aqueous electrolyte.Above methods vastly improve energy density,power density and cycling life of hybrid supercapacitors.The specific research contents is as follows(1)we ground the pristine micro-sized LiMn2O4 powders and dispersed them in graphene sheet simultaneously by a facile ball milling process.The as-prepared nanocomposite were named as BGLMO.Nanocrystallization can keep structure stability of spinel LiMn2O4 and provide a mass of active sites which vastly shorten the transport path.What's more,graphene network greatly increases the conductivity among spinel LiMn2O4 particles and hinders the reuniting process.These characters bring splendid rate capability of electrode materials.(2)Comparing the electrochemical performance of LiMn2O4@graphene nanocomposite in 1M Li2SO4 electrolyte and 5M LiTFSI electrolyte,it is clearly to find that the latter is better especially at low scan rate.The content of Li+in 5M LiTFSI electrolyte is more than in 1M Li2SO4 electrolyte,which benefits faradaic pseudocapacitance.To the contrary,the content of free water molecules in the solvated sheath of Li+is poor,reducing the electrochemical ativity and increasing the decomposition voltage of electrolyte.The hybrid device,LiMn2O4@graphene//active carbon in 5M LiTFSI,delivers high energy density,power density,as well as long cycling life.The energy density can reach to 20 Wh kg-1 at the power density of 2571 W kg-1,meanwhile,the capacitance maintains 98%after 7000 cycles.(3)LiMn2O4@C nanocomposite was prepared by coating carbon film on the surface of spinel LiMn2O4 with dopamine.The excellent performance with high specific capacitance and rate capability is attribute to the carbon coating,which avoids reuniting process of spinel LiMn2O4 particles to keep structure stability and simultaneously improves the conductivity among spinel LiMn2O4 particles to accelerate the ion transmission rate.Therefore,it can be found that the electrochemical performance of LiMn2O4@C in 5M LiTFSI electrolyte is better than that in 1M Li2SO4 eletrolyte.