Construction And Electrochemical-performance Research Of Aqueous Energy-storage Electrode Materials Based On Manganese Oxides And Nickel Hydroxide

Electrical energy storage devices with fast energy collection,high energy density,and excellent cyclic stability are urgently needed for the sake of integration of intermittent and uneven renewables into electric grid.Aqueous electrolytes is beneficial for rapid energy collection because of the much higher ionic conductivity than organic liquid electrolytes.B esides,aqueous electrolytes are hypotoxic,safe and low cost.Therefore,aqueous supercapacitors and aqueous ion batteries have broad development prospects and significance.Manganese oxides and nickel hydroxide are promising electrode material due to their high theoretical specific capacitances.However,they suffer from poor rate capability because of low ionic conductivity so that they are unable to meet demands for rapid energy storage.In addition,on the one hand,the conventional electrode-preparation method involve the use of an insulating polymer?as a binder?to ensure the bonding between active materials and current collectors.In this way,not only the internal resistance of the electrode is increased but also the utilization ratio of the active material is reduced.On the other hand,energy densities of current aqueous energy storage devices are low due to the decomposition voltage of water?1.23 V?.For improving ionic conductivity,amorphous active materials with hierarchically porous structure are synthesized by intense interference in crystallization process and electrochemical activation.For ameliorating electronic conductivity and binding between active materials and current collectors,freestanding electrodes are prepared by in-situ growth in hydrothermal environment and electrochemical deposition.For widening potential window,core-shell structural materials are constructed,inhibiting the oxygen evolution and hydrogen evolution process on the electrode surface.The research contents and main results of present work are as follows:?1?Constructing amorphous manganese dioxide electrodes for better electrochemical performances than that of crystalline manganese dioxide.Amorphous manganese dioxide nano-particles with hierarchical pores are synthesized in order to improve ionic conductivity.Morphology and pore structure of manganese dioxides can be adjusted by intense stirring,controlling feeding speed of oxidation and reaction duration.The amorphous manganese dioxide with hierarchical pores presents a specific capacitance of 405.2 F·g^-1 at a scan rate of 1 mV·s^-1 and a good rate capability of 75.3%retention at 10 mV·s^-1.Meanwhile,it shows a capacitance of 252.2 F·g^-1 at a current density of 1 A·g^-1 and a great rate capability of 77.3%retention at 5 A·g^-1.Moreover,the amorphous-phase nanoparticle electrode possesses an excellent cycle life with 95.1%retention of its initial capacitance after 5200 cycles.It can be found that the ionic conductivity of the amorphous manganese dioxide is superior to that of crystalline ones so that rate capabilities and cyclic performances of the amorphous manganese dioxide are more admirable.Furthermore,electrochemical-performance mechanism of the amorphous manganese dioxide in various aqueous electrolytes are elaborated to provide references for the matching of electrode mat erials and electrolytes.?2?Constructing freestanding electrodes with core-shell structure to ameliorate the rate capability and cyclic stability,as well as energy density.Nickel hydroxide arrays grow on current collector by hydrothermal reaction.Then,electrodes with nickel hydroxide/manganese dioxide core-shell structure are constructed for widening potential window.The nickel hydroxide/manganese dioxide electrode obtained by the constant-voltage electrodeposition presents a high specific capacity of 642.3 C·g^-1 at 1 mV·s^-1and a long cyclic life without capacity fading after 20000 cycles.Remarkably,an asymmetric supercapacitor constructed by nickel hydroxide/manganese dioxide electrode and activated carbon electrode delivers a high energy density of 66.7 Wh·kg^-1at a power density of 485.7 W·kg^-1 under potential window of 1.7 V.Furthermore,the asymmetric supercapacitor exhibits an energy density of 17.8 Wh·kg^-1 at a high power density of 14571.4 W·kg^-1 and a capacitance retention of 101.4%after 15000 cycles,showing great rate capability and stable cyclic performance.The nickel hydroxide@manganese dioxide electrode prepared by the constant-current electrodeposition shows great rate capability and long cyclic life.In addition,the coulombic efficiency are about 100%at every current density.It can be found that in-situ growth of active materials on current collector not only achieves great binding but also enables excellent structural stability.Furthermore,manganese dioxide as shell successfully inhibits the oxygen evolution reaction process at high potential so that widened potential window and high energy density are achieved.?3?Constructing freestanding electrodes with core-shell structure and amorphous phase for an aqueous symmetric potassium-ion battery with high energy/power density and ultra-long cyclic life.A freestanding electrode using novel amorphous NiMn_1.57O_4.13 solid solution microsphere is fabricated by electrochemical activating the nickel hydroxide@manganese dioxide electrode.The NiMn_1.57O_4.13 electrode presents high specific capacities of 1637.2 C·g^-1 at1 mV·s^-1 and 1390 C·g^-1 at 5 A·g^-1.An aqueous symmetric potassium-ion battery using NiMn_1.57O_4.13 electrodes delivers a high energy density of 115.7 Wh·kg^-1 at a power density of 1875.0 W·kg^-1 and an energy density of 44.3 Wh·kg^-1 at a high power density of 18750.0 W·kg^-1 under potential window of 1.5 V.Furthermore,the aqueous symmetric potassium-ion battery shows ultra-long cyclic life and enables current fluctuations.The capacity retention is 106.1%after 100000 cycles in spite of many changes of current density during cycles.?4?Constructing flexible and freestanding electrodes with high specific capacity.Manganese dioxide nano-fibers and micro-belts are prepared to connect carbon fibers and fill the gap between them.Then the flexible electrode with nickel hydroxide coated manganese dioxide modified-carbon cloth is fabricated by constant-current deposition,displaying enhanced volume utilization and high specific capacity.The flexible electrode exhibits a specific capacity of 1272.9 C·g^-1 at a scan rate of1 mV·s^-1.When the current density increases from 1 A·g^-1 to 10 A·g^-1,the specific capacitance changes from 892.1 C·g^-1 to 612.0 C·g^-1,showing good rate capability.Moreover,the capacity retention is 99%after 2000 cycles at a current density of 2A·g^-1.However,the capacity retention decrease to 70%after 2000 cycles at a current density of 5 A·g^-1,indicating that the operating current density should not ex ceed 5A·g^-1.