Application Of MnO_x-C Composites In Asymmetric Supercapacitors

People pay more and more attention to the development of energy-saving,high-efficiency,and pollution-free supercapacitors,but the energy density of this energy storage device is limited,which restricts its further practical application.Balancing the capacity and storage kinetics between positive and negative electrode materials is an important factor to obtain high energy density capacitors.MnO has the characteristics of abundant resource reserves,high theoretical specific capacity（756 m Ah/g）,and low charging voltage plateau（<1.5V）.However,it has poor electrical conductivity and is prone to volume expansion during charging and discharging.This phenomenon leads to the low specific capacity,poor rate performance and cycle performance of MnO_x-based capacitors,which makes it impossible to obtain capacitor energy storage devices with high energy density.Carbon materials have high specific surface area,abundant pore volume and excellent electrical conductivity.Therefore,we perform nanostructure design and carbon coating on MnO_xto comprehensively improve its electrochemical performance.In this paper,focusing on assembling asymmetric capacitors,a series of carbon-coated MnO_xcomposites（MnO_x-C）were obtained by different preparation methods and used as electrode materials for capacitors,and then the materials were capacity-matched with pitch-based carbon materials.,assembled into an asymmetric full capacitor,tested its electrochemical performance,and explored the energy storage mechanism of different MnO_x-C composites in capacitors.（1）Controllable preparation of porous-structured MnO-C composites and their application in aqueous asymmetric supercapacitors.Combining tablet forming and one-step activation,using manganese acetate（C₄H₆MnO₄·4H₂O）as manganese source and petroleum-based pitch as carbon source,porous carbon-MnO composites with large specific surface area,rich meso-and macroporous structure,and uniform MnO loading were obtained.In the three-electrode electrochemical performance test system,the PAC@MnO-0.3 electrode has a specific capacity as high as 345.5 F/g at 0.5 A/g,and still has a high specific capacity of 190 F/g even at a high current density of 20 A/g.specific capacitance,showing excellent rate performance.The PAC@MnO-0.3 was assembled with porous carbon to form an aqueous asymmetric supercapacitor,which was cycled for 3000 cycles at 5 A/g,and its capacity retention rate was as high as87.24%,showing excellent cycling stability.The results show that the porous carbon-rich pore channels enhance the electrical conductivity of MnO,and its pore structure also provides more storage space for potassium ions.The effect between the double-layer capacitance of the synergistic porous carbon and the pseudocapacitance from the redox reaction of MnO is the reason for the excellent electrochemical performance of this capacitor.（2）Controllable preparation of spherical MnO-C composites and their application in all-lithium-ion capacitors.Using coal-based pitch as the carbon source and KOH as the activator,the mixture was uniformly mixed,the mass ratio of the two was adjusted,and the embryo body（10 Mpa）was pressed by the"embossing method",and then a step of calcination was performed to obtain the graded porous activated carbon,among which,CAC-3 has a high specific area of 2868 m²/g and abundant mesoporous structure,which exhibits excellent electrochemical performance in both aqueous and organic electrolytes.In addition,using manganese sulfate as the manganese source,glucose as the carbon source,and ammonium bicarbonate as the nitrogen source and precipitating agent,the hydrothermal and high-temperature calcination methods were used to obtain the obtained products with good morphologies.of spherical MnO-C composites.As a negative electrode material for lithium-ion capacitors,its specific capacity is 737 m Ah/g at 0.5 A/g,and its capacity retention rate is 82.27%at 2 A/g for 1000 cycles.Matching the MnO-C anode material with the porous carbon cathode material with the best electrochemical performance for an asymmetric all-Li-ion capacitor,the results show that its energy density is as high as 100 Wh/kg at a low power density of 1502 W/kg.MnO-C has carbon coating with excellent conductivity and stable spherical structure,which greatly improves the rate capability and cycling stability of the electrode.（3）In order to realize the green and low-cost production process,we explored the material composition in the reaction process of preparing activated carbon,and effectively recovered the KOH activator.According to the K₂CO₃and KOH generated during the activation process,quantitative analysis was carried out to explore the law to provide data for the experiment,and to investigate the feasibility of the KOH recovery method.The filtrate of the washed product activated carbon was recovered,reacted with calcium hydroxide,filtered,concentrated and dried to recover KOH powder and the yield was calculated.Through data analysis,it is judged whether the process is feasible.Taking the MnO_x-C material as the starting point,different types of MnO_x-C composites were synthesized by the most convenient one-step co-activation method and hydrothermal high temperature post-calcination.On the one hand,it enriches the types of manganese oxides,and on the other hand,it responds to the national new energy strategy to provide high-quality electrode materials for the new energy field while recycling the activator,which has good industrial application prospects.