Controllable Preparation And Electrochemical Energy Storage Performance Of Manganese Dioxide/Carbon Composite Electrode Materials

As an important electrochemical energy storage electrode material,manganese dioxide?MnO₂?was intensively used in the field of energy storage such as supercapacitor,lithium-ion batteries and Zn-Mn batteries etc,due to their abundant resource,low cost,environment friendly and high theoretical capacity.However,the low conductivity of MnO₂ result in the low usage efficiency of the material,poor cycle stability and rate capability.Therfore,the practical capacity of MnO₂ as electrode material is often far below its theoretical capacity.By designing MnO₂ electrode material with high specific surface area and porous nanostructure,the effective utilisation of electrode material could be improved,thus enhanced its electrochemical performance.In addition,in order to improve the electronic conductivity of the Mn O₂ material,the researchers further combined the MnO₂ with highly conductive materials and prepared the composite system of MnO₂/conductive materials.Although the electrochemical performance of MnO₂ electrode material have been greatly promoted by the above treatment,it has been found that the MnO₂-based composite electrode material even with same chemical compositions may exhibit significant different cycling stability.Similar phenomena are also widespread in other oxide and hydroxide pseudocapacitive electrode materials.The in-depth analysis and full understanding of the above problems are crucial for exploring the relationship between the microstructure and electrochemical properties of electrode materials,as well as the rational design of high-performance electrode materials.In this paper,amorphous MnO_2/C composite were firstly prepared,and two different dry processes,conventional air drying and freeze drying,were employed to manipulate the particle agglomeration of samples.The influence of particle agglomeration on electrochemical performance of samples was investigated in the supercapacitor and Zn-Mn battery.On this basis,to solve the problem of high resistance state interface and large interfacial resistance between granular carbon particles,carbon nanotubes?CNTs?with high length-diameter ratio were used to replace carbon particles,and further prepared the CNT/MnO₂composite material with excellent storage of Zn²⁺.The main research contents and results of this paper are as follows:1.Amorphous MnO_2/C composite was prepared by the redox reaction between potassium permanganate?KMnO₄?and black pen ink.The agglomeration properties of the particles were manipulated by two different dry processes?conventional air drying and freeze drying?.In the freeze drying process,water in the samples from the solid ice sublimate directly to gaseous water molecule,and the capillary force caused by the aqueous solution is absent in the drying process,resulting in loose and porous samples.The specific surface area and pore volume are 127.8 m²/g and 0.322 cm³/g,respectively.However,in the conventional air drying process,the capillary force of liquid water would cause tightly shrinkage among particles,leading to more tightly particle agglomeration samples along with lower specific surface area and pore volume?103.6 m²/g,0.298 cm³/g?.The results of supercapacitor test indicated that the specific capacitance of the freeze drying sample was superior to conventional air drying sample?492 F/g vs 440.5 F/g?,which is attributed to large specific surface area and pore volume in freeze drying sample;however,conventional air drying sample showed better cycling stability?capacity retention:70.9 vs 60.7%after 5000 cycles?due to strong particle agglomeration.2.In order to verify the universality of the influence of particle agglomeration on electrochemical performance of the electrode materials,the MnO₂/C sample prepared by two different drying methods were used as cathode materials of Zn-Mn batteries,and further explored their Zn²⁺storage.The electrochemical performance test of Zn-Mn batteries indicated that the initial discharge capacity of freeze drying sample with large specific surface area and pore volume higher than that of conventional drying sample?109 mAh/g vs 68 mAh/g,at 1 A/g?,while the cycling stability was inferior to conventional air drying sample?47.7%vs 77.9%after 400 cycles?.The results showed that the particle agglomeration of MnO₂/C sample has a very similar influence on the electrochemical performance of the electrode material both supercapacitor and Zn-Mn batteries.The study effectively demonstrate that rational regulate the relationship between the particle agglomeration and specific surface area is the key of obtaining high specific capacitance/specific capacity and long cycling stability electrode materials.3.Considering the disadvantages of high interfacial resistance and large internal resistance between carbon particles,the high length-diameter ratio of CNT was further used to replace carbon particles,and reduce interfacial resistance and obtain high performance CNT/MnO₂ cathode material.In 3 M ZnSO₄+0.1 M MnSO₄ electrolyte,the discharge capacity of CNT/MnO₂ cathode material is 468 mAh/g at current density of 50 mA/g,along with 83.42%intial Coulomb efficiency.When the current density increases to 1 A/g,its discharge capacity remains still 188 mAh/g.Meanwhile,the CNT/MnO₂ cathode material retains 96.5%of its initial capacity after 350 cycles,which higher than that of pure phase MnO₂ electrode?27.9%,300 cycles?.When the loading mass of CNT/MnO₂ cathode material is 5.0 mg,the energy density based on the total mass of positive and negative electrodes is up to 65 Wh/kg,which is superior to the traditional lead-acid battery.The excellent Zn²⁺storage performance of CNT/MnO₂cathode material are attributed to the synergistic effect between the high electrical conductivity of CNT and the high theoretical capacity of MnO₂.