Preparation Of Battery-grade MnO₂ By Liquid Phase Method And Study Of Its Performanc

With the rise of new energy vehicles,metal ion batteries have become the focus of people’s research as a kind of energy storage method with superior performance.As one of the raw materials of lithium batteries,manganese dioxide is mainly produced by electrolysis in industry,with high cost and energy consumption,long production cycle and low production efficiency.In this paper,a more advantageous liquid phase method is proposed to prepare manganese dioxide,to find an alternative for the production of battery-grade manganese dioxide and to discuss the application of manganese dioxide in supercapacitors,battery cathode materials and battery anode materials.In this paper,manganese dioxide was prepared by liquid phase method.The physical and chemical properties of manganese dioxide were analyzed by XRD,SEM,BET,XPS and FT-IR.The electrochemical properties of manganese dioxide were tested by three electrodes of aqueous solution,positive electrode and negative electrode of lithiumion battery.The main research contents are as follows:（1）Five crystal forms of?、?、?、?、?manganese dioxide were prepared by liquid phase method.The phase,morphology,specific surface area and surface structure were characterized and the specific capacitance was tested.The results show that?-Mn O₂ has the characteristics of layered crystal structure with appropriate size,spherical shape,large specific surface area,abundant oxygen vacancies and more interlayer bound water,and the specific capacitance is 73.25 F/g.The crystal structure of?-Mn O₂ is not conducive to ion intercalation,but its large specific surface area,pore size,pore volume,sufficient oxygen vacancies,and small particle size are conducive to ion transport and storage,and the specific capacitance is 66.15 F/g.?-Mn O₂ has poor crystal structure and small specific surface area,but the surface oxygen vacancy concentration is very high,and the specific capacity is the third.The crystal structure of?-Mn O₂ is large,which is conducive to ion penetration,but the specific surface area is small,the surface oxygen vacancies are less,the active sites are less,and the electrochemical performance is poor.The crystal structure of?-Mn O₂is disordered,and the specific capacitance of?-Mn O₂ and?-Mn O₂ is relatively small.Among the five crystalline manganese dioxides prepared by liquid phase,?-Mn O₂ and?-Mn O₂ have the best electrochemical performance.The CV curves of different scan rates were calculated by kinetic analysis to obtain the specific capacitance composition of five crystal forms.The capacitance contribution of?-Mn O₂in the specific capacitance is extremely high,which can be used as a capacitor material for rapid charge and discharge.The capacitance contribution and diffusion contribution of?-Mn O₂ at low scan rate are almost equal,and the ion occurs bulk phase diffusion,which has better battery material performance.（2）?-Mn O₂ was prepared by controlling different conditions,and the phase and microstructure of the product were characterized.The battery anode material was prepared by using it to carry out constant current charge-discharge cycle and different rate specific capacity test,and the performance difference of manganese dioxide under different preparation conditions was analyzed.The results show that Na OH is used as the precipitant to obtain the product with high crystallinity,pure phase and relatively maximum specific capacity.The shorter the reaction time,the greater the specific capacity;the temperature has a great influence on the crystallinity and microstructure of manganese dioxide.At 50°C,the product has good crystallinity,relatively dispersed nanorods,which can provide more active sites,shorter intraparticle diffusion distance and larger specific capacity.When the p H value increases,the crystallinity of the product decreases,the proportion of?-Mn O₂ in the phase decreases,and the specific capacity increases first and then decreases.The specific capacity is the largest at p H=2,and the p H=1 is lower than the former is caused by the strong acidic condition of heating and dissolving manganese dioxide.As the concentration of manganese ions increases,the specific capacity decreases and the cycle stability decreases.The low concentration of 10g/L has the maximum cycle specific capacity.It is concluded that the preparation conditions of manganese dioxide with better electrochemical performance are as follows:manganese ion concentration of 10g/L,30g/L sodium hydroxide as precipitant,oxidation reaction for 1h,p H=2,50°C water bath for 1h.The prepared manganese dioxide as anode has a specific capacity of 272m Ah/g after 100 cycles at a current density of 0.1A/g,which is the best electrochemical performance among all manganese dioxide,but it is far from the theoretical value and should not be used as anode material.（3）The effects of calcination conditions and?-Mn O₂ preparation conditions on the performance of lithium manganate were studied.The phase,crystallinity,unit cell parameters,grain size,microstructure,particle dispersion and cycle specific capacity of the product were tested and analyzed.The mechanism of preparation conditions and electrochemical performance was revealed.The results showed that the concentration of Mn²⁺was 20g/L,the precipitant was 30g/L Na OH,the p H of the solution was controlled to 8,the oxidation reaction was 1h,and the p H was adjusted to 3 in a 50°C water bath for 1h to obtain manganese dioxide.It was mixed with lithium hydroxide at a molar ratio of Mn:Li=2:1.Lithium manganate was obtained by calcination at 700°C for 2h.The product has high crystallinity,suitable grain size,uniform particles,less agglomeration,and a relatively regular polyhedron morphology.It can provide a stable structure required for lithiumion insertion process.The ion transport distance is suitable,which is beneficial to the insertion of ions into the deep layer of the grain.It has the best electrochemical performance:the specific capacity is125.79m Ah/g after 200 cycles at 1C,and the capacity retention rate is 95.98%.After10 cycles at 0.5C,1C,2C,5C and 0.5C,the specific capacities are 134.95,130.98,125.55,111.41and 132.40m Ah/g,respectively.Studies have shown that?-Mn O₂ is more suitable as a capacitor material among the five crystalline manganese dioxides prepared in the liquid phase.Lithium manganate synthesized by?-Mn O₂ and lithium hydroxide has excellent electrochemical performance as a battery cathode material.The mechanism of the influence of preparation conditions on the electrochemical performance of manganese dioxide is clarified,which provides sufficient theoretical support for the liquid phase preparation of battery-grade Mn O₂.