Synthesis And Mechanism Study Of Ultrafine Cobalt Oxide And Its Composites Via An Enhanced Solid State Reaction

Ultrafine cobalt oxide and its composite oxide powders have important application in the fields of electrode materials,supercapacitors,sensors,catalysts and so on.It is one of the key directions to explore a new,short flow,high efficiency and environmental friendly method for the preparation of these powders to accurately control the particle size,phase composition and structure of the powders.At present,the liquid phase route(co-precipitation,hydrothermal method,spray pyrolysis,etc.)and the solid state reaction route are the main methods for preparation of ultrafine cobalt oxide and its composite oxide powders.However,the liquid phase method shows some disadvantages,such as complex reaction process,environmental pollution,serious powder agglomeration,difficult to control the composition and phase structure of powders and so on.These problems are more significant in the synthesis of nanosized powders.And there are some difficulties when the composite oxide powders prepared by the solid state method,such as high reaction temperature,coarse particle size,poor dispersibility of powders and so on.Oxides can be directly obtained by thermal decomposition of metal carbonates and ions show a fast diffusion rate in nanoscale materials.A new method was studied in this work to directly synthesize ultrafine cobalt oxide and its composites.In this method,the thermal decomposition temperature of carbonates is reduced and the thermal decomposition rate is increased by mechanical activation/nanocrystallization of metal carbonates.Ultrafine Co₃O₄,LiCoO2 and LiN1/3Co1/3Mn1/3O2 powders were selected as the research objects in this paper.And carbonates were used as the precursors to study the mechanical activation effect on the thermal decomposition behaviors,phase compositions,crystal structures,electrochemical properties and related mechanisms of nanoscale carbonates.The main research contents and conclusions are as follows:(1)Ultrafine Co₃O₄ powders with particle size distribution in 10?20 nm were prepared by the enhanced thermal decomposition reaction.The effects of ball size,ball-to-material ratio and milling time on the crystal structure and micro-morphology of Co₃O₄ powders were studied.And the effect of mechanical activation treatment on the kinetic behavior and mechanism of thermal decomposition of CoCO3 powders was also studied.The results showed that small size grinding ball(5 mm),ball-to-material ratio at 15:1 and pretreatment time for 10 h were the optimal high energy ball milling parameters.The thermal decomposition reaction temperature was reduced,the reaction time was shortened and the thermal decomposition efficiency was increased by 18.2%using the mechanical activation treatment.The thermal decomposition apparent activation energy of CoCO3 powders with mechanical activation treatment was 72.7?82.8 kJ/mol,and the kinetic equation of the thermal decomposition was as follows: d?/dt=(2.42×106?2.19×)exp[(8.74×103?9.96×103)/T]4/3(1-?)[-ln(1-?)]1/4 The thermal decomposition apparent activation energy of CoCO3 powders without mechanical activation treatment was 118.7?125.4 kJ/mol,and the kinetic equation of the thermal decomposition was as follows: d?/dt=(1.08×1010?1.07×1011)exp[(1.43×104?1.51×104)/T]2/3?-1/2 The results indicate that the thermal decomposition apparent activation energy of CoCO3 powders,pretreated by the mechanical activation treatment,decline by approximately 33.2%.And the reaction mechanism is also changed from the n=3/2 to the random nucleation and subsequent growth,n=3/4.(2)Ultrafine HT-LiCoO2 powders were prepared by enhanced solid state reaction.The effects of mechanical activation treatment on the crystal structure,microstructure and electrochemical properties of the products were studied.The effect of mechanical activation treatment on the thermal decomposition and solid state diffusion reaction apparent activation energy of raw material powder was also discussed.The results showed that the morphology of LCO-BM10 was quasi-spherical and the particle size ranged from 200 nm to 400 nm.The thermal decomposition and solid state diffusion apparent activation energies of LCO-BM10 powders calculated by Kissinger method were 70.3 kJ/mol and 122.2 kJ/mol,respectively.(3)The initial discharge capacity and coulomb efficiency of LCO-BM10 powders were 175.2 mAh/g and 94.0%at 3.3?4.3 V.In terms of rate performance,the discharge capacity was 106.6 mAh/g even at 2740 mA/g(10C).The discharge capacity retention of LCO-BM10 was 97.6%when the current density returned to low values.After 100 cycles,the capacity retentions of LCO-BM10 at 137 mA/g(0.5C)and 822 mA/g(3C)were 87.8%and 78.1%,respectively.(4)Ultrafine LiNi_1/3Co_1/3Mn_1/3O₂ powders were prepared by enhanced solid state reaction.The effects of calcination method,calcination time,calcination temperature and mechanical activation time on the crystal structure,microstructure and electrochemical properties of the products were investigated.The phase transformation of mixed powders in enhanced solid state reaction was also studied.The optimal mechanical activation time of 10 h results in more stable and integrated structured ultrafine LiNi_1/3Co_1/3Mn_1/3O₂ powders with the average diameter of 200?500 nm.The thermal decomposition efficiency of the mixed raw materials was 35.7%higher compared to the ball-milling-free process.The phase transformation law of ultrafine LiNi_1/3Co_1/3Mn_1/3O₂ powders prepared by enhanced solid state reaction was as follows:First of all,the basic nickel carbonate and cobalt carbonate completely decomposed to nickel oxide and cobalt tetroxide;Subsequently,when manganese carbonate decomposed to manganese dioxide,solid state diffusion occurred between the intermediate products obtained by thermal decomposition,and the LiNi_1/3Co_1/3Mn_1/3O₂ powders began to form(566?);In the first holding stage(700?/7h),the content of LiNi_1/3Co_1/3Mn_1/3O₂ phase in the system increased gradually;In the second holding stage(950?/2h),the LiNi_1/3Co_1/3Mn_1/3O₂ powders with stable crystal structure were obtained.(5)Ultrafine LiNi_1/3Co_1/3Mn_1/3O₂ powders(NCM-BM10)prepared by the optimized process showed excellent electrochemical properties.The initial discharge capacity of NCM-BM10 at 2.5?4.3 V and 2.5?4.5 V was 156.1 and 187.1 mAh/g;NCM-BM10 exhibited a high reversible capacity of 114.3 mAh/g and 140.9 mAh/g at 1620 mA/g(6C)in the voltage range of 2.5?4.3 V and 2.5?4.5 V,respectively;NCM-BM10 exhibited capacity retentions of 80.8%(2.5?4.3 V)and 83.3%(2.5?4.5 V)at 270 mA/g after 200 cycles.