| With the development of electronic equipment,the demand for secondary batteries with high energy density is increasing.Lithium ion batteries is relatively safe and has long cycle life,which have taken control of global rechargeable battery market,and widely used in portable electronic devices,electric vehicles(EVs),renewable energy systems with intermittent energy sources(wind,solar,etc.)power,space technology and national defense industry.The current cathode material of commercial LIBs,LiCo02,has some shortcomings,such as high cost,high toxicity and limited resources of cobalt ore.In the cathode candidates,one of the most attractive material to replace LiCoO2 is manganese oxide with layered and spinel structure,because the manganese oxide is low cost,non-toxic,high working voltage and can provide various variant[1.2]In recent years,Li/MnO2 battery with high energy density,voltage stability,long service life,environmental friendly,high security etc.,has became the most widely used lithium primary battery.Electrolytic manganese dioxide(EMD)with high purity,low cost and strong electrochemical activity is a key component for cathode material of Li/MnO2 battery,and has a decisive influence on battery performance.Recently,researchers have tried to expand the application of electrolytic manganese dioxide in secondary battery,however,due to its low conductivity and unstable structure,the energy output in its actual working process is limited.Researches on sodium ion batteries have been reignited by the driver of new applications that are different from the demand for portable electronic products and concern about Li reserve.Sodium ion batteries in a more sustainable green strategy can promote the developlment of alternative energy storage technology,restrict environment pollution and have huge advantages and commercial prospect.In this paper,the electrolytic manganese dioxide has been studied through a large number of experiments to improve its discharge capacity and cycle performance.For example,nano electrolytic manganese dioxide with mesoporous structure has been prepared to increase the active sites;or prepared its compound with lithium salt to improve its conductivity and stability.Based on electrolytic manganese dioxide,this paper has prepared a series of nanoscale manganese oxide used as cathode materials of lithium ion batteries and sodium ion batteries.The main research contents include:1.The electrochemical properties of lithium ion battery for electrolytic manganese dioxide are investigated:The prepared EMD1 nanoparticles are irregular nanoparticles,and can be prepared on a large scale by the method;XRD and TEM tests show that EMD1 is interconnected intergrowth of the a-y-MnO2;BET tests shows that the BET surface area of EMD1 is much larger than that of commercial EMD0;Cyclic voltammetry test indicates that the electrochemical reaction of EMD1 in Li/MnO2 battery is reversible,and the irreversible phase transition and electrochemical activation process can be completed after several cycles;By comparing the charge and discharge curves of commercial EMD0 and nano EMD1 cathodes in Li/MnO2 batteries,the initial discharge capacity and platform of nano EMD1 cathode are higher than the commercial EMD0 at the larger current density;By studying cycle performance of nano EMD1 cathode in the Li/MnO2 secondary battery,the capacity retention ratio is 65%over 400 cycles at the high current density of 755 mA·g-1,showing good cycle performance;By studying the rate performance of nano EMD1 cathode,the capacities are stable under different current densities,without significant capacity loss;Electrochemical impedance spectroscopy(EIS)test find that the charge transfer resistance of nano EMD,cathode is 142.6 ?.The nano EMD1 cathode with high surface area,mesoporous structure and the stable frame channel of ?·?-MnO2 can enhance the migration of lithium ions and improve its structural stability and rate performance.2.The electrochemical properties of sodium ion battery for electrolytic manganese dioxide are investigated:The EMD2 is ?·?-Mn02 of "thorn ball" consisting of regular nanorods;BET test shows that the BET surface area of EMD2 is higher,which is conducive to the diffusion of sodium ions;By comparing the cyclic voltammetry tests of nano EMD2 cathode in Li/MnO2 battery and Na/MnO2 battery,the electrochemical reaction reversibility of nano EMD2 cathode in Na/MnO2 battery is not very good;By comparing the charge and discharge curves of nano EMD2 cathodes in Li/MnO2 and Na/MnO2 batteries,the difference of the discharge platforms is about 1 V,and the amount of reversible inserted sodium is lower;By comparing the charge and discharge curves in Na/MnO2 batteries at different current densities,there are two obvious charge platforms and a not obvious discharge platform(except initial discharge).As increase of current density,the voltage platforms become less obvious,showing that nano EMD2 cathode has certain recharge ability;By researching the cycle performances of nano EMD2 cathodes in Na/MnO2 batteries at different current densities,nano EMD2 cathode has a large capacity loss and low reversible capacity under low current density.However,the discharge capacity of 2nd cycle is as high as 123.6 mAh·g-1 under the high current density of 0.5mA·cm-2,without obvious capacity loss,showing good fast charge and discharge performance;XRD diagram analysis of nano EMD2 cathode after cycles find that the?-MnO2 of nano EMD2 cathode will collapse into a smaller[2×1]channel after multiple cycles,which will make EMD2 mainly going to be y-MnO2;EIS tests find that the charge transfer resistance of the nano EMD2 cathode is smaller after cycles,but sodium ions diffusion slows down.The nano EMD2 cathode with high surface area,mesoporous structure and the stable frame channel of a y-MnO2 is beneficial to the fast migration of sodium ions,however,Na+ with larger ion radius can cause greater volume expansion/contraction in the cycle process,which makes the larger channels collapsed into smaller to stabilize its structure and reduce the reversible sodium storage capacity.3.The preparation and electrochemical properties of Al-doped lithium manganate are investigated:The prepared Al-doped lithium manganate is homogeneous spinel nanoparticles with partial aggregation;The crystal cell shrinks with the increase of Al content,and the optimum content is 0.14;Cyclic voltammetry tests show that the electrochemical reactions of LiMn2O4 and LiAlo.14Mn1,86O4 are reversible;By comparing the charge and discharge curves of LiMn2O4 and LiAl0.14Mni.86O4 cathodes,all the charge and discharge curves shape are similar,but the platform of charge and discharge curves for LiAl0.i4Mni.86O4 are more obvious,and initial coulomb efficiencies are higher than 92%,showing excellent cycle stability;By studying the cycle performances of LiAlxMn2-xO4(x=0,0.1,0.14,0.18)cathodes,the discharge capacities and cycle performances of LiMn2O4 and LiAl0.i4Mni.86O4 are good.The capacity of LiAl0.14Mn1.86O4 is 100 mAh·g-1 after 500 cycles,and the good cycle performance can meet the requirement of power lithium manganite;By comparing the rate performances of LiMn2O4 and LiAl0.14Mn1.86O4,the capacities are fairly stable under 0.1,0.5,1.2,5 and 0.1 C,without significant capacity loss,and the capacity of LiAl0.14Mn1-86O4 under 0.1C is close to the theoretical specific capacity of spinel lithium manganate;EIS tests find that the charge transfer resistance of LiAl0.14Mni.86O4 cathode decreases after cycles,and the rate of lithium ion diffusion is constant,which will improve its cycle performance and fast charge and discharge capacity.The high surface area and the more stable spinel structure of nano Al-doped lithium manganate cathode can promote the migration of lithium ions,and improve the structural stability and rate performance of lithium manganate. |
PDF Full Text Request