Synthesis And Electrochemical Performance Of Al₂O₃@LiMn₂O₄ And La_0.65Sr_0.35MnO₃@LiMn₂O₄ Cathode Materials For Lithium-ion Battery

With the fast development of society,the consumption of coal,oil and gas increase,the serious problems such as energy crisis,environmental pollution,global warming have already threatened the human living and its development.Petrol cars not only expend the petrochemical energy but also exhaust lots of toxic emissions,and the toxic emissions is to blame for the petrochemical energy crisis and environment pollution.In the future,new energy vehicles will replace the petrol cars,slow down the fossil energy crisis,eliminate pollution and renew blue shy.The development of new energy vehicles is high-profile,and the key of new energy vehicles is the power batteries!Lithium-ion battery has many advantages,such as high discharge voltage,large capacity,long circle-life,high energy density,high power density,low self discharge,environmentally friendly and no memory effect,it becomes the first choice of driving power for new energy vehicles.Spinel LiMn2O4 cathode material has the advantages of low-cost,high coulomb efficiency,good rate performance,environmental friendliness and better safety.Therefore it is a promising candidate for cathode materials in lithium-ion batteries.However,severe capacity fading,high-temperature performance and aside performances are severe problems of this cathode material,which have greatly limited spinel LiMn2O4 in large-scale applications for power battery in new energy vehicles.This paper reports on the preparation of Al2O3@LiMn2O4 and La0.65Sr0.35MnO3 materials,in order to solve the existing problems of spinel LiMn2O4 cathode material.At the same time,this paper studies the research on the preparation and electrochemical properties of the composites with coating layers,thereby obtaining LiMn2O4 coating cathode material with excellent electrochemical performances.The main research contents and results are as follows:(1)Al2O3@LiMn2O4 composite was prepared by sol-gel method using Al(NO3)3·9H2O and LiMn2O4 as the starting materials and NH3-H2O as the precipitator.The structure was investigated by X-ray diffraction(XRD),scanning electron microscope(SEM)and transmission electrin microscope(TEM).The electrochemical properties of the assembled button batteries using the as-prepared composite as cathode material.Based on the structure and electrochemical properties,the optimum sintering temperature and the best mass ratio of Al2O3 and LiMn2O4 are ascertained.The results show that Al2O3 coating does not change the structure of LiMn2O4 after calcination under the experimental conditions of 400,500 and 600℃,with the Al2O3 and LiMn2O4 mass ratio of 0.5,1.0,1.5,2.0 and 2.5%.The sample prepared at 500℃ with mass ratio of 1.0%presents a uniform Al2O3 layer wholly coats the surface of LiMn2O4,and the nanoscale Al2O3 shows an unordered and two dimension structure.Under 0.1 C rate at 25℃,the first discharge specific capacity is 112.7 mAh·g-1 and the capacity retention is 78.35%after 400 cycles at 1 C rate.When cycling at 55 ℃,the composite shows a capacity retention of 96.96%after 200 cycles.While the first discharge capacity of pristine LiMn2O4 is 124.2 mAh-g-1,but the capacity retention is 52.89%after 400 cycles at 1 C and 61.29%after 200 cycles at 55℃.The Al2O3-coated LiMn2O4 sample prepared at 500℃ with mass ratio of 1.0%has the best cycle and high temperature performances,which is considered as the optimized preparation condition.The uniform and complete Al2O3 coating can effectively prevent direct contact of electrolyte and LiMn2O4,thereby improving the electrochemical performances.(2)With the Al(NO3)3·9H2O and LiMn2O4 as raw materials,the Al2O3@LiMn2O4 composite materials prepared respectively by the methods of sol-gel,hydrothermal and micro emulsion,and the comparison study on the different methods were conducted.The structure and electrochemical properties were investigated by XRD,SEM and electrochemical tests.The results show the Al2O3 coating by sol-gel method is well distribured with uniform thickness and completely coated on the surface of LiMn2O4.The first discharge capacity is 122.9 mAh.g-1 at 0.1 C rate under 25℃;and the capacity retention is 86.53%after 450 cycles at 1 C under 25℃,and 85.46%after 200 cycles at 55℃ under the same rate.The Al2O3-coated LiMn2O4 cathode material prepared by sol-gel method has good cycle and high temperature performances.(3)LSM@LiMn2O4(LLMO)composite material was prepared by sol-gel method using La(CH3COO)3·2H2O,Sr(CH3COO)3·l/2H2O,Mn(CH3COO)2·4H2O and LiMn2O4 as the starting materials.The structure and morphology were investigated by XRD,SEM,TEM,XPS and Raman;the conductivity of LLMO and pritine LiMn2O4 samples were tested by DC Four-Point Probes;the electrochemical performances were tested on the assembled button batteries with the as prepared composite and pristine LiMn2O4 as cathode materials.The results show the chemical formula of LSM layer is La0.65Sr0.35MnO3.The close and uniform LSM thin layer wholly coats the surface of spinel LiMn2O4 with particle size of～40 nm.The electrochemical results reveal the as-prepared LSM-coated sample exhabits an cycle stabiltiy under high rates at elevated temperature.The specific discharge capacity is 129.9 mAh g-1 at 0.1 C and exhibits a capacity retention 90.6%after 500 cycles under 1 C rate.When cycling at 55℃,the composite shows a capacity retention of 93.6%after 130 cycles.The value of electrical conductivity increases from 5.31×10-5 S cm-1(BLMO)to 8.42×10-4 S cm-1(LLMO).The LLMO sample has excellent electrochemical performances,especially the cycle performance and high temperature performance.The LSM film layer can effectively prevent direct contact of electrolyte and LiMn2O4,thereby improving the electrochemical performances.The research that La0.65Sr0.35MnO3 is introduced as a coating material for LiMn2O4 cathode material has not been reported in open literature.