| In the thesis,The different synthesis methods(such as rheological phase method,hydrothermal combined solid phase reaction method,co-precipitation combined solid ph ase reaction method)were used to synthesize ternary LiNi1/3Co1/3Mn1/3O2 cathode mate rials with improved electrochemical performance.The ele-ctrochemical properties of Li Ni1/3Co1/3Mn1/3O2 cathode materials were improved by doping and coating methods.The modern test techniques(X-ray diffraction(XRD),scanning electron microscope(SEM),inductively coupled plasma emission spectrometry(ICP),and cyclic voltammetry(CV))were used to characterize structure and composite as well as morphology for expecte d compounds.Then the electrochemical properties of the compounds synthesized were evaluated as a cathode in lithium ion battery system.The electrochemical reaction and mechanism were studied also.The important and valuble conclutions were obtained in our work.They were described as follows.The LiNi1/3Co1/3Mn1/3O2cathode materials were synthesized by rheological phase method using lithium hydroxide,nickel acetate and cobalt acetate as raw materials.At first,the raw materials were mixed to form the precursors at 120? and then the precursors were heated at 800,850,and 900 ?,respectively to form expected compounds.The expected compounds were characterized by XRD,SEM,ICP,and CV techniques.The properties were tested by battery comprehensive system.The results show that rheological phase method is effective method to prepare LiNi1/3Co1/3Mn1/3O2 cathode materials.It was found that synthesized temperature and treatment time are very important factor for synthesis process.When temperature was controlled at 850 ? for 12 h,the prepared sample behaved best electrochemical performances,Its specific capacity reached at 170 mAh/g after 50 cycles at a current density of 20 mA/g between 2.5-4.5 V.In addition,the sample as cathode have specific capacity of 157,132,111,and 70 mAh/g at diffrent current densities(50,100,200,and 400 mA/g respectively).The specific capacity recovered to 174 mAh/g when the current density went back to 20 mA/g.It was concluded that the cathode material prepared at 850 ?by rheological phase method behaved the best rate and cycle performance among all samples synthesized at orther temperature.Besides,in order to improve electrochemical performances of LiNi1/3Co1/3Mn1/3O2 cathode materials,doping and coating methods were used to prepare modified compounds.In respect of doping,the MgO;Al2O3 or ZnO was used as doped materials to synthesize ternary cathode materials doped with metal ions by rheological phase method at 850 ?.Different ratios of metal ions were employed to prepare different doped samples.For example,the LiNi1/3Co1/3Mn1/3O2 was doped with metal ions(Al3+;Mg2+ or Zn2+)with different ratios;The expected compounds doped with metal ions still maintain layer structure which is same as that of ?-NaFeO2(R3m).With the increasing of the dopant,the sizes of precursors and ion shuffling degree are also increased.When doped metal ions(Al3+;Mg2+ or Zn2+)ratio was 1%,the sample showed the best electrochemical performance among all doped compounds.The rate and cycle performance for LiNi1/3Co1/3Mn1/3O2 doped with metal ions were improved greatly.In respect of caoting,the AlPO4;carbon or polyaniline(PANI)was used to coat ternary cathode materials synthesized by rheological phase reaction method.The LiNi1/3Co1/3Mn1/3O2 coated with carbon was prepared using glucose as carbon source at 850?.The above coating materials were used to coat LiNi1/3Co1/3Mn1/3O2 cathod material separately,the similar results were obtained.The results showed that ratio of coating material to LiNi1/3Co1/3Mn1/3O2 was key facotor to its electrochemical properties.When the ratio of AlPO4 or polyaniline(PANI)was 1%,the modified compound bahaved better electrochemical properties than that of pure LiNi1/3Co1/3Mn1/3O2.Its charge and discharge capacity are about 176 and 174 mAh/g after 100 cycles.The coulombic efficiency was 99.8%.To LiNi1/3Co1/3Mn1/3O coated with carbon,when curent density are increased,although the discharge and charge capacity were decreased,cycle performance still behaved good stability.That is,the rate capability and cycle performance for coated LiNi1/3Co1/3Mn1/3O2 cathod materials were also greatly improved.The modification mechanisms for doping and coating were studied by electrochemical methods,the results showed that both doping and coating in LiNi1/3Co1/3Mn1/3O2 materials led increase of conductivity and diffusion coefficient of Lithium ion in the cathod material,which improved its rate specific capacity and cylcle performances.Co-precipitation method was used to prepare the precursors of LiNi1/3Co1/3Mn1/3O2.Then,these precursors were mixed with lithium salts to be calcinated at different temperatures,including 800,850,and 900 ?.The expected samples were characterized and investigated as cathode for LIBs.The sample prepared at 850 ? showed highest capacity.The capacity decreased from 176 mAh/g to 158 mAh/g after 20 cycles when current density was at 20 mA/g.When tested curent density was at 50;100;and 200 mA/g,the specific capacities were 134,112,and 98 mAh/g,and then the specific capacities dropped to 123,111,and 92 mAh/g,respectively.The capacity went back to 157 mAh/g when the current density changed to 20 mA/g.The cathode materials prepared at 850 ? showed highest reversibility and best capacity among all samples synthesized at other temperature..Hydrothermal method was also used to prepare precursors of the ternary cathode material.Then the precursors was mixed with Li2CO3 to prepare LiNi1/3Co1/3Mn1/3O2 by using solid phase method at different temperatures such as 800,850,and 900 ?.XRD,SEM,and TEM techniques were used to investigate its structure and morphology.Then,the expected samples were investigated as cathode materials for lithium ion battery.At range of 2.5V?4.5V,the sample of LiNi1/3Co1/3Mn1/3O2-850 showed best electrochemical performance.The initial capacity was about 186 mAh/g at the current density of 20 mA/g and dropped to 180mAh/g after 50 cycles.The sample synthesized by hydrothermalcombined solid phase reaction method behaved excellent electrochemical performance,so it is an effective method to prepare LiNi1/3Co1/3Mn1/3O2 cathod materials for lithium ion battery application. |
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