| Based on the demand of energy,environment and economy,it is inevitable to replace the existing transportation system for power vehicles(EVs)and hybrid electric vehicles(HEVs)which require better lithium-ion batteries.Howerer,the cathode materials of lithium-ion batteries restrict its development and application to EVs and HEVs.The Ni-rich layered cathode material of LiNixCoyMn1-x-yO2 is given high expectation due to its high theoretical capacity which can meet the energy requirements of the power car.But LiNixCoyMn1-x-yO2 materials suffer from several problems including the rapid capacity deterioration,phase transition of the structure and release of oxygen from the main structure during the cycling.There are two most effecttive methods that are bulk doping and surface coating to improve the properties of LiNixCoyMn1-x-yO2.The full concentration-gradient materials of lithium-ion batteries in which the chemical composition and element distribution continuously change are derived from the coating.Therefore,this kind of materials can effectively improve the electrochemical and thermodynamic properties and have broad application prospects.The full concentration-gradient cathode materials were synthesized and characterized in this paper.The electrochemical tests were carried out to study the rate capability,cycling and safety performance of the concentration-gradient cathode material.The full concentration-gradient hydroxide precursor was prepared via co-precipitation method and technique of "Tubular synthesis" from mole ratio of Ni/Co/Mn=8:1:1 and Ni/Co/Mn=5:0:5 solution as initial material.The characterization results of the concentration-gradient precursor show that the content of Ni/Co/Mn in the precursor particles is graded variation.And its XRD diffraction peaks were correspond to the typical hydroxide precursors,no impurity.The properties of final lithiated products were affected by high temperaturesolid-state sintering process.In addition to the full concentration-gradient cathode material,the problems about ion-gradient disappearance and Li/Ni disorder should be considered.The Ni0.8Co0.iMn0.1(OH)2 precursor was used to conduct sintering tests by the uniform design method and the relationship between influencing factors and sintering results was established as a result.It was found that the ratio of lithium and precursor,calcination temperature and oxygen flow rate were the most important.The optimum condition of regression equation that the amount of lithium was 1.065,the pre-heating temperature was about 500?,the pre-heating time was 4h,the calcination temperature was 770 ?,the calcination time was 22h and the oxygen flow rate was 40mL·min-1 was applied to precursor to obtain the full concentration-gradien cathode material.The chemical composition of cathode material with concentration-gradient was LiNi0.643Co0.055Mn0.302O2 measured by gravimetric method,redox titration and ICP-AES.The LiNi0.643Co0.055Mn0.302O2 particle was nearly spherical and had the size of 5?m in diameter with narrow particle distribution.The results of XRD showed that LiNi0.643Co0.055Mn0.302O2 is a-NaFeO2 layered structure with space group R 3m and I(003)/I(104)peak intensity ratio was>1.2 indicating Li/Ni mixed degree was low.The Ni/Co/Mn content changes in the full concentration-gradient cathode material and the corresponding precursor was same,but the change degree was lower abrupt because of metal ions internal diffusion at high sintering temperature.The initial discharge capacity of LiNi0.643Co0.055Mn0.302O2 was about 187.68mAh g-1,slightly less than LiNi0.8Co0.1Mn0.1O2,but its capacity retention at 3C rate was close to LiNio.5Co0.5O2.Cycling at 1C rate for 200 times,the capacity retention of LiNi0.643Co0.055Mn0.302O2 was about 86.90%,while LiNi0.8Co0.1Mn0.1O2 was only about 77.24%,which was attributed to the rich-Mn surface in the cathode material with full concentration gradient.The cycling performance and rate capability of concentration-gradient cathode material LiNi0.643Co0.055Mn0.302O2 could be effectively improved with the small capacity as the sacrifice. |
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