| Intensive research and development work has been conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials.LiCoO2 has been used a major cathode material in commercial lithium ion battery production.However,due to its disavantages of high cost,toxicity,and instability at high potential windows,considerable effort has been expended to find possible alternative to LiCoO2.Ternary transition metal oxide,Li-Ni-Co-Mn-O was regarded as the possible substitute for LiCoO2,owing to its excellent performance.Extensive researches have been made to LiNi1-x-yCoxMnyO2 at different ratio of Ni,Co and Mn.This work emphasized layed LiNi1/2Co1/6Mn1/3O2 material possessing superior performance and lower cost,based on scientific literature review and experiments verification.The synthesis,morphology,crystal structure and electrochemical behaviors of layered LiNi1/2CO1/6Mn(1/3O2 cathod material were studied in detial using X-ray diffraction,scanning electron microscope, galvanostatic charge-discharge tests and electrochemical impedance spectroscopy. LiNi1/2Co1/6Mn1/3O2 was synthesized by mixing Ni1/2Co1/6Mn1/3(OH)2 with LiOH·H2O followed by heat-treatment.The precursor Ni1/2Co1/6Mn1/3(OH)2 was prepared via hydroxide co-precipitation using ammonia as complexant. The effect of synthesis method and synthesis conditions,such as the pH value of the aqueous solution,dosage of ammonia,time of reaction were studied. The results indicated that the optimum conditions for Ni1/2Co1/6Mn1/3(OH)2 were that the pH value was 10.5,the NH3/M was 2.25 and the reaction time was 12h.In the optimization for high temperature solid state reaction of Ni1/2Co1/6Mn1/3(OH)2 and LiOH·H2O,pre-heated temperatrue was selected at 450℃by thermogravimetric analysis and the effects of pre-heated time, calcination temperature,calcination time and Li/(Ni+Mn+Co)ratio on the electrochemical properties of LiNi1/2Co1/6Mn1/3O2 were characterized by orthogonal experiments.The results showed that LiNi1/3Co1/3Mn1/3O2 which were synthesized from the mixture of Ni1/2Co1/6Mn1/3(OH)2 precursor and LiOH·H2O(Li/(Ni+Mn+Co)=1.05)sintered at 850℃for 12h after pre-heat 4～8h at 450℃in the air,exhibited better electrochemical performance.Multiple co-precipitation route was devised to improve the efficiency of the first charge-discharge cycle.Gradient precursor Ni1/2Co1/6Mn1/3(OH)2 was prepared via multiple coprecipitation of solutions with different composition of Ni2+,Co2+,Mn2+.The deposition with lower Co content was covered by Ni1-x-yCoxMny(OH)2 with higher Co content in appropriate conditons.Gradient LiNi1/2CO1/6Mn1/3O2 in which the content of Co was increasing from interior to exterior was received by solid state reactions of this kind of precursor and LiOH·H2O.Tests showed that the gradient material possessed perfect layered crystal structure and eximious electrochemical performance.Not only the first discharge specific capacity of the gradient material was higher than the homogeneous material,but the cyclic stability of the gradient material is improved greatly.The gradient material sintered at 800℃delivered high discharge specific capacity of 171.2 mA·h·g-1after multiple cycles,which was higher than that of commercialized LiCoO2.The gradient LiNi1/2Co1/6Mn1/3O2 material was modified by doping F-. The results illuminated that the tap density of the material doped F- was improved to approximately 2.41 g·cm-1,closed to commercialized LiCoO2.
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