Studies On Multi-doped Lithium Nickel Cobalt Oxides As Cathode Materials For Lithium Secondary Batteries

In recent decade, lithium ion batteries have been rapidly developed because of their advantages on high cell voltage, high energy density, slow self-discharge, no-memory effect and etc. Till now, lithium ion batteries have occupied the main status in small portable electronic equipment, such as laptop computers, mobile phones and vidicons. At the same time, lithium ion batteries show promising prospects of application in electric vehicle, space technique and military field.Up to now, most commercialized lithium ion batteries use lithium cobalt oxides, LiCoO₂, as their positive material. But it suffers from high price and high toxicity. LiNiO₂ has been ever considered to be a promising substitute, which is with a larger discharge capacity and lower price. However, the preparation difficulties and poor safety limits its practical application. Basing upon the above two materials, LiNi_1-yCo_yO₂ was exploited. Comparing with its end compound-LiCoO₂ and LiNiO₂, LiNi_1-yCo_yO₂ shows a higher capacity and better cycleability, moreover its electrochemical performance and safety property can be further improved through doping treatment. So it won great attention since it was firstly proposed.In this thesis, a rheological phase method was adopted in the sample preparation. The influences of the synthetic condition were comprehensively studied through structural and electrochemical characterization. Basing on the establishment of the optimum preparation condition, several doping phases were further obtained. Their structure character, surface property, thermal stability and electrochemical performance was investigated. And the main results are as follows: i. A series of layered LiNi_1-yCo_yO₂ compounds was synthesized through rheological phase method and an optimum preparation condition was established.LiNi_1-yCo_yO₂ was obtained through the calcination of the precursor that was prepared by rheological phase method. An optimum preparation condition wasdetermined through the structural and electrochemical analysis of the sample that was obtained under different calcinations temperature and with different Co/Ni molar ration. The result of XRD measurement and cycling test shows that the resulting LiNio.85Coo.15O2 sample presents an ordered layered structure and best electrochemical performance when the precursor was calcined in oxygen atmosphere for 6h. The first discharge capacity of LiNio.85Coo.15O2 reaches 198.2 mAh/g, and more than 90% initial capacity can be retained after 20 cycles. Comparing with the sample synthesized by the solid-state method, the sample prepared by the rheological phase method shows an obviously improved structural and electrochemical property.ii. Investigation on Al-doped LiNio.85Coo.i5-yAly02 phaseA series of LiNio85Coo.i5-yAly02 samples was prepared through rheological phase method. The effect of Al-doping on the structure, electrochemical character and thermal property was discussed. It was found from structure analysis that the lattice parameters "a" and "c" of LiNio85Coo.i5-yAly02 increase with the increasing of y value, and a corresponding slight increase in the c/a ratio was also observed. The variation results from the substitution of Al for Co. In the DSC measurement, it was noticed that the Al-doping phase exhibited an improved thermal stability. It can be explained by the following reasons. The first, the structural stability of the de-lithiated phase is enhanced and some unfavorable phase transformation is depressed after Al is introduced in the framework of LiNio.85Coo.15O2; the second, the existence of A12O3 facilitates the heat release considering its good heat conductibility. Comparing to the undoped LiNio.85Coo.15O2, the Al-doped phase shows an improved cycleability. The smaller capacity fading suggests that Al-doping did favor the maintenance of the layer crystal structure during repeated cycling.iii. Investigation on other doping phase- LiNio.85Coo.i25Mo.o2502(M=Ca, Ga, Ti)LiNio.85Coo.i25Mo.o2502 (M=Ca, Ga, Ti) was also synthesized through the rheological phase method. CV and AC impedance technique was applied to examine the Li+ intercalation/de-intercalation behavior. CV measurement shows that dopingLiNio.85Coo.15O2 with Ga3+ or Ti4+ can depress the phase transformation that occurs during the charge and discharge process, especially for the Ti-doped phase. It also implies that Ga3+ or Ti4+ can enter into the crystal cell and the resulting variation in microstructure brings about the difference in Li+ insertion/ejection process. According to the result of CV experiment, it is suggested that Ca2+ is unable to be introduced into the framework of LiNio.g5Coo.15O2 due to its large ion radius, instead it exists as CaO. Although the mechanism for the notable improved cycleabilty for LiNio.85Coo.i5.x02-xCaO phase is not clear yet, it is speculated from the preliminary analysis of EIS that the role of CaO is similar to that of the coating layer. The existence of CaO is in favor of the formation of a stable interface between the LiNio.85Coo.15O2 electrode and the electrolyte, thus the decomposition of the electrolyte can be greatly avoided, which finally results in the enhancement of the cycle life of LiNixCoi.xO2.material.