Synthesis And Modification Of LiNi_0.5Mn_1.5O₄ Cathode Material

Since the commercialization of the secondary lithium ion battery in 1991, it has been widely used in portable electronic products, electric vehicles, and energy storge systems. Nowadays, the energy density is the key factor to restrict the further development of the lithium ion battery, and how to increase the capacity of cathode and anode materials appears to be of great urgency. In this regard, the high-voltage cathode materials have attracted intensive attention due to their high voltage plateaus and large energy density. Among them, the spinel structured LiNi_0.5Mn_1.5O₄ cathode material is a promising candidate. However, this material also has the problems such as effecting by Jahn-Teller effect and dissolving manganese, leading to the poor cycling ability, compared with commercialized cathode materials. Therefore, this thesis describes the studies on how to control the formation of precipitates in the co-precipitation synthesis method, thus providing cathode materials with improved properties during cycling.The main contents are: controlled synthesis of the precursors of the high voltage cathode material LiNi_0.5Mn_1.5O₄ and according characterization of physical and electrochemical properties. Micro-nanostructural LiNi_0.5Mn_1.5O₄ cathode material was prepared by two steps: in the first step, the precursor of（Ni0.25Mn0.75）CO3 nanoparticles or microspheres with good crystallinity were prepared by coprecipitation and subsequent hydrothermal; in the second step, the as-prepared precursor was mixed with lithium salt by a solid phase calcination to obtain the final LiNi_0.5Mn_1.5O₄ cathode material. In the first step, by controlling the feeding way, choosing proper precipitating agent or adding surfactant hexadecyl trimethyl ammonium bromide（CTAB） can obtain a series of final products with different crystal structure, morphology. Also, phase, morphology and electrochemical properties of the final product Li Ni0.5Mn1.5O4 cathode materials were conducted. This thesis consists of four parts, as described below:1. The progress of high voltage cathode material is summarized in particularly the high-voltage cathode material Li Ni0.5Mn1.5O4, including crystal characteristics, preparation methods and modification. Besides, the significance of this research is also included.2. The effect of adding order of reactants to form precipitation on the synthesis of LiNi_0.5Mn_1.5O₄ has been studied. The results indicate that the cathode material prepared by adding the solution of metal salts into the precipitant solution does not influence the size and morphology of the resulting crystals, compared with the material prepared with the opposite addition order. But the former shows less aggregation of particles than the later.3. The effects of surfactant CTAB on the synthesis of LiNi_0.5Mn_1.5O₄ have been studied. It turns out that with an increasing concentration of CTAB form 0 to 0.12 mol/L to 0.24 mol/L, the resulting cathode materials show the capacity retention of 84.3%, 85.7%, 96.2%, respectively, after 50 cycles at the charging/discharging rate of 0.2 C. In addition, with the increasing of CTAB concentration, the aggregation of primary particle is inhibited, resulting in better particle dispersion, more desirable crystalline, and thus better electrochemical properties.4. The formation of LiNi_0.5Mn_1.5O₄ by using different precipitations has also been studied. Using Na2CO3, NH4HCO3 and CO（NH2）2 results in materials with layered crystalline, polyhedron, microsphere morphology, respectively, after 50 cycles of charging/discharging at 0.2 C. The Li Ni0.5Mn1.5O4 materials formed by using NH4HCO3 and CO（NH2）2 show capacity retention of 95.1% and 98.1%, respectively, after 100 cycles of charging/discharging at a rate of 2 C.