| Spinel LiNi0.5Mn1.5O4 has a high discharge potential plateau of 4.7V (vs. Li+/Li), it can effectively elevate the energy and power densities of the batteries when it is used as a cathode material of lithium ion batteries. In LiNi0.5Mn1.5O4, the function of Mn4+is to stabilize the structure of the material, and that of Ni2+ is to perform the oxidation/reductuin reaction, so it has excellent electrochemical cycling performance. While this material is heat treated at high temperature, the oxygen is going to be lost, and the structure will change from perfect P4332 to nonstoichiometric Fd 3 m. For maintaining electronic neutrality of the whole material, the valence of part of Mn in the material has to be changed from+4 to+3. In addition, due to the relative high discharge potential as compared to the stable potential windows of commercial electrolyte and the dissolution of Mn3+produced during the heat treatment at high temperature, the electrochemical performance of LiNi0.5Mn1.5O4 at high temperature become worse obviously.Anode material Li4Ti5O12 gives no obvious structure changes and no volume expansions or contractions during charge and discharge process, so it is normally called "Zero-Strain" material. Li4Ti5O12 has not only the excellent electrochemical cycling performance, but also high rate performance due to the three dimensional lithium ion diffusion path. However, Li4Ti5O12 has relatively higher working potential (1.55V (vs. Li+/Li)) than traditional carbon anode, so only when Li4Ti5O12 is matched with a cathode material which has higher working potential than traditional one then can a battery be obtained with a voltage higher than 3V. LiNi0.5Mn1.5O4 positive electrode material with high working potential can be matched with a Li4Ti5O12 anode material to assemble a battery with working voltage of 3.2V.Aimed at solving these problems and satisfying the requirements mentioned above, this work is focused on the following three aspects:the optimization of synthetic method, performance improvement by doping modification and the matching of anode and cathode in the lithium ion batteries. The main contents are as follows:1. Spinel type LiNi0.5Mn1.5O4 materials with excellent electrochemical performance are synthesized by optimizing the ratio of raw materials and heat treating temperature in the sol-gel process. On the basis of traditional sol-gel method, more uniform LiNi0.5Mn1.5O4 material with better electrochemical performance is obtained by applying ultrasonic treatment in the synthesizing process. Based on the above synthesizing process, the heat treatment temperature has also been optimized, and different LiNi0.5Mn1.5O4 materials are obtained with heat treatments at different temperature (650,750,850,950 and 1000℃), respectively. The results show that the XRD diffraction peaks increased as the heat treatment temperature is raised, and the peaks induced by impurities resulting from the oxygen lost during the high temperature treatment also increase as the heat treatment temperatures is raised. The particle size of the synthesized materials grows larger as the temperature is raised. It can be seen from the electrochemical test results that the material synthesized at 850℃gives the highest discharge capacity and most excellent rate performance. The discharge capacities are 111.4,103.2 and 99.3 mAh g-1 at the rate of 5C,10C,15C, respectively.2. Spinel LiNi0.5Mn1.5O4 with Fd3m structure is obtained by one step precipitation method under different raw materials concentrations (i.e.0.05 mol L-1,0.1 mol L-1,0.2 mol L-1 and 0.3 mol L-1) and different synthesizing temperatures (25℃,50℃and 80℃). The materials obtained by this method have stable structure and high electrochemical reversibility. The results show that different raw material concentrations have little effect on the structure of the materials, while the particle size becomes smaller as the raw material concentrations are increased. Among the four materials synthesized by four raw material concentrations, the material (AO.2-80) synthesized with the concentration of 0.2 mol L-1 has the best electrochemical performance. Its initial discharge capacity is 131.5 mAh g-1 and remained 128.5 mAh g-1 after 50 cycles. Sample A0.2-80 shows obvious advantage in rate test compared with the other three materials, it is about 102.3 mAh g-1 and 86.9mAh g-1 at 10C and 15C rate, respectively. When the material is discharged at low current after high rate test, the discharge capacity of all the four samples can revert back to above 95% of the initial discharge capacity. This is an indication that the structure of the four samples is not destroyed during the high rate tests. When hydrothermal treatment is added after the precipitation step, it can result in better crystalline of the materials. The materials synthesized with the addition of hydrothermal treatment give larger particle size and a more regular morphology. Conversely, the materials synthesized without hydrothermal treatment have smaller particle size with serious glomeration and bad electrochemical performance. In addition, higher synthesis temperature gives high reactivity of the precursor, so the material synthesized at higher temperature shows more excellent electrochemical performance and stable structure.3. When LiNi0.5Mn1.5O4 is used as a cathode material and tested at high temperature (55℃), it shows bad cycling performance and its discharge capacity is kept only 65.9% of the initial discharge capacity after 50 cycles. The charge and discharge tests among different voltage range show that no structure damage occurs during the charge and discharge process, while the cycling performance is aggravated along with the increasing of the cutoff voltage for charging. The material LiNi0.5Mn1.5O4 calcinated under oxygen atmosphere contains less amount of Mn3+, however this feature cannot improve the high temperature electrochemical performance of the material but promote its degradation. So we can deduce that the contents of Mn3+ in the material and the dissolution of Mn from the material are not the main reasons for the bad performance of LiNi0.5Mn1.5O4 at high temperature. The results of doping with different elements of LiNi0.5Mn1.5O4 show that Co-doping can improve the cycling performance of this material remarkably at room temperature, while Y-doping has decreased the discharge capacity of the material, which can be explained by the reason that there is a great difference in ion radius between Y3+ and Ni2+, Mn4+. And there is an impurity phase Y2O3 emerged when 5% Y has doped. Although both the Co and Y doping can decrease the discharge capacity of LiNi0.5Mn1.5O4 at high temperature, it can also improve the cycling performance of this material at 55℃.4. Li4Ti5O12 is synthesized by a solid state method at high temperature with different lithium salts and different Li/Ti ratio. The results show that the material Li4Ti5O12 synthesized with lithium hydroxide monohydrate and [n (Li+)/n(Ti+)]=1.1 gives the best electrochemical performance. The full lithium ion battery comprised with Li4Ti5O12 synthesized by solid state method as the anode material and LiNi0.5Mn1.5O4 synthesized by ultrasonic assistant sol-gel method as the cathode material is assembled with different anode and cathode capacity ratio. It is found that, different anode and cathode capacity ratio gives quite different electrochemical performance. When the anode capacity is larger than that of cathode (P/N<1), the cycling performance of the full battery is bad and it is getting worse along with the decreasing of P/N ratio. Conversely, when the cathode capacity is larger than that of anode (P/N>1), the cycling performance of the full battery among the ratiol<P/N<1.5 is better than that of the battery with a ratio of P/N<1. More significantly it shows great improvement when the ratio is kept at P/N>1.5 and its discharge capacity gives almost no decay after 40 cycles. However, when the ratio is in the range of P/N>1.5, the discharge capacity shows a decline. |
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