Syntheses And Electrochemical Properties Of Spinel-type Electrode Materials For High Power Lithium Ion Batteries

Lithium ion batteries of high voltage and high energy density are used in mobile phones and other small electronics. With the development of the society, new demands to the batteries come out for different purposes. For example, smart phones consume electric energy faster than traditional phones and need to be recharged every day, which is troublesome and require batteries with high capacity. When used in electric cars, the batteries should be able to be discharged at high current densities to achieve high power, in addition to high capacity. New energy technologies and energy-saving technologies like solar cell, wind energy and electronic car are developing fast nowadays for the increasing demands of energy density. But the solar power and wind energy is not stable so that energy storage devices are needed to stabilize the output of these energy sources. Batteries for energy storage should be cheaper and with higher power density. Commercial lithium ion batteries comprises graphite as anode and lithium cobalt oxide as cathode. Its cost, energy density, safety and rate performance cannot meet the the demands of energy storage devices and electronic cars. The research on new electrode materials draws more and more attentions. In this thesis, our aim is to improve the power density of lithium ion batteries using new electrode materials. Lithium nickel manganese oxide (LiNi_0.5Mn_1.5O₄) with high potential （4.7 V vs. Li） is used as a cathode and lithium titanium oxide (Li₄Ti₅O₁₂) is used as an anode material. The electronic conductivity and Li ion conductivity of LiNi_0.5Mn_1.5O₄ are both high so that this material shows high rate performance. On the other hand, there is no SEI interphase formed on the surface of Li4i5O12 during cycling, which makes Li₄Ti₅O₁₂ performs better rate performance than graphite. It is coincidence that both these two materials are with spinel-type structures. In this thesis, the syntheses and modifications of these two spinel electrode materials (LiNi_0.5Mn_1.5O₄ and Li₄Ti₅O₁₂) are studied and the rate performance is improved. Then, these two materials are coupled to obtain a full cell with high rate performance. In addition, we have also studied another spinel-type anode material （Co₃O₄） and another novel cathode material （Cr₂O₅）.In Chapter, a general introduction is given on the following aspects:the charge-discharge mechanism, configurations and electrode materials of lithium ion batteries. A review of some spinel materials like LiMn2O4 and Li₄Ti₅O₁₂ which are used as electrode materials is given. We also introduce the demands and current situation of electric cars. The background of this thesis is also given.In Chapter 2, raw materials and equipment used in the project of this thesis are briefly introduced. The details of assembling process of coin cells, common electrochemical and structure test method are introduced too.In Chapter 3, LiNi_0.5Mn_1.5O₄ spinel samples with good electrochemical performance are synthesized by a two-step solid-state reaction method. First, we have surveyed literature on LiNi_0.5Mn_1.5O₄ and found that the sample obtained from co-precipitation method shows the highest capacity, being almost 140 mAh g^-1. The difference of co-precipitation method is that nickel and manganese precursors are mixed first, then the lithium precursor is added next. In our experiments, manganese acetate and nickel acetate are mixed together first and react at 500℃ to form a mixed Ni-Mn oxide, which then reacts with lithium acetate at 900℃ to form a LiNi_0.5Mn_1.5O₄ powder. For comparison, manganese acetate, nickel acetate and lithium acetate are mixed together in a molar ratio of 3:1:2 directly and sintered at 900℃. The LiNi_0.5Mn_1.5O₄ powder synthesized from the two-step process shows enhanced conductivity and improved rate performance. XRD and Raman analyses indicate that the former sample has a space group of Fd-3m while the latter sample has a space group of P4332. The cyclic voltammetry test has confirmed that the former sample has a better conductivity. Then, we also investigate the structure and electrochemical performance with different lithium contents in LixNi0.5Mn1.5O4 and Cr doping to replace some of nickel and manganese.The advantage of the solid-state reaction method is low cost and simplicity. But when a solid-station reaction process is used to synthesize LiNi_0.5Mn_1.5O₄, some impurity of Li_xN_1-xO appears at high temperature and it is hard to overcome. To decrease the amount of Li_xN_1-xO, we use citrate method to obtain LiNi_0.5Mn_1.5O₄ in Chapter 4. Traditionally, a citrate method is time-consuming. As a contrast, we use microwave to heat the solution containing citric acid. After calcining at a high temperature, LiNi_0.5Mn_1.5O₄ with excellent rate performance is obtained. In addition, we also investigate the change of the structure and impedance spectra of LiNi_0.5Mn_1.5O₄ after being cycled at high discharge current.In Chapter 5, chromium-doped Li₄Ti₅O₁₂ （LiCrTiO₄） is synthesized using acrylic thermal polymerization method. After the calcination at high temperature, LiCrTiO₄ is obtained with an impurity like LiCrxOy, in which there is some high valence chromium Cr⁶⁺. This impurity is converted into a new material after the lithiation in the first discharge process and can lower the impedance of LiCrTiO₄, which makes LiCrTiO₄ with excellent rate performance.Considering a Cr⁶⁺-containing compound can improve the rate performance of LiCrTiO₄, in Chapter 6, we use a solution of CrO₃ to treat Li₄Ti₅O₁₂ and test the electrochemical performance of modified samples. After the modification, there are some new impurities like TiO₂, Li₂CrO₄ and possible Cr₂O₅ generated in the products. These impurities are resulted from ion exchange process. CrO₃ is converted into chromic acid when dissolved in deioned water, while H⁺ in chromic acid would exchange with Li⁺ in Li₄Ti₅O₁₂ to form H₄Ti₅O₁₂ and Li₂Cr₂O₇. After the calcination at 350℃, H₄Ti₅O₁₂ changes into TiO₂ and Li₂Cr₂O₇ is converted into LiCrO₄ and Cr₂O₅. As expected, the modified Li₄Ti₅O₁₂ shows better rate performance. To find the cause of improvement, other three samples are prepared using TiO₂, LiCrO₄ and Cr₂O₅ coated on the surface of Li₄Ti₅O₁₂. Except for TiO₂, other two impurities are both found to play a positive role. Li₂CrO₄ and Cr₂O₅ can react with lithium and generate a low potential material, which can make some Li_4+xTi₅O₁₂ on the surface stable even after being charged to 2.5 V. It can improve the electric conductivity and makes the rate performance better.In Chapter 7, we couple LiCrTiO₄ and LiNi_0.5Mn_1.5O₄ with perfect rate performance to obtain full cells with perfect electrochemical performance. But when discharged at high current, the capacity is lost obviously when the cycle number is increased. The reason of capacity loss is the increase of impedance.In Chapter 8, basic cobaltous carbonate with different morphologies are synthesized during a hydrothermal process. After calcination, these basic cobaltous carbonate decompose into Co₃O₄ with retained morphology. These obtained Co₃O₄ shows very low initial capacity loss （16%） and good cycling and rate performances. There are some difference between Co₃O₄ with different morphology; the nanosheet Co₃O₄ shows better mechanical strength and the morphology is retained even after grinding. The integrity of structure leads to better rate performance.Cr₂O₅ is irreversible between 1.0-2.5 V, but if charged to a higher voltage （4.5 V）, it shows high reversible capacity. In Chapter 9, Cr₂O₅ was prepared by a thermal decomposition of CrO₃ at different temperatures and the electrochemical performances are tested. When cycled between 1.0-4.5 V, Cr₂O₅ shows a capacity of over 300 mAh g^-1, but the capacity between 1.0-2.0 V decreases fast. To improve the cycling performance, the lower cut-off voltage is increased to 2.0 V, over 96% capacity can be retained after 100 cycles. During the first charge and discharge processes, Cr₂O₅ is first converted to a partially lithiated LixCr₂O₅ and then fully lithiated to an unknow phase （LiyCr₂O₅）. In the following cycles, LiyCr₂O₅ is charged and discharged following a solid solution process. Cr₂O₅ can be only used as a cathode material when coupled with lithium for there is no lithium source in this material. Thus we also synthesize lithiated Cr₂O₅ using chemical lithiation methods. BuLi and LiI are usually used as lithium source for chemical lithiation. The potential of BuLi is about 1.0 V versus Li and it is suitable for chemical lithiation of Cr₂O₅. BuLi should be 80% excess or the reaction time should be very long （over 6 days） to obtain completely lithiated Cr₂O₅. The as-synthesized LiyCr₂O₅ shows a high capacity of over 190 mAh g^-1 and is stable below 500℃. When calcined over 600℃, the is decomposed into LiCrO2. Besides BuLi, we also used Lil to synthesize Li yCr₂O₅. The potential of LiI is about 2.8 V versus Li and is too high to reduce Cr₂O₅. We calcine the mixture of LiI and Cr₂O₅ at higher temperature of 350℃ for chemical lithiation.At last, we make a brief summary of the innovations and shortcomings of this thesis work. We also point out the possible direction and aims of the related research in the future.