Preparation of Cathode Material LiMn2O4 for Lithium Ion Batteries by Solvent-Gel Method and Its Properties
Abstract: In this experiment, the spinel LiMn2O4 was synthesized by solvent-gel method using CH3COOLi and Mn(CH3COO)2 as raw materials. This experiment mainly studied the physicochemical properties of the spinel powder LiMn2O4 synthesized at 850 °C calcination temperature. The product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and tested for electrochemical performance.
Keywords: spinel LiMn2O4; solvent gel method; cathode material; lithium ion battery
Lithium-ion battery is a green high-energy rechargeable battery that appeared in the early 1990s. Because of its high voltage, large specific energy, long life of charge and discharge, stable discharge performance, relatively safe, and no pollution , it is well received by the society and users. At present, the biggest use is in mobile phones and notebook computers, which fully demonstrates the superiority of high-power batteries, and has become the main power source for these two types of electrical appliances. It is also the portable electrical appliances such as mobile phones and computers that have created opportunities for the development of lithium-ion batteries.
The spinel lithium manganate cathode material is considered to be one of the most promising cathode materials for lithium ion batteries today due to its abundant resources, low cost, easy preparation, high safety, no toxicity and no pollution. . There are many ways to synthesize spinel LiMn2O4, and it is crucial to choose an optimal way to prepare it. To date, there have been many methods to be implemented, such as high temperature solid phase synthesis, mechanical activation, molten salt, microwave, solid phase coordination, Penehini, sol-gel coprecipitation, emulsification. Drying method, spray drying method, and the like.
In this experiment, we mainly use solvent-gel method to prepare spinel lithium manganate . This method of preparing colloidal chemistry is different from traditional sintering, melting and other physical methods, which overcomes the high temperature. The shortcomings of the solid phase reaction, the outstanding advantages of the method are: the components of the raw materials can achieve uniform mixing at the molecular level, the product has good chemical uniformity, high purity, the stoichiometric ratio can be precisely controlled, the heat treatment temperature can be significantly reduced, and the heat treatment time can be Significantly shortened, suitable for the production of nano-scale, sub-micron powder.
The experiment used solvent-gel method to synthesize LiMn2O4 at 850 °C, and characterized its morphology and structure by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Chemical properties . The synthesis method is as follows: CH3COOLi, Mn(CH3COO)2 is placed in a beaker, dissolved in a small amount of distilled water, a complexing agent (citric acid) is added, and a gel is formed through a gelation and gelation process, followed by baking. Dry and high temperature calcination gives LiMn2O4 cathode material.
Weigh 0.01g of LiMn2O4 powder and mix it with 0.006g of Teflonized ethylene carbon black (TAB), and then press it into the cathode of the battery; use the metal lithium/polypropylene film to make the anode and insulating diaphragm of the battery separately; 1 mixed solution of ethylene carbonate and dimethyl dimethyl carbonate in LiPF6. It was made into a button battery of CR2032 for further electrochemical testing.
3 Results and Discussion
Figure 1 is a scanning electron micrograph of the produced product particles. It can be seen from the SEM image that the LiMn2O4 crystal prepared by the solvent-gel method has agglomeration phenomenon , and the particle size of the crystal agglomerated is between 100-130 μm. This is because the preparation of LiMn2O4 by solvent-gel method is to first polymerize organic matter and metal cations to form macromolecules, and then burn the organic matter by calcination, thus causing the crystal to agglomerate to form polymerized macromolecules, and the macromolecular crystals formed by polymerization are The action of the electrolyte protects Mn, which can reduce the contact between the solid particles and the electrolyte, and reduce the dissolution of the metal cation in the electrolyte. In addition, it can be seen from the figure that the macromolecule agglomerated by LiMn2O4 is composed of numerous small crystal particles of uniform size and regular shape. The particle size of the small crystal particles is about 500 nm, and the crystal form is relatively perfect, showing a relatively regular pattern. There are many regular channels between the small crystals due to the solvent-gel process which is formed by the carbon dioxide and water vapor generated when the organic matter is calcined. Regular channels during charge and discharge can reduce the barrier of lithium ion migration and enhance the conductivity of the material . It can be seen that although a large number of particles are united together, the particle size distribution is relatively uniform and the particle size is about 500 nm. It facilitates the extraction, embedding and enhancement of the anti-distortion ability of lithium ions, thereby enhancing the electrical conductivity of the material.
At present, the production process of lithium ion battery cathode materials at home and abroad is mainly based on high temperature solid phase method, which is also a mature method for preparing lithium ion cathode materials . The method is simple in process and easy to operate, but the phase of the obtained product is not uniform, the particles have irregular shapes, the grain boundary size is large, and the particle distribution range is wide, resulting in poor electrochemical performance. The reason is that lithium acetate and manganese acetate are not It can be fully contacted, and the reaction temperature is high and the time is long. In order to complete the reaction, the material must be treated, which makes the reaction energy consumption large, lithium loss is serious, it is difficult to control the stoichiometric ratio, and it is easy to form a heterophase. Compared with the solid phase method, the solvent gel method can mainly be realized. The atomic level of the reactants is uniformly mixed and the synthesis temperature is low, so the prepared product has small particle size, good uniformity, large specific surface area, easy to control morphology and composition, but the preparation process is complicated, and the control requirements of solvent-gel preparation conditions are required. Higher. The temperature of the precursor is relatively low when sintered, and the particle size of the synthesized material powder is relatively small, usually a nano material.
Figure 2 is a LiMn2O4 X-ray diffraction pattern, and Figure 2(b) shows that the sample is a spinel structure compound and does not contain an impurity phase. The result is consistent with the standard spectrum of spinel lithium manganate, and its peak type comparison Narrow, peak intensity is relatively high, indicating that the crystallinity of LiMn2O4 crystal is better, so we can determine that the solution-gel method can synthesize a single crystalline spinel LiMn2O4 product under the experimental conditions.
4 Cyclic performance test
The battery with LiMn2O4 as electrode material has an initial capacity of 123.5 mAh/g, and after 50 cycles, it is reduced to 91 mAh/g, and its capacity retention rate is 91/123.5=0.7368.
There are two main reasons for the low capacity retention rate: (1) Jahn-Taller effect. The octahedral Mn3+ is the main cause of the Jahn-Taller effect. In the ratio of LiMn2O4, the average valence of manganese is 3.5. With the intercalation of lithium, the valence of manganese decreases, resulting in the Jahn-Taller effect, which changes from cubic to tetragonal with low symmetry and increased disorder. Crystal system.
For the preparation of lithium ion batteries by solvent-gel method, the particles are small, so the contact area between crystal and electrolyte is relatively large, and the dissolution of Mn ions is also a major factor leading to the decrease of capacity. Therefore, in future experiments, we use surface coating to modify the material.
LiMn2O4 was obtained by solvent-gel method at 850 °C. The results showed that: (1) Lithium manganate was successfully prepared by solvent gel method, and the obtained product lithium manganate particles were uniform. The crystallinity is good, and it is proved that the preparation of lithium manganate by solvent gel method is an ideal method. (2) The electrochemical performance of the obtained product is acceptable through electrochemical analysis, but if the reaction conditions are optimized or the surface of the electrode material is modified or the electrode material is more exbatteryent in performance.
JEChilton Jr.GMCook, in Abstract: Lithium Nonaqueous Secondary Batteries, ECS fall Meeting, ECS fall Meeting, Boston, 1962: 90-91.
 Guo Bingyan, Xu Hui, Wang Xianyou, et al. Lithium-ion battery [M]. Changsha: Central South University Press, 2002: 1-193.
 Uchida Takayuki. Battery [M]. Guo Chengyan, translated. Beijing: Science Press , 2004, 68-69.
 Wu Yuping, Dai Xiaobing, Ma Junqi, et al. Lithium-ion battery - applied in practice [M]. Beijing: Chemical Industry Press, 2004: 3. [br\u003e [ 5] Wu Yuping, Wan Chunrong, Jiang Changyin, et al. Lithium ion secondary battery [M]. Beijing: Chemical Industry Press, 2002.
Liu Yi, Fujiwara T, Yukawa H, Morinaga M.Electronic Structure of lithium manganese oxides for rechargeable lithium bakery electrodes[J].Solid State Ionics 1999,126(3):209.
Shi Yanhui, Feng Shouhua. Lithium battery cathode material Li(CoxAI1-x)O2 Synthesis and Characterization by Gel Method[J].Chemical Research In Chinese Universities,2000,21(4):497-500.\u003cb r\u003e  Xiongg Wang, Xiangying Chen et. Citric acid-assisted sol-gel synthesis of nanocrystallineLiMn2O4 spinel as cathode material [J]. Journal of Crystal Growth, 2003 (256): 123-127.