The concept of mechanochemistry was first proposed by Peter in the early 1960s, when he defined it as "a phenomenon in which a substance undergoes a chemical change or a physical and chemical change under the action of mechanical force". From the viewpoint of energy conversion, it can be understood that the energy of mechanical force is converted into chemical energy. In fact, the discovery of mechanochemical effects can be traced back to 1893. Lea observed a small amount of Cl2 escape when grinding HgCl2, indicating that HgCl2 was partially decomposed. In the field of materials science, research on mechanochemical effects in the 1950s, when Taka-hashi grinds clay for a long time, he found that the clay not only partially dehydrates, but also the structure has changed.
Mechanical chemical activation (mechanochemistry, also known as high-energy ball milling) is a mechanochemical reaction triggered by a physical process under mechanical stress. Study the physical and chemical properties and structural changes of solid materials induced and effected by mechanical forces. In the process of mechanical activation, a considerable part of the mechanical energy consumed by the system remains in the pretreated solid. The mechanical energy is converted into surface energy and lattice defect energy and stored in this form for a long time. We call this free contact between solid particles "mechanical activation".
The multi-phase chemical reaction process in which the solid phase participates is a process in which the solid-phase reactants overcome the reaction barrier and reach the atomic level combination to cause a chemical reaction. Its characteristic is that there is an interface between the reactants. The factors affecting the reaction rate include the change of free energy in the reaction process, temperature, interface characteristics, diffusion rate and thickness of the diffusion layer. In the process of mechanical ball milling, the powder particles are strongly plastically deformed to produce stress and strain, and a large number of defects are generated in the particles, which significantly reduces the diffusion activation energy of the elements, so that the atoms or ions can be diffused significantly at room temperature between the components; the particles are continuously cold welded and fractured, and the structure is refined, forming countless diffusion/reaction couples, and the diffusion distance is also greatly shortened; the generation of stress, strain, defects and a large number of nano-grain boundaries and phase boundaries make the system energy storage very high, reaching more than ten kJ/mol, and the activity of the powder particles is greatly improved; the interface temperature rises at the moment when the ball and the powder particles collide. Mechanochemical activation involves many disciplines such as solid chemistry, materials science, mechanical engineering, and surface chemistry.
The synthesis of new materials, the formation of new substances, the transformation of crystal forms or the deformation of crystal lattices are all achieved through high temperature (thermal energy) or chemical changes. It is a new idea that mechanical energy directly participates in or triggers chemical reactions. The basic principle of mechanochemistry is to use mechanical energy to induce chemical reactions or to induce changes in material organization, structure and properties. As a new technology, mechanochemical activation can significantly reduce the activation energy of the reaction, refine the crystal grains, increase the powder activity, improve the uniformity of particle distribution and the combination of the interface between the reinforcement and the matrix, promote the diffusion of solid ions, induce low-temperature chemical reactions, and improve the material Its compactness, electricity, heat and other properties.
Various functional and structural materials such as supersaturated solid solutions, intermetallic compounds, and amorphous alloys have been developed by mechanochemical methods, and have been applied in many researches on highly active ceramic powders and nano-ceramic matrix composite materials.
In the process of crushing solid materials, not only the particle size of the material changes after crushing, but its physical and chemical properties are also significantly different. The main characteristics of the mechanochemistry of solid materials are as follows.
① Changes in particle structure, such as spontaneous reorganization of the surface structure, forming an amorphous structure or recrystallization;
② Changes in the physical and chemical properties of the particle surface, such as surface electrical properties, physical and chemical adsorption, solubility, dispersion and agglomeration properties;
③ A chemical reaction occurs in the area subject to repeated stress, such as changing from one substance to another, releasing gas and foreign ions into the crystal structure to cause a change in the chemical composition of the original material.
Preparation of cathode material by mechanochemical method
The process flow of LiMxMn2-xO4 prepared by mechanical activation-high temperature solid phase method is: Lithium source + manganese source → ball milling → Roasting → washing → Sieving → product.
In the process of ball mill activation, the solid material is not just a simple reduction in the particle size of the material, it also contains many complex changes in the physical and chemical properties and crystal structure of the powder-mechanochemical changes, that is, the action of mechanical energy causes changes in the properties and structure of the powder, causes the material to undergo crystalline transformation, and induces chemical reactions. The combination of mechanical ball milling and chemical synthesis has proven to be a very effective method in preparing positive electrode materials with new structures and properties. For example, it has been reported in the study of preparing LiMn2O4 and LiNiO2 cathode materials, in the subsequent solid-phase synthesis process, the reaction temperature is significantly lower than conventional methods.
Now take the mechanochemical method to synthesize LiNiO2 cathode material as an example. Using Ni0.8Co0.2(OH)2 and LiOH·H2O (the molar ratio of which is 1:1.05) as raw materials for synthesis, on a planetary ball mill with a speed of 400r/min and a ball/material ratio of 10:1, it is activated by one-time ball milling for 0.5-4h. Under the conditions of an oxygen atmosphere and a solid-phase reaction temperature of 600℃ and a synthesis time of 8~16h, the synthesized LiNi0.8Co0.2O2 cathode active material has high first charge-discharge specific capacity and charge-discharge efficiency, and has good electrochemical cycling performance. When the secondary solid-phase reaction temperature is 750℃, the activation time of ball milling should be 1h. Under these conditions, the initial charge-discharge capacities of the samples obtained were 143.4mA·h/g and 127.8mA·h/g, respectively. The charge-discharge specific capacity of the fifth cycle was 85.9mA·h/g and 80.2mA·h/g, respectively, and the discharge specific capacity decay rate per cycle was 7.45%.
After 8 hours of mechanical activation, the LiCo1-xNixO2 material with layered structure was synthesized by combining with the oxygen atmosphere baking method. With the increase of work, the cell parameters a and c and the discharge capacity increase, the thermal stability and cycle performance decrease with the increase of x, and the working voltage decreases. In LiCo1-xNixO2, Ni also participates in the electrochemical reaction. After Ni doping, the discharge capacity and cycle performance of the compound can be improved.
The mechanochemical method can modify the cathode material. For example, in the process of preparing LiMn2O4 by solid-phase reaction, combining the mechanochemical reaction with the solid-phase reaction can make up for some of the shortcomings of the mechanochemical reaction. The nanometer LixMn2-xO4 material is obtained by ball milling. Although the obtained LixMn2-xO4 also has the Jahn-Teller effect, the cycle performance is significantly improved.
The doped LiFePO4 cathode material was prepared by a combination of mechanochemical method and solid-phase method. The first discharge specific capacity reached 144mA·h/g, and during the cycle, the capacity remained stable without significant attenuation.