- 1. Lithium alloy
- 2. Comparison of preparation methods of various alloy anode materials
To a certain extent, nano-alloys can weaken the volume change of alloy materials in the process of deintercalating lithium, but the violent agglomeration during the electrochemical reaction limits the further improvement of the properties of nano-alloy materials. By combining nano-alloys with other materials, especially carbon materials, composite materials with high capacity and good cycle performance can be obtained. On the one hand, they benefit from the high capacity of alloy materials, and on the other hand, they also benefit from the cycle of carbon materials. The structural stability.
Nanostructured electrode materials
In the research of composite materials, SnSb/HCS composite materials are more representative, in which HCS is a nanoporous carbon microsphere with a diameter of 5-20μm. The inside of the sphere is an amorphous structure composed of a single graphite layer. 0.5~3nm nanopores. Using HCS as the framework, the nano-SnSb alloy particles are evenly pinned on the surface, so that the nano-alloy particles rarely fuse and agglomerate during the charging and discharging process, which has good cycle performance. The reversible capacity of 35 cycles is stable at 500mA ·About h/g.
In the preparation of composites, there is a type of composites that are nanoalloys combined with some inert materials, such as SiO2, Al2O3, etc. The purpose of adding inert materials is to buffer the volume expansion of the active material on the one hand, and to prevent the nanoalloy from reacting on the other hand. Reunion in the process. For example, graphite/Si/SiO2 composites are prepared by the sol-gel method, in which graphite and silicon are wrapped in the Si-O network structure of SiO2. The addition of SiO2 increases the resistance of the material, reduces the reversible capacity, but stabilizes the material After 30 cycles, the reversible capacity is 200mA·h/g. The Si/TiN nanocomposite is prepared by high-energy ball milling. After 12h ball milling, the first discharge capacity of the composite with a silicon content of 33.3% (molar fraction) is about 300mA·h/g. h/g, the capacity attenuation per cycle is only 0.36%, showing good cycle performance.
1. Lithium alloy
Similar to the hydrogen storage alloys in nickel-metal hydride batteries, the above types of alloy materials can be collectively referred to as lithium storage alloys. In order to form a battery with no lithium source cathode materials, such as MnO2, S, V2O5, Li1+xV3O8, etc., it is necessary to consider adding lithium to the lithium storage alloy material. On the one hand, it can solve the problem of lithium source, and on the other hand, it can also compensate The first irreversible capacity of lithium storage alloy materials. Li-Mg, Li-Al, Li-Cr-Si, Li-Cu-Sn, etc. have been reported. The most representative one is the Li-Al expanded metal ("EXMET") prepared by Zaghib et al. The process is to press lithium aluminum and expanded aluminum metal (porosity 50%) together, and heat treatment at 80°C for 1 hour to form Expanded lithium aluminum alloy material, this alloy material has a very high porosity, can buffer the volume expansion of the alloy material in the reaction process, has high structural stability, matches with the V2O5 cathode material, and shows good electrochemical performance .
2. Comparison of preparation methods of various alloy anode materials
The preparation method of alloy anode material is more high-energy ball milling method, and most of the alloy materials can be made by ball milling method. In addition, the hot melt method, chemical reduction method, electrodeposition method and reverse micelle microemulsion method are used to prepare alloy materials. These methods have their own characteristics and can prepare corresponding alloy materials under specific conditions.
(1) High-energy ball milling method
The high-energy ball milling method uses the rotation or vibration of the ball mill to make the hard balls strongly impact, grind and agitate the raw materials to pulverize the metal or alloy powder into nano-sized particles. It is also called mechanical alloying. In 1988, Shingu and others of Kyoto University in Japan first reported the preparation of Al-Fe nanocrystalline materials by high-energy ball milling, which found a practical way for the preparation of nanomaterials. The main feature of the high-energy ball milling method is that it is widely used and can be used to prepare a variety of nano-alloy materials and composite materials, especially the alloy nano-materials with high melting point that are difficult to obtain by conventional methods, and the alloy powder prepared by the high-energy ball milling method, its structure and The composition distribution is relatively uniform. Compared with other physical methods, this method is simple and practical, and can prepare nanocrystalline metal alloys under relatively mild conditions. Almost all the alloy materials reported in the literature can be prepared by high-energy ball milling.
High-energy ball milling method
The main disadvantage of the high-energy ball milling method is that it is easy to introduce certain impurities, especially the presence of impurity oxygen, which makes the surface of the nano-alloy very easy to be oxidized during the ball milling process. The introduction of impurity oxygen makes the alloy material undergo an irreversible reduction and decomposition reaction during the lithium insertion process, thereby bringing about a larger irreversible capacity.
(2) Hot melt method
The hot melt method is a traditional method for preparing alloy materials. The alloy materials are obtained by mixing, smelting, and annealing the metal raw materials. Its main advantage lies in the simple equipment and process, especially in the preparation of lithium alloys. Almost all lithium alloy materials reported in the literature are prepared by hot melt method. The main disadvantage of the hot-melt method is that it is difficult to obtain nano-alloy materials. Generally, high-energy ball milling is required. For some metals with high melting points and immiscible metals on the phase diagram, it is difficult to obtain their alloy materials by conventional hot-melt methods. .
(3) Chemical reduction method
Chemical reduction method is one of the effective and commonly used methods for preparing alloy ultrafine powder. By selecting appropriate complexing agents and reducing agents, the co-reduction of metal elements with relatively close reduction potentials can be achieved, thereby preparing alloy materials. Chemical reduction can be carried out in an aqueous solution or in an organic solvent. The main advantage of the chemical reduction method is that it is simple and easy to implement, has lower requirements for equipment, and is convenient for industrial production. Commonly used reducing agents include hydrazine hydrate, sodium borohydride, sodium hypophosphite or active metals. At present, the chemical reduction method has prepared nano-Sn-Cu, Sn-Sb, Sn-Ag and other alloy materials. The main disadvantage of the chemical reduction method is that it is very limited. For some metals with relatively negative reduction potentials and large potential differences, it is difficult for general reducing agents to reduce or co-reduce them.
(4) Electrodeposition method
As a method of preparing nano-alloy materials, electrodeposition has gradually attracted people's attention. By increasing the deposition current density to be higher than the limiting current density, nanocrystalline alloy materials can be obtained.
The lithium battery alloy anode material prepared by the electrodeposition process does not need to use conductive agents and binders. The electrode has a larger volumetric capacity and a lower cost. The bonding force of the alloy material and the substrate is better than the traditional paste process. Sn-Cu, Sn-Ni, Sn-Co, Sn-Fe, Sn-Sb, Sn-Ag, Sn-Sb-Cu and other alloy anode materials can be prepared by electrodeposition. One of the more characteristic features is the SnSbCu/graphite composite material. Through high current deposition (400mA/cm²), a layer of nanocrystalline porous SnSbCu alloy material is deposited on the copper foil, and then a layer of graphite PVDF composite is coated on the surface of the alloy. , 0.2mA/cm² charge and discharge between 2.0~0.02V, reversible capacity 495mA·h/g, capacity attenuation of 35 cycles is 0.48% per cycle, the main disadvantage of electrodeposition method is that there are many influencing factors of electrodeposition process , The process control is more complicated, especially the mechanism of preparing nanomaterials by electrodeposition method, the current understanding is not deep.
(5) Reverse micelle microemulsion method
Inverted micelle microemulsion, that is, water-in-oil microemulsion, refers to a dispersion system in which water-insoluble non-polar substances (oil phase) are used as the dispersion medium and polar substances (water phase) are used as the dispersed phase. The milk nucleus in the reverse micelle microemulsion can be regarded as a micro-reactor or nanoreactor, and nano-alloy particles can be prepared relatively easily by the reverse micelle microemulsion method. Tin/graphite and SnO2/graphite nanocomposites can be prepared by the reverse micelle microemulsion method. The particle size of tin and SnO2 is 7~10nm. The experimental device of the microemulsion method is simple, easy to operate, and can artificially control the synthesis of particles However, due to the limited applicable range of the microemulsion method, there is less aqueous phase in the system (about 1/10 volume), resulting in less output per unit volume, and high cost, which is not suitable for industrial production.
Reverse micelle microemulsion method