Tin-based alloys are also the most important and widely studied anode materials for lithium-ion battery alloys. Tin-based alloys mainly use tin energy and lithium to form alloys as high as Li22Sn5, and the material has a high theoretical capacity.
Tin-based alloys mainly use tin energy to form an alloy with lithium as high as Li22Sn5, so the theoretical capacity is high. Tin-based alloys are also the most important and widely studied anode materials for lithium-ion battery alloys.
SnSb has a rhombohedral phase structure. Sn and bromide atoms are alternately arranged along the c-axis direction. With the insertion of lithium, the crystal structure gradually changes to the multiphase coexistence of Li3Sb and Li-Sn alloy, and returns to the SnSb phase with the release of lithium. Someone separately prepared Sn-SnSb alloy materials with different particle sizes by electrochemical deposition and aqueous chemical reduction methods. The experimental results show that there is a multi-phase structure in Sn-SnSb (tin element and SnSb alloy). The smaller the particles, the better the cycle performance. When the particles are less than 300nm, it will maintain 360mA·h/g after 200 cycles. The nanocrystalline Sn-SnSb alloy was also prepared by the method of chemical reduction in aqueous solution, and the capacity of 50 cycles was stabilized at 500-600mA·h/g, comparing the electrochemical performance of Sn-SnSb, SnSb, Sb-SnSb three materials. The results show that the cycle performance of SnSnSb is the best, and the cycle performance of Sb-Snsb is the worst. The reason is that the elemental Sb in Sb-SnSb and the SnSb alloy react with lithium at the same potential, and the volume effect is larger (see Figure 1).
Figure 1 - Cycle performance of Sn-SnSb, SnSb and Sb-SnSb
A solvothermal method can be used to prepare a pure phase nano Snsb alloy with a dendritic structure under low temperature conditions. By analyzing the lithium insertion mechanism of the nano-SnSb alloy and the reason for the capacity decay, it is found that the gradual agglomeration of the nano-SnSb alloy into large particles is the main reason for the capacity decay.
Prepare nano-Snsb alloy by electrodeposition in acid system, the deposition current is (245±5) mA/cm2, the deposition time is 2 min, and the reversible capacity of 30 cycles is 400 mA·h/g. With the increase of the paving content, the reversible capacity of the material decreases.
The Cu6Sn5 alloy has a NiAs-type structure (see Figure 2), with tin atoms arranged in layers, sandwiched between copper atomic sheets. When lithium is inserted into Cu6Sn5, a phase change occurs. After two steps, Li2CuSn and Cu6Sn5 coexist first, and when potassium continues to be inserted, a lithium-rich phase Li4.4Sn coexists with Cu; during the escape, lithium is first extracted from Li4.4Sn, and then Li4.4-xSn reacts with Cu to form Li2CuSn, and then lithium is extracted from Li2CuSn to form Li2CuSn with vacancies, and further to form Cu6Sn5.
Figure 2 - Schematic diagram of Cu6Sn5 structure
The black balls represent tin atoms and the white balls represent copper atoms
The copper powder and tin powder of different stoichiometric ratios are mixed, pressed into small balls, and heat-treated at 400°C for 12 hours in an argon atmosphere to obtain three alloy materials of Cu6Sn6, Cu6Sn5, and Cu6Sn4. Among them, Cu6Sn6 and Cu6Sn4 are composite materials of Sn/Cu6Sn5 and Cu/Cu6Sn5, respectively. The experimental results show that Cu6Sn4 has the highest reversible capacity and the best cycle stability, and the reversible capacity of 20 cycles reaches 200mA·h/g. The reason is that the presence of elemental copper can make the active tin particles generated during the decomposition of Cu6Sn5 lithium insertion smaller, and the large specific surface area corresponding to the small particles is more conducive to the diffusion of lithium.
The nano-Cu6Sn5 alloy is prepared by high-energy ball milling with a grain size of 5~10nm, and the first discharge capacity reaches nearly 690mA·h/g, which is higher than the theoretical capacity of Cu6Sn5 of 608mA·h/g. The reason is that some oxide impurities are generated during the ball milling process. The presence of these oxide impurities makes the alloy and lithium undergo an irreversible reduction reaction during the first discharge. At the same time, the lithium insertion mechanism of nano-Cu6Sn5 is different from that of crystalline Cu6Sn5. Nano-Cu6Sn5 does not generate intermediate LixCu6Sn5 during the lithium insertion process, but directly generates LixSn alloy and copper. The performance of nano-Cu6Sn5 is better than that of crystalline Cu6Sn5, and the reversible capacity of 20 cycles reaches 200mA·h/g. If the Cu6Sn5 flakes with a thickness of less than 1μm are prepared by the ball milling method, the reversible capacity at 0.2~1.5V 50 cycles can reach 200mA·h/g. Using NaBH as the reducing agent, the nano Cu6Sn5 alloy material was reduced from the aqueous solution, the particle size was 20-40nm, and the reversible capacity of 80 cycles was more than 200mA.h/g. The nano-Cu6Sn5 alloy is reduced in an organic solvent, the particle size is 30-40nm, and the reversible capacity of 100 cycles reaches 1450mA·h/mL. The acid tin plating system electroplated a layer of active tin on the copper foil. After heat treatment, two alloy phases Cu6Sn5 and Cu3Sn are formed between the tin and the copper matrix. The experimental results show that the heat treatment process enhances the bonding force between the active material and the copper matrix, the first discharge capacity reaches 900mA·h/g Sn or more, and the capacity retention rate after 10 cycles is 94%. Different ratios of copper-tin alloys were deposited from a single plating solution by pulse deposition. When the copper-tin atomic ratio was 3.83, the reversible capacity for 40 cycles was 200mA·h/g, and the capacity retention rate was 80%. The Cu(II), Sn(IV) salt and NaOH undergo solid-phase reaction at room temperature to prepare a copper-tin composite, and then reduce by heating and passing H2 to obtain Cu-Sn nano-alloy material. The reversible capacity of 10 cycles is maintained above 280mA·h/g (see Figure 3).
Figure 3 - Cyclic performance of nanometer and crystalline Cu6Sn5
The structure of Ni3Sn2 alloy is similar to Cu6Sn5, and the reversible insertion and extraction of lithium can also occur. The Ni3Sn2 alloy material was prepared by high-energy ball milling. The cycle performance of the material is good, and the reversible capacity reaches 327mA·h/g or 2740mA·h/cm2, which is 4 times higher than the existing carbon materials. The nanocrystalline Ni3Sn2 alloy was prepared by the high-energy ball milling method, and the first discharge capacity was as high as 1520mA·h/g, which exceeded the theoretical capacity of Ni3Sn2. The reason is that the large number of grain boundaries of the nanocrystalline grains can hold more lithium, but the cycle performance of this material is very poor, and the capacity is only 35mA·h/g after 40 cycles. After heat treatment at 1000°C for 10 hours under argon, the first discharge capacity of the material drops to 590mA·h/g, and the reversible capacity for 40 cycles is 245mA·h/g. When Sn-Ni alloys with different atomic ratios are prepared by electrodeposition, the material with a tin atomic ratio of 62% has the best cycle performance, and the reversible capacity for 70 cycles is 650mA·h/g.
In addition to Sn-Sb, Sn-Cu, Sn-Ni alloys, tin-based alloys reported in the literature include SnCa, Mg2Sn, SnCo, SnMn, SnFe, SnAg, SnS, SnZn, etc. The cycle performance of these materials is far better than elemental tin, and compared with tin oxide, the irreversible capacity is greatly reduced. However, the electrochemical performance of these materials is still far from industrialization.