Tin salt and stannate
What is tin salt and stannate and what is silicon? In addition to oxides, tin salts can also be used as negative electrode materials for lithium-ion batteries, such as SnSO4 and SnS2. With SnSO4 as the negative electrode material, the maximum reversible capacity can reach more than 600mA·h/g. According to the alloy type mechanism, not only SnSO4 and SnS2 can be used as active materials for lithium storage, but other tin salts are also on the list, such as Sn2PO4Cl whose capacity can be stabilized at 300mA·h/g after 40 cycles.
The reactions of the insertion and escape of lithium in SnSO4 are as follows:
In the lithium deintercalation reaction, the metal tin particles produced are very small (may be nano-sized), and the reaction is the essential reason for the capacity. The alloy formed by lithium and tin has an amorphous structure, and its amorphous structure is not easy to be destroyed in the subsequent cycle process, and the cycle performance is better during the charge and discharge process. The results of X-ray diffraction and Mubauer spectroscopy proved the above reaction process. The performance of SnSO4 electrode has a lot to do with the composition of the electrode. For example, adding acetylene black can help improve its cycle performance, as shown in Figure 1.
Figure 1 - Cycle performance of SnSO4 electrodes with different ratios
Like SnO2, tin sulfide SnS2 mainly stores lithium based on an alloying mechanism: Li2S is formed first, and then alloyed with tin. This compound has a high reversible capacity. The nanoparticle capacity can reach 620mA·h/g, and the stability is also good.
In addition to tin salts, stannate salts can also be used as anode materials for lithium-ion batteries, such as MgSnO3, CaSnO3 and so on. The charge-discharge mechanism of stannate follows the alloy-type mechanism, and the formed nano-tin particles are the main basis for increasing its reversible capacity. The first delithiation capacity of amorphous MgSnO3 is 635mA·h/g. After 20 charge-discharge cycles, the charge capacity is 488mA·h/g and the average decay rate is 1.16%. CaSnO3 prepared by wet chemical method has a reversible capacity of over 469mA·h/g, and the capacity can maintain 95% and 94% after 40 and 50 cycles.
Silicon
Silicon generally exists in two forms: crystalline and amorphous. As the negative electrode material of lithium-ion batteries, amorphous silicon has better performance. The reason why it is used as a lithium storage material is that lithium can react with silicon to form Li12Si7, Li13Si4, Li7Si3, and Li22Si4. The theoretical specific capacity of silicon as a negative electrode material is as high as 4200mA·h/g.
As the negative electrode material of lithium-ion batteries, the main characteristics of silicon include: ①It has a capacity advantage that other high-capacity materials (except lithium metal) cannot match; ② Its microstructure transforms into an amorphous form after the first lithium insertion, and this amorphous form has been maintained during the subsequent cycles. From this point of view, it can be considered that it has relative structural stability; ③In the process of electrochemical deintercalation of lithium, the material is not easy to agglomerate; ④The discharge platform is slightly higher than that of carbon-based materials. Therefore, it is not easy to cause the formation of lithium dendrites on the electrode surface during the charge and discharge process.
The electrochemical performance of silicon is related to its morphology, particle size and operating voltage window. From the morphological point of view, silicon used as an electrode can be divided into main body material and thin film material. The host material can be prepared by ball milling and high-temperature solid phase method; the thin film material can be prepared by physical or chemical vapor deposition, sputtering, etc. Silicon-based materials have a serious volume effect under the condition of highly deintercalating lithium, which easily causes the structure of the material to collapse, resulting in poor cycle stability of the electrode. The binary phase diagram of the Li-Si compound is shown in Figure 2. The film material can alleviate the volume effect to a certain extent and increase the cycle life of the electrode. For example, a silicon film made by a vacuum deposition method can maintain a capacity above 1000mA•h/g in a PC-based electrolyte after 700 cycles. On the other hand, the use of nanomaterials, taking advantage of its large specific surface area, can improve the cycle stability of the material to a certain extent. However, because nanomaterials are easy to agglomerate, after several cycles, the cycle stability of the materials cannot be fundamentally solved. The potential window also greatly affects the cycle performance of the material. Using Si2H6 as the reaction gas, the amorphous silicon film prepared by chemical vapor deposition method, when the potential window is 0~3V, the first discharge capacity can reach 4000mA•h/g, but the capacity drops sharply after 20 cycles, and there is almost no discharge capacity after 40 cycles; however, if the potential range is between 0 and 0.2V, after the electrode is cycled for more than 400 times, the capacity can still maintain about 400mA•h/g. As seen in Figure 3.
Figure 2 - Binary phase diagram of Li-Si compound
Figure 3 - Comparison of electrochemical cycling performance of silicon with different particle sizes
1, 2—Ordinary silicon powder; 3—Nano silicon powder, 0~2.0V, 0.1mA/cm2;
4, 6—Nano silicon powder (different electrode composition), 0~0.8V, 0.1mA/cm2;
5—Nano silicon powder, 0~0.8V, 0.8mA/cm2