Li, Li-Al alloys and Li-Si alloys have all been considered as negative electrode materials to increase the capacity of lithium secondary batteries. However, due to the safety issues caused by the formation of lithium metal dendrites, metal lithium has not been truly commercialized. change. In order to overcome this problem, other metals (Si, In, Pb, Ga, Ge, Sn, Al, Bi, S, etc.) that can form alloys with lithium have been proposed. During the charging process, these metals react with lithium within a certain voltage range to become alloys, and return to the initial state during discharge, so continuous reversible charging and discharging can be carried out. Unlike the intercalation/extraction reaction in graphite, the reversible reaction referred to here is an alloying/dealloying process related to lithium. The charge and discharge reactions of electrodes involving metal elements are as follows:
xLi＋＋xe﹣+M↔LixM (→charge, ←discharge)
Alloying occurs during charging. The metal is neutralized by accepting electrons and lithium ions. However, dealloying is a completely opposite process, that is, it becomes the original metal by returning lithium ions. The capacity per unit mass or unit volume of the alloy (LixM) formed by the metal element and lithium is shown in Figure 1.
The specific capacity of most metals is higher than that of graphite (Li6C), and the theoretical capacity of Si is even higher than 4000 mA/g. At the same time, we should also pay attention to the working voltage of these materials. As shown in Figure 2, the metal-lithium reaction occurs at a relatively low potential, which is different from graphite. If these metals are selected as negative electrode materials for lithium secondary batteries, as the battery voltage decreases, even if the capacity per unit mass is high, the energy density of the battery will also decrease. However, with the recent development, with the reduction of energy consumption and working voltage, these high-capacity alloys have become extremely promising anode materials. Because past research has focused on Si and Sn, more research is now on Ge, Pb and Al.
From the equilibrium phase diagram, we can infer the phase and composition of the alloy formed by the reaction of metal and lithium.
For example, when Li is added to pure Sn, the final compounds are Li2Sn5, LiSn, Li5Sn2, Li13Sn5 and Li22Sn5. The phase of each compound can be inferred from the phase diagram in Figure 3. These metals show a high specific capacity because when lithium is added, an alloy containing multiple components can be formed. At the same time, a large amount of lithium is involved in the reaction process, and one lithium ion corresponds to 6 carbon atoms to bond with it. In the case of equilibrium conditions, the potential of the lithium-metal reaction can be determined. Figure 4 compares the changes in the potential of the reaction between Sn, Si and different lithium components.
The potential of Sn-Li is higher than that of Si-Li, but when the two phases coexist, both maintain a constant potential. During the charging process, these materials form an alloy with lithium, and the reaction is accompanied by a decrease in voltage and an expansion in volume. If the working voltage is high, the voltage drop will not cause problems in actual operation. However, the change in volume is detrimental to the performance of the battery.
The volume expansion of the metal negative electrode is attributed to the larger lattice constant, because during alloying, the positions between the metal atoms are filled. Since one Si atom can react with up to 4.4 lithium ions, the volume expansion may reach 400%. Under the pressure caused by volume expansion, the weak ionic bond between metal and lithium is easily broken. For inorganic materials composed of ionic bonds, the critical point of volume expansion is 5%. Figure 5 shows the metal rupture caused by volume expansion during charging.
In the early stages of charging, the alloying of metal and lithium caused excessive volume expansion and particle breakage. The further alloying reaction creates a new surface layer, which finally forms an SEI film together with the decomposition of the electrolyte. Because the rupture of the particles is not always in the radial direction, some parts of the particles are not in contact with the electrolyte.
These isolated fragments do not participate in the electrochemical reaction, causing a huge loss of capacity. Figure 6 is a comparison of two SEI films formed on the surface of graphite and on the surface of metal particles.
In order to prevent the metal particles from breaking, we should explore some methods to suppress the volume expansion of the negative electrode material.
Methods to reduce volume expansion: ① Refine metal particles that react with lithium; @ Multiphase alloying with lithium reaction; ③ Use active/inactive metal composite materials; ④ Form lithium alloy/carbon composite materials.