It is possible to obtain better performance by compounding the metal that reacts with lithium and the metal that does not react with lithium to prepare composite materials instead of pure metals. Because the active phase is surrounded by the inactive phase, volume expansion and contraction are alleviated. This concept can be illustrated in Figure 1.
In order to minimize the volume expansion, the metal particles of the active phase should be well dispersed in the inactive phase.
In the early cycle of the lithium reaction, tin oxides, such as SnO, SnO2 and SnxAlyBzPpOn, participate in the irreversible reaction and form Li2O and Sn. In the continuous phase of Li2O, nano-metal Sn particles exist in the dispersed phase and form an active phase (Sn ) And inactive phase (Li2O) composite material. As the cycle continues, Sn and Li participate in a reversible reaction.
Since the irreversible reaction related to the formation of Li2O causes a huge capacity loss, it is difficult to use stannous oxide (SnO) in actual lithium secondary batteries. The use of composite materials with lithium-reactive metals and non-lithium-reactive metals may be able to solve the problem of irreversible capacity loss. Examples of active metals are Sn and Si, and inactive metals refer to transition metals such as Fe, Ni, Mn, and Co. The reactions of these composite materials with lithium are as follows:
The formation process of composite materials is the same as that of oxides. In the initial reaction between lithium ion and SnM composite material, SnM decomposes to form LixSn and M. Because the active metal product (LixSn) is distributed in the continuous phase of the transition metal M, the volume change during the reaction with lithium is suppressed, so the structural stability of the electrode is improved. Compared with Li2O, inactive metal (M) has better electronic conductivity, but it hinders the movement of lithium ions and reduces the rate performance. Figure 2 depicts the reaction between Sn-M and Li.
By uniformly distributing metals that react with lithium (such as Si, Sn, etc.) in the continuous phase of metals that do not react with lithium, the volume change during charging can be alleviated or suppressed. Metals that do not react with lithium should have high mechanical strength and elasticity, so that they can withstand the pressure caused by volume expansion, and their excellent electronic conductivity facilitates the migration of electrons. When Si that reacts with lithium and metals that do not react with lithium, such as TiN, TiB2, and SiC, react to form a composite material, better charge-discharge cycle performance can be obtained. However, the mass and volume of the metal that does not react with lithium may make the composite material have a smaller lithium storage capacity, and because it inhibits the flow of lithium ions, it limits the reaction between Li and Si.
The phase diagram of a metal alloy can be used to design a substance composed of an active phase and an inactive phase. An alloy with a eutectic composition melts and undergoes rapid solidification, and the metal alloy exhibits various forms of microstructure. For example, in the Co-Si phase diagram shown in Figure 3, Co-58Si melts at temperature a, then rapidly solidifies by cooling to temperature b, and then forms a metal composite with a variety of microstructures. Figure 4 shows the cross-section of the metal composite particles obtained in this way. The particles are spherical, which is a characteristic of the particles formed by atomization. Si particles are uniformly distributed in the Co-Si matrix phase. The metal composite material improves the cycle life by suppressing the volume expansion caused by the reaction between metal particles and nano-lithium ions. However, this manufacturing method requires heating the metal to a high temperature, and it has not yet been commercialized due to the complicated process and high manufacturing cost.