①Metal particles in lithium reaction
When metal reacts with lithium, the degree of volume expansion is affected by the size of the metal particles. Making the particle size as small as possible is recognized as an effective way to reduce the pressure caused by volume expansion. Excessive changes in volume will destroy the active material. During the reaction between metal and lithium, the huge change in volume can cause the metal particles to break. As the number of lithium ions increases, more ruptures occur and a new SEI film is formed. Ultimately, the isolation of the charged body reduces the battery capacity, which in turn affects the battery performance.
As shown in Figure 1, the loose arrangement of metal particles on the electrode ensures the stability of charge and discharge by alleviating the pressure caused by the volume expansion during the lithium reaction. Keeping the size of the metal particles to a minimum not only inhibits the breakage of the particles, but also accelerates the reaction between the metal and lithium. If the size of the metal particles is kept below the critical point, the occurrence of particle breakage can be avoided. When the particle ruptures, the strain energy caused by the volume expansion is greater than or equal to the surface energy of the particle. The critical size of metal particles can be obtained by the following formula:
dcrit = 32.2γ(1 -2v) 2 V02/E△V2
In the formula, dcrit is the critical size of the particle; γ is the surface energy; v is the Poisson’s ratio; V0 is the initial volume; V is the volume change.
In the above theoretical calculations, if every factor is estimated, this will increase the inaccuracy of the results. However, the critical value obtained in the actual calculation is smaller than the unit character size of the metal, which indicates that only by minimizing the particle size, the particle breakage cannot be avoided.
②Multiphase lithium alloy
The reaction of metals (such as Sn) and lithium in a single phase can only occur at a specific potential, while the reaction of multiphase metals (such as multiphase Sn/SnSb) with lithium occurs within a range of potentials. As shown in Figure 2, the initial reaction of the Sn-Sb alloy with lithium ions occurs in the 800-850 mV range, while the remaining Sn reaction occurs in the lower 650-700 mV range. By alleviating the phase volume expansion that occurs when reacting with lithium ions, the cycle characteristics can be improved.
It can be seen from the above description that the gradual reaction with lithium can effectively suppress volume expansion. However, the resulting lithium metal alloy continues to react with large amounts of lithium, resulting in limitations in controlling volume expansion.
Ag and Sn are other substances that have similar reactions. As a metal, Ag has electronic conductivity and forms LiAg through a stepwise reaction with lithium ions. LiAg reduces volume expansion by suppressing the reaction between lithium ions and Sn.
③Metal film electrode
The metal active material can be used as a metal thin film electrode without a binder. Figure 3 shows the typical volume changes of thin-film electrodes during charging (alloying with lithium) and discharging. The battery is charged first and then discharged, and the thickness of the film increases from 6 μm to 11 μm. After the second cycle, its thickness further increased to 17 μm. The surface of the copper is covered with a layer of cylindrical tin particles, which absorbs the pressure caused by the volume expansion. Since there is enough space to accommodate the volume expansion in the horizontal direction, particle breakage will not occur. It can be seen that considering the volume change will help the design of the electrode shape. In terms of broader commercialization, chemical methods should be developed to suppress volume expansion.