Stress evolution during the two-step charging of high-capacity electrode materials

It has been well recognized that the lithiation of crystalline silicon advances via a one-step two-phase mechanism. In stark contrast, other high-capacity anode materials including amorphous silicon, germanium, and tin for lithium-ion and sodium-ion batteries are charged through a two-step process. That is, the first step of charging advances by the movement of a reaction interface that separates a pristine phase and a partially charged phase until the pristine anode material is fully consumed. Then the second step of charging sets in without a visible interface, eventually resulting in the fully charged anode. Lithiation and associated stress generation in crystalline silicon have been extensively studied by experiments and simulations. However, little attention has been given to the charging mechanics of anodes undergoing two-step charging. In this work, by resorting to the finite element method, we develop a model to simulate the stress generation in nanoparticle anodes during the two-step charging process. The model accounts for the unique charge-carrier concentration profile during two-step charging and the elastic softening of anode materials. We find that these two factors, in concert, lead to effective stress mitigation during the second step of charging, yielding the tough charging behavior of anodes featuring the two-step charging mechanism.

Automated summary

It has been well established that lithiation of crystalline silicon occurs through a one-step two-phase mechanism, while other high-capacity anode materials go through a two-step charging process. Little attention has been given to the charging mechanics of anodes undergoing two-step charging, but a model developed in this study using the finite element method reveals that the unique charge-carrier concentration profile and elastic softening of anode materials lead to effective stress mitigation during the second step of charging, resulting in tough charging behavior.