Effects of Stress and State-of-Charge on Lithium Transport Behavior in Silicon Electrodes

In recent years, silicon (Si) has been proposed as a promising negative electrode material because of  its high specific capacity, low discharge voltage, and abundance. However, the large volume change and high stress during lithiation and de-lithiation of Si^1-3, which can lead to cracking and severe degradation, are impeding the commercialization of Si-based electrodes. Furthermore, Li-ion transport behavior, which affects stress generation⁴ and rate performance of the electrode, must be understood in order to design Si electrodes with improved performance.

In this study, we investigated the transport mechanism of Li in both crystalline and amorphous lithiated Si  in the framework of Density Functional Theory as implemented in the Vienna ab initio Simulation Package (VASP)^{5, 6}. The effects of stress/strain and state-of-charge (SOC) on ionic transport were examined. In the study of crystalline phases, we used the Climbing Nudged Elastic Band (CNEB) method to estimate the diffusion barrier for several possible diffusion paths. We prepared amorphous Li-Si systems (Figure 1) by simulated annealing within ab initio Molecular Dynamics (AIMD). We used the Stokes-Einstein relationship⁷ to obtain diffusion coefficients from AIMD trajectories (Figure 2).

We found that, in crystalline Li-Si alloys, the interstitial diffusion barrier of Li-ion increases monotonically as the system is compressed. However, in amorphous systems, the effect of stress is not monotonic. One possible mechanism of the non-monotonic behavior is the change of local structure, such as the coordination number, in the amorphous Li-Si alloys as a function of stress. From the view of activation volume, this behavior could be explained by the factor that the activation volume due to Li-ion migration is negative in amorphous Li-Si structures. In addition, we investigated the effects of SOC on Li-ion transport using AIMD calculations and Darken’s equation for chemical diffusion. This study provides new insights into the effects of stress and composition on lithium transport in Si.

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