Impact of local amorphous environment on the diffusion of sodium ions at the solid electrolyte interface in sodium-ion batteries

The in-depth study of the transport properties of the solid electrolyte interface (SEI) is crucial for the development of ultra-high-rate, and long lifespan sodium-ion batteries (SIBs). However, there remains a lack of theoretical investigation into the transport mechanisms of the main inorganic components of the SEI, namely NaF, Na 2 O, and Na 2 CO 3 . To address this research gap, we performed classical molecular dynamics simulations in this work to study the diffusion mechanisms of sodium ions in these inorganic components of the SEI, with special emphasis on the impact of the amorphous SEI environment on the diffusion behavior of sodium ions. The results have shown that amorphous SEI components significantly enhance the diffusion rate of sodium ions at room temperature compared to crystalline components. Within these amorphous SEI components, we reveal that the diffusion coefficients of sodium ions in amorphous Na 2 O and Na 2 CO 3 are more than an order of magnitude higher than that of NaF, suggesting that amorphous Na 2 O and Na 2 CO 3 are more effective in facilitating the Na ion diffusion. Analysis of the local atomic structure indicates that the amorphous local structures are dominant in Na 2 O and Na 2 CO 3 at room temperature, maintaining a disordered amorphous phase. In contrast, amorphous NaF undergoes a spontaneously transformation into an ordered structure, exhibiting crystalline characteristics that restrict the diffusion of sodium ions. In summary, our work provides atomic insights into the impact of local amorphous environments on Na ion diffusion in SEI and suggests that amorphous SEI components play a critical role in improving battery performance. Classical molecular dynamics simulations investigate sodium ion diffusion mechanisms in crystalline and amorphous SEI films, focusing on three main components: NaF, Na 2 O, and Na 2 CO 3 . The study emphasizes the effect of the amorphous SEI environment on sodium ion diffusion, providing insights into ion transport at the atomic level within the SEI.