Sodium ion diffusion in Al₂O₃: A distinct perspective compared with lithium ion diffusion

Surface coating of active materials has been one of the most effective strategies to mitigate undesirable side reactions and thereby improve the overall battery performance. In this direction, aluminum oxide (Al₂O₃) is one of the most widely adopted coating materials due to its easy synthesis and low material cost. Nevertheless, the effect of Al₂O₃ coating on carrier ion diffusion has been investigated mainly for Li ion batteries, and the corresponding understanding for emerging Na ion batteries is currently missing. Using ab initio molecular dynamics calculations, herein, we first find that, unlike lithiation, sodiation of Al₂O₃ is thermodynamically unfavorable. Nonetheless, there can still exist a threshold in the Na ion content in Al₂O₃ before further diffusion into the adjacent active material, delivering a new insight that both thermodynamics and kinetics should be taken into account to describe ionic diffusion in any material media. Furthermore, Na ion diffusivity in Na_xAl₂O₃ turns out to be much higher than Li ion diffusivity in Li_xAl₂O₃, a result opposite to the conventional stereotype based on the atomic radius consideration. While hopping between the O-rich trapping sites via an Na-O bond breaking/making process is identified as the main Na ion diffusion mechanism, the weaker Na-O bond strength than the Li-O counterpart turns out to be the origin of the superior diffusivity of Na ions.