Aluminum doping improves the energetics of lithium, sodium, and magnesium storage in silicon

TL;DR

Aluminum doping in silicon enhances its ability to store sodium and magnesium by improving thermodynamic stability, although diffusion barriers remain high, which may limit practical electrochemical applications.

Contribution

This study demonstrates through density functional theory that Al doping significantly improves the energetics for Na and Mg insertion in silicon, a novel approach for multi-ion battery anodes.

Findings

Al doping makes Na and Mg insertion thermodynamically favorable in silicon.

Mg energy in Al-doped Si approaches its cohesive energy at high Al concentrations.

Diffusion barriers for Li, Na, and Mg remain high in Al-doped Si, potentially limiting ion mobility.

Abstract

While Si is an effective insertion type anode for Li-ion batteries, crystalline Si has been shown to be unsuitable for Na and Mg storage due, in particular, to insufficient binding strength. It has recently been reported that Si nanowires could be synthesized with high-concentration (several atomic %) and dispersed Al doping. Here we show based on density functional theory calculations that Al doping significantly improves the energetics for Na and Mg insertion, specifically, making it thermodynamically favored versus vacuum reference states. For high Al concentrations, the energy of Mg in Al-doped Si approaches the cohesive energy of Mg. However, the migration barriers for the diffusion of Li (0.57-0.70 eV), Na (1.07-1.19 eV) and Mg (0.97-1.18 eV) in Al-doped Si are found to remain about as high as in pure Si, likely preventing effective electrochemical sodiation and magnesiation.