Li+ diffusion in crystalline lithium silicides : influence of extrinsic point defects and structural complexity

Abstract

Li15Si4, which forms in Si anodes for Li-ion batteries upon full lithiation, belongs to a class of binary crystalline lithium-silicon compounds LixSiy. In this study, by combining ab initio molecular dynamics simulations and nudged elastic band calculations, the impact of isovalent extrinsic point defects on Li diffusion in Li1Si1, Li12Si7, Li13Si4, and Li15Si4 is explored through the incorporation of Na and Ge atoms. Since Na interstitials turn out to be unstable, larger elements are identified as substitutional atoms in these compounds. Overall, both cationic and anionic substitution have only minor effects on Li mobility. Energy barriers for Li migration marginally increase near Na and Ge atoms and decrease at larger distances. Na migration energies are approximately 30% higher than those of Li atoms. Moreover, defect formation energies in stoichiometric LixSiy systems are shown to be inversely correlated with crystal structure complexity.