Lithiation behavior of silicon-rich oxide (SiO_1/3): A first-principles study

Silicon suboxides (SiO_x, x < 2) have been recognized as a promising anode material for high-performance Li-ion batteries (LIBs), especially when the O content is relatively low. To better understand the lithiation behavior in partially oxidized silicon at the atomistic level, we perform density functional theory calculations to examine the structural evolution, bonding mechanism, mechanical property, and voltage profile of lithiated a-SiO_1/3. With lithiation, the a-SiO_1/3 host matrix gradually disintegrates as Li atoms are accommodated by both Si and O atoms. Interestingly, we find that the Si-Li coordination number (CN) monotonically increases up to CN_Si-Li ≈ 10 in a-Li ₄SiO_1/3, whereas CN_O-Li tends to saturate far before full lithiation at CN_O-Li ≈ 6; the formation mechanism of such intriguing Li₆O complexes with O_h symmetry is investigated via detailed electronic structure analyses. Li incorporation in the a-SiO_1/3 matrix is predicted to be highly favorable with a capacity comparable to that of fully lithiated Si (Li:Si ratio ≈ 4); additionally, the approximated lithiation voltage between 0.2 and 0.8 V is also well within the desirable range for LIB anode applications. Our study highlights the importance of controlling the Si:O ratio as well as O spatial distribution in order to tailor the desired lithiation properties; such a realization may benefit the rational design and development of high-performance silicon suboxide based anodes via fine-tuning of the oxidation conditions.