Different behavior of lithium interaction with SiO2 and Al2O3

Lithiation of SiO2 and lithium intercalation in Al2O3 is studied both theoretically and experimentally. Lithium interacts with these two types of oxides in distinctly different behaviors. Reversible insertion/extraction of lithium in SiO2 up to a Li density of 2/3 Li per Si are demonstrated experimentally. Density-functional-theory (DFT) calculation shows that neither free interstitial Li atoms (no reduction) nor formation of a local Li2O cluster plus a Si-Si bond (full reduction) is energetically favorable. However, two Li atoms can effectively break a Si-O bond and be stabilized between the Si and O atoms. Such a defect, representing a state of partial reduction of SiO2, is energetically favorable. DFT simulation shows that intercalation of SiO2 at high Li density through partial reduction results in crystalline compounds LixSiO2 (x \textless 2/3) with tunable band-gaps in the range of 2-3.4 eV. In sharp contrast, Al2O3 is very stable against lithiation through any form of reduction. However, good conductivity of Li ions is shown in porous Al2O3.