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.