Tracing Reversible and Irreversible Li Insertion in SiCO Ceramics with Modeling and Ab-Initio Simulations

Abstract

ABSTRACTWe present combined modeling and simulation studies of Li insertion in amorphous SiCO ceramics. Atomistic models of amorphous SiCO with and without so-called “free” carbon have been crafted using a network modeling approach and subsequently relaxed and optimized within density functional theory.In a first series, Li atoms are randomly inserted to test for sites with high binding energy. We find that Li prefers always bonding to O. The enthalpy of insertion, however, depends strongly on electronic states available in the SiCO host matrix. While Li inserted into insulating SiO2 is an energetically unfavorable process, the enthalpy of insertion in SiCO glass is decreased. Some glassy SiCO models exhibit favorable sites for Li insertion, which share a common motif in their local atomic environment. The presence of “free” carbon in SiCO then promotes Li insertion, because carbon-related structural imperfections give rise for low-lying unfilled electronic states. Consequently, strong and irreversible bonding of Li into SiCO is provided, if bonding of Li-cations in Li-O bonds outweighs the promotion energy for the electron to occupy unfilled electronic states.