Reaction mechanism of metal silicide Mg2Si for Li insertion

Electrochemical Li insertion into the anti-fluorite-type Mg2Si was performed and the structural variation during the insertion was examined. It was found that lithium insertion into Mg2Si proceeds stepwise according to the following reactions: Mg2Si+2Li(+)+2e(-) --> Li2MgSi+Mg, and Mg+yLi(+)+ye(-) --> LiyMg. Li2MgSi has the 2a(F)x2a(F)x2a(F) structure with space group Fm-3m, where a(F) represents a lattice length of the fundamental fluorite unit cell. DV-X alpha calculation showed that charge compensation in Mg2Si accompanied by the Li insertion into the vacant octahedral site occurs by indroduction of electrons into Mg3s orbitals, resulting in a reduction of Mg2+ ions. Thus, one-half of the Mg2+ ions discharge from Mg2Si and the Li+ ion is incorporated into the missing site to form Li2MgSi. (C) 2000 Academic Press.