Reaction mechanism of metal silicide Mg2Si for Li insertion

Electrochemical Li insertion into the anti-fluorite-type Mg2Si was performe d and the structural variation during the insertion was examined. It was fo und that lithium insertion into Mg2Si proceeds stepwise according to the fo llowing 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-3 m, where a(F) represents a lattice length of the fundamental fluorite unit cell. DV-X alpha calculation showed that charge compensation in Mg2Si accom panied by the Li insertion into the vacant octahedral site occurs by indrod uction of electrons into Mg3s orbitals, resulting in a reduction of Mg2+ io ns. 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 Pres s.