The tin reduction during lithium insertion in spinel sulfide compounds is investigated. Mössbauer and RMN spectroscopy are combined with tight binding calculations to analyze the reduction mechanism. It is shown that the formal reaction SnIV + 2e- → SnII is involved without an intermediate Sn oxidation state. The following mechanism is proposed:  lithium ions create shallow and deep-defect donor states; the trapping of two electrons on the deep one, which exhibits a negative U behavior, induces the reduction of the corresponding tin atom.