Suppression of thermal runaway by continuous heat generation using porous silicon covered with a thin oxide layer

Abstract The safety of the negative electrode was improved by a thin oxide covered porous silicon, which was synthesized by the thermal oxidation of Mg2Si. The reaction of lithiated porous Si with a 1 M LiPF6 ethylene carbonate–diethyl carbonate electrolyte was compared with that of lithiated bulk Si in the same electrolyte by differential scanning calorimetry at temperatures from room temperature to 450 °C. The total heat value of bulk Si was 4444 J g−1, whereas that of porous Si was 3121 J g−1. This difference was comparable to the heat value caused by reaction between fully Li intercalated graphite (C6Li) and the electrolyte. In situ heating X-ray diffraction showed that a reaction between Li in the porous Si electrode and the electrolyte progressed from 200 °C or lower, and Li was consumed to form Li2CO3 and LiOH with a mildly exothermic reaction. Above 200 °C, LiF with high formation entropy was formed by a reaction of PF5 derived from LiPF6 and Li in the electrode. In the case of porous Si, it was revealed that the small amount of LiF formation and its low crystallinity suppressed the exothermic reaction in the high temperature range.