Surface Area Increase of Silicon Alloys in Li-Ion Full Cells Measured by Isothermal Heat Flow Calorimetry

Li-ion pouch cells utilizing a negative electrode formulated with 15 wt% of an engineered Si alloy in a graphite composite electrode were cycled in an isothermal heat flow calorimeter against a LiCoO 2 positive electrode. Two different electrolytes were investigated: a blend of ethylene carbonate and ethyl methyl carbonate (3EC:7EMC) and a blend of ethylene carbonate, ethyl methyl carbonate and 1-fluoro ethylene carbonate (27EC:63EMC:10FEC). Both electrolytes were 1 M in LiPF 6 salt. The parasitic thermal power and coulombic efficiency was derived from isothermal heat flow measurements and high precision current-source meters. Cells without FEC showed high parasitic thermal power which increased with cycle number indicative of a surface area increase which was confirmed by post-cycling scanning electron micrographs and surface area measurements. Cells with FEC showed relatively stable parasitic thermal power. These measurements demonstrate the surprising function of FEC in controlling or attenuating the evolution of surface area in Si alloys. Vinylene carbonate was also found to be effective at controlling the increase in alloy surface area.