Electrolyte design for high-energy density Lithium-ion battery with pure Silicon anode

Abstract

Silicon anodes can boost the energy density of lithium-ion batteries due to its high theoretical capacity up to ~3600 mAh/g. However, a challenge of the use of silicon anode is 300% swelling/shrink upon lithiation/delithiation, which is a major cause of battery failure. A lithium-ion battery with silicon anode requires the formation of a flexible and stable solid electrolyte interphase (SEI) on the anode’s surface to prevent continuous electrolyte decomposition, to mitigate the volume changes and to enable good Li­-ion transportation. It is still a challenge to develop new electrolyte compositions to mitigate the continuous SEI formation and, consequently, enhance the cycle life. In this thesis, a 100% pure amorphous silicon anode is investigated in lithium-ion battery cells with an energy density up to 1350 Wh/L. The effect of different electrolyte compositions on the electrochemical behaviour and cycle life is studied. The results show that the addition of fluoroethylene carbonate (FEC) and vinylene carbonate (VC) to an electrolyte mixture of LiPF6 and a pure linear carbonate solvent improves the capacity retention for over 100 cycles. The addition of co-solvents, propylene carbonate(PC) or ethylene carbonate (EC), improves the silicon utilisation level from ~1500 to ~1700 mAh/g. The diallyl pyrocarbonate (DAPC) additive in the electrolyte improves the capacity retention at 100 cycles from 67.7% to 72.2% in a full NMC­622/Si coin cell and from 84.2% to 90.8% in a full pouch cell. This study demonstrates that the electrolyte composition has an effect on the cycle life of lithium-ion batteries with silicon anode, likely by SEI formation from preferable decomposition products and from a complementary mixture of electrolyte components.