Carbonate-Grafted Polysilane as a New Additive for Elevated-Temperature Lithium-Ion Batteries

New additives are required to meet the challenge for Li-ion batteries working at elevated temperature. Here, we report ethylene-carbonate-grafted polysilane (PMSVC) as an additive of commercial carbonate-based electrolytes. PMSVC is synthesized by using graft copolymerization of ethylene carbonate onto polymethylsilane within tetrahydrofuran solvent and azodiiso-butyronitrile initiator. Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy show that the substitution degree of Si-H in final product is about 5%. Electrochemical impedance spectra and charge-discharge tests of the cell with 0.5 wt% additive show that the film on a positive electrode was mainly formed during the initial five cycles at 0.2 C. The capacity retention of the cell with additive is about 87% after 60 cycles at 60 degrees C, whereas the cell with baseline electrolyte is about 80% under the same conditions. The results suggest that the addition of PMSVC is beneficial to the cycling stability of the cell at elevated temperature. X-ray photoelectron spectroscopy analysis identified the compounds on a cycled LiCoO2 electrode, which are associated to the degradation, oxidation-crosslink reaction, and unreacted groups of PMSVC. The mechanism to form the protective film on positive electrode is also further discussed in this work.