Oligo(ethylene glycol)-functionalized disiloxanes as electrolytes for lithium-ion batteries

Functionalized disiloxane compounds were synthesized by attaching oligo(ethylene glycol) chains, -(CH2CH2O)-(n), n = 2-7, via hydrosilation, dehydrocoupling, and nucleophilic substitution reactions and were examined as non-aqueous electrolyte solvents in lithium-ion cells. The compounds were fully characterized by (1) H, C-13, and Si-29 nuclear magnetic resonance (NMR) spectroscopy. Upon doping with lithium bis(oxalato)borate (LiBOB) or LiPF6, the disiloxane electrolytes showed conductivities up to 6.2 x 10(-4) S cm(-1) at room temperature. The thermal behavior of the electrolytes was studied by differential scanning calorimetry, which revealed very low glass transition temperatures before and after LiBOB doping and much higher thermal stability compared to organic carbonate electrolytes. Cyclic voltammetry measurements showed that disiloxane-based electrolytes with 0.8 M LiBOB salt concentration are stable to 4.7 V. The LiBOB/disiloxane combinations were found to be good electrolytes for lithium-ion cells; unlike LiPF6, LiBOB can provide a good passivation film on the graphite anode. The LiPF6/disiloxane electrolyte was enabled in lithium-ion cells by adding 1 wt% vinyl ethylene carbonate (VEC). Full cell performance tests with LiNi0.80Co0.15Al0.05O2 as the cathode and mesocarbon microbead (MCMB) graphite as the anode show stable cyclability. The results demonstrate that disiloxane-based electrolytes have considerable potential as electrolytes for use in lithium-ion batteries. (C) 2009 Elsevier B.V. All rights reserved.