SiO₂-modified separators: Stability in LiPF₆-containing electrolyte solutions and effect on cycling performance of Li batteries

Significant improvement in the performance of cells comprising Li metal anodes and LiNi_0.6Co_0.2Mn_0.2O₂ (NCM 622) cathodes with practical loading of the electrodes material (3.3 mAh cm⁻²) and low amount of the electrolyte solution (22 μl cm⁻²) was achieved with the use of SiO₂ filled polyethylene separators. The effects of cycling and storage on the stability of the SiO₂ filled separators in fluoroethylene carbonate (FEC) and LiPF₆-containing electrolyte solutions was studied with the use of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. We analyzed the composition of the separators and the gas phase after storage and cycling of these cells. Although SiO₂ reacts with LiPF₆ when the separator is in contact with the electrolyte solution, we showed that it is fully stable during cycling in the Li-NCM622 cells. The formation of lithium hexafluorosilicate (Li₂SiF₆) on SiO₂ filled separators and tetrafluorosilane (SiF4) in the gas phase around them was detected when these separators were stored in the electrolyte solutions without cycling. The effect of the electrodes and the operation condition on the stability of SiO₂ containing separators is discussed.