Blends of Pegylated Polyoctahedralsilsesquioxanes (POSS-PEG) and Methyl Cellulose as Solid Polymer Electrolytes for Lithium Batteries

Solid polymer electrolytes (SPEs) were prepared by blending LiClO4, methyl cellulose (MC) and an oligometric dendritic polyethylene glycol (PEG), formed from polyoctahedralsilsesquioxane functionalized with ∼ 8 PEG side chains (POSS-PEG) on the SiO1.5 core. Compositions with 80/20, 70/30 and 60/40 POSS-PEG/MC, and LiClO4 (O/Li = 16/1) were investigated for their electrochemical, mechanical and morphological properties, where O/Li indicates the molar ratio of ether oxygens from POSS-PEG to Li+ ions. Since compatible blends can be made over a range of compositions, ranging from "tacky" with low amounts of MC, to "hard" with high percentages of MC, this system is a paradigm for preparing batteries with continuous interfaces from the electrodes to the SPE. SEM and TEM images indicate that POSS-PEG/LiClO4(O/Li = 16/1)/MC form microporous structures in which the pore walls are microphase separated. In these microphase separated blends, the LiClO4 partitions into the POSS-PEG phase and is amorphous between −100 °C and its decomposition temperature, Td ∼ 300 °C, as evidenced by an increased glass transition temperature (Tg) for the POSS-PEG/LiClO4 phase, while the MC phase is semicrystalline, as evidenced by x-ray diffraction data, showing the persistance of crystalline regions of MC in the blends. At temperatures below the glass transition temperature (Tg) of POSS-PEG/LiClO4(O/Li = 16/1) the moduli of the blends increase with increasing POSS-PEG content, indicating that the POSS-PEG component behaves like high molar mass polyethylene oxide (PEO) reinforced with SiO1.5. At temperatures T > Tg (POSS-PEG/LiClO4), where POSS-PEG/LiClO4 is highly viscous (does not flow under its own weight) but not a true solid, the moduli of the blends increase with MC content, consistent with the high modulus MC forming cross-link sites to the dominant POSS-PEG/LiClO4 phase. It is proposed that these crosslink sites are due to residual primary OH groups from MC that form hydrogen bonds with ether oxygens of POSS-PEG/LiClO4 and result in a rubbery morphology (32 - 156 MPa) at room temperature, with the Li+ ions preferentially binding to the ether oxygens in the interior of the microphase separated regions. Conductivities of 1.6 × 10−5 and 1.1 × 10−6 S/cm were measured at 30 °C and 0 °C, respectively, for the 80/20 POSS-PEG/LiClO4 (O/Li = 16/1)/MC blend. Stability and reversibility of the blends at 50 °C were observed in the range 1.5 to 4.2 V.