PEO-based polymer electrolytes for secondary lithium batteries

Polyethers mixed with lithium salts are excellent candidates for electrolytes in rechargeable lithium batteries. Polyether systems with low crystallinity result in fast ion mobility and therefore high conductivities. In this work the properties of several poly(ethylene oxide) based electrolytes are examined with an emphasis on systems with reduced crystallinity including, composite polymer electrolytes, oligomeric polyethers, and (AB) microblock copolymers.; Highly conductive and processable composite polymer electrolytes were made using surface functionalized fumed silica fillers and PEGDME-500 (LiClO ₄, O/Li = 20). The fillers were both hydrophobic and cross-linkable and formed an open three-dimensional network in the electrolytes due to van der Waals forces. The open network allowed for high ionic mobility and provided for the mechanical stability of the composite. Methacrylate monomers of differing hydrophobicity were added to cross-link the silica network and impart permanent mechanical stability. The optical, conductive, thermal, mechanical, and kinetic properties of the composites are examined as a function of monomer hydrophobicity and filler surface chemistry. It was found that hydrophobic monomers such as butyl methacrylate and octyl methacrylate preferentially phase separate onto the filler surface while hydrophilic methyl methacrylate is soluble in the electrolyte phase. The composites were both photochemically and thermally cured to 85–95% conversion of monomer to polymer. Hydrophilic monomers such as methyl methacrylate are more compatible with the electrolyte after polymerization and therefore provide for better mechanical properties in the composite. However, unpolymerized methyl methacrylate can react at the electrodes resulting in increased interfacial resistance.; A branched oligomeric polyether, star(12)PEO, was prepared and characterized. Electrolytes formed from star(12)PEO and LiClO₄ were characterized by DSC and variable temperature impedance spectroscopy. The properties of the branched system were compared to linear PEGDME-500. It was found that branching completely inhibits crystallinity and improves the low temperature properties of polyether-based electrolytes. In addition, star(12)PEO added to high molecular weight poly(ethylene oxide) help to decrease crystallinity. Finally, electrolytes were made from LiClO₄ and (AB)_n microblock copolymers that contain a repeating pattern of exact length segments of poly(ethylene oxide) and poly(ethylene). The thermal and conductive properties were studied as a function of the polyether block length. The conductivity of the electrolytes increases with the mole fraction of the polyether block, but electrolytes with >10 ether segments crystallized, leading to decreases in conductivity.