High lithium ionic conductivity of poly(ethylene oxide)s having sulfonate groups on their chain ends

Poly(ethylene oxide) oligomers having sulfonate groups on one or both chain ends [PEO–(SO 3 M) n (n=1 or 2); M=Li, Na, K, Rb or Cs] with different PEO molecular masses (150–2000) have been prepared as new single-ion conductive matrices. High lithium ionic conductivity (4.45×10 -6 S cm -1 , at 30 °C) was obtained in PEO–SO 3 Li with PEO molecular mass of 350, the transport number of lithium ion (t Li+ ) was ca. 0.75 at 20 °C. The glass transition temperature (T g ) of these hybrid salts depended significantly on the molecular mass of the PEO and its molecular structure. The PEO oligomers containing a sulfonate group on only one end showed higher ionic conductivity than those containing sulfonate groups on both ends at the same salt fraction up to 50% per OE unit (i.e. M w >500). In both types of hybrid salts, the PEO–sulfonate with relatively high molecular mass around 2000 showed a crystalline phase at room temperature reflecting the crystallinity of the PEO component. On the other hand, the hybrid having PEO with low molecular mass of ca. 200 showed an amorphous phase but had a very high T g , considerably reflecting the characteristics of the salt unit. PEO–sulfonates with molecular masses of PEO between 350 and 1000 were confirmed not only to show an amorphous phase over a wide temperature range but also to have low glass transition temperatures at ca. -40 °C.