A polymer electrolyte includes a substrate polymer, a branched polymer, and a
lithium
salt. The branched polymer has a main chain whose repeating unit is composed of
an oligoethylene oxide chain and a connector molecule bonded to the oligoethylene
oxide chain. The branched polymer can be a hyperbranched polymer. The polymer electrolyte
can further include a composite oxide and/or a boroxine compound. The polymer electrolyte
is good in terms of the ionic conductivity, and exhibits a high ionic conductivity
especially at low temperatures. When the polymer electrolyte is used to make polymer
lithium batteries, the resulting polymer lithium batteries shows improved charge-discharge
cycle characteristics. In particular, it is possible to operate the polymer lithium
batteries at low temperatures.
