  A safe lithium-sulfur (Li-S) battery employs a composite polymer electrolyte
based on a poly(ethylene glycol) dimethyl ether (PEGDME) solid at room
temperature. The electrolyte membrane enables a stable and reversible Li-S
electrochemical process already at 50{\deg}C, with low resistance at the
electrode/electrolyte interphase and fast Li+ transport. The relatively low
molecular weight of the PEGDME and the optimal membrane composition in terms of
salts and ceramic allow a liquid-like Li-S conversion reaction by heating at
moderately high temperature, still holding the solid-like polymer state of the
cell. Therefore, the electrochemical reaction of the polymer Li-S cell is
characterized by the typical dissolution of lithium polysulfides into the
electrolyte medium during discharge and the subsequent deposition of sulfur at
the electrode/electrolyte interphase during charge. On the other hand, the
remarkable thermal stability of the composite polymer electrolyte (up to
300{\deg}C) suggests a lithium-metal battery with safety content significantly
higher than that using the common, flammable liquid solutions. Hence, the Li-S
polymer battery delivers at 50{\deg}C and 2 V a stable capacity approaching 700
mAhgS-1, with a steady-state coulombic efficiency of 98%. These results suggest
a novel, alternative approach to achieve safe, high energy batteries with solid
polymer configuration.
