Overcharge Protection for Rechargeable Lithium Batteries Using Electroactive Polymers

A conducting polymer shunt capable of providing overcharge protection for rechargeable lithium batteries via a reversible, internal, self-actuating mechanism is described. A microporous separator was impregnated with poly(3-butylthiophene), an electrochemically active polymer that becomes electronically conducting when oxidized. The morphology of the resulting composite membrane and its electrochemistry in a lithium battery electrolyte were examined. The composite membrane was introduced into a TiS 2 -Li cell as an overcharge protection separator. While the discharge capacity of an unprotected cell was rapidly degraded when it was charged above 4 V, the protected cell was undamaged by overcharging by as much as ten times the normal capacity. The excess charge current was carried by the polymer, which limited the charging potential to about 3.2 V.