Electroactive Polymer Separator to Protect from Overcharging in Lithium Ion Batteries

Battery packs for hybrid electric vehicles need to be low cost and have a 15-year lifetime. However, the cycle life of lithium-ions cells, which are configured in series, is reduced by the overcharging of individual cells. The current method of overcharge control, which uses external control circuitry, has a gravimetric and cost burden, and is ineffective at eliminating all of the cell damage during overcharge. This project addresses the overcharging issue by developing an alternative separator that becomes temporarily conductive when a cell reaches an over-voltage condition, shunting electrons between the electrodes. This separator/shunt will keep the cell voltage below the value at which deleterious reactions occur. When the charging current is no longer applied to the cell at higher voltages, the separator will switch to an insulator and allows normal battery operation. The process will be fully reversible and will not limit the number of charging cycles that can be applied to the cell. In Phase I, the fabricated separator demonstrated the technical feasibility for controlling overcharge. At over-voltage conditions (> 3.9V), the electroactive polymer separator functioned as a current shunt at current densities up to 10 mA/cm2. At cell operating voltages, the separator performed as an ion shuttle as the cell charged and discharged, maintaining cell capacity at typical C-rates. In Phase II, the polymer blend components that make up the separator will be modified to enable 10 mA/cm2 current on overcharge and to allow performance as a typical separator with a fully charged open circuit voltage of 4.2 volts. The selected polymer blend will be scaled-up via film production on continuous process, solvent casting equipment. Experimental and control separators will be tested in 240 mAh pouch cells for overcharge and cycle life. Commercial Applications and other Benefits as described by the awardee: The new separator technology should be appropriate for lithium ion battery stacks that require greater cycle life than what is available currently. The electroactive separator is a cost-effective and gravimetrically-effective means of protecting the electrodes locally for improved cycle life. In addition to the HEV and electric vehicle markets for this separator, other potential consumer applications include laptop computer batteries.