Composite Polymer Electrolytes for Lithium Batteries

Abstract The flexible and less flammable solid polymer electrolytes are attractive candidates for high specific energy density lithium batteries. However, the use of polymer electrolytes in rechargeable batteries for EVs is hindered because of their low lithium‐ion conductivity at room temperature and lithium dendrite formation during lithium deposition at high current density. To improve the room temperature conductivity of polymer electrolytes, composite lithium‐ion conductors consisting of polymer and ionic liquids or inorganic solid lithium ion conductors have been examined over the past two decades. The polymer electrolyte with ionic liquid showed ionic conductivity of more than 10 −4 S cm −1 at room temperature, but lithium dendrite‐free composite electrolytes have not yet been reported at room temperature and a high current density. The flexible composite electrolytes of polymers and lithium‐ion conductive solid electrolytes showed high ionic conductivity of more than 2×10 −4 S cm −1 and no dendrite short‐circuit was observed at room temperature and 1.0 mA cm −1 for long cycling. The suppression of lithium dendrite formation in the composite electrolyte is due to the formation of a stable interface layer between the lithium electrode and composite electrolyte. At present, no room temperature all‐solid‐state batteries have been developed with performance comparable to conventional lithium‐ion batteries with liquid electrolytes. One of the disadvantage of the lithium batteries with the composite polymer electrolyte is higher mass of the electrolyte than that of a liquid electrolyte with a porous separator. The specific energy density of the batteries depends on the thickness of the electrolyte and the specific area capacity. At present, the thickness of the polymer composite electrolytes is in a range of 50–200 μm. The specific energy density of the lithium battery with a 100 μm thick composite electrolyte is 430 Wh kg −1 at 10 mAh cm −2 , which is 1.4 times higher than that of the conventional Li‐ion battery. A high specific area capacity battery with a thin polymer composite electrolyte should be developed to obtain a high energy density battery. We are anxiously expecting a mechanically stable composite polymer electrolyte thin film of less than 100 μm thick with high Li‐ion conductivity more than 10 −3 S cm −1 at room temperature and low interface resistance with Li electrode. The addition of small amount of a low molecular weight additive into the composite electrolytes is effective to improve the interface resistance between the lithium electrode and composite electrolyte, which results in no dendrite formation at high current density and room temperature.