ChemMatCARS Nuggets - Materials Science

Lithium-Polymer Electrolyte Batteries: New Process, Larger Molecules Overturn Conventional Wisdom 

Accomplishment
This work presents a new approach to preparing polymer electrolytes for lithium–polymer electrolyte batteries. Instead of using pure polyethylene oxide, the new method introduces two new twists.

• Key difference 1: Soft, conductive polyethylene oxide is combined with stiff, nonconductive polystyrene (the main component of motorcycle and bicycle helmets) to form a diblock copolymer—a composite of two polymers that segregate into discrete regions.
• Key difference 2: The copolymer is blended with a lithium salt so that the electrolyte is already doped with lithium ions before being paired with its lithium electrode in a battery.

The electrical and mechanical properties were tested extensively, and the results were surprising in several ways.

• Surprise 1: The stiffer the polystyrene "framework," the better the conductivity of the overall membrane. Conventional wisdom said the opposite.
• Surprise 2: Bigger polyethylene oxide molecules provided better conductivity. Conventional wisdom predicted a plateau at high molecular weights.
• Surprise 3: The copolymer did not have a structured network of channels (as confirmed by X-ray and electron microscopy measurements), yet conductivity was still high. Conventional wisdom held that networks were necessary to ion transport.

Secondary peaks are missing in small angle x-ray scattering data, showing copolymer did not form network structures

Impact
The new synthesis approach may overcome performance barriers that have, so far, made lighweight lithium–polymer electrolyte batteries impractical for use in electric vehicles. In these batteries, the lithium anode is separated from the cathode by a film of pure polyethylene oxide. In the past, such batteries developed shorts after repeated charging and discharging, as “dendrites” grew from the anode out into the film and eventually reached the cathode. Without this limitation, such batteries could exceed the U.S. Department of Energy’s targets for electric vehicle range and acceleration.

Because the new electrolyte does not require special processing to align the polymer domains, a wide range of possible compositions and structures can be explored to achieve optimal conductivities. The method is also important because it confirms theoretical studies that predicted that increasing the shear modulus (rigidity) of the electrolyte would stop the formation of dendrites. The results also show that the mechanical and electrical properties of polymer electrolytes can be manipulated separately.

Principal Investigator: Nitash P. Balsara, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, and Department of Chemical Engineering, UC Berkeley
Published in: M.J. Park, K.H. Downing, A. Jackson, et al. "Increased Water Retention in Polymer Electrolyte Membranes at Elevated Temperatures Assisted by Capillary Condensation," Nano Lett. 2007, 7(11), 3547-3552, and in M. Singh, O. Odusanya, G.M. Wilmes, et al., "Effect of Molecular Weight on the Mechanical and ElectricalProperties of Block Copolymer Electrolytes," Macromolecules 2007, 40(13), 4578-4585.

October 2007

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