Ion Trek Through Polymer Offers Better Batteries

The INEEL team, led by inorganic chemist Thomas Luther, discovered how lithium ions move through the flexible membrane that powers their patented rechargeable lithium battery. Research results are currently published online, and in the April 24, 2003, print issue of the Journal of Physical Chemistry B.

Luther calls their translucent polymer membrane an 'inorganic version of plastic kitchen wrap.' The team, including chemists Luther, Mason Harrup and Fred Stewart, created it in 2000 by adding a ceramic powder to a material called MEEP ([bis(methoxyethoxyethoxy) phosphazene]), an oozy, thick oil. The resulting solid, pliable membrane lets positively charged lithium ions pass through to create the electrical circuit that powers the battery, but rebuffs negatively charged electrons. This keeps the battery from running down while it sits on the shelf-overcoming a major battery-life storage problem.

For years, rechargeable lithium battery performance has been disappointing because the batteries needed recharging every few days. After conquering the discharge challenge, INEEL's team attacked the need for greater battery power to be commercially competitive. Their membrane didn't allow sufficient passage of lithium ions to produce enough power, so they needed to understand exactly how the lithium ions move through the membrane on a molecular level.

First, they analyzed the MEEP membrane using nuclear magnetic resonance-the equivalent of a hospital MRI-to zero in on the best lithium ion travel routes. The results supported the team's suspicion that the lithium ions travel along the 'backbone' of the membrane. The MEEP membrane has a backbone of alternating phosphorus and nitrogen molecules, with oxygen-laden 'ribs' attached to the phosphorus molecules.

Further analysis using infrared and raman spectroscopy (techniques that measure vibrational frequencies and the bonds between different nuclei) helped confirm that the lithium ions are most mobile when interacting with nitrogen. Lithium prefers to nestle into a "pocket" created by a nitrogen molecule on the bottom with oxygen molecules from a MEEP rib on either side.

Armed with this new understanding of how lithium moves through the solid MEEP membrane, the team has already starting making new membrane versions to optimize lithium ion flow. And that should make the team's lithium batteries much more powerful.

The team's research results are a major departure from the conventionally accepted explanation of lithium ion transport that proposed the lithium/MEEP transport mechanism as jumping from one rib to the next using the oxygen molecules as stepping stones.

Harrup, Stewart and Luther are optimistic their battery design will ultimately change the battery industry. The team projects that its polymer membrane will be so efficient at preventing battery run down, that batteries could sit unused for up to 500 months between charges with no loss of charge. Since the membrane is a flexible solid, it can be molded into any shape-which could open up new applications for batteries. And the membrane is very temperature tolerant-with the potential to solve portable power need problems in the frigid cold of space. The team is already working with several federal agencies on applications for its lithium battery designs.

The reference for the paper describing this research is "On the Mechanism of Ion Transport Through Polyphosphazene Solid Polymer Electrolytes: NMR, IR, and Raman Spectroscopic Studies and Computational Analysis of 15N Labeled Polyphosphazenes," Journal of Physical Chemistry B. INEEL authors include Thomas Luther, Fred Stewart, Randall A. LaViolette, William Bauer and Mason K. Harrup. The work was also supported by Christopher Allen of the University of Vermont in Burlington, Vt.

The INEEL is a science-based applied engineering national laboratory dedicated to supporting the U.S. Department of Energy's missions in environment, energy, science and national defense. The INEEL is operated for the DOE by Bechtel BWXT Idaho, LLC.