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battery, which renders most conducting polymers insulators.”

One such experimental polymer, called PAN (poly- aniline), has positive charges; it starts out as a con- ductor but quickly loses conductivity. An ideal con- ducting polymer should readily acquire electrons, rendering it conducting in the anode’s reducing environment.

The signature of a promising polymer would be one with a low value of the state called the “lowest unoc- cupied molecular orbital,” where electrons can easily reside and move freely. Ideally, electrons would be acquired from the lithium atoms during the initial charging process. Liu and his postdoctoral fellow Shidi Xun in EETD designed a series of such polyfluo- rene-based conducting polymers – PFs for short.

When Liu discussed the excellent performance of the PFs with Wanli Yang of Berkeley Lab’s Advanced Light Source (ALS), a scientific collaboration emer- ged to understand the new materials. Yang suggested conducting soft x-ray absorption spectroscopy on Liu and Xun’s candidate polymers using ALS beamline 8.0.1 to determine their key electronic properties.

Says Yang, “Gao wanted to know where the ions and electrons are and where they move. Soft x-ray spec- troscopy has the power to deliver exactly this kind of crucial information.”

Compared with the electronic structure of PAN, the absorption spectra Yang obtained for the PFs stood out immediately. The differences were greatest in PFs incorporating a carbon-oxygen functional group (carbonyl).

11-09 :: September 2011

“We had the experimental evidence,” says Yang, “but to understand what we were seeing, and its relevance to the conductivity of the polymer, we needed a theoretical explanation, starting from first principles.” He asked Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD) to join the research collaboration.

Wang and his postdoctoral fellow, Nenad Vukmiro- vic, conducted ab initio calculations of the promi- sing polymers at the Lab’s National Energy Research Scientific Computing Center (NERSC). Wang says, “The calculation tells you what’s really going on – including precisely how the lithium ions attach to the polymer, and why the added carbonyl functional group improves the process. It was quite impressive that the calculations matched the experiments so beautifully.”

The simulation did indeed reveal “what’s really going on” with the type of PF that includes the carbonyl functional group, and showed why the system works so well. The lithium ions interact with the polymer first, and afterward bind to the silicon particles. When a lithium atom binds to the polymer through the carbonyl group, it gives its electron to the poly- mer – a doping process that significantly improves the polymer’s electrical conductivity, facilitating elec- tron and ion transport to the silicon particles.

Having gone through one cycle of material synthesis at EETD, experimental analysis at the ALS, and theo- retical simulation at MSD, the positive results trigge- red a new cycle of improvements. Almost as im- portant as its electrical properties are the polymer’s physical properties, to which Liu now added another functional group, producing a polymer that can