New conductive polymer addresses volumetric change issue with silicon anodes for Li-ion batteries; high-capacity and longer cycle life reported
23 September 2011
A team of scientists at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has developed new polyfluorene-based conductive polymers that address the long-standing issue of volumetric change in high-capacity silicon (Si) anodes for Li-ion batteries. A combination of synthesis, spectroscopy and simulation techniques tailors the electronic structure of the polymer to enable in situ lithium doping.
The tailored conductive polymer continues to bind closely to the lithium-storing silicon particles even as they expand to more than three times their volume during charging and then shrink again during discharge. Composite anodes based on this polymer and commercial Si particles exhibit 2,100 mAh g−1 in Si after 650 cycles without any conductive additive. A paper on their work is published in the journal Advanced Materials.
High-capacity lithium-ion anode materials have always confronted the challenge of volume change—swelling—when electrodes absorb lithium. Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly when housing the ions between its graphene layers. Silicon can store 10 times more—it has by far the highest capacity among lithium-ion storage materials—but it swells to more than three times its volume when fully charged.—Gao Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD)
Many approaches have been proposed to solving the volumetric change issue, which causes rapid capacity fading; some are prohibitively costly. One less-expensive approach has been to mix silicon particles in a flexible polymer binder, with carbon black added to the mix to conduct electricity. Unfortunately, the repeated swelling and shrinking of the silicon particles as they acquire and release lithium ions eventually push away the added carbon particles. What’s needed is a flexible binder that can conduct electricity by itself, without the added carbon.
Conducting polymers aren’t a new idea, but previous efforts haven’t worked well, because they haven’t taken into account the severe reducing environment on the anode side of a lithium-ion battery, which renders most conducting polymers insulators.—Gao Liu
One such experimental polymer, called PAN (polyaniline), has positive charges; it starts out as a conductor but quickly loses conductivity. An ideal conducting 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 unoccupied 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 polyfluorene-based conducting polymers (PFs).
When Liu discussed the excellent performance of the PFs with Wanli Yang of Berkeley Lab’s Advanced Light Source (ALS), a scientific collaboration emerged 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. Soft x-ray spectroscopy can tell the researchers where the ions and electrons are and where they move, notes Yang.
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).
Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD) and his postdoctoral fellow, Nenad Vukmirovic, conducted ab initio calculations of the polymers at the Lab’s National Energy Research Scientific Computing Center (NERSC).
The simulation revealed that 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 polymer—a doping process that significantly improves the polymer’s electrical conductivity, facilitating electron and ion transport to the silicon particles.
Having gone through one cycle of material synthesis at EETD, experimental analysis at the ALS, and theoretical simulation at MSD, the positive results triggered a new cycle of improvements. Almost as important as its electrical properties are the polymer’s physical properties, to which Liu now added another functional group, producing a polymer that can adhere tightly to the silicon particles as they acquire or lose lithium ions and undergo repeated changes in volume.
Scanning electron microscopy and transmission electron microscopy at the National Center for Electron Microscopy (NCEM), showing the anodes after 32 charge-discharge cycles, confirmed that the modified polymer adhered strongly throughout the battery operation even as the silicon particles repeatedly expanded and contracted. Tests at the ALS and simulations confirmed that the added mechanical properties did not affect the polymer’s superior electrical properties.
The new PF-based anode is not only superior but economical.
Using commercial silicon particles and without any conductive additive, our composite anode exhibits the best performance so far. The whole manufacturing process is low cost and compatible with established manufacturing technologies. The commercial value of the polymer has already been recognized by major companies, and its possible applications extend beyond silicon anodes.—Gao Liu
This achievement provides a rare scientific showcase, combining advanced tools of synthesis, characterization, and simulation in a novel approach to materials development. The cyclic approach can lead to the discovery of new practical materials with a fundamental understanding of their properties.—Zahid Hussain, the ALS Division Deputy for Scientific Support and Scientific Support Group Leader
Gao Liu, Shidi Xun, Nenad Vukmirovic, Xiangyun Song, Paul Olalde-Velasco, Honghe Zheng, Vince S. Battaglia, Lin-Wang Wang, and Wanli Yang (2011) Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes. Advanced Materials DOI: 10.1002/adma.201102421
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