Silicon/polymer combo offers lithium-ion batteries higher capacity

September 26, 2011 — Lawrence Berkeley National Laboratory scientists designed a tailored polymer that enables increased energy storage in lithium-ion (Li-ion) batteries with silicon anodes, maintaining the increased energy capacity in tests over a year later with many hundreds of charge-discharge cycles.

The tailored polymer conducts electricity and binds closely to lithium-storing silicon particles, which expand to more than 3x their volume during charging and shrink during discharge. The anodes are made from low-cost materials, compatible with standard lithium-battery manufacturing technologies.

Figure 1. At left, the traditional approach to composite anodes using silicon (blue spheres) for higher energy capacity has a polymer binder such as PVDF (light brown) plus added particles of carbon to conduct electricity (dark brown spheres). Silicon swells and shrinks while acquiring and releasing lithium ions, and repeated swelling and shrinking eventually break contacts among the conducting carbon particles. At right, the new Berkeley Lab polymer (purple) is itself conductive and continues to bind tightly to the silicon particles despite repeated swelling and shrinking.

Lithium-ion batteries typically have graphite anodes, expanding modestly when housing the ions between its graphene layers. Silicon can store 10X more, but it swells to more than three times its volume when fully charged, said Gao Liu of Berkeley Lab’s Environmental Energy Technologies Division (EETD), a member of the BATT program (Batteries for Advanced Transportation Technologies) managed by the Lab and supported by DOE’s Office of Vehicle Technologies. This swelling breaks the anode’s electrical contacts. Some companies mix silicon particles into a flexible polymer binder with carbon black for conductivity. The Berkeley Lab researchers decided to use this concept without the carbon materials, which suffer from pump out after repeated charges. 

Figure 2. At top, spectra of a series of polymers obtained with soft x-ray absorption spectroscopy at ALS beamline 8.0.1 show a lower “lowest unoccupied molecular orbital” for the new Berkeley Lab polymer, PFFOMB (red), than other polymers (purple), indicating better potential conductivity. Here the peak on the absorption curve reveals the lower key electronic state. At bottom, simulations disclose the virtually complete, two-stage electron charge transfer when lithium ions bind to the new polymer.

PAN (polyaniline) polymer 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.

A polymer with a low value of the “lowest unoccupied molecular orbital,” where electrons can easily reside and move freely best suits the Li-ion battery. 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).

Wanli Yang of Berkeley Lab’s Advanced Light Source (ALS) used 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 has the power to track where the ions and electrons are and where they move, Yang said.

Compared with the electronic structure of PAN, the absorption spectra Yang obtained for the PFs differed, most notably in PFs incorporating a carbon-oxygen functional group (carbonyl).

Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD) joined the research collaboration with his postdoctoral fellow, Nenad Vukmirovic, to conduct ab initio calculations of the polymers at the Lab’s National Energy Research Scientific Computing Center (NERSC). They determined precisely how the lithium ions attach to the polymer, and why the added carbonyl functional group improves the process.

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.

Figure 3. Transmission electron microscopy reveals the new conducting polymer’s improved binding properties. At left, silicon particles embedded in the binder are shown before cycling through charges and discharges (closer view at bottom). At right, after 32 charge-discharge cycles, the polymer is still tightly bound to the silicon particles, showing why the energy capacity of the new anodes remains much higher than graphite anodes after more than 650 charge-discharge cycles during testing.

To tune the polymer’s physical properties, Liu 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.

"Using commercial silicon particles and without any conductive additive, our composite anode exhibits the best performance so far," says Liu. "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."

The research collaboration is now studying other battery components, including cathodes.

The research team reports its findings in Advanced Materials: "Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes," by Gao Liu, Shidi Xun, Nenad Vukmirovic, Xiangyun Song, Paul Olalde-Velasco, Honghe Zheng, Vince S. Battaglia, Lin-Wang Wang, and Wanli Yang. Access it at http://onlinelibrary.wiley.com/doi/10.1002/adma.201102421/abstract.

Materials research for this work in the BATT program was supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy. The ALS, NCEM, and NERSC are national scientific user facilities supported by DOE’s Office of Science. Visit http://batt.lbl.gov/, http://www-als.lbl.gov/, http://ncem.lbl.gov/, or http://www.nersc.gov/.www.lbl.gov.

Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.energy.gov.

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