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A Better Anode Design to Improve Lithium-Ion Batteries Print
Friday, 23 March 2012 13:53

Lithium-ion batteries are in smart phones, laptops, most other consumer electronics, and the newest electric cars. Good as these batteries are, the need for energy storage in batteries is surpassing current technologies. In a lithium-ion battery, charge moves from the cathode to the anode, a critical component for storing energy. A team of Berkeley Lab scientists has designed a new kind of anode that absorbs eight times the lithium of current designs, and has maintained its greatly increased energy capacity after more than a year of testing and many hundreds of charge-discharge cycles.

Cyclical Science Succeeds

The anode achievement described in this highlight provides a rare scientific showcase, combining advanced tools of synthesis, characterization, and simulation in a novel approach to materials development. Gao Liu’s original research team, part of Berkeley Lab’s Environmental Energy Technologies Division (EETD), got the ball rolling by designing the original series of polyfluorene-based conducting polymers. Then, Wanli Yang of the ALS suggested soft x-ray absorption spectroscopy to determine their key electronic properties. To better understand these results, and their relevance to the conductivity of the polymer, the growing team sought a theoretical explanation from Lin-Wang Wang of Berkeley Lab’s Materials Sciences Division (MSD). By conducting calculations on the promising polymers at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), the team gained insight into what was really happening in the PF with the carbonyl functional group, singling it out for further development.

After one cycle of material synthesis at EETD, experimental analysis at the ALS, and theoretical simulation at MSD using NERSC, the positive results triggered a new cycle of improvements, wherein Liu added another functional group to the PF. Scanning and transmission electron microscopy done at the National Center for Electron Microscopy (NCEM) confirmed strong adherence of the modified polymer throughout numerous charge-discharge cycles.


Use of this collaborative approach can lead to the discovery of new practical materials with a fundamental understanding of their properties, informing modifications in the next generation of polymers. Already this research collaboration is pushing to the next step, studying other battery components including cathodes.

When building a charge, anode materials swell to store lithium ions. The swelling that accompanies lithium absorption has always been a challenge when designing high-capacity lithium-ion anode materials. Most of today’s lithium-ion batteries have anodes made of graphite, which is electrically conducting and expands only modestly. 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. This swelling quickly breaks the electrical contacts in the anode, so researchers have concentrated on finding ways to use silicon while maintaining anode conductivity.

At left, the traditional approach to composite anodes using silicon (blue spheres) for higher energy capacity has a polymer binder (light brown) plus added particles of carbon to conduct electricity (dark brown spheres). Silicon swells and shrinks while acquiring and releasing lithium ions. 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.

One approach mixes silicon particles in a flexible polymer binder, adding carbon to the mix to conduct electricity. Unfortunately the repeated swelling and shrinking of silicon as it acquires and releases lithium ions eventually push away the carbon particles. What’s needed is a flexible binder that can conduct electricity by itself, without added carbon.

The secret to the newly-designed anode is a tailored polymer that conducts electricity and binds closely to lithium-storing silicon particles, even as they expand to more than three times their volume during charging and then shrink again during discharge. Anodes made from these conducting polymers have low-cost materials and are compatible with standard lithium-battery manufacturing technologies. However, previous efforts did not take into account the severe reducing environment on the anode side of a lithium-ion battery, which renders most conducting polymers insulators. An ideal conducting polymer should readily acquire electrons, rendering it conducting in the anode’s reducing environment, and allow electrons to reside and move freely. Electrons would be acquired from the lithium atoms during the initial charging process.

Gao Liu, a member of the Batteries for Advanced Transportation Technologies (BATT) program, and his team designed a series of polyfluorene-based conducting polymers—PFs for short—that performed excellently. Upon hearing about the PF’s success, ALS beamline scientist Wanli Yang suggested candidate polymers be tested to determine their key electronic properties, embarking on a scientific collaboration to better understand the new materials.

 

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.

Soft x-ray absorption spectroscopy conducted on ALS Beamline 8.0.1 revealed where the ions and electrons are and where they move. The absorption spectra obtained for the PFs stood out from those of other candidate anode materials immediately. The differences were greatest in PFs incorporating a carbon-oxygen functional group (carbonyl). Theoretical calculations made via computer simulation exposed the functionality behind the PF, revealing precisely how the lithium ions attach to the polymer and why the added carbonyl functional group improves the process. The calculations matched the experiments beautifully.

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.

 

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.

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. After 32 charge-discharge cycles, scanning and transmission electron microscopy confirmed that the modified polymer adhered strongly throughout battery operation even as the silicon particles repeatedly expanded and contracted. Additional testing and simulation confirmed that the added mechanical properties did not affect the polymer’s superior electrical properties.

Using commercial silicon particles and without any conductive additive (like carbon), this composite anode exhibits the best performance so far in lithium-ion batteries, while retaining an economical cost and compatibility with existing manufacturing technologies.

Read the Berkeley Lab News Release


 

Research conducted by: G. Liu, S. Xun, X. Song, H. Zheng, and V.S. Battaglia (EETD, Berkeley Lab), P. Olalde-Velasco and W. Yang (ALS, Berkeley Lab), L.-W. Wang (MSD, Berkeley Lab), and N. Vukmirovic (MSD, Berkeley Lab, and University of Belgrade).

Research funding: 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. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: G. Liu, S. Xun, N. Vukmirovic, X. Song, P. Olalde-Velasco, H. Zheng,V.S. Battaglia, L.-W. Wang, and W. Yang, “Polymers with Tailored Electronic Structure for High Capacity Lithium Battery Electrodes,” Advanced Materials 23, 4679 (2011).

ALS Science Highlight #245

 

ALSNews Vol. 329