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A better lithium-ion battery on the way

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. Image courtesy of Berkeley Labs.

Lithium-ion batteries power a wide variety of consumer electronics, including smartphones, laptops, and the electric cars that are increasingly attractive as gas prices soar. Inevitably, the limitations of lithium-ion batteries also limit the devices we use.

Ultimately, those limitations come down to how much energy can be safely packed into a small battery, and how rapidly that battery's capacity to store energy will decrease with each cycle of charge and discharge. Now, researchers have designed a new type of anode - a critical battery component - that could vastly improve lithium-ion batteries on both fronts.

The new anode type, developed by a team of scientists at Lawrence Berkeley National Laboratory with the help of high-performance computing systems, is capable of absorbing eight times the lithium of current designs. This in turn translates into a battery that can store a great deal more energy. The new type of anode has also maintained that larger energy capacity after many hundreds of charge-discharge cycles.

The secret is a tailored low-cost 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.

"Conducting polymers aren't a new idea," Liu said, "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."

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 for which the energy of the lowest unoccupied molecular orbital is particularly low. In search of polymers with these qualities, Liu and his postdoctoral fellow Shidi Xun designed a series of polymers based on something called polyfluorene - PFs for short. Xun and Liu in turn partnered with Wanli Yang of Berkeley Lab's Advanced Light Source, to study the material using soft x-ray absorption spectroscopy via ALS' beamline.

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. Image courtesy of Berkeley Lab.

"Gao wanted to know where the ions and electrons are and where they move. Soft x-ray spectroscopy has the power to deliver exactly this kind of crucial information," Yang said, referring to Liu.

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

"We had the experimental evidence," Yang said, "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."

In search of a theoretical explanation, Yang reached out to Berkeley Lab material scientist Lin-Wang Wang. Wang and his postdoctoral fellow, Nenad Vukmirovic, conducted ab initio calculations of the promising polymers at the Berkeley Lab-based US National Energy Research Scientific Computing Center (NERSC).

"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," Wang said. "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. 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, experimental analysis at the Advanced Light Source, and theoretical simulation at NERSC, the positive results triggered a new cycle of improvements. Almost as important as its electrical properties are the polymer's physical properties.

"High-capacity lithium-ion anode materials have always confronted the challenge of volume change - swelling - when electrodes absorb lithium," Liu said. "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."

This kind of swelling quickly breaks the electrical contacts in the anode, so researchers have concentrated on finding other ways to use silicon while maintaining anode conductivity. Many approaches have been proposed, some of which are prohibitively costly.

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. Image courtesy of Berkeley Lab.

One less-expensive approach is 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.

Liu's team had produced a polymer that could conduct electricity, so now they turned their attention to making the PFs more effective binders. As with the carbonyl, the team found that by adding a functional group they produced 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, they examined their test anodes using scanning electron microscopes and transmission electron microscopes at the US National Center for Electron Microscopy. What they saw confirmed that the modified polymer adhered strongly throughout the battery operation even as the silicon particles repeatedly expanded and contracted. Further tests at the ALS and simulations confirmed that the added mechanical properties did not affect the polymer's superior electrical properties. The icing on the anode cake is that 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," Liu said. "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."

Anodes are a key component of lithium-ion battery technology, but far from the only challenge. The research collaboration has already begun the next step, studying other batter components such as cathodes.

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