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Charging up to build a better battery

EMSL develops new capabilities to better understand batteries

It’s been said, build a better mousetrap and the world will beat a path to your door. When it comes to energy issues, building a better battery could bring the world to your door.

At EMSL, a national scientific user facility, researchers are committed to improving batteries, technically known as electrochemical energy storage devices. Today’s batteries aren’t powerful enough, require frequent recharging, have to be periodically replaced, and are expensive to produce. To help solve these problems and others, scientists need to understand how batteries work and why they gradually wear out.

Improvements in current systems and advanced concepts for lithium-based electrochemical storage devices are getting a lot of research attention, including lithium-ion, or Li-ion, batteries. Li-ion batteries are used in many battery-powered devices, such as cell phones and laptop computers. With improved performance, they hold promise for electric vehicles and other energy-storage requirements. Li-ion batteries are an efficient way to store energy, but they specifically suffer from stability, storage capacity and other technical problems.

Scientists at EMSL have made significant contributions to lithium-based battery research in the past several years. They have developed technology to study batteries at a microscopic level under operating conditions, or in situ. They are using this capability to test new materials to eliminate many of the problems associated with today’s batteries.

“I want to find efficient ways to store energy,” said EMSL Senior Scientist Chongmin Wang. “Lithium-ion batteries are one of best examples – currently it’s a good technology. But we need to understand it more, and along the way we might find new technologies to store energy even better.”

It Started with EMSL’s Intramural Program

EMSL launched the Intramural Research and Capability Development Program in 2007 to facilitate the development of new research tools and to enable its staff to develop skills and expertise to enhance its user program.

Wang saw EMSL’s Intramural Program as a forum to develop critical new capabilities to study actual working batteries. In 2008, he proposed developing in situ microscopy capabilities to observe a battery during actual operation. Wang and a team of researchers from Pacific Northwest National Laboratory, or PNNL, and Hummingbird Scientific LLC got to work.

 “When we started our research, if you wanted to study a battery you dismantled it and observed the structure,” said Wang. “The ideal way to study a battery is in situ – or as it’s operating in real-world conditions. That way you can see how it functions. At that time, not a lot of people were doing this kind of research.”

Within two years, the team developed what was dubbed “the smallest working battery in the world.” It comprised a single tin dioxide, or SnO2, nanowire as the anode, ionic liquid as the electrolyte and lithium cobalt oxide as the cathode. PNNL Scientist Wu Xu devised the key idea of using an ionic electrolyte, allowing the battery to be studied in vacuum. The small size enabled in situ transmission electron microscopy, or TEM, imaging of the electrode during the battery’s operation.

The TEM is a powerful tool for material research because it generates high spatial resolution images. The direct observation of chemical and material transformations is essential to achieve a confident level of control over complex systems, such as a battery. According to Wang, the TEM is the right tool to see the atomic resolution of both structure and chemical composition and how the structure changes.

The team’s in situ TEM battery testing capability technique was not an instant success; it took a lot testing and research. The technique greatly benefitted from the on-going battery research supported by PNNL internal investment; and the Department of Energy’s Office of Energy Efficiency & Renewable Energy, Office of Electricity Delivery & Energy Reliability, and Office of Basic Energy Sciences. The team’s work was further helped by the guidance of PNNL battery experts like Jason Zhang.

“The technique took us about two years to develop,” said Wang. “We wanted to directly observe how a battery works, and why it wears out gradually and eventually fails. But we didn’t know how to do it at first. We approached this with different concepts along the way and eventually found a way to make it work.”

Much of the testing for the in situ TEM battery was done at EMSL. Some high resolution experiments were conducted with the first successful battery using instruments at the Center for Integrated Nanotechnologies at Sandia National Laboratory. Results from this research were published in Science.

The initial battery concept has been extended and developed enough so that EMSL has made the team’s in situ TEM capability available to other scientists through the lab’s user proposal system. The project’s prototype battery concept for the in situ TEM study has been adopted by researchers worldwide.

“The work we’ve been doing at EMSL is a major accomplishment,” said PNNL Laboratory Fellow Jun Liu. “We developed a tool to actually look at how the material is changing while we charge and discharge the battery directly on the electron microscope. We’re pioneers in that area.”

Testing Other Materials

During the Intramural project, the in situ TEM battery testing capability used a SnO2 anode to test the tool. According to Wang, the team members knew a SnO2 anode would work, but they wanted to know if the tool would work with other anode materials under normal battery operating conditions.

The team member’s success with the SnO2 experiments motivated them to test the next element – silicon. Silicon is a useful material to replace the graphite currently used in Li-ion batteries, because it has a higher electrical capacity and is plentiful – the second most abundant element in the earth’s crust. In addition, silicon-related technology is well developed. The semiconductor industry is based around silicon, and much of the needed infrastructure already exists.

“Silicon in theory is a very good material for Li-ion batteries. In practice, it’s difficult to use,” said Wang. “During charging, the lithium causes the silicon to expand by up to 400 percent and explode or fall apart. We know silicon is a useful material, but making it work is a difficult issue. We have to find a way to mitigate the expansion problem during charging.”

Others doing work in this area include Oak Ridge National Laboratory and General Motors. They had developed a Li-ion battery using carbon-coated fiber with amorphous silicon, but they never studied the battery in situ. In 2011, they began collaborating with Wang, Liu and other researchers from EMSL, PNNL, Stanford University and Applied Science Inc. The group tested the micro-battery in situ at EMSL and found the silicon-carbon material worked better than silicon alone. The addition of carbon sped up the charging process. Unfortunately, the silicon expansion problem during charging still occurred.

The study also provided the researchers with a better understanding of how the combination of lithium and silicon forms an unstructured liquid glassy layer that crystalizes during charging and returns to an unstructured liquid glassy layer during discharge. This process, called congruent phase transition, occurs at the critical points when a liquid changes from disorder to crystalline order then back to disorder.

“Learning more about the nature of the transformation of the silicon from a liquid glassy layer to crystalized layer was an important discovery,” said Wang. “Before the study, we didn’t know what went on between the liquid and the solid states, but using our in situ technique, we found the transformation is a spontaneous process.”

The team’s silicon-carbon research could lead to longer-lasting, cheaper batteries. Their findings were featured in the journal Nano Letters, and TEM videos showing the charge/discharge process generated considerable attention. Because of these and similar findings using EMSL’s in situ TEM battery testing capability, the scientific community’s interest in the technique is growing.

Researchers continue to look for better ways to use silicon-carbon and other alloy-type anode materials in Li-ion batteries. Wang recently collaborated with scientists from Stanford University and SLAC National Accelerator Laboratory to develop a “yoke-shell” structure for the anodes. The scientists sealed silicon nanoparticles in carbon shells, allowing enough space inside for the silicon to expand during charging without rupturing the carbon shell. The team was able to watch the yoke-shell anodes operate using EMSL’s in situ TEM technique. Published in Nano Letters, the team’s findings showed the yoke-shell design has excellent capacity and allows the silicon particles to expand without deforming the carbon shell.

“Scientists are making steady progress to mitigate the problems associated with using silicon in batteries,” said Wang. “We already know a lot about silicon behavior. The expansion problems and other issues will be resolved. I think we’ll eventually use silicon in batteries.”

In addition to silicon, researchers at PNNL and elsewhere are testing other elements to improve electrochemical energy storage devices. Some of these included sodium, sulfur, nanoparticles and nanostructure materials. Scientists also are exploring other types of batteries beyond the solid-type Li-ion, such as liquid and air batteries. The goal is to make the best possible batteries.

“There’s no such thing as ‘the’ perfect battery,” said Liu. “We need to design a battery that is perfect for a particular application. People are always thinking they can make a single battery for every application, but that won’t happen.”

Depending on its use, some of the best battery characteristics include: high capacity, low cost, long lasting, quick recharge, fast recycle (charge/discharge), safe operation, easily expandable and environmentally friendly. Unfortunately, delivering all these characteristics in a single type of battery may not be possible.

“Battery research will never stop,” said Wang. “Energy is a long-term issue. Our energy problems will not end soon and we must deal with them. Energy research may take different directions, but it will continue.”

Released: August 07, 2012