Roots of the Lithium Battery Problem: Berkeley Lab Researchers Find Dendrites Start Below the Surface

The lithium-ion batteries that power our laptops, smartphones and electric vehicles could have significantly higher energy density if their graphite anodes were to be replaced by lithium metal anodes. Hampering this change, however, has been the so-called dendrite problem. Over the course of several battery charge/discharge cycles, particularly when the battery is cycled at a fast rate, microscopic fibers of lithium, called “dendrites,” sprout from the surface of the lithium electrode and spread like kudzu across the electrolyte until they reach the other electrode. An electrical current passing through these dendrites can short-circuit the battery, causing it to rapidly overheat and in some instances catch fire. Efforts to solve the problem by curtailing dendrite growth have met with limited success, perhaps because they’ve just been scratching the surface of the problem.

These 3D reconstructions show how dendritic structures that can short-circuit a battery form deep within a lithium electrode, break through the surface and spread across the electrolyte.

Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered that during the early stages of development, the bulk of dendrite material lies below the surface of the lithium electrode, underneath the electrode/electrolyte interface. Using X-ray microtomography at Berkeley Lab’s Advanced Light Source (ALS), a team led by Nitash Balsara, a faculty scientist with Berkeley Lab’s Materials Sciences Division, observed the seeds of dendrites forming in lithium anodes and growing out into a polymer electrolyte during cycling. It was not until the advanced stages of development that the bulk of dendrite material was in the electrolyte. Balsara and his colleagues suspect that non-conductive contaminants in the lithium anode trigger dendrite nucleation.

Nitash Balsara and Katherine Harry at ALS beamline 8.3.2 where they shed important new light on the dendrite problem in lithium batteries. (Photo by Roy Kaltschmidt)

“Contrary to conventional wisdom, it seems that preventing dendrite formation in polymer electrolytes depends on inhibiting the formation of subsurface dendritic structures in the lithium electrode,” Balsara says. “In showing that dendrites are not simple protrusions emanating from the lithium electrode surface and that subsurface non-conductive contaminants might be the source of dendritic structures, our results provide a clear prescription for the path forward to enabling the widespread use of lithium anodes.”

Balsara, who is a professor of chemical engineering at the University of California (UC) Berkeley, is the corresponding author of a paper describing this research in Nature Materials titled “Detection of subsurface structures underneath dendrites formed on cycled lithium metal electrodes.” Co-authors are Katherine Harry, Daniel Hallinan, Dilworth Parkinson and, Alastair MacDowell.

The tremendous capacity of lithium and the metal’s remarkable ability to move lithium ions and electrodes in and out of an electrode as it cycles through charge/discharge make it an ideal anode material. Until now, researchers have studied the dendrite problem using various forms of electron microscopy. This is the first study to employ microtomography using monochromatic beams of high energy or “hard” X-rays, ranging from 22 to 25 keV, at  ALS beamline 8.3.2. This technique allows non-destructive three-dimensional imaging of solid objects at a resolution of approximately one micron.

“We observed crystalline contaminants in the lithium anode that appeared at the base of every dendrite as a bright speck,” says Katherine Harry, a member of Balsara’s research group and the lead author of the Nature Materials paper. “The lithium foils we used in this study contained a number of elements other than lithium with the most abundant being nitrogen. We can’t say definitively that these contaminants are responsible for dendrite nucleation but we plan to address this issue by conducting in situ X-ray microtomography.”

Balsara and his group also plan further study of the role played by the electrolyte in dendrite growth, and they have begun to investigate ways to eliminate non-conductive impurities from lithium anodes.

This research was funded by the DOE Office of Science.

New Twist in the Graphene Story

13 August 2013

The tiny twist that may solve a mystery

 Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered a unique new twist to the story of graphene, sheets of pure carbon just one atom thick, and in the process appear to have solved a mystery that has held back device development.
Working at Berkeley Lab’s Advanced Light Source (ALS), a DOE national user facility, a research team led by ALS scientist Aaron Bostwick has discovered that in the stacking of graphene monolayers subtle misalignments arise, creating an almost imperceptible twist in the final bilayer graphene. Tiny as it is – as small as 0.1 degree – this twist can lead to surprisingly strong changes in the bilayer graphene’s electronic properties.

 

 
Monolayers of graphene have no bandgaps – ranges of energy in which no electron states can exist. Without a bandgap, there is no way to control or modulate electron current and therefore no way to fully realize the enormous promise of graphene in electronic and photonic devices. Berkeley Lab researchers have been able to engineer precisely controlled bandgaps in bilayer graphene through the application of an external electric field. However, when devices were made with these engineered bandgaps, the devices behaved strangely, as if conduction in those bandgaps had not been stopped.

 

 

 
To get to the bottom of this mystery, Rotenberg, Bostwick, Kim and their co-authors performed a series of angle-resolved photoemission spectroscopy (ARPES) experiments at ALS beamline 7.0.1. ARPES is a technique for studying the electronic states of a solid material in which a beam of X-ray photons striking the material’s surface causes the photoemission of electrons. The kinetic energy of these photoelectrons and the angles at which they are ejected are then measured to obtain an electronic spectrum.

 

 
Massless Dirac fermions, electrons that essentially behave as if they were photons, are not subject to the same bandgap constraints as conventional electrons. In their Nature Materials paper, the authors state that the twists that generate this massless Dirac fermion spectrum may be nearly inevitable in the making of bilayer graphene and can be introduced as a result of only ten atomic misfits in a square micron of bilayer graphene.

 

 
Beyond solving a bilayer graphene mystery, the researchers say the discovery of the twist establishes a new framework on which various fundamental properties of bilayer graphene can be more accurately predicted.

 

 
This story is reprinted from material from Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

Plastic electronics made easy

(Nanowerk News)  Scientists have discovered a way to  better exploit a process that could revolutionise the way that electronic  products are made.

The scientists from Imperial College London say improving the  industrial process, which is called crystallisation, could revolutionise the way  we produce electronic products, leading to advances across a whole range of  fields; including reducing the cost and improving the design of plastic solar  cells.

The process of making many well-known products from plastics  involves controlling the way that microscopic crystals are formed within the  material. By controlling the way that these crystals are grown engineers can  determine the properties they want such as transparency and toughness.  Controlling the growth of these crystals involves engineers adding small amounts  of chemical additives to plastic formulations. This approach is used in making  food boxes and other transparent plastic containers, but up until now it has not  been used in the electronics industry.

The team from Imperial have now demonstrated that these  additives can also be used to improve how an advanced type of flexible circuitry  called plastic electronics is made.

The team found that when the additives were included in the  formulation of plastic electronic circuitry they could be printed more reliably  and over larger areas, which would reduce fabrication costs in the industry.

The team reported their findings this month in the journal  Nature Materials (“Microstructure formation in molecular and polymer  semiconductors assisted by nucleation agents”).

Dr Natalie Stingelin, the leader of the study from  the Department of Materials and Centre of Plastic Electronics at Imperial, says:

“Essentially, we have demonstrated a simple way to gain control  over how crystals grow in electrically conducting ‘plastic’ semiconductors. Not  only will this help industry fabricate plastic electronic devices like solar  cells and sensors more efficiently. I believe it will also help scientists  experimenting in other areas, such as protein crystallisation, an important part  of the drug development process.”

Dr Stingelin and research associate Neil Treat looked at two  additives, sold under the names IrgaclearÒ XT 386 and MilladÒ 3988, which are  commonly used in industry. These chemicals are, for example, some of the  ingredients used to improve the transparency of plastic drinking bottles. The  researchers experimented with adding tiny amounts of these chemicals to the  formulas of several different electrically conducting plastics, which are used  in technologies such as security key cards, solar cells and displays.

The researchers found the additives gave them precise control  over where crystals would form, meaning they could also control which parts of  the printed material would conduct electricity. In addition, the  crystallisations happened faster than normal. Usually plastic electronics are  exposed to high temperatures to speed up the crystallisation process, but this  can degrade the materials. This heat treatment treatment is no longer necessary  if the additives are used.

Another industrially important advantage of using small amounts  of the additives was that the crystallisation process happened more uniformly  throughout the plastics, giving a consistent distribution of crystals.  The team  say this could enable circuits in plastic electronics to be produced quickly and  easily with roll-to-roll printing procedures similar to those used in the  newspaper industry. This has been very challenging to achieve previously.

Dr Treat says: “Our work clearly shows that these additives are  really good at controlling how materials crystallise. We have shown that printed  electronics can be fabricated more reliably using this strategy. But what’s  particularly exciting about all this is that the additives showed fantastic  performance in many different types of conducting plastics. So I’m excited about  the possibilities that this strategy could have in a wide range of materials.”

Dr Stingelin and Dr Treat collaborated with scientists from the  University of California Santa Barbara, and the National Renewable Energy  Laboratory in Golden, US, and the Swiss Federal Institute of Technology on this  study. The team are planning to continue working together to see if subtle  chemical changes to the additives improve their effects – and design new  additives.

They will be working with the new Engineering and Physical  Sciences Research Council (EPSRC)-funded Centre for Innovative Manufacturing in  Large Area Electronics in order to drive the industrial exploitation of their  process. The £5.6 million of funding for this centre, to be led by researchers  from Cambridge University, was announced earlier this year. They are also  exploring collaborations with printing companies with a view to further  developing their circuit printing technique.

Controlling crystals

Here are some of the technologies that could benefit from Drs  Treat and Stingelin’s research:

Improving drugs

Most drugs work by blocking or activating proteins in our  bodies. To develop better drugs, scientists must understand what these proteins  look like. The work carried out by the Imperial team could enable researchers in  the future to develop more accurate models of proteins, by converting them into  a crystalline form.

More efficient solar technology

Solar cells are made from a solid mixture of electrically  conducting crystalline chemicals. Currently these cells only convert about 10%  of the Sun’s energy into electricity. Dr Treat and Stingelin’s additives may  provide a way of improving crystal growth in solar cells, which could improve  the amount of energy they convert.

New flexible electronics

Flexible semiconductor films can be made by methods such as  inkjet printing. Using additives that control how inkjet-printed droplets of  semiconductors crystallise will mean they crystallise in evenly distributed  patterns that conduct electricity efficiently. This means industry can produce  these printed electronics more easily and cheaply.

Source: By Joshua Howgego, Imperial College London

Read more: http://www.nanowerk.com/news2/newsid=31181.php#ixzz2YPLhjKF8

Solution coating the easy way

Researchers in the US and China have developed the first solution-coating technique capable of producing high-quality, large-area single-crystalline organic semiconductor thin films suitable for high-performance, low power and inexpensive printed electronic circuits. The technique, dubbed FLUENCE (fluid-enhanced crystal engineering) can be used to make thin film organic semiconductors with record charge carrier mobilities.

Fluid flow around micropillars

Solution coating of organic semiconductors is an excellent method for making large-area and flexible electronic materials. However, it is not at all good for making aligned single-crystalline thin films – the ideal form for organic semiconductors and that have the best electronic properties. Aligned crystals are preferred in these materials because charge carrier transport through these structures depends on the crystal orientation.

A team led by Zhenan Bao at Stanford University and Stefan Mannsfeld of the SLAC National Accelerator Laboratory is now reporting on a new solution-coating method that can produce high-quality, millimetre-wide and centimetre-long highly aligned single-crystalline organic semiconductor thin films. The essence of FLUENCE is that we are able to control the flow of liquid in which the organic semiconductor is dissolved, explains team member Ying Diao. During fast printing, this “ink” often distributes itself unevenly – something that leads to defects and other structural imperfections quickly appearing in the semiconducting crystals.

Single-crystal OFET

FLUENCE tackles this problem from two angles, she says. First, it works using a microstructured printing blade containing tiny pillars that mixes the ink uniformly. Second, specially designed chemical patterns on the substrate prevent the crystals from aligning randomly or “stochastically” in a direction that would be the opposite to that in which printing is taking place. These two methods combined lead to large-area highly aligned single crystalline films that are much more structurally perfect.

To prove that their technique works, the researchers fabricated an organic semiconductor made from TIPS-pentacene, a routinely used and much studied organic semiconductor material, and found a charge carrier mobility of as high as 11 cm² V⁻¹ s⁻¹. This is the first time a mobility of greater than 10 cm² V⁻¹ s⁻¹ has been reported for TIPS-pentacene.

Micropillar-patterned blade

“The concepts we have developed in FLUENCE could easily be scaled up and applied to commercial printing methods,” said Diao. “The significant improvement in structural quality and electrical performance of the thin films printed with our method could allow to make higher performance, lower power, small and inexpensive organic circuitry,” she told nanotechweb.org. “We hope that our work will help advance such a morphology-by-design approach to make organic semiconductors for high-performance, large-area printed electronics.”

The team, which includes researchers from Nanjing University in China, says that it will now look at pattering aligned crystals at length scales suitable for making sub-micron devices.

The present work is reported in Nature Materials.

About the author

Belle Dumé is contributing editor at nanotechweb.org.

Quantum Dots that Assemble Themselves

A paper on the new technology, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics,” appears in the current issue of the scientific journal Nature Materials. Quantum dots are tiny crystals of semiconductor a few billionths of a meter in diameter. At that size they exhibit beneficial behaviors of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The scientists demonstrated how quantum dots can self-assemble at the apex of the gallium arsenide/aluminum gallium arsenide core/shell nanowire interface. Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s center. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the gallium-arsenide core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometers from the surface – a feature that even surprised the scientists. “Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it,” said NREL senior scientist Jun-Wei Luo, one of the co-authors of the study. Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium Arsenide core, the thin aluminum-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminum-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanowires raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

More information: http://www.nature.com/nmat/journal/vaop/ncurrent/fig_tab/nmat3557_F4.htmlJournal reference: Nature Materials Provided by National Renewable Energy Laboratory

Read more at: http://phys.org/news/2013-04-team-quantum-dots.html#jCp

A Magic Formula to Predict Fracture in Steel

22.11.12 – EPFL researchers have elucidated a century-old mystery: how hydrogen destroys steels. A new mathematical model predicts this failure in the presence of the destructive atoms.

A veritable gangrene for steels and other structural metals, hydrogen is one of the most important causes of ruptures in industrial parts, such as pipelines. At the slightest defect in a material, these atoms introduce themselves in the crack and weaken the structure dramatically, making it brittle. The material need only be in contact with aggressive substances or placed in an aqueous environment from which for the dangerous hydrogen atoms enter the material. This phenomenon of “hydrogen embrittlement” has been known for many years, but so far no one managed to capture the physical process or predict when hydrogen embrittlement will occur. Bill Curtin of the Laboratory of Multiscale Mechanical Modeling at EPFL and his collaborator Prof. Jun Song at McGill, tackled this problem and developed a mathematical model to understand the behavior of hydrogen atoms in iron-based steels and thus to predict steel fracture. This is revolutionary in the world of materials, and serves as the subject of an article in the journal Nature Materials.

Hydrogen Attracted by Fractures To establish their equation, the researchers studied the behavior of iron at the atomic level. They showed that the reason hydrogen weakens the materials comes from the tendency of hydrogen atoms to cluster at the tip of a crack. “In the absence of hydrogen, dislocation defects form around a crack, allowing it to relax the stress in the material and preventing the crack from growing, making the material more resilient or tougher, explained Bill Curtin. By grouping around the crack, the hydrogen atoms prevent the creation of these dislocations, and prevent the stress relaxation, allowing the crack to grow and the material becomes extremely brittle.”

A mathematical model that predicts the fracture Using their simulations, the scientists were able to establish a complex mathematical model that calculates when a material in contact with hydrogen will start to break. Several factors are taken into account, such as the concentration of hydrogen in the environment, the speed at which the hydrogen molecules move toward the crack, type of steel, and the load on the structure. If a combination of these parameters attains a critical value, computed from the simulations, then the material will break. Using the model, they predicted the breaking point for a various steels under various conditions. “Our predictions coincided with the experiments in 9 out of 10 cases, rejoiced Bill Curtin. And the 10th case was right on the border”.

This knowledge should allow scientists to tackle the problem armed with new weapons. It will become easier to identify adverse operating modes and to construct materials that are more resistant to this type of deterioration.

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How does hydrogen come into contact with a material?

•When welding in damp conditions (presence of H2O). •When steels are used in the presence of hydrogen or hydrogenated gas mixtures (hydrocarbons in pipelines, for example) •Hydrogen can originate from corrosion in an aqueous environment, for example.

Additional information: Atomic mechanism and prediction of hydrogen embrittlement in iron