Researchers improve power storage capabilities

Battery update: Several universities have been working on advances to improve battery technologies and make them safer and more reliable for industrial and manufacturing applications. Click on links for more information.

By Chris Vavra June 11, 2019

Batteries have become more energy efficient over the years, but researchers continue to look for ways to improve on the original design. Batteries remain the power source for many everyday devices whether they’re rechargeable or implanted in a machine. Electric cars and robots, in particular, are emerging technologies that rely on batteries to function. Improving on battery designs to extend battery life remains a top priority, and several universities are leading the charge to improve the technology, which, at its core, hasn’t changed much (see links highlighted for more information about their research).

Solid-state battery improvements

Cornell University researchers have advanced the design of solid-state batteries by starting with liquid electrolytes and then transforming them into solid polymers inside the electrochemical cell. The researchers take advantage of both liquid and solid properties to overcome key limitations in current battery designs. This research coincides with industries looking for rechargeable battery technology that can safely power next-generation technology such as electric cars, autonomous vehicles and robots.

Qing Zhao, a Cornell University postdoctoral researcher, said in a press release, “Imagine a glass full of ice cubes: some of the ice will contact the glass, but there are gaps. But if you fill the glass with water and freeze it, the interfaces will be fully coated, and you establish a strong connection between the solid surface of the glass and its liquid contents.” She added, “This same general concept in a battery facilitates high rates of ion transfer across the solid surfaces of a battery electrode to an electrolyte without needing a combustible liquid to operate.”

The process introduces special molecules capable of initiating polymerization inside the electrochemical cell, without compromising other cell functions. If the electrolyte is a cyclic ether, the initiator can be designed to rip open the ring, which produces reactive monomer strands that bond together to create long chain-like molecules with essentially the same chemistry as the ether. This now-solid polymer retains the tight connections at the metal interfaces, much like the ice inside a glass.

Solid-state electrolytes, apart from improving battery safety, also are beneficial for enabling next-generation batteries that use metals, including lithium and aluminum, as anodes for achieving far more energy storage than today’s state-of-the-art battery technology. In this context, the solid-state electrolyte prevents the metal from forming dendrites, a phenomenon that can short circuit a battery and lead to overheating and failure.
Solid-state batteries, in spite of potential advantages, have not been produced at a large scale. Reasons include high manufacturing costs and the previous designs’ poor interfacial properties. A solid-state system also circumvents the need for battery cooling by providing stability to thermal changes.

“Our findings open an entirely new pathway to create practical solid-state batteries that can be used in a range of applications,” said senior author Lynden Archer, also in a Cornell press release.

According to Archer, the strategy for creating solid polymer electrolytes is exciting because it shows promise for extending cycle life and recharging capabilities of high-energy-density rechargeable metal batteries.

Hybrid cathode in lithium batteries

Researchers at MIT and in China have developed a new version of a key component for lithium batteries: the cathode. This development is part of wide-ranging research to develop batteries that are smaller, lighter and run for longer periods of time.

The research team describes their concept as a “hybrid” cathode because it combines aspects of two prior approaches, one to increase the energy output per pound (gravimetric energy density), the other for the energy per liter (volumetric energy density). The synergistic combination, they say, produces a version that provides the benefits of both, and more.

Today’s lithium-ion batteries tend to use cathodes (one of the two electrodes in a battery) made of a transition metal oxide, but batteries with cathodes made of sulfur are considered a promising alternative to reduce weight. Today, the designers of lithium-sulfur batteries face a tradeoff.

The cathodes of such batteries usually are made in one of two ways, known as intercalation types or conversion types. Intercalation types, which use compounds such as lithium cobalt oxide, provide a high volumetric energy density—packing a lot of punch per volume because of their high densities. These cathodes can maintain their structure and dimensions while incorporating lithium atoms into their crystalline structure.

The other cathode approach, called the conversion type, uses sulfur that gets transformed structurally and even is dissolved temporarily in the electrolyte.

have very good gravimetric energy density,” said Ju Li, MIT professor of nuclear science and engineering and of materials science and engineering, “but the volumetric density is low.” This is partly because they tend to require a lot of extra materials including an excess of electrolyte and carbon, which is used to provide conductivity.

In the hybrid system, the researchers have managed to combine the two approaches into a new cathode that incorporates both a type of molybdenum sulfide called Chevrel phase, and pure sulfur, which together appear to provide the best aspects of both. They used particles of the two materials and compressed them to make the solid cathode.

“It is like the primer and TNT in an explosive, one fast-acting, and one with higher energy per weight,” said Li.

Among other advantages, the electrical conductivity of the combined material is relatively high, thus reducing the need for carbon and lowering the overall volume, Li said. Typical sulfur cathodes are made up of 20% to 30% carbon, he said, but the new version needs only 10% carbon.

The net effect of using the new material is substantial. Today’s commercial lithium-ion batteries can have energy densities of about 250 watt-hours per kilogram and 700 watt-hours per liter, whereas lithium-sulfur batteries top out at about 400 watt-hours per kilogram but only 400 watt-hours per liter. The new initial version, which has not yet gone through an optimization process, already can reach more than 360 watt-hours per kilogram and 581 watt-hours per liter, Li said. It can beat both lithium-ion and lithium-sulfur batteries in energy densities.

With further work, Li said, “We think we can get to 400 watt-hours per kilogram and 700 watt-hours per liter,” with that latter figure equaling that of lithium-ion. Already, the team has gone a step further than many laboratory experiments aimed at developing a large-scale battery prototype: instead of testing small coin cells with capacities of only several milliamp-hours, they have produced a three-layer pouch cell (a standard subunit in batteries for products such as electric vehicles) with a capacity of more than 1,000 milliamp-hours. This is comparable to some commercial batteries, indicating that the new device does match its predicted characteristics.

So far, the new cell can’t quite live up to the longevity of lithium-ion batteries in the number of charge-discharge cycles before losing too much power to be useful. In this case, Li stated that the limit is based on cell design rather than cathode, and that, “we’re working on that.” Even in its present early form, “This may be useful for some niche applications, like a drone with long range,” where both weight and volume matter more than longevity.

Predict battery life with AI, machine learning

If cell phone battery manufacturers could tell which cells will last at least two years, then they could sell only those to phone makers and send the rest to makers of less demanding devices. New research shows how manufacturers could do this. The technique could be used to sort manufactured cells and help new battery designs reach the market more quickly.

Combining comprehensive experimental data and artificial intelligence revealed the key for accurately predicting the useful life of lithium-ion batteries before their capacities start to wane, scientists at Stanford University, the Massachusetts Institute of Technology (MIT) and the Toyota Research Institute discovered.
After the researchers trained their machine learning model with a few hundred million data points of batteries charging and discharging, the algorithm predicted how many more cycles each battery would last, based on voltage declines and a few other factors among the early cycles.

The predictions were within 9% of the number of cycles the cells actually lasted. Separately, the algorithm categorized batteries as either long or short life expectancy based on the first five charge/discharge cycles. Here, the predictions were correct 95% of the time.

This machine learning method could accelerate research and development of new battery designs and reduce the time and cost of production, among other applications.

“The standard way to test new battery designs is to charge and discharge the cells until they fail. Since batteries have a long lifetime, this process can take many months and even years,” said Peter Attia, Stanford doctoral candidate in materials science and engineering. “It’s an expensive bottleneck in battery research.”

The method has many potential applications, Attia said. For example, it can shorten the time for validating new types of batteries, which is especially important given rapid advances in materials. With the sorting technique, batteries designed for electric vehicles but determined to have shorter lifespans could be used instead to power streetlights or back-up data centers. Recyclers could find cells from used EV battery packs with enough capacity left for a second life.

Yet another possibility is optimizing battery manufacturing. “The last step in manufacturing batteries is called ‘formation,’ which can take days to weeks,” Attia said. “Using our approach could shorten that significantly and lower the production cost.”

The researchers now are using their model to optimize ways of charging batteries in just 10 minutes, which they say will cut the process by more than a factor of 10.

Rechargeable lithium batteries

Researchers at Penn State University are developing rechargeable lithium metal batteries with increased energy density, performance and safety by using a solid-electrolyte interphase (SEI). The SEI’s stability has been a critical issue as demand for higher density lithium metal batteries increases. Advancement has been halted because a salt layer on the surface of the battery’s lithium electrode insulates it and conducts lithium ions, the researchers said.

“This layer is very important and is naturally formed by the reaction between the lithium and the electrolyte in the battery,” said Donghai Wang, professor of mechanical and chemical engineering. “But it doesn’t behave very well, which causes a lot of problems.”

One of the least understood components of lithium metal batteries, the degradation of the SEI, contributes to the development of dendrites, which negatively affect performance and safety.

“This is why lithium metal batteries don’t last longer, the interphase grows, and it’s not stable,” Wang said. “In this project, we used a polymer composite to create a much better SEI.”

Led by chemistry doctoral student Yue Gao, the enhanced SEI is a reactive polymer composite consisting of polymeric lithium salt, lithium fluoride nanoparticles, and graphene oxide sheets.

Using chemistry and engineering design, the collaboration between fields enabled the technology to control the lithium surface at the atomic scale. The reactive polymer also decreases the weight and manufacturing cost, further enhancing the future of lithium metal batteries.

“With a more stable SEI, it’s possible to double the energy density of current batteries, while making them last longer and be safer,” Wang said.

Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.