Are batteries safe?
The importance of further improvements to turn Nobel Prize-worthy research into commercially viable products became evident approximately 10 years ago. In the early 2010s, pictures of lithium-ion batteries in smartphones and tablets that had heated up to the point of bursting into flames circulated on social media. When a Tesla S burned out after an accident in October 2013, some US$3 billion of market capital were annihilated for the U.S. car manufacturer within a short time in a dramatic drop of the company’s stock price.
The topic had drawn global attention in January of the same year, when newly developed lithium-ion batteries, designed for Boeing 787 Dreamliner aircraft, caught fire in two airplanes within a few days of one another. The U.S. Federal Aviation Administration (FAA) followed up by demanding proof of safe battery operation. Authorities in other parts of the world followed suit, leading to a virtual global flying ban for Dreamliners. Boeing had to equip its planes with a new battery system before they were allowed to take off again in April.
“That was a critical moment for lithium-ion battery technology. It ran the risk of being rejected as unsafe by consumers,” says Guido Skudlarek, the head of Evonik’s special oxides business. The events caused major battery manufacturers to move further safety improvements to the top of their priority list worldwide.
This effort quickly led to the question of improving the separator, a thin, micro-porous film that electrically insulates the two electrodes from each other, but allows the ions to flow between the cathode and the anode. In lithium-ion batteries, separators typically are made of thermoplastic materials such as polyethylene and/or polypropylene. “These materials only have limited heat resistance and shrink under thermal influence,” Skudlarek explains. This can lead to short-circuits and thermal runaways of batteries when the electrodes come in direct contact.
Contribution of additives
Ceramic coatings with fumed metal oxides such as AEROXIDE® prevent exactly those scenarios by improving the thermal and mechanical properties of the separator. In an event of a thermal runaway, at first the micro-pores in the polymeric membrane close and interrupt the flow of lithium ions. The AEROXIDE® coating prevents the shrinkage of the separator film and thus the direct short circuit of the battery. The energy storage system simply stops working and slowly cools down instead of spectacularly bursting into flames. There are also considerations to no longer coat the separator, but to introduce the metal oxide particles directly into the separator during the manufacturing process, so that they are later distributed homogenously throughout the interior of the separator . “Such a procedure would be especially resource-efficient because the coating process is eliminated,” says Skudlarek. At the same time, the battery design would be even thinner and take up less space – an important gain for mobile applications.
Optimizations have also involved the electrodes, not just the separator. By now, petroleum coke is no longer used at the anode and has been replaced with a mixture of graphite and silicon. Likewise, cathodes are no longer manufactured from pure cobalt oxide, but now consist of mixed oxides, which contain manganese and nickel in addition to cobalt. In the latest generation of such NMC(Nickel-Manganese-Cobalt) cathode active materials, the cobalt share in the cathode is just ten percent. “The trend is currently towards even higher nickel content to further increase the energy density. On the downside, that makes material less structurally stable and the surface even more reactive.” Skudlarek explains. AEROXIDE® additives from Evonik can also help in that case: When the NMC particles are coated with a thin layer of nano-structured aluminum oxide (Al2O3) and/or titanium dioxide (TiO2), this oxide layer partially reacts with the electrolyte to form a protective shell, a so-called defined solid electrolyte interface (SEI). Such an artificial layer protects the cathode material from decomposition while allowing Li-ions to pass. This keeps the battery at a higher capacity throughout its lifecycle.
Improvements in the area of electrolytes also offer greater safety. The most common solution currently in use involves liquid electrolytes, for example, lithium salts dissolved in aprotic solvents such as the toxic lithium hexafluorophosphate (LiPF6). Understandably, battery manufacturers are looking for ways to prevent electrolyte leakage in case of damaged battery cells. This can be achieved, for example, with a special surface-modified form of AEROXIDE®. The additive is applied to the separator as a ceramic coating as before. However, thanks to its special properties, it reacts with a component of the electrolyte solution. Based on cross-linking reaction with this substance, the reactive liquid turns into a gel-like electrolyte based on a polymer composite, further increasing the safety of the lithium-ion battery.
The technology for lithium-ion batteries has made huge advances since the first steps in the 1970s. It appears so advanced now that some professionals only see limited possibilities for further improvement. Nevertheless, it will be a few more years for the next generation of small, powerful storage devices such as solid-state batteries to emerge.
Lithium-ion batteries make a crucial contribution to the use of cleaner energy technologies and electromobility, helping to limit greenhouse gas emissions. As they have become mainstream, in the best sense of the word, they are found not only in submarines and satellites, but also in electric toothbrushes, pacemakers and home appliances. “In my opinion, that makes the well-deserved award of the Nobel Prize even more special,” says Evonik manager Skudlarek. “The prize in the past has frequently recognized basic principles with a hidden connection to products or processes, or referred to very specific developments that we occasionally encounter in our lives. This is very different.”
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