Rechargeable lithium-ion batteries are an extraordinary technology and they are becoming increasingly important in the transition from fossil fuel burning to renewable energy. But if this shift is to be fast enough to save the planet, the batteries have to be better and they too have to be sourced and manufactured sustainably.
Batteries are a pioneer of a sustainable society
Chris Stumpf, Waters Corporation
The demand for batteries with high energy density is unstoppable. According to a recent article in nature, the number of electric vehicles in use worldwide is estimated to increase by a factor of 72 and reach almost 1 billion vehicles between 2020 and 20501. And all of these vehicles will need batteries. “Batteries make a sustainable society possible,” said Chris Stumpf, senior manager of the materials science division at Waters Corporation. Not to forget that lithium-ion batteries also provide power to our mobile devices and computers.
The lithium-ion battery is so transformative, so important in our society, that the scientists who pioneered the technology, John B. Goodenough, M. Stanley Whittingham and Akira Yoshino, were jointly awarded the Nobel Prize in Chemistry in 2019 for Scientific Brilliance As an ubiquitous energy-giving technology, it will be necessary to open the lid of these batteries and make the next generation more environmentally friendly and efficient than their predecessors. A look into the battery is only possible with the most modern characterization instruments. The techniques range from mass spectrometry and thermal analysis to electron microscopy.
At the most basic level, batteries are made up of two electrodes – the negatively charged anode and the positively charged cathode. Then there is the electrolyte through which the critical ions flow. As the ions flow, electrons go through an external circuit and generate electricity. A lithium-ion battery also has a separator that lets ions pass through but keeps the electrodes far apart.
Cobalt is a problematic element. His chemistry is great, his origins less so
One starting point for improving battery performance and sustainability is the cathode. In the lithium-ion battery version developed by Nobel Prize winner John Goodenough, the electrodes are made of lithium cobalt oxide. And most commercial batteries use some form of cobalt-containing cathode. But cobalt is a problematic element. His chemistry is great, his origins less so. According to the United States Geological Survey2, Cobalt is a relatively scarce element and reserves exist mainly in the Democratic Republic of the Congo (DRC). Mining here is extremely problematic, with allegations of human rights violations, including child labor and dangerous working conditions.
For these reasons, researchers are working hard to create alternative cathode materials that outperform traditional lithium cobalt oxide cathodes in terms of performance and how much or little cobalt they contain. Detailed characterization is essential to understand the performance of these new and innovative materials.
The research efforts go far beyond optimizing the composition of current cathodes and include the formulation of new high-performance materials. Using the Waters Xevo G2-XS QTof mass spectrometer, Yongzhu Fu and colleagues from Zhengzhou University in China demonstrated earlier this year that a promising new class of compounds, organosulfides, could be successful cathode materials in rechargeable batteries3rd.
While organosulfides are a tempting alternative as a cathode, they have some serious limitations, including poor conductivity and solubility in the electrolyte, to name a few. Fu’s team overcame this with tin or copper benzene-1,2-dithiolato complexes. They found that making these complexes with metals gives the organosulfides a different dimension – makes them less soluble and, thanks to the metal centers, can pack energy into them. This system was only properly understood through detailed characterization. The performance of the QTof mass spectrometer can be further increased when it is combined with a chromatography system that enables unique insights into the electrolyte, says Stumpf.
Waters has a number of techniques that together can follow the electrolyte and any additives used in it as they go through the charge and discharge cycles in a working system. It’s a powerful combination recently tested with an electrolyte solution of dimethyl carbonate with diethyl carbonate, ethyl methyl carbonate, fluoroethyl carbonate, lithium hexafluorophosphate and proprietary additives4th. When charging and discharging, these electrolytes contain volatile and non-volatile components.
we can provide niche insights into the electrolyte
It is not easy to analyze these clearly different phases in one fell swoop. A gentle atmospheric pressure gas chromatograph (APGC) delivered ions from the electrolyte to the Xevo G2-XS QTof mass spectrometer, which was designed to take the most complex samples with high resolution. Over a series of 200 charge / discharge cycles, this kit was able to see the electrolyte degrade by precisely determining the formation of certain breakdown products. In conjunction with powerful analysis software and the detailed chemistry of the electrolyte, how it worked could be followed in a single device setup. The potential of understanding how electrolytes work and testing new systems like this holds great promise for making more efficient and better understood batteries. “We can provide niche insights into the electrolyte,” says Stumpf.
In order to get the maximum energy out of a lithium-ion battery, lithium metal would theoretically be the ideal material. However, lithium is so reactive that the practicality of using solid lithium in most commercial rechargeable batteries has been overlooked in favor of graphite. Lithium metal can form dendrites – tiny lithium fingers that grow into the battery electrolyte – that can cause out of control reactions and fires. But by calming down with a passivation layer on the lithium metal, these anodes look more promising. But this passivation layer also poses a characterization problem.
Scanning electron microscopy of the anode surface is the best way to understand what is happening, but preparing a lithium-based sample in-situ is a major challenge. Late last year, however, a team led by Karim Zaghlib of McGill University in Montreal, Canada, which included 2019 Nobel Prize winner John Goodenough, of the University of Texas at Austin, took a major step forward5. Using a SEM equipped with EDS and EBSD detectors from Oxford Instruments (OI), the team was able to analyze a lithium-magnesium alloy at high temperatures in a next-generation battery technology – a solid-state battery. This alloy forms a passivation layer and with the help of OI’s in-situ detector, the team was able to track the alloy as a passivation layer. OI’s Ultim Extreme detector, which can detect low energy X-rays, won the Queen’s Award for Business in 20196th.
As the temperature was raised, they could see that the passivation layer remained unchanged even if the alloy it was protecting melted. This critical insight into material behavior will help make lithium metal-based anodes safer that avoid runaway reactions and short circuits.
Atomic force microscopy is another powerful tool. And OI’s Cypher Electrochemical Cell AFM was used by a team in Germany to study different formulations of a polymer gel electrolyte as they work to see what happens to the passivation layer as it forms. They were able to trace the mechanical stability, thickness, and morphology of this layer in each different formulation to see which would work best in a working battery. The entire AFM can be housed in a glove box, explains OI’s Christan Lang, which allows for a number of interesting shots while a battery is in use, he says.
Knowing how a battery can change over the course of its useful life will be key to figuring out how best to reuse it
The characterization is not limited to mass spectrometry and microscopy. NMR spectroscopy can help detect impurities in electrolytes or track reaction products to see how battery components change over their lifespan and is a growing field in battery research. The benchtop NMR spectrometers and analyzers from OI are already in use in research groups.
Characterization technologies can provide an analytical perspective on research and development, manufacturing, and recycling. This last point plays a big role in battery development. Recycling plants are big, dirty places. Having a state-of-the-art mass spectrometer for testing old batteries doesn’t make sense. “Recycling has to be initiated earlier,” says Stumpf. All of the analytical techniques used to aid in the design of better battery components will be critical to future efforts to ensure that batteries are recycled efficiently. Knowing how a battery can change over the course of its useful life will be key to figuring out how best to reuse it.
In order to achieve the sustainable society our planet needs, batteries will undoubtedly be a driving force. But behind the scenes, the often-forgotten analytical tools work quietly and heroically, providing the critical data needed to make these batteries function better, made from more sustainable materials, and recycled or reused to the best of our ability.