Lithium News

A research team has conducted a systematic analysis of thermal safety in lithium-sulfur pouch cells. The team focused their study on the exothermic reactions which leads to the temperature rise of lithium-sulfur batteries after several charge-discharge cycles. Their work contributes to the understanding on thermal behaviors of lithium-sulfur batteries and offers a path toward safer, practical applications for these batteries.

The team published their findings in the journal Particuology on December 8, 2022.

Lithium metal batteries, especially lithium–sulfur batteries, have attracted significant attention because they have high theoretically specific energy density, are cost effective, and environmentally friendly. Because of their high energy density, these batteries hold the potential for use in electric vehicles and high-end portable devices.

However, there are many challenges to be addressed before practical applications of lithium-sulfur batteries can be achieved. These challenges include the “shuttle effect” of the intermediate lithium polysulfides between anodes and cathodes, the intrinsic insulation property of sulfur, and dendrite issues of lithium anode during cycles.

More importantly, the thermal safety issue that occurs because of the highly active lithium metal is an essential bottleneck for the practical applications of lithium-sulfur batteries. “Therefore, it is of great importance to investigate the key exothermic reactions which lead to the dramatic and abnormal temperature rise of lithium-sulfur batteries after several charge-discharge cycles,” said Xin-Bing Cheng, a professor at Southeast University.

So the research team set out to investigate the thermal safety of deeply cycled lithium-sulfur pouch cells. They conducted a comprehensive analysis of the thermal runaway mechanisms of lithium-sulfur batteries. Thermal runaway describes the process where the battery overheats. They found that the reactions between dissolved higher-order polysulfides and lithium metal were the origins for thermal runaway in the lithium-sulfur pouch cells.

A 16-cycle pouch cell shows high safety, heating from 30 to 300°C without a dramatic temperature increase during the accelerating rate calorimeter test. By comparison, the team discovered that when 16-cycle pouch cell with additional electrolyte is heated to 147.9°C, violent exothermic reactions and the generation of tremendous heat occur.

The various thermal behaviors of these batteries are attributed to the different viscosity of electrolytes. The high viscosity of the electrolyte leads to the poor solvent’s diffusion at the interface of the electrodes and electrolytes. This poor solvent’s diffusion contributes to improved thermal stability between battery components at high temperature.

In contrast, the thermal behaviors for 45-cycle pouch cells with additional electrolyte is consistent with that for cells without additional electrolyte.

In their study of the 45-cycle pouch cell with an additional electrolyte, the team observed that during the accelerating rate calorimeter test, the highest temperature rate for the lithium-sulfur pouch cell after 45 cycles is just 1.72°C min−1 and its highest temperature is only 304.0°C, while the numbers are 162.4°C s−1 and 436°C respectively for the 16-cycle lithium-sulfur cells with an additional electrolyte. The various thermal behaviors of batteries are attributed to the different polysulfides species in cycled electrolytes.

“We discovered that the strong exothermic reactions between dendritic lithium and dissolved higher-order polysulfide rather than lower-order polysulfide drive the dramatic and abnormal temperature rise of deeply cycled lithium-sulfur pouch cells. Therefore, from a safety viewpoint, it is essential to inhibit the polysulfide shuttle for lithium-sulfur batteries,” said Cheng.

Through their study, the team systematically revealed the thermal runaway mechanisms of cycled lithium-sulfur pouch cells. “It is important to develop effective strategies to inhibit the undesirable polysulfide shuttle and dendrite growth in Li-S batteries. Once the polysulfide shuttle and dendrite growth in lithium-sulfur batteries is suppressed completely, the thermal safety of lithium-sulfur batteries can be greatly improved,” said Cheng.

Mining companies and governments in Africa are calling for stronger trade ties with the United States after a new climate law set out incentives for US carmakers sourcing battery materials from trade partners.

The $430 billion Inflation Reduction Act (IRA) has been criticised by the European Union and South Korea, who say it could hurt their car industries.

It could also negatively impact African nations that produce battery materials.

The United States has a Free Trade Agreement in place with only one African country, Morocco. Yet the continent is a key copper producer and Democratic Republic of Congo produces most of the world’s cobalt.

Battery materials and trade are set to be a focus at next week’s US-Africa Leaders’ Summit in Washington where President Joe Biden will meet presidents of African countries including Congo.

“The IRA was intended to push out China, and what it’s ended up doing is pushing out the DRC, and the EU, and South Korea,” said Indigo Ellis, managing director at consultancy Africa Matters Limited, who will attend the Dec. 13-15 summit.

Under IRA, US carmakers will get tax credits if they source at least 40% of battery materials domestically or from American free-trade partners. This risks carmakers replacing Congolese cobalt with Australian, Canadian, Moroccan, or US cobalt.

Congo produced 74% of the world’s mined cobalt last year while the next-biggest single producer, Australia, was responsible for just 3%, according to a Cobalt Institute report.

An adviser to Congo’s President Felix Tshisekedi said a USA-DRC Free Trade Agreement “is an option for the medium to long-term, but in the short term other avenues will be explored”.

A spokesperson for the US Trade Representative (USTR) said “we look forward to discussing ways to strengthen and deepen our trade and investment ties with our partners throughout Africa” during the summit.

The IRA aims to boost US mining and processing, which some companies fear could come at the expense of value-added processing in Africa.

“The West needs to work with us to build some value-add,” said George Roach, CEO of Premier African Minerals, which has a lithium project in Zimbabwe.

His is one of many projects across sub-Saharan Africa aiming to produce battery materials like lithium, nickel and graphite.

Joe Walsh, managing director at Australia-listed Lepidico, which is building a lithium mine in Namibia and chemical plant in Abu Dhabi, said the IRA makes the United States a more attractive location for a planned second plant.

“The US is not going to be able to incentivize the development of a significant battery raw material production base of its own without ruffling a few feathers along the way.”

Water and wood may one day be all that’s needed to provide electrical power for a household. At a time when energy is a critical issue for many millions of people worldwide, scientists in Sweden have managed to generate electricity with the help of these two renewable resources.

The method reported by researchers at KTH Royal Institute of Technology focuses on what naturally happens after wood is placed in water, and the water evaporates. Transpiration, a process in which water moves through a plant, is constantly occurring in nature. And it produces small amounts of electricity, known as bioelectricity.

Yuanyuan Li, assistant professor at the Division of Biocomposites at KTH, says that with some nanoengineering of wood—and pH tuning—small but promising amounts of electricity can now be harvested.

“At the moment we can run small devices such as an LED lamp or a calculator,” Li says. “If we wanted to power a laptop, we would need about one square meter of wood about one centimeter thick, and about 2 liters of water.”

“For a normal household we’d need far more material and water than that, so more research is needed.”

By altering the nanoscale composition of wood, the researchers improved its properties in terms of surface area, porosity (or density), surface charge, how readily water can pass through the material and the water solution itself—all of which are factors that influence electricity generation in wood.

“We compared the porous structure in regular wood with the material we improved with regard to surface, porosity, surface charge and water transportation. Our measurements showed electricity generation that’s 10 times higher than with natural wood,” Li says.

She says that further tuning the pH difference between wood and water, due to an ion concentration gradient, achieves a potential of up to 1 volt and a remarkable power output of 1.35 microwatts per square centimeter.

Li says that to date, the wood manages to deliver high voltage for about 2-3 hours, before it begins to wane. So far the wood has managed 10 cycles with water, without a decline in the material’s performance, she says.

“The great advantage of this technology is that the wood can readily be used for other purposes once it’s depleted as an energy source, such as transparent paper, wood-based foam and different biocomposites.”

The findings are published in the journal Advanced Functional Materials.

Researchers at Oxford University and Exciton Science have demonstrated a new way to create stable perovskite solar cells, with fewer defects and the potential to finally rival silicon’s durability.

By removing the solvent dimethyl-sulfoxide and introducing dimethylammonium chloride as a crystallization agent, the researchers were able to better control the intermediate phases of the perovskite crystallization process, leading to thin films of greater quality, with reduced defects and enhanced stability.

Large groups of up to 138 sample devices were then subjected to a rigorous accelerated aging and testing process at high temperatures and in real-world conditions.

Formamidinium-cesium perovskite solar cells created using the new synthesis process significantly outperformed the control group and demonstrated resistance to thermal, humidity and light degradation.

This is a strong step forward to matching commercial silicon’s stability and makes perovskite-silicon tandem devices a much more realistic candidate for becoming the dominant next-generation solar cell.

Led by Professor Henry Snaith (Oxford University) and Professor Udo Bach (Monash University), the work has been published in the journal Nature Materials.

Oxford University Ph.D. student Philippe Holzhey, a Marie Curie Early Stage Researcher and joint first author on the work, said, “It’s really important that people start shifting to realize there is no value in performance if it’s not a stable performance.

“If the device lasts for a day or a week or something, there’s not so much value in it. It has to last for years.”

During testing, the best device operated above the T80 threshold for over 1,400 hours under simulated sunlight at 65°C. T80 is the time it takes for a solar cell to reduce to 80% of its initial efficiency, a common benchmark within the research field.

Beyond 1,600 hours, the control device fabricated using the conventional dimethyl-sulfoxide approach stopped functioning, while devices fabricated with the new, improved design retained 70% of their original efficiency, under accelerated aging conditions.

The same degradation study was performed on a group of devices at the very high temperature of 85°C, with the new cells again outperforming the control group.

Extrapolating from the data, the researchers calculated that the new cells age by a factor of 1.7 for each 10°C increase in the temperature they are exposed to, which is close to the 2-fold increase expected of commercial silicon devices.

Dr. David McMeekin, the corresponding and joint first author on the paper, was an Australian Center for Advanced Photovoltaics (ACAP) Postdoctoral Fellow at Monash University and is now a Marie Skłodowska-Curie Postdoctoral Fellow at Oxford University.

He said, “I think what separates us from other studies is that we’ve done a lot of accelerated aging. We’ve aged the cells at 65°C and 85°C under the whole light spectrum.”

The number of devices used in the study is also significant, with many other perovskite research projects limited to just one or two prototypes.

“Most studies only show one curve without any standard deviation or any kind of statistical approach to determine if this design is more stable than the other,” David added.

The researchers hope their work will encourage a greater focus on the intermediate phase of perovskite crystallization as an important factor in achieving greater stability and commercial viability.

Background: About perovskites

Artificially synthesized in laboratory conditions, semiconductor thin films made up of perovskite compounds are far cheaper to make than silicon solar cells, with greater flexibility and a tunable band gap.

They emerged unexpectedly in the last decade and have reached impressive power-conversion efficiencies of over 25%.

However, too much focus has been placed on creating the most efficient perovskite solar cell, rather than resolving the fundamental problems inhibiting the material from being used in widespread commercial applications.

Compared to silicon, perovskites can degrade rapidly in real world conditions, with exposure to heat and moisture causing damage and negatively impacting device performance.

Solving these stability issues is the key challenge for perovskites in their quest to take on, or “boost” silicon via a tandem architecture and take their place in the commercial photovoltaics landscape.