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Lithium-ion batteries have quickly taken over the market due to their high energy density. But due to the surge in demand for car battery production, the price of lithium is very high today: around 45,000 euros per tonne (or about 360% more expensive than a year ago). In this regard, a group from the Massachusetts Institute of Technology has developed a battery from common and inexpensive materials: aluminum, sulfur and salt. This new battery is not only cheaper to produce, but also very durable.
Another major disadvantage of lithium-ion batteries is that they contain a highly flammable electrolyte, so they can catch fire or explode if damaged or misused. However, demand will not decrease in the near future: the massive deployment of solar panels and wind turbines must be accompanied by backup systems to meet the needs in adverse climatic conditions. As the world attempts to reduce CO2 emissions by increasing the use of renewable energy and electric vehicles, it is important to find safer and less expensive storage alternatives.
The battery developed by Professor Donald Sadoway and his colleagues is ideal for this need. It uses aluminum and sulfur as electrode materials separated by molten salt electrolyte. Aluminum has quickly become the material of choice: it is the most abundant metal on Earth and also the second metal on the market (after iron). Sulfur is the cheapest of all non-metals; occurs in nature in large quantities in the form of sulfides and sulfates, and is also a waste from oil refining.
Particularly fast charging speeds
The use of volatile and flammable organic liquids as an electrolyte was excluded. So the researchers tested various molten salts that had a relatively low melting point — the goal was to overcome the insulating and anti-corrosion measures, Sadoway explains in a press release. Their choice fell on molten salt, consisting of NaCl-KCl-AlCl3. The team explains that molten chloraluminates contain AlnCl3(n+1) compounds in chain form, whose Al-Cl-Al bonds provide easy Al3+ ion desolvation kinetics, resulting in high Faraday exchange current rates, forming the basis for high-speed battery charging.
“We show that a multi-step aluminum-to-chalcogen pathway enables fast charging up to 200°C, and that the battery can withstand hundreds of cycles at very high charge rates,” the researchers report in the journal Nature. From their experiments, they found that the charging rate strongly depends on the operating temperature: thus, at 110°C, the rate was 25 times higher than at 25°C. But the team clarifies that the battery doesn’t need an external heat source to reach that temperature: the heat generated naturally during the charge and discharge cycle is sufficient.
In addition, the chloraluminate salt chosen as the electrolyte had the unexpected benefit of preventing the formation of aluminum dendrites, narrow spikes of metal that build up over time on one electrode and then grow until they come into contact with another electrode, which causes short circuits. closures. scheme. This phenomenon greatly affects the efficiency of lithium-ion batteries. Even at very high charge rates (charging less than a minute), the team did not observe a short circuit. In short, a new example of a happy accident. “If we started by trying to prevent shortening of the dendrites, I’m not sure I would know how to achieve this,” Sadoway said.
Solving the problem of mass introduction of electric vehicles
Another advantage (and not least) is that the cost of an aluminum-sulfur cell should be less than one-sixth that of a similarly sized lithium-ion cell. “Composed of earth elements that can be ethically sourced and used at moderately high temperatures, just above the boiling point of water, this chemistry exhibits all the characteristics of a cheap, rechargeable, fireproof and recyclable material,” the team concludes.
The researchers believe that their battery would be ideal for installations on the order of several tens of kilowatt-hours (for example, for storing energy at home or a small business running on renewable energy sources).
Thanks to fast charging, they will also be very practical for charging stations for electric vehicles. If the latter become the majority on the roads, we will really need more and faster charging points. However, the capacity of the lines feeding the stations today is insufficient for this use. Having such batteries to store energy and quickly release it when needed will avoid the installation of costly new transmission lines.
The aluminum-sulphur battery patents have been licensed to a subsidiary of Avanti, co-founded by Sadoway and Luis Ortiz. The company’s first goal is to demonstrate that the system works at scale and then run a series of stress tests.
For larger grid-scale requirements (up to several hundred megawatt-hours, for example), other technologies may be more effective, including liquid metal batteries that Sadoway and his students developed a few years ago and will soon be sold by Ambri (a company also co-founded in 2010 by Sadoway and Ortiz).