This article is taken from the monthly journal Sciences et Avenir – La Recherche #908 of October 2022.
“Goodbye pump, hello cork!” This is the brief message of the European Union to motorists. New light thermal or battery hybrid vehicles will be banned in the European Union from 2035, according to a text adopted in June. There will be adjustments and exceptions, but you have to get used to the idea of driving a 100% electric car one day.
If 82% of the French say they are ready to fight global warming (Ipsos survey of November 2021 for the Avere-France association), autonomy – 46% of respondents believe that it should be at least 500 kilometers – and the high price remain the main obstacles to buying . Suffice it to say that the future of the electric car is largely based on batteries. Thus, the sector is in full bloom, encouraged by the state.
“40 million euros will be allocated to public laboratories as part of the France 2030 plan to develop new batteries,” says Helene Bourlet, an expert on new energy technologies at the Commissariat for Atomic Energy and Alternative Energy (CEA). This is about basic research, but we also work hand in hand with manufacturers, car or battery manufacturers to meet their specific needs.” Okay, but can we really revolutionize batteries? Yes, the scientists say. But carefully … Because if the laws of chemistry are merciless – not a single battery will be charged in a few minutes to cover 1000 km – there really is room for improvement.
What can be improved?
To understand them, let’s go back to the basics. The battery consists of several accumulators. Each consists of two electrodes filled with electrolyte. In the case of the more common lithium-ion technology, when discharged, the lithium atoms in the negative electrode (anode) donate Li+ ions and electrons. The electrons circulate outside the battery and make up the electrical current that powers the circuit. Lithium ions pass through the electrolyte and attach to the positive electrode (cathode), where they find electrons. These reactions are reversible, allowing the battery to be recharged.
Lithium-Ion Batteries vs. “All Solids” During a discharge in lithium-ion technology, electrons pass from the anode to the cathode through an external circuit. Lithium ions leave the anode and travel to the cathode, attracted by negative electron charges, and circulate through the liquid electrolyte. In the “fully solid” case, the anode is lithium metal. The liquid electrolyte is replaced by ceramics, allowing lithium ions to pass from the anode to the cathode. More compact “solid” batteries allow you to store more energy in a safer environment, especially in fire hazard environments. Computer graphics: Bruno Bourgeois
What can be improved in this simple process? “Firstly, the capacity of the batteries, in other words, the amount of electrical charge they can store,” explains Jean-Marie Tarascon, research coordinator for electrochemical energy storage for CNRS, winner of the organization’s 2022 gold medal for work on lithium-ion batteries. For this requires the development of electrodes with a material rich in vacancies (atomic-scale holes, approx. ed.), each of which contains a lithium ion. However, in twenty-five years, the energy density has more than doubled thanks to the use of new materials derived from LiCoO (cobalt and lithium dioxide, editor’s note) and made from NMC sheets (nickel, manganese and cobalt).”
The most important safety issue
Another element is increasingly attracting the attention of laboratories: the electrolyte. “The question is important because it is directly related to the issue of safety,” says Helene Bourlet. “Some liquid electrolytes are highly flammable. By developing a non-flammable composition, it becomes possible to increase the amount of energy on board. Enough to double the autonomy of current batteries.” Several cases of ignition of lithium batteries in the past due to thermal runaway was due to the formation of dendrites or lithium filaments that grow at the level of the negative electrode, pierce the separator and cause a short circuit.
One solution, mentioned as early as the 1970s, is to develop a solid electrolyte that would block the growth of dendrites. This concept is making a comeback with the discovery in recent years of solid materials that are good ion conductors. “There are several families: ceramics, polymers or sulfur-based compounds,” says Helene Bourlet. Each of them has its own advantages and disadvantages. solid polymer. But it needs to be heated to 60-80°C to make it ionically conductive. A store of energy that reduces autonomy. Ceramics, on the other hand, are fragile and therefore difficult to fit. Finally, sulfide research has only just begun.”
However, the main problem of lithium-ion technology remains parasitic reactions at the electrode-electrolyte interface, whether solid or liquid. “Without them, the battery would last forever,” recalls Jean-Marie Tarascon. “But due to the voltage applied to the battery terminals, we are always outside the zone of thermodynamic stability of materials. nightmare for electrochemists!”
Another difficulty in developing a solid state battery is that the thickness of its lithium anode changes during charging and discharging. “However, the correct functioning of the battery depends on the contact between the various interfaces,” recalls Jean-Marie Tarascon. “It’s easier to maintain with a liquid electrolyte that adapts to volume changes than with a solid case.”
That is why, in anticipation of “semi-solid”, we are seeing the emergence of “semi-finished products” with batteries that mix solid and liquid electrolyte, or gels. “It’s so difficult that research is progressing in stages,” concludes Helene Bourlet. “You have to study so many combinations of materials to determine the best one.” atom to avoid the famous parasitic reactions. A tedious job that should be made easier by using artificial intelligence to automate research as part of the Battery 2030+ project, which brings together research efforts at a European level.
Have the ability to monitor battery status in real time
Intelligence is also a matter of putting it into the batteries themselves. An idea put forward by Jean-Marie Tarascon in 2017 is starting to take shape in his laboratory at the Collège de France. “Batteries are only used to about 70% of their capacity because there is a safety concern beyond that. If we had access to “observable parameters” such as internal temperature or the presence of parasitic chemical reactions, we could monitor the “battery in real time”. health status and use it with maximum efficiency. We are currently testing various fiber optic cables to gain access to the inside of working batteries.”
Finally, the last element shook the sector: Elon Musk. In 2021, the founder of Tesla delivered 910,000 electric vehicles, an 87% increase from 2020. The strike force of the American changed the rules of the game. “Elon Musk doesn’t innovate, but because he’s the market leader, he sets the trends for what we design and what we don’t,” says Jean-Marie Tarascon. — For example, he decided to exchange nickel and cobalt electrodes for cheaper materials. thus reviving lithium iron phosphate batteries, or LFPs, which were already in use ten years ago. Less efficient, but also less expensive, they could be fitted to entry-level electric vehicles.” In general, from 1200 euros in the early 2010s during the days of the first Teslas, the price of a “kWh battery” is now around 130 euros. According to Elon Musk, by 2025 it should be halved. This is undoubtedly the most convincing argument in favor of the emergence of a 100% electric vehicle.
Supercapacitors, another promising area
Supercapacitors store energy by trapping ions inside the pores of carbon electrodes. But unlike batteries, they charge in minutes. This is because the accumulation of charges on the surface of the electrodes is electrostatic, without resorting to chemical reactions to form ions, as in batteries. On the other hand, if they are more powerful and more stable, they contain less energy, which limits their use.
Therefore, research is devoted to increasing the charge density carried by carbon electrodes. The goal is to have supercapacitors that provide, for example, 250 km of battery life, but recharge in less than ten minutes. The goal, however, experts consider ambitious.