This article is taken from the monthly Sciences et Avenir – La Recherche n°908, dated October 2022.
“Goodbye pump, hello plug!“This is the European Union’s message to motorists in summary. From 2035, new light combustion or rechargeable hybrid vehicles will be banned from the European Union, according to a text voted in June. There will be adjustments and exceptions, but you have to get used to the idea of one day driving a 100% electric car.
If 82% of French people say they are ready to fight against 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 purchase. Suffice to say that the future of the electric car is largely based on batteries. The sector is thus in full effervescence, encouraged by the State.
“40 million euros will be granted to public laboratories as part of the France 2030 plan to develop new batteries, welcomes Hélène Burlet, expert in new energy technologies at the French Alternative Energies and Atomic Energy Commission (CEA). This concerns basic research, but we also work hand in hand with manufacturers, car manufacturers or battery manufacturers, in order to meet their specific needs. “Okay, but can we really revolutionize batteries? Yes, answer the scientists. But gently… Because if the laws of chemistry are ruthless – no battery will be recharged in a few minutes to cover 1000 km -, there is indeed room for improvement.
What can be improved?
To understand them, let’s go back to basics. A battery consists of several accumulators. Each consists of two electrodes bathed in an electrolyte. In the case of the more common lithium-ion technology, upon discharge, the lithium atoms in the negative electrode (anode) donate Li+ ions and electrons. The electrons circulate outside the accumulator and constitute the electric current supplying the circuit. The lithium ions pass through the electrolyte and reach the positive electrode (the cathode) where they find the electrons. These reactions are reversible, allowing the battery to be recharged.
Lithium-ion batteries versus “all solid” When discharging, in lithium-ion technology, electrons pass from the anode to the cathode through an external circuit. The lithium ions leave the anode for the cathode, attracted by the negative charges of the electrons and circulate through a liquid electrolyte. In the “all solid” case, the anode is metallic lithium. The liquid electrolyte is replaced by a ceramic allowing the lithium ions to pass from the anode to the cathode. The more compact “all-solid” batteries make it possible to store more energy in safer conditions, particularly in the face of the risk of fire. Computer graphics: Bruno Bourgeois
What can be improved in this simple process? “First the capacity of the batteries, in other words the quantity of electrical charges that they can store, explains Jean-Marie Tarascon, coordinator of research on the electrochemical storage of energy for the CNRS, gold medal 2022 of the organization for his work on lithium-ion batteries. This requires developing electrodes with a material rich in vacancies (holes on the atomic scale, editor’s note), each harboring 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 up of sheets of NMC (nickel, manganese and cobalt). “
The overriding issue of safety
Another element is increasingly attracting the attention of laboratories: the electrolyte. “The issue is important because it is directly linked to the question of security, says Hélène Burlet. Some liquid electrolytes are flammable. By developing a non-flammable compound, it becomes possible to increase the amount of energy on board. Enough to double the autonomy of current batteries. “The few lithium battery fires in the past due to thermal runaways have come from the formation of dendrites, or lithium filaments. They grow at the negative electrode, pierce the separator and cause a short circuit.
One solution, mentioned as early as the 1970s, consists in developing a solid electrolyte, which would block the growth of dendrites. The concept is making a strong 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, explains Hélène Burlet. They each have their advantages and disadvantages. For example, the batteries that fitted Bolloré’s Bluecar electric cars (former Autolib in Paris, editor’s note) used a solid polymer. But it must be heated to 60-80°C to make it ionically conductive. A supply of energy that reduces autonomy. Ceramics, on the other hand, are fragile and therefore difficult to implement. Finally, research on sulphides has only just begun. “
However, the major problem with lithium-ion technology remains the parasitic reactions at the electrode-electrolyte interface, whether solid or liquid. “Without them, a battery would be eternal, recalls Jean-Marie Tarascon. But because of the voltage applied to the terminals of the accumulator, we are always outside the zone of thermodynamic stability of the materials. Parasitic chemical reactions inevitably take place, a nightmare for electrochemists! “
Another difficulty for the design of a solid state battery comes from the fact that the thickness of its lithium anode varies during charging and discharging. “However, the proper functioning of a battery depends on the contact between the different interfaces recalls Jean-Marie Tarascon. It is easier to maintain it with a liquid electrolyte, which adapts to variations in volume, than in the solid case. “
This is why, while waiting for the “all solid”, we see the emergence of the “semi” with batteries mixing solid and liquid electrolyte, or gels. “It’s so complicated that the research advances in stages, summarizes Hélène Burlet. It is necessary to study very many combinations of materials to determine the best one. “The problem is also the same for the electrodes, where we test the addition of such and such an atom in order to avoid the famous parasitic reactions. A tedious work which should be simplified by the use of artificial intelligence in order to automate research, as part of the Battery 2030+ project, which brings together research efforts at European level.
Be able to monitor battery health status in real time
Intelligence, it is also a question of putting it in the batteries themselves. The idea put forward by Jean-Marie Tarascon in 2017 is beginning to take shape in his laboratory at the Collège de France. “Batteries are only used at around 70% of their capacity because beyond that there is concern for safety. If we had access to ‘observables’ such as internal temperature or the presence of parasitic chemical reactions, we could control the ‘health of the battery in real time and use it at its best. We are currently testing different optical fibers in order to access the interior of the batteries in operation. “
Finally, one last element came to shake up the sector: Elon Musk. In 2021, the founder of Tesla delivered 910,000 electric cars, or 87% more than in 2020. The American’s strike force was a game-changer. “Elon Musk does not innovate, but since he is a leader in this market, it gives the trends of what we develop or not, believes Jean-Marie Tarascon. For example, he decided to swap nickel and cobalt electrodes for cheaper materials. It is thus relaunching lithium iron phosphate batteries, or LFP, which were already in use ten years ago. Less efficient but less expensive, they could equip entry-level electric cars. “And since Tesla still produces a lot, the impact on prices is immediate. Overall, from 1200 euros in the early 2010s when the first Teslas were produced, the price of the “kWh-battery” today is worth around 130 euros. According to Elon Musk, it should be halved again by 2025. This is undoubtedly the most convincing argument for the advent of the 100% electric car.
Supercapacitors, another promising avenue
Supercapacitors store energy by capturing ions inside the pores of carbon electrodes. But unlike batteries, they recharge in minutes. This comes from the fact that the storage of charges on the surface of the electrodes is electrostatic, it does not have recourse to chemical reactions to produce ions, as with batteries. If they are more powerful and more resistant, they contain on the other hand less energy, which limits their use.
Research is therefore devoted to increasing the charge density carried by the carbon electrodes. The objective would be to achieve supercapacitors offering, for example, 250 km of autonomy, but recharging in less than ten minutes. A goal, however, considered ambitious by the experts.
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