MIT discovery paves the way for safe solid-state lithium batteries
A new discovery from the Massachusetts Institute of Technology could finally usher in the development of solid-state lithium batteries, which would be lighter, more compact and safer than current lithium batteries. The growth of metal filaments called dendrites within solid electrolyte has long been a barrier to their development, but the new study explains how dendrites form and how to manipulate them, resulting in the long-desired long-lasting solid-state batteries.
The key to this potential leap in battery technology is the replacement of the liquid electrolyte that sits between the positive and negative electrodes with a much thinner and lighter layer of solid ceramic material, and the replacement of one of the electrodes with metallic lithium. solid. This would significantly reduce the size and overall weight of the battery and eliminate the safety risk associated with liquid electrolytes, which are flammable.
Dendrites , whose name derives from the Latin and means branches, are metal projections that can build up on the surface of the lithium and penetrate the solid electrolyte, then extend from one electrode to another and short out the battery cell. Researchers have failed to agree on what causes these metallic filaments, nor has much progress been made on how to prevent them and thus make lightweight solid-state batteries a practical option.
The new research , published in the journal Joule in an article by MIT professor Yet-Ming Chiang, lead researcher Cole Fincher and five other researchers from MIT and Brown University, appears to resolve the question of what causes dendrites to form and how to prevent them from bringing the cathode and anode into contact.
Chiang said in previous work the team made an "amazing and unexpected" discovery, which is that the hard, solid electrolyte material used for a solid-state battery can be passed through by lithium, which is a very soft metal, during the process. charging and discharging of the battery, as the lithium ions move between the electrodes.
This displacement of ions causes the volume of the electrodes to change. This inevitably causes voltages in the solid electrolyte, which must remain in full contact with both electrodes between which it is inserted. “To deposit this metal, there has to be an expansion of volume because new mass is added,” Chiang said. “So, there is a volume increase on the side of the cell where the lithium is deposited. And if even microscopic defects are present, this generates pressure on these defects which can cause cracks where dendrites develop."
These stresses cause the cracks which allow the formation of dendrites. The solution to the problem turns out to be more stress, applied in the right direction and with the right amount of force.
While some researchers previously thought that dendrites form through a purely electrochemical process, rather than a mechanical one, the team's experiments show that mechanical stresses are causing the problem.
The process of dendrite formation normally takes place deep within the opaque materials of the battery cell and cannot be observed directly, so Fincher developed a way to make thin cells using a transparent electrolyte, which allows the dendrites to be directly seen and recorded. whole process. “You can see what happens when you put compression on the system and you can see if the dendrites behave in a way that is commensurate with a corrosion or fracture process,” he said.
The team demonstrated that they could directly manipulate dendrite growth simply by applying and releasing pressure, causing the dendrites to zig and zag in perfect alignment with the direction of the force .
The application of mechanical stresses to the solid electrolyte does not eliminate the formation of dendrites, but controls the direction of their growth. This means they can be directed to remain parallel to the two electrodes, preventing them from crossing to the other side and therefore rendering them harmless.
In their tests, the researchers used the pressure induced by bending the material, which was formed into a beam with a weight at one end. But they say that, in practice, there could be many different ways to produce the necessary stresses. For example, the electrolyte could be made with two layers of material that have different amounts of thermal expansion, causing the material to inherently deflect, as occurs in some thermostats.
Another approach could be to "dope" the material with atoms which, once added, distort it and leave it in a state of permanent stress. It's the same method used to produce the super-hard glass used in smartphone and tablet screens, Chiang explained. The pressure needed isn't extreme: Experiments showed that a pressure of 150 to 200 megapascals was enough to keep the dendrites from passing through the electrolyte.
The pressure required is "commensurate with the stresses commonly induced in commercial film growth processes and many other manufacturing processes," so it shouldn't be difficult to implement in practice, Fincher added.
Fischer explained that actually a different type of stress, called stack pressure, is often applied to battery cells, essentially squashing the material in the direction perpendicular to the battery plates – a bit like compressing a sandwich by placing a weight on it. This was thought to help prevent the layers from separating. But experiments have shown that pressure in that direction actually exacerbates dendrite formation, so it was the battery's own manufacturing process that was preventing the solid-state battery from working.
Instead, pressure is needed along the plane of the plates, as if the sandwich were being compressed from the sides. “What we have shown in this work is that when you apply a compressive force you can force the dendrites to travel in the direction of compression,” Fincher said, and if this direction is along the plane of the plates, the dendrites “will not reach never to the other side”.
This could finally make it practical to produce batteries with solid electrolytes and lithium metal electrodes. Not only could these batteries hold more energy in a given volume and weight, but they would eliminate the need to use liquid electrolytes, which are flammable materials.
After demonstrating the basic principles, the team's next step will be to try to apply them to the creation of a functional battery prototype, overcoming what has so far been the major limitation to the production of expensive batteries, because they contain lithium, but safe.
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The MIT Discovery article paves the way for safe, solid-state lithium batteries comes from Economic Scenarios .
This is a machine translation of a post published on Scenari Economici at the URL https://scenarieconomici.it/scoperta-del-mit-apre-la-strada-a-batterie-al-litio-allo-stato-solido-e-sicure/ on Fri, 25 Nov 2022 10:00:46 +0000.