BEIJING, Nov. 20 (Xinhua) -- MIT researchers have explained the formation of dendrites in rechargeable lithium batteries and how to prevent them from crossing the electrolyte, according to the latest issue of the journal Joule. The discovery could eventually open the door to the design of a new type of rechargeable lithium battery that is lighter, more compact and safer than current versions.

　　One of the reasons that rechargeable lithium metal batteries have had limited commercial use so far is dendrites. Dendrites can build up on the surface of the lithium, penetrate the solid electrolyte, and eventually cross from one electrode to another, shorting out the cell.

　　Early research at MIT found that lithium ion solid electrolyte material travels back and forth during battery charging and discharging, causing a change in the volume of the electrodes. This inevitably creates stress in the solid electrolyte, which must remain in full contact with the two electrodes sandwiched in between. "In order to deposit this metal, it is necessary to expand the volume, because the new mass is increasing. As a result, the volume of one side of the lithium battery increases. If there are even minor defects present, there will be pressure on those defects, which will lead to cracking."

　　The research team has now found that these pressures can lead to cracks, which can lead to the formation of dendrites. The solution to the problem proved to be to apply pressure in the right direction and with the right force.

　　Previously, some researchers believed that dendrites were formed by a purely electrochemical process rather than a mechanical one, but the team's experiments showed that it was mechanical stresses that were causing the problem.

　　The process of cell dendrite formation usually occurs deep in opaque materials and cannot be directly observed, so the researchers developed a method for making thin cells using a transparent electrolyte that allows the entire process to be seen and recorded directly.

　　The team demonstrated that they could directly control the growth of dendrites by simply applying and releasing pressure so that the dendrites were perfectly aligned with the direction of the force. Applying mechanical stress to a solid electrolyte does not eliminate the formation of dendrites, but it does control the direction of their growth. This means that they can be guided to remain parallel to the two electrodes and prevented from crossing to the other side, thus becoming harmless.

　　Another approach is to "dope" the material with embedded atoms, deforming it and putting it in a permanent stress state. Experiments have shown that a pressure of 150 to 200 MPa is sufficient to prevent dendrites from crossing the electrolyte.

　　Previously, it was thought that a sandwich-like multilayer structure would prevent dendritic structures from being created. However, new experiments have shown that squeezing the material in a direction perpendicular to the cell plate actually exacerbates the formation of dendritic structures. Instead, pressure should be applied along the plane, as if squeezing from the side of the sandwich.

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