A team led by the University of California, San Diego (UCSD) found that the root cause of lithium metal battery failure is that during the discharge of the battery, a small amount of metal lithium deposits are detached from the surface of the negative electrode and trapped, becoming unusable. "Dead" or inactive lithium.
According to reports, the research team led by the University of California, San Diego (UCSD) found that the root cause of the failure of lithium metal batteries is that during the discharge of the battery, a small amount of metal lithium deposits are detached from the surface of the negative electrode and become trapped. Reused "dead" or inactive lithium.
This finding challenges the traditional view that lithium metal battery failure is due to the solid electrolyte interface film (SEI) formed between the lithium negative electrode and the electrolyte. Researchers have developed a technique to measure the amount of inactive lithium on the negative electrode and study its micro-nanostructure to arrive at this conclusion. This is the first time in the field of battery research. These findings may pave the way for the commercialization of rechargeable lithium metal batteries.
Lithium metal batteries (made of lithium metal as a negative electrode) are an important part of next-generation battery technology, and their energy density is twice that of current lithium-ion batteries (usually graphite negative electrodes). As a result, longer life and lighter weight may double the cruising range of an electric car. However, lithium metal batteries have an important problem, that is, the coulombic efficiency is low, and the number of cycles of the battery can be limited. This is because the active lithium and electrolyte stored in the battery are exhausted during the cycle.
Battery researchers have long suspected that this is due to the solid electrolyte interface film (SEI) formed between the negative electrode and the electrolyte. However, Y. Shirley Meng, a professor of nanoengineering at the University of California, San Diego, said that although researchers have developed various methods to control and stabilize the SEI layer, this problem cannot be completely solved. "These batteries will still fail because a lot of inactive lithium is forming in the battery. So there should be another important aspect that has been overlooked."
The researchers found that the main culprit in the failure of lithium metal batteries was lithium metal deposition. When the battery is discharged, lithium metal deposition falls off from the negative electrode, and then trapped in the SEI layer, losing electrical connection with the negative electrode, becoming inactive lithium that cannot participate in battery cycling. These trapped lithium greatly reduces the coulombic efficiency of the battery. Researchers have invented a method to measure how much unreacted lithium metal is trapped as inactive lithium to determine the cause of battery failure. They added water to a sealed flask containing an inactive lithium sample formed in the half cell cycle. All unreacted lithium metal reacts with water to produce hydrogen. By measuring how much gas is produced, the researchers can calculate the amount of lithium trapped in the metal. Inactive lithium also includes another component: lithium ions, which are also part of the SEI layer. The amount of unreacted lithium metal can be subtracted from the total amount of inactive lithium to calculate their amount.
In testing lithium metal half-cells, the researchers found that unreacted lithium metal is the main component of inactive lithium. As it is formed, the coulombic efficiency gradually decreases. At the same time, the lithium ion content of the SEI layer is always kept at a low level. These results can be observed in eight different electrolytes. "This finding is important. It can be seen that the main failure product of lithium metal batteries is unreacted lithium metal, not SEI," said researcher Fang. "This method is used to quantify the two components of active lithium. Can achieve ultra-high precision and achieve reliable results. This is something that other characterization tools can't do."
“From a chemical point of view, metallic lithium is aggressive, so this task is extremely challenging. Metal lithium will have many different types of parasitic reactions at the same time, and it is almost impossible to distinguish various inactive lithium.” US Army Kang Xu, a researcher at the research lab, said, "The advanced approach of this research provides a very powerful tool to do this in a precise and reliable way." The team at Xu's lab provides advanced research for this research. Electrolyte formulation.
Researchers hope their approach can be a new standard for evaluating the efficiency of lithium metal batteries.
By studying the micro-nanostructures of lithium deposition in different electrolytes, the researchers answered another important question: why some electrolytes improve coulombic efficiency, while others do not.
This is related to the way lithium is deposited on the negative electrode during battery charging. In some electrolytes, lithium can form micro-nanostructures to improve battery performance. For example, densely packed columnar lithium deposits are produced in electrolytes specifically designed by GM researchers. In the discharge process, benefiting from this structure, as the inactive lithium, the unreacted lithium metal trapped in the SEI layer is less. According to the test results, the coulombic efficiency of the first cycle was 96%. “This superior performance is attributed to the columnar microstructure formed on the surface of the current collector, which has the least curvature and greatly enhances the structural connection,” said Mei Cais of GM's Global R&D Center. His team developed advanced electrolytes to deposit lithium in an "ideal" microstructure.
In contrast, when using commercial carbonate electrolytes, lithium deposition exhibits a distorted, whisker-like morphology. This structure causes more lithium metal to be trapped in the SEI during the detachment process, reducing the coulombic efficiency to 85%.
Next, the research team proposed methods to control the deposition and detachment of metallic lithium, including applying pressure to the electrode stack, forming a uniform and mechanically elastic SEI layer, and using a 3D current collector. “The key is to control the micro-nano structure,” Meng said. “I hope that our findings will inspire new research directions and advance the development of rechargeable lithium metal batteries.”