This is the first time that scientists have observed the growth of tiny metal thorns known as dendrites grow within lithium-ion batteries thus making the batteries short-circuit. Their results published Mar. 12 in the journal Science illuminate the hitherto unrecognized mechanical aspects of the lithium dendrites during their development.
Lithium dendrites have been the subject of study of scientists since a long time, yet their behavior within batteries has not been well understood. Dendrites are developed at the nanoscale; development is difficult to monitor in a closed system such as a working battery, but has been associated with battery degradation and failure.
The new work, an international alliance of scholars at the U.S. and Singapore universities, simulated and experimented and came up with the first view on how dendrites crystalize, according to co-lead author Xing Liu, an assistant professor of mechanical and industrial engineering at New Jersey Institute of Technology and head of the NJIT Computational Mechanics and Physics Lab.
He says that it is a result of a close collaboration between experimental and computational mechanics and possibly could be used to make batteries safer.
Co-author Qing Ai, a former research scientist at Rice University, says: “The basic nanomechanical behavior of lithium dendrites has been a riddle of decades.”
Customized platforms
Lithium dendrites (named after the Latin word for branch) are about 100 times narrower than the thickness of a human hair and they are spouting out of anodes, which are negative terminals in lithium-ion batteries. The branches of dendrites may extend into an electrolyte in a lithium cell; in case the dendrites run to the negatively charged anode, and extend to the positively charged cathode, they may short out the battery.
Lithium dendrites are commonly known to be one of the largest impediments to commercialization of lithium-metal batteries, Liu says. Under battery operation, it is possible to have lithium dendrites form, break and be electrically isolated to the lithium metal anode to form so-called dead lithium. This is what causes a progressive depletion of battery capacity with time. Moreover, the dendrites may tunnel through the separator, and form an internal short between the anode and cathode. Capacity loss and short-circuit dendrite risks tend to be common in laboratory experiments.
Better still, lithium dendrites become almost impossible to eliminate in a battery once they develop.
At this point in time, says Liu, “there is no empirical way to cleanse dendrites of a working battery cell.”
In the new study, scientists at the Rice University together with their counterparts in Georgia Institute of Technology, the University of Houston and the Nanyang Technological University in Singapore extracted dendrites of working batteries to see whether they were mechanically strong or not.
“In order to make the quantitative study of lithium dendrites possible, we constructed specialized sample preparation and mechanical characterization stations of such delicate work,” says Boyu Zhang, a Rice doctoral graduate and a co-lead author on the work.
Rice Karl F. Hasselmann Professor of Materials Science and Nanoengineering co-corresponding author Jun Lou headed a team at the Nanomaterials, Nanomechanics and Nanodevices lab in performing a direct probe into the mechanical behavior of dendrites as they grew in real batteries. The extremely delicate experiments were done by Ai and Zhang, former members of the lab of Lou with the help of study co-corresponding author Hua Guo and co-author Wenhua Guo of the Rice University Shared Equipment Authority.
In order to execute the experiments, they made air-tight platforms to prepare and study the samples since lithium is a highly reactive element that changes chemically and structurally due to the amount of air it is exposed to. The nature of the deformation of individual dendrites to controlled stresses was then exposed using high-resolution electron microscopy.
‘Like dry spaghetti’
Lithium bulk is soft and cushy; the dendrites of lithium, consequently, were supposed to be soft as well. The experiments however indicated otherwise. This observation of the failure of dendrites in real-time under the operation of a battery through the University of Houston team under the leadership of one of the co-corresponding authors Yan Yao, a professor at the Department of Electrical and Computer Engineering, supported the idea that dendrites are brittle in liquid as well as solid electrolyte systems.
Liu says that for long it has been thought that the lithium dendrites are soft and ductile, resembling Play-Doh. However, it seems to us that they can be tough and brittle, too, and break like dry spaghetti.
Data on the observations was then modeled and theoretically analyzed by teams of NJIT and Georgia Tech.
To answer the question, Liu says that they did scale-bridging simulations to understand the reason lithium dendrites act contrary to expectations.
They discovered that when dendrites are growing in a battery cell, they will be covered by a thin coating of solid electrolyte interphase, known as SEI. The SEI coating causes the dendrites to become rigid and needle like and are able to pierce battery cells separators and electrolytes and are likely to break under stress and accumulate in the battery cell as lithium dead time fragments and lead to battery failure.
Liu explains that by knowing about the physics behind it, soon it will be possible to develop methods of making dendrites less susceptible to brittle fracture, such as; utilizing lithium alloy anodes. To scholars in the field of computational mechanics, the mechanisms to be found in the experiment, like the way that structures defame, or the reasons why they break and break down, are like musical notes and can be added to a symphony of high-performance materials and high-energy storage systems.
“The strengthening mechanism we identified in lithium dendrites adds a new note to this composition,” Liu says.
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