Sep 2 2016
At right, scanning electron microscope (SEM) images show the two types of lithium deposits, the bulky, mossy type (at top), which grows from its base, and the needle-like dendritic type (bottom), which grows from the tips. At left, SEM images show the effect of a blocking layer of ceramic material that limits the growth of the mossy deposits. Courtesy of Peng Bai
Lithium metal electrodes have been considered a key contender by battery researchers, as these electrodes enhance the amount of energy that batteries are capable of storing without increasing their weight.
An increasing amount of effort put into this research was unsuccessful due to the problem that exists in this metallic form of lithium. As the batteries charge, finger-like lithium deposits develop on the metal surface, which can hinder the performance and also result in short circuits that disable or damage the battery.
Researchers from MIT state that they have executed the most in depth analysis on the formation of these deposits. They also report that on the existence of two completely different mechanisms at work. Both forms of deposits are made up of lithium filaments and the manner in which they grow depends on the current applied.
Mossy, clustered deposits that develop at low rates, grow from their roots and can be controlled effortlessly. The dendritic projections, that are rapidly advancing and a lot sparser, grow only at their tips. The team has difficulty dealing with the dendritic variety as it is the cause of many problems.
The team’s findings are reported in the recent issue of the journal Energy and Environmental Science, in a paper by Peng Bai, a senior postdoc; Ju Li, the Battelle Energy Alliance Professor of Nuclear Science and Engineering and a professor of materials science and engineering; Fikile Brushett, an assistant professor of chemical engineering; and Martin Z. Bazant, the E. G. Roos (1944) Professor of Chemical Engineering and a professor of mathematics.
[This research provides] fundamental experimental and theoretical insights into the growth of lithium metal, showing that there are really two different kinds of growth.
Martin Z. Bazant, Professor, MIT
Even though it was already established that such growth takes place on lithium surfaces, this is the first study to reveal two different varieties - dendritic, which extends extremely fast from the growing tips, and mossy, which grows slowly from the base.
He further states that these two types of growth were earlier lumped together under the common term ‘dendrites’, but this new study illustrates the accurate conditions for each individual growth mode that occurs and also explains how to control the mossy type.
The root-growing mossy growth is capable of being blocked by introducing a separator layer developed from a nanoporous ceramic material - a sponge-like material comprising extremely small pores at the nanometer scale, or billionths of a meter across. In comparison to this, the tip-growing dendritic growth cannot be blocked very easily but should never occur in practical batteries.
The regular working currents of these batteries are increasingly lower than the characteristic current linked to tip-growing deposits, and these deposits will not develop unless major degradation of the electrolyte has taken place.
Bai explains that, in theory, replacing standard carbon-based anodes with lithium metal will result in halving the volume and weight of lithium-ion batteries, for a specific amount of output current and storage capacity. However, the poor understanding of these surface deposits during the recharge process has been a key hindrance to the advancement of such batteries.
Unless they are somehow monitored, Bai says, “those small fibers can go right through the separator [layer inside the battery] and cause explosions or fires.”
Even when such destruction is found to be limited, the filaments slowly reduce the battery’s storage capacity, resulting it to gradually degrade. This current research proves the possibility of controlling these growths in an efficient manner, for a specific cell capacity. The work also illustrates the expected upper limits on the performance of the battery in order to avoid the occurrence of the truly damaging dendritic filaments.
The separators capable of blocking the mossy growth are developed from anodic aluminum oxide (AAO), which is 60 micrometers thick and has straight, well-arranged nanopores across its thickness.
It’s a big discovery, because it answers the question of why you sometimes have better cycling [charging and discharging] performance when you use ceramic separators.
Peng Bai, Senior Postdoc, MIT
The current work points out that flexible composite ceramic separators, referring to those produced by coating ceramic particles onto standard polyolefin separators, should be applied in lithium metal batteries in order to block the root-growing mossy lithium.
Bazant states that previous studies on the applications of lithium metal anodes were performed at low battery capacities or low current levels, and this has prevented a reliable observation of the second type of growth mechanism. The researchers performed tests at higher current levels that efficiently revealed the two separate types of growth.
He further states that the findings were successfully obtained because of his team’s development of a glass capillary cell, which is an innovative laboratory setup, that “allows you to see the growth, and you can see where there is this transition from one kind of growth to the other.”
Earlier research work relied mostly on electrical measurements to determine the physical activities inside a battery, but observing the whole thing in action made the difference clearer. The mossy, slow growth takes place for a limited period and then at a particular level of current, “all of a sudden, this little finger [of lithium] snaps out. It allows you to see exactly when the dendrites begin.”
Battery researchers will now use these new findings to enhance their understanding of the fundamental scientific principles. The new findings will also show “what are the limitations on rates and capacity that are achievable,” Bazant says.