Researchers at MIT have carried out the most detailed analysis yet of lithium dendrite formation from lithium anodes in batteries and have found that there are two entirely different mechanisms at work. While both forms of deposits are composed of lithium filaments, the way they grow depends on the applied current.
Clustered, “mossy” deposits, which form at low rates, turn out to grow from their roots and can be relatively easy to control. More sparse and rapidly advancing “dendritic” projections grow only at their tips. The dendritic type, the researchers say, are harder to deal with and are responsible for most of the problems dendrites cause: degraded performance and short-circuits that damage or disable the battery. Their findings are reported in an open-access paper in the RSC journal Energy and Environmental Science.
To develop batteries with higher energy density, such as Li–O2, Li–S, and other Li metal batteries using intercalation cathodes, lithium is believed to be the ideal anode material for its extremely high theoretical specific capacity (3860 mA h g-1), low density (0.59 g cm-3) and the lowest negative electrochemical potential ( 3.04 V vs. the standard hydrogen electrode). Unfortunately, lithium growth is unstable during battery recharging and leads to rough, mossy deposits, whose fresh surfaces consume the electrolyte to form solid–electrolyte interphase layers, resulting in high internal resistance, low Coulombic efficiency and short cycle life. Finger-like lithium dendrites can also short-circuit the cell by penetrating the porous separator, leading to catastrophic accidents. Controlling such hazardous instabilities requires accurately determining their mechanisms, which are more complex than the well-studied diffusion-limited growth of copper or zinc from aqueous solutions. Such fundamental understanding is critical for the success of the lithium metal anode and could provide guidance for the optimal design and operation of rechargeable lithium metal batteries.—Bai et al.
The new study is the first to show the two different types of dendritic growth: mossy, which grows slowly from the base, and dendritic, which extends rapidly from the growing tips.
|The “mossy" type of root-based growth is followed by the faster, tip-based needle-like dendritic growth. (Courtesy of Peng Bai) Click to enlarge.|
While previous research has always lumped the two types of growth together under the blanket term “dendrites”, the new work demonstrates the precise conditions for each distinct growth mode to occur, and how the mossy type can be relatively easily controlled.
The root-growing mossy growth, the team found, can be blocked by adding a separator layer made of a nanoporous ceramic material (a sponge-like material with tiny pores at the nanometer scale, or billionths of a meter across). The tip-growing dendritic growth, by contrast, cannot be easily blocked, but fortunately should not occur in practical batteries.
The normal working currents of these batteries are much lower than the characteristic current associated with the tip-growing deposits, so these deposits will not form unless significant degradation of the electrolyte has occurred.
The research shows that dendritic growths can be effectively controlled at lower current levels, for a given cell capacity, and demonstrates what the upper limits on battery performance would need to be in order to prevent the truly damaging dendritic filaments.
We have demonstrated that lithium growth in liquid electrolytes follows two different mechanisms, depending on the applied current and capacity. Below Sand’s capacity, reaction-limited mossy lithium mainly grows from the roots and cannot penetrate hard ceramic nanopores in a sandwich cell. Above Sand’s capacity, transport-limited dendritic lithium grows at the tips and can easily cross the separator to short the cell. Our results suggest maximizing Sand’s capacity by increasing the salt concentration in the electrolyte. Electrolyte degradation should also be monitored to prevent dendrites by keeping the cycled capacity below Sand’s capacity.
Ceramic separators with pores smaller than mossy lithium whiskers could replace conventional polyolefin separators with flexible large pores to enhance safety and cycle life, and the effect could be further reinforced with lithium salts and solvents that favor thicker columnar deposits. To the broader field of electrodeposition, our results clarify the physical connections between lithium and copper/zinc dendrites formed in liquid electrolytes. Mechanisms and mathematical models of copper/zinc dendrite growths cannot be and should not be applied to explain either the development or the suppression of lithium whiskers.Future theoretical investigations should take into account the dynamics of SEI formation during both the root-growth and tip-growth processes of lithium electrodeposition.—Bai et al.
The separators that could block the mossy growth are made of anodic aluminum oxide, or AAO, which is 60 micrometers thick and has well-aligned, straight 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 Pai, senior postdoc
The research suggests that flexible composite ceramic separators, such as those made by coating ceramic particles onto conventional polyolefin separators, should be used in lithium metal batteries to help block the root-growing mossy lithium.
Martin Z. Bazant, the E. G. Roos (1944) Professor of Chemical Engineering and a professor of mathematics explained that most previous research on the use of lithium metal anodes has been carried out at low current levels or low battery capacities, and because of that the second type of growth mechanism had not been reliably observed. The MIT team carried out tests at higher current levels that clearly revealed the two distinct types of growth.
He said that the findings were made possible by his team’s development of an innovative laboratory setup, a glass capillary cell. Previous research had mostly relied on electrical measurements to infer what was taking place physically inside the battery, but seeing it in action made the differences very clear.
The new findings will now provide battery researchers with a better understanding of the underlying scientific principles, and show the limitations on rates and capacity that are achievable.
The work was supported by Robert Bosch LLC through the MIT Energy Initiative.
Peng Bai, Ju Li, Fikile R. Brushett and Martin Z. Bazant (2016) “Transition of lithium growth mechanisms in liquid electrolytes” Energy Environ. Sci. doi: 10.1039/C6EE01674J