Lithium metal, having a high theoretical specific capacity of 3,860 mAh g^-1 and the most negative electrochemical potential among anode materials, has been considered an ideal anode in lithium battery systems over the past four decades. The recent emerging demand for extended-range electric vehicles has stimulated the development of high-energy storage systems, especially the highly promising lithium–sulfur and lithium–air batteries, in which lithium metal anodes are employed. However, practical applications of rechargeable lithium metal-based batteries have been hindered by the formation of dendritic and mossy lithium and associated electrolyte decomposition, resulting in safety concerns and low Coulombic efficiency.

Uncontrolled growth of lithium dendrites originates from the repeated stripping/plating of a lithium layer during cycling, associated with which is a large volume change that causes cracks in the solid-electrolyte interphase (SEI) that exposes fresh lithium metal to the electrolyte, resulting in continuous electrolyte decomposition and rapid loss of both working lithium and electrolyte. … the formation of a SEI layer with high uniformity and stability is essential to ensure high Coulombic efficiency, long cycle life and safety in lithium metal-based batteries.

… Herein, we demonstrate that the parasitic reaction between lithium polysulfide and lithium can instead be used to effectively suppress the growth of lithium dendrites. Both lithium polysulfide (Li₂S₈) and lithium nitrate (LiNO₃) were used as additives to the ether-based electrolyte, which enables a synergetic effect leading to the formation of a stable and uniform SEI layer on lithium surface that can greatly minimize the electrolyte decomposition and prevent dendrites from shooting out.

—Li et al.

Top: (Left) Dendrites grow from the surface of a battery anode and penetrate the separator between the battery’s halves. When this happens the battery can short-circuit, overheat and burst into flame. A study by SLAC and Stanford found that adding two chemicals to the battery’s electrolyte could prevent this growth in next-generation lithium metal batteries. (Right). Credit: SLAC. Bottom: Schematic illustration showing the morphology difference of lithium deposited on the stainless steel substrate in the two electrolytes (both contain lithium nitrate), but (a) without lithium polysulfide (b) containing lithium polysulfide. Credit: Li et al. Click to enlarge.

Lithium nitrate has been under investigation for a long time as an additive to improve battery performance. Lithium polysulfide, on the other hand, has been considered a nuisance; formed when a sulfur electrode degrades, it travels to the lithium metal electrode and wrecks it, Cui said.

In brainstorming sessions, the research team realized their combined effect had not been studied before; together the chemicals could potentially react with lithium metal to form a stable, solid interface between the electrode and the electrolyte.

The researchers assembled coin cells using various concentrations of the two chemicals to the ether-based electrolyte. They found that by simply manipulating the concentrations of Li₂S₈ and LiNO₃, they could prevent the formation of lithium dendrites at a practical current density of 2 mA cm^-2 up to a deposited areal capacity of 6 mAh cm^-2.

They also demonstrated excellent cyclability: the Coulombic efficiency can be maintained at >99% for more than 300 cycles at 2 mA cm^-2 with a deposited capacity of 1 mAh cm^-2. Even for cyclic deposition of a high areal capacity of 3 mAh cm^-2, the average Coulombic efficiency can be as high as 98.5% over 200 cycles (and is higher at ~99.0% between 70 and 200 cycles).

Using in-situ optical imaging, the researchers were able to observe that the polysulfide additive smooths the lithium dendrites through an etching effect.

Our findings show that the reaction that has long been considered a critical flaw in lithium–sulfur batteries can actually benefit lithium metal-based battery systems when it is properly controlled. This illustrates a new strategy for solving dendrite issues associated with lithium metal anodes, which could be applied to next-generation high-energy-density battery systems such as lithium–sulfur and lithium–air batteries, as well as other metal–anode battery chemistries.

—Li et al.

This is a really exciting observation. We had been doing experiments all along with these two chemicals in there, but this was the first time we looked at the synergistic effect. This does not completely solve all the problems associated with lithium metal batteries, but it’s an important step.

—Fiona (Weiyang) Li, first author

Yet-Ming Chiang said the next step is to see if this approach can prevent dendrite formation in larger-scale cells that are closer to being practical batteries. It may also work for electrodes made of other metals, such as magnesium, calcium or aluminum, that also have potential for storing much more energy than today’s batteries.

Funding for the project was provided by the Joint Center for Energy Storage Research (JCESR), a Department of Energy Innovation Hub, and Cui and Chiang are both JCESR principal investigators.

Resources

• Weiyang Li, Hongbin Yao, Kai Yan, Guangyuan Zheng, Zheng Liang, Yet-Ming Chiang & Yi Cui (2015) “The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth” Nature Communications 6, Article number: 7436 doi: 10.1038/ncomms8436