KEY CONCEPTS
• A new study has determined the cause of failure of lithium-metal batteries. 
• Evaluation of lithium-copper half-cells over multiple cycles led to the conclusion that the formation of unreacted lithium metal is probably the main cause of loss of battery capacity. 
• To retain lithium metal in an active state, a columnar lithium metal microstructure with a large granular size and homogeneous distribution of SEI components is desired. 

This column has discussed the advantages and disadvantages of using lithium-ion batteries as an energy source. Operating concerns have led researchers to look at other options such as lithium-sulfur batteries where the battery’s anode is lithium metal instead of the graphite used in lithium-ion batteries.

One issue that remained with lithium-metal batteries is the formation of dendrites generated from the lithium metal anode during use. In a previous TLT article (1), researchers indicated that deposition of multiwalled carbon nanotubes on the lithium metal anode can minimize dendrite formation. The carbon nanotubes become lithiated and act as an enhanced solid electrolyte interphase (SEI) to reduce the possibility of dendrite formation.

Ying Shirley Meng, Zable Endowed chair professor in energy technologies and professor of nanoengineering and materials science at the University of California San Diego in La Jolla, Calif., says, “Using lithium as the anode increases the potential energy density of the battery compared to lithium-ion batteries because graphite is a fluffy material that just occupies a lot of volume but does not contribute to the battery’s energy density. Lithium ions need to move in and out of the anode in a process known as intercalation that is inefficient.”

After a lithium-metal battery starts to cycle, a SEI forms between lithium metal and the electrolyte. Meng says, “The SEI layers are byproducts formed during the electrochemical operation of a battery. The formation of these byproducts consumes the active lithium metal.”

Past research has speculated that the main cause for lithium-metal battery failure is due to the continuous growth of the SEI layer. Meng says, “In evaluating past work, we found that the data generated in evaluating lithium-metal batteries was not consistent. While the SEI has been thought to be the cause for battery failure, no clear data has been presented to verify this hypothesis.”

Meng and her colleagues have undertaken a new study to determine a better procedure for evaluating the performance of lithium-metal batteries. During this study, they have found that battery failure is due to a different cause.

Inactive lithium metal
The researchers evaluated the Columbic efficiency of lithium-copper half-cells prepared with a high-concentration electrolyte and a commercial carbonate electrolyte and determined that battery efficiency decreases as the amount of inactive lithium metal increases. Meng says, “Inactive lithium metal is material that degrades during battery operation leading to an increased resistance to battery performance. This is in contrast to active lithium metal that participates in the operation of the battery.”

The key analytical technique used to identify high levels of inactive lithium metal as the source of battery failure is titration gas chromatography. Meng says, “We initially discharged the half-cell by plating lithium metal to the copper cathode, then the battery was charged to create inactive lithium. To determine the amount of inactive lithium metal present, we reacted the whole inactive lithium with water in a process known as titration gas chromatography (TGC).”

The researchers took advantage of the reaction of lithium metal with water that produces hydrogen gas. TGC measured the concentration of hydrogen gas produced from the reaction, which the researchers then used to stoichiometrically determine the amount of inactive lithium metal in the whole inactive lithium. Based on this information, the researchers also calculated the amount of lithium ions that form SEI which is the difference between the total inactive lithium and the amount of inactive lithium metal.

In evaluating the half-cells over multiple charge/discharge cycles, the researchers found that the main loss of battery capacity is primarily due to the formation unreacted lithium metal. In fact, there is a linear relationship between reduction of battery capacity and amount of unreacted lithium metal. 

The researchers then used a technique known as cryogenic electron microscopy (both scanning and transmission) to learn more about the formation mechanism of unreacted lithium metal. Meng says, “We need to evaluate the structure of inactive lithium metal at low temperature because the morphology of samples at room temperature changes rapidly and cannot be resolved.”

An image of the microstructure of inactive lithium metal is shown in Figure 1.


Figure 1. This image of the microstructure shows that inactive lithium becomes isolated and is not able to participate in the battery’s charge/discharge cycles. (Figure courtesy of the University of California San Diego.) 

Meng says, “We determined that inactive lithium metal is wrapped in the SEI, which contains electrolyte salts such as lithium carbonate and lithium oxides leaving the lithium metal isolated and not able to participate in the battery’s charge/discharge cycles.”

Meng emphasized that a columnar microstructure with large granular size and a homogeneous distribution of SEI components is desired to retain lithium metal in an active state. She says, “We are now applying our finding to figure out how to reconnect inactive lithium metal back into an active state. Our finding is that using a columnar 3D scaffolding in the electrode might be useful in minimizing the generation of inactive lithium metal during battery operation.”

Meng also points out that ultimately the answer for more stable lithium-metal batteries is the removal of the anode at the start (so-called anode-free battery). She says, “In our view, the ideal battery infrastructure contains only the electrolyte and a lithium-containing cathode.” 

Additional information on this research can be found in a recent article (2) or by contacting Meng at shirleymeng@ucsd.edu. 

REFERENCES
1. Canter, N. (2019), “Minimization of dendrite formation in lithium-sulfur batteries,” TLT, 75 (2), pp. 10-11.
2. Fang, C., Li, J., Zhang, M., Zhang, Y., Yang, F., Lee, J., Lee, M., Alvarado, J., Schroeder, M., Yang, Y., Lu, B., Williams, N., Ceja, M., Yang, L., Cai, M., Gu, J., Xu, K., Wang, X. and Meng, Y. (2019), “Quantifying inactive lithium in lithium metal batteries,” Nature, 572 (7770), pp. 511-515.