KEY CONCEPTS
• Interest in lithium-sulfur batteries is growing due to the potential for improved capacity and energy density.
• While dendrite formation can remain a problem, researchers have found that deposition of multiwalled carbon nanotubes on the lithium anode minimizes their formation.
• The multiwalled carbon nanotubes become lithiated and control the dendrite formation by acting as an ion reservoir. 

Development of lithium-ion batteries for use in a variety of applications from cell phones to automobiles is continuing. The initial focus for more than 25 years has been to determine how to boost the performance and efficiency of this technology. 

Unfortunately, lithium-ion batteries are hindered by a lack of energy storage capacity due to the intercalation of lithium ions between layers of graphite in the battery’s anode. Researchers have moved to post lithium-ion batteries where the anode is now lithium metal instead of graphite. 

Dr. Rodrigo Salvatierra, post-doctoral researcher at Rice University in Houston, says, “With the presence of lithium metal-based anodes and not lithium ion-based, the active material capacity can be 10 times higher. While this does not mean the overall battery capacity is 10 times higher, since the anode represents a fraction of the battery weight, the use of a lithium metal-based anode with a high capacity sulfur cathode instead of lithium cobalt oxide (the main material used in lithium-ion batteries) represents a meaningful weight reduction that improves battery capacity and energy density.”

One concern still faced with lithium metal anodes is the possibility of dendrite formation. Salvatierra says, “After the first cycles, a protective layer is formed over the lithium metal surface. This layer is called a solid electrolyte interphase (SEI) and is formed drawing components from the battery electrolyte. A common problem is that this protective layer does not form uniformly and, through the battery cycles, the inhomogeneities of the SEI can promote very local metal deposits or even rupture of the SEI layer causing development of needle-like structures known as dendrites. If left unchecked, dendrites can continue to grow toward the cathode. Upon reaching the cathode, an electric current can flow and lead to a short circuit that may cause the battery to overheat and potentially catch on fire.”

Several approaches have been taken to minimize dendrite formation. They involve use of additives in the electrolyte to improve lithium ion movement and passivate the electrode surfaces. Two other ideas include use of solid electrolytes as physical barriers and interfaces or three-dimensional structures to control lithium deposition. 

Salvatierra says, “Some of these methods showed interesting proof of concept and promising performances. However, in most cases, they were demonstrated under very mild battery operating conditions such as a low charge rate or very low capacity per area. These do not realistically simulate the conditions used in commercial batteries.”

A past TLT article discussed the use of a solid electrolyte based on aramid nanofibers and poly (ethylene oxide) (1). The researchers demonstrated that dendrite formation was suppressed. 

A new approach now has been developed to minimize dendrite formation through the modification of the SEI surrounding the lithium anode. 

Lithiated multiwalled carbon nanotubes
Salvatierra, James Tour, T.T. and W.F. Chao professor of chemistry, computer science and materials science and nanoengineering at Rice University, and their colleagues discovered that deposition of multiwalled carbon nanotubes on the lithium metal anode minimizes the formation of dendrites, and the nanotubes immediately become lithiated when placed atop lithium metal. Salvatierra says, “We discovered this dendrite-suppression effect when we were trying to solve another problem, the leakage of lithium polysulfides from sulfur cathodes. Instead, we found that the multiwalled carbon nanotube film became lithiated when placed on top of the lithium metal protecting the lithium metal against dendrite formation and parasitic reactions from lithium polysulfides in the electrolyte.”

The researchers knew that a reaction had occurred after the multiwalled carbon nanotubes wetted with a highly concentrated electrolyte [lithium bis (fluorosulfonyl imide)] were placed on top of a lithium metal foil. Within 30 minutes, the color of the multiwalled carbon nanotubes changed from black to red. 

The color change is caused by the lithiation of the multiwalled carbon nanotubes due to intercalation of lithium ions. Simultaneously, the multiwalled carbon nanotubes are reduced. 

The mechanism for how the lithiated multiwalled carbon nanotubes prevents dendrite formation is shown in Figure 1. This material acts as an enhanced SEI layer controlling the lithium plating/stripping occurring during charge-discharge cycles. During charging, lithium ions plate onto the lithium anode from the multiwalled carbon nanotubes but are replaced by lithium ions present in the electrolyte. 


Figure 1. Lithiated multiwalled carbon nanotubes act as an enhanced solid electrolyte interphase by controlling the lithium plating/stripping that occurs during charge-discharge cycles. (Figure courtesy of Rice University.)

The process reverses during discharging as lithium ions moving into the electrode are replaced by lithium present in the anode. Salvatierra says, “The lithiated multiwalled carbon nanotube interphase plays a role as an ion reservoir, which prevents dendrites from forming and growing.”

The researchers demonstrated this effect during charge/discharge cycles at current densities between 2 and 4 milliamps per square centimeter. 

Several analytical techniques were used to evaluate the effectiveness of lithiated multiwalled carbon nanotubes. Salvatierra says, “We found that operando Raman spectroelectrochemical methods enabled us to investigate the lithiated carbon layer through the charge/discharge cycles.”

The researchers evaluated using the lithiated multiwalled carbon nanotube SEI on the lithium anode of a lithium-sulfur battery. Salvatierra says, “We conducted several tests such as continuous and pulse discharges. The batteries could deliver 70% of the capacity when the power consumption increased over 60 times, from 0.05 C (20 hours) to 3 C (20 minutes). Also, the battery could withstand 500 cycles of continuous charge and discharge.”

Salvatierra also indicated that no charge loss was found when a fully charged lithium-sulfur battery was stored for one month.

For the future, Tour indicated that the researchers will assess the performance of lithiated multiwalled carbon nanotubes in coin battery cells such as those used in watches and in large format devices such as the pouch cells used in cell phone batteries. 

Additional information can be found in a recent paper (2) or by contacting Tour at tour@rice.edu. 

REFERENCES
1. Canter, N. (2015), “Dendrite-suppressing battery technology,” TLT, 71 (4), pp. 14-15.
2. Salvatierra, R., López-Silva, G., Jalilov, A., Yoon, J., Wu, G., Tsai, A. and Tour, J. (2018), “Suppressing Li Metal Dendrites Through a Solid Li-Ion Backup Layer,” Advanced Materials, 30 (50), 1803869.