Safer lithium-ion batteries

Dr. Neil Canter, Contributing Editor | TLT Tech Beat May 2014

A gummy electrolyte forms a non-conductive wax layer at high temperatures to minimize the risk of thermal runaway.

KEY CONCEPTS
• The concern with using lithium-ion batteries is the remote possibility of a thermal runaway resulting in smoke or fire that is due to the liquid electrolyte currently used.
• A new gummy electrolyte that is a hybrid between liquid and solid electrolytes improves the safety properties of the electrolyte.
• The solid component in the gummy electrolyte melts at a sufficiently high temperature to form a non-conductive layer on electrodes to effectively shut down the battery.

THE DEMAND FOR BATTERIES that exhibit better performance and are more durable is ongoing as they are being used and under development in a number of applications such as automobiles and electronics. With the need to boost fuel economy, battery-powered vehicles are in greater demand, but weight and performance issues need to be addressed.

Lithium-ion batteries have been focused on because they have the potential to deliver higher energy density compared to most other battery types without the need for a high degree of maintenance. One area where there has been much interest in upgrading lithium-ion batteries is the development of improved battery anodes. In a previous TLT article, researchers developed a lithium-ion battery using the readily available byproduct of paper manufacturing, lignin (1). This natural material was converted through a series of process steps into a lignin-based carbon fiber that displays very promising performance as an anode material. Better performance was determined as compared to the incumbent anode material, graphite.

But one of the biggest problems in using lithium-ion batteries is the potential safety concerns that can occasionally occur, leading to smoke and fires. Katie Zhong, Westinghouse Distinguished Professor in the school of mechanical and materials engineering at Washington State University in Pullman, Wa., says, “The root of the safety problem with lithium-ion batteries is the electrolyte used as a medium through which ions can move between the anode and the cathode to create electricity. Leakage of the electrolyte or gas-generating reactions can lead to the remote possibility of thermal runaway, resulting potentially in smoke or fire.”

Two approaches have been tried to eliminate the safety problems with lithium-ion batteries. Zhong says, “Temperature sensors and flame-retardant additives have been introduced into lithium-ion batteries but with limited success.”

The other idea has been to replace liquid electrolyte commonly used with solid polymer electrolytes. Zhong says, “Solid electrolytes offer better safety because leakage is eliminated as a problem, but they also exhibit inferior performance. Two areas where they are deficient are poor ionic conductivity and very poor contact with the electrode where there are lots of gaps and adhesion is inadequate.”

Gel electrolytes have been evaluated but also suffer from safety concerns due to a high level of liquid electrolyte and unstable retention of the liquid during deformation. There is a need for a more stable electrolyte that will combine the features of good mechanical properties so that it can handle deformation and a more effective interface with electrodes. Such an electrolyte has been developed.

GUMMY ELECTROLYTE
Zhong and her associates have developed a new type of lithium-ion battery electrolyte that combines good performance comparable to a liquid electrolyte, better adhesive properties to electrodes and enhanced safety. She says, “The origin of the idea for a gummy electrolyte came at a conference I was attending on battery performance where adhesion properties were a big issue. I noted the presence of chewing gum on the bottom of a shoe and figured this might solve the problem.”

The gummy electrolyte is a hybrid-containing liquid and solid electrolytes. A wax emulsion method was used to prepare this electrolyte. The liquid electrolyte is lithium chlorate in propylene carbonate and the solid electrolyte consisting of high molecular weight polyethylene oxide. To improve the safety properties of the electrolyte, a core shell structure was developed using wax particles.

In effect, the liquid electrolyte facilitates the migration of ions, while the solid is present to ensure that no leakage occurs. Zhong says, “The wax particles act to shut down ion flow through the electrolyte by melting at a sufficiently high temperature to form a non-conductive layer on the electrode. This step blocks the ions from reaching the electrodes and shuts down the battery before a safety problem can be reached.”

Figure 1 shows an image of the waxy layer formed between a thermally destroyed gummy electrolyte and an electrode. The researchers assessed the ionic conductivity of the gummy electrolyte as the temperature increased and noted a significant reduction in conductivity once the wax melted. Zhong indicates that the type of wax can be changed so that the gummy electrolyte can be tailored to the conditions encountered in a specific application.


Figure 1. At high temperatures, the solid component in the gummy electrolyte will melt and form a non-conductive layer on the electrode preventing the battery from continuing to operate and minimizing the occurrence of a safety problem. (Courtesy of Washington State University)

The adhesive properties of the gummy properties were evaluated and found to be twice as sticky as real gum. Good adhesion to electrodes was seen with no evidence of any gaps.

Mechanical properties for the gummy electrolyte proved that no leakage will occur during twisting and compression tests. Zhong says, “We believe that the gummy electrolyte shows good durability based on the mechanical testing, but we’ll initiate work shortly to evaluate the performance of the gummy electrolyte in batteries to prove that this concept will work over a long operating period.”

The researchers are looking for commercial partners to evaluate battery performance with the gummy electrolyte. Additional information about this technology can be found in a recent paper (2) or by contacting Zhong at katie_zhong@wsu.edu.

REFERENCES
1. Canter, N. (2013), “Preparation of Lithium-Ion Battery Anodes using Lignin,” TLT, 69 (12), pp. 16-17.
2. Wang, Y., Li, B., Ji, J., Eyler, A. and Zhong, W. (2013), “A Gum-Like Electrolyte: Safety of a Solid, Performance of a Liquid,” Advanced Energy Materials, 3 (12), pp. 1557-1562.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.