October 10, 2017
A team of scientists at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland, has partnered with researchers from the University of Maryland (UMD) and the Army Research Laboratory (ARL) to develop a new type of flexible lithium-ion battery that can operate under extreme conditions, including cutting, submersion and ballistic impact. The team recently published their discovery in the journal Advanced Materials.
Li-ion batteries have become the energy storage source of choice for multiple applications, ranging from consumer electronics to military and aerospace systems, due to their energy and power performance. Despite these benefits, potential safety hazards associated with the organic electrolytes that are used in Li-ion battery cells remain an ongoing concern. These electrolytes are highly flammable, toxic, and moisture sensitive, limiting the forms in which a Li-ion battery can be manufactured.
In the recently published paper “Flexible Aqueous Li-ion Battery with High Energy and Power Densities” in Advanced Materials, a team of scientists from UMD, APL, and ARL have demonstrated a new type of flexible Li-ion battery that is not hazardous and also can continue to operate even under severe mechanical abuse.
The work builds upon a novel aqueous electrolyte referred to as “water-in-salt” developed in 2015 by UMD and ARL. This highly concentrated water-based electrolyte can address the key issue associated with the use of water in Li-ion batteries, which is the low electrochemical stability window of roughly 1.2 volts. By expanding this window to 3 volts, the water-in-salt enables much higher energy density aqueous Li-ion batteries.
“In recent years, UMD and ARL have explored several anode and cathode combinations that can be used within the stability window of our electrolyte. By collaborating with APL, we are starting to transition this technology into novel battery architectures and demonstrate its practical true potential,” said Chunsheng Wang, professor of chemical and biomolecular engineering at UMD and corresponding author of the paper.
In this new research, the team is embedding the water-in-salt electrolyte in a polyvinyl alcohol (PVA) polymer matrix, forming a gel polymer electrolyte (GPE). This GPE is even more stable than the liquid counterpart, and enables integration into a flexible battery configuration. “What limits the form factor of current Li-ion batteries is the flammable organic electrolytes. To ensure safety, you need sufficient packaging and protective measures. When the water-in-salt electrolyte was introduced, I thought that making a stable polymer version would radically change the way that Li-ion batteries are made and used,” said Kostas Gerasopoulos, senior research scientist and principal investigator at APL.
“By expanding the window of the electrolyte and improving its stability, we are also expanding the list of available materials that can be used to make working cells with long cycle life,” said Kang Xu, electrochemistry team leader and fellow at ARL. The team’s flexible battery uses LiVPO4F as the single active material in both the anode and cathode, forming a symmetric Li-ion battery. The “LiVPO4F is not a new material. It is well established as a Li-ion battery cathode. What makes it attractive for us is that it can be used as both anode and cathode within the stability window of the water-in-salt GPE, or alternatively, it can be matched with other high-voltage cathodes to achieve high energy density,” says Dr. Chongyin Yang, assistant research scientist at UMD and first author of the Advanced Materials article.
The team operated the flexible Li-ion battery in open air with minimal packaging, using only some electronically insulating heat-resistant tape to keep the flexible substrate in place. In their demonstration, the battery powered a significant motor load without any safety concerns. To demonstrate the full safety potential, the team attempted further tests that are not possible with today’s Li-ion batteries. These tests were performed while the battery was in operation and included cutting in air, immersing in sea water, and even subjecting it to ballistic testing at an APL facility. Impressively, not only do these abuse tests cause no catastrophic failure, but the battery maintains its performance and continues to power the load even when damaged and completely exposed to air and water.
The extraordinary safety of the water-in-salt GPE in the flexible cell stems from the fact that the water is strongly bound to the salt and that the water-in-salt GPE is slightly hydrophobic. “We wanted to show the real implications of this technology in practical applications. Particularly for our military, with our warfighters exposed to extreme conditions and environments during their missions, the capability to maintain both safety and performance is unprecedented,” said Gerasopoulos. “By making the batteries flexible and lighter compared to the devices currently used in the field, you can significantly decrease the burden to the warfighter,” added Xu.
The current generation of flexible batteries shows considerable potential, but they are still in the prototype phase. The team is looking for opportunities to transition the technology to make it available to the military. “We want to increase the robustness of the GPE and the energy density of the batteries even further. This work though proves the concept that we can build safe Li-ion batteries that can survive mechanical abuse,” said Gerasopoulos.
“Our team is currently working on several key innovations both in the materials and manufacturing,” said Jeffrey P. Maranchi, Signature, Energy and Materials Science Program Manager at APL. “We are interacting closely with the defense community, and we are very encouraged by the feedback we are receiving. We are not that far away from testing in the field. The sky is the limit for this technology.”
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.