For the flexible battery cell, the team, led by Konstantinos Gerasopoulos of APL's Research and Exploratory Development Department, developed a new class of "water-in-salt" (WiS) and "water-in-bisalt" (WiBS) electrolytes that were incorporated into a polymer matrix. This expands the electrochemical stability window to 4.1 V and dramatically improves cycle life in full cells with lithium titanate anodes compared to liquid water-based electrolytes say the team.
Using an aqueous electrolyte avoids the need for flammable, toxic, and highly reactive solvents present in current Li-ion cells and cauase problems for flexible battery developments.
"Li-ion batteries are already a constant presence in our daily lives, from our phones to our cars, and continuing to improve their safety is paramount to further advancing energy storage technology," said Gerasopoulos, senior research scientist and principal investigator at APL. "Li-ion battery form factors have not changed much since their commercialization in the early 1990s; we still use the same cylindrical or prismatic cell types. The liquid electrolyte and required hermetic packaging have a lot to do with that."
"Our team's efforts have generally been focused on replacing the flammable liquid with a polymer that improves safety and form factor. We are excited about where we are today. Our recent paper shows improved usability and performance of water-based flexible polymer Li-ion batteries that can be built and operated in open air."
The work builds on work in 2017. "The first generation of flexible batteries were not as dimensionally stable as those we are making today," said Gerasopoulos.
The team used LTO and LMO electrodes with areal capacities of 0.4 mAh/cm2 and 0.46 mAh/cm2 from Saft America to test the technoclogy, and the next move is to build prototype cells. "Our team is continuously improving the safety and performance of flexible Li-ion batteries," said Jeff Maranchi, the program area manager for materials science at APL. "We hope to transition this new research to prototyping within the year."