Among the most promising replacements for the current commercial lithium ion chemistry is lithium sulfur (Li-S). Already in limited production, the performance potential for the use of a sulfur cathode was proven in 2008, when the battery chemistry was used in a solar-powered unmanned aircraft that set altitude and endurance records. Lithium sulfur cells offer higher energy density and lower cost than current metal oxide cathode chemistries. The problem is that Li-S batteries have trouble maintaining their superiority beyond just a few recharge cycles. But a solution to that problem may have been found with new research at Drexel’s College of Engineering.
Lighter Weight, Higher Performance
Recall that commercial lithium ion batteries have three main components. The positive electrode (cathode) is a combination of metal oxides. The negative electrode (anode) is usually made from carbon graphite, which allows storage of lithium ions between the graphite layers during charging. An organic solvent liquid electrolyte allows the flow of lithium ions back and forth between the cathode and anode during charging and discharging. The metal oxides of the cathode—typically nickel, manganese, and cobalt—are both heavy and relatively expensive. Sulfur, on the other hand, is both light and cheap.
The lighter weight makes sulfur attractive for electric aircraft batteries—particularly those designed for use in unmanned aerial vehicles (UAVs) or drones. The best Li-S batteries offer specific energy levels of 500 Wh/kg (watt-hours per kilogram), while most commercial lithium ion batteries are in the 150 to 250 Wh/kg range. Several commercial Li-S batteries are available in limited production, primarily for aviation and aerospace applications.
Li-S Limitations
Li-S battery limitations result from the migration of sulfur away from the electrode in the form of intermediate products called polysulfides. Significant research around the world has been undertaken to control the formation and distribution of these polysulfides, which are the primary reason that Li-S cells fade after several recharge cycles. Until now, coatings and passivation layers that reduce polysulfide formation have shown the most promise.
Drexel’s solution, described in a university news release, is to create a structure within the sulfur cathode that traps the polysulfides and holds them in place. “We have created freestanding porous titanium monoxide nanofiber mat as a cathode host material in lithium-sulfur batteries,” said Vibha Kalra, PhD, an associate professor in the College of Engineering. “This is a significant development because we have found that our titanium monoxide-sulfur cathode is both highly conductive and able to bind polysulfides via strong chemical interactions, which means it can augment the battery’s specific capacity while preserving its impressive performance through hundreds of cycles,” he added in the Drexel release.
Not only does the porous nanofiber mat hold onto the polysulfides; it also helps reduce the weight of the battery. “We can also demonstrate the complete elimination of binders and current collector on the cathode side that account for 30-50 percent of the electrode weight—and our method takes just seconds to create the sulfur cathode, when the current standard can take nearly half a day,” said Kalra.
