High-Energy, High-Power Lithium-Sulfur Batteries

There is immense interest in developing rechargeable battery systems with high energy density and long cycle life at an affordable cost for portable electronic devices, electric vehicles, and stationary storage of electricity produced by renewable sources like solar and wind. The current lithium-ion technology based on lithium-insertion compound electrodes is limited in energy, so cathodes like sulfur and oxygen have become appealing as they offer an order of magnitude higher capacity and are abundant. The reaction of sulfur with lithium to provide Li₂S, invoving two electrons per sulfur, provides a capacity of 1,672 mAh/g, which can enable packaged lithium-sulfur cells with two to three times higher energy density than the current lithium-ion batteries. However, several challenges prevent the practical application of Li-S batteries: high resistivity of sulfur and discharged products, dissolution of intermediate polysulfides, and shuttling effect of polysulfides, which result in low utilization of sulfur and poor cycle life. Significant improvements have been made in recent years, but further improvements and a better understanding of the Li-S batteries are still needed. This presentation will focus on Li-S systems with a carbon interlayer between the cathode and the separator, Li₂S cathodes, and polysulfide cathodes.

Our group has developed a novel Li-S cell configuration utilizing a bifunctional carbon interlayer between the separator and regular sulfur electrode for improving the cyclability and rate capability. The interlayer effectively decreases the resistance of the sulfur electrode, facilitates the absorption of soluble polysulfide shuttling in the electrolyte, and acts as a second current collector for accommodating the migrating active material from the sulfur electrode. This novel approach not only simplifies the processing without requiring elaborate synthesis of composites and surface chemistry modification, but also improves the rate capability and cycle life. A variety of carbon interlayers have been developed for this approach, including binder-free carbon nanotube (CNT) paper, microporous carbon paper, and naturally abundant materials like carbonized eggshell membranes and plant leaves.

Li/dissolved polysulfide batteries have the advantage of utilizing dissolved active material, which could enhance the kinetics of the redox reactions of sulfur. However, the carbon electrode used as a current collector plays a key role in the cell performance. We utilized a binder-free, free-standing CNT paper as the current collector, which can effectively trap the charged and discharged products in amorphous state. The nanoscaled structure of the CNT paper ensures electron and ion access to the active material upon cycling, leading to unprecedented high capacities of 1,400 mAh/g with good cycle life. Other binder-free carbon papers such as carbon nanofiber can also work effectively with this system.

Lithium metal or lithiated anodes are required to be used in Li-S batteries since the sulfur cathodes do not have any lithium in the initial stage, which poses a significant safety hazard. Li₂S with a theoretical capacity of 1,169 mAh/g, is more desirable as the cathode material than sulfur. However, it is not straightforward to make Li₂S cathode because of its high resistivity and reactivity in air. Our group has developed a simple Li₂S-CNT sandwiched electrode which can effectively trap the air-sensitive Li₂S in the electrode with good electron and ion accessibility. A high capacity of around 800 mAh/g was obtained at a rate of C/10 over 100 cycles. Also, another approach to chemically synthesize Li₂S in situ by using a lithiated graphite as a lithium donor to reduce polysulfides will be presented.