Lithium sulfur batteries are of great interest due to their high specific energy and relatively low cost (e.g., earlier post). However, Li-S batteries exhibit significant capacity decay over cycling. A team at Stanford University led by Profesor Yi Cui has now identified a new capacity fading mechanism of the sulfur cathodes and developed a new approach to overcoming this mechanism. A paper on their work is published in the ACS journal Nano Letters.
The new capacity fading mechanism relates to the detachment of lithium sulfide from the carbon surface during the discharge process. To overcome this mechanism, they introduced amphiphilic polymers to modify the carbon surface. The modified sulfur cathode shows excellent cycling performance with specific capacity close to 1180 mAh/g at C/5 current rate. Capacity retention of 80% is achieved over 300 cycles at C/2.
Sulfur cathode has a specific capacity of around 1673 mAh/g, which gives lithium sulfur batteries a specific energy of around 2600 Wh/kg, much higher than the conventional lithium ion batteries based on metal oxide cathodes and graphite anodes. Commercial applications of lithium sulfur batteries have not been very successful despite several decades of research. The major problems of sulfur cathode include low active material utilization, poor cycling performance and low Coulombic efficiency. Much effort has thus been put into improving the electrochemical performance of the sulfur cathode.
Recently, our group demonstrated a hollow carbon nanofiber/ sulfur composite cathode structure that exhibited a high specific capacity of around 1500 mAh/g and improved cycle life. [Earlier post.]...In this work, we investigated the structural change of the sulfur cathode using the hollow carbon nanofibers. It was observed that lithiation of sulfur resulted in the detachment of the lithium sulfide from the carbon surface, indicating the importance of interfacial effect in contributing to the sulfur cathode decay.
We performed first-principles calculations to study how lithiation changes the chemical interaction between sulfur and the carbon surface. The results showed a significant decrease in binding energy between the lithium sulfide and the carbon. In light of this new understanding, we modified the interface between the carbon and sulfur with amphiphilic polymers and showed a much-improved cycling performance of the modified electrode.—Zheng et al.
The team fabricated a nanofiber sulfur cathode using their earlier method and assembled it into a 2032-type coin cell (MTI) with lithium metal as the counter electrode. The battery was discharged at C/5 current rate to 1.7 V and held at this voltage for another 24 h until the discharge current was smaller than 5 μA. TEM imaging showed clear shrinking of lithium sulfide away from the carbon wall along the length of the hollow nanofiber.
This observation is surprising as the density of lithium sulfide is lower than that of sulfur, which means that lithiated sulfur undergoes volumetric expansion. Separation of lithium sulfide from the carbon wall means that the intermediate polysulfides could have leaked out from the hollow carbon nanofibers through the openings. The extra Li2S could have precipitated and segregated from the carbon matrix, resulting in the loss of electrical contact and capacity decay.—Zheng et al.
Results of DFT simulation further suggested that the interfacial effect between the lithium sulfide and the carbon can play important role in sulfur cathode degradation.
To investigate the effect of adding amphiphilic polymers—polymers composed of hydrophilic (water-loving) and hydrophobic (water-hating) parts—in modifying the interface between sulfur and the hollow carbon nanofiber, they used polyvinylpyrrolidone (PVP) due to its simple molecular structure and availability. PVP also has strong binding with a carbon surface from aqueous solution due to a strong thermodynamic driving force in eliminating the hydrophobic interface.
The electrochemical performance of the modified hollow carbon nanofiber/sulfur cathode showed marked improvement compared to the unmodified material. At C/5, a specific capacity of around 1180 mAh/g was achieved. The specific capacities were around 920 mAh/g and 820 mAh/g at C/2 and 1C, respectively.
Instead of the rapid initial decay generally observed in the unmodified electrodes, the first few cycles showed a slight increase in specific capacity from 828 to 838 mAh/g. The amphiphilic polymers provide anchoring points that allow lithium sulfides to bind strongly with the carbon surface. Subsequent cycles showed very stable performance, with less than 3% decay over the first 100 cycles. The capacity retention was over 80% for more than 300 cycles of charge/ discharge, with Coulombic efficiency at around 99%.—Zheng et al.
Guangyuan Zheng, Qianfan Zhang, Judy J. Cha, Yuan Yang, Weiyang Li, Zhi Wei Seh, and Yi Cui (2013) Amphiphilic Surface Modification of Hollow Carbon Nanofibers for Improved Cycle Life of Lithium Sulfur Batteries. Nano Letters doi: 10.1021/nl304795g