Lithium-sulfur batteries pack up to four times more energy than lithium-ion batteries – and could allow electric-vehicle drivers to go longer distances without needing to recharge. Standing in the way of this promise, however, is the short lifetime of existing lithium-sulfur batteries.
Scientists at the U.S. Dept, of Energy’sPacific Northwest National Laboratory (PNNL) have created a new cathode material that contains nanoparticles of a nickel-based metal organic framework (MOF). A lab-scale battery made with this new cathode maintained 89% of its energy capacity after 200 charge-discharge cycles. The performance of a traditional carbon-sulfur cathode decays by more than 30% after 200 cycles.
“Lithium-sulfur batteries have the potential to power tomorrow’s electric vehicles, but they need to last longer after each charge and be able to be repeatedly recharged,” says Jie Xiao, a materials scientist at PNNL. “Our metal organic framework may offer a new way to make that happen.”
The short lifetime of the lithiumsulfur battery relates to the electrochemical reactions that take place in it. During discharge, lithium from the anode travels through the electrolyte to the cathode, where it reacts with sulfur to form Li2S2 or Li2S (both are insoluble in the electrolyte), releasing two electrons that flow through an external circuit. During charging, lithium plates the anode surface and the process begins again.
If these were the only reactions that took place, the battery would likely live up to its potential. They are not. Side reactions at the cathode produce long-chain polysulfides that dissolve in the electrolyte and cause the cathode to disintegrate. These polysulfides can travel to the anode, where they react with lithium atoms on the anode surface, rendering those atoms unavailable for energy storage. The polysulfides eventually form a thin film at the anode-electrolyte interface called a solid-electrolyte interface (SEI), which causes the battery to rapidly lose its energy capacity.
Research aimed at addressing this problem is taking several different paths. Previous work by the PNNL team, for example, designed a new anode material to shift the redox reaction away from the lithium-metal surface {CEP, Feb. 2014, p. 6). The anode consists of graphite and lithium metal foil connected electrically in parallel. When immersed in the electrolyte, lithium ions immediately intercalate into the graphite to form lithiated graphite. The lithiated graphite functions as an artificial SEI that supplies lithium ions on demand while minimizing direct contact between the polysulfides and the lithium metal surface.
In this most recent work, the PNNL researchers focused on the other electrode. They added a porous material to the cathode to prevent the polysulfides from traveling through the electrolyte to the anode. The additive is a type of MOF – a three-dimensional porous material consisting of metal ions (in this case, iron) coordinated to organic ligands (or linkers).
While several types of porous materials have been explored to trap polysulfides and prevent them from dissolving in the electrolyte, this nickel-based MOF has a unique benefit: Polysulfides are strongly attracted to this porous material. The MOF’s different-sized pores (mesopores and micropores) form an ideal matrix to confine the polysulfides, and its positively charged nickel (II) center tightly binds to the polysulfides, thereby slowing down their migration out of the pores.
The scientists characterized the pores of the MOF as well as the interaction between the nickel and the soluble polysulfides to confirm their understanding of how this porous additive addresses the short-lifetime issue of the lithium-sulfur battery. They now plan to improve the cathode materials to increase the battery’s storage capacity. In addition, they will test the cathode materials on larger prototypes to evaluate the cathode’s performance in conditions that more closely match those of commercial applications.
source: http://www.hispanicbusiness.com/2014/5/24/lithium-sulfur_battery_drives_the_distance.htm
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