October 30, 2013

Lithium-sulfur battery breakthroughs for holding good charges for up to 200 recharges

Cornell researchers’ improvement of the performance of lithium-sulfur batteries, a promising alternative to today’s lithium-ion batteries.

Two recently published papers, both originating from the lab of Hector Abruña, the Emile M. Chamot Professor of Chemistry and Chemical Biology, describe breakthroughs in the durability and performance of lithium-sulfur battery cathodes, one by using a component of corn starch, and the other, by modeling a nanocomposite material after the yolk-shell structure of eggs.

“Lithium-sulfur batteries could potentially offer about five times the energy density of today’s typically used lithium-ion batteries,” said Yingchao Yu, Ph.D. student with Abruña, and co-first author on the JACS publication. “But a lithium-sulfur battery is not a stable system, as its capacity tends to fade over a short period of time.”

After about 50 charge cycles, the energy density of a lithium-sulfur battery decreases rapidly due to a phenomenon called the polysulfide shuttling effect, which is when the polysulfide chains in the battery’s cathode (positive end) dissolve in the electrolyte, the ionizing liquid that allows electrons to flow.

To combat this problem and stabilize the sulfur, the researchers used amylopectin, a polysaccharide that’s a main component of corn starch.

Top left, false-colored energy dispersive X-ray mapping of a sulfur-polyaniline core-shell nanocomposite, next to a scanning electron microscopy image of the core shells cracked after five cycles. Bottom left is a transmission electron microscopy image of a yolk-shell structure coating with polyaniline, and, right, its preserved morphology after five charge cycles.

ACS Nano - Amylopectin Wrapped Graphene Oxide/Sulfur for Improved Cyclability of Lithium–Sulfur Battery

Journal of the American Chemical Society - Yolk–Shell Structure of Polyaniline-Coated Sulfur for Lithium–Sulfur Batteries



After about 50 charge cycles, the energy density of a lithium-sulfur battery decreases rapidly due to a phenomenon called the polysulfide shuttling effect, which is when the polysulfide chains in the battery’s cathode (positive end) dissolve in the electrolyte, the ionizing liquid that allows electrons to flow.

To combat this problem and stabilize the sulfur, the researchers used amylopectin, a polysaccharide that’s a main component of corn starch.

“The corn starch can effectively wrap the graphene oxide-sulfide composite through the hydrogen bonding to confine the polysulfide among the carbon layers,” said Hao Chen, co-first author of the ACS Nano publication and a former Ph.D. student in the lab of Francis DiSalvo, paper co-author and the John A. Newman Professor of Chemistry and Chemical Biology. “As an additive, it greatly improves the cycling stability of the battery.”

Charge/discharge capacities vs cycle numbers of GO-S-Amy at different sulfur loadings. Credit: ACS, Zhou et al. 2013

In another approach to improving lithium-sulfur battery durability, the researchers also published a new way to make lithium-sulfur cathodes by synthesizing a nanocomposite, consisting of sulfur coated with a common, inexpensive, conductive polymer called polyaniline, and modeled after the way an egg is encased in a shell but has room to move around inside. Similar sulfur-polyaniline composites have previously been synthesized in a “core-shell” structure, but the new method provides an internal void within the polymer shell, called a “yolk-shell” structure.

“When the lithium-sulfur battery was fully discharged, the volume of the sulfur expanded dramatically to 200 percent. If you think about the beauty of an egg yolk, there is some empty space inside, with space for the yolk to expand,” Yu said.

The polyaniline coating, which chemically is a benzene ring with ammonium and strung into cross-linked chains, is also soft and flexible, and can protect against the “shell” cracking during long cycling.

Abstract

An amylopectin wrapped graphene oxide-sulfur composite was prepared to construct a 3-dimensionally cross-linked structure through the interaction between amylopectin and graphene oxide, for stabilizing lithium sulfur batteries. With the help of this cross-linked structure, the sulfur particles could be confined much better among the layers of graphene oxide and exhibited significantly improved cyclability, compared with the unwrapped graphene oxide-sulfur composite. The effect of the electrode mass loading on electrochemical performance was investigated as well. In the lower sulfur mass loading cells, such as 2 mg cm–2, both the capacity and the efficiency were obviously better than those of the higher sulfur mass loading cells, such as 6 mg cm–2.

5 pages of supporting info

Lithium–sulfur batteries have attracted much attention in recent years due to their high theoretical capacity of 1672 mAh g–1 and low cost. However, a rapid capacity fade is normally observed, attributed mainly to polysulfide dissolution and volume expansion. Although many strategies have been reported to prolong the cyclability, the high cost and complex preparation processes still hinder their practical application. Here, we report the synthesis of a polyaniline–sulfur yolk–shell nanocomposite through a heating vulcanization of a polyaniline–sulfur core–shell structure. We observed that this heating treatment was much more effective than chemical leaching to prepare uniform yolk–shell structures. Compared with its sulfur–polyaniline core–shell counterparts, the yolk–shell nanostructures delivered much improved cyclability owing to the presence of internal void space inside the polymer shell to accommodate the volume expansion of sulfur during lithiation. The yolk–shell material exhibited a stable capacity of 765 mAh g–1 at 0.2 C after 200 cycles, representing a promising future for industrial scale Li–S batteries.

5 pages of supporting material

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