Sulfur, a light and abundant element capable of gaining multiple electrons, is a promising alternative cathode material for high-energy-density rechargeable batteries (i.e., Li–S battery) due to its high theoretical capacity of 1,672 mAh g⁻¹. However,the cycle life of existing Li–S batteries still suffers from significant capacity loss of their sulfur cathodes during cycling, due to dissolution and migration of the formed lithium polysulfide (LPS) intermediates (Li₂S_x, 4 ≤ x ≤8) during the battery cycling process.

Thus far, confining LPS has been regarded as one of the most effective ways of increasing sulfur electrode cyclability. … Another strategy is to insert a polysulfide diffusion barrier interlayer in the battery structure, which separates the polysulfide-confining function from the electrode itself and can thus be applicable to essentially any type of sulfur cathode material.

… In this work, we develop a composite thin film comprising naphthalimide-functionalized poly(amidoamine) (PAMAM) G4 dendrimer (Naph-Den) and mildly oxidized graphene oxide (mGO) as an LPS-confining interlayer to realize high-performance sulfur cathodes that can be stably cycled.

— Liu et al.

The three components of the film have distinct functions:

• the amide-containing dendrimer molecules effectively trap polysulfides via strong chemical binding which is further enhanced by the branched dendrimer structure;

• the terminal naphthalimide groups attached to the dendrimer structure interact with mGO via π–π stacking and enable composite formation; and

• the mGO nanosheets impart mechanical strength and durability to the 100-nm-thick film.

With a sulfur content of 76 wt % in the electrode material and a sulfur mass loading of 2 mg cm⁻² on the electrode, the coated cathode exhibits a capacity decay rate as low as 0.008% per cycle over 500 cycles. As a result of the negligible thickness/weight of the composite film, the battery energy density is hardly affected. According to the researchers, sulfur cathodes coated with the material can be stably discharged and recharged for more than 1,000 cycles, enhancing the battery’s efficiency and number of cycles.

Electrochemical performance of the Naph-Den-mGO/GO-S electrode compared with a GO–S electrode. (A) Representative discharging and charging voltage profiles for the Naph-Den–mGO/GO–S electrode at various rates. (B) Discharging specific capacities at various rates. (C) Cycling stability for 560 cycles at 1.0 C and consecutive 665 cycles at 2.0 C. The GO–S electrode material has a sulfur content of 65 wt %, and the sulfur mass loading on the cathode is 1 mg cm−2. Source: Liu et al.Click to enlarge.

Our approach is general in that [the film] can be integrated with virtually any kind of sulfur electrode to increase cycling stability. The developed film is so thin and light it will not affect the overall size or weight of the battery, and thus it will function without compromising the energy and power density of the device.

—Hailiang Wang, lead investigator of the study

The gel-like slurry can be readily coated as a 100-nanometer-thin film onto sulfur electrodes.

The corresponding authors of the study are Gary Brudvig, the Benjamin Silliman Professor and chair of chemistry, professor of molecular biophysics and biochemistry, and director of the Yale Energy Sciences Institute at Yale West Campus; Yale chemistry professor Victor Batista; and Wang.

Resources

• Wen Liu, Jianbing Jiang, Ke R. Yang, Yingying Mi, Piranavan Kumaravadivel, Yiren Zhong, Qi Fan, Zhe Weng, Zishan Wu, Judy J. Cha, Henghui Zhou, Victor S. Batista, Gary W. Brudvig, and Hailiang Wang (2017) “Ultrathin dendrimer–graphene oxide composite film for stable cycling lithium–sulfur batteries” PNAS doi: 10.1073/pnas.1620809114