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SINANO team demonstrates sulfur’s theoretical capacity in Li-S cells using ultra-small nanoparticles at low discharging rate

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Theoretical discharging behavior and high cycle stability of 5 nm NPs. (a) Discharge/charge profile of 5 nm S NPs at 0.1 C. (b) XRD patterns of discharge products of 20 and 5 nm S NPs discharged to 1.0 V. (c) XPS spectra of discharge products of 5 nm S NPs at DoD of 72 and 100%. (d) Cycling performance of 5 nm SNPs at 0.5 C and 1 C. Credit: ACS, Chen et al. Click to enlarge.

A team at the Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO) has demonstrated that the electrochemical performance of sulfur nanoparticles (NPs) in the cathode of a Lithium-sulfur battery is critically dependent on the sulfur particle size. Further, they demonstrated that sulfur’s theoretical discharging behavior can be experimentally realized with ultra-small sulfur nanoparticles.

In a paper published in the ACS journal Nano Letters, they report that 5 nm S NPs display sulfur’s theoretical discharging/charging capacity of 1672 mAh g–1 at a 0.1 C rate and a discharge capacity of 1089 mAh g–1 even at 4 C. Specific capacity remained at 1017 and 965 mAh g−1 after 500 cycles at 0.5 C and 1 C, respectively.

Sulfur is a promising cathode material with a high theoretical specific capacity of 1672 mAh g-1. When paired with lithium metal anode to form rechargeable lithium−sulfur (Li−S) batteries, the theoretical specific energy density can be as high as 2600 Wh kg−1. These estimations have inspired high expectations in meeting future large-scale energy storage needs; however, low sulfur utilization, short cycle life, and low Coulombic efficiency have hindered practical application of Li−S batteries.

A major bottleneck is the slow charge transfer kinetics resulting from poor electronic conductivity of sulfur and the final discharge product, Li2S. Similar challenges were encountered in LiFePO4-based cathode materials of lithium ion battery and are successfully overcome via preparation of nanosized particles with conductive carbon coating. The lithium sulfur battery field has made remarkable progresses via a related approach, that is, by using nanostructured carbon−sulfur composite cathode materials, which typically consist of sulfur active material infiltrated in nanoporous conducting carbon frameworks. However, unlike the intercalation/deintercalation process in LiFePO4, the Li−S system is based on complicated multistep redox reactions between lithium and sulfur involving a solid−solution−solid transition. The influence of materials size in such a complicated battery system has not been investigated and thoroughly understood.

… Because these processes can be profoundly affected by the size of sulfur materials, we hypothesize that there exists nanosize effect in Li−S batteries, that is, the size of sulfur particles may significantly impact the discharging behavior. … small sulfur particle size may lead to better sulfur utilization, and the larger specific surface area leads to smaller effective current density and enables faster discharging kinetics. The trade-off between high specific capacity and high rate performance may thus be resolved in ultra-small sulfur nanoparticles.

—Chen et al.

In the study, the SINANO team first synthesized a series of mono-dispersed sulfur nanoparticles with different diameters (5, 10, 20, 40 and 150 nm) on reduced graphene oxide (rGO). They then assembled the rGO-S nanoparticle composites in CR-2025 coin cells to examine electrochemical performance.

Testing showed that sulfur nanoparticles exhibit higher specific capacity and better rate performance with decreasing particle size. Large particle size with a high discharging rate resulted in high Li+ concentrations at the surface of the sulfur particle, with the resulting formation of an insulating Li2S2 and Li2S blocking layer.

Small sulfur particle size and low discharging rate allowed for Li+ diffusion into the interior of sulfur particle, thereby delaying the formation of the insulating Li2S2 and Li2S and improving the sulfur utilization.

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Electrochemical performance of S NPs in Li−S batteries. (a) Initial galvanostatic discharge/charge profiles under 0.25 C, (b) electrochemical impedance spectra of as-prepared batteries, (c) cycling performance and Coulombic efficiency under 0.25 C, (d) rate performance at current density from 0.1 to 4 C. Credit: ACS, Chen et al.Click to enlarge.

These results indicate that the charge transfer and mass transport in Li−S cathode is a kinetic process highly dependent on the particle size and discharge/charge rate. Reducing the size of sulfur particles down to a few nanometers can greatly improve the utilization of sulfur and enhance the cycle stability and rate performance of Li−S batteries.

—Chen et al.

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Schematic illustrations of the nanosize effect in Li−S cathodes. (a) Theoretical and typical experimental discharge curves of S8-based cathodes. (b) Dissolution of S8 in discharge stage I, loss of electrical contact with electrode in large S particles causes incomplete utilization of S; (c) diffusion of polysulfides in discharge stage I and II; (d) reduction of polysulfides to Li2S2 and Li2S, large particle size may block the electron transfer; (e) large flux of Li+ forms a Li2S blocking layer on S particle surface although this effect is less severe in small particles due to large specific surface area; and (f) formation of Li2S blocking layer on Li2S2 surface, similar to the process in (e).

Credit: ACS, Chen et al. Click to enlarge.

Resources

  • Hongwei Chen, Changhong Wang, Weiling Dong, Wei Lu, Zhaolong Du, and Liwei Chen (2014) “Monodispersed Sulfur Nanoparticles for Lithium–Sulfur Batteries with Theoretical Performance” Nano Letters doi: 10.1021/nl504963e

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