In Situ TEM Study of Volume Expansion in Porous Carbon Nanofiber/Sulfur Cathodes with Exceptional High‐Rate Performance

Abstract

Although lithium sulfur batteries (LSBs) have attracted much interest owing to their high energy densities, synthesis of high-rate cathodes and understanding their volume expansion behavior still remain challenging. Herein, electrospinning is used to prepare porous carbon nanofiber (PCNF) hosts, where both the pore volume and surface area are tailored by optimizing the sacrificial agent content and the activation temperature. Benefiting from the ameliorating functional features of high electrical conductivity, large pore volume, and Li ion permselective micropores, the PCNF/A550/S electrode activated at 550 °C exhibits a high sulfur loading of 71 wt%, a high capacity of 945 mA h g −1 at 1 C, and excellent high-rate capability. The in situ transmission electron microscope examination reveals that the lithiation product, Li 2 S, is contained within the electrode with only ≈35% volume expansion and the carbon host remains intact without fracture. In contrast, the PCNF/A750/S electrode with damaged carbon spheres exhibits sulfur sublimation, a larger volume expansion of over 61%, and overflowing of Li 2 S, a testament to its poor cyclic stability. These findings provide, for the first time, a new insight into the correlation between volume expansion and electrochemical performance of the electrode, offering a potential design strategy to synthesize high-rate and stable LSB cathodes.

Novel 2D Sb2S3 Nanosheet/CNT Coupling Layer for Exceptional Polysulfide Recycling Performance

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capacity of 1675 mAh g −1 . They include: i) the inherently low electrical conductivity of S, i.e., 5 × 10 −30 S cm −1 at room temperature; ii) the formation of polysulfides leading to high overpotentials and low utilization of active materials [14] ; iii) the large volume expansion of ≈80% during lithiation, causing large mechanical stresses to build up on the cathode [15] ; and iv) the dissolution of intermediate polysulfide products in a shuttling effect, resulting in fast capacity degradation and poor Coulombic efficiencies. They include: i) the inherently low electrical conductivity of S, i.e., 5 × 10 −30 S cm −1 at room temperature; ii) the formation of polysulfides leading to high overpotentials and low utilization of active materials [14] ; iii) the large volume expansion of ≈80% during lithiation, causing large mechanical stresses to build up on the cathode [15] ; and iv) the dissolution of intermediate polysulfide products in a shuttling effect, resulting in fast capacity degradation and poor Coulombic efficiencies.

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Interfacial Incompatibility and Internal Stresses in All‐Solid‐State Lithium Ion Batteries

“…Taking S as an example, it was lithiated upon discharging, leading to a series of lithium polysulfides Li 2 S x (where x typically ranged from 4 to 5) and finally to Li 2 S, as has been exhibited from typical plateaus of the discharge curve. [305] The large strain arising from the dramatic volumetric change will produce pulverization and cause the structural collapse of the active cathode materials and then capacity fading of the batteries. [304] A volumetric expansion of 35% was inspected with the aid of in situ TEM examination when S was fully converted to lithiated products of Li 2 S in the porous carbon nanofiber/S electrode.…”

Nitrogen doped hierarchically structured porous carbon fibers with an ultrahigh specific surface area for removal of organic dyes

“…These featured characteristics make the porous carbon materials promising candidates for wastewater treatment, energy storage electrodes, electrochemistry etc. [16][17][18][19] However, the comparatively low SSA of carbon materials (around hundreds of square meters per gram) may seriously affect their performance. 16,20,21 Polybenzoxazine (PBZ), as a new kind of phenolic resin, has been widely used to fabricate porous carbon materials with hierarchical structures by virtue of a wide range of intriguing features including near-zero volumetric shrinkage, high glass-transition temperatures and high carbon yields.…”