Structural Design of High Surface Area Core-Shell Carbon Fibers for All-Solid-State Li-S Batteries with Enhanced Reaction Kinetics

Northeastern University Background
All-solid-state lithium-sulfur batteries (ASSLSBs) have attracted attention due to their remarkable safety, and ultrahigh theoretical energy density (2600 Wh kg-1) which is ten times higher than that of current commercial lithium-ion batteries. In comparison with Li-S batteries which use organic liquid electrolytes, ASSLSBs improve cycling stability by successfully avoiding the active material loss caused by shuttle effects of polysulfides. However, the energy densities of current ASSLSBs are still much lower than the theoretical value. The most significant challenge in ASSLSBs is the sluggish charge transfer, including electrons and ions, resulting in high overpotential, low voltage efficiency, poor energy efficiency, and low sulfur utilization. Additionally, due to the huge volume expansion and shrinking of sulfur in the cathode during cycling, the electrodes are eroded, resulting in a fast capacity decay. Also, the lithium-ion transfer inside the sulfur electrode and at the sulfur/electrolyte interface is another main limitation due to the point-point contact in solid electrolyte. Therefore, it is crucial to create advanced cathode electrodes with improved electron and ion transfer in ASSLSBs. 
Technology Overview
For the first time, Northeastern University researchers unveiled the structural design criteria of porous carbon used in ASSLSBs. The ideal porous carbon should meet the following requirements: 

A large specific surface area 
Pores localized at the surface 
Easy to form percolation. 

Polyacrylonitrile-derived porous carbon fibers (PPCF) meeting the above requirements were successfully prepared through electrospinning. Each has its own unique core-shell structure where a layer of the micropores is covers the dense core. A high specific surface area of 1519 m2 g-1 was achieved, and the micropores effectively confine the sulfur avoiding the formation of detrimental bulky sulfur. 
More importantly, the pores were distributed on the outer surface which rendered remarkable ion access to sulfur, contributing to high sulfur utilization. This fibrous morphology in turn enabled efficient electron conduction paths through effective percolation. Compared with the vapor-grown carbon fibers (VGCF), common carbon fiber with no surface pores, the cathode using PPCF exhibited improved reaction kinetics with an overpotential reduction by 149 mV.
Benefits

Reduced overpotential that results in higher utilization of sulfur in S-PPCF-SE
Improved reaction kinetics 
High and stable capacity
Enhanced charge transfer and electrochemical reaction kinetics
Accelerated electron conduction in the entire electrode
Remarkable rate and cycling performances of ASSLSBs utilizing PPCF as carbon additives

Applications
High energy density batteries for: 

Portable electronics
Electrical vehicle
Grid station energy storage 

Opportunity

Research Collaboration
Licensing
Commercial partner

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