Li-S Cathode Composition to Suppress Polysulfide Shuttle
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Solution Overview
Problem
Lithium-sulfur (Li-S) electrochemical cells face challenges with low coulombic efficiency, low sulfur cathode utilization, and short cycle life due to the 'shuttle' mechanism of polysulfide diffusion, which is not effectively mitigated by existing solutions like microporous carbon or graphene layers.
Innovation Solution
The use of a cathode comprising microporous carbon particles with a specific particle size distribution and elemental sulfur with a controlled oxidation degree, combined with an electrolyte containing at least one fluorinated carbonate, significantly reduces the formation of polysulfides and enhances the gravimetric capacity and cycle life of Li-S cells.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If microporous carbon is used in the cathode to reduce polysulfide formation, then the shuttle mechanism is reduced, but the reversible capacity is limited
Solution Approach 1:
The patent changes the particle size parameter of microporous carbon from conventional fine particles to larger particles with Dv50 between 1-5 μm. This parameter change reduces the total surface area, thereby reducing polysulfide formation and the shuttle mechanism, while maintaining adequate reversible capacity through the optimized particle size distribution
Solution Approach 2:
The patent applies local quality by creating a specific particle size distribution where larger particles (Dv50 1-5 μm) are used to reduce overall polysulfide formation, while the microporous structure within these particles provides localized sites for sulfur accommodation and electrochemical reactions
2Reliability
If sulfur is intercalated between graphene layers to prevent shuttle mechanism, then polysulfide diffusion is reduced, but the distance between layers must be precisely controlled
Solution Approach 1:
The patent extracts the sulfur accommodation function from the interlayer space of graphene and relocates it to the micropores of carbon particles. This eliminates the need for precise interlayer distance control while maintaining the ability to accommodate sulfur and prevent polysulfide diffusion
3Productivity
If long-chain lithium polysulfides are formed during discharge, then sulfur reduction is achieved, but they dissolve in electrolyte and cause shuttle mechanism
Solution Approach 1:
The patent converts the harmful dissolution of polysulfides in electrolyte by using the microporous carbon structure to adsorb and retain polysulfides within its pores. The micropores act as confinement sites that prevent polysulfide diffusion while maintaining the electrochemical activity of sulfur
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration achieves a substantial reduction in the 'shuttle' mechanism, leading to improved gravimetric capacity, extended cycle life, and increased energy density, with the Li-S cells exhibiting capacities over 600 mAh/g of sulfur and potentially up to 1200 mAh/g.
Implementation Method 1
the use of microporous carbon in the cathode of Li—S cells has been found to greatly reduce the amount of polysulfides in the electrolyte during the charge-discharge process, thanks to a specific mechanism through a 'quasi solid-state reaction'
Implementation Method 2
an electrolyte containing at least one fluorinated carbonate, significantly reduces the formation of polysulfides and enhances the gravimetric capacity and cycle life of Li-S cells
Implementation Method 3
During discharge of the cell, elemental solid sulfur at the cathode reduces into lithium sulfide Li2S. Lithium metal (or the lithium alloy) oxidizes at the anode
Implementation Method 4
A separator prevents electrical contact between the cathode and the anode but allows ionic transport between the cathode and the anode
Data Source
AI summary
An electrochemical cell of the lithium/sulfur type comprising:a) a cathode comprising:i) microporous carbon in the form of particles, the particle size distribution being characterized by a median volume diameter Dv50 of the particles which is greater than or equal to 1 μm and less than or equal to 5 μm;ii) sulfur having an oxidation degree in the range of 0 to −2 in the pores of the microporous carbon;b) an electrolyte comprising a solvent, said solvent comprising at least one fluorinated carbonate;c) an anode composed of lithium metal or of a lithium metal alloy.The electrochemical cell exhibits an improved specific gravimetry and an improved life cycle.