Expanded Graphite Cathode for Lithium-Sulphur Battery Polysulfide Retention
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Solution Overview
Problem
Lithium-sulphur batteries face issues with polysulfides diffusing from the cathode to the anode, leading to a loss of active material and reduced capacity, and require an electrically conductive agent to overcome the insulating nature of sulphur, which can lower discharge voltage and energy density.
Innovation Solution
A solid composite for the cathode comprising 1-75 wt.% expanded graphite or graphene, 25-99 wt.% sulphur, and optional conductive agents and binders, which improves sulphur retention and contact with the current collector and electrolyte, eliminating the need for metal oxides and enhancing cycle stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal oxides are used to hold polysulfides within the cathode, then polysulfide retention is improved, but discharge voltage is reduced and gravimetric energy density is lower
Solution Approach 1:
The patent introduces a porous carbon material as an intermediary substance that physically traps polysulfides within its pore structure. This carbon mediator holds polysulfides through physical confinement rather than chemical bonding, avoiding the voltage reduction caused by metal oxides while maintaining polysulfide retention. The porous structure acts as a bridge between the cathode matrix and polysulfide molecules.
Solution Approach 2:
The patent creates a composite cathode structure combining sulfur, conductive carbon material, and porous carbon material. This composite approach allows the porous carbon to provide physical confinement for polysulfides while the conductive carbon maintains electrical conductivity and the sulfur provides active material, achieving polysulfide retention without the drawbacks of metal oxide additives.
2Use of energy by moving object
If sulphur is used as the main cathode material, then theoretical specific energy is four times higher than Li-ion batteries, but polysulfides are soluble in electrolyte and diffuse to anode causing capacity loss
Solution Approach 1:
The patent employs porous carbon material with controlled pore size and distribution to physically confine polysulfides. The porous structure provides a three-dimensional network that traps polysulfides within the cathode, preventing their dissolution and diffusion into the electrolyte. This maintains the high energy density of sulfur while solving the polysulfide shuttle problem.
Solution Approach 2:
The porous carbon material serves as an intermediary between the sulfur active material and the electrolyte, creating a physical barrier that prevents direct contact between polysulfides and the bulk electrolyte. This intermediary layer allows ionic transport while blocking polysulfide diffusion, maintaining both high capacity and long cycle life.
3Reliability
If conventional carbon materials are used as conductive agents, then electrical conductivity is improved, but contact with current collector and electrolyte is insufficient
Solution Approach 1:
The patent uses porous carbon material that provides both electrical conductivity and enhanced surface area for contact. The porous structure creates numerous contact points with the current collector and electrolyte, improving interfacial contact while maintaining electrical conductivity. The three-dimensional porous network facilitates better ion transport and electrical connection compared to conventional flat carbon materials.
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
The composite allows for higher sulphur usage and longer cycle stability, maintaining high discharge voltage and specific capacity, with expanded graphite providing a cost-effective and environmentally friendly solution.
Implementation Method 1
the use of a solid composite for a cathode of a lithium-sulphur battery which comprises 1 to 75 wt.-% of expanded graphite and 25 to 99 wt.-% of sulphur
Implementation Method 2
improves sulphur retention and contact with the current collector and electrolyte
Implementation Method 3
the sulphur is initially reduced to polysulfides like Li2S8, Li2S6, Li2S4, and Li2S3
Implementation Method 4
In the discharge modus Li0 dissociates into an electron and a Li+-ion
Implementation Method 5
the good solubility of the polysulfides in the electrolyte which may diffuse from the cathodic region into the anodic region
Implementation Method 6
The Li+-ion is removed from the electrolyte and precipitated on the anode, thereby. This called lithium plating
Data Source
AI summary
The present invention relates to a solid composite for use in the cathode of a lithium-sulphur electric current producing cell wherein the solid composite comprises 1 to 75 wt.-% of expanded graphite, 25 to 99 wt.-% of sulphur, 0 to 50 wt.-% of one or more further conductive agents other than expanded graphite, and 0 to 50 wt.-% one or more binder, based on the total amount of the solid composite, a lithium-sulphur electric current producing cell comprising (i) a cathode comprising the solid composite, (ii) an anode and (iii) an electrolyte interposed between said cathode and said anode, and a process for preparing the solid composite comprising the steps (I) preparing a slurry comprising sulphur, expanded graphite, and optionally further components in a liquid medium by dispersing the sulphur, the expanded graphite, and optionally the further components in the liquid medium; (II) casting the slurry provided in step (I) on a substrate or placing the slurry provided in step (I) into a mold; and (III) removing some or all of the liquid medium from the slurry cast in step (II) to form a solid composite.