Lithium-Sulphur Cell Cycling Method for Capacity Fade Reduction
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Solution Overview
Problem
Lithium-sulphur cells experience capacity fade due to over-charging, leading to increased internal resistance and degradation, as existing methods like monitoring voltage for sharp increases may not be applicable to all cells.
Innovation Solution
A method involving discharging lithium-sulphur cells to a threshold voltage of 1.5 to 2.1V and charging to a threshold voltage of 2.3 to 2.4V, with the option to under-charge and under-discharge to reduce non-conducting species deposition, thereby minimizing resistance and capacity fade.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If the cell is charged to a fixed cut-off voltage (2.45-2.8V), then the cell capacity is maximized, but the cell experiences over-charging and capacity fade due to increased internal resistance and degradation
Solution Approach 1:
The patent changes the voltage parameter from a fixed high cut-off (2.45-2.8V) to a dynamic range (2.0-2.4V) based on charge state monitoring. By adjusting the charging voltage threshold dynamically and monitoring charge capacity, the system prevents over-charging while maintaining effective capacity utilization, thereby reducing internal resistance buildup and extending cell life.
2Measurement precision
If voltage monitoring is used to detect full charge (sharp voltage increase), then charging termination is precise, but the method is not applicable to all lithium-sulphur cells that do not exhibit sharp voltage increases
Solution Approach 1:
The patent creates a universal charging termination method that works for all lithium-sulphur cells regardless of their specific voltage profile characteristics. By combining voltage threshold monitoring (2.0-2.4V range) with charge capacity tracking, the system achieves reliable full-charge detection for cells with or without sharp voltage increases, making the method universally applicable across different cell designs and chemistries.
3Productivity
If repeated charging to fixed cut-off voltage is performed, then the cell reaches full capacity each cycle, but the cell becomes over-charged and experiences detrimental chemical reactions leading to electrode and electrolyte degradation
Solution Approach 1:
The patent implements feedback control by continuously monitoring both voltage and charge capacity during charging. When the voltage reaches the 2.0-2.4V threshold or charge capacity indicates full charge, the system automatically terminates charging. This feedback mechanism prevents over-charging and the associated harmful chemical reactions that would otherwise degrade electrodes and electrolyte, while maintaining high charging efficiency through optimized charge termination.
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 approach reduces the rate of capacity fade by maintaining lower resistance and prolonging the cell's lifespan through controlled charge and discharge cycles.
Implementation Method 1
When a lithium-sulphur cell is discharged, the sulphur in the cathode is reduced in two-stages. In the first stage, the sulphur (e.g. elemental sulphur) is reduced to polysulphide species, Sn2− (n≥2). These species are generally soluble in the electrolyte. In the second stage of discharge, the polysulphide species are reduced to lithium sulphide, Li2S
Implementation Method 2
When the cell is charged, the two-stage mechanism occurs in reverse, with the lithium sulphide being oxidised to lithium polysulphide and thereafter to lithium and sulphur
Implementation Method 3
In the first stage, the sulphur (e.g. elemental sulphur) is reduced to polysulphide species, Sn2− (n≥2). These species are generally soluble in the electrolyte
Data Source
AI summary
A method for cycling a lithium-sulphur cell, said method comprising discharging a lithium-sulphur cell, terminating the discharge when the voltage of the cell reaches a threshold discharge voltage that is in the range of 1.5 to 2.1V, charging the lithium-sulphur cell, and terminating the charge when the voltage of the cell reaches a threshold charge voltage that is in the range of 2.3 to 2.4V, wherein the lithium-sulphur cell is not fully charged at the threshold charge voltage, and wherein the lithium-sulphur cell is not fully discharged at the threshold discharge voltage.