Dynamic Lithium-Sulphur Cell Charging Control
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Solution Overview
Problem
Lithium-sulphur batteries face capacity fade and potential overcharging due to increasing internal resistance, leading to detrimental chemical reactions and reduced longevity, as conventional charging methods rely on fixed cut-off voltages that do not account for varying cell characteristics.
Innovation Solution
A method involving monitoring the voltage and capacity of lithium-sulphur cells during charging to determine a reference capacity at which the maximum dV/dt or dV/dQ occurs, allowing for dynamic termination of charging before the second stage, thereby reducing the risk of overcharging and capacity fade.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a fixed cut-off voltage is used for charging, then the charging process is simple to control, but the cell may be over-charged leading to detrimental chemical reactions and reduced longevity
Solution Approach 1:
The patent applies dynamics by transitioning from a static fixed cut-off voltage to a dynamic cut-off voltage that adapts based on the cell's charging characteristics. The cut-off voltage is determined by monitoring dV/dt during charging and identifying the inflexion point, then applying a scaling factor to establish a dynamic termination voltage. This allows the charging system to adapt to changing cell conditions and internal resistance, preventing over-charging while maintaining simple control operation.
Solution Approach 2:
The patent implements feedback by continuously monitoring the cell voltage and its rate of change (dV/dt) during charging. The system uses this feedback to identify the inflexion point in the charging curve and dynamically adjust the cut-off voltage accordingly. This closed-loop feedback mechanism ensures that charging terminates at the optimal point, preventing over-charging damage while accounting for variations in cell characteristics and aging.
2Ease of manufacture
If charging is terminated at a fixed voltage, then the charging method is easy to implement, but capacity fade occurs due to over-charging from increasing internal resistance
Solution Approach 1:
The patent applies parameter changes by modifying the charging termination parameter from a fixed voltage value to a dynamically calculated voltage based on dV/dt characteristics. The system monitors the rate of voltage change during charging, identifies the inflexion point where dV/dt reaches a maximum, and scales this reference voltage by a factor (1.05-1.15) to determine the optimal cut-off voltage. This parameter adaptation prevents capacity fade caused by over-charging while maintaining ease of implementation through automated monitoring.
3Productivity
If the cell is charged to a fixed cut-off voltage repeatedly, then the charging process is consistent and simple, but undesirable chemical reactions damage the electrodes and electrolytes
Solution Approach 1:
The patent applies preliminary action by proactively identifying the optimal charging termination point through dV/dt monitoring before over-charging damage can occur. The system continuously tracks the charging curve and detects the inflexion point in advance, then terminates charging at the scaled reference voltage before detrimental chemical reactions can damage the electrodes and electrolytes. This preventive approach maintains high charging efficiency while eliminating harmful over-charging effects.
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 effectively reduces the risk of overcharging by adjusting the cut-off voltage based on current charging characteristics, thereby extending the battery's lifespan by preventing undesirable chemical reactions.
Implementation Method 1
monitoring the voltage, V, of a cell during charge as a function of time (t) or capacity (Q), determining in a voltage region in which the cell transitions between the first stage and second stage of charge the reference capacity, Qref, of the cell at which dV/dt or dV/dQ is at a maximum
Implementation Method 2
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 3
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
Data Source
AI summary
A method for charging a lithium-sulphur cell, said method comprising: monitoring the voltage, V, of a cell during charge as a function of time, t, or capacity, Q, determining, in a voltage region in which the cell transitions between the first stage and second stage of charge, the reference capacity, Qref, of the cell at which dV/dt or dV/dQ is at a maximum, terminating charge when the capacity of the cell reaches a.Qref, where a is 1.1 to 1.4.