Lithium-Ion Cell Diagnostics for Early SEI Layer Degradation Detection
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Solution Overview
Problem
Lithium-ion battery systems in electrified vehicles face issues due to the thickening of the solid electrolyte interphase (SEI) layer, leading to reduced performance, cycle life, and potential internal shorting, which can cause thermal events and cell destruction.
Innovation Solution
A method to determine the state-of-health of each cell in a lithium-ion battery pack by modeling each cell using a relationship involving terminal voltage, current, and their derivatives, and estimating coefficients to diagnose unacceptable SEI layer health, internal resistance, and charge transfer resistance, allowing for early detection of degradation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If the lithium-ion battery operates for extended service life, then the SEI layer naturally forms and provides protection, but the SEI layer thickens over time causing increased internal resistance and reduced cell performance
Solution Approach 1:
The patent applies preliminary action by detecting SEI layer thickening through voltage derivative analysis before it causes significant performance degradation or thermal events. The system continuously monitors the first and second derivatives of terminal voltage to identify early signs of abnormal SEI growth, enabling preventive intervention before the thickening leads to cell failure or safety issues.
2Loss of energy
If the SEI layer is allowed to thicken naturally during aging, then the battery maintains initial low internal resistance, but excessive thickening leads to localized heating and potential thermal events
Solution Approach 1:
The patent implements feedback by continuously monitoring the relationship between terminal voltage derivatives and current derivatives to detect abnormal SEI layer growth patterns. When the analysis indicates excessive thickening that could lead to thermal events, the system provides feedback signals to the battery management system, enabling real-time adjustments to charging/discharging rates or thermal management strategies to prevent harmful thermal events.
3Measurement precision
If advanced diagnostic methods are implemented to detect SEI layer thickening, then early intervention opportunities are provided, but the system complexity and measurement requirements increase
Solution Approach 1:
The patent uses the relationship between voltage derivatives and current derivatives as an intermediary to indirectly measure SEI layer thickness without requiring direct physical measurement of the SEI layer. By analyzing how changes in terminal voltage respond to changes in current through the mathematical relationship dVT/dt = -a1*dIb/dt - a3*Ib, the system derives SEI layer state information from standard voltage and current measurements already available in typical battery management systems.
Data Source
AI summary
A Thevenin equivalent model of a lithium-ion battery cell provides the basis for a simplified cell diagnostic relying on cell current and cell terminal voltage measurements.