Thermal runaway in lithium-ion batteries represents a critical safety concern characterized by uncontrolled temperature and pressure increases leading to catastrophic failure. Early detection of warning signs is essential for preventing hazardous events. Several measurable indicators precede thermal runaway, each tied to specific electrochemical and physical degradation processes. Understanding these signals, their origins, and their temporal progression enables timely intervention.

Voltage drop serves as one of the earliest detectable indicators of impending thermal runaway. During normal operation, lithium-ion cells maintain stable voltage within defined limits. However, internal short circuits, lithium plating, or electrode degradation disrupt this equilibrium. When localized heating initiates due to a short, the affected region experiences increased resistance, causing a sudden voltage decline. This phenomenon occurs because the short circuit creates an alternative current path, diverting energy from the main circuit and reducing usable voltage. Research shows that voltage drop can precede thermal runaway by several minutes, providing a critical window for mitigation.

Temperature rise follows as a secondary but equally critical warning sign. Exothermic reactions begin when the internal temperature exceeds safe thresholds, typically around 90°C to 120°C. At this stage, the solid electrolyte interphase (SEI) layer decomposes, reacting with the electrolyte to release heat. As temperatures climb further, anode materials react with the electrolyte, and cathode decomposition accelerates. These reactions form a feedback loop: each exothermic process raises the temperature, triggering more severe reactions. Temperature sensors placed near hot spots can detect abnormal increases seconds to minutes before thermal runaway becomes irreversible.

Gas generation accompanies temperature rise due to electrolyte decomposition. Common gases include carbon dioxide, carbon monoxide, hydrogen, and hydrocarbons, produced when organic solvents break down under heat. Pressure buildup within the cell often precedes venting, making gas sensors an effective detection method. For instance, ethylene and dimethyl carbonate decomposition begins around 120°C, while more severe breakdown occurs above 200°C. Gas sensors capable of detecting specific compounds can identify these precursors before venting or rupture occurs. The timing varies but typically falls within a few minutes of the initial temperature rise.

Acoustic emissions provide another early signal tied to mechanical changes within the cell. As internal pressure builds, structural components such as separators and casings undergo stress, producing high-frequency sounds. Microphones or piezoelectric sensors can detect these acoustic waves, which often manifest before visible swelling or venting. Additionally, the breakdown of materials like lithium metal or delamination of electrodes generates distinct acoustic signatures. Studies indicate that these emissions may precede catastrophic failure by seconds to minutes, depending on the failure mechanism.

The sequence of these warning signs follows a predictable pattern tied to the stages of thermal runaway. Initially, voltage drop indicates internal faults, followed by gradual temperature increases as exothermic reactions begin. Gas emissions and acoustic signals emerge as decomposition accelerates, culminating in rapid temperature spikes and cell failure. The exact timing depends on factors like cell chemistry, state of charge, and external conditions, but the general progression remains consistent.

Detection systems leveraging multiple signals improve reliability. Voltage and temperature monitoring form the first line of defense, while gas and acoustic sensors add redundancy. Advanced battery management systems integrate these inputs to trigger alarms or disconnection before thermal runaway becomes unstoppable. For example, a sudden voltage drop combined with rising temperatures may activate cooling systems or isolate the cell from the pack.

Understanding the electrochemical basis of these signals enhances detection accuracy. SEI decomposition, lithium plating, and cathode breakdown each produce distinct thermal and gas profiles. Nickel-rich cathodes, for instance, release oxygen at high temperatures, while lithium metal anodes exhibit more pronounced acoustic activity due to dendrite formation. Tailoring detection methods to specific chemistries improves sensitivity and reduces false positives.

In summary, early warning systems for thermal runaway rely on monitoring voltage, temperature, gas emissions, and acoustic signals. Each indicator corresponds to specific degradation mechanisms, offering a multi-layered approach to prevention. By recognizing these signs and their temporal relationships, safety systems can intervene before catastrophic failure occurs. The integration of real-time data analytics further refines detection, enabling proactive measures in high-risk applications such as electric vehicles and grid storage.

The development of robust early warning systems remains an active area of research, with advancements in sensor technology and machine learning improving predictive capabilities. Future systems may incorporate even earlier indicators, such as subtle impedance changes or localized strain measurements, to extend the detection window. For now, a combination of existing signals provides a reliable foundation for thermal runaway prevention.