The rapid evolution of energy storage systems is pivotal for the global transition towards sustainable energy. Among various technologies, the lithium-ion battery stands out due to its high energy density, extended cycle life, and declining cost, making it the cornerstone for applications ranging from portable electronics to electric vehicles and grid-scale storage. However, the widespread adoption of lithium-ion battery technology is persistently shadowed by critical safety concerns. Catastrophic failures, often culminating in fire or explosion, pose significant risks. These incidents are frequently triggered under abuse conditions—thermal, electrical, or mechanical—that drive the battery into an uncontrollable self-heating state known as thermal runaway (TR).
Mechanical abuse, such as penetration from collisions or external impact, represents one of the most severe and instantaneous triggers for thermal runaway in a lithium-ion battery. It directly breaches the internal structure, causing a hard internal short circuit (ISC) with very low resistance. This event unleashes a complex and violent chain of electrochemical and thermal reactions. Understanding the precise evolution from the initial short circuit to full-scale thermal runaway is essential for developing effective safety management systems. This article delves into the characteristics of nail penetration-induced thermal runaway, a standardized test method for mechanical abuse. Through experimental analysis and coupled electro-thermal modeling, we delineate the stages of failure, identify critical intervention points, and systematically evaluate the influence of key factors like state of charge (SOC) and cooling conditions on the progression and severity of thermal runaway in a lithium-ion battery.
Experimental Investigation of Nail Penetration Thermal Runaway
The nail penetration test serves as a critical experimental method to simulate a severe, localized internal short circuit in a lithium-ion battery. In our experimental setup, a prismatic lithium-ion battery with a Lithium Cobalt Oxide (LCO) cathode was subjected to penetration by a steel nail at a controlled speed. The battery’s voltage and surface temperatures at multiple points were recorded synchronously with high-speed imaging to capture the failure dynamics.
The experimental data reveals a distinct, four-stage evolution of thermal runaway in the lithium-ion battery following nail penetration:
- Stage 1: Structural Breach. The nail makes initial contact and penetrates the battery casing. During this phase, the internal cell components are compressed but not yet severely compromised. No significant change in voltage or temperature is observed.
- Stage 2: Direct Internal Short Circuit. The nail tip fully breaches the electrode stack, creating a direct, low-resistance electrical path between the cathode and anode. This results in an abrupt voltage drop. The localized Joule heating at the short-circuit point initiates rapid temperature rise at the penetration site. However, the battery exterior remains largely unchanged visually.
- Stage 3: Widespread Indirect Internal Short Circuit. This is the pivotal escalation phase. The intense local heat from Stage 2 causes the separator material around the nail to melt and shrink. Concurrently, the heat triggers decomposition of the Solid Electrolyte Interphase (SEI) and other exothermic reactions, generating gas and causing the battery to swell. The combined effect of separator failure and mechanical stress from swelling creates extensive new internal short circuits far beyond the original nail path. This is marked by a second, steeper voltage collapse (e.g., a drop rate exceeding 1 V/s) and a violent jet of ejecta from the battery. The surface temperature rise rate reaches its global maximum during this stage.
- Stage 4: Delamination and Complete Failure. The exothermic reactions propagate throughout the entire lithium-ion battery. The electrode layers delaminate, and the cell experiences massive expansion. Voltage falls to near zero, and the ejection of smoke and particles continues at a diminishing rate until the cell is fully destroyed.
Post-test analysis of the failed lithium-ion battery confirms this progression. The electrode jellyroll shows severe deformation not only at the penetration point but also in remote areas, with visible cracks and layer separation, evidencing the widespread nature of the final failure.
Temperature and Voltage Signature Analysis
The synchronized voltage and temperature profiles provide a diagnostic signature for thermal runaway in a lithium-ion battery. The voltage curve shows two characteristic drops: a first drop corresponding to the direct short (Stage 2) and a catastrophic second drop signaling the widespread short (Stage 3).
More critically, the analysis of the temperature rise rate (dT/dt) unveils two distinct peaks, as conceptually summarized below:
| Peak | Magnitude | Corresponding Stage | Primary Heat Source |
|---|---|---|---|
| First Peak | Moderate (e.g., ~55 °C/s) | Stage 2: Direct ISC | Joule heating from the nail-induced short circuit. |
| Second Peak | High (e.g., ~100 °C/s) | Stage 3: Widespread ISC | Combined Joule heating from multiple shorts and intense exothermic chemical reactions (SEI decomposition, anode-electrolyte reaction, etc.). |
The second peak is typically significantly larger than the first. This indicates that the heat generated by the triggered chemical chain reactions far surpasses the initial electrical fault’s heating. This distinction is crucial for safety management. The period surrounding the first temperature rise rate peak is the critical window for intervention. Once the battery progresses beyond this point into the chemical reaction-dominated Stage 3, the process becomes extremely difficult to arrest, leading inevitably to catastrophic thermal runaway of the lithium-ion battery.
Electro-Thermal Coupling Model for Simulation
To quantitatively analyze the influencing factors, a coupled electro-thermal model for a lithium-ion battery under internal short circuit was established. The model integrates an electrical circuit with a thermal network.
Electrical Model: The battery is represented by its open-circuit voltage $U_{oc}$ and ohmic resistance $R_{cell}$. The nail penetration is modeled as a fixed short-circuit resistance $R_{short}$ connected directly across the terminals. The terminal voltage $U_t$ is given by:
$$U_t = U_{oc} – I_{short}R_{cell} – I_{short}R_{short}$$
where $I_{short}$ is the short-circuit current. The heat generation rate $Q_{short}$ from this electrical fault is:
$$Q_{short} = (U_{oc} – U_t)I_{short} = I_{short}^2 (R_{cell} + R_{short})$$
Thermal Model: A lumped thermal model considers the battery’s internal temperature $T_c$ and surface temperature $T_s$. The heat balance includes internal heat generation, conduction resistance $R_{in}$ between core and surface, and convective cooling $h$ (or thermal resistance $R_{out}$) to the ambient at $T_{air}$.
$$\frac{dT_c}{dt} = \frac{1}{C_{in}} \left( Q_{total} – \frac{T_c – T_s}{R_{in}} \right)$$
$$\frac{dT_s}{dt} = \frac{1}{C_{out}} \left( \frac{T_c – T_s}{R_{in}} – \frac{T_s – T_{air}}{R_{out}} \right)$$
where $C_{in}$ and $C_{out}$ are thermal capacitances, and $R_{out} = 1/(hA)$.
Exothermic Reaction Model: The total heat $Q_{total}$ is the sum of electrical heat $Q_{short}$ and chemical reaction heat $Q_{chem}$. The major exothermic reactions in a lithium-ion battery are modeled using Arrhenius equations:
- SEI Decomposition (starts ~70-120°C):
$$ \frac{dc_{sei}}{dt} = -A_{sei} c_{sei}^{m_{sei}} \exp\left(\frac{-E_{a,sei}}{RT_c}\right); \quad Q_{sei} = H_{sei} W_c \left(-\frac{dc_{sei}}{dt}\right) $$ - Anode-Electrolyte Reaction (starts >120°C):
$$ \frac{dc_{neg}}{dt} = -A_{neg} c_{neg}^{m_{neg}} \exp\left(\frac{-E_{a,neg}}{RT_c}\right); \quad Q_{neg} = H_{neg} W_c \left(-\frac{dc_{neg}}{dt}\right) $$ - Electrolyte Decomposition (starts >200°C):
$$ \frac{dc_{e}}{dt} = -A_{e} c_{e}^{m_{e}} \exp\left(\frac{-E_{a,e}}{RT_c}\right); \quad Q_{e} = H_{e} W_e \left(-\frac{dc_{e}}{dt}\right) $$ - Cathode-Electrolyte Reaction (starts >170°C):
$$ \frac{d\alpha}{dt} = A_{pe} (1-\alpha)^{m_{pe}} \exp\left(\frac{-E_{a,pe}}{RT_c}\right); \quad Q_{pe} = H_{pe} W_p \frac{d\alpha}{dt} $$
The model parameters are listed below:
| Parameter | Symbol | Value |
|---|---|---|
| SEI Decomposition Pre-exponential Factor | $A_{sei}$ | 1.667 × 1015 s-1 |
| Anode Reaction Pre-exponential Factor | $A_{neg}$ | 2.5 × 1013 s-1 |
| Electrolyte Decomposition Pre-exponential Factor | $A_{e}$ | 5.14 × 1025 s-1 |
| SEI Decomposition Activation Energy | $E_{a,sei}$ | 135.08 kJ/mol |
| Anode Reaction Activation Energy | $E_{a,neg}$ | 135.08 kJ/mol |
| Electrolyte Decomposition Activation Energy | $E_{a,e}$ | 274 kJ/mol |
| SEI Reaction Enthalpy | $H_{sei}$ | 257 J/g |
| Anode Reaction Enthalpy | $H_{neg}$ | 1714 J/g |
This comprehensive model was validated against experimental data, showing good agreement in predicting the surface temperature trajectory and the onset of thermal runaway in the lithium-ion battery.
Impact of State of Charge (SOC)
The State of Charge is arguably the most significant factor determining the severity and probability of thermal runaway in a lithium-ion battery. Simulations were conducted for SOC levels of 100%, 80%, 50%, 20%, and 0%.
The results demonstrate a dramatic dependence:
$$ T_{max} \propto SOC $$
For the high SOC (100%) condition, the simulated core temperature exceeded 500°C, indicative of violent thermal runaway. As the SOC decreased, the peak temperatures dropped substantially. At 20% SOC, the peak temperature was significantly lower (~225°C), and the heating process was much slower. At 0% SOC, the temperature rise was minimal, and no exothermic reactions were triggered beyond minor SEI decomposition.
The underlying mechanism is directly linked to the availability of reactive lithium. The reaction kinetics for the anode-electrolyte and other exothermic processes depend on the concentration of intercalated lithium $c_{neg}$:
$$ \frac{dc_{neg}}{dt} = f(SOC, T) $$
At lower SOC, $c_{neg}$ is smaller, and the chemical reaction rates are inherently slower. This delays and reduces the total heat release from $Q_{chem}$, allowing more time for heat dissipation and preventing the self-accelerating feedback loop necessary for full thermal runaway in the lithium-ion battery.
Impact of Heat Transfer Conditions
Thermal management systems aim to enhance heat dissipation, typically by increasing the effective heat transfer coefficient $h$. The simulation analyzed the effect of varying $h$ for both high-SOC (100%) and medium/low-SOC (25%) lithium-ion battery scenarios.
The findings reveal a nuanced impact:
- For High-SOC Lithium-ion Batteries: Increasing the cooling coefficient ($h$ from 10 to 60 W/m²K) had a negligible effect on the maximum internal core temperature, which remained near 510°C. Its primary effect was to significantly lower the surface temperature and steepen the internal-to-surface temperature gradient. This is because once triggered, the exothermic reactions in a high-SOC cell are so intense and rapid that external cooling cannot extract heat quickly enough to quench the internal reaction front.
- For Medium/Low-SOC Lithium-ion Batteries: Enhancing heat transfer had a pronounced and critical effect. For the 25% SOC case, increasing $h$ from 10 to 60 W/m²K reduced the peak core temperature from ~275°C to ~230°C and accelerated the cooling phase, effectively preventing a full thermal runaway event. The slower reaction kinetics at lower SOC allow external cooling to compete with internal heat generation, making thermal management an effective mitigation strategy.
This dichotomy highlights a crucial safety design principle: for a lithium-ion battery already at high risk (e.g., damaged at high SOC), conventional cooling may be insufficient to stop thermal runaway. However, if the reaction kinetics can be slowed first—most effectively by rapidly reducing the SOC—then enhanced cooling becomes highly effective.
Conclusions and Implications for Safety Management
This investigation into the nail penetration-induced failure of a lithium-ion battery elucidates a clear four-stage progression from structural breach to complete thermal runaway. The identification of the two characteristic peaks in the temperature rise rate signature is vital. The first peak, associated with the initial direct internal short circuit, represents the final practical window for intervention in the failure of a lithium-ion battery.
The parametric studies lead to two fundamental conclusions for the safety of a lithium-ion battery system:
- The State of Charge is the Dominant Risk Factor: Lowering the SOC drastically reduces the intensity and likelihood of thermal runaway by slowing the kinetics of the key exothermic chemical reactions.
- The Efficacy of Cooling is SOC-Dependent: Enhanced heat dissipation is largely ineffective at arresting thermal runaway in a high-SOC lithium-ion battery but becomes a powerful suppression tool for cells at medium or low SOC.
These insights directly inform advanced safety management strategies for lithium-ion battery packs. A promising approach involves integrated fault diagnosis and proactive mitigation. Upon early detection of a hazard (e.g., an internal short circuit precursor), the battery management system (BMS) could initiate an emergency procedure to rapidly discharge the affected cell or module to a low SOC state. This action directly attacks the root cause by depleting the source of reactive lithium. Following this rapid discharge, active cooling can then effectively manage the remaining heat, thereby preventing the escalation to catastrophic thermal runaway. This two-step strategy—rapid energy depletion followed by thermal management—could significantly enhance the inherent safety and resilience of systems based on lithium-ion battery technology.