The rise of new energy vehicles represents a pivotal shift in global transportation, driven by the urgent need to address environmental pollution and fossil fuel dependency. At the heart of this transition lies the lithium-ion battery, a power source lauded for its high energy density, superior power output, and extended cycle life. Its dominance in automotive applications is unquestioned. However, the inherent safety of these energy-dense systems remains a critical concern. Composed of flammable electrolytes and highly reactive electrode materials, lithium-ion batteries are susceptible to catastrophic failure under abusive conditions. Among these, internal short circuit (ISC) stands out as a primary and notoriously dangerous failure mode. An ISC can bypass the intended electrochemical pathway, leading to localized, rapid energy release, uncontrolled temperature rise, and potentially thermal runaway—a sequence involving fire or explosion. Therefore, a profound understanding of ISC mechanisms is not merely academic; it is fundamental to developing safer, high-energy-density lithium-ion batteries and ensuring the sustainable growth of the electric vehicle industry.
ISC is a complex, multi-scale phenomenon involving intricate couplings between electrochemical, thermal, and, in cases triggered by mechanical intrusion, mechanical fields. Nail penetration testing is a standard safety evaluation method that physically induces an ISC. While useful for observing macroscopic outcomes like smoke or fire, these tests offer limited insight into the critical transient processes—the evolving current pathways, localized heat generation, and chemical reactions—occurring inside the sealed battery. To bridge this gap, high-fidelity multi-physics simulation becomes an indispensable tool. Previous modeling efforts have provided valuable insights but often contained simplifications, such as using empirical functions for short-circuit resistance or lacking full integration between the ISC trigger and the core electrochemical-thermal response. This work aims to construct a more dynamically coupled model. Specifically, it focuses on simulating the nail penetration-induced ISC within a multi-layer pouch cell structure, systematically analyzing the coupled electrical and thermal response characteristics under this progressive failure scenario.
1. Model Framework and Numerical Methodology
1.1 Geometric Model and Equivalent Circuit Representation
A commercial lithium-ion battery is a complex assembly of dozens or hundreds of repeating cell units. To balance computational fidelity with efficiency, a representative 3D stacked model comprising five unit cells was constructed. Each unit cell consists of an aluminum current collector (positive tab side), a cathode layer (e.g., NCM111), a separator, an anode layer (e.g., graphite), and a copper current collector (negative tab side). Stacking these five units creates a model with six electrode layers, six current collector layers, and five separator layers. The nail, modeled as a cylindrical conductor, penetrates through the center of this stack at a predefined velocity.
The key innovation in representing the ISC process lies in the equivalent circuit approach for the penetrated layers. As the nail pierces the stack, it sequentially creates electrical short circuits between the cathode (Al current collector) and anode (Cu current collector) of each unit cell. In the model, each potential short-circuit site is associated with a switch (S) and a short-circuit resistance (R_s). The short-circuit resistance for a given layer is calculated based on the nail’s material conductivity (σ_steel) and the instantaneous contact geometry as it penetrates:
$$ R_s = \frac{L_{contact}}{\sigma_{steel} \cdot A_{cross}} $$
where \(L_{contact}\) is the effective conductive length along the nail within the shorted layer, and \(A_{cross}\) is the cross-sectional area of the nail. Initially, all switches are open. When the nail physically breaches the separator of the first unit cell, switch S1 closes. This creates the first short-circuit path, allowing a short-circuit current (I_s1) to flow, governed by the potential difference between the electrodes and the total resistance in that loop (including R_s1 and the internal impedance of the cell unit, R_in). As penetration continues, switches S2 through S5 close sequentially. The total short-circuit current (I_total) drawn from the lithium-ion battery becomes the parallel sum of the individual layer short-circuit currents:
$$ I_{total} = \sum_{i=1}^{n} I_{si} = \sum_{i=1}^{n} \frac{\nabla \phi_i}{R_{si}} $$
where \( \nabla \phi_i \) is the potential gradient along the i-th short-circuit path, and \(n\) is the number of penetrated layers (1 ≤ n ≤ 5). This formulation dynamically couples the mechanical progression of the nail (which closes switches and defines R_s) with the electrochemical state of the lithium-ion battery.
1.2 Multi-Physics Coupling: Electrochemical, Thermal, and Short-Circuit Models
The core of the simulation is a tightly coupled electrochemical-thermal model. The electrochemical behavior is described using a pseudo-two-dimensional (P2D) framework, which resolves lithium concentration in the solid particles and electrolyte, as well as potential distributions. The thermal model is based on the energy conservation equation, accounting for heat generation and dissipation:
$$ \rho C_p \frac{\partial T}{\partial t} = \nabla \cdot (k \nabla T) + \dot{Q}_{gen} $$
where \( \rho \), \( C_p \), and \( k \) are the density, heat capacity, and thermal conductivity of the cell components, respectively. \( \dot{Q}_{gen} \) is the total volumetric heat generation rate, which is the critical link between the models. It has three primary components:
1. Electrochemical Heat (Q_ec): Generated from reversible (entropic) and irreversible (overpotential) reactions during normal operation or discharge via the ISC path.
2. Joule Heat from ISC (Q_short): The dominant heat source during the short-circuit event, calculated from the short-circuit current and resistance:
$$ \dot{Q}_{short} = \frac{I_{total}^2 \cdot R_{eff}}{V_{short}} $$
Here, \( R_{eff} \) is the effective parallel resistance of all active short circuits, and \( V_{short} \) is the volume of the nail/shorted region.
3. Exothermic Side Reaction Heat (Q_side): Triggered when local temperature exceeds certain thresholds. This includes Solid Electrolyte Interphase (SEI) decomposition, reaction between anode and electrolyte, cathode decomposition, and electrolyte decomposition. These reactions are typically modeled using Arrhenius expressions, and their initiation marks the onset of thermal runaway.
The coupling is bidirectional: temperature (T) from the thermal model affects the reaction kinetics and transport properties in the electrochemical model, which in turn determines the heat generation rate (Q_gen). The ISC model acts as a boundary condition driver, forcing a high-rate discharge through the short-circuit resistors and injecting Q_short directly into the thermal model at the penetration site.
1.3 Numerical Implementation in COMSOL Multiphysics
The coupled model was implemented and solved using COMSOL Multiphysics software. The “Lithium-Ion Battery” physics interface was used for the electrochemical P2D model, and the “Heat Transfer in Solids” interface was used for the thermal model. The nail penetration and short-circuit logic were implemented using time-dependent variables and event functions. The electrical conductivity of the nail material was controlled by a step function tied to its position, effectively “activating” the short-circuit path for a layer once the nail tip passed through its separator. The external surfaces of the cell stack were subjected to natural convection boundary conditions. Key simulation parameters for the base case are summarized in the table below.
| Parameter Category | Symbol | Value | Unit |
|---|---|---|---|
| Cell Geometry | |||
| Number of Unit Cells | n | 5 | – |
| Cell Thickness (per unit) | d_cell | ~0.154 | mm |
| Nail Properties | |||
| Radius | r_nail | 2.0 | mm |
| Penetration Speed (Base) | v | 2.0 | mm/s |
| Material Conductivity | σ_steel | 4.03e6 | S/m |
| Electrochemical | |||
| Initial State of Charge | SOC | 30%, 90% | – |
| Cathode Material | – | NCM111 | – |
| Anode Material | – | Graphite | – |
| Thermal (Ambient) | |||
| Initial Temperature | T_0 | 298.15 | K |
| Convective Heat Transfer Coef. | h | 5 | W/(m²·K) |
2. Simulation Results and Analysis of Internal Short Circuit Behavior
2.1 The Progressive Internal Short Circuit Process
The simulation vividly captures the dynamic, layer-by-layer failure of the lithium-ion battery under nail penetration. The results for a cell at 30% State of Charge (SOC) with a 2 mm radius nail moving at 2 mm/s serve as a baseline. The evolution of terminal voltage and maximum cell temperature versus penetration depth is critical for understanding the ISC stages.
Initially, as the nail tip contacts and compresses the first few layers (Penetration Depth, d < 0.154 mm), the voltage remains stable. This corresponds to the mechanical intrusion phase before the first separator is breached; no electrical short circuit exists yet. A significant and abrupt voltage drop occurs precisely when the nail pierces through the first separator (d ≈ 0.154 mm). This event closes the first equivalent circuit switch (S1), creating a sudden low-resistance path for discharge. The terminal voltage instantly drops by approximately 1.5 V. Concurrently, a sharp spike in the maximum temperature is observed, with localized heat at the puncture site raising temperatures significantly above ambient.
As penetration continues, each subsequent breach of a separator (at d ≈ 0.308 mm, 0.462 mm, etc.) triggers another discrete voltage drop. This results in a characteristic “staircase” voltage profile, where each step down corresponds to the addition of another unit cell into the parallel short-circuit network. The total current draw increases with each added parallel path, accelerating the discharge of the lithium-ion battery. The temperature response is cumulative but complex. Each new short adds a new source of Joule heating (Q_short). However, the nail also acts as a heat sink and conductor, potentially spreading heat from earlier shorts. The competition between concentrated heat generation at new puncture sites and the overall thermal diffusion dictates the local and global temperature field. This progressive nature highlights that an internal short circuit in a multi-layer lithium-ion battery is not a single event but a cascading failure, where the severity escalates with the number of compromised layers.
2.2 Influence of State of Charge (SOC) and Nail Radius
The initial energy state of the lithium-ion battery profoundly impacts the consequences of an internal short circuit. Simulations comparing 30% SOC and 90% SOC conditions reveal stark differences. At 90% SOC, the lithium-ion battery possesses a much higher chemical potential and stored energy. When an ISC path is created, the driving force for current flow is greater, leading to significantly higher short-circuit currents (I_s). According to the Joule heating term ( \( \dot{Q}_{short} \propto I_{total}^2 \) ), this results in substantially more intense localized heating. In our simulations, the peak temperature for the 90% SOC case exceeded 500 K, far surpassing the peak observed at 30% SOC. This high temperature readily crosses the activation thresholds for exothermic side reactions (e.g., SEI decomposition at ~80-120°C), dramatically increasing the risk of a full, uncontrollable thermal runaway in the lithium-ion battery.
The radius of the penetrating nail (r_nail) directly influences the short-circuit resistance (R_s ∝ 1/A_cross ∝ 1/r_nail²). A larger nail radius creates a lower-resistance, higher-current short. The table below summarizes the thermal response under different SOC and nail radius conditions for a fixed penetration speed.
| SOC | Nail Radius (mm) | Peak Temp. (K) | Thermal Runaway Risk | Key Observation |
|---|---|---|---|---|
| 30% | 1.2 | ~380 | Low | Multiple temp. peaks observed; heat dissipation competes with generation. |
| 30% | 2.0 | < 350 | Very Low | Larger nail improves heat sinking; temperature stabilizes. |
| 90% | 1.2 | > 500 | Very High | Rapid temperature rise, clear triggering of side reactions. |
| 90% | 2.0 | > 480 | High | High peak temperature, but slightly moderated compared to smaller radius due to better heat conduction along nail. |
An interesting phenomenon observed, particularly at r_nail = 1.2 mm, was the occurrence of multiple temperature peaks. After an initial sharp rise, the temperature would briefly dip before rising again to a second or even third peak. This can be attributed to the dynamic competition between heat sources and sinks. The initial peak is driven by the first, highly localized Joule heating from the ISC. This heat begins to dissipate via conduction and convection. However, if the temperature was sufficiently high to initiate slow chemical side reactions (like electrolyte decomposition), the subsequent heat release from these reactions (Q_side) could cause a secondary temperature rise, especially as the exothermic reactions self-accelerate. This multi-stage heating process underscores the complex, time-dependent interplay between electrical abuse and chemical kinetics in a failing lithium-ion battery.
2.3 Influence of Penetration Speed
The speed at which the nail penetrates the lithium-ion battery (v) is a critical but often overlooked factor in ISC severity. Simulations comparing “fast” penetration (v = 2.5 mm/s) with “slow” penetration (v = 1.2 mm/s) yielded significant insights. In the fast penetration case, all five layers are breached in a very short timeframe (~1 second). This means the short-circuit switches (S1 to S5) close in rapid succession, almost simultaneously from a thermal diffusion perspective. The total short-circuit current is distributed almost instantly across five parallel paths. While the cumulative current is high, the duration of the current pulse through any single, newly created short-circuit spot is extremely brief before the nail moves on. This limits the time for intense heat to build up locally at each new puncture site before it starts being conducted away by the advancing metal nail. Consequently, the peak temperatures recorded were lower.
In contrast, slow penetration creates a more severe thermal scenario. When the nail breaches the first separator, it remains in that position for a longer period before moving on to the second layer. This allows a sustained, high-current pulse to flow through the first short-circuit path for an extended time. The localized Joule heating at this initial site has ample time to raise the temperature significantly, potentially initiating exothermic side reactions locally. By the time the nail progresses to the second layer, the surrounding material is already preheated. This process repeats for each subsequent layer, leading to a cumulative and more severe heating effect. In our simulation for a high-SOC lithium-ion battery, the peak temperature for the slow penetration case was approximately 35% higher than for the fast penetration case. The slow penetration effectively provides a “dwell time” at each short-circuit location, maximizing energy deposition and minimizing the cooling effect of the nail’s motion, thereby posing a substantially greater thermal runaway hazard for the lithium-ion battery.
3. Discussion on Thermal Runaway Mechanisms and Model Implications
The simulation results collectively paint a detailed picture of the pathway from mechanical intrusion to potential thermal runaway in a multi-layer lithium-ion battery. The internal short circuit initiated by the nail is the trigger, but the ultimate outcome is determined by the race between heat generation (Q_gen = Q_short + Q_side) and heat dissipation (∇·(k∇T) + convection). The model demonstrates that factors which increase the magnitude or concentration of Q_short—such as high SOC (higher driving voltage) or small nail radius (higher current density)—push the system toward thermal runaway. Similarly, factors that increase the duration of intense heating at a spot—like slow penetration speed—also dramatically increase the risk.
The staged voltage drop is a clear electrical signature of a progressive internal short circuit in a multi-layer lithium-ion battery. This could, in principle, be used as a diagnostic signal in a Battery Management System (BMS) to detect severe mechanical damage before temperatures become critical. The thermal response, particularly the multiple temperature peaks, illustrates the transition from an abuse-driven event (Joule heating from ISC) to a chemistry-driven catastrophe (self-sustaining side reactions). The model successfully captures the coupling between these phases.
This work’s model framework, integrating a dynamic equivalent circuit for the penetrating nail with a fully coupled electrochemical-thermal model, provides a powerful tool for analyzing lithium-ion battery safety. It moves beyond static short-circuit assumptions and allows for the investigation of complex, real-world failure scenarios. The findings emphasize that safety testing standards for lithium-ion batteries, which often specify a single nail speed, may need to consider the worse-case scenario of slower intrusion speeds. Furthermore, the design of lithium-ion battery modules and packs should account for the progressive nature of internal short circuits; containment strategies that can isolate a single failing cell unit before it cascades to neighbors are crucial.
4. Conclusions and Future Perspectives
This study established a multi-physics simulation framework to investigate the nail penetration-induced internal short circuit in a multi-layer lithium-ion battery. By employing an equivalent circuit model dynamically coupled with electrochemical and thermal physics, the simulation elucidated the progressive failure mechanism. The key findings are: (1) The terminal voltage exhibits a distinct staircase decline, with each step corresponding to the breach of an additional cell layer, serving as a potential indicator of severe internal short circuit progression. (2) The initial State of Charge is a paramount factor; a high-SOC lithium-ion battery experiences drastically higher short-circuit currents and temperatures, exceeding 500 K and readily crossing into the thermal runaway regime. (3) Contrary to intuition, slower nail penetration speeds create more severe thermal conditions than faster speeds, as they allow sustained localized heating at each short-circuit site, increasing the peak temperature by approximately 35% in the studied cases.
The model provides significant insights into the coupled electrical and thermal behavior of lithium-ion batteries under mechanical abuse. Future work will focus on extending this framework. This includes modeling more complex internal short circuit types (e.g., anode-to-cathode, cathode-to-current collector) within a single layer, simulating the behavior of larger-format or different chemistry lithium-ion batteries, and incorporating more detailed mechanical deformation and failure models for the electrodes and separator. Ultimately, such high-fidelity simulation tools are indispensable for advancing the intrinsic safety of lithium-ion batteries, guiding safer design principles, and developing more effective failure detection and mitigation strategies for the new energy vehicle industry.