The proliferation of electric vehicles (EVs) and large-scale energy storage systems has placed stringent demands on the performance and safety of electrochemical storage devices. Among these, the capacitive lithium-ion battery represents a critical technological advancement, designed to bridge the gap between high-energy lithium-ion chemistries and high-power supercapacitors. By incorporating materials like activated carbon composites in the cathode and soft carbon/graphite in the anode, alongside innovative designs such as tabless or full-tab configurations, these batteries achieve exceptional power density and cycle life. However, their safety under mechanical abuse, particularly localized loading akin to collision scenarios, remains a paramount concern that requires detailed investigation.
In real-world accidents, a lithium-ion battery pack is rarely subjected to uniform compression. More commonly, intrusion from vehicle components or debris leads to highly localized stress concentrations, effectively simulating a “ball-head” or “spherical indentation” condition. This type of loading creates a complex, multi-axial stress state within the jelly roll, differing significantly from planar compression. It can induce simultaneous shear, tensile, and compressive failures in the electrode and separator layers, potentially triggering internal short circuits (ISC) with catastrophic consequences like thermal runaway (TR). While extensive research exists on the mechanical abuse of energy-dense lithium-ion batteries, the failure mechanisms and safety boundaries for high-power capacitive lithium-ion batteries under such localized indentation are not fully understood. This work aims to elucidate these mechanisms by experimentally probing the mechanical, thermal, and electrochemical responses of 18650-format capacitive lithium-ion batteries subjected to spherical indentation, providing a foundation for safer battery pack design.
1. Experimental Methodology
1.1 Test Specimen Preparation
The subject of this investigation is a commercial 18650 cylindrical capacitive lithium-ion battery. Its key specifications are summarized in Table 1. The battery features a composite positive electrode (LiNi0.6Co0.2Mn0.2O2 blended with activated carbon) and a composite negative electrode (soft carbon/graphite), optimized for high-rate capability. A tabless or full-tab current collection design is employed to minimize internal resistance.
Prior to mechanical testing, batteries were conditioned to specific States of Charge (SOC) using a standard constant-current constant-voltage (CC-CV) protocol on a battery cycler. SOC levels of 0%, 20%, 40%, 60%, 80%, and 100% were prepared to evaluate the influence of electrochemical energy content on failure behavior.
| Parameter | Specification |
|---|---|
| Format | 18650 Cylindrical |
| Dimensions (Diameter × Height) | 18 mm × 65 mm |
| Chemistry (Cathode // Anode) | LiNi0.6Co0.2Mn0.2O2@AC // Soft Carbon/Graphite |
| Nominal Capacity | 1500 mAh |
| Voltage Window | 2.5 V – 4.2 V |
| Power Density | ≥ 10.5 kW/kg |
| Energy Density | ~127 Wh/kg |
1.2 Ball-Head Indentation Test Setup
Localized mechanical abuse was applied using a universal testing machine equipped with a 1 mm diameter spherical indenter made of high-temperature-resistant steel. The test setup, as schematically represented, integrated synchronized data acquisition systems for force, displacement, voltage, and temperature. Batteries were placed horizontally on a flat platen. The indenter was driven vertically into the battery casing at a constant speed of 1 mm/min. Two testing protocols were employed:
- Continuous Indentation: The indenter compressed the battery until a predefined displacement (typically 9 mm) or catastrophic failure occurred.
- Stepwise Indentation: The battery was indented in 1 mm increments. After each step, the machine paused for 4 minutes to monitor voltage relaxation and temperature equilibrium before proceeding. This was used for post-damage electrochemical analysis.
Voltage across the battery terminals was monitored using a high-precision voltmeter. Three K-type thermocouples were attached to the battery surface at the positive terminal (T1), the geometric center (T2), and the negative terminal (T3) to capture spatial temperature evolution. A high-speed camera recorded visual phenomena such as venting, smoke, and fire. Post-mortem analysis involved computed tomography (CT) scanning to examine internal structural damage and scanning electron microscopy (SEM) to observe electrode morphology changes.
2. Results and Discussion
2.1 Influence of State of Charge (SOC)
The safety outcome of the indentation test was critically dependent on the initial SOC of the lithium-ion battery. Visual observations revealed a distinct transition from benign failure to violent thermal runaway as SOC increased.
- Low SOC (0%, 20%, 40%): Batteries exhibited electrolyte leakage and mild gas venting. At 40% SOC, visible smoke was emitted, but no open flame or significant temperature spike leading to thermal runaway was observed.
- High SOC (≥60%): A dramatic shift occurred. Internal short circuits led to rapid joule heating, triggering the safety vent. This was followed by jetting of sparks and, at 80% and 100% SOC, sustained flaming combustion for several seconds, confirming the onset of full thermal runaway.
The mechanical and electrical responses, plotted as force-displacement and voltage-displacement curves, provide quantitative insight into this transition. A typical force-displacement profile for the lithium-ion battery under indentation is characterized by an initial linear-elastic region, a plateau representing layer compaction and buckling, followed by a steep rise as the densely packed core is compressed, culminating in a peak force just before catastrophic failure.
The key parameters extracted from these curves are the peak indentation force ($F_{peak}$) and the critical indentation displacement at the onset of internal short circuit ($\delta_{ISC}$). Analysis shows a clear trend:
$$ F_{peak} \propto -\alpha \cdot SOC $$
$$ \delta_{ISC} \propto -\beta \cdot SOC $$
where $\alpha$ and $\beta$ are positive constants. In simpler terms, both the maximum load the battery can withstand and the deformation it tolerates before shorting decrease with increasing SOC. For instance, the 0% SOC battery failed at a displacement of ~7.50 mm with a peak force of 12.41 kN, while the 100% SOC battery failed earlier at ~6.26 mm with a lower peak force of 9.62 kN. This can be attributed to the increased stiffness and altered mechanical properties of the lithiated electrodes (especially the graphite anode), making them more brittle and prone to fracture under stress.
The voltage response during failure also evolved with SOC. For batteries that did not undergo thermal runaway, a two-stage voltage drop was often observed: a gradual, small decrease (“soft short”) followed by an abrupt plunge to near-zero voltage (“hard short”). The soft short is likely due to separator thinning and micro-shorts, while the hard short signifies a major, established short-circuit path. In batteries undergoing thermal runaway, the voltage collapse was instantaneous and concurrent with the force drop.
The most critical safety metric, temperature rise, showed a stark SOC dependence. The maximum surface temperature ($T_{max}$) and its rate of increase ($dT/dt$) escalated dramatically above 60% SOC, as summarized in Table 2.
| SOC (%) | Peak Force (kN) | $\delta_{ISC}$ (mm) | $T_{max}$ (°C) | Approx. $dT/dt_{max}$ (°C/s) | Outcome |
|---|---|---|---|---|---|
| 0 | 12.41 | 7.50 | ~50 | < 1 | Leakage |
| 20 | 11.88 | 7.20 | ~55 | < 1 | Leakage |
| 40 | 11.05 | 6.85 | 135.1 | ~5 | Smoke, Venting |
| 60 | 10.52 | 6.60 | >250 | ~54 (T1) | Thermal Runaway (Spark Jetting) |
| 80 | 9.98 | 6.42 | >250 | >60 | Thermal Runaway (Flame) |
| 100 | 9.62 | 6.26 | >250 | >60 (T2) | Thermal Runaway (Flame) |
2.2 Failure Process Analysis via Thermal Runaway Signatures
Analyzing the high-speed thermal data reveals the intricate sequence of events during the failure of a high-SOC lithium-ion battery. For a 60% SOC battery, the moment of internal short circuit is marked by an instantaneous, sharp temperature spike at the indentation point (T2) and the positive terminal (T1), with rates exceeding 50 °C/s. This corresponds to the concentrated joule heating at the short site. The safety valve ruptures, often accompanied by a brief jet of sparks. A temporary temperature drop may follow as hot gases are expelled.
Subsequently, a second, more sustained temperature rise wave propagates. This is fueled by exothermic chemical reactions triggered by the initial heat: decomposition of the Solid-Electrolyte Interphase (SEI), reaction of the lithiated anode with electrolyte, and finally, decomposition of the cathode material and electrolyte. This chain reaction causes thermal runaway to propagate from the localized short point throughout the entire jelly roll, leading to temperatures exceeding 250°C and, if electrolyte vapors ignite, open flame. The process for a 100% SOC battery is even more violent, with higher initial heating rates and more intense combustion.
Post-mortem CT analysis elucidates the physical damage mechanism. The spherical indenter imposes a complex stress field. The radial component causes global bending and layer buckling around the indentation site. The axial (through-thickness) component generates intense shear stresses between layers, leading to localized tearing and punching of the electrode sheets. This combined loading creates a concentrated “fracture zone” where the separator is completely compromised, and the cathode and anode materials are forced into direct electrical contact, establishing the primary internal short circuit. This damaged zone then acts as a hotspot, initiating the thermal runaway sequence described above.
2.3 Effect of Indentation Location
The cylindrical lithium-ion battery is not structurally homogeneous along its axis. The ends, particularly the positive cap with its safety valve and the negative cap, have different internal geometries and mechanical constraints compared to the mid-section. Indentation tests were conducted at three locations: the positive terminal region, the geometric center, and the negative terminal region, all on batteries at 60% SOC.
The results indicate significant variation. Indentation near the rigid terminal ends required higher peak forces (14.36 kN at positive end, 16.14 kN at negative end) compared to the mid-section (11.44 kN). This is because the midsection has a slight gap between the jelly roll and the casing, offering less support. However, from a safety hazard perspective, indentation near the positive terminal proved most dangerous. This location exhibited:
- The highest voltage drop rate, indicating a more severe or rapidly propagating short.
- The highest peak temperature rise rate.
- A greater propensity for immediate and violent venting/sparking.
This increased hazard is likely due to proximity to the safety valve and the different internal current path in a tabless design. A short circuit initiated near the positive terminal may involve a larger effective electrode area more quickly and provide a direct path for hot ejecta to escape, immediately compromising the safety vent. This finding is crucial for battery pack design, suggesting that protecting the positive terminal area from potential intrusions is especially important.
2.4 Electrochemical Performance of Damaged Batteries and Implications for Secondary Use
A critical, practical question arises from non-catastrophic indentation events: Can a mechanically damaged lithium-ion battery be used safely, even if it doesn’t immediately go into thermal runaway? To answer this, batteries at 0% SOC were indented to various depths (1-7 mm) using the stepwise protocol. After each increment, the battery was charged to 100% SOC and then discharged at a high rate (7C) to assess its capacity and internal resistance. It was also subjected to a long-term open-circuit voltage (OCV) monitoring to evaluate self-discharge.
The results, summarized in Table 3, define a clear degradation threshold. Up to 4-5 mm of indentation, the lithium-ion battery retained over 90% of its original capacity, and the internal resistance increase was moderate (~120% of initial). Self-discharge over 1000 hours was slightly elevated but not catastrophic. These batteries, while compromised, might still function in a degraded state.
Beyond 6 mm of indentation, a sharp decline occurred. Capacity retention fell below 80%, internal resistance nearly doubled, and self-discharge became severe, with OCV dropping to zero within a few hundred hours for the 7 mm case. More alarmingly, during attempted cycling, some batteries in the 6-7 mm damage group experienced sudden failure (“charging explosion”), indicating latent instability.
This behavior can be modeled by considering the damaged lithium-ion battery as having a parallel leakage resistance ($R_{leak}$) across its terminals, which dominates its performance after severe indentation. The self-discharge current ($I_{sd}$) and capacity loss ($\Delta Q$) can be approximated by:
$$ I_{sd} \approx \frac{V_{OCV}}{R_{leak}} $$
$$ \Delta Q \propto \int I_{sd} dt $$
where $R_{leak}$ decreases sharply with indentation depth beyond the critical threshold (5-6 mm), leading to exponential growth in self-discharge and capacity fade. This critical depth corresponds to the point where CT scans show extensive layer buckling, electrode fracture, and separator integrity loss, creating numerous micro-short circuits that collectively act as a low $R_{leak}$.
| Indentation Depth (mm) | Capacity Retention (%) | Internal Resistance (% of Fresh) | Self-Discharge after 1000h (OCV, V) | Safety during Cycling |
|---|---|---|---|---|
| 1-3 | > 95% | 105-110% | > 4.0 | Stable |
| 4-5 | ~90% | ~120% | 3.24 – 3.47 | Stable |
| 6 | ~77.5% | ~160% | ~2.5 (after 624h) | Unstable (Risk of TR) |
| 7 | ~56.8% | ~192% | 0.0 (after 624h) | Highly Unstable (TR observed) |
3. Conclusion
This comprehensive experimental study on capacitive lithium-ion batteries under spherical indentation provides critical insights into their failure mechanisms and safety boundaries under localized mechanical abuse, a key scenario in electric vehicle collisions. The primary conclusions are as follows:
- The State of Charge (SOC) is the dominant factor determining the severity of failure. A critical threshold exists around 60% SOC, above which localized indentation reliably triggers thermal runaway. Both the mechanical strength (peak force) and the tolerable deformation before internal short circuit decrease linearly with increasing SOC for this lithium-ion battery.
- The failure process is initiated by a complex multi-axial stress state that causes simultaneous layer buckling and shear-driven electrode fracture, creating a concentrated internal short circuit hotspot. This hotspot then ignites a cascade of exothermic reactions leading to thermal runaway, with temperature propagation rates exceeding 50 °C/s.
- The location of indentation significantly influences the hazard level. While the midsection is mechanically weakest, indentation near the positive terminal of the lithium-ion battery presents the highest thermal and electrical hazard, likely due to its proximity to the safety vent and internal current collection geometry in a tabless design.
- There exists a critical indentation damage threshold (approximately 5-6 mm for this cell format) beyond which the lithium-ion battery is no longer safe for secondary use. Below this threshold, capacity and resistance may be acceptable, but above it, severe self-discharge, massive capacity fade, and a high risk of latent thermal runaway during subsequent cycling render the battery unsafe and unusable.
These findings underscore the necessity for robust mechanical protection systems in battery packs, with particular attention to shielding against localized impacts on high-SOC cells and the vulnerable positive terminal region. Furthermore, they provide essential criteria for diagnosing and handling potentially damaged lithium-ion batteries post-incident, helping to prevent secondary accidents. Future work will involve integrating these experimental results into multi-physics models to predict the safety of capacitive lithium-ion batteries under a wider range of abuse conditions.