With the global energy transition accelerating toward cleaner and low-carbon systems, the demand for high-safety, long-life energy storage lithium batteries has become increasingly critical in applications such as grid-scale energy storage and electric vehicles. However, safety incidents involving thermal runaway, fires, and explosions in energy storage lithium battery systems have raised significant concerns. The separator, as the sole physical barrier between the cathode and anode in lithium-ion batteries, plays a pivotal role in determining safety boundaries through its thermal stability, mechanical strength, and electrochemical properties. In particular, the adhesive layer coated on separators—often composed of polymers like polyvinylidene fluoride (PVDF) or polymethyl methacrylate (PMMA)—enhances bonding with electrodes, improves mechanical integrity, and influences thermal runaway behavior. This study systematically investigates how PVDF and PMMA adhesive layers affect the safety performance of 280 A·h lithium iron phosphate (LiFePO4) energy storage lithium batteries under abusive conditions, including overcharge, external heating, and nail penetration. By comparing key safety parameters such as valve activation time, voltage drop, maximum temperature during thermal runaway, and temperature rise rates, we provide experimental insights for optimizing separator materials in energy storage lithium battery designs to mitigate risks.
The development of separator coatings for energy storage lithium batteries has evolved through distinct phases: early separators lacked additional layers, but to address thermal instability and poor electrolyte wettability, inorganic coatings like alumina and boehmite were introduced. More recently, polymer adhesive layers superimposed on these inorganic coatings have become mainstream, as they enhance adhesion between the separator and electrodes, reduce internal resistance, and improve cycle life. Among these, PVDF and PMMA are commonly used, but their comparative effects on safety under extreme conditions remain unclear for large-format energy storage lithium batteries. We evaluated two types of separators from the same manufacturer, designated as Type 1 (with PVDF adhesive layer) and Type 2 (with PMMA adhesive layer), both featuring a 9+2+1 structure (9 μm base, 2 μm alumina coating, 1 μm adhesive layer). Fundamental properties were assessed to understand their intrinsic characteristics, as summarized in Table 1.
| Property | Type 1 (PVDF) | Type 2 (PMMA) |
|---|---|---|
| Air Permeability [s·(100 mL)−1] | 192 | 194 |
| Tensile Strength [N/mm²] (Longitudinal) | 0.2513 | 0.2512 |
| Tensile Strength [N/mm²] (Transverse) | 0.2448 | 0.2390 |
| Thermal Conductivity [W/(m·K)] | 0.126 | 0.128 |
| Puncture Strength [N] | 4.86 | 4.61 |
| Thermal Shrinkage [%] at 130°C for 1 h (Longitudinal) | 0.75 | 1.00 |
| Thermal Shrinkage [%] at 130°C for 1 h (Transverse) | 0.59 | 0.84 |
As shown in Table 1, the Type 1 separator with PVDF exhibited superior puncture strength, transverse tensile strength, and lower thermal shrinkage compared to the Type 2 separator with PMMA. These differences stem from the inherent material properties: PVDF has a higher melting point and better mechanical robustness, which contribute to enhanced safety in energy storage lithium batteries. The thermal conductivity was similar for both, but the reduced shrinkage in Type 1 indicates better dimensional stability under heat, a critical factor in preventing internal short circuits. To quantify the safety implications, we fabricated 280 A·h prismatic LiFePO4 energy storage lithium battery samples using identical electrodes and electrolytes, with the separator as the only variable. The batteries were conditioned to 100% state of charge (SOC) before testing, and safety evaluations were conducted under overcharge, external heating, and nail penetration scenarios, with data collected on voltage, temperature, and thermal runaway parameters using specialized equipment.
Overcharge testing simulates electrical abuse scenarios where the battery management system fails, leading to continuous charging beyond design limits. For energy storage lithium batteries, this can trigger exothermic reactions, gas generation, and thermal runaway. We performed overcharge tests at a constant current of 0.5 C (140 A) until thermal runaway occurred. The voltage (V), temperature (T), and temperature rise rate (dT/dt) were monitored, and the results are presented in Table 2 and discussed below. The thermal runaway process can be modeled using energy balance equations, where the heat generation rate (Q_gen) during overcharge includes contributions from side reactions and ohmic heating:
$$ \frac{dT}{dt} = \frac{Q_{\text{gen}} – Q_{\text{loss}}}{m C_p} $$
Here, m is the battery mass, C_p is the specific heat capacity, and Q_loss represents heat dissipation to the environment. For the Type 2 separator battery, safety valve activation occurred at 12.6 minutes, followed by voltage drop at 24.4 minutes (120% SOC), and thermal runaway onset at 27.4 minutes, with a maximum temperature (T_TRmax) of 290.2°C and maximum temperature rise rate (dT/dt_max) of 5.28°C/s. In contrast, the Type 1 separator battery delayed valve opening to 14.1 minutes, voltage drop to 25.9 minutes (121% SOC), and thermal runaway to 28.0 minutes, achieving a lower T_TRmax of 279.2°C and dT/dt_max of 5.17°C/s. This demonstrates that the PVDF-based separator in energy storage lithium batteries provides a longer safety margin by resisting internal short circuits and reducing heat accumulation, attributable to its higher mechanical strength and thermal stability.
| Parameter | Type 1 (PVDF) | Type 2 (PMMA) |
|---|---|---|
| Valve Opening Time (min) | 14.1 | 12.6 |
| Voltage Drop Time (min) | 25.9 | 24.4 |
| Thermal Runaway Onset Time (min) | 28.0 | 27.4 |
| Maximum Temperature T_TRmax (°C) | 279.2 | 290.2 |
| Maximum Temperature Rise Rate dT/dt_max (°C/s) | 5.17 | 5.28 |
External heating tests evaluate the response of energy storage lithium batteries to thermal abuse, such as exposure to fire or hot environments in energy storage systems. We used a heating plate to gradually increase the battery temperature until thermal runaway, monitoring key events. The Type 2 separator battery showed earlier safety valve activation at 28.4 minutes, voltage drop at 38.5 minutes, and thermal runaway at 40.7 minutes, with T_TRmax of 295.2°C and a sharp dT/dt_max of 8.82°C/s. Notably, flames were observed at the valve opening, indicating severe gas ejection and combustion. For the Type 1 separator battery, valve opening occurred later at 35.9 minutes, voltage drop at 49.0 minutes, and thermal runaway at 52.6 minutes, with T_TRmax of 293.7°C and a milder dT/dt_max of 5.64°C/s. The delayed responses in Type 1 can be explained by its lower thermal shrinkage and better adhesion, which slow down separator collapse and internal short circuit formation. The heat transfer during external heating can be described by Fourier’s law, and the temperature evolution follows:
$$ \rho C_p \frac{\partial T}{\partial t} = k \nabla^2 T + q_{\text{abuse}} $$
where ρ is density, k is thermal conductivity, and q_abuse is the abusive heat flux. The PVDF-based separator’s properties help dissipate heat more effectively, reducing the risk of catastrophic failure in energy storage lithium batteries.
| Parameter | Type 1 (PVDF) | Type 2 (PMMA) |
|---|---|---|
| Valve Opening Time (min) | 35.9 | 28.4 |
| Voltage Drop Time (min) | 49.0 | 38.5 |
| Thermal Runaway Onset Time (min) | 52.6 | 40.7 |
| Maximum Temperature T_TRmax (°C) | 293.7 | 295.2 |
| Maximum Temperature Rise Rate dT/dt_max (°C/s) | 5.64 | 8.82 |
Nail penetration tests simulate mechanical abuse, such as internal short circuits caused by impact or crushing, which are critical scenarios for energy storage lithium battery safety. A steel needle with a diameter of 6 mm and a tip angle of 45° was driven into the battery at 25 mm/s, and the results are summarized in Table 4. The Type 2 separator battery experienced valve opening within 10.9 seconds, voltage drop to near zero in 0.76 minutes, and thermal runaway at 4.2 minutes, reaching T_TRmax of 192.4°C and dT/dt_max of 3.12°C/s. In comparison, the Type 1 separator battery delayed valve opening to 15.1 seconds, voltage drop to 1.11 minutes, and thermal runaway at 4.2 minutes, but with a lower T_TRmax of 175.7°C and dT/dt_max of 2.27°C/s. Throughout the test, the Type 2 battery maintained higher temperatures, with a maximum difference of approximately 28°C, highlighting the superior safety of PVDF in mitigating short-circuit severity. The mechanical strength of the separator influences the internal short-circuit resistance (R_short), which can be approximated by:
$$ R_{\text{short}} = \frac{\rho_{\text{sep}} L}{A} $$
where ρ_sep is the resistivity of the separator, L is the thickness, and A is the contact area. The higher puncture strength of Type 1 reduces L and A under penetration, thereby limiting heat generation (Q = I²R_short t) and slowing thermal escalation in energy storage lithium batteries.
| Parameter | Type 1 (PVDF) | Type 2 (PMMA) |
|---|---|---|
| Valve Opening Time (s) | 15.1 | 10.9 |
| Voltage Drop Time (min) | 1.11 | 0.76 |
| Thermal Runaway Onset Time (min) | 4.2 | 4.2 |
| Maximum Temperature T_TRmax (°C) | 175.7 | 192.4 |
| Maximum Temperature Rise Rate dT/dt_max (°C/s) | 2.27 | 3.12 |
In summary, this comprehensive study demonstrates that the choice of separator adhesive layer material significantly impacts the safety of energy storage lithium batteries under abusive conditions. The PVDF-based separator (Type 1) consistently outperformed the PMMA-based separator (Type 2) across overcharge, external heating, and nail penetration tests, by delaying critical safety events and reducing thermal runaway severity. Specifically, PVDF delayed valve opening by 0.1–7.5 minutes, voltage drop by 0.1–10.5 minutes, and lowered maximum temperatures by 1.5–16.7°C, while also reducing maximum temperature rise rates by 0.11–3.18°C/s. These advantages stem from PVDF’s superior thermal stability, mechanical strength, and synergistic interactions with inorganic coatings, which collectively enhance the safety margins of energy storage lithium batteries. As the demand for reliable energy storage solutions grows, optimizing separator materials like PVDF can play a crucial role in preventing incidents and extending battery life. Future work could explore hybrid adhesive layers or novel polymers to further advance the safety of energy storage lithium battery systems.