The escalating stringency of global carbon emission regulations has catalyzed the adoption of lithium-ion battery technology within the maritime sector. To meet the high voltage, energy, and power demands of electric propulsion and shipboard services, substantial quantities of lithium-ion cells are configured into packs through series and parallel connections. In marine applications, the total energy storage capacity of a battery system can reach megawatt-hour or even tens of megawatt-hour levels. The confined nature of shipboard spaces presents a critical safety challenge: in the event of a thermal runaway incident, personnel may have limited means of escape. Consequently, marine lithium-ion batteries must adhere to exceptionally high safety standards. A fundamental requirement is that the thermal runaway of a single cell within a pack, induced by abuse conditions, should not propagate to adjacent cells, thereby preventing a cascading failure. At its core, the safety issue of lithium-ion batteries is a thermal management problem. This article delves into the physicochemical changes and thermal processes occurring within the cell materials during the overcharge-induced thermal runaway of a marine-grade lithium iron phosphate (LiFePO4) battery pack. The insights gained aim to provide a theoretical foundation for the development of active and passive safety technologies, as well as for research into intrinsic safety measures for lithium-ion battery systems.
The experimental setup utilized a commercial marine battery pack enclosure as the test platform. The interior was populated with four authentic 280 Ah prismatic aluminum-can LiFePO4 cells, with the remaining slots occupied by dummies of identical dimensions. The central cell was designated as the target for overcharging. Comprehensive monitoring was employed: K-type thermocouples were attached to measure temperatures at the cell terminals, the pressure relief valve (PRV), and the center of the large sidewall. Voltage taps were connected via wires soldered to the aluminum busbars welded to the cell terminals, enabling precise voltage tracking of each cell throughout the test. Prior to overcharge testing, the target cell underwent a standardized initialization procedure involving two full charge-discharge cycles at a 0.25C rate to establish a consistent baseline state of charge (SOC).
The core of the investigation involved subjecting the target LiFePO4 cell within the pack to a continuous overcharge at a constant current of 0.5C (140 A). The ensuing thermal runaway process was meticulously analyzed through synchronized voltage and temperature data, revealing a sequence of six distinct stages characterized by specific internal reactions and thermal signatures.
| Stage | Voltage Signature | Key Internal Processes | Primary Heat Sources |
|---|---|---|---|
| a) Normal Overcharge | Voltage rises steadily towards upper limit. | Continued lithium intercalation into graphite anode; minor electrode redundancy exploited. | Ohmic (Joule) heating and reversible reaction heat. |
| b) Voltage Inflection & Plateau | Voltage reaches inflection point (Vip), then forms a plateau (Vp). | Over-delithiation of LiFePO4 cathode; lithium plating on anode; lithium dendrite formation. | Increased polarization heat; onset of minor exothermic side reactions. |
| c) Initial Side Reactions & First Voltage Peak | Voltage rises to first critical peak (Vcr). | Decomposition of Solid Electrolyte Interphase (SEI); reaction of plated lithium with electrolyte. | Exothermic SEI decomposition and lithium-electrolyte reactions. |
| d) Gas Generation & Venting | Voltage decreases slowly from peak. | Severe reactions between plated Li and electrolyte; massive gas generation (hydrocarbons, CO2). | Highly exothermic gas-producing reactions. |
| e) Post-Venting Transient | Voltage rises slightly after venting. | Pressure release slows internal reactions; temporary re-stabilization. | Continued, but reduced, exothermic side reactions. |
| f) Thermal Runaway & Short Circuit | Voltage spikes sharply, then collapses to ~0V. | Separator meltdown/internal short circuit (ISC); electrolyte & binder decomposition; violent reactions. | Intense exothermic reactions from ISC and material decomposition. |
The initial stage (a) represents the period where the cell, despite being at nominal 100% SOC, continues to accept charge. Lithium ions intercalate into the graphite anode, and the temperature rise is modest, governed primarily by Joule heating and the reversible heat of the electrochemical reaction. The reaction can be summarized as:
$$ \text{LiFePO}_4 + 6\text{C} \rightarrow \text{FePO}_4 + \text{LiC}_6 $$
The heat generation rate at this stage can be approximated by:
$$ \dot{Q} = I(V – U) + I T \frac{dU}{dT} $$
where $I$ is current, $V$ is terminal voltage, $U$ is open-circuit voltage, and $T$ is temperature.
Stage (b) commences when the cathode is severely over-delithiated. The voltage inflection point (Vip) signifies that lithium ions can no longer be supplied from the cathode and must come from the electrolyte. This leads to lithium metal plating on the anode surface:
$$ \text{Li}^+ + e^- \rightarrow \text{Li}_{(s)} $$
The plated lithium often forms dendrites. The associated change in anode potential manifests as a voltage plateau (Vp).
The onset of significant exothermic side reactions defines stage (c). The metastable SEI layer, a passivation film on the anode, begins to decompose around 80-120°C. Concurrently, the plated lithium metal reacts with the electrolyte solvents (e.g., ethylene carbonate (EC), dimethyl carbonate (DMC)). Key initial reactions include:
$$ (\text{CH}_2\text{OCO}_2\text{Li})_2 \rightarrow \text{Li}_2\text{CO}_3 + \text{C}_2\text{H}_4 + \text{CO}_2 + 0.5\text{O}_2 $$
$$ 2\text{Li} + (\text{CH}_2\text{OCO}_2\text{Li})_2 \rightarrow 2\text{Li}_2\text{CO}_3 + \text{C}_2\text{H}_4 $$
The dynamic balance between lithium plating and consumption leads to the first voltage peak (Vcr). Temperature at the terminals and PRV of the target LiFePO4 battery begins a noticeable, steady climb.
Stage (d) is characterized by accelerated gas generation. As temperature increases further, reactions between plated lithium and electrolyte become more violent, producing hydrocarbon gases and increasing internal pressure dramatically.
| Electrolyte Component | Reaction with Plated Lithium (Li) | Main Gas Products |
|---|---|---|
| Ethyl Methyl Carbonate (EMC) | $\text{Li} + \text{CH}_4\text{H}_8\text{O}_3 \rightarrow \text{CH}_3\text{CHOCO}_2\text{Li} + \text{CH}_4$ | Methane (CH4) |
| Dimethyl Carbonate (DMC) | $2\text{Li} + \text{C}_3\text{H}_6\text{O}_3 \rightarrow \text{Li}_2\text{CO}_3 + \text{C}_2\text{H}_6$ | Ethane (C2H6) |
| Ethylene Carbonate (EC) | $2\text{Li} + \text{C}_3\text{H}_4\text{O}_3 \rightarrow \text{Li}_2\text{CO}_3 + \text{C}_2\text{H}_4$ | Ethylene (C2H4) |
The substantial gas pressure eventually forces the Pressure Relief Valve (PRV) to open, releasing smoke and flammable gases. This pressure release causes a momentary reduction in reaction intensity, leading to stage (e), where the voltage may exhibit a slight recovery.
The final and most catastrophic stage (f) is triggered by separator failure. The intense heat causes the polyolefin separator (e.g., polyethylene, PE) to melt and shrink, leading to a large-scale internal short circuit (ISC). The voltage spikes momentarily due to increased internal resistance and electrode detachment before plummeting to near zero. The ISC releases a massive amount of heat locally. Subsequently, at temperatures exceeding 200°C, the electrolyte salt (LiPF6) and the polyvinylidene fluoride (PVDF) binder decompose exothermally:
$$ \text{LiPF}_6 \leftrightarrow \text{LiF} + \text{PF}_5 $$
$$ \text{PF}_5 + \text{Solvent} \rightarrow \text{Various phosphorus oxyfluorides} + \text{HF} + \text{CO}_2 + \text{Hydrocarbons} $$
$$ -(\text{CH}_2-\text{CF}_2)- + \text{Li} \rightarrow \text{LiF} + -(\text{CH}=\text{CF})- + 0.5\text{H}_2 $$
These reactions create a thermal avalanche. The temperature of the failing LiFePO4 battery skyrockets, with the PRV area exceeding 800°C, ejecting jet flames and dense smoke. This intense heat flux poses a severe propagation threat to neighboring cells, as evidenced by their sharply rising temperatures.
The detailed stage-wise analysis provides a clear roadmap for implementing targeted safety strategies in LiFePO4 battery pack design. Interventions can be categorized based on the thermal runaway stage they aim to mitigate.
| Safety Strategy Category | Targeted Stage(s) | Example Measures | Mechanism of Action |
|---|---|---|---|
| Active Safety / Early Intervention | (a), (b), (c) | Advanced Battery Management System (BMS) with overcharge detection; Immersion cooling; Enhanced liquid cooling plates. | Prevents conditions leading to lithium plating; rapidly removes heat from cell to suppress onset and early growth of exothermic reactions. |
| Passive Safety / Propagation Mitigation | (d), (f) (Post-ignition) | Fire-resistant module/pack materials; Intumescent barriers; Automatic fire suppression (e.g., aerosol, water mist, perfluorohexanone). | Contains flames and hot particles; physically isolates failing cell; rapidly cools the module to absorb heat and delay thermal propagation. |
| Intrinsic Safety / Material-Level | All stages (fundamental) | Thermally stable separators (ceramic-coated); Flame-retardant electrolyte additives (e.g., FEC, HFPM); Solid-state/semi-solid electrolytes. | Raises the onset temperature of key failure reactions (SEI分解, separator melt); eliminates or reduces flammable organic electrolyte. |
The efficacy of an active strategy like immersion cooling can be modeled by considering enhanced heat dissipation. The modified heat balance equation during stages (b)-(c) becomes:
$$ m C_p \frac{dT}{dt} = \dot{Q}_{gen} – h A (T – T_{coolant}) $$
where $m$ is cell mass, $C_p$ is heat capacity, $\dot{Q}_{gen}$ is internal heat generation rate, $h$ is the significantly improved heat transfer coefficient with immersion, $A$ is surface area, and $T_{coolant}$ is the coolant temperature. A high $hA$ value can keep $T$ below the threshold for severe SEI decomposition and lithium-electrolyte reactions, effectively prolonging the time to venting and runaway.
Similarly, the use of a flame-retardant additive like fluoroethylene carbonate (FEC) modifies the SEI composition and stability, altering the reaction kinetics in stage (c). It may promote the formation of a more robust, LiF-rich SEI layer that decomposes at a higher temperature, effectively increasing the thermal stability window of the LiFePO4 battery anode.
In conclusion, the overcharge-induced thermal runaway of a marine LiFePO4 battery pack is a sequential, staged process governed by well-defined physicochemical reactions. Each transition—from lithium plating and SEI decomposition to gas generation, venting, and final catastrophic decomposition—leaves distinct voltage and thermal signatures. This detailed understanding is paramount for designing effective multi-layered safety architectures. By mapping specific protective technologies—whether active thermal management, passive fire containment, or intrinsic material modifications—to the vulnerable phases of this failure cascade, engineers can develop robust LiFePO4 battery systems that significantly delay the onset of thermal runaway, prevent cell-to-cell propagation, and enhance the overall safety resilience critical for maritime applications. The pursuit of such integrated safety solutions, grounded in a fundamental understanding of the thermal runaway process, remains essential for the secure and sustainable electrification of the shipping industry.