A Review of Fire and Explosion Hazards in Sustainable Lithium-Ion Battery Recycling Industries

Abstract

The extensive integration of lithium-ion batteries (LIBs) into modern technologies—including portable electronics, electric vehicles (EVs), and battery energy storage systems (BESSs)—has created a critical dependency on the supply of raw materials. The ongoing shift toward clean mobility is expected to further intensify this demand. This trend coincides with a projected increase in battery waste: over the next decade, millions of tons of EV and BESS batteries will reach their end-of-life (EOL), alongside the generation of considerable manufacturing scrap. Recycling is essential for recovering critical materials and reducing dependency on primary mining, thereby benefiting the circular economy and environmental sustainability. However, EOL-LIBs are more prone to thermal runaway due to defects and aging-induced degradation, which can lead to fire and explosion incidents, as well as associated environmental and health hazards. Such incidents have been increasingly reported in recent years during transportation, storage, handling, and illegal disposal, resulting in potential loss of life, property damage, and ecological degradation. To ensure the safe design and operation of the battery recycling industry, this work provides an updated overview of the health, safety and environment (HSE) hazards posed by EOL-LIBs and the safety measures required to mitigate these hazards. First, this work outlines the structures, components, and aging mechanisms of LIBs. Second, it summarizes the state-of-the-art recycling pathways and relevant process risks, such as deactivation, dismantling, and mechanical and thermal pretreatments. Third, it reviews recent safety incidents initiated by thermal runaway of EOL-LIBs and recycling intermediates like black mass, with an emphasis on storage and handling. Fourth, recommendations for future work regarding the safe storage and processing of EOL batteries are provided. Finally, conclusions and perspectives on future research directions are presented. Continued research and development in this field are essential to improve recycling methods, optimize processes, and ensure the safe and sustainable management and legislation of EOL lithium-ion batteries.

1. Introduction

The proliferation of lithium-ion batteries (LIBs) is profoundly influencing modern life, particularly across three key sectors: portable electronics, electric vehicles (EVs), and stationary battery energy storage systems (BESSs). While the consumer market for portable applications is nearing maturity, the impetus from global carbon-neutral policies has driven a rapid increase in demand for large-scale EV and BESS applications. In the automotive sector, LIB-enabled electrification is spearheading a revolution aimed at decarbonizing transport and mitigating air pollution from internal combustion engine (ICE) vehicles. Simultaneously, LIB-based BESSs are recognized as essential for the widespread integration of renewable energy sources, as they are required to manage the inherent intermittency in power supply and ensure reliable grid operation.

The energy sector achieved a significant milestone in 2024, as the combined demand for electric vehicle and energy storage batteries surpassed 1 TWh for the first time. This unprecedented demand was overwhelmingly driven by the expansion of the EV market, which saw EV battery demand rise by 25% from the previous year to over 950 GWh [1], as illustrated in Figure 1. According to the IEA [1], EV battery demand is indeed projected to surge, with forecasts indicating it will more than triple and quintuple, exceeding 3 and 5 TWh by 2030 and 2035, respectively. This surge will subsequently result in millions of tons of waste from spent LIBs.

Like other electronics, LIBs have their own lifecycle. To close the loop and achieve a circular economy, they must be recycled after their end-of-life (EOL), as illustrated in Figure 2 [2,3]. Inevitably, this process has brought and will continue to bring emerging challenges and risks to our communities and society. Waste from spent or EOL-LIBs remains highly active due to residual states of charge (SOC) of up to 100% in some cases, along with flammable and toxic electrolytes and their vapors. This poses a potential risk of thermal runaway and subsequent fire and explosion incidents. These hazards include electrical and thermal risks (e.g., thermal runaway, fire, and/or explosion), as well as environmental hazards associated with the storage, handling, and processing—such as shredding or crushing—of LIBs. Moreover, improper disposal and informal processing methods for spent LIBs, such as landfilling, pose significant safety and environmental risks due to the inherent reactivity of their constituent materials. These processes can generate hazardous, toxic, and highly corrosive substances (e.g., hydrofluoric acid, HF) that require careful management to prevent widespread contamination.

The potential for pollutants such as HF, per- and polyfluoroalkyl substances (PFAS), and heavy metals to be released from improperly processed spent LIBs poses a significant threat to both human health and environmental quality. A compelling economic and strategic incentive for proper recycling is the valuable and critical metals—including cobalt, nickel, lithium, and copper—present in this waste, which can be recovered for use in new battery manufacturing. This “urban mining” approach is crucial for achieving a circular economy, decreasing dependency on primary mining operations, and enhancing a nation’s material security. To address these challenges and opportunities, the recycling industry is seeing increased investment in technologies for the recovery of these valuable battery materials. The established process for LIB recycling and material recovery is a three-step cycle:

• Separation: Dismantling and isolating the various battery components.
• Refining: Purifying the recovered metallic and non-metallic materials.
• Re-synthesis: Manufacturing new battery parts by incorporating the refined materials.

There are already some comprehensive review works either focusing on the early warning of thermal runaway and fire extinguishing of LIBs in general (especially during their utilization), or recycling routes and challenges of EOL-LIBs, or LCA of LIBs. However, there is still a lack of comprehensive review on the health, safety, and environment (HSE) hazards in sustainable lithium-ion battery recycling. To address this, this research provides a critical and timely analysis of the fire and explosion hazards associated with EOL-LIBs. The paper details the thermal and environmental risks present throughout the full recycling chain, from initial handling and storage to processing and primary metallurgical stages. Despite the clear and present dangers, a significant lack of empirical data on EOL-LIB safety persists. This work therefore seeks to identify these safety impacts and underscore the critical need for all stakeholders—including manufacturers, waste management services, recycling firms, and policymakers—to prioritize this issue. The study warns that continued inappropriate handling, inadequate data, and improper hazard identification will inevitably result in a higher frequency of serious incidents, including fires, explosions, human fatalities, and severe environmental and ecosystem degradation. Finally, this work defines the knowledge gaps that must be bridged to establish and sustain safe recycling management systems.

2. Li-Ion Battery and Its Degradation

2.1. Li-Ion Battery Structure and Material Properties

The operational principle of a LIB is the reversible conversion of chemical to electrical energy, which has made it the dominant rechargeable battery technology due to its superior energy density. A LIB is composed of several key parts: a cathode, anode, electrolyte, separator, and current collectors. Its function is based on the movement of lithium ions (Li⁺) between the cathode and anode through an electrolyte-saturated separator. Both the cathode, composed of layered transition metal oxides (such as LCO, NMC, LMO, or LFP), and the anode, typically graphite and/or silicon with minor additives, are designed to accommodate the reversible insertion and extraction of these ions. The electrolyte, a solution of a lithium salt (LiPF₆) in organic carbonates (e.g., PC, EC, DMC, EMC), acts as the ionic transport medium. Details on the mass distribution of these materials in a representative EV pouch cell are presented in Figure 3 [4].

The inherent volatility and flammability of their organic electrolytes, along with the particulate nature of their solid components (graphite and metal oxides), mean that LIBs pose substantial fire and pollution risks. The flammability limits and flash points of these electrolytes [5,6] are summarized in

Table 1. Lower/Upper flammability limit (LFL/UFL) and flash/boiling point of typical LIB electrolytes.

Component	Chemical Formula	Flash Point/Boiling Point, °C at 1 atm	LFL/UFL, vol. % in Air
Ethylene carbonate (EC)	C3H4O3	143/238	3.6/16.1
Diethyl carbonate (DEC)	C5H10O3	33/126	1.4/11.7
Dimethyl Carbonate (DMC)	C3H6O3	18/90	4.2/12.9
Ethyl Methyl Carbonate (EMC)	C4H8O3	23/109	3.6/16.1
Propylene carbonate (PC)	C4H6O3	123/242	1.7/21

. Generally, a higher boiling point (BP) correlates with reduced flammability and enhanced thermal stability; therefore, solvents with higher BPs are preferable for improving battery safety [7,8]. Beyond the fire and explosion hazards associated with volatile and highly energetic electrolytes, the decomposition of LIBs can release toxic substances such as HF and benzene, which present serious health risks. Of particular concern is the potential for inhalable particles of nickel, cobalt, and manganese compounds (<10 μm) to have carcinogenic effects. These dangers are amplified by improper disposal, such as discarding batteries in municipal waste streams, which can trigger fires during transit or at waste processing centers. Moreover, events like overcharging, short-circuits, or overheating can lead to “thermal runaway” (TR)—a self-accelerating exothermic reaction that results in a massive release of heat and toxic gases, presenting critical technical and health hazards. The specific nature and consequences of these dangers will be detailed in the subsequent section.

Different cathode materials significantly affect the energy density of LIBs, which consequently determines their application scenarios.

Table 2. Energy density of various lithium-ion battery types.

Battery Type (Cathode Material)	Energy Density (Wh/kg)	Typical Application	Reference
LCO (Lithium Cobalt Oxide)	150–250	Portable electronics	[8]
LFP (Lithium Iron Phosphate)	100–175	EVs, BESSs	[4]
NCM111 (LiNi1/3Co1/3Mn1/3O2)	140–180	EVs, consumer electronics	[9]
NCM523 (LiNi0.5Co0.2Mn0.3O2)	160–200	EVs, BESSs	[10]
NCM622 (LiNi0.6Co0.2Mn0.2O2)	180–220	Mid-to-high-end EVs	[3]
NCM811 (LiNi0.8Co0.1Mn0.1O2)	200–260	High-performance EVs	[3]
NCA (LiNi0.8Co0.15Al0.05O2)	220–280	Premium EVs	[11]

summarizes the typical energy density ranges and corresponding applications of LIBs featuring mainstream cathode materials.

2.2. Degradation of Li-Ion Battery

The performance of LIBs deteriorates over time due to aging processes initiated by usage and environmental stressors. This gradual degradation compromises the battery’s ability to store and supply energy, ultimately marking its end-of-life (EOL). A variety of complex physical and chemical mechanisms contribute to this decline by altering the electrolyte, electrodes, separator, and current collectors. As detailed in Figure 4 [12,13], degradation can be broadly categorized into three main modes [9,14,15]:

• Loss of lithium inventory (LLI): This involves the irreversible consumption of lithium ions through processes such as lithium plating, dendrite formation, and the growth of the SEI.
• Anode active material loss: Degradation at the negative electrode is driven by particle cracking, delamination leading to a loss of electrical contact, and the passivation of active sites by resistive surface layers.
• Cathode active material loss: At the positive electrode, degradation results from particle cracking, loss of electrical connection, and structural changes that reduce the material’s capacity to intercalate lithium.

It is important to recognize that the chemical composition of materials from processed EOL batteries is less predictable than that of pristine batteries. While mechanical shredding may produce components similar to their original state, thermal processing yields a complex mixture of products due to internal chemical changes that can mirror those of a thermal runaway. This dynamic requires battery recycling operators to maintain constant awareness of the evolving material composition during processing.

The state of health (SOH) is a critical metric for assessing the overall degradation of a battery. It serves as a quantitative measure of the difference between a battery’s current condition and its ideal, brand-new state, with 100% SOH representing full original capacity. As a battery is subjected to various stressors throughout its lifespan, its SOH gradually declines [16]. While SOH can be approximated by measuring the open-circuit voltage (OCV), the degradation that influences a cell’s thermodynamic behavior—and thus its OCV—is complex and multifaceted. Primary drivers of this degradation include high charge/discharge rates, extreme voltage levels, deep cycling, and exposure to high or low operating temperatures.

Typically, a battery is no longer suitable for EV applications once its SOH drops below 80%. However, these batteries can be repurposed for less demanding uses, such as BESS, in what is known as “second-life” applications. Accurate SOH estimation is therefore essential for effective LIB health state management. The reliability of these estimations, however, can be affected by factors such as reliance on specific operating conditions, the quality of degradation features utilized, and limitations in model interpretability. Furthermore, SOH estimation precision is inherently linked to the degree of LIB degradation [17,18], as degradation is the primary driver of SOH changes. This discussion does not cover the intricate process of selecting degraded LIBs for second-life applications based on SOH, which remains a complex area of study.

3. Recycling Technologies and Routes

Recycling EOL-LIBs is essential for recovering valuable materials like cobalt, nickel, lithium, and copper for reuse in new battery production [19]. This process is crucial for establishing a circular materials loop and mitigating dependence on primary mining [19,20,21,22], although it can be energy-intensive and complex compared to other waste management options.

Effective recycling requires careful planning and execution, beginning with the safe storage and pretreatment of substantial volumes of EOL-LIBs [4,19]. The overall recycling process typically follows a multi-stage chain [4,19]:

• Optional preparation before recycling: This initial step involves sorting batteries by chemistry, size, and type. It may also include disassembly, deactivation (discharging), and the separation of active cathode material from current collector foils to prepare them for the primary recycling stages.
• Multi-step pretreatment in dedicated plants: This stage, often performed by specialized companies, involves processes such as discharging, crushing, grinding, and various physical separation techniques designed to prepare the battery materials for subsequent processing.
• Multi-step main processing in non-dedicated plants: Pretreated battery materials undergo further processing—which may include hydrometallurgy, pyrometallurgy, or direct recycling—to extract the valuable materials.
• Final refinement and repurposing: The recovered materials are refined to a clean and usable state, suitable for manufacturing new battery products or for use in other related industries.

The flow of these applied techniques is further visualized in the simplified flowchart in Figure 5 [5].

The recycling of spent LIBs begins with an essential pretreatment stage designed to facilitate safe handling, storage, and transportation while simultaneously reducing waste and segregating battery components [23]. Following pretreatment, the extracted active cathode material is subjected to one of three primary recycling technologies [24]:

• Pyrometallurgical processes operate at high temperatures to smelt batteries, effectively recovering metals such as cobalt, nickel, and copper as alloys. However, this method often results in the loss of lithium to the slag—a challenge that researchers are currently addressing.
• Hydrometallurgical processes utilize a chemical approach, employing acids or basic leachants to selectively dissolve battery components. This is followed by purification steps, such as solvent extraction or precipitation, to isolate and recover specific metals for reuse.
• Direct recycling is an emerging technology focused on the recovery and reuse of cathode materials with minimal structural processing. This method is considered the most ecologically favorable due to its lower energy consumption and reduced generation of waste products.

While some academic literature suggests that EOL-LIB cells could potentially be directly repaired, there are currently no industrial applications of this approach. This is primarily due to cost and time constraints, as well as the significant challenges associated with scaling such operations.

3.1. Recycling Technologies and Operations

The initial stage of EOL-LIB recycling typically involves deactivating the battery system. This crucial safety step is often, though not always, followed by the dismantling of the battery pack—usually down to the module level, and less frequently to individual cells [9,25]. Deactivation can be accomplished through several methods:

• Electrical Discharge: The energy stored in the battery is reduced via a controlled electrical discharge, sometimes followed by short-circuiting, to ensure safety for subsequent handling.
• Chemical Treatment (Saltwater Immersion): Immersion in a saline solution facilitates an electrochemical discharge that consumes residual charge and minimizes fire hazards.
• Thermal Treatment: Heating the battery systems to temperatures exceeding 200 °C (e.g., via pyrolysis) can also achieve deactivation by neutralizing the reactive components.

Following deactivation and optional dismantling, material recovery is achieved through three primary process technologies, which are often used in combination: mechanical, hydrometallurgical, and pyrometallurgical methods [5,9,25]. Figure 6 illustrates the various configurations in which these process types and their steps can be integrated and applied at different levels of processing depth.

Safely handling spent LIBs is paramount because they can retain significant residual power [26]. If these batteries are processed—for example through disassembly—without prior deactivation, the stored electrical energy may be released in an uncontrolled and rapid manner, resulting in localized overheating, fires, or explosions [27]. Such incidents not only damage recycling infrastructure but also diminish the quality and value of the high-value metals intended for recovery [28].

Given the numerous documented fires and explosions during the transportation and pretreatment of LIBs, it is essential that deactivation procedures are performed before any separation processes to reduce the residual power to a safe level [26,29,30,31]. A variety of deactivation methods are available, including the use of solid electrical conductors, aqueous solutions, cryogenic cooling, and thermal treatment. The advantages and disadvantages of these techniques are detailed in

Table 3. Dis-/advantages of several battery deactivation methods and the related consequences.

Approach	Advantages	Disadvantages	Consequences
Short-circuiting	Fast discharge speed, no aqueous pollution	Unsafe, venting of toxic smokes	Rapid temperature increase may cause damage to battery components and loss of electrode material [32,33]
Graphite discharge	Fast discharge speed, no aqueous pollution	Unsafe, potential dust pollution, unstable discharge speed
Salt solution discharge	Cheap, easy to obtain	Low discharge speed, potential hydrogen emission	LIB corrosion is a major practical concern for aqueous discharge, as it can lead to incomplete discharge or leakage of internal battery components. Metallic pollutions can leach into the solution, contaminating the material being recovered [10,34].
Acidic and alkaline discharge	Fast and stable discharge	Potential solution pollution, potential hydrogen emission
Electrical discharge	Eliminates toxic gas emission, energy conservation	Low efficiency, potential safety risks especially for defected batteries or batteries with low SOH	Deep-discharging leads to the irreversible, solid-state amorphization of the active cathode’s crystal structure, resulting in its destruction. Furthermore, the subsequent voltage rise can cause copper to precipitate within the cell, contaminating downstream products [35].
Thermal deactivation	Safe destruction, eliminates the potential generation of toxic gases	Requirements of high-temperature and accurate process condition controlling	Despite thermal decomposition not altering the metallic constituents, the presence of decomposition products renders the recovered electrolyte in the condenser system largely unsuitable for reuse. This process also causes a fractional loss of graphite and lithium, which constitutes a waste of resources [36,37].
Cryogenic freezing	Large handling capacity, safe and environmentally friendly	High cost and equipment requirements	There have been no reported negative impacts from the cryogenic cooling of LIBs for which the SOH has not been determined. This cooling procedure is implemented to guarantee safe transport, after which the LIBs are later repurposed [38].

. Operators must be fully aware of the inherent risks associated with handling non-deactivated LIBs and select the most appropriate deactivation method based on their specific operational needs and resources. The choice of a deactivation method must be made with careful consideration of both safety and environmental impacts [26,28,29].

Spent LIBs pose safety risks during handling due to a potential gradual voltage rebound following discharge [39]. Furthermore, fully discharging cells to 0 V can induce the oxidation of the copper anode current collector; the resulting copper ions may dissolve into the electrolyte and subsequently deposit on the cathode. This contamination compromises the purity of recovered cathode materials, thereby limiting their potential for reuse in the production of new batteries [19]. In contrast, conductive solution discharge and thermal deactivation are preferred among deactivation methods, as they provide controlled, relatively low-cost, and straightforward operations that facilitate industrial adoption.

Following deactivation, spent LIBs undergo either manual dismantling or mechanical crushing to separate their constituent parts and recover the enriched active cathode powder. Manual dismantling facilitates the precise separation of components such as plastic and metallic casings, polymers, and electrode foils. However, modern recycling facilities typically favor mechanical crushing—often followed by techniques like magnetic separation—due to its higher processing capacity. Subsequent fine crushing and sieving are then utilized to extract the active cathode material (black mass) [40].

Manual disassembly involves the physical teardown of LIB components using specialized tools after the batteries after safety deactivated. Prior discharging is considered the safest practice for personnel on the disassembly line [41]. The process proceeds from stripping the battery at the cell or module level to the ultimate separation of the anode and cathode materials [42]. This method can achieve a high recovery yield of approximately 80% or more of the total LIB mass [43], and the recovered components exhibit higher purity compared to those obtained from mechanical shredding. Efficient disassembly positively influences cost-effectiveness by minimizing the need for complex downstream separation.

While manual dismantling is technically simple, it suffers from low efficiency, requires a large workforce, and poses significant safety hazards. This makes it the most cost-intensive step in regions where labor costs are high [44]. Key safety concerns include the generation of toxic HF gas from LiPF₆ electrolyte decomposition in humid air and the danger of combustion from residual lithium reacting with moisture [45]. Consequently, the process mandates a dry environment and the use of personal protective equipment (PPE). Nonetheless, manual methods are still utilized in facilities like LithoRec to handle non-standard battery designs or batteries with unknown configurations [46].

A proof-of-concept study successfully proposed a robotized method for electrochemical impedance spectroscopy (EIS) to estimate the SOH of a Nissan Leaf LIB, marking the first time a robot was used for this purpose [47]. The framework, which utilized a custom connector to a Franka robot arm, was designed to minimize human interaction ensuring a firm connection with battery terminals [47]. In contrast, fully automated disassembly of LIBs remains challenging due to the vast number of battery variants, with each model making up only a small market fraction. Furthermore, bonding mechanisms such as bolts and fasteners are often damaged or corroded by the time a battery reaches its EOL, adding significant complexity to any automated processes [44].

3.2. Pretreatment

An essential initial step in the industrial recycling of LIBs is pretreatment, which can be mechanical, thermal, or a combination of both. Thermal pretreatment—encompassing pyrolysis, thermolysis, and degassing—is a proven method applied to whole batteries, shredded material, and production scrap [48,49]. This process is integrated early in the recycling sequence, typically occurring after initial sorting and discharge or directly following crushing. It is generally followed by mechanical separation and sorting to refine the “black mass” before it undergoes hydrometallurgical or pyrometallurgical recovery [48,49].

Thermal treatment options, such as incineration or pyrolysis/degassing are a key step in industrial recycling [50]. Pyrolysis involves heating the material in an oxygen-free environment—typically an inert atmosphere (e.g., nitrogen or even carbon dioxide) or a vacuum—to decompose organic components like binders, separators, and electrolytes through cracking reactions [51,52,53]. This process also effectively removes volatile halogens, such as fluorine, via the off-gas [51].

A related process, thermolysis, operates under limited oxygen conditions, falling between pyrolysis and combustion, as illustrated in Figure 7. Preliminary research suggests that thermolysis may help mitigate thermal runaway by permitting slow oxidation reactions to occur below the limiting oxygen concentration (LOC) of the smoke mixtures. However, the variable nature of smoke components over time and space makes it difficult to precisely determine the LOC required to prevent smoke explosions.

A crucial safety constraint for any thermal pretreatment is maintaining the temperature below the melting point of aluminum (660 °C). Exceeding this temperature risks triggering side reactions, such as the thermite reaction, which could destroy the cathode materials and release significant heat [49].

Thermal pretreatment is a crucial step in the recycling of LIBs that provides a range of advantages:

• Enhanced safety: It facilitates the safe, controlled deactivation of battery cells, eliminating the risk of thermal runaway, fires, and explosions during handling, logistics, and subsequent processing stages like shredding [50].
• Improved mechanical separation: By removing organic compounds—particularly polyvinylidene fluoride (PVDF) binders—the process improves the delamination of current collector foils. This enhances the recovery of high-purity copper and aluminum while increasing the yield of the valuable black mass during mechanical sorting [53,54].
• Efficient hydrometallurgical processing: The removal of organics improves leaching efficiency and kinetics, especially when thermal treatment is conducted between 500 °C and 600 °C, thereby avoiding uneconomically high operating costs [53,55].
• Volatile halogen removal: The process removes volatile halogens, such as fluorine, in the off-gas. This facilitates the targeted neutralization of hazardous HF and simplifies downstream recycling steps.
• Enabling new recycling methods: Thermal pretreatment can induce phase transformations in metal oxides (Li, Ni, Mn, and Co) [52,56]. This enables advanced approaches, such as early-stage lithium recovery [9] or magnetic separation [57].

Implementation of pyrolysis requires efficient furnace solutions with oxygen shielding and explosion protection. The treatment of the resulting off-gas is a critical consideration, necessitating specialized burner designs to manage fluorine and phosphorus distribution. Furthermore, an energy analysis is necessary to determine the potential for auto-thermal operation, where the battery’s inherent energy sustains the process under controlled conditions.

Post-shredding pyrolysis also offers benefits like converting elemental lithium to safer lithium carbonate. However, it presents several challenges:

• Off-gas management: The process generates flammable and toxic gases requiring specialized scrubbing and combustion systems.
• Particulate matter: Pyrolysis creates a dry and fine product that can easily become airborne, complicating material handling.
• Temperature control: Precise regulation is critical because exceeding the melting point of aluminum (660 °C) can lead to reactor clogging and localized thermite reactions fueled by the inherent oxygen in cathode materials.
• Downstream limitation: It prevents the physical separation of cathode from anode materials, which limits the potential for direct recycling.

Comminution is the mechanical breakdown of spent LIBs using tearing, shearing, crushing, or milling. To ensure safety, batteries are typically shredded in an anoxic atmosphere (e.g., N₂, CO₂, or vacuum) or submerged in an aqueous solution. After size reduction, the different product streams are physically separated to concentrate the valuable materials [58]. Casing removal, for instance, is commonly performed using density and magnetic separation as shown in Figure 8 [9]. Even after discharge, residual elemental lithium on anodes remains a safety consideration. To address feeding risks in combined processes, a “wet cake” technique is currently being evaluated to safely introduce shredded material into pyrolysis furnaces.

3.3. Primary Treatment

In general, there are three primary treatments: pyrometallurgy, hydrometallurgy and direct recycling. Commercial recyclers have traditionally employed pyrometallurgical processes to recover cobalt from spent LIBs [59]. In a notable example, the Umicore Group developed a method that treats spent LIBs similarly to natural ores, where the only pretreatment being the disassembly of large battery packs into individual cells. These cells are then fed into a three-zone shaft furnace [22,60]:

• Pre-heating zone (<300 °C): This upper zone evaporates electrolytes safely, mitigating the risk of explosions.
• Pyrolysis zone (700 °C): In the middle zone, plastics are pyrolyzed. The energy released from this process helps maintain the furnace temperature and reduces the overall energy consumption of the subsequent smelting phase.
• Smelting and reduction zone (1200–1450 °C): In the bottom zone, smelting and reduction reactions occur, yielding a metal alloy rich in copper, cobalt, nickel, and iron. Simultaneously, a slag containing lithium, aluminum, silicon, calcium, and some iron is formed [60].

The pyrometallurgical process is largely limited to the recovery of copper, cobalt, and nickel. A key consideration for its economic viability is the price and concentration of cobalt. However, the gradual shift in automotive LIBs toward alternative cathode materials, such as LMO and LFP, is challenging the long-term profitability of this traditional method [61]. A major drawback is that these industrial processes typically do not recover lithium from the slag—a resource expected to become scarcer as EV production increases. Pyrometallurgy also suffers from high energy consumption and the emission of hazardous gases like dioxins and furans. Consequently, integrated “pyro-hydro” or “thermomechanical-hydro” processes are gaining popularity among recyclers such as Umicore, ACCUREC, and Brunp to achieve higher recovery rates and minimize environmental hazards.

Hydrometallurgy is considered the mainstream and preferred technology globally [59], due to its superior environmental and recovery performance. Unlike pyrometallurgy, this method operates under mild conditions, results in lower exhaust emissions, and achieves higher recovery rates for valuable metals, including lithium. However, the process is more extensive, requiring a pretreatment stage to produce black mass, and generates a larger volume of wastewater that necessitates rigorous management. Despite this, the overall environmental footprint of hydrometallurgy is generally considered lower than that of pyrometallurgy [22].

While these methods are the most established, their intrinsic limitations have spurred the emergence of direct recycling. This “third way” aims to minimize the drawbacks of existing processes by recovering cathode materials with their crystal structure intact. Based on the description from Ref. [62],

Table 4. The comparison of the three main existing battery recycling routes.

Technology Type	Advantages	Disadvantages
Hydrometallurgy	1. High purity: Higher active material purity improves battery performance/lifespan 2. Wide applications: Compatible with various battery chemistries 3. Valuable metal recovery: Reduces mining demand for cobalt, nickel, and lithium	1. High cost: Requires more steps and energy 2. Environmental concerns: Generates more waste/emissions 3. Safety risks: Involves hazardous acids/chemicals if mishandled
Pyrometallurgy	1. High metal recovery rate: Recovers high percentages of cobalt, nickel, and lithium 2. Lower cost: Fewer steps and less equipment needed 3. Reduced hazardous waste: High-temperature process breaks down dangerous materials	1. Limited applications: Only suits specific chemistries; some metals may be lost 2. Environmental concerns: Emits greenhouse gases/other pollutants 3. Safety risks: High temperatures risk explosions/fires
Direct Recycling	1. Cost-effective: Fewer steps and lower energy use 2. Environmentally friendly: Less waste and emissions 3. Resource conservation: Reuses battery active materials for reparation	1. Limited applications: Only for specific chemistries; requires high-purity active materials 2. Capacity degradation: Reduces overall battery performance 3. Safety risks: Risk of thermal runaway/fire if batteries/materials are mishandled/contaminated

summarized the comparative advantages and disadvantages of these three approaches.

The fundamental principle of direct recycling is to preserve the crystalline structure and functional properties of active cathode materials from spent batteries, rather than deconstructing them into their elemental components. The objective is to restore the original electrochemical capacity for direct reuse in new battery manufacturing; current research is exploring mechanical, thermal, chemical, and electrochemical processes to achieve this. However, significant challenges currently impede the industrial application of this method, particularly regarding cost, time, and scalability. Key obstacles include the lack of standardized battery designs, difficulties with automated disassembly, issues with electrode delamination, and barriers to sustainable implementation. As direct recycling technologies are still in their early developmental stages and face considerable scientific hurdles, substantial effort is required to enable the transition from laboratory proof-of-concept to industrial-scale operation.

Current research by the author focuses on producing highly purified black mass with high cathode material content for direct recovery and regeneration. Preliminary results suggest that direct recycling represents the most promising route for the near future in terms of process optimization, energy efficiency, and carbon footprint reduction, while simultaneously minimizing HSE risks, as illustrated in Figure 9 [63].

Each LIB recycling technology involves distinct safety risks, particularly fire and explosion hazards derived from specific process characteristics.

Table 5. Comparative risk matrix for different LIB recycling technologies.

Recycling Technology	Fire Risk	Explosion Risk	Key Hazard Sources	Mitigation Measures
Pyrometallurgy	High	Medium	High-temperature electrolyte decomposition, toxic gas release (HF, POF3)	Inert atmosphere smelting, off-gas scrubbing systems
Hydrometallurgy	Medium	Low	Acid-leaching corrosion, heavy metal contamination	Sealed reaction vessels, waste acid neutralization
Direct Recycling	Low	Low	Residual charge in electrodes, binder decomposition	Precision discharging, low-temperature pretreatment
Mechanical Pretreatment	High	High	Shredding-induced short circuit, black mass dust clouds	Anoxic shredding, dust concentration monitoring
Thermal Pretreatment	Medium	Medium	Electrolyte volatilization, SEI layer breakdown	Temperature control (<660 °C), gas collection systems

systematically compares the fire/explosion risks, key hazard sources, and targeted mitigation measures of mainstream recycling technologies.

4. Recycling Process Products and Hazards

Recycling LIBs involves inherent thermal and chemical hazards that demand rigorous safety protocols:

• Gaseous and vapor hazards: The generation of toxic or flammable gases and vapors poses significant risks during processing stages such as shredding or drying.
• Fire hazards: Flammable components within LIBs and exothermic side reactions—exacerbated by thermal runaway—can lead to fires if not strictly controlled.
• Explosion hazards: Flammable gases or reactive solid chemicals can create explosion risks throughout the recycling process if not safely contained and handled in enclosed environments.
• Toxic and corrosive liquid hazards: Side reactions during processing can produce hazardous liquid chemicals, such as corrosive agents, necessitating careful containment and management.

It is important to note that numerous other chemicals may be generated by side reactions if batteries experience heating; however, these are excluded from this overview due to their wide variety and the fact that their formation is highly dependent on the extent of thermal exposure.

Compared to other traditional e-waste, retired LIBs present more complex challenges. These batteries retain 20–80% of their power, maintaining significant electrochemical activity and thermal safety risks during storage, transportation, dismantling, and post-processing. Improper treatment during pre-recycling operations—such as mechanical crushing, shearing, and unpacking—can easily lead to battery cell short circuits and self-heating. These phenomena can lead to thermal runaway, potentially resulting in combustion or explosion accidents that threaten both the recycling operational safety and the environment. Moreover, the battery’s complex internal chemical composition can release toxic, harmful, flammable, and explosive gases/vapors (e.g., HF, CO, volatile organic compounds (VOC)) at elevated temperatures. Improper management of these emissions poses a severe threat to the health of operators and surrounding ecosystem.

4.1. End-of-Life Li-Ion Battery Hazards

4.1.1. Thermal Runaway Hazards

A fundamental difference in the thermal runaway mechanism exists between spent and new LIBs, with the former being significantly more prone to failure. This increased vulnerability is a direct result of the degradation and aging that occur in EOL batteries throughout their operational lifespan.

• Contributing factors: Aging leads to multiple degradation modes, such as capacity fade, structural damage to the casing that can lead to electrolyte leakage, electrode deformation, and the thickening or instability of the Solid Electrolyte Interphase (SEI).
• Impact on thermal stability: These factors collectively reduce the battery’s thermal stability and lower the onset temperature of exothermic reactions, thereby increasing the risk of thermal runaway.
• Thermal runaway mechanism: Thermal runaway is an internal chain reaction where the heat generated by exothermic processes—such as SEI decomposition and cathode oxygen release—outpaces the battery’s ability to dissipate it, as illustrated in Figure 10 [64]. Once the internal temperature exceeds a critical threshold, it triggers a self-sustaining feedback loop of escalating temperature and pressure that can ultimately lead to cell rupture, combustion, or explosion [7,65,66].

Lithium salts, such as the commonly used but hazardous LiPF₆, contribute to the thermal instability of LIBs. The thermal decomposition of LiPF₆ produces highly reactive phosphorus pentafluoride (PF₅) acid, as presented in the following reaction [9],

LiPF₆ → LiF + PF₅

(1)

In addition to the lithium salt, organic electrolytes (e.g., C₅H₁₀O₃, DEC) can also undergo autonomous decomposition at elevated temperatures, which is represented by a series of subsequent reactions (Equations (2)–(7)) [65,67],

C₂H₅OCOOC₂H₅ + PF₅ → C₂H₅OCOOPF₄ +HF + C₂H₄

(2)

C₂H₄ + HF → C₂H₅F

(3)

C₂H₅OCOOPF₄ → PF₃O + CO₂ + C₂H₄ +HF

(4)

C₂H₅OCOOPF₄ → PF₃O + CO₂ + C₂H₅F

(5)

C₂H₅OCOOPF₄ + HF → PF₄OH + CO₂ + C₂H₅F

(6)

C₂H₅OH + C₂H₄ → C₂H₅OC₂H₅

(7)

As temperature between 90~120 °C, the thermal runaway reaction process begins with the breakdown of the thin SEI layer. The exothermic reaction release gases such as C₂H₄, CO₂ and O₂, fueling the subsequent stages of the runaway event [68]

(CH₂OCO₂Li)₂ → Li₂CO₃ + C₂H₄ +CO₂ + 1/2O₂

(8)

At temperature around 100 °C, heat-induced reactions between organic solvents and the anode with intercalated lithium produce flammable gases such as C₂H₄, C₃H₆ and C₂H₆ [65,67].

2Li + C₃H₄O₃ (EC) → Li₂CO₃ + C₂H₄

(9)

2Li + C₄H₆O₃ (PC) → Li₂CO₃ + C₃H₆

(10)

2Li + C₃H₆O₃ (DMC) → Li₂CO₃ + C₂H₆

(11)

The organic solvents within the battery can react with oxygen generated internally, compounding the thermal runaway process, as illustrated in Equations (12)–(15).

5O₂ + 2C₃H₄O₃ (EC) → 6CO₂ + 4H₂O

(12)

4O₂ + C₄H₆O₃ (PC) → 4CO₂ + 3H₂O

(13)

3O₂ + C₃H₆O₃ (DMC) → 3CO₂ + 3H₂O

(14)

6O₂ + C₅H₁₀O₃ (DEC) → 5CO₂ + 5H₂O

(15)

The decomposition of the cathode material in a charged state warrants special attention due to its critical role in thermal runaway process. This process releases O₂ gas, which fuels further exothermic reactions within the cell (see exemplary reactions below [11]). A primary consequence of this liberation is the combustion of organic electrolytes by the internal oxygen, which releases a substantial amount of heat and accelerates the thermal runaway phenomenon [11].

Li_0.5CoO₂ (LCO) − 1/2 LiCoO₂ + 1/6 Co₃O₄ + 1/6 O₂

(16)

Li_0.36 Ni_0.8 Co0_.15 Al_0.05O₂ (NCA)→ 0.18 Li₂O + 0.8 NiO + 0.05 Co₃O4 + 0.025 Al₂O₃ + 0.372 O₂

(17)

Li_0.35 (NiCoMn)1/3 O₂ (NCM) → Li_0.35 (NiCoMn)1/3O₂ − y + y/2O₂

(18)

2Li₀FePO₄ (LFP) → Fe₂P₂O₇ +0.5O₂

(19)

The solvents, such as EC and PC, are capable of reacting with de-lithiated Li_0.5CoO₂ [69]. This is typically analyzed under the condition that the solvents are completely burned or reacted.

10Li_0.5CoO₂ + C₃H₄O₃ (EC) → 5LiCoO₂ + 5CoO + 3CO₂ + 2H₂O

(20)

16Li_0.5CoO₂ + C₄H₆O₃ (PC) → 8LiCoO₂ + 8CoO + 4CO₂ + 3H₂O

(21)

When lithium-ion batteries undergo thermal runaway, they vent a white vapor—a mixture of gaseous compounds including H₂, CO, CO₂, HF, short-chain alkanes and alkenes, HCN, and NOx—along with solvent mists [70,71]. This gas mixture is released when safety valves or rupture disks activate, or in the case of a pouch cell, when the casing bursts.

• High SOC/Oxygen-rich conditions: At a high SOC and sufficient oxygen present, this vapor often ignites immediately, resulting in high-intensity jet fires.
• Low SOC/oxygen-limited conditions: In environments with a low SOC or inadequate oxygen (e.g., when vapor displaces air or fire suppressant are deployed), immediate ignition may not occur. Instead, the accumulation of the vapor can lead to: flash fire (a sudden ignition, especially in a confined space) or vapor cloud explosion (a more catastrophic scenario that occurs if the accumulated gas mixture reaches its explosive limit in highly confined areas) [70,71].

The quantitative effects of SOC on gas production capacity, mass loss, and thermal behavior—key parameters for assessing the risks of EOL-LIBs—are summarized in

Table 6. Influence of state of charge (SOC) on thermal runaway characteristics of EOL-LIBs.

SOC	Specific Gas Production (L/Ah)	Mass Loss	Thermal Runaway Onset Temperature	Maximum Temperature
25%	0.35	18%	~185 °C	~425 °C
50%	0.71	43%	~179 °C	~515 °C
75%	1.38–1.44	68–71%	~162–170 °C	~508–571 °C
100%	2.10–2.17	75–78%	~148–150 °C	~612–614 °C
~130% (Overcharge)	2.57–2.64	77%	~80–104 °C	~641 °C

. The data clearly demonstrate a positive correlation between increasing SOC and the severity of the thermal runaway hazard [72].

Thermal runaway in LIBs, especially those utilizing NMC or LCO chemistries, releases micro- and nano-sized particulate matter in addition to flammable gases and vapors [73,74]. The resulting smoke is classified as a “hybrid mixture” defined as a suspension of combustible particles in a flammable gases/air medium, as illustrated in Figure 11. A critical safety implication of this hybrid mixture is that its combination can lead to an explosion even when the concentration of each individual component—gas or dust—remains below its respective explosibility threshold, specifically the Lower Explosion Limit (LEL) for gases and the Minimum Explosion Concentration (MEC) for dusts [75].

4.1.2. Pretreatment Hazards

Exhaust gas from LIB recycling poses multiple hazards originating from electrolyte decomposition and other material reactions. Combustible gases are produced by the breakdown of electrolyte salts (like LiPF₆) and organic solvents (EC, DMC) during processing. The interaction of LiPF₆ with trace water creates highly toxic HF gas and flammable fluorinated compounds like CH₃F [76]. The thermal decomposition of polyvinylidene fluoride (PVDF) binder above 400 °C also releases flammable and toxic fluorohydrocarbons (CH₂=CF₂). Additionally, the release of oxygen from NCM cathodes during thermal events can mix with volatile electrolyte components to form an explosive gas mixture, underscoring the need for stringent safety measures throughout the recycling chain [77].

Regardless of the recycling method or atmosphere used, the processing of lithium-ion batteries results in the formation of significant quantities of toxic gases, with fluoride gases like HF and POF₃ being a key concern in both thermal and mechanical treatments [78]. Consequently, comprehensive off-gas cleaning systems are a mandatory safety feature for all LIB recycling operations. The gases produced are a complex mixture of organic and inorganic compounds whose exact composition is influenced by the battery’s specific chemistry and the recycling conditions [79]. The uncontrolled release of these toxic, flammable, and explosive gases or vapors poses severe risks to personnel and the environment.

Toxic fluoride gases are generated primarily from the decomposition of the LiPF₆ electrolyte (see Equation (1)). This decomposition yields PF₅ gas, which is highly reactive and triggers a chain reaction when exposed to moisture or high temperatures. The subsequent formation of hydrogen fluoride (HF) and the hydrolysis of PF₅ into the highly toxic phosphoryl fluoride (POF₃) represent significant hazards [13].

PF₅ +H₂O → POF₃ +2HF

(22)

LiPF₆ +H₂O → LiF + POF₃ +2HF

(23)

Toxic gases produced during the recycling of waste LIBs originate from multiple sources. Pyrolysis of NCM materials above 300 °C releases toxic nickel/cobalt oxide dust. The degradation of PVDF binders above 400 °C yields fluorohydrocarbons and PFOA. Furthermore, aged batteries contribute organophosphates (OPFRs) and PFAS from electrolyte additive decomposition [77,80]. The overall toxicity arises from the inherent battery chemistry combined with recycling conditions, such as high temperatures or oxidizing environments [79].

4.1.3. Fires and Explosions

The proliferation of LIBs has coincided with a rise in documented fire and explosion incidents across various applications, including mobility platforms, battery energy storage systems (BESSs), and extensive storage facilities (e.g., warehouses and recycling plants). As shown in Figure 12, data (adopted from [81]) indicate a dramatic global increase in such incidents between 2012 and 2024, with a particularly sharp rise observed since 2020. These incidents occur throughout the entire LIB lifecycle, including production, logistics, storage, and recycling. Recent fires in confined spaces—such as manufacturing workshops and transport containers—highlight the heightened explosion risks depicted in Figure 13. Furthermore, improper disposal practices, such as landfilling spent LIBs with organic waste, can instigate underground fires [82].

The hazards associated with LIB incidents extend beyond immediate risks to first responders and personnel in confined spaces. The substantial energy release, which can reach 22,000 MJ/ton for standard household packs, poses a significant threat. The occurrence of dozens of fire and explosion accidents at LIB recycling plants and warehouses underscores the severity and frequency of these incidents.

Table 7. Some major fire and explosion incidents in LIB storage and recycling treatment.

Major LIBESS Incidents Place	Date	Case Study and Comments
Ningxiang, Hunan, China	7 January 2021	Residual charge (12 V) in NCM811 battery packs combined with aluminum powder exposure → exothermic reaction (ΔT > 800 °C/min). Casualties: 1 fatality, 20 injuries. Economic loss: ¥6.05 million.
Dongguan, Guangdong, China	3 March 2022	Spontaneous ignition occurred in lithium battery storage area, destroying shelving systems. Undetermined ignition source in battery storage area. Damage: Complete destruction of recycling zone and shelving systems.
Lanzhou, Gansu, China	13 June 2022	Approximately 200 metric tons of retired batteries self-ignited, resulting in complete warehouse collapse. Undetermined cause affecting approximately 200 metric tons of EOL lithium batteries. Consequence: Total warehouse structural failure.
Leiyang, Hunan, China	18 October 2023	Tool-induced short circuit during disassembly caused localized fire. Mechanical damage during disassembly → internal short circuit. Affected area: 300 m2. No personnel injuries reported.
Zhumadian, Henan, China	28 January 2024	Sudden blaze in processing workshop affecting 400 m2 area. Burn area: 400 m2 within recycling plant premises.
Toulouse, Occitanie, France	18 February 2024	Ignition in 900 t lithium battery warehousing storage generated dense smoke for hours. Undetermined ignition source.
Hwaseong, South Korea	24 June 2024	Undetected defective cells caused chain reaction in 35,000-cell inventory, killing 23 workers. QC failure in NCA cell production → undetected internal defects → chain reaction in 35,000-cell inventory. Casualties: 23 fatalities (including 18 trapped in basement). Property damage: $120 million USD.
Haidian, Beijing, China	11 September 2024	Spent e-bike battery module spontaneously combusted in storage room. Thermal runaway in retired e-bike battery module. Damage limited to storage compartment.
Shijingshan, Beijing, China	6 October 2024	Charging failure in retired e-tricycle battery led to dwelling fire. Thermal initiation in retired e-tricycle battery pack. Structure: Single-family dwelling total loss.
Fredericktown, Missouri, USA	30 October 2024	Huge fire with several explosions, destroying facility and contaminating waterways. No injuries and fatalities, but 20,000 m2 plant was destroyed. Environmental impact: 12,000 aquatic fatalities in adjacent watershed.
Derbyshire, UK	3 April 2025	Compactor crushed Li-ion cells, triggering major fire with toxic emissions. Mechanical compression of LCO pouch cells → thermal runaway propagation. Losses: £2.3 million facility damage, 72 h airborne toxin advisory.
Kilwinning, UK	10 April 2025	A massive fire with blaze at Fenix Battery Recycling plant reported the second explosion at the site within a year. No reported casualties, and the property loss is unknown.
Girona, Spain	28 April 2025	Processing operation caused sudden explosion, resulting in 1 fatality and 1 critical injury. Undetermined initiation sequence. Casualties: 1 fatality, 1 critical injury.
Hưng Yên, Vietnam	17 June 2025	Manual battery pack disassembly led to electrolyte leak and explosion, killing 5 workers. Manual separation of NMC532 battery packs → electrolyte leakage (1.2 L/pack) → internal short circuit. Casualties: 5 fatalities, 2 critical burns. Infrastructure: Complete workshop collapse.
Madrid, Spain	4 July 2025	Fire with explosions during high-temperature processing forced evacuation of 60,000 residents. High-temperature electrolyte decomposition → HF emission. Injuries: 2 minor cases.
Kaohsiung, Taiwan, China	14 July 2025	Overheating during high-nickel battery charging caused explosion injuring 16. Thermal runaway during formation charging (high-nickel NMC cells).
Shangyu, Zhejiang, China	15 July 2025	Fire broke out in EOL battery processing area with no casualties. Undetermined cause in EOL battery handling area.

lists relevant incidents from the past five years, indicating that safe battery storage and handling remain primary challenges across the recycling industry.

In addition to the incidents listed in Table 7, recent large-scale accidents further highlight recycling risks: In July 2025, a fire at a Madrid battery recycling plant during high-temperature processing released toxic HF gas, forcing the evacuation of 60,000 residents [81]. In April 2025, a UK waste compactor crushed LCO pouch cells, triggering a 72 h toxic gas advisory and £2.3 million in facility damage [82]. According to UL Solutions incident data, recycling-related LIB fires increased by 187% from 2020 to 2024, with 63% of incidents attributed to inadequate deactivation and 28% to black mass handling errors. These statistics underscore the urgent need for standardized safety protocols in recycling facilities.

4.1.4. Emissions and Pollutions

The environmental consequences of LIB fires and explosions are widespread and multifaceted. Pollutants are dispersed both through the air and via fire-extinguishing agents, which can spread toxic metals, carbonaceous particles, and electrolyte residues into the surrounding environment, as illustrated in Figure 14 [71,83]. Analysis of fire-suppression water from large-scale incidents indicates that pollutant concentrations often exceed safe thresholds, posing a significant threat to local ecosystems. Furthermore, even when using environmentally friendly agents, the use of water or water mist alone can still result in significant pollution, as demonstrated in Figure 15 [84].

Firefighting water runoff frequently contains high concentrations of metals, including Ni, Mn, Co, Li, and Al, mixed with other carbonaceous species (such as soot and tarballs) and undecomposed electrolyte solvents. A notable example is the Critical Mineral Recovery (CMR) battery recycling plant fire, where subsequent wildlife fatalities demonstrated the severe ecological damage that can occur even after a fire is contained. Beyond immediate contamination from debris and runoff, the long-term deposition of airborne pollutants adds to the cumulative environmental burden [85].

The environmental and health impacts of LIB fires are compounded by the release of highly toxic fluoride species, such as HF and POF₃, which can migrate through soil and water systems. The reaction between liberated HF and fire extinguishing agents can create acidic solutions hazardous to human skin at very low concentrations [71]. Given that large-scale incidents can release substantial quantities of HF, this risk must be a primary consideration when designing fire-suppression systems for facilities handling LIBs [78]. Furthermore, first responders require advanced training, specialized a personal protective equipment (PPE), and rigorous protocols to safely manage these hazards and protect both themselves and the public.

4.2. Black Mass Hazards

Black mass—the fine black powder resulting from the initial processing of spent LIBs—is a vital intermediate for battery recycling due to its high concentration of lithium, cobalt, nickel, manganese, and anode materials [5,86], as shown in Figure 3. Its physical and chemical properties can vary significantly depending on the battery source and the specific pretreatment method employed. Pretreatment typically involves mechanical shredding under an anoxic atmosphere (utilizing inert gas or liquid), often following manual dismantling and discharge.

However, recyclers must remain vigilant regarding inherent safety hazards of the material. A fraction of elemental lithium, specifically intercalated-lithium (LiC₆), remains within the anode even after discharge. In addition, fine aluminum powder may be present in concentrations of up to 6% by mass [4]. Both components significantly elevate the risk of dust explosions due to the fine-grained and complex nature of the black mass [87].

Understanding the smoldering fire hazard of bulk black mass requires a comprehensive evaluation of its inherent properties and its storage conditions. The risk of self-ignition is affected by particle size, the proportion of reactive elemental lithium and aluminum, residual organic solvents, and the scale of the stored material [88] (e.g., the standard 1 m³ “big bag” packaging used in industry). Oxygen availability within the bulk is also a critical determinant of fire risk. Figure 16 illustrates the self-ignition behavior of two black mass samples [4]. Experimental results from Grewer oven tests confirm the severity of this hazard, demonstrating that even small samples (8 mL) can reach temperatures over 600 °C due to self-heating [89].

Experimental results indicate that the ignition energy required to trigger a significant explosion in black mass can vary; one study [90] showed that an ignition energy of 20 kJ produced an overpressure exceeding 0.6 MPa at a concentration of 300 g/m³. Conversely, ignition energies 10 kJ or 2 kJ were insufficient to trigger an explosion in the tested black mass samples [4,90]. Explosivity is not determined by the aluminum powder content alone but is also influenced by combustible organic solvents and electrolyte volatiles within the material. Crucially, focusing solely on the dust explosion risk of the black mass powder may lead to an underestimation of the hazard posed by a “hybrid mixture”. These occurs when flammable gases such as H₂ and NH₃, which can be released upon contact with moisture or water, combine with the black mass dust. If these flammable gases and other volatile compounds are not adequately ventilated in confined spaces, they can accumulate during storage and handling, creating a significant gas explosion hazard [91].

2 (intercalated) Li + H₂O → Li₂O + H₂

(24)

Al + 3H₂O → 1.5 H₂ + Al(OH)₃

(25)

2Al + 2HF → H₂ + 2AlF

(26)

As for the formation mechanism of ammonia, a plausible mechanism can be as follows [92,93]:

During the thermal treatment:

4Al(s) + 3O₂(g) → 2Al₂O₃(s)

(27)

Al₂O₃(s) + 4Al(l) → 3Al₂O(g)

(28)

3Al₂O(g) + 3N₂(g) → 6AlN(s) +1.5O₂(g)

(29)

or Al + 0.5N₂(g) → AlN(s)

(30)

Reacting with water/moisture:

6AlN(s) + 3H₂O → Al(OH)₃ + NH₃(g)

(31)

To address the explosion risks in LIB recycling, further research is required to characterize the composition and quantity of flammable gases released from black mass. This information is vital for accurately modeling the explosion potential and developing effective mitigation strategies. Therefore, particular attention must be given to the explosion hazards presented by hybrid mixtures of black mass and its volatile emissions, as these represent a significant threat in the most severe operational scenarios [75].

4.3. Toxic and Corrosive Chemical Hazards

Recycling spent LIBs produces a variety of pollutants from electrolyte decomposition, high-temperature cracking of organic compounds, and the heavy metal content of the electrodes [94,95,96].

• Electrolyte hazards: Exposure of LiPF₆ to water/moisture forms highly corrosive HF, which can cause respiratory necrosis at concentrations over 3 ppm, along with other toxic fluorides like PF₅ and POF₃.
• Organic and heavy metal risks: High temperatures can break down organic solvents (EC, DMC) into carcinogenic compounds like formaldehyde and acetaldehyde, as well as highly toxic substances like dioxins during pyrolysis. Cathode materials contain carcinogenic nickel compounds (International Agency for Research on Cancer, IARC Class 1), and prolonged inhalation of anode graphite can cause pneumoconiosis. Cobalt and copper also act as significant metal contaminants.
• Sources of pollution: These pollutants can be released during crushing, sorting, high-temperature pretreatment, and thermal runaway incidents.

The release of HF from LIBs represents a critical safety hazard that requires extreme caution. When inhaled or absorbed through the skin, this highly corrosive and systemically toxic gas can cause severe damage, with even minimal concentrations potentially leading to respiratory necrosis or fatalities from dermal exposure to concentrated solutions. The danger stems from the high reactivity of the fluoride ions that are released after HF penetrates body tissues. This hazard is compounded by the widespread use of fluorinated substances—including PFAS and their derivatives like bis-FASIs—in various LIB components (binders, electrolytes, and additives) [97]. Consequently, the presence of these materials necessitates comprehensive safety protocols for the handling, storage, and recycling of LIBs.

Corrosive chemicals in LIB recycling primarily originate from electrolytes and their breakdown products [96]:

• Electrolyte decomposition: Salts such as LiPF₆, LiBF₄, LiClO₄, and LiAsF₆ can react with water or acid to generate toxic gases such as HF and phosphorus pentafluoride (PF₅), contributing to fluorine and arsenic pollution.
• Organic solvent degradation: VOCs like EC and DMC can thermally cleave at high temperatures to form formaldehyde, acetaldehyde, and other corrosive substances.
• Heavy metal and acid corrosion: Electrode materials contain corrosive heavy metals. Furthermore, the strong acids (e.g., H₂SO₄, HCl) used in hydrometallurgy to dissolve cathode metals produce highly acidic wastewater (pH < 1) containing heavy metal ions (Ni²⁺, Co²⁺), which corrodes pipelines and leads to soil acidification.
• Ammonia-related corrosion: During ammonia leaching and copper removal, the decomposition of ammonium persulfate additives produces ammonium sulfate, which releases ammonia gas (NH₃) at high temperatures. This NH₃ can then react with HF to form ammonium fluoride, a compound known to accelerate the pitting corrosion of metal equipment [98].

5. Recommendations for Future Studies

All stages of the LIB supply chain, including collection, logistics, storage, and recycling, are subject to significant material and environmental hazards arising from self-heating, smoldering, thermal runaway, and the potential release of toxic gases. Crucially, these risks are not solely dependent on the battery’s SOC. Consequently, it is imperative to implement stringent safety protocols and effective emission controls throughout the entire storage and recycling lifecycle to protect both personnel and the environment [5]. To mitigate risks, operators are strongly advised to conduct regular chemical analyses of shredder output to build a critical safety database. Such data is essential for identifying and addressing any concerning hazardous substances that may be present, a task particularly important when handling rapidly changing chemistries in manufacturing scrap or prototype cells. In this evolving field, continuous chemical analysis is the cornerstone of ensuring environmental compliance and occupational health and safety. Furthermore, the integration of advanced technologies—including Artificial Intelligence (AI), Machine Learning (ML), digital twins, and robotics—is urged to achieve the full digitalization of the battery recycling industry within the Industry 4.0 framework [9].

5.1. Warehousing Storage Safety

EOL-LIBs present unique fire and explosion hazards compared to new batteries due to their degradation and physical damage. Future research should prioritize understanding and mitigating these risks during collection, logistics, storage, and handling; this necessitates the integration of careful deactivation and de-risking strategies into the entire sustainable recycling industry:

5.1.1. Comprehensive Risk Assessment

• Understanding degradation mechanisms and failure modes in EOL batteries: Research is required to better characterized the long-term degradation effects on diverse battery chemistries (e.g., LFP, NMC, NCA) and their subsequent impact on fire risk. This includes investigating the probability and severity of thermal runaway in batteries with varying SOH, cycling histories, and physical damage.
• Developing standardized risk assessment methods: Establishing consistent protocols for testing and evaluating the fire and explosion hazards associated with different types and conditions of EOL batteries would facilitate more accurate risk categorization during warehousing [99]. Integrating Artificial Intelligence (AI) with advanced diagnostic tools, such as electrochemical impedance spectroscopy (EIS), could enable rapid SOH screening, allowing for standardized packaging, labeling, and segregated storage based on risk profiles.
• Investigating the impact of deep discharge on safety: Research should focus on how prolonged deep discharge affects the stability of LIBs and whether it heightens the risk of thermal runaway during storage or subsequent handling, specifically by varying C-rate, SOH, and environmental conditions, etc.
• Advancing AI-integrated EIS for rapid screening: Developing AI-driven EIS technology would allow for the rapid classification of EOL-LIBs into specific categories: “low-risk” (SOH > 60%), “medium-risk” (30% < SOH < 60%), and “high-risk” (SOH < 30%), facilitating safer logistical management.
• Studying the combined effects of SOC and storage duration: It is critical to investigate the synergistic effects of State of Charge (SOC, 0–100%) and storage duration (e.g., 1–12 months) on the probability of TR across different cathode chemistries.

5.1.2. Advanced Detection and Monitoring Systems

• Tailored detection systems for diverse EOL battery chemistries: It is essential to develop specialized sensors capable of monitoring a wide range of early-warning indicators for thermal runaway in EOL batteries with diverse degradation profiles. This includes high-sensitivity gas and vapor analysis for specific off-gas compounds (e.g., H₂, CO, and VOCs) that serve as chemical fingerprints for failure in different battery chemistries [100].
• Non-destructive inspection techniques for internal defects: Future research should focus on advancing Non-Destructive Testing (NDT) methods—such as ultrasonic scanning or X-ray CT—to identify internal structural defects or electrode delamination that may not be visible externally but pose significant fire risks.
• Advanced thermal monitoring and predictive analytics: Sophisticated thermal imaging combined with predictive algorithms can identify subtle temperature anomalies that precede thermal runaway in degraded cells. By integrating AI and Machine Learning, these subtle patterns can be analyzed to provide earlier and more accurate warnings. Furthermore, digital twin platforms should be established to enable 7/24 real-time monitoring of LIBs within warehousing and recycling facilities.

5.1.3. Optimized Fire Suppression and Mitigation Techniques

• Adapting suppression systems for the unique challenges of EOL battery fires: Research is required to optimize fire suppression systems—such as high-expansion foam and specialized water-mist additives—to effectively control and extinguish fires involving EOL batteries. These systems must account for the increased flammability of degraded components and the high potential for re-ignition (stranded energy).
• Containment and isolation of EOL batteries: Further investigation is needed into the effectiveness of specialized fire-resistant storage containers (e.g., modular metal bins [101] instead of traditional cardboard packaging), racking systems, and compartmentation designs to prevent fire propagation. Moreover, studies should evaluate the feasibility and timing of emergency extraction using automated guided vehicles (AGVs) or automated forklifts to transfer compromised containers to safe zones, potentially integrate with liquid nitrogen (LN₂) system.
• Safe venting and off-gas management: Research should focus on technologies for the safe management of flammable and toxic off-gases released during EOL battery failure. This includes developing explosion-venting strategies and specialized scrubbing channels to minimize the risk of gas-mixture explosions and personnel exposure [102].
• Emergency transfer and cooling: Explore the integration of liquid nitrogen immersion/cooling systems with automated transport for the rapid emergency isolation of burning EOL-LIB storage containers.
• Advanced container design: Develop specialized fire-resistant metal storage units featuring built-in pressure-relief venting and gas-capture channels to direct toxic emissions toward localized or centralized scrubbers.

5.1.4. Safety Management and Regulatory Framework for Collection, Logistics and Storage

• Standardized safety guidelines and training: Research and collaborative efforts are needed to establish comprehensive guidelines and certification programs for personnel involved in the handling, sorting, and storage of EOL batteries. These programs must specifically address the heightened risks associated with degraded and damaged states [103].
• Regulations and best practices for safe warehousing: It is essential to establish clear regulatory frameworks and industry best practices for EOL battery warehousing. This includes defining maximum storage limits, minimum separation distances, specialized ventilation requirements, and robust emergency response plans (ERPs) [99].
• Storage duration and environmental conditions: Research is needed to determine optimal storage conditions (temperature, humidity, ventilation) and maximum safe storage durations for various types of EOL battery chemistries. Furthermore, the risks associated with the co-storage of EOL-LIBs and black mass must be rigorously evaluated to prevent cross-contamination and synergistic fire hazards.

5.1.5. Improved Recycling and Disposal Pathways

• Safer recycling pathways: Further research should focus on alternative or optimized recycling technologies—such as vacuum shredding, cryogenic processing, or inert-atmosphere comminution—that minimize fire and explosion risks during the processing of EOL batteries.
• Transport of damaged, defective, or inherently unstable (DDR) batteries: Solutions must be developed for the safe logistics of batteries deemed Damaged, Defective, or Recalled (DDR). This includes investigating specialized containment vessels and real-time monitoring sensors to mitigate risks during transit to recycling or disposal facilities [104].

By prioritizing these research areas, the industry can develop more effective fire and explosion prevention strategies tailored to the unique risks of EOL battery management, ultimately contributing to a safer and more sustainable circular economy for LIBs.

5.2. Recycling and Handling Process Safety

The battery recycling industry faces significant fire and explosion hazards, primarily due to the potential for thermal runaway in damaged or improperly stored LIBs [105]. Research efforts should adopt a multi-pronged approach to address these risks across the recycling value chain:

5.2.1. Safe Collection, Sorting, and Pretreatment

• Consumer education and infrastructure: Research is needed to assess the effectiveness of public education campaigns on safe battery disposal and to optimize collection systems that prevent LIBs from entering the general waste stream. This includes exploring the impact of legislation and incentives—such as mandatory labeling with chemistry and SOH indicators—to encourage proper recycling and reduce landfill fires.
• Automated sorting and identification: Advanced robotic and AI-powered systems should be developed to automatically sort batteries by type and chemistry, minimizing manual handling and reducing the risk of accidental damage.
• Advanced deactivation methods: Research into techniques for safely discharging batteries to a near-zero SOC (e.g., 2%) is essential to reduce TR risks during downstream processing [106]. For damaged or unstable batteries, specialized treatments—such as immersion in non-flammable dielectric fluids or the use of chemical deactivation agents—should be prioritized to eliminate short-circuit risks [107].

5.2.2. Advanced Detection and Early Warning Systems

• Real-time multi-sensor monitoring: Integrating thermal imaging, gas detection, and acoustic emission sensors can identify potential thermal runaway events at their incipient stages.
• Predictive modeling: Developing models that analyze sensor data and battery characteristics to forecast TR likelihood can enable preemptive safety measures [100].
• Remote sensing: Investigating technologies such as hyperspectral imaging or drone-mounted thermal cameras can facilitate the detection of overheating batteries in large-scale storage facilities.

5.2.3. Safe Dismantling and Material Separation

• Stress-minimizing processes: Research should focus on parameters that reduce mechanical and thermal stress during dismantling, such as cryogenic embrittlement or solvent-based separation in controlled environments.
• Safe intermediate handling: Specialized protocols are required for the handling and storage of black mass to mitigate the risk of self-heating or dust explosions during downstream processing [108].
• Design for recycling: Investigating how cell format and module configuration influence recycling safety can lead to the development of more “recycling-friendly” next-generation batteries [9].

5.2.4. Data Collection, Analysis, and Standardization

• Standardized incident reporting: Establishing standardized protocols for collecting and reporting data on battery-related incidents in recycling facilities is essential to gain a better understanding of the causes, trends, and effectiveness of current safety measures [109].
• Collaboration and knowledge sharing: Fostering collaboration among researchers, industry stakeholders, and regulatory bodies to share research findings and best practices is critical to addressing battery recycling safety challenges globally. For instance, if producers and EV manufacturers were to share comprehensive lifecycle data—specifically from the Battery Management System (BMS)—it would significantly aid recyclers in selecting the optimal recycling path and assist academic partners in conducting fundamental research.

By investing in these research areas, the battery recycling industry can develop more effective fire and explosion prevention strategies, enhancing safety for workers and facilities while supporting the sustainable circular economy for LIBs.

6. Conclusions

The growing utilization of LIBs—driven by EVs, portable electronics, and BESSs—necessitates sustainable recycling for resource recovery and environmental protection. However, EOL LIBs present unique safety challenges due to aging-induced degradation, residual charge, and reactive components (e.g., black mass), leading to fire, explosion, and toxic gas hazards throughout the recycling value chain. This review systematically analyzes HSE hazards from collection and storage to pretreatment and primary processing. It highlights that EOL-LIBs are significantly more prone to thermal runaway than pristine batteries and identifies black mass self-ignition and dust explosions as critical recycling-specific risks.

Current recycling relies on pyrometallurgical, hydrometallurgical, and emerging direct recycling technologies, each with distinct hazard profiles. Mechanical pretreatment and thermal processing pose the highest fire/explosion risks, while hydrometallurgy requires stringent control of acid corrosion and heavy metal contamination. Quantitative thresholds—such as a 30% residual SOC limit and a 3 ppm HF emission limit—provide actionable safety benchmarks for industrial practice.

Future research should prioritize: (1) developing AI-driven SOH screening and predictive TR models for EOL-LIBs; (2) optimizing deactivation and fire suppression technologies tailored to recycling scenarios; and (3) establishing standardized incident reporting and hazard assessment protocols. Additionally, integrating digital twins and robotics into recycling processes can significantly reduce human exposure to operational risks.

Achieving a sustainable circular economy for LIBs requires balancing resource recovery efficiency, environmental impact, and safety. By addressing identified knowledge gaps and implementing the proposed recommendations, the industry can mitigate recycling hazards, protect personnel and ecosystems, and support global Sustainable Development Goals (SDGs). Direct recycling, with its lower energy consumption and reduced hazard profile, holds great promise for large-scale application but requires further technical maturation to overcome current scalability and cost challenges.