The Urgent Need for Flame Retardancy in Batteries
Battery electrolytes (especially conventional organic carbonate electrolytes used in Li-ion cells) are a significant contributor to thermal runaway and fire risk. Over the last decade researchers and formulators have therefore developed several classes of flame-retardant (FR) additives and alternative electrolyte chemistries to lower flammability while retaining electrochemical performance.
In lithium-ion batteries, the main flammable material causing fires is the organic liquid electrolyte used. Linear carbonate solvents frequently employed in commercial formulations — including ethylene carbonate, dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate — generally have low flash points (commonly not exceeding ~30 °C), with reported flash-point ranges roughly between −10 °C and +25 °C. Those values lie far below typical thermal-runaway onset temperatures (>150 °C), so under modest heating these solvents volatilize and generate flammable vapor clouds that readily ignite. Therefore, reducing the flammability of the electrolyte through additives, solvent substitution, or engineered separators directly addresses one of the most important root causes of battery fires and is a priority means of achieving safer battery designs.
Common Flame-Retardant Additives for Electrolytes
Fig 1. Typical phosphorus-based flame retardants used as electrolytes. [1]
Phosphorus chemistry is the most widely studied in battery FR research as it suppresses flames in the gas phase while also promoting char/SEI formation in the condensed phase.
- Trialkyl/Triaryl Phosphates: triethyl phosphate (TEP), trimethyl phosphate (TMP), triphenyl phosphate (TPP). TEP/TMP are common co-solvents; TPP has historical usage as an additive. These can be used as co-solvents (partial replacement of carbonates) or as discrete additives.
- Phosphazene Derivatives: Cyclotriphosphazene compounds represent an innovative class of flame retardants that combine phosphorus and nitrogen in a synergistic structure. Phosphazene-based flame retardants achieve good flame-retardant effects with only small amounts added. Commonly used phosphazene-based flame retardants include ethoxy(pentafluoro)cyclotriphosphazene (PFPN), phenoxy(pentafluoro)cyclotriphosphazene (FPPN), hexaphenoxycyclotriphosphazene (HPCTP), hexakis(methoxy)cyclotriphosphazene (HMPN), etc.
- DOPO and DOPO-Derivatives: widely used as phosphorus FR units in polymers and separators and increasingly explored for multifunctional electrolyte additives or separator coatings.
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Fluorinated Phosphorus Compounds
The introduction of fluorine atoms into phosphorus-based flame retardants creates a synergistic effect that enhances both flame retardancy and electrochemical compatibility. Tris(2,2,2-trifluoroethyl) phosphate (TFEP), tris(2,2,2-trifluoroethyl) phosphite (TTFP), and trifluoroethoxy pentafluorocyclotriphosphazene (TFPN) are typical examples of this class of compounds. For example, at 15% concentration in PC/EC/EMC solvents, TTFP is able to create an effectively non-flammable electrolyte with SET values close to zero, while also improving cycling performance in nickel-based oxide/graphite cells. [2]
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Ionic liquids such as pyrrolidinium or imidazolium salts with non-coordinating anions (e.g., TFSI, FSI) are non-volatile, low-flammability liquids and can be used as full-replacement electrolytes or co-solvents. They can often improve thermal stability, though can be viscous and may sometimes limit low-temperature performance. [3]
How to Select the Right Flame-Retardant Additive?
When choosing an FR for an electrolyte, evaluate all of the following for your target cell chemistry (cathode, anode, separator, operating voltage/temperature):
1. Electrochemical stability window — additive must be stable (or form a stable, passivating decomposition product) within your cell's voltage range. Phosphate esters can be oxidatively sensitive at very high voltages.
2. Compatibility with salt & Li source — interactions with LiPF6 (and generated HF) can consume additive or produce undesired by-products. Some FRs require modified salts or scavengers.
3. Ionic conductivity & viscosity effects — co-solvent loading changes conductivity and rate capability (ILs and sulfones raise viscosity). Balance safety vs rate requirements.
4. SEI/CEI performance — does additive improve or damage SEI on graphite/Si or CEI on high-voltage cathodes? Some phosphates form beneficial LiF- or phosphate-rich layers.
5. Dosage required for non-flammability — practical ranges vary widely by chemistry; verify via standard flammability tests.
6. Cost, scale-up and regulatory profile — availability, toxicity, odor, and regulatory acceptance matter for commercial use.
7. Synergy with other mitigations — separator coatings, high-concentration electrolyte strategies, or flame-retardant polymer gels may reduce required additive level.
Usage Methods and Recommended Dosage Ranges
Two main strategies: (A) additive/co-solvent (partial replacement) and (B) solvent replacement (majority or all solvent replaced). There are also (C) gel/polymer/solid electrolytes and (D) hybrid approaches (microcapsules, coated separators).
A. Additive / co-solvent (most common in R&D & supply)
- Dosage: It is commonly reported as ~2–20 wt% additive; many effective systems are in the 5–15 wt% range. Some specialized additives (e.g., certain cyclic phosphates or engineered phosphazenes) can work at lower loadings if they form strong protective films.
- Examples:
- Triethyl phosphate (TEP): used as a co-solvent at 5–20 wt% in many studies; TEP at ~10 wt% has improved safety vs performance in some Si/carbon–NMC cells.
- Trimethyl phosphate (TMP) and other phosphate esters: some researches indicate >15 wt% TMP is necessary to reach truly non-flammable behaviour in certain carbonate blends. Verify per cell chemistry.
- Triphenyl phosphate (TPP): has been evaluated historically as a flame-retardant additive with reasonable electrochemical compatibility; often screened at low to moderate wt%.
B. Solvent replacement (partial or full)
- Ionic liquids: used as partial replacements (e.g., 10–50 wt% IL) or pure IL electrolytes. As replacement, ILs can render electrolytes essentially non-flammable but can increase viscosity and reduce conductivity at low temperature.
- TEP/sulfolane-based electrolytes: full TEP-based electrolytes have been demonstrated (with modified salts/additives) as non-flammable; percent is ~100% replacement (i.e., Li salt dissolved in TEP). Some formulations require film forming additives to preserve cycle life.
C. Gel / polymer electrolytes & coatings
- Incorporating phosphorus- or nitrogen-type flame retardants into gel or polymer matrices is a robust approach for pouch and Li-metal cells. Dosage is formulation dependent.
D. Microencapsulation and composite separator approaches
- Microcapsules containing FR molecules (or inorganic fillers like ATH) can be included in separator layers to provide localised flame suppression without greatly changing bulk electrolyte properties.
References
- Mu, Xiaowei, et al. Small Structures 4.12 (2023): 2300179.
- Chen, Zhiqi, et al. Advanced Science 8.11 (2021): 2003694.
- Santiago-Alonso, Antía, et al. Batteries 10.9 (2024): 319.
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