Flame-Proof Phosphate Electrolytes May Unlock Safer Sodium Batteries

The batteries that could one day store electricity for entire cities face a stubborn enemy: fire. Sodium-ion and sodium-metal batteries are widely seen as the logical successors to lithium-ion technology for grid-scale energy storage, thanks to the abundance and low cost of sodium, yet most current designs still rely on flammable carbonate solvents inherited directly from lithium battery chemistry. Now, a comprehensive review published in Discover Electrochemistry argues that a family of molecules once dismissed as electrochemical failures may hold the key to sodium batteries that are intrinsically safe, long-lived, and capable of operating at high voltages. The study, led by Xuemei Xu, Youchen Hao, Sennan Cao, Teng Liu, Jiajia Zhang and Ding Zhang of the Wuhan Institute of Technology, systematically surveys the rise of phosphate ester solvents as the backbone of flame-retardant sodium electrolytes.

The story of phosphate esters in batteries has long been a tale of ironic failure. Molecules such as trimethyl phosphate (TMP) and triethyl phosphate (TEP) burn with remarkable reluctance, a property that made them obvious candidates for safer electrolytes in lithium-ion systems. Yet in molecular orbital terms, their lowest unoccupied molecular orbital, or LUMO, sits at an energy level that makes them easy to reduce. At the low potentials demanded by graphite and lithium-metal anodes, phosphate solvents decompose relentlessly, failing to form the dense, protective solid electrolyte interphase (SEI) that conventional carbonates achieve at ordinary salt concentrations. Their highest occupied molecular orbital, meanwhile, is too high in energy, limiting oxidation resistance to roughly 4.2 to 4.5 volts against lithium and causing them to degrade at high-voltage cathodes. Even used as additives at about one percent, they failed to make lithium batteries fully non-flammable while still degrading performance.

Sodium changes the thermodynamic picture in a way that inverts these liabilities. Sodium’s redox potential of −2.71 volts, compared with −3.04 volts for lithium, sits closer to the reduction threshold of phosphate esters, so the molecules are far less prone to reductive decomposition against a sodium metal anode. The larger ionic radius of sodium ions also alters interfacial chemistry, promoting faster ion transport at the electrode surface and helping to build a compact, inorganic-rich SEI in which phosphate compounds participate more actively. Meanwhile, the moderate oxidation potential of phosphate solvents aligns comfortably with the cathode windows of sodium cells, which typically do not exceed 4.0 volts against sodium. The result is a solvent whose weaknesses in lithium chemistry become strengths in sodium chemistry, allowing stable interfaces to form without the extreme salt concentrations of 5 molar and above that lithium systems required.

The review breaks down the three workhorse solvents of the field in detail. TMP offers a dielectric constant near 21, facilitating sodium salt dissociation, with low viscosity of roughly 1.8 millipascal-seconds at 25 degrees Celsius and a freezing point of −46 degrees Celsius that suits cold-climate operation. TEP trades some dielectric strength for superior thermal stability, boiling at 219.3 degrees Celsius and remaining liquid to −56.5 degrees Celsius. Tris(2,2,2-trifluoroethyl) phosphate, or TFEP, adds fluorination to the mix, raising thermal stability with a flash point above 150 degrees Celsius and promoting the formation of protective sodium fluoride-rich interfaces. According to the review’s analysis, dielectric constant and viscosity govern not merely how fast ions travel through the bulk electrolyte but also how the solvation shell is constructed, which in turn determines whether solvent molecules or salt anions decompose first at the electrode surface.

The flame-retardant mechanism itself operates on two fronts. When phosphate esters are heated during combustion, they release phosphorus-containing radicals such as PO· and HPO2·, which quench the high-energy H· and OH· radicals that sustain gas-phase flame chains through reactions like PO· + H· → HPO and PO· + OH· → HPO2·. Simultaneously, in the condensed phase, phosphates promote a cross-linked carbon char layer that acts as a physical barrier to flammable volatile release. TFEP compounds this defense by releasing fluorine radicals that further enhance gas-phase quenching. This dual mechanism, the authors note, is what distinguishes phosphates from simple additives and underpins their role as primary solvents rather than supplements.

Turning theory into working cells has required a toolbox of interfacial engineering strategies. The pioneering formulation of Yuliang Cao’s team, 0.8 molar NaPF6 in TMP with 10 volume percent fluoroethylene carbonate (FEC), delivered ionic conductivity of 5.41 millisiemens per centimeter across a 0 to 4.5 volt window, allowing an antimony anode to retain 94 percent capacity over 80 cycles. Subsequent refinements tuned salt-to-solvent molar ratios to improve hard carbon compatibility, achieving 84 percent capacity retention after 1,500 cycles with coulombic efficiency above 99.8 percent. Additives such as vinylene carbonate (VC), which stabilizes the cathode interface, and sodium bis(oxalato)borate (NaBOB) salts, which enable fluorine-free, low-cost formulations, extended cycling further. In one system, a hard carbon cathode against Prussian white retained 73.7 percent capacity after 1,000 cycles at a demanding 10C rate.

For sodium-metal batteries, where dendrites and interfacial instability loom largest, the field has converged on locally high-concentration electrolytes (LHCEs). By diluting concentrated salt solutions with fluorinated ethers such as bis(2,2,2-trifluoroethyl) ether (TTE), researchers steer sodium ions into anion-rich solvation shells that decompose into protective inorganic layers rather than dissolving solvent. A dual-salt system of 0.8 molar NaPF6 plus 0.1 molar NaDFOB in TMP/TTE produced a phosphorus- and boron-rich gradient cathode interphase that preserved 85.2 percent capacity after 800 cycles at 4.2 volts. Other groups pushed oxidation resistance to 4.7 volts, with cells surviving over 1,000 cycles, while a ‘salt-as-diluent’ strategy using sodium nitrate delivered 5.99 millisiemens per centimeter conductivity and 80 percent retention after 500 cycles at a fraction of typical cost.

TEP-based systems have followed a parallel trajectory with distinctive advantages at elevated temperatures. A 1 molar NaPF6 electrolyte in TEP/FEC with the scavenger additive TMSPi enabled a sodium half-cell to retain 84 percent capacity after 1,000 cycles at 60 degrees Celsius and 86 percent after 100 cycles at 70 degrees Celsius, marking the first demonstration of a sodium-ion full cell cycling stably beyond 100 cycles at that temperature. Systematic salt screening by Van Ekeren and colleagues identified NaFSI as the optimal pairing with TEP, yielding 6.5 millisiemens per centimeter conductivity and 88 percent retention in 1 ampere-hour pouch cells. Perhaps most strikingly, a formulation balancing solvent dielectric constant and binding energy produced a 6.61 ampere-hour pouch cell with an energy density of 152.3 watt-hours per kilogram that showed no smoke or expansion during nail penetration testing, the brutal industry benchmark for thermal abuse.

Gel polymer architectures push the safety envelope further still. Encapsulating TEP within crosslinked polymer networks produced electrolytes with transference numbers as high as 0.61, enabling sodium symmetric cells to run for over 1,000 hours dendrite-free and full cells to retain 87.8 percent capacity across 1,000 cycles at 2C. In-situ gelated formulations built around TFEP and phosphorus-containing monomers achieved capacity decay of only 0.0035 percent per cycle over 1,000 cycles in 2.4 ampere-hour pouch cells, with oxidation windows approaching 5.0 volts. Beyond liquid and gel systems, mixed-solvent formulations blending phosphates with glyme ethers and alkyl-chain-extended derivatives such as tris(2-ethylhexyl) phosphate (TOP) are broadening the design space, with TOP-based electrolytes sustaining 95.7 percent capacity retention over 1,800 cycles at room temperature.

The review’s authors are candid about the remaining obstacles. Anion-rich solvation structures, so often celebrated as a design goal, do not guarantee stable interfaces if the decomposition products dissolve or if reaction kinetics are too sluggish to passivate the electrode before further solvent degradation. Most phosphorus-based flame retardants still compromise oxidative stability to some degree, and the complex influence of phosphates on sodium ion desolvation and interfacial transport remains incompletely understood. On the industrial side, advanced phosphate formulations can cost an order of magnitude more than carbonate systems once expensive NaFSI salts and fluorinated diluents are counted, a decisive factor for grid storage economics. The authors propose machine-learning-guided molecular discovery, operando atomic-scale interfacial diagnostics, realistic large-format cell validation across −40 to 60 degrees Celsius, and greener synthesis routes as the pillars of future progress. If those threads converge, the fireproof chemistry that lithium batteries could not tame may prove to be exactly what sodium batteries needed all along.

Subject of Research: Phosphate ester based nonaqueous electrolytes for safe, high-performance sodium batteries

Article Title: Phosphate ester based nonaqueous electrolytes for sodium batteries

Article References: Xu, X., Hao, Y., Cao, S., Liu, T., Zhang, J., & Zhang, D. (2026). Phosphate ester based nonaqueous electrolytes for sodium batteries. Discover Electrochemistry, 3(1), Article 80. https://doi.org/10.1007/s44373-026-00167-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00167-6

Keywords: sodium-ion batteries, sodium-metal batteries, phosphate ester electrolytes, flame-retardant electrolytes, solid electrolyte interphase, trimethyl phosphate, triethyl phosphate, TFEP, locally high-concentration electrolytes, gel polymer electrolytes, battery safety, grid energy storage