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

Conventional quasi-solid-state electrolytes (QSEs), while offering some advantages in terms of safety and mechanical integrity compared to liquid electrolytes, are significantly hindered by two primary bottlenecks. First, the transport of sodium ions (Na⁺) through the bulk electrolyte is inhibited due to the dominant movement of anions, resulting in reduced Na⁺ transference numbers typically ranging between 0.4 to 0.7. This imbalance precipitates concentration polarization, reducing the effective ionic mobility at high current densities and limiting ultrafast charging capabilities. Second, ionic diffusion at the interfaces between electrolyte and electrodes—the bilateral interphases—is often sluggish, fostering dendrite formation on the anode and accelerating electrolyte degradation, thereby compromising both longevity and safety of SMBs.

Shattering these limitations, a research consortium from Southeast University, in partnership with HiNa Battery Technology Co., Ltd. and Yangzhou University, has introduced an innovative dual interlocked mediator electrolyte system. This novel quasi-solid-state electrolyte, designated as Sn-FB QSE, achieves near-unity Na⁺ transference numbers alongside exceptional ionic conductivity without resorting to complex polymer functionalizations typically required in single-ion conducting strategies. The secret lies in the synergistic engineering of two mediators—cationic Sn²⁺ ions and anionic difluoro(oxalato)borate (DFOB⁻)—that simultaneously modulate the bulk electrolyte structure and interfacial chemistry, delivering unprecedented electrochemical performance tailored for ultrafast charging and extended battery life.

The dual interlocked mediator mechanism operates on two intertwined fronts. During the synthesis phase, Sn²⁺ initiates a controlled in situ cationic polymerization of 1,3-dioxolane (PDOL), constructing a uniformly cross-linked amorphous polymer network that imparts mechanical strength while facilitating ion transport. Simultaneously, DFOB⁻ acts as a polymerization retarder, preventing excessive cross-linking and maintaining an optimal network polydispersity index around 1.6—a value significantly lower than single-mediator systems—thus balancing mechanical robustness with ion mobility. This finely tuned polymer matrix strengthens puncture resistance to 8.5 kPa, crucial for preventing dendrite penetration while supporting flexible form factors.

At the molecular level, sophisticated simulations reveal that DFOB⁻ preferentially coordinates with Na⁺ ions, effectively attenuating the strong Na⁺-polymer oxygen interactions that traditionally bind salts tightly within polymer matrices. This chemical modulation reduces the average coordination number from 4.87 to 2.81, liberating a substantial fraction of free Na⁺ ions that are free to migrate swiftly through the electrolyte. The resulting diffusion coefficient, calculated at 16.8 Ų/ns, marks a sixfold enhancement over conventional liquid electrolytes, thereby enabling rapid Na⁺ conduction even under aggressive charging regimes.

Upon cell operation, an elegant interfacial transformation ensues shaped by the distinct frontier orbital energies of the two mediators. Sn²⁺$, possessing a low LUMO energy level of −4.87 eV, is preferentially reduced at the sodium metal anode surface, forming a hybrid solid-electrolyte interphase (SEI) composed of nano-scale NaSn alloys embedded within inorganic-rich matrices. This SEI effectively homogenizes local electric fields, dramatically reducing nucleation overpotentials to approximately 50 mV and creating a mechanically stable protective barrier that mitigates dendrite initiation and growth. Concurrently, the DFOB⁻ anion, with its higher HOMO energy of −8.12 eV, undergoes sacrificial oxidation at the cathode to establish a thin yet resilient cathode–electrolyte interphase (CEI) approximately 14 nm thick. This CEI exhibits an extraordinary Young’s modulus near 8.9 GPa, an order of magnitude greater than single-mediator counterparts, mitigating mechanical degradation during repeated cycling.

Electrochemical testing validates the transformative impact of this dual mediator approach. Symmetric Na|Na cells sustain stable cycling over an unprecedented 6000 hours at 0.1 mA cm⁻² with minimal polarization (~0.1 V) and no dendritic short-circuit events, comparable to nearly continuous operation for over eight months. The critical current density surges to 3.0 mA cm⁻², while the exchange current density rises to 10 μA cm⁻², reflecting enhanced interfacial kinetics. When paired with Na₃V₂(PO₄)₃ (NVP) cathodes, full cells demonstrate retention of 90% capacity after 2000 cycles at a rapid 3C charge-discharge rate, retaining 80.1 mAh g⁻¹ at an extraordinary 15C, and maintaining 53.4 mAh g⁻¹ after 800 cycles even at 5C. The electrochemical stability window is also broadly expanded to 4.7 V vs. Na⁺/Na, paving the way for compatibility with high-voltage cathode materials.

To bridge the gap between laboratory innovation and practical application, the research team scaled their Sn-FB QSE technology into high-mass-loading full cells containing 5 mg cm⁻² NVP cathodes, achieving 75% capacity retention after 500 cycles at 1C. Pouch cells without applied pressure, measuring 4 × 5 cm², demonstrated impressive mechanical resilience by retaining 84% capacity after 19 cycles and powering smartphones continuously even through repeated full folding. Additionally, compatibility with advanced sodium nickel iron manganese oxide (NaNi₁/₃Fe₁/₃Mn₁/₃O₂, NFM) cathodes with high mass loading (17.54 mg cm⁻²) was confirmed, showcasing initial capacities of 129.9 mAh g⁻¹ and stable cycling performance over multiple cycles, indicating versatility across diverse cathode chemistries.

This pioneering dual interlocked mediator electrolyte paradigm overturns the long-standing trade-offs in electrolyte design—simultaneously achieving single-ion conduction, high mechanical strength, and adaptive bilateral interphases, properties traditionally viewed as mutually exclusive. By harnessing the complementary chemical and electronic properties of the Sn²⁺ and DFOB⁻ mediators, the approach delivers holistic control over ion transport and interfacial stability, unlocking performance metrics previously deemed unattainable for quasi-solid-state sodium electrolytes. Moreover, its intrinsic scalability via in situ polymerization and compatibility with existing battery manufacturing infrastructures spotlight this innovation as a viable candidate for commercial deployment.

Looking forward, this versatile mediator strategy harbors significant potential beyond sodium systems. Its principles may be extended to lithium and potassium metal batteries, where similar challenges in ion selectivity and interface stability prevail. Moreover, integrating this dual mediator system into fully solid-state configurations could yield safer, denser energy storage solutions with ultrafast charging capabilities. Concurrently, advancing mechanistic understanding through AI-guided frontier orbital screening may expedite the discovery of new mediator pairs optimized for specific chemistries, ushering an era of rational electrolyte design tailored to next-generation battery demands.

In essence, the dual interlocked mediator engineering approach pioneers a transformative paradigm for battery electrolytes that bridges performance, safety, and manufacturability. By breaking free from the restrictions imposed by traditional electrolyte designs, sodium metal batteries can now realistically aspire to meet the rigorous demands of ultrafast charging, long cycle life, and intrinsic safety at scale. This breakthrough marks a critical milestone propelling sodium batteries from a niche laboratory curiosity to a formidable contender in the mainstream energy storage landscape, drawing us closer to a sustainable energy future predicated on earth-abundant and cost-effective materials.

Subject of Research:
Article Title: Dual Interlocked Mediators Enable Single‑Ion‑Conducting Quasi‑Solid‑State Electrolytes for Ultrafast‑Charging Long‑Life Sodium Metal Batteries
News Publication Date: 21-May-2026
Web References: http://dx.doi.org/10.1007/s40820-026-02236-2
Image Credits: Yuan Zhang, Long Pan, Cheong Wa Leong, Xing-Guo Qi, Xiaozhong Huang, Xinyi Cai, Mufan Cao, Min Gao, Haoyu Zhang, Dawei Sha, Yang Zhou, ZhengMing Sun*

Keywords

Sodium Metal Batteries, Quasi-Solid-State Electrolytes, Single-Ion Conduction, Dual Interlocked Mediators, Sn-FB QSE, Polymer Electrolytes, Solid-Electrolyte Interphase, Cathode-Electrolyte Interphase, Ultrafast Charging, Electrochemical Stability, Ion Transport, Battery Cycle Life