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

Wang succeeds Judy Jeevarajan, Ph.D., who will remain with UL Research Institutes as vice president and distinguished scientific advisor. In his new role, Wang will direct research strategy and operations at the Electrochemical Safety Research Institute, an organization focused on understanding why electrochemical energy-storage systems fail and how those failures can be prevented. The institute investigates the links between battery chemistry, mechanical design, electrical control, manufacturing quality, and real-world operating conditions. Its work is intended to influence safer products, testing methods, engineering practices, and standards as energy systems become more densely packed and increasingly dependent on rechargeable batteries.

Before joining UL Research Institutes, Wang spent more than three decades at Pennsylvania State University, where he served as the William E. Diefenderfer Chair in Mechanical Engineering, professor of mechanical engineering, chemical engineering, and materials science and engineering, director of the Electrochemical Engine Center, and co-director of the Battery and Energy Storage Technologies Center. His research career has covered the full battery-development chain, from fundamental electrochemical reactions and heat generation to manufacturing, fast charging, system integration, and commercialization. That breadth has made him a prominent figure in a field where the performance of a battery cannot be separated from its thermal behavior, structural integrity, production consistency, and control software.

One of Wang’s most important contributions has been the development of electrochemical-thermal modeling methods for batteries. These models combine the movement of ions and electrons inside a cell with the heat produced by electrochemical reactions, electrical resistance, and transport limitations. As a battery charges or discharges, local variations in current density, temperature, and chemical concentration can create areas of stress that are invisible from the outside. Electrochemical-thermal models allow researchers to predict how these internal conditions evolve, helping engineers design cells and battery packs that operate within safer limits. The approach has influenced battery development across transportation, consumer electronics, defense, and stationary energy storage.

Wang also invented a self-heating, all-climate battery designed to maintain performance in extremely cold conditions. Conventional lithium-ion batteries can lose power at low temperatures because ion transport slows and the internal resistance of the cell rises. Charging a cold battery can be particularly hazardous, as lithium plating may occur on the anode surface instead of lithium ions being safely stored within the electrode structure. Wang’s technology uses the battery’s own electrical energy to generate controlled internal heat, bringing the cell rapidly to an operating temperature at which it can deliver power and accept charge more efficiently. The technology enabled electric buses to operate during the 2022 Winter Olympics and has since been commercialized for transportation, defense, and energy-storage applications.

His work on ultrafast charging also drew international attention after being recognized by The Guardian as one of the world’s leading science stories of 2022. Fast charging is not simply a matter of supplying more electrical current. High charging rates can produce heat, accelerate unwanted chemical reactions, and cause lithium ions to accumulate as metallic deposits on the anode. These deposits can reduce capacity and, in extreme cases, create internal pathways that trigger a short circuit. Wang’s research has explored how electrode architecture, thermal management, charging protocols, and cell chemistry can be coordinated to reduce these risks while shortening the time required to recharge a battery.

More recently, Wang has focused on lithium-metal and solid-state batteries, two technologies widely viewed as possible successors to today’s dominant lithium-ion systems. Lithium-metal anodes can store substantially more charge by replacing conventional graphite, potentially increasing energy density and extending the range of electric vehicles. However, lithium can form needle-like structures known as dendrites during charging. If dendrites penetrate a separator and reach the opposite electrode, they can cause an internal short circuit. Solid-state batteries replace the flammable liquid electrolyte used in many conventional cells with a solid ion-conducting material, but they introduce their own challenges, including interfacial resistance, cracking, contact loss, and mechanical instability. Wang’s research has examined the safety mechanisms behind these emerging systems and contributed to the design of batteries intended to be intrinsically safer rather than merely protected by external controls.

“Dr. Wang is among the world’s foremost authorities on battery technology and electrochemical energy systems,” said James J. Hudgens, Ph.D., president and chief executive officer of UL Research Institutes. Hudgens said Wang’s scientific leadership, entrepreneurial approach, and focus on battery safety made him especially qualified to lead the Electrochemical Safety Research Institute as demand for energy storage accelerates. Wang said batteries are fundamental to the future of transportation, infrastructure, and energy systems, and that he would work with colleagues across UL Research Institutes to advance research that improves the safety, reliability, and sustainability of energy technologies worldwide. His responsibilities will include expanding experimental and computational capabilities and strengthening partnerships with industry, government, universities, and standards-development organizations.

Wang’s appointment also brings an unusually extensive record of invention and technology transfer to a research institute whose findings are intended to inform public safety. He is a fellow of the National Academy of Inventors, the Electrochemical Society, and the American Society of Mechanical Engineers, holds approximately 140 issued patents, and has authored research cited more than 50,000 times. He earned bachelor’s and master’s degrees in mechanical engineering from Zhejiang University and a doctorate in mechanical engineering from the University of Iowa. Throughout his career, he has founded companies and helped move laboratory discoveries into commercial products. At UL Research Institutes, that combination of fundamental science, engineering, and commercialization could help close the gap between promising battery concepts and the safety requirements of technologies deployed at global scale. The institute, part of the nonprofit UL Research Institutes, conducts independent research across electrochemical safety, fire safety, chemical insights, materials discovery, digital safety, and research education, publishing findings openly to support safer standards, policies, products, and communities.

Subject of Research: People

Article Title: Battery Safety Pioneer Chao-Yang Wang Appointed to Lead UL Research Institutes’ Electrochemical Safety Research Institute

Web References: Chao-Yang Wang, Ph.D.; Electrochemical Safety Research Institute; James J. Hudgens, Ph.D.

Image Credits: UL Research Institutes

Keywords

Battery safety, electrochemical energy storage, lithium-ion batteries, lithium-metal batteries, solid-state batteries, ultrafast charging, thermal runaway, battery research, electric vehicles, energy storage safety