A Humble Phosphate Salt Could Unlock Cheaper, Cooler Sodium Batteries

Molten sodium batteries have long been celebrated as the workhorses of grid-scale energy storage: they rely on abundant, inexpensive sodium, they are inherently resistant to thermal runaway, and they can last more than two decades in service. Yet their Achilles heel has always been heat. Conventional sodium-metal-halide and sodium-sulfur cells must run at 260 to 350 degrees Celsius to keep their ceramic electrolytes and molten sodium electrode working smoothly, and that operating temperature drives up manufacturing cost. Now a team at Pacific Northwest National Laboratory has shown that a remarkably simple ingredient, an ordinary inorganic phosphate salt, could help push these batteries toward cooler, cheaper operation.

The challenge lies at the heart of every sodium-beta battery: the interface between liquid sodium and the beta-alumina solid-state electrolyte, or BASE, a ceramic that separates the molten sodium anode from the cathode. BASE conducts sodium ions with exceptional efficiency and remains stable in contact with molten metal, but molten sodium itself has high surface tension while the ceramic surface has high surface energy, a combination that makes wetting notoriously poor. Worse, wettability deteriorates further as temperature drops. Without intimate physical contact, the battery suffers high interfacial resistance and underutilized cathode materials, eroding the very performance advantages that make sodium-beta chemistry attractive.

Engineers have attacked this problem before. Metallic coatings of tin, lead, bismuth, and nickel on BASE can dramatically improve sodium wetting, and lead particles formed from lead acetate decomposition have enabled excellent wetting at temperatures as low as 120 degrees Celsius. Alloying molten sodium with potassium, rubidium, or cesium raises the work of adhesion, as described by the Young-Dupré relation. More recently, carbon-based layers have emerged as a low-cost, scalable alternative. Researchers have demonstrated sparked reduced graphene oxide that wets completely at 175 degrees Celsius, porous carbon films made from glucose templates, three-dimensional carbon fiber networks with tin particles that reduced interface resistance to 6.6 ohm cm2, and carbon-coated BASE sustaining sodium plating and stripping beyond 1000 mA/cm2. A metal-free, intrinsically sodiophilic carbon black wetting layer from the same PNNL group even achieved low area-specific resistance near 110 degrees Celsius.

But carbon layers carry a hidden weakness. To create the well-dispersed carbon inks needed for spray coating, organic surfactants and binders such as Tergitol and hydroxypropyl methylcellulose are mixed into the ink. During heat treatment at 350 degrees Celsius in air, these organics burn off, and that burnout is precisely what creates the meso- and macroporous network that molten sodium needs to spread rapidly. The catch is that removing the organic binders also strips away the van der Waals forces that hold the carbon layer to the ceramic, leaving coatings prone to cracks, voids, and peeling. The PNNL team, led by Henry Han, Mark Weller, and Guosheng Li, therefore sought a secondary binder that could survive the high-temperature treatment and reinforce the coating without sacrificing wettability.

Their solution was to add conventional sodium salts directly to the carbon ink. The selection criteria were pragmatic: the salts had to contain sodium as the cation to avoid undesirable ion exchange with the BASE, they had to be chemically simple and inexpensive, and they had to dissolve well in water so that aqueous carbon inks could be spray-coated. The team tested sodium chloride, sodium nitrate, sodium carbonate, and sodium hexametaphosphate, Na6(PO3)6, known as SHMP, at a salt-to-carbon weight ratio of 1:3. Each ink, containing Vulcan carbon black, surfactant, binder, and salt in a 90 percent deionized water and 10 percent ethanol solvent blend, was sprayed through shadow masks onto glass, molybdenum foil, and BASE disks, then heat-treated in air at 350 degrees Celsius.

The first test was brutally simple: press Scotch tape onto each circular carbon layer under a 350 gram load and peel it off. Layers containing SHMP and sodium chloride clung to the substrate far better than the pristine salt-free coating, which shed significant carbon onto the tape. Sodium nitrate performed slightly worse than the salt-free layer, and sodium carbonate was catastrophic. Carbon and sodium carbonate react at elevated temperatures to form gaseous carbon monoxide and carbon dioxide, and the carbonate-containing layer emerged from the furnace visibly lighter, having lost most of its material and nearly all of its adhesion.

Wettability told a more nuanced story. Because molten sodium wets good carbon layers almost instantaneously, conventional contact angle measurements could not distinguish between treatments, so the researchers instead tracked how fast a 25 microliter droplet of molten sodium spread along a rectangular wetting layer at 120 degrees Celsius. The SHMP-containing layer spread at 0.128 mm/s, 14 percent faster than the salt-free layer at 0.112 mm/s, while the sodium chloride layer, despite its strong adhesion, spread 13 percent slower at 0.097 mm/s. Layers with sodium nitrate or sodium carbonate showed neither instantaneous wetting nor spreading at all. Scanning electron microscopy explained why: sodium chloride and sodium carbonate precipitated as submicron cubic particles and aggregates that blocked the carbon layer’s pores, while SHMP left the meso- and macroporous network intact, closely resembling the additive-free morphology.

The decisive evidence came from sodium symmetric cells, in which carbon wetting layers were spray-coated on both sides of BASE disks and molten sodium was applied at 110 degrees Celsius. In a current-staircase experiment ramping from 1 to 15 mA/cm2 at 120 degrees Celsius, both cells delivered stable voltage profiles, but the SHMP cell showed markedly lower overpotential. Linear fits of averaged voltage against current density revealed an overall resistance of 2.8 ohms for the SHMP cell, roughly half the 5.5 ohms of the salt-free cell. Electrochemical impedance spectroscopy from 1 MHz to 0.01 Hz confirmed the split: at 120 degrees Celsius the salt-free cell showed a high-frequency intercept of 5.16 ohms and charge-transfer resistance of 0.40 ohms, while the SHMP cell measured 2.58 ohms and 0.20 ohms respectively, with both components falling further as temperature rose.

Cross-sectional analysis with plasma focused-ion beam SEM and energy-dispersive X-ray mapping uncovered the mechanism behind this performance gap. The salt-free carbon layers contained cracks running through the coating and voids at the carbon-substrate interface, imperfections far larger than the porous network itself. X-ray photoelectron spectroscopy showed that SHMP did not chemically alter the carbon black, with comparable carbonyl and C 1s spectra in both samples, and instead introduced the characteristic P-O bonding environments of hexametaphosphate. The SHMP aggregates appear to act as microscale structural reinforcements, or inorganic binders, that enhance internal cohesion and substrate adhesion while leaving the porous architecture open. In the salt-free cell, similar voids at the carbon-BASE interface likely disrupted the continuity of the porous network, leaving regions that molten sodium could not wet and reducing the effective area available for sodium ion transfer, which explains the doubled resistance.

The implications reach beyond a single additive. SHMP is non-toxic, inexpensive, water-soluble, and sodium-based, checking every box for manufacturable grid batteries, and it demonstrates that strategic inorganic additives can simultaneously solve adhesion, wettability, and interfacial resistance in microscale carbon layers. By enabling reliable molten sodium contact at temperatures near 120 degrees Celsius rather than 350, such interface engineering could slash the cost of sodium-metal-halide and sodium-sulfur systems, technologies whose non-flammable ceramic and molten salt electrolytes already offer safety advantages over lithium-ion chemistry. As grid storage demand surges and extreme weather strains electrical infrastructure, cooler-running sodium batteries reinforced by a humble phosphate could prove a quietly transformative ingredient in the quest for durable, affordable stationary energy storage.

Subject of Research: A sodium hexametaphosphate additive in carbon wetting layers on beta-alumina solid electrolyte improves adhesion and molten sodium wettability, halving interfacial resistance for low-temperature sodium batteries.

Article Title: Designing carbon wetting layers with inorganic salts as additives for enhanced adhesion and molten sodium wettability in electrochemical application

Article References: Han, H. H., Weller, J. M., Quinn, J. P., Sivakumar, B. M., Polikarpov, E., Reed, D. M., & Li, G. (2026). Designing carbon wetting layers with inorganic salts as additives for enhanced adhesion and molten sodium wettability in electrochemical application. Frontiers in Batteries and Electrochemistry, 5, Article 1759353. https://doi.org/10.3389/fbael.2026.1759353

Image Credits: AI Generated

DOI: 10.3389/fbael.2026.1759353

Keywords: molten sodium batteries, beta-alumina solid electrolyte, carbon wetting layer, sodium hexametaphosphate, grid energy storage, interfacial resistance, spray coating, sodium-metal halide batteries, Designing, carbon, wetting, layers