Structural Sodium Batteries Step Forward with Electrografted Polymer Electrolyte

Researchers at KTH Royal Institute of Technology in Stockholm have demonstrated a single-step method for coating individual carbon fibres with a chemically bonded solid polymer electrolyte (SPE), enabling the fabrication of separator-less structural sodium batteries. 

The work, published in EES Batteries, uses cathodic electrografting to deposit a 1.1 μm thick PEG-acrylate/NaTFSI-based coating uniformly across every fibre in a carbon fibre tow — eliminating the need for a conventional separator and opening new pathways for lightweight, high-performance energy storage.

Science Made Simple

Sodium-ion batteries work like lithium-ion batteries but use sodium instead of lithium — a far more abundant and affordable element. Both types need a separator, a thin physical barrier between the two electrodes that prevents short circuits but allows ions to pass through. That separator adds weight, takes up space, and contributes nothing structurally.

The KTH team found a way to replace the separator with an ultra-thin polymer coating applied directly onto the carbon fibre electrodes. Think of it as a layer of functional varnish, less than one and a half micrometres thick, bonded to the surface of each individual fibre. It acts as both electrolyte and separator in one, cutting out the bulk of a conventional separator entirely.

Why Structural Batteries Need a Different Approach

Conventional batteries are passive components: they store energy, but their packaging and electrodes carry no mechanical load. Structural batteries change that by integrating energy storage into load-bearing parts of a vehicle or aircraft, reducing total system weight by replacing monofunctional components with multifunctional ones. Carbon fibres are central to this concept because they combine high mechanical strength with the hard-carbon-like microstructure needed for sodium or lithium ion storage.

The standard structural battery design stacks laminated carbon fibre electrodes infused with a structural battery electrolyte. But conventional separators in those stacks — typically 20 to 200 μm thick — add inactive material, reduce the carbon fibre volume fraction, and degrade both energy density and mechanical performance. Replacing them with a nanoscale surface coating is a meaningful architectural gain.

Electrografting: One Step, 250 Seconds

Cathodic electrografting works by applying a precise potential to an electrode immersed in a solution of acrylate monomer, solvent, and supporting electrolyte. The monomer chemisorbs onto the electrode surface and polymerises by an anionic mechanism, forming covalently bonded polymer brushes. Because propagation is kinetically limited, coating thickness can be controlled by adjusting monomer concentration.

Because the process is single-step and operates directly on the electrode surface without requiring complex tooling or pre-treatment, it is also considered scalable — an important practical consideration for any battery manufacturing route moving beyond the lab.

In this study, the team used a 1:1 ethylene carbonate/propylene carbonate solvent with 0.8 M NaTFSI — a system chosen because it also functions as a conventional liquid sodium-ion electrolyte, simplifying post-processing before cell assembly. Carbon fibre tows were attached to 20 μm copper current collectors supplied by Advent Research Materials and submerged in a three-electrode Teflon cell. The grafting potential was identified by linear sweep voltammetry, and the electrografting was completed in 250 seconds. Passivation of the carbon fibre surface was essentially complete within 150 seconds, confirmed by cyclic voltammetry showing near-zero residual grafting current by the third cycle.

Scanning electron microscopy and energy-dispersive X-ray spectroscopy confirmed a homogeneous 1.1 μm SPE coating on every individual fibre in the tow. Thermogravimetric analysis corroborated this thickness and showed that no salt enrichment occurred during grafting, giving an ethylene oxide-to-sodium ratio of approximately 30:1. The bulk ionic conductivity was 6 × 10⁻⁵ S cm⁻¹ at 25°C, with thermal decomposition at 390°C — well above typical battery operating temperatures.

Why Leaching Matters

Not all monomers react during electrografting — some remain unreacted within the film. The team confirmed this using ¹H NMR spectroscopy, finding that simply rinsing the coated fibres left residual monomer in place. During subsequent electrochemical cycling, this monomer reacted at the carbon fibre surface, densifying the polymer and raising ionic resistance. Prolonged leaching — submerging the coated fibres in solvent for 48 hours before cell assembly — removed the residual monomer and substantially reduced first-cycle irreversible capacity loss compared to rinsed samples. This post-synthesis leaching step is described as critical to achieving consistent, low-resistance performance.

Performance and Cycling Stability

SPE-coated, leached carbon fibres assembled in sodium half-cells delivered specific capacities of approximately 150 mAh/g at low current rates, compared with 200 mAh/g for uncoated fibres. The researchers attribute most of this difference — around 23% — to a loss of electrochemically active mass during the grafting process, where the polymer coating disrupts fibre-to-fibre contact points, electrically isolating some fibres that previously relied on indirect connections. When accounting for this active mass reduction, the SPE-coated fibres perform comparably to uncoated fibres across all tested current rates.

Long-term cycling at approximately C/10 demonstrated coulombic efficiencies exceeding 99% sustained over 100 cycles after initial stabilisation. The initial coulombic efficiency for leached SPE-coated fibres (58%) closely matched that of uncoated fibres (60%), indicating that the polymer coating does not significantly alter solid electrolyte interphase formation at the carbon fibre surface. Capacity also showed a gradual increase over the first 40 cycles before stabilising, a behaviour the authors attribute to voltage drift of the sodium counter electrode rather than to any degradation.

Outlook

The KTH team notes that because electrografting addresses the electrode/electrolyte interfacial contact challenge that typically limits solid-state batteries, and because it produces a low overall internal cell resistance, the approach has relevance well beyond structural batteries. 

It could equally benefit conventional solid-state sodium-ion battery designs where achieving reliable electrode/electrolyte contact at complex electrode geometries remains an open problem. The three-dimensional nature of coated carbon fibres also gives them high areal energy density, making them attractive candidates for microbattery applications where volumetric performance is paramount. 

The authors identify electric transport, aerospace, and portable electronics as the primary sectors that stand to benefit from this combination of energy density improvement, system-level weight reduction, and scalable electrode processing.

 Source: Lind et al., EES Batteries, 2026, 2, 541–551. DOI: 10.1039/d5eb00212e