Case Study | No Separator Needed: How KTH's Electrografting Breakthrough Used Advent Copper Foil

Why Structural Batteries Matter 

Batteries are usually passengers. They ride inside vehicles, aircraft, and portable devices, adding weight without carrying any structural load. A structural battery changes that bargain entirely — it stores energy and bears mechanical stress at the same time, replacing passive packaging with active material and reducing total system weight. 

That ambition has driven years of research at KTH Royal Institute of Technology in Stockholm, and a 2026 paper in EES Batteries marks one of the clearest steps forward yet.

Science Made Simple

A normal battery is a dead weight. It stores energy, but the casing, electrodes, and packaging carry no load — they just sit there, adding mass. A structural battery solves that by making the battery itself part of the structure, so the same material that holds the vehicle together also powers it. Less total weight, same energy.

To do that, you need electrodes made from something mechanically strong. Carbon fibre is the obvious choice — it's already used in aircraft and racing cars because of its exceptional strength-to-weight ratio. The challenge is that a battery also needs a separator between its two electrodes to stop them short-circuiting, while still letting ions pass through. Conventional separators are bulky, add inactive mass, and don't contribute structurally.

The KTH team replaced the separator with a coating — an ultra-thin layer of polymer bonded directly onto the surface of each individual carbon fibre. Think of it like a precisely applied varnish, less than 1.1 micrometres thick (roughly one hundredth the width of a human hair), that acts as both electrolyte and separator at once. The process that applies it — electrografting — uses an electrical charge to grow the coating directly on the fibre surface in a single step, in around four minutes.

The result is a carbon fibre electrode that carries load, stores energy, and no longer needs a separate separator layer at all.

Coating Carbon Fibres at the Fibre Level

The challenge the KTH team set out to solve was specific: how to coat individual carbon fibres with a solid polymer electrolyte thin enough to be functionally invisible, yet reliable enough to replace a conventional separator entirely. 

Carbon fibres are the obvious electrode material for a structural battery — they combine excellent mechanical strength with the hard-carbon microstructure needed for sodium or lithium ion storage. But coating them uniformly, at the fibre level, without complex multi-step processing, had proved difficult.

Electrografting: A Single-Step Solid Polymer Electrolyte Coating

Their answer was electrografting — a process in which an applied electrical potential causes an acrylate monomer to polymerise directly onto the surface of an electrode, forming a covalently bonded coating in a single step. 

In 250 seconds, the team deposited a 1.1 μm layer of PEG-acrylate/NaTFSI-based solid polymer electrolyte onto every individual fibre in a carbon fibre tow. Scanning electron microscopy confirmed the coating was continuous and homogeneous across the entire tow cross-section. The resulting layer acts simultaneously as electrolyte and separator, eliminating the bulk separator that typically accounts for a significant fraction of inactive material in a conventional stack.

High-Purity Copper Foil and the Critical Leaching Step

To prepare the electrode assemblies for electrochemical testing, the carbon fibre tows were attached to 20 μm copper current collectors supplied by Advent Research Materials. 

The precision copper foil served as the current collector, providing the electrical connection between the carbon fibre tows and the external circuit during electrochemical testing.

 An additional leaching step, in which coated fibres were submerged in solvent for 48 hours before cell assembly, proved critical: it removed residual unreacted monomer that would otherwise increase internal resistance and cause irreversible capacity loss on the first cycle.

The results? 150 mAh g⁻¹ and 99% Coulombic Efficiency Over 100 Cycles

The results of this careful preparation were striking. 

SPE-coated carbon fibre electrodes delivered specific capacities of 150 mAh g⁻¹ and coulombic efficiencies exceeding 99% sustained over 100 cycles. Post-leaching cells showed coulombic efficiency on the first cycle closely matching that of uncoated fibres — confirming that the polymer coating does not significantly disrupt solid electrolyte interphase formation. 

Capacity showed a gradual increase over the first 40 cycles before stabilising — a behaviour the authors attribute to voltage drift in the sodium counter electrode rather than any degradation of the SPE-coated fibres themselves, confirming the electrode coating remained stable throughout extended cycling.

Significance for Electric Transport, Aerospace, and Portable Electronics

A structural battery that integrates energy storage and mechanical load-bearing into a single carbon fibre component could reduce the total weight of electric aircraft, vehicles, and portable devices in ways that passive battery packs simply cannot match. 

The electrografting route is scalable and operates directly on the electrode surface without specialist tooling — a practical advantage that makes it a realistic candidate for production scale-up rather than a laboratory curiosity. The KTH team identifies electric transport, aerospace, and portable electronics as the primary sectors that stand to benefit.

This work adds to a growing body of research in which precise, well-characterised materials from Advent Research Materials underpin battery science at the forefront of energy storage innovation — providing the reliable inputs that researchers need to generate results they can trust.

Citation:
Title: Electrografting solid polymer electrolytes for separator-less structural sodium batteries
Journal: EES Batteries
Authors: Elvira Lind, Vincent Nieboer, Martina Cattaruzza, Mats Johansson, Karin Odelius, Dan Zenkert, and Göran Lindbergh
Published: 9 February 2026 (online first); 20 April 2026 (issue)
DOI: 10.1039/D5EB00212E