Toray Cracks the Anode Problem: A Resin Film That Could Shave 10% Off Battery Cell Weight

The anode current collector is one of those components that battery engineers have long wanted to fix but couldn't. It sits inside every lithium-ion cell, doing nothing but conducting current - pure dead weight. Copper foil, the conventional material, accounts for roughly 10% of total cell weight, and it contributes nothing to capacity. The cathode side solved this years ago by switching to a polymer-backed film. The anode side couldn't follow because the electrochemical environment there is far harsher.

Toray Industries announced on July 29 that it has solved that problem.

What Toray Actually Built

Toray said it has developed what it calls the world's first anode current-collector film that meets four key requirements for lithium-ion battery current-collector materials, designed to replace conventional copper foil in anode current collectors.

The four requirements - resistance to oxidation and reduction, adhesion to the metal layer, mechanical strength, and thermal stability - are not new targets. The industry has known them for years. The obstacle has always been chemistry. Polyethylene terephthalate (PET) film works on the cathode side but breaks down in the strongly reducing conditions at the anode, and no substrate had previously combined reduction resistance with adhesion, strength, and thermal stability.

Toray's answer is its polymer alloy technology: blending two or more polymers to reach properties that no single resin can achieve on its own. The resulting substrate resists reduction, bonds strongly to the copper layer deposited on top of it, matches copper foil in mechanical strength, and shows low thermal shrinkage. Cycling tests on single-layer laminate cells (30 mAh capacity) showed performance comparable to conventional copper-foil current collectors.

Photo: Auguste Lavi / Unsplash

The Numbers, Carefully Read

A film collector carries a copper layer of around one micron on a 4.5-micron substrate, against copper foil of six microns or more, cutting collector weight by 50% to 60% and cell weight by roughly 10%.

That 10% cell-weight reduction is real and meaningful. But Toray's headline range claim - up to 10% more electric vehicle range - deserves more scrutiny. The range claim needs reading carefully. Toray estimates up to 10% more electric vehicle range, but only if the weight saved is spent on additional active material rather than taken as a lighter pack, and under conditions it does not fully specify.

In other words: if a cell designer pockets the weight saving as a lighter pack, range stays flat. The 10% range extension only materializes if the freed-up mass budget is reinvested in more cathode active material, more anode active material, and more electrolyte - keeping the negative-to-positive capacity ratio constant throughout. That's a design choice, not a guaranteed outcome. Pack engineers at automakers will make that call, not Toray.

Why the Anode Side Was So Hard

Aluminum and copper are the most commonly used current collector materials in lithium-ion batteries - aluminum for the cathode, copper for the anode. The cathode operates at high potential, where aluminum is stable. The anode operates at low potential - a strongly reducing environment - where aluminum dissolves and PET degrades.

Enlarging energy density by reducing the thickness of the current collector is feasible, but the thicknesses of copper foil and aluminum foil as anode and cathode current collectors have been reduced from 20 µm and 18 µm at an early stage to 6 µm and 10 µm at the current stage, respectively - and conventional rolling and electrodeposition processes struggle to go much thinner without tearing. That physical limit is part of why the industry has been looking at composite film collectors for years.

Composite current collectors, prepared by plating a metal layer on both sides of a polymer base such as PET, polypropylene, or polyimide, have been waiting for commercialization due to their promising performance in increasing battery energy density. The cathode side got there first. Toray's announcement is the anode side catching up.

Where Toray Is Targeting First

Automotive is not the first target. Toray plans to commercialize the film for small and medium batteries in mobile devices, drones, and electric vertical take-off and landing aircraft, while it considers larger applications in vehicles and energy storage.

That sequencing makes sense. The weight penalty of copper foil matters most where every gram counts. For eVTOL aircraft, engineers focus on weight and heat as much as aerodynamics. Most current eVTOL aircraft use lithium-ion batteries with nickel-manganese-cobalt chemistry, offering energy density in the range of 250 to 300 Wh/kg. eVTOLs require 10-15x more power for takeoff than ground vehicles, demanding energy densities exceeding 400 Wh/kg for commercial viability - and current lithium-ion batteries max out at 250-300 Wh/kg, forcing trade-offs between payload and flight time.

A 10% cell-weight reduction doesn't close that gap on its own, but it is a meaningful contribution to a problem where every Wh/kg matters. For drones and eVTOLs, the film could extend flight time or increase payload without a chemistry overhaul.

Sample Stage, Not Commercial Stage

Toray has produced the film on mass-production equipment and begun supplying samples to customers. It is working to make the film thinner than the current 4.5-micron standard grade.

That distinction matters. Samples on mass-production equipment is a better signal than lab-scale prototypes - it means the process is at least compatible with high-volume manufacturing. But it is not a signed supply agreement, not a qualified cell, and not a vehicle program. The path from sample qualification to cell-level certification to pack integration at an OEM runs to years, not months.

The EV application is further out still. Large-format cells for automotive packs face more demanding mechanical and thermal cycling requirements than small cells for consumer electronics. Toray says it is "studying" the use in larger batteries - which is the appropriate word for where that work stands.

The Broader Context

Toray is not the only company working on composite film current collectors. The optical-grade base film for composite current collectors has high technical barriers and is highly dependent on Japanese and Korean manufacturers, including Toray, Mitsubishi, and SKC. The cathode-side version of this technology - PET film coated with aluminum - has been in development for several years and is approaching commercialization in some cell formats.

What Toray has done is extend the concept to the anode side, where the chemistry is harder. If the cycling data holds at larger cell formats and over longer cycle counts, this is a genuine materials advance - not a chemistry breakthrough, but a manufacturing one. The copper foil is still there; it's just thinner and sitting on a lighter substrate. The active materials, the separator, the electrolyte - none of those change. That's both the limitation and the appeal: it's a drop-in improvement to existing cell architectures, not a platform bet on a new chemistry.

For applications where weight is the binding constraint - drones, eVTOLs, wearables - that kind of incremental but real gain is exactly what the market needs right now.