A battery separator forecast to 2035 leaves out China's giants

The product story is more interesting than the headline number. IndexBox expects ultra-thin films with ceramic coatings to go from about a third of the market to more than half over the decade. Thinner film leaves more room inside a cell for the material that stores energy. The coating helps the film survive the heat of fast charging. The firm also flags the risks: volatile plastic prices, trade friction, approval cycles that can take a battery maker a year or two, and the chance that solid-state batteries change what a separator needs to do.

On geography, the report says Asia-Pacific is about 70% of the market and that China, Japan and South Korea hold nearly all production capacity between them. Keep that in mind. It makes the company list even harder to explain.

The math doesn't hold together

The separate Asia report, sold through the IndexBox store, says the region's volume will grow 22% to 28% a year over the next decade. It also says volume will end up 2.5 to 3.5 times where it started.

It can't be both. A market that grows by roughly a quarter every year for ten years ends up close to ten times its starting size, not three. Tripling over a decade works out to growth of just over 10% a year. That conflict sits inside the same product description.

The global forecast helps settle it. If Asia is most of the world market and grew anywhere near 25% a year, the world total couldn't land where IndexBox puts it. The slower global rate fits the smaller Asia multiple, so the faster figure is probably the error. That's our reading. IndexBox hasn't said which number is right.

This isn't nitpicking. Forecasts like these get pasted into investor decks, economic development pitches and other outlets' stories, usually with only the most exciting number kept. Someone who planned around the faster Asia rate would be planning for a market several times bigger than the one the same firm describes elsewhere.

The world's biggest maker isn't on the list

IndexBox's top ten includes Asahi Kasei, Toray, SK IE Technology, Celgard, W-Scope, Ube, Sumitomo Chemical, Mitsubishi Chemical, Entek and Freudenberg. Most are Japanese, with a few American, Korean and German names mixed in. None is Chinese. Two of those names also overlap: Celgard has belonged to Asahi Kasei since the Japanese group bought its parent, Polypore, in 2015.

Industry trackers in China tell a very different story. The research firm EVTank found that Chinese companies shipped close to four of every five square meters of separator film sold worldwide in 2024. The leader, SEMCORP (the trading name of Yunnan Energy New Material), shipped about 9 billion square meters on its own, more than 40% above the year before. Behind it come names most Western drivers have never heard of, such as Senior Technology Material, Gellec and Sinoma.

SEMCORP isn't a newcomer either. It passed Asahi Kasei as the world's largest separator supplier years ago, and its customers include CATL and BYD, the two biggest battery makers on the planet. A top-ten list of separator makers without SEMCORP leaves out the market leader.

There is a charitable reading. Maybe IndexBox defines "composite laminated" narrowly, as a premium layered product where Japanese firms are strong. But the report describes some of its leaders, including Asahi Kasei and Entek, simply as makers of wet-process polyethylene film. That's the everyday grade Chinese factories turn out by the billion square meters. The report never defines its category tightly enough to settle the question.

Other sources don't agree on China's exact share either. When the US Department of Energy paid out the first slice of a loan to Entek, it described the film as made mostly in Asia, with about 41% coming from China. EVTank counts shipments in a single year, and DOE doesn't say what its figure measures. Either way, the honest answer to where an EV's separator comes from starts with China.

A glut, not a shortage

Growth forecasts can leave the impression that the world is short of separator film. Right now it's the opposite. EVTank describes fierce competition, factories with spare capacity and battery makers squeezing suppliers until prices fell. Chinese plants were built faster than demand grew.

That makes life harder for the plants North America is building. They aren't filling a gap. They're trying to sell a non-Chinese alternative at a time when Chinese film is cheap.

In Terre Haute, Indiana, Entek is building a wet-process separator plant backed by a federal loan of up to $1.3 billion. DOE says the project will make Entek the only US-owned producer of this kind of film based in the country. The company now targets the start of production for the fourth quarter of 2026, after earlier target dates slipped. At full build-out the plant is meant to make about 1.4 billion square meters of film a year, which Entek says is enough for about 160 gigawatt-hours of cells. That's real capacity, but SEMCORP alone shipped about 9 billion square meters in 2024.

In Port Colborne, Ontario, Asahi Kasei's C$1.56 billion plant was supposed to feed Honda, which joined as a partner. Then in May 2025, Honda Canada put its C$15 billion EV plans on hold indefinitely. Asahi Kasei says construction continues, but commercial production has slipped by roughly two years to 2029, "to align with evolving market conditions," a spokesperson told CBC. The town's mayor said about 925 people a day were working on the site. Those workers are building a factory whose main customer has stepped back.

Why you should care about a film you'll never see

The separator does two jobs you'd notice right away if it failed. It lets lithium ions pass between the electrodes while you charge and drive, and it keeps those electrodes apart so the cell doesn't short. The shift to thinner coated film that IndexBox expects is part of how future packs get more range and handle faster charging without growing bigger.

Where that film comes from also shapes what your car costs. Washington has framed separator plants as a matter of securing domestic supply chains, which is why taxpayers are lending money to build one. Film made in North America is unlikely to be the cheapest in the world while China has a glut. Someone eventually pays that difference: the automaker, the government or you.

The next few months will show how things stand. When SEMCORP and its Chinese rivals report full-year 2026 results in spring 2027, we'll see whether the price war EVTank described has started to ease. Closer to home, Entek's first phase is due in the fourth quarter of 2026, and another slip would say a lot about how hard it is to compete with cheap imports. And Honda's promised 2027 review of the EV market will decide whether the Port Colborne plant gets the customer it was built for.