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Idemitsu Kosan (TSE: 5019): Lithium Sulfide & Solid Electrolyte Supply for Toyota's ASSB
An oil refiner that spent three decades and thirteen years alongside Toyota turning a waste byproduct into the battery industry's most-watched feedstock.
Who they are
Idemitsu Kosan is one of Japan's largest oil refiners and energy companies — not the profile you'd expect on a battery-materials chain, but the connection makes more sense once you know the chemistry. Idemitsu discovered in the mid-1990s that sulfur byproducts from its petroleum refining process could be converted into lithium sulfide, the core feedstock for sulfide-based solid electrolytes, and has spent three decades since developing that discovery into a commercial-grade material.
What they do
Idemitsu has partnered with Toyota since 2013 on sulfide solid electrolyte development, formalizing a mass-production collaboration in October 2023. On January 29, 2026, the company broke ground on a large-scale pilot plant for solid electrolytes at its Ichihara site in Chiba Prefecture — designed for an annual capacity of several hundred tonnes — following validation at two smaller demonstration plants. A separate, previously announced lithium sulfide production plant is targeting 1,000 tonnes per year capacity, also aimed for 2027 completion. The technical target for the resulting Toyota battery: 450–500 Wh/kg energy density (roughly double today's lithium-ion cells), 10-minute 10-80% fast charging, and a 1,000–1,200 km driving range.
How they make money
Idemitsu's overwhelming revenue base remains conventional energy: petroleum refining, fuel retail, and petrochemicals. Solid electrolyte production is a strategic diversification project backed partly by Japan's Green Innovation Fund and NEDO government support, framed by the company as building an integrated value chain from raw sulfur byproducts through to finished battery materials — a long-term bet on where a state-backed "future of energy" business could sit once EVs displace some of Idemitsu's traditional fuel demand.
Where it sits in the value chain
Idemitsu sits in Solid Electrolyte Materials, and is unusual in this tier for having arguably the deepest and longest-running automaker partnership of any materials supplier in the chain — 13 years with Toyota, versus the more recently formed relationships most Korean materials suppliers have with their own domestic cell makers.
The bigger trend
Idemitsu-Toyota is arguably the most tightly integrated pairing in this entire value chain — a single materials supplier locked into a single automaker's roadmap since 2013, rather than the more common pattern of one materials company sampling to several potential cell-maker customers. That concentration cuts both ways: if Toyota's 2027–2028 ASSB launch holds, Idemitsu has a clear, large anchor customer already locked in; if Toyota's timeline slips (as most of the industry's solid-state targets have, repeatedly), Idemitsu doesn't have an obvious backup buyer at the same scale. The January 2026 groundbreaking is a concrete signal the partnership is still moving on schedule.
What to watch
- Whether the Ichihara pilot plant and the separate 1,000 t/yr lithium sulfide plant both complete on their targeted 2027 timelines.
- Whether Toyota's 2027–2028 ASSB commercialization window holds, since Idemitsu's solid electrolyte business is unusually concentrated around this one customer relationship.
- Whether Idemitsu diversifies its solid electrolyte customer base beyond Toyota once production scales up, or remains a single-customer supplier.
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FAQ
Why is an oil company making solid-state battery materials?
Idemitsu Kosan's petroleum refining operations produce sulfur as a byproduct, and the company discovered in the mid-1990s it could convert that sulfur into lithium sulfide, the core feedstock for sulfide-based solid electrolytes — turning a low-value refining byproduct into a high-margin battery material.
Is Idemitsu's solid electrolyte business tied to just one customer?
Its most prominent, publicly disclosed relationship is with Toyota, dating back to 2013 and formalized as a mass-production collaboration in October 2023. The company hasn't publicly named other large battery-maker customers for its solid electrolyte materials as of mid-2026.
When will Idemitsu's solid electrolyte plants be operational?
The company broke ground on its large-scale Ichihara, Chiba pilot facility in January 2026, and a separate 1,000 t/yr lithium sulfide plant is also targeting 2027 completion — both timed to support Toyota's stated 2027–2028 commercialization window for all-solid-state EV batteries.