Summary

Idemitsu Kosan is a Japanese petroleum and chemicals company that has spent over 30 years developing sulfide solid electrolyte technology — a quiet, deep-tech R&D arc that most battery coverage ignores. The company is now converting that expertise into industrial production infrastructure as Toyota’s exclusive solid-state electrolyte supplier. Idemitsu is building a ¥21.3 billion lithium sulfide plant (completion June 2027, 1,000 MT/year capacity serving 50,000–60,000 EVs), and in January 2026 broke ground on a large-scale solid electrolyte pilot facility. This makes Idemitsu one of the most operationally advanced solid electrolyte material suppliers in the world, and its performance matters as much as Toyota’s cell chemistry to whether Toyota hits its 2027–2028 mass production target.

Key Facts

  • HQ: Tokyo, Japan
  • Type: Public (Tokyo Stock Exchange: 5019)
  • Sector: Petroleum / specialty chemicals (pivoting to battery materials)
  • Solid electrolyte R&D history: 30+ years; collaboration with Toyota formalised 2013; co-development of mass production technology from 2023
  • Key product: Lithium sulfide (Li₂S) — precursor for sulfide solid electrolytes; and finished sulfide solid electrolyte material
  • Lithium sulfide plant: ¥21.3B investment; annual capacity 1,000 MT; serves 50,000–60,000 EVs/year; target completion June 2027
  • Solid electrolyte pilot plant: Construction began January 29, 2026; capacity of several hundred MT/year; designed to de-risk mass manufacturing before full ramp
  • Primary customer: Toyota Motor Corporation (exclusive supply relationship)
  • Commercialization target: 2027–2028 (aligned with Toyota’s solid-state EV launch timeline)

What It Is / How It Works

Sulfide solid electrolytes are the leading candidate for high-performance all-solid-state batteries because their ionic conductivity approaches that of liquid electrolytes. But sulfide materials are chemically sensitive — they react with moisture and oxygen in air, making manufacturing in large quantities technically demanding. This is the problem Idemitsu has been working on for three decades.

Idemitsu’s role in the solid-state supply chain is as a raw material and electrolyte precursor producer, not a cell manufacturer. Lithium sulfide (Li₂S) is the key precursor compound from which sulfide solid electrolytes (such as Li₆PS₅Cl argyrodite and related compounds) are synthesized. Toyota and its cell-manufacturing partners take Idemitsu’s materials and process them into solid electrolyte films that go into the battery cell.

The 1,000 MT/year lithium sulfide plant capacity is calibrated against Toyota’s initial EV target: at the energy densities Toyota is projecting for its solid-state cells (~1,000 km range), 1,000 MT of Li₂S would cover approximately 50,000–60,000 vehicles per year in the initial ramp. This is a small fraction of Toyota’s total production volume but a meaningful launch scale for an entirely new battery chemistry.

The January 2026 pilot facility for finished solid electrolyte is the next step in the supply chain: converting Li₂S feedstock into the electrolyte material Toyota needs in a controlled, large-format process. This facility is explicitly designed to address the manufacturing scale-up challenges (particle size control, moisture exclusion, cost reduction) before the full production ramp.

What makes Idemitsu interesting beyond its Toyota relationship is the precedent: it is an oil major that has spent decades positioning for the battery transition and now has a technically defensible role in the most challenging part of the supply chain (sulfide electrolyte precursors). The company’s 30-year R&D investment is now a genuine moat.

Notable Developments

  • 2026-01-29: Idemitsu officially begins construction of large-scale solid electrolyte pilot facility; designed for several hundred MT/year capacity; targets cost and manufacturability challenges for mass production. (Electrive)
  • 2025-02: ¥21.3 billion lithium sulfide production unit construction begins; 1,000 MT/year capacity; completion targeted June 2027. (Battery-News)
  • 2023: Toyota and Idemitsu begin co-development of mass production technology for sulfide solid electrolytes.
  • 2013: Formal collaboration with Toyota begins on solid electrolyte development.
  • ~1990s: Idemitsu begins internal R&D on sulfide solid electrolytes — over 30 years before the current industry spotlight.

Key People / Key Organizations

Tetsuji Mishina — Executive Officer; Manager, Lithium Battery Material Department

  • LinkedIn: No confirmed public profile found; Japanese executives at Idemitsu rarely maintain public English-language LinkedIn profiles
  • Role: Executive Officer; leads the Lithium Battery Material Department; primary external spokesperson for Idemitsu’s solid electrolyte commercialization program; publicly announced the ¥21.3B Li₂S plant capacity (1,000 MT/year) and final investment decisions for the solid electrolyte pilot facility
  • Background: Long-tenure Idemitsu career through the petroleum/chemicals division into battery materials; no external employment history available from public sources. His public statements indicate deep technical familiarity with the sulfide electrolyte manufacturing challenges, suggesting direct R&D involvement over multiple years.

Nakata — Development Leader, Solid Electrolyte Program

  • LinkedIn: No confirmed public profile; full given name not available from public sources
  • Role: Development leader for solid electrolyte project within Idemitsu’s Battery Material Development Center; named in Idemitsu’s published accounts of the solid electrolyte program history
  • Background: Idemitsu career spanning the early R&D phase of the solid electrolyte program; part of the team that worked on sulfide electrolytes starting in the 1990s, over 30 years before the current industry spotlight

Key Partner Organizations

  • Toyota Motor Corporation — Primary customer; 2027–2028 solid-state EV mass production target; strategic collaboration formalised 2013; mass production co-development from 2023
  • Sumitomo Metal Mining — Secondary materials supply agreement with Toyota for cathode materials (separate from Idemitsu’s electrolyte role)

People — Last Reviewed: 2026-03-24

Supply Chain Position

Idemitsu Kosan operates at the Precursor Chemicals → Electrolyte/Active Materials layers — it is a raw material and electrolyte precursor producer, not a cell manufacturer. Its unique position is vertical integration from petroleum refining (producing sulfur as a byproduct) through lithium sulfide (Li₂S) production to finished sulfide solid electrolyte material. Upstream, Idemitsu sources lithium from commodity markets and sulfur from its own refining operations. Downstream customer is Toyota (exclusive relationship). This exclusivity means Idemitsu is not a disclosed supplier to QuantumScape or Solid Power as of 2026, even though those companies use similar sulfide electrolyte chemistries requiring Li₂S. ⚑ Key bottleneck: Idemitsu’s 1,000 MT/year Li₂S capacity (by June 2027) is the only documented large-scale dedicated Li₂S production investment; it is committed to Toyota. As the broader sulfide electrolyte industry scales, additional Li₂S producers will be required.

Sources