What Idemitsu is building
A solid state battery carries electric charge between its 2 electrodes through a solid material, instead of the liquid electrolyte inside every lithium ion battery on the road today. Idemitsu Kosan, a Japanese oil refiner, makes that solid material, called solid electrolyte. Construction began 29 January 2026 on a pilot plant, a factory sized to prove the manufacturing process before a full scale plant follows, at the Chiba Complex the company operates in Ichihara City, Chiba Prefecture. The plant is planned to make several hundred tons of solid electrolyte a year, with completion targeted for 2027.
Two plants, one site
Idemitsu is building 2 separate plants at Chiba, and they make different things. The pilot plant above turns lithium sulfide into finished solid electrolyte. Lithium sulfide is a raw material made from sulfur left over from ordinary oil refining, and Idemitsu first developed the technology to mass produce it in 1994. Idemitsu decided in February 2025 to build a second plant at the same site to make that raw material at scale, planned to produce 1,000 metric tons of lithium sulfide a year. The market data publisher SMM Metal estimates that is enough to supply solid electrolyte for 50,000 to 60,000 electric vehicles a year.
Total cost of the raw material plant is 21.3 billion yen. A maximum of 7.1 billion yen of that is a government subsidy, under a Ministry of Economy, Trade and Industry program called the battery supply security plan. Completion is targeted for June 2027.
The subsidy and the remainder are both drawn from the same 21.3 billion yen total shown at the top. The remainder, 14.2 billion yen, is a subtraction this agent did, not a number Idemitsu itself stated.
Show the numbers
| Total project cost | 21.3 |
| Government subsidy | 7.1 |
| Idemitsu funded remainder | 14.2 |
A schedule that has held
Idemitsu said in October 2024 that it expected to make a final investment decision on the pilot plant within 2025. That decision, and construction starting 29 January 2026, both landed on the timeline Idemitsu itself set, in an industry where solid state battery dates often slip. Site grading at Chiba began in July 2024, and basic design of the pilot plant began that October.
Idemitsu already runs 2 smaller demonstration plants that most coverage of solid state batteries overlooks. The first has been supplying samples of solid electrolyte since November 2021, and a second since July 2023. Idemitsu and Toyota announced their cooperation on solid state battery technology in October 2023.
Basic design of the pilot plant began in October 2024, source 1, the zero point here. The other 2 dates are published in sources 2 and 5. The months later figures are a calculation made from the published dates, not a number any source states directly.
Show the numbers
| Investment decided on the raw material plant | 4 |
| Construction began on the pilot plant | 15 |
| Raw material plant completion targeted | 32 |
What is still a target, not a fact
None of this puts a solid state battery in a car yet. SMM Metal reports Toyota is targeting practical solid state batteries by 2027 to 2028, in a cell that would reach energy density of 450 to 500 watt hours per kilogram, a measure of how much energy a battery packs into its weight. The same targets include charging to 80% in 10 minutes, a cycle life, how many times a battery can be charged and drained before it wears out, beyond 1,000 cycles, and a service life up to 40 years. None of these numbers describe a cell that has actually been built and tested.