Automakers do not need another slide about "solid-state someday." They need a cell they can put in a development car this decade without rebuilding every gigafactory. That is the bet Factorial Energy (Nasdaq: FAC) is selling with two named platforms: FEST, a quasi-solid lithium-metal design built to ride existing lithium-ion lines, and Solstice, a sulfide all-solid-state path now getting Mitsui Kinzoku electrolyte muscle.
This guide covers Factorial as of September 2026: what FEST and Solstice actually are, the Mercedes and Stellantis vehicle disclosures, the July 29 SK On manufacturing MoU, the September 17 Mitsui Kinzoku joint development agreement, the capital-light public-company model after the June 8 Nasdaq listing, and the honest limits that still sit between road tests and series production. For the ceramic-separator, licensing-heavy contrast case, start with Inside Deep Tech's QuantumScape QSE-5 full guide. Numbers below come from Factorial IR, automaker releases, and partner primary pages, not invented datasheets.
Key Takeaways
- FEST is quasi-solid lithium-metal; Solstice is sulfide all-solid-state.
- Mercedes EQS road test: 1,205 km Stuttgart to Malmo on FEST cells (late Aug 2025).
- Stellantis validated 77 Ah FEST at 375 Wh/kg; Charger Daytona road tests from June 2026.
- SK On MoU (July 29) targets FEST scale on existing Li-ion plants; non-binding.
- Mitsui Kinzoku JDA (Sept 17) industrializes Solstice with A-SOLiD sulfide electrolyte.
Two platforms, two manufacturing clocks
Factorial's public story is easy to flatten into one word: solid-state. The company's own technology page splits the work into two platforms with different materials and different factory implications.
FEST (Factorial Electrolyte System Technology) pairs a lithium-metal anode with a quasi-solid polymer electrolyte and a high-capacity cathode. Factorial markets it as a high drop-in for lithium-ion manufacturing. The Q2 2026 shareholder letter (August 11, 2026) says FEST is compatible with up to 80% of existing lithium-ion manufacturing equipment and frames it as the high-power platform for drones, high-performance data centers, and other power-dense uses, while automotive validation continues in parallel.
Solstice is the sulfide-based all-solid-state platform: zero liquid electrolyte on the company description, dry coating / dry cathode processing, and thermal stability claims that include maintaining operating temperature over 90 °C on the technology page. Factorial introduced Solstice in September 2024 with Mercedes-Benz as a key customer and development partner, and published a company target of up to 450 Wh/kg at introduction. Treat that as a vendor target unless a later OEM validation names Solstice specifically.
| Attribute | FEST (company framing) | Solstice (company framing) |
|---|---|---|
| Electrolyte class | Quasi-solid polymer | Zero-liquid sulfide all-solid |
| Anode / cathode pitch | Lithium-metal + high-capacity cathode | High-capacity materials on sulfide stack |
| Manufacturing pitch | High drop-in on Li-ion lines (up to ~80% equipment reuse) | Novel dry coating / dry cathode process |
| Near-term vehicle proof | Mercedes EQS + Stellantis Charger Daytona (FEST cells) | OEM development partner (Mercedes at intro); no mass SOP disclosed |
| Sept 2026 industrialization news | SK On MoU evaluates FEST scale-up | Mitsui Kinzoku JDA on A-SOLiD + Solstice processes |
| Published density anchors | Stellantis: 375 Wh/kg on 77 Ah cells (Apr 2025) | Intro target up to 450 Wh/kg (Sept 2024); see platform notes below |
Source: Factorial technology page, Stellantis, April 24, 2025, Factorial Q2 2026 letter.
Here's why that split matters. If you are an OEM program manager, quasi-solid first is often the only path that fits a 2026-2030 vehicle calendar without a clean-sheet solid-electrolyte plant. FEST is Factorial's answer to that calendar. Solstice is the longer all-solid bet that still needs electrolyte supply, process yield, and a different cost curve.
What the automaker tests actually showed
Mercedes-Benz: EQS, FEST cells, 1,205 km
Mercedes-Benz and Factorial have been in a joint development relationship since 2021. Mercedes-Benz and Stellantis led a $200 million Series D round for Factorial in January 2022. In summer 2024, Factorial said it delivered lithium-metal solid-state B-sample cells on the FEST platform to Mercedes, described as a first lithium-metal solid-state B-sample shipment to a global OEM.
Road testing in a lightly modified EQS began in February 2025 after late-2024 lab vehicle work. Mercedes-AMG High Performance Powertrains (HPP) co-developed the pack architecture, including mechanical management of lithium-metal cell expansion. Mercedes said the solid-state pack could deliver up to 25% more driving range at comparable battery weight and size versus a corresponding standard EQS pack, with a development target above 1,000 km.
The public long-distance proof came at the end of August 2025: the EQS covered 1,205 km from Stuttgart to Malmo without a charging stop and arrived with about 137 km of indicated range remaining. Factorial's June 8, 2026 Nasdaq listing release repeats that Q3 2025 milestone. It is a demonstration vehicle result under Mercedes' validation program, not a certified production WLTP figure for a catalog EQS.
Stellantis: 375 Wh/kg cells, then a Charger Daytona on the road
On April 24, 2025, Stellantis and Factorial published validation of automotive-sized 77 Ah FEST cells at 375 Wh/kg with more than 600 cycles progressing toward automotive qualification, charge from 15% to over 90% in 18 minutes at room temperature, operation from -30 °C to 45 °C, and discharge rates up to 4C. Stellantis also restated its $75 million 2021 investment and a plan to put Factorial cells into a demonstration fleet by 2026.
That fleet step showed up on June 11, 2026. Stellantis and Factorial said FEST cells had been integrated into a Dodge Charger Daytona development vehicle on the STLA Large architecture, with road testing underway in North America. Stellantis designed a patented mechanical pack architecture for the solid-state cells and adapted controls for automotive safety and durability. Factorial calls it the first automotive integration of the technology in North America.
Read those two milestones as a sequence, not a finish line. Cell validation plus one development vehicle is how OEMs learn pack pressure, thermal edges, and BMS behavior. It is not the same as a production offtake announcement.
SK On MoU: FEST on someone else's gigafactory footprint
On July 29, 2026, Factorial announced a Memorandum of Understanding with SK On. The companies will jointly evaluate technical feasibility and manufacturing considerations for Factorial's FEST solid-state battery technologies on SK On's network. Factorial's release cites SK On capacity of more than 200 GWh annually worldwide, including about 100 GWh in the United States (with 22 GWh called out in Commerce, Georgia, plus Tennessee operations).
The strategic point is explicit in both the July release and the August 11 Q2 letter: Factorial wants a capital-light industrialization path that uses existing lithium-ion infrastructure instead of funding a Factorial-owned GWh plant first. The Q2 letter calls the SK On MoU Factorial's second manufacturing-focused partnership with a global battery producer and notes Factorial's own lines have already produced and shipped thousands of automotive-sized FEST cells to OEM partners for testing.
A note on dates: some secondary feeds recirculated the SK On MoU around September 25, 2026. The primary Factorial GlobeNewswire timestamp is July 29, 2026. Use July 29.
Mitsui Kinzoku: electrolyte supply for the Solstice clock
On September 17, 2026, Factorial announced a joint development agreement with Mitsui Kinzoku to support industrialization of the Solstice all-solid-state platform. Mitsui Kinzoku's parallel company release (posted around September 18 Japan time) says the work centers on battery cell manufacturing processes using Mitsui's A-SOLiD sulfide solid electrolyte, which Mitsui has been advancing toward mass production at a dedicated plant in Saitama, Japan.
Roles are clearly split. Factorial remains the fabless innovator across the platform: cell design, process development, supply-chain qualification, line integration, and manufacturing validation. Mitsui brings materials and manufacturing technology for the sulfide electrolyte and layer-formation work. Factorial's release also leans on Mitsui's broader materials franchise, including ultra-thin copper foil leadership in semiconductors, as context for why a materials giant belongs in an ASSB coalition.
That means... Solstice's bottleneck is not only cell design IP. It is whether sulfide electrolyte, dry electrode processes, and interface control can hit cost and yield at automotive scale. Mitsui is a supply-chain answer to that bottleneck. It is still a development agreement, not a published GWh offtake.
Public company facts that matter for a capital-light battery story
Factorial listed on Nasdaq on June 8, 2026 as Factorial Energy Inc. after completing a business combination with Cartesian Growth Corporation III. Series A common stock and warrants began trading as FAC and FACWW. The listing release said the transaction implied about $1.3 billion of equity value and provided more than $100 million in gross proceeds for commercialization across defense and aerospace, hyperscale data centers, and e-mobility.
Backers named across Factorial materials include Mercedes-Benz, Stellantis, Hyundai, Kia, and In-Q-Tel (IQT). The listing release also flagged a Karma Automotive solid-state production program announcement in Q1 2026, drone-integrator partnerships, and board additions including Dieter Zetsche (former Daimler / Mercedes-Benz Cars chair), who the Q2 letter says joined the board after the business combination closed.
| Metric (company-reported) | Figure | As-of / source |
|---|---|---|
| Ticker | FAC (warrants FACWW) | Nasdaq listing, June 8, 2026 |
| Implied equity value at listing | ~$1.3B | June 8, 2026 listing release |
| Gross proceeds (listing) | >$100M | June 8, 2026 listing release |
| Cash and cash equivalents | ~$112.8M | June 30, 2026 (Q2 letter) |
| Q2 2026 GAAP net loss | $11.3M | Q2 letter, Aug 11, 2026 |
| Q2 2026 operating expenses | $13.0M | Q2 letter |
| H1 2026 GAAP net loss | $19.9M | Q2 letter |
| H1 2026 capex | $0.6M | Q2 letter (capital-light framing) |
| FY2026 non-GAAP OpEx guidance | ~$40M | Q2 letter |
| FY2026 capex guidance | ~$13M | Q2 letter |
| First commercial revenue (company expectation) | 2027 | Q2 letter (drone + supercar orders) |
| Fab capacity expansion phases | Through end of 2028 | Q2 letter |
| Patents / applications | >150 | Q2 letter |
| Platform energy density (OEM cells) | 390 Wh/kg demonstrated | Q2 letter (cells to a global OEM) |
| Drone path density target | ~450 Wh/kg by year end 2026 | Q2 letter (company expectation) |
Source: Factorial Q2 2026 shareholder letter, Factorial Nasdaq listing release, June 8, 2026.
The financial shape matches the partnership story. Capex is small relative to a gigafactory builder. Cash around $113 million mid-2026 funds R&D, sampling, and phased fab expansion while SK On and Mitsui are supposed to carry materials and volume-manufacturing questions. First commercial revenue guided to 2027 is tied to high-spec orders (drone battery order in July 2026; a supercar battery order earlier in 2026), not to mass EV SOP.
For readers mapping Factorial against other deep-tech industrials, Inside Deep Tech's Top 100 Deep Tech companies list and the September 2026 Physical AI Market Report are useful adjacent context. Robotics and drones are where Factorial says near-term demand moves faster than automotive qualification cycles.
How Factorial compares with the QuantumScape licensing model
Both companies sell a capital-light story. The mechanics differ.
| Lens | Factorial (FEST / Solstice) | QuantumScape (QSE-5) |
|---|---|---|
| Near-term cell class | Quasi-solid FEST already in OEM demo cars | Anode-free solid-state lithium-metal with ceramic separator |
| All-solid path | Solstice + Mitsui A-SOLiD (Sept 2026 JDA) | Core product is the solid ceramic platform |
| Scale partner type | Cell maker MoU (SK On) + materials JDA (Mitsui) | OEM battery co. licensing (PowerCo) + Honda JRA |
| Public vehicle demos | Mercedes EQS 1,205 km; Stellantis Charger Daytona | Partner-factory / sample path; no Factorial-style EQS road headline |
| Revenue timing (company) | First commercial revenue expected 2027 (high-spec) | Royalty scale tied to partner SOP discussion around 2029 |
| Honest shared risk | MoUs and demos are not GWh offtake | License execution and royalties still milestone-gated |
If you want the PowerCo licensing math, Eagle Line pilot, and 2029 cash-runway detail, read the QuantumScape QSE-5 guide. The short version for this article: Factorial is racing to prove quasi-solid vehicles and high-spec revenue first, while QuantumScape is racing ceramic separator transfer into a licensee factory.
Honest limits: who this does not work for yet
Name the downside. FEST is quasi-solid, not an all-solid cell. Marketing language that collapses FEST and Solstice into one "solid-state" brand obscures the manufacturing and safety cases investors and engineers actually have to diligence.
OEM road tests are not SOP. Mercedes' 1,205 km drive and Stellantis' Charger Daytona program prove integration progress. Neither disclosure, as of the sources above, announces a production vehicle year with Factorial cells as the standard pack.
Partnership paper is staged. The SK On MoU is non-binding. The Mitsui deal is a joint development agreement. Factorial's Q2 letter itself says automotive volume production should develop over a longer timeframe consistent with industry qualification cycles, while high-spec markets (drones, aerospace, defense-aligned) are the nearer commercial path.
Density figures need condition labels. Stellantis' 375 Wh/kg is a partner-validated 77 Ah cell result from April 2025. Factorial's Q2 letter cites 390 Wh/kg in cells delivered to a global OEM and a drone-program expectation toward about 450 Wh/kg by year-end 2026. Solstice's original "up to 450 Wh/kg" was a 2024 company target. None of those replace fleet-validated pack Wh/kg.
And the public-company clock is young. FAC listed in June 2026 with roughly $113 million of cash at mid-year and a 2027 first-revenue expectation. That can fund a capital-light plan. It does not erase execution risk if MoUs stall or if high-spec orders stay small.
FAQ
What is Factorial Energy's FEST technology?
FEST (Factorial Electrolyte System Technology) is Factorial's quasi-solid lithium-metal platform: lithium-metal anode, quasi-solid polymer electrolyte, and high-capacity cathode. Factorial markets it for high drop-in compatibility with existing lithium-ion manufacturing, including up to about 80% equipment reuse in the Q2 2026 shareholder letter.
How is Solstice different from FEST?
Solstice is Factorial's sulfide-based all-solid-state platform with zero liquid electrolyte on the company description and a dry coating process. FEST is the nearer quasi-solid path already in Mercedes and Stellantis development vehicles. Solstice is the longer all-solid bet now tied to the September 2026 Mitsui Kinzoku A-SOLiD joint development agreement.
Did a Mercedes EQS really drive 1,205 km on Factorial cells?
Yes, according to Mercedes and Factorial disclosures: a lightly modified EQS with Factorial FEST lithium-metal cells completed 1,205 km from Stuttgart to Malmo without charging in late August 2025 and arrived with about 137 km of indicated range remaining. It is a demonstration validation result, not a production catalog rating.
What did Stellantis validate on Factorial cells?
In April 2025, Stellantis and Factorial said automotive-sized 77 Ah FEST cells reached 375 Wh/kg with more than 600 cycles progressing toward automotive qualification, 15% to over 90% charge in 18 minutes at room temperature, -30 °C to 45 °C operation, and up to 4C discharge. In June 2026 they reported FEST cells integrated into a Dodge Charger Daytona development vehicle for North American road testing.
Is the SK On deal a binding manufacturing contract?
No. Factorial's July 29, 2026 release describes a Memorandum of Understanding that is non-binding except for certain customary provisions, with definitive agreements still expected later. The work is to evaluate putting FEST technology onto SK On's existing manufacturing footprint.
What did Mitsui Kinzoku agree to on September 17, 2026?
Factorial and Mitsui Kinzoku signed a joint development agreement to industrialize Factorial's Solstice all-solid-state platform using Mitsui's A-SOLiD sulfide solid electrolyte and related cell manufacturing process development. Factorial stays the fabless cell-technology lead; Mitsui brings electrolyte materials and manufacturing expertise.
When does Factorial expect commercial revenue?
In the August 11, 2026 Q2 shareholder letter, Factorial said it expects first commercial revenue in 2027, driven by a first commercial drone battery order announced in July 2026 and a supercar battery order from earlier in 2026. Automotive volume is described as a longer-cycle path.
Should readers treat FEST as a full solid-state battery?
No. Factorial's own technology materials classify FEST as quasi-solid. Solstice is the all-solid sulfide platform. For diligence, keep those labels separate even when press shorthand says "solid-state."
What to watch next
The next honest milestones are operational. Watch whether the SK On MoU converts into a definitive manufacturing agreement with named process gates. Watch whether Mitsui A-SOLiD work produces Solstice cells that an OEM is willing to put on a public validation calendar. Watch whether Stellantis and Mercedes move from development vehicles to a dated series program. And watch whether Factorial's 2027 revenue guidance shows up as recognizable high-spec shipments rather than another year of sampling language.
If those gates clear, Factorial's thesis holds: automakers take quasi-solid FEST first because the factory math is less violent, while Solstice rides a materials partnership into a later all-solid wave. If they slip, FAC becomes another deep-tech battery story where demos outrun industrialization.
For the ceramic all-solid contrast and the licensing P&L shape, stay with Inside Deep Tech's QuantumScape QSE-5 guide. For the machines that will need denser packs next, the Physical AI Market Report is the right adjacent brief.
