Semi-Solid State Battery - Battery Design

Semi-Solid State batteries use some liquid electrolyte to overcome the interface issues with the true solid state battery.

In-Situ Solidified Electrolyte

The in-situ solidified electrolyte is a WeLion developed and patented technique. The original idea developed for the in-situ solidification was to solve the solid to solid contact issues that arise when assembly solid layers. The simple steps to the approach is to coat the solid electrolyte on the electrode material particles, combine all of the layers, add liquid electrolyte and then to polymerize this as a built up stack into a semi-solid cell.

Sodium-ion & Semi-Solid Technologies

Desay introduced a SSSB reaching 420 Wh/kg, positioning itself between conventional LIB and ASSB. The cell (30 Ah-class) was first shown in China in April 2026 and is reported to have passed nail penetration and 300 °C hot plate tests, according to a company announcement (Energy-Storage.News, June 2026).

Key characteristics:

1000+ validated cycles
Passes nail penetration & 300°C thermal tests (no fire, no explosion)
Wide temperature adaptability
Core Technology Stack:
- Lean electrolyte fast-ion pathways
- In-situ dense interface engineering
- Intrinsic + active safety system
- Proprietary solid electrolyte membrane (thin, high conductivity, flame-resistant)

Needs review: The cycle-life figure (1000+ validated cycles) and “Core Technology Stack” details above come from Desay’s own marketing material; only the 420 Wh/kg energy density and the nail penetration / 300°C hot plate test results could be corroborated against an independent trade-press source. Verify the cycle-life claim against a datasheet or independent test before treating it as established.

Why go semi-solid at all? A fully solid cathode composite needs the catholyte percolation network and high stack pressure covered on the site’s other solid-state pages; adding back a small amount of liquid or ionic-liquid electrolyte at just the cathode interface is a deliberate engineering shortcut around both problems. Wetting the cathode composite with a liquid closes the solid-solid contact gaps between electrolyte and active-material particles far more completely than any dry catholyte can, and it does so without needing the tens of MPa of stack pressure a fully solid cathode requires. Reported liquid or ionic-liquid loadings for this purpose are typically low — figures in the low single-digit to low double-digit weight-percent range are common in the literature, with some formulations remaining leak-free under pressure once optimised (dynamically-wetting interface design study) — enough to solve the wetting problem while keeping the separator itself solid, which is what preserves the safety case: even if the cathode-side interlayer carries a trace of liquid, there’s no continuous liquid path across the separator for a fire to propagate through.

SES AI takes a hybrid lithium-metal approach: a high-concentration “solvent-in-salt” liquid electrolyte, paired with a protective polymer coating on the lithium metal anode. The coating doesn’t try to prevent dendrites outright — it changes their growth mode from sharp, needle-like filaments that can pierce a separator into flatter, denser deposits that are far less likely to cause an internal short (SES AI, Hermes/Apollo cells). SES’s own pitch for the hybrid approach is manufacturing continuity: it’s compatible with existing lithium-ion production infrastructure, which a fully solid design generally is not.

Factorial Energy’s FEST (Factorial Electrolyte System Technology) platform is explicitly described by the company as “quasi-solid-state”: a lithium-metal anode and high-capacity cathode built around a solid polymer electrolyte carrying a small amount of liquid electrolyte, rather than a fully dry solid separator. Factorial’s stated rationale mirrors SES’s: FEST is designed to integrate into existing roll-to-roll lithium-ion manufacturing lines with minimal retooling, avoiding the capital cost of a purpose-built solid-state line (Factorial Energy).

Needs review: ProLogium is sometimes grouped with semi-solid/hybrid designs in press coverage, but the company explicitly positions its own current technology (a “superfluidized” electrolyte, all-ceramic separator, all-silicon anode) as fully inorganic and distinct from semi-solid or gel-type approaches, which it says still carry organic electrolyte and the thermal-runaway risk that goes with it. Worth checking ProLogium’s latest technical claims directly before citing them either way, since the company’s own framing has been a moving target across announcements.