CATL’s sulfide solid-electrolyte cathode patent (published application US20250364543A1, PCT/CN2025/086345 family) discloses a chemical approach that raises interfacial stability by designing the cathode active material as a three-layer structure of substrate, cobalt-rich layer, and fluorine-based lithium-salt coating layer. However, chemical stabilization alone does not resolve the interfacial contact problem of sulfide solid electrolytes. Sulfide electrolytes exhibit rising interfacial resistance when in contact with lithium metal or high-nickel cathodes, requiring continuous high-tonnage compression throughout cell life to maintain physical contact. This is where a mechanical contradiction arises. High compression to maintain interfacial contact makes the cell case and pressing frame heavier, which erodes the energy-density advantage that all-solid-state batteries must secure. What uniform compression, at the several-kN level, must the pressing-stack jig achieve, and within what deviation (mm)?
Kinematic Analysis
The stack-compression jig is a structure that applies uniform surface pressure across the front face of multiple stacked cells. In a frame composed of tie bars and top/bottom plates, an eccentric load causes localized stress concentration, which directly leads to interfacial delamination. The basic load path distributes from the 4 tie-bar points to the top plate and then across the front face of the cell stack; the fewer the tie bars, the larger the surface-pressure deviation from plate deflection.
Calculation / Formula Verification
The relationship between surface pressure $P$ (MPa), pressing force $F$ (kN), and effective area $A$ (mm²) is as follows.
$$P = \frac{F \times 10^{3}}{A}$$
Assuming an effective area of 200mm×150mm(=30,000mm²) for a 60Ah-class automotive cell stack and a target surface pressure of 5MPa (estimated: the level typically required to maintain sulfide interfacial contact — confirm after field measurement), the required force is $F = 5 \times 30000 / 1000 = 150$kN. The tie bars carrying this load must dually secure a primary safety factor of 3.0 or higher against yield strength and a secondary safety factor of 1.5 or higher against long-term creep deformation. If either safety factor falls short, the tie-bar cross-section must be enlarged or the material grade upgraded.
Shop-notes
High-tensile steel (confirm after field measurement) should be reviewed first for the tie-bar material; a fillet radius of at least 3mm at the threaded section should be secured to relieve stress concentration and prevent cracking during machining (machinability note). An alternative of individual hydraulic-cylinder pressing was reviewed but not adopted. The reason for non-adoption is that managing pressure deviation across multiple cylinders carries a greater maintenance burden than a mechanical tie-bar frame. Whether CATL’s patent claims are infringed can only be judged from the published application, and this article does not make a determination of infringement.
Design-Reflection Checklist
- Target surface pressure and effective area re-calculated
- Tie-bar dual safety-factor verification completed
- Fillet-radius machining allowance reflected in drawing
- Tie-bar material grade confirmed on site
One-line summary: Interfacial stability of the sulfide solid-electrolyte stack ultimately comes down to a surface-pressure uniformity design problem for the pressing jig.