Why Do All-Solid-State Batteries Require Specific Sealing Pressure? Master The Critical Solid-To-Solid Interface

Unlike traditional liquid batteries that rely on permeation, all-solid-state batteries (ASSBs) depend entirely on physical solid-to-solid contact for ion conduction. While liquid electrolytes naturally fill microscopic voids between electrodes, solid electrolytes remain rigid, requiring the application of specific, high sealing pressure via a laboratory press to force components together.

This pressure serves two immediate functions: ensuring the battery casing is tightly engaged to create an airtight seal (typically around 4.9 MPa) and applying initial "pre-tensioning" to the internal components. This pre-tensioning is the critical variable that guarantees the consistency and repeatability of testing data across different samples.

The Core Takeaway In solid-state battery assembly, pressure is a functional component of the electrochemical system, not just a packaging step. Without precise mechanical compression to eliminate voids and establish "pre-tensioning," ions cannot traverse the rigid interfaces, rendering the battery inoperable or ensuring the resulting data is unreliable.

The Fundamental Barrier: Solid-to-Solid Contact

The transition from liquid to solid electrolytes introduces a massive physical challenge: contact resistance.

The Limitation of Solids

Liquids possess "wettability"—they flow into porous electrodes and establish contact automatically. Solid electrolytes are rigid.

When a solid electrolyte meets a solid electrode, they touch only at rough, microscopic peaks. Without intervention, this results in significant gaps (voids) where no ion transfer can occur.

The Necessity of Pre-Tensioning

To bridge these gaps, a laboratory press applies a specific sealing pressure. As noted in your primary context, a pressure of roughly 4.9 MPa acts as a "pre-tensioning" force.

This force ensures that the internal stack is compressed enough to function as a cohesive unit before the battery ever cycles.

Reproducibility of Data

For research and development, consistency is paramount. Variations in assembly pressure lead to variations in internal resistance.

Using a precision laboratory press ensures that every coin cell or stack starts with the exact same mechanical baseline, allowing for valid comparisons between different material samples.

Mechanics of the Interface

Beyond simply closing the case, the pressure applied during assembly drives the physical mechanisms required for electrochemistry.

Densification of Materials

High mechanical pressure (often higher during pellet formation, around 80 MPa) compresses cathode powders and electrolyte materials into dense structures.

This minimizes the porosity between particles, establishing continuous "highways" for ion transport.

Plastic Deformation for "Wetting"

For batteries utilizing lithium metal anodes, the pressure serves a unique purpose. Lithium metal is relatively soft.

Under high pressure from the press, the lithium undergoes plastic deformation. It physically flows into the microscopic surface irregularities of the electrolyte, effectively "wetting" the surface without a liquid.

Reducing Interfacial Impedance

The primary result of eliminating voids and improving contact is a drastic reduction in interfacial impedance (resistance).

Low impedance is the fundamental physical requirement for efficient operation; without it, the battery suffers from high voltage drop and poor efficiency.

Common Pitfalls and Trade-offs

While pressure is essential, it introduces complexity that does not exist in liquid battery manufacturing.

The Risk of Delamination

Batteries "breathe" during operation; electrode materials expand and contract as they charge and discharge.

If the assembly pressure is not maintained (using specialized fixtures or clamps), these volume changes can cause the layers to separate (delaminate), leading to sudden contact failure.

Bipolar Stack Sensitivity

In bipolar configurations (cells in series), pressure control is even more stringent.

Because the current must pass through every layer sequentially, a single poor interface caused by uneven pressure causes a surge in internal resistance for the entire module.

Dendrite Formation

Ironically, while pressure helps, uneven pressure can hurt.

If contact is poor in specific spots, current concentrates there. This uneven distribution promotes the growth of lithium dendrites (metal needles) which can penetrate the electrolyte and short-circuit the cell.

Making the Right Choice for Your Goal

When selecting a laboratory press or defining your assembly protocol, consider your specific objective:

If your primary focus is Data Consistency: Prioritize a press with highly repeatable force control (e.g., capable of exactly 4.9 MPa every time) to ensure your pre-tensioning is identical across all samples.
If your primary focus is Cycle Life: Ensure your assembly process transitions into a fixture that maintains stable external pressure (e.g., 1 MPa) during operation to counteract volume expansion and prevent delamination.
If your primary focus is Interface Optimization: You may require a press capable of higher pressures (up to 80 MPa) to densify pellets and force plastic deformation of the lithium anode before final sealing.

Ultimately, the laboratory press in solid-state assembly acts as a surrogate for the wettability of liquid electrolytes, forcing rigid materials to behave as a unified electrochemical system.

Summary Table:

Feature	Liquid Li-ion Batteries	All-Solid-State Batteries (ASSBs)
Electrolyte Form	Liquid (flows into pores)	Rigid Solid (requires compression)
Interface Mechanism	Natural wetting/permeation	Mechanical solid-to-solid contact
Assembly Pressure	Minimal (casing seal only)	High (sealing + pre-tensioning)
Critical Goal	Leak prevention	Interfacial impedance reduction
Key Component	Electrolyte filler	Laboratory Press & Fixtures

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References

Alexander Beutl, Artur Tron. Round‐robin test of all‐solid‐state battery with sulfide electrolyte assembly in coin‐type cell configuration. DOI: 10.1002/elsa.202400004

This article is also based on technical information from Kintek Press Knowledge Base .