Why Must A Specific External Pressure Be Maintained When Assembling All-Solid-State Sodium Batteries (Assb)?

External pressure is a critical engineering requirement for the successful assembly and operation of all-solid-state sodium batteries (ASSBs). Because these batteries rely on solid-to-solid interfaces rather than liquid electrolytes, a dedicated pressure system is the only mechanism available to ensure tight physical contact between the sodium anode, solid electrolyte, and cathode. Without this mechanical constraint, the internal components would remain electrically isolated or detach during use, rendering the battery non-functional.

In the absence of liquid electrolytes to naturally "wet" surfaces and fill gaps, external pressure serves as the vital bridge for ion transport. It eliminates microscopic voids during assembly and acts as a stabilizing force to prevent component delamination caused by volume expansion during cycling.

The Fundamental Challenge: Solid-Solid Interfaces

Overcoming the Lack of Wetting

In traditional batteries, liquid electrolytes flow into porous electrodes, creating an immediate and comprehensive path for ions to move. Solid-state batteries lack this "wetting" effect entirely.

Without external pressure, the contact between the electrode and the solid electrolyte is poor, leading to extremely high interfacial resistance.

Eliminating Surface Roughness

On a microscopic level, the surfaces of cathodes and solid electrolytes are rough and uneven. Simply placing them together leaves gaps and voids where air is trapped.

A pressure system (often requiring high pressures like 70-74 MPa during initial assembly) forces these materials together, crushing irregularities to create a void-free, intimate interface.

Maximizing Contact Points

High stack pressure increases the actual surface area of contact between the active materials and the electrolyte. This is necessary to facilitate atomic-level contact.

By increasing these contact points, you drastically reduce the ionic transport resistance, enabling the battery to function with low internal impedance.

Managing Dynamics During Operation

Counteracting Volume Changes

During charge and discharge cycles, electrode materials—particularly the sodium metal anode—undergo significant expansion and contraction.

If the battery is not constrained, this "breathing" causes the layers to separate physically. A pressure fixture acts as a spring, compensating for volume changes to maintain connection during stripping and deposition.

Preventing Delamination

Because solid electrolytes cannot flow to self-repair gaps, any separation between layers is permanent without external force.

Continuous pressure prevents this interfacial delamination, ensuring that the impedance remains stable and low throughout the battery's lifespan.

Mitigating Current Constriction

When contact is spotty, current is forced to flow through very small, specific points, a phenomenon known as current constriction.

High local current density at these points increases the risk of dendrite growth. Proper pressure ensures uniform current distribution, guiding safer lateral expansion of material rather than vertical penetration.

Common Pitfalls to Avoid

Insufficient Initial Pressure

Applying too little pressure during the initial stacking phase is a primary cause of poor activation. If the initial "stacking pressure" is not high enough to create intimate adhesion, the battery will exhibit high resistance immediately, regardless of the materials used.

Ignoring Cycle-Life Dynamics

Testing a solid-state battery without a fixture that maintains pressure during cycling renders the data unreliable. A simple press during assembly is not enough; the pressure must be maintained continuously (e.g., via a spring-loaded frame) to prevent immediate degradation as the anode volume shifts.

Making the Right Choice for Your Goal

To ensure the viability of your all-solid-state sodium battery project, apply pressure strategies based on your specific phase of development:

If your primary focus is Assembly and Activation: Apply a high "stacking pressure" (e.g., ~70-74 MPa) using a hydraulic press to force air out and establish atomic-level contact between layers.
If your primary focus is Long-Term Cycling Stability: Utilize a fixture or pressure frame that maintains a constant, lower operational pressure (e.g., ~15 MPa) to compensate for volume expansion and suppress void formation over time.

Ultimately, the mechanical pressure system is not just an accessory; it is an active component of the battery that dictates its efficiency, safety, and longevity.

Summary Table:

Factor	Requirement	Impact on Battery Performance
Interface Contact	High Stacking Pressure (70-74 MPa)	Eliminates voids and ensures atomic-level contact.
Volume Expansion	Constant Operational Pressure (~15 MPa)	Compensates for material "breathing" and prevents delamination.
Current Flow	Uniform Distribution	Reduces current constriction and mitigates dendrite growth.
Ionic Transport	Intimate Contact Points	Drastically lowers internal impedance and interfacial resistance.

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Whether you are focusing on initial assembly or long-term cycling stability, our equipment ensures the mechanical consistency your sodium battery projects require.

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References

Hao Guo, Matteo Bianchini. Structure and Ionic Conductivity of Halide Solid Electrolytes Based on NaAlCl 4 and Na 2 ZnCl 4. DOI: 10.1002/advs.202507224

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

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Why must a specific external pressure be maintained when assembling all-solid-state sodium batteries (ASSB)?