Why Is It Necessary To Apply A Constant Limiting Pressure Via A Mold Device During The Cycling Of Asslsb?

The application of constant limiting pressure is a fundamental requirement for all-solid-state lithium-sulfur batteries due to the massive volumetric changes inherent to sulfur chemistry. During the lithiation and delithiation processes, sulfur expands and contracts by up to 78 percent, creating a mechanical instability that creates gaps between internal components. A mold device applying significant pressure (typically around 60 MPa) is necessary to physically constrain this expansion, prevent material detachment, and maintain the essential contact required for the battery to cycle effectively.

Core Insight: Unlike liquid electrolyte systems that can flow to fill voids, solid-state batteries possess rigid interfaces that cannot self-heal. External pressure acts as a mechanical bridge, forcing active materials to maintain the continuous contact necessary for ion transport despite the drastic physical swelling and shrinking of the sulfur cathode.

The Mechanics of Volume Management

Controlling Massive Expansion

The primary driver for this requirement is the nature of sulfur itself. As sulfur reacts with lithium, it undergoes a volume change of nearly 78 percent.

Without external confinement, this expansion pushes components apart. The pressure mold acts as a containment vessel, ensuring the overall geometry of the cell remains stable despite internal fluctuations.

Suppressing Material Detachment

When the sulfur contracts during delithiation, it naturally pulls away from the electrolyte and conductive additives.

This leads to "island formation," where active material becomes electrically isolated and inactive. Constant limiting pressure effectively suppresses this detachment, forcing the materials to stay in proximity and reducing rapid capacity decay.

Optimizing the Solid-Solid Interface

Overcoming Interface Rigidity

In solid-state batteries, the interface between the cathode, anode, and electrolyte consists of rigid solids rather than adaptable liquids.

These solids have microscopic roughness that prevents perfect contact. High pressure (often around 80 MPa in testing) is required to deform these materials slightly, minimizing physical gaps and establishing a continuous path for lithium ions.

Minimizing Interfacial Resistance

Physical gaps at the interface act as barriers to ion movement, drastically increasing interfacial resistance.

By forcing full contact at these organic/inorganic boundaries, pressure ensures that lithium ions can migrate smoothly. This is critical for achieving acceptable current densities and ensuring the battery does not fail due to high impedance.

Utilizing Lithium Creep

During discharge, lithium is stripped from the anode, potentially creating voids that break contact.

External pressure leverages the creep properties of lithium metal, essentially squeezing the lithium to fill these voids as they form. This self-healing mechanism, driven by pressure, is vital for maintaining long-term cycling stability.

Understanding the Trade-offs

The Weight and Volume Penalty

While high pressure (60–80 MPa) solves electrochemical problems, it introduces significant engineering challenges.

The heavy steel molds or hydraulic presses required to maintain this force add immense weight and volume. This creates a disparity between the high energy density of the material level and the potentially low energy density of the complete system level.

Scalability Concerns

Replicating a constant 60 MPa pressure environment outside of a laboratory press is difficult for commercial applications.

Standard battery packs in electric vehicles cannot easily accommodate the heavy clamping mechanisms used in lab testing. This necessitates the search for solid electrolytes that can function at lower pressures or new cell designs that apply force more efficiently.

Making the Right Choice for Your Goal

When designing your testing protocols or battery architecture, the application of pressure dictates your results.

If your primary focus is fundamental material research: Apply high constant pressure (60–80 MPa) to eliminate contact resistance as a variable and isolate the true electrochemical capability of your materials.
If your primary focus is commercial viability: Experiment with the lowest functional pressure thresholds to identify the minimum mechanical overhead required for a practical cell design.

Ultimately, the pressure mold is not just a testing accessory; it is an integral component of the battery's operating system, compensating for the lack of fluidity in solid-state chemistry.

Summary Table:

Factor	Impact on Solid-State Li-S Batteries	Role of Constant Pressure
Sulfur Volume Change	Up to 78% expansion/contraction	Constrains expansion & prevents structural failure
Interface Contact	Rigid solids create gaps/voids	Forces physical contact for ion transport
Material Attachment	Active materials detach (island formation)	Suppresses detachment to maintain conductivity
Interfacial Resistance	Increases significantly without contact	Minimizes resistance by closing microscopic gaps
Lithium Anode	Void formation during stripping	Leverages lithium creep to self-heal voids

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References

Yuta Kimura, Saneyuki Ohno. Unraveling Asymmetric Macroscopic Reaction Dynamics in Solid‐State Li–S Batteries During Charge–Discharge Cycles: Visualizing Ionic Transport Limitations with <i>Operando</i> X‐Ray Computed Tomography. DOI: 10.1002/aenm.202503863

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