A laboratory press increases the relative density of slurry-processed Li7SiPS8 pellets by applying the mechanical force necessary to overcome the adhesive, "fixing" effect of binders. By driving particle rearrangement and plastic deformation, the press allows these composite pellets to achieve relative densities of approximately 94%, significantly reducing internal porosity.
The presence of binders creates a structural resistance that prevents electrolyte particles from settling naturally. The laboratory press solves this by mechanically forcing particles into intimate contact, reducing void space and establishing the continuous pathways required for efficient ion transport.
The Mechanism of Densification
Overcoming the "Fixing Effect"
In slurry-processed pellets, binders act as a stabilizing agent. While necessary for processing, they lock solid electrolyte particles in place, often leaving gaps between them.
The laboratory press applies stack pressure to break this stasis. It overrides the binder's hold, forcing the solid components to move closer together than they would under gravity or light compaction alone.
Promoting Particle Rearrangement
Once the binder's resistance is overcome, the applied force causes the Li7SiPS8 particles to physically shift. They slide past one another to fill the interstitial voids left by solvent evaporation.
This rearrangement is critical for reaching high relative densities, such as those seen in pellets with a 98:2 wt% electrolyte-to-binder ratio.
Inducing Plastic Deformation
To reach the upper limits of density (around 94%), simple rearrangement is not enough. The press exerts enough force to cause plastic deformation.
The electrolyte particles physically change shape, flattening against one another. This eliminates microscopic pores that rearrangement alone cannot fill, ensuring a solid, cohesive pellet structure.
Impact on Battery Performance
Reducing Internal Pores
The primary physical result of this pressing process is the drastic reduction of internal porosity. Voids are effectively crushed out of the structure.
Improving Ion Transport Continuity
For a solid-state battery to function, ions must move through a continuous material. Pores act as roadblocks.
By creating a dense, non-porous structure, the press ensures continuity of ion transport channels. This intimate contact between particles is the defining factor in maximizing the material's ionic conductivity.
Understanding the Trade-offs
The Consequence of Excessive Pressure
While pressure is essential, more is not always better. When extreme pressure (such as 1.5 GPa) is applied, the mechanical stress can exceed the material's structural limits.
This is particularly relevant for Li7SiPS8 particles with grain sizes exceeding 100 μm. Under extreme load, these large grains undergo significant fragmentation, breaking down into a uniform population of much smaller particles.
The Conductivity Paradox
Fragmentation increases macroscopic density, but it introduces a hidden cost. Breaking large grains creates a higher volume of grain boundaries.
These boundaries can act as resistance points for ions. Therefore, while the pellet may look denser physically, the increased number of interfaces can negatively impact the overall ionic conductivity.
Making the Right Choice for Your Goal
Achieving the optimal pellet requires balancing density against particle integrity.
- If your primary focus is Physical Density: Apply sufficient pressure to induce plastic deformation and overcome the binder's fixing effect to reach ~94% relative density.
- If your primary focus is Ionic Conductivity: Select a compaction pressure that maximizes density but remains below the threshold where significant fragmentation of large grains occurs.
The goal is to use the laboratory press to close pores, not to crush the conductive pathways within the material itself.
Summary Table:
| Mechanism | Action on Li7SiPS8 Pellets | Impact on Performance |
|---|---|---|
| Overcoming Fixing Effect | Breaks binder-induced structural resistance | Initiates particle contact |
| Particle Rearrangement | Particles shift to fill interstitial voids | Increases physical density |
| Plastic Deformation | Particles flatten and change shape | Eliminates microscopic pores |
| Controlled Pressure | Balances density vs. grain fragmentation | Maximizes ionic conductivity |
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References
- Duc Hien Nguyen, Bettina V. Lotsch. Effect of Stack Pressure on the Microstructure and Ionic Conductivity of the Slurry‐Processed Solid Electrolyte Li <sub>7</sub> SiPS <sub>8</sub>. DOI: 10.1002/admi.202500845
This article is also based on technical information from Kintek Press Knowledge Base .
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