Pressing a solid-electrolyte pellet involves two pressures: the one that makes the pellet, and the much lower one kept on it while it is measured. Soft sulfides densify cold at a few hundred megapascals; stiff oxide powders pressed dry leave the die as a green pellet, and their density is made afterwards, conventionally in a furnace. Both pressures, and the density of the pellet, belong to every conductivity value that is reported. This guide covers which press makes which pellet, how megapascals become tonnes on a given die, how to keep air-sensitive powders protected, and what to write down.
In one paragraph: the material class decides the route. Sulfides are soft enough to densify at room temperature under a few hundred megapascals, and warm pressing at 150 to 200 °C closes more of the remaining porosity or fuses the grains. Halides can also be pressed cold. Oxides are stiff and brittle: pressed dry, they give a green pellet, and the density conventionally comes from sintering or from hot pressing at furnace temperatures. Cold sintering, in which a transient liquid lets a powder densify in a heated die (LATP at 140 to 280 °C in the study cited below), is a separate route that depends on the material. Polymer and composite electrolytes are hot-pressed into films at moderate temperatures, 75 °C in one published example. In every case keep two numbers apart: the pressure that made the pellet and the pressure on it during the measurement. Convert both to megapascals, report both, and measure the pellet’s density. Sulfides and moisture-sensitive halides must be kept away from moisture throughout, which usually means pressing inside the glove box.
How much pellet preparation can matter was shown when samples of the same lithium thiophosphate electrolytes were sent around a group of laboratories for impedance measurements: the conductivities that came back for one and the same material spread over a range of up to 4.5 mS/cm, with relative standard deviations of 35 to 50 %.1 The samples had been sent out as powders, so everything between the vial and the impedance analyzer was each laboratory’s own: how the powder was pressed, how dense the pellet came out, how it was contacted, and how much pressure sat on it during the measurement.
1. The material class decides the route
| Electrolyte class | What the press does | Conditions in the work cited here | Where the density comes from |
|---|---|---|---|
| Sulfides | Makes the finished pellet, cold or warm | 125 to 370 MPa at room temperature; 300 to 370 MPa at 150 to 200 °C | The pressing |
| Halides | Makes the finished pellet, cold | Cold-pressed; no pressure quoted | The pressing |
| Oxides, pressed dry | Makes the green pellet | 100 MPa, then sintering at 760 to 840 °C (LATP); or 1000 °C and 40 MPa in a furnace hot press (LLZO) | The furnace |
| Oxides, cold-sintered with a transient liquid | Densifies the pellet in a heated die | 140 to 280 °C under 510 to 600 MPa (LATP) | The pressing, with the liquid; the conductivity stayed below the sintered reference |
| Polymers and composites | Presses a film between platens | Hot-pressed; 75 °C in one example | Softening and flow |
Each figure belongs to the study it comes from and is cited in the section on that class below.
2. Two pressures that must not be confused
A pressed-powder electrolyte meets pressure twice. The first time it is made into a pellet. The second time it is held between current collectors while its impedance is measured or a cell is cycled. The two do different things, and they differ by one to two orders of magnitude.
A study that varied both for a sulfide electrolyte separated their roles. The fabrication pressure sets the porosity of the electrolyte layer and through it the performance of the cell. A low stack pressure lowers the apparent ionic conductivity, because the electrolyte is in poor contact with the current collectors, without harming cyclability. The authors traced inconsistencies in the literature to this and asked for test conditions to be standardized and reported.2
More stack pressure is not simply better. With the argyrodite Li6PS5Cl and lithium metal electrodes, a stack pressure of 5 MPa allowed lithium to be plated and stripped for more than 1000 hours. At 25 MPa the cell short-circuited after about 48 hours, and at 75 MPa it was shorted before cycling began, as lithium was pushed into the pellet. The pellets in that work were made from 200 mg of powder in a 13 mm die at 370 MPa and reached a relative density of about 82 %.3
Put those numbers into force and the practical point appears.
| Pressure | Force on a 10 mm die | Force on a 13 mm die | |
|---|---|---|---|
| Fabrication | 370 MPa | 3.0 t | 5.0 t |
| 250 MPa | 2.0 t | 3.4 t | |
| Stack | 5 MPa | 40 kg | 68 kg |
| 1 MPa | 8 kg | 14 kg |
Force = pressure × area ÷ 9800, in tonnes.
Fabrication is hydraulic-press work: tonnes, applied once. Stack pressure is tens of kilograms, held for days, and belongs to a spring-loaded or bolted cell fixture. A laboratory press is the wrong instrument for it: the force range printed for an automatic 10 T press starts at 0.1 t, which is already 12.5 MPa on a 10 mm die.
3. Sulfides: soft enough to press cold
Sulfide electrolytes densify at room temperature because they are mechanically soft. Their Young’s moduli were measured at about 20 GPa, between typical oxides and polymers, and the behavior was named room-temperature pressure sintering. For a 75Li2S·25P2S5 glass the relative density rose with molding pressure and passed 90 % above 350 MPa, while the conductivity rose steeply to about 0.1 mS/cm at 70 MPa and then gradually to 0.31 mS/cm at 360 MPa.4
In that study most of the conductivity arrived early, as particles came into contact, and the last points of density were the expensive ones. The numbers do not carry over to other powders. For argyrodite, one group reports 97.8 % relative density after pressing at 250 MPa and 92.2 % at 125 MPa;5 another reports about 82 % at 370 MPa.3 Different powders, particle sizes and dies give different results, and the two cannot be read as points on one line. The density of your pellet is something to measure, not to assume from the pressure.
Pressing warm
Heat helps where cold pressing stalls, and the temperatures are within reach of a laboratory heated die.
- Sulfide glass at its glass transition. Hot-pressed at 200 °C, a 75–25 Li2S–P2S5 glass reached 98 % relative density while staying amorphous. Its modulus rose to about 30 GPa and its conductivity rose five-fold to 1.1 mS/cm compared with room-temperature molding; cold pressing the same powder at 360 MPa gave about 89 %.6
- Argyrodite at 150 °C. Pellets of Li6PS5Cl made at 150 °C and 300 MPa reached more than 2 mS/cm at 20 °C with less than 1 MPa on them, where pellets pressed at room temperature usually need more than 10 MPa, sputtered metal electrodes or both to exceed 1 mS/cm. The densities of the two kinds of pellet were nearly identical; electron microscopy showed fused grains only in the pellets pressed at 150 °C, and their conductivity was nearly independent of the operating pressure.7
- Cathode composites. Cell stacks formed at room temperature typically keep 15 to 30 % porosity. Hot pressing a glassy sulfide electrolyte above its glass transition brought that below 10 %. Of the cathode composites compared, the one that kept the best electrochemical function, NCM622 with Li3PS4, had been hot-pressed at 200 °C and 370 MPa for 10 minutes.8
Three hundred megapascals on a 13 mm die is 4.1 t, and 200 °C is inside the 300 °C class. The comparison of heated dies and heated platens explains the equipment.
4. Oxides: pressed dry, a green pellet
Crystalline oxide electrolytes are another kind of solid. Their Young’s moduli are about 200 GPa for the perovskite LLTO, 150 GPa for the garnet LLZO and 115 GPa for the NASICON-type LATP, and their fracture toughness is about 1 MPa·m1/2: stiff and brittle.9 Particles like that do not flow into one another at room temperature. Pressing the dry powder packs them, and the pellet that comes out of the die is a green body whose density and conductivity are still to be made. Conventionally that is done in a furnace; the third route below does it in the die, with a liquid.
- Pressureless sintering. In one study, LATP powder mixed with a lithium carbonate sintering additive was compacted uniaxially at 100 MPa without heating and then sintered for an hour at 760 to 840 °C. The pellets reached 80 % density at 760 °C and 92 to 95 % at 780 to 840 °C, and those sintered at 780 to 840 °C had conductivities of 0.155 to 0.338 mS/cm.10
- Hot pressing in a furnace. An aluminum-doped cubic LLZO powder hot-pressed at 1000 °C and 40 MPa reached 98 % relative density and 0.40 mS/cm at room temperature.11 Induction hot pressing and spark plasma sintering have both brought garnet to about 98 % within five minutes.12 None of that is in the range of a 500 °C laboratory press.
- Cold sintering, a separate route. With a transient liquid, LATP has been densified in a heated die at 140 to 280 °C under 510 to 600 MPa to 90 to 98 % density. At 200 °C and 510 MPa the density was 94 % but the conductivity only 0.0126 mS/cm, well below the sintered reference, which the authors attributed to amorphous phases between the grains. For the runs at 200 °C and above they lowered the pressure from 600 to 510 MPa, because of the limit that the maker of their die set for its mechanical stability at temperature.10 High density is not the same as good grain boundaries. The result belongs to that material and that liquid. The pressure a heated die can take can also depend on its temperature. No PressPro™ press or die has been verified for this process.
In the conventional route, then, the laboratory press has one job for an oxide, and it is worth doing well: a green pellet of even density, free of laminations, that shrinks uniformly when fired. The powder compaction guide covers the pressing, and the uniaxial and isostatic comparison explains when a cold isostatic step after the die is worth adding.
5. Halides, polymers and composites
Halides
Halide electrolytes such as Li3YCl6 and Li3YBr6 are deformable, and their cold-pressed powders exceed 1 mS/cm at room temperature without additional resistance between the grains.13 Like sulfides, they can be pressed cold. Halides can be sensitive to moisture too: Li3InCl6 takes up water from air, part of it reacts and the rest forms a hydrate, and the conductivity falls; it is much more stable at low humidity and in a dry room.14
Polymers and composites
Polymer electrolytes are made as films, and hot pressing makes them without solvent. Composite membranes of polyethylene oxide, a lithium salt and a ceramic nanofiller have been prepared by hot pressing the dry components,15 and a thermoplastic electrolyte of polyethylene oxide, LiTFSI, an ionic liquid and a clay filler was finished by hot pressing at 75 °C after extrusion.16 This is platen work at low pressure: the thickness is usually set by a frame or shims, and the temperature has to pass the softening point of the polymer.
6. Air and moisture: where the press has to stand
Sulfide electrolytes react with water to form hydrogen sulfide gas and solid by-products that raise the resistance at interfaces. Laboratory work is typically done in glove boxes with less than 1 ppm of water; a battery dry room at a dew point of −40 °C holds about 126 ppm. After 30 minutes in such a dry room, the powder of one Li2S–P2S5-based electrolyte had released 0.1 cm3 of hydrogen sulfide per gram and lost more than half of its conductivity.17 Hydrogen sulfide is toxic. Both the chemistry and the safety case say the same thing: the powder should not meet room air between the vial and the measurement.
For the pressing step there are three ways to arrange that.
- Press inside the glove box. A small manual press goes in with the die. The PressPro™ two-column presses are the compact frames, hand-pumped and with no mains supply, and are described as suited to glove-box use. The 12 and 15 T frames weigh about 28 and 55 kg; check their dimensions against your antechamber. The digital-gauge version reads to 0.01 t, far better than a pointer at these forces; the digital gauge is electronic, so plan how it will be powered inside the box and check it against the rules for your box.
- Load in the box, press outside. Only with a die or container shown to keep air out for the whole time, and only where your safety assessment allows it. An ordinary pellet die is not gas-tight, and a sulfide that meets room air degrades and releases hydrogen sulfide. For sulfides, pressing inside the glove box is the safer default.
- Hot pressing without air. For films and layers pressed warm, the vacuum hot press has its own chamber, which is pumped to −0.1 MPa gauge with a vacuum pump (an option) and can be back-filled with inert gas. For sulfides and other materials that react with traces of moisture, verify the chamber atmosphere with your own oxygen and moisture readings before relying on it.
7. Pellet pressure calculator
Enter the die and the pressure, and say whether it is the pressure for making the pellet or for holding it. The calculator gives the force, shows where it falls on the gauges of the compact manual presses and against the lowest setting of the automatic ones, and, if you add the mass, the thickness and the theoretical density, the relative density of the pellet.
8. Isostatic pressing of cells and sheets
A die presses along one axis. For a layered cell, pressure from all sides acts evenly on every layer. Isostatic pressing is being developed as a processing route for solid-state battery components and their integration.18 A widely cited all-solid-state pouch cell with an argyrodite electrolyte and a silver–carbon anode layer used warm isostatic pressing to improve the contact between electrode and electrolyte.19 At the level of a single component, slurry-cast composite electrode sheets that combine a sulfide electrolyte with a positive-electrode material chosen because it does not change its dimensions, densified by cold isostatic pressing, cycled stably under a stack pressure below 0.5 MPa.20 The cell or sheet is sealed in a bag and pressed in liquid; CIP, WIP and HIP are compared in their own guide.
9. Which press for which step
| Step | What it needs | PressPro™ equipment |
|---|---|---|
| Cold pellet of a sulfide (or of a halide, at the pressure your source gives) | 2 to 6 t on a 10 to 13 mm die; inert atmosphere | Two-column manual press, 12 or 15 T, with digital gauge, in the glove box |
| The same, as a repeatable series | Identical force and hold on every pellet; the inert atmosphere still has to be provided | Automatic powder press, 10 T; lowest setting 0.1 t. It is not described as a glove-box machine |
| Warm pressing of a powder | 150 to 200 °C with 300 to 370 MPa | Heated-die press, 15 T, die between 3 and 20 mm. It presses in air, so an air-sensitive powder needs a die shown to keep air out. The pressures in this guide are nominal and do not determine the allowable pressure of the heated die |
| Green pellet of an oxide, to be sintered | About 100 MPa or more; even density | A powder press, for example the four-column manual press; a cold isostatic press, for example the manual model, to even it out |
| Polymer or composite film | Low pressure, moderate heat (75 °C in the example cited), set thickness | Platen hot press; vacuum hot press for air-sensitive films |
| Densifying a sealed cell or sheet | Pressure from all sides, cold or warm | Cold isostatic press or warm isostatic press |
| Holding stack pressure during a measurement | Tens of kilograms, for hours or days | A cell fixture, not a press |
Two notes on dies. For pressing, use a steel or carbide die and keep the pressure inside its working band; PressPro™ steel dies are intended for use below 800 MPa of sample pressure, which on a 10 mm bore is 6.4 t. For measuring under pressure, an insulating fixture is needed so that the pellet can be contacted while it is held. A demountable test die in PEEK for solid-state cells, rated to 20 MPa, with cores from 8 to 25 mm, is available on request. Twenty megapascals is a stack-pressure rating: such a fixture holds a pellet for measurement and is not the die in which the pellet is pressed at several hundred.
10. What to measure and report
The interlaboratory study sent the samples out as powders so that each laboratory’s own preparation would show in the result: the densification procedure, the relative density, the pressure applied during the measurement and the way the pellet was contacted. No single one of these explained the spread; the authors concluded that several influences were mixed together. They asked for measurements in triplicate and an accurate description of consolidation, contacting and measurement conditions, and noted that conductivity may need to be reported as a function of the applied pressure.1 A review of inorganic solid electrolytes likewise counts the maintenance of physical contact among the main open problems of the field.21 In practice that comes to a short list.
- Die. Diameter and material.
- Powder. Mass, and how it was handled before pressing.
- Fabrication. Pressure in megapascals, hold time, temperature, atmosphere. Give the pressure, not only the tonnes.
- Pellet. Thickness and relative density: mass divided by area, thickness and theoretical density.
- Contacts. Electrode material and how it was applied.
- Measurement. Stack pressure in megapascals, temperature, and whether the pressure was varied.
11. Related guides and equipment
- Battery Lab Tools — punching, crimping and opening coin cells, and where presses are used.
- Heated Dies vs Heated Platens — warm pressing of powders and hot pressing of films.
- CIP vs WIP vs HIP — isostatic pressing for cells and green bodies.
- Uniaxial vs Isostatic Pressing — density gradients in a die and what removes them.
- Tonnage-to-MPa Calculator — force, gauge reading and pressure for any die and frame.
- Powder Compaction Guide — pressing, holding and releasing a pellet that survives.
- How to Choose a Laboratory Hydraulic Press — the whole PressPro™ range by purpose.
- Vacuum vs Ambient Hot Pressing — what a sealed chamber changes, what it does not, and when room air is enough.
- How to Specify a Pellet-Press Cycle — the numbers that define a pressing cycle, and a method line another laboratory can follow.
- Equipment: two-column and automatic powder presses; heated-die press; vacuum hot press; warm isostatic press.
12. FAQ
What pressure is used to press a sulfide solid electrolyte pellet?
Published work uses a few hundred megapascals at room temperature: 250 to 370 MPa appears repeatedly. On a 10 mm die that is 2 to 3 t and on a 13 mm die 3.4 to 5 t. The density reached at a given pressure varies between powders, so measure it.
What is the difference between fabrication pressure and stack pressure?
Fabrication pressure is applied once in a die to make the pellet and is hundreds of megapascals. Stack pressure is kept on the pellet or cell during measurement or cycling and is far lower: from under 1 MPa to a few tens of megapascals in the studies cited here. Both affect the result, and both should be reported.
Can LLZO be densified in a laboratory press?
Not by pressing alone, in the work cited here. Garnet is stiff and brittle; a press makes the green pellet, and density comes from sintering or from hot pressing at around 1000 °C, which is furnace equipment. The cold-sintering result in this guide is for LATP, not for garnet.
Does higher stack pressure give better results?
It improves contact, and with lithium metal it can cause short circuits. In one study a cell at 5 MPa ran for more than 1000 hours, while one at 25 MPa shorted after about 48.
Why does my pellet’s conductivity change with the pressure on it?
Because the contacts between grains, and between pellet and electrodes, improve under load. Cold-pressed sulfide pellets show this strongly. Pellets pressed warm, in which the grains have fused, show it much less.
Do I need to press in a glove box?
For sulfides and moisture-sensitive halides, the powder should not see moist air at any point. Pressing in the glove box with a compact manual press is a common arrangement, and for sulfides, which release hydrogen sulfide in moist air, it is the safer one.
How do I calculate relative density?
Divide the pellet mass by its volume, which is the die area times the measured thickness, and divide the result by the theoretical density of the material. A 150 mg pellet 10 mm across and 1.2 mm thick has a density of 1.59 g/cm³; if the theoretical density of the material were 1.85 g/cm³, that would be a relative density of 86 %.
Is warm pressing worth it for sulfides?
The published evidence says it can be: higher density for a sulfide glass at its glass transition, and fused grains in argyrodite at 150 °C that made conductivity nearly independent of the pressure applied afterward. It needs a heated die and an inert atmosphere.