Learning objectives
By the end of this lesson you will be able to:
- describe how one five-layer repeat unit becomes a stacked or wound cell;
- name the ingredients of a composite electrode and the job of each;
- explain where electrons and ions travel inside a porous electrode, and where the reaction happens;
- explain the trade-off porosity sets between energy and power;
- describe what the separator and the electrolyte are made of, and why.
Before you start: Lesson 1 · Cell anatomy for the five parts and the two paths.
01From one sandwich to a whole cell
The repeating unit of a lithium-ion cell is five layers: copper foil, negative electrode, separator, positive electrode, aluminium foil. Together they are only around a tenth to a fifth of a millimetre thick. A single unit would store very little, so manufacturers make the most of every foil by coating it on both faces and then repeating the arrangement many times:
- Stacked — the electrodes are cut into sheets and laid on top of each other with separator between them. This is the usual route for large pouch cells, such as those in electric cars, and for some prismatic cells. A large pouch cell can hold dozens of electrode sheets; many small pouch cells, in phones for example, are wound flat instead.
- Wound — long strips of both electrodes and two separators are rolled into a spiral called a jelly roll. This is how cylindrical cells, and many prismatic cells, are built.
Either way, every negative electrode faces a positive electrode across a separator, and all the negative foils are joined to one terminal and all the positive foils to the other, so the repeat units work in parallel. Lesson 3 shows the machines that do the stacking and winding.
02Interactive tutorial
Build the five-layer stack one layer at a time, then zoom into a layer, then into the liquid inside one of its pores. Keep an eye on the rail at the top: it shows where in the stack each zoom comes from.
03Inside a composite electrode
An electrode coating is a composite: a mixture of several solids, pressed into a thin porous layer on the foil. Three ingredients do three different jobs:
| Ingredient | Job | Negative electrode | Positive electrode |
|---|---|---|---|
| Active material | Stores lithium — the only part that holds charge | graphite, 95 % | NMC oxide, 95 % |
| Conductive additive | Carbon black that links particles into an electronic network | 1 % | 1 % |
| Binder | Glues particles to each other and to the foil | CMC 2 % + SBR 2 % | PVDF 4 % |
| Solvent (removed) | Turns the powders into a coatable slurry, then is dried off | water | NMP |
Graphite and oxide particles are typically around 5–20 µm across; lithium iron phosphate particles are much smaller, often below a micrometre. The conductive carbon is far finer and coats the gaps between them, because many active materials conduct electrons poorly on their own. The binder is a polymer: PVDF dissolved in the solvent NMP for most positive electrodes, and a water-based pair, CMC and SBR, for graphite. Water is cheaper and safer than NMP, but many positive-electrode materials, especially nickel-rich oxides, react with moisture, which is one reason the two sides are still often made differently.
Everything that is not active material adds mass and volume without storing charge. Cell designers squeeze the binder and carbon down to the few percent the electrode needs to stay mechanically and electrically connected for thousands of cycles.
04Why the holes matter
After it is pressed in manufacturing, a typical electrode is still roughly 20–35 % empty space by volume — its porosity. Those pores are connected, and the electrolyte fills them. That is deliberate. The electrochemical reaction can only happen where three things meet: an active particle that can take or give lithium, an electronic connection to bring or remove the electron, and electrolyte to bring or remove the ion. Pores carry electrolyte to particles deep inside the coating, so the whole thickness takes part, not just the surface facing the separator.
| Electrons | Lithium ions | |
|---|---|---|
| Travel through | Active particles, the carbon network and the metal foil | Electrolyte in the pores, then into the particle |
| Blocked by | Electrolyte, separator, loose or isolated particles | Solid binder, the foil, closed or dry pores |
| Slowed by | Poor particle contact, too little carbon | Long, winding pore paths — high tortuosity — and thick coatings |
Here is the trade-off. Less porosity and thicker coatings put more active material into each cubic centimetre, so the cell stores more energy. But the ions then have to travel further, through narrower and more winding pores, so the far side of the electrode cannot keep up at high current. That is why energy cells, for example in an electric car meant for long range, use thick, dense electrodes, while power cells, for power tools or hybrids, use thin, more open ones. The speed of that ion supply is a diffusion problem, which Fick’s first law starts to describe, and the pressing step that sets porosity is calendering, in Lesson 3.
05The separator
The separator is a thin plastic film, usually polyethylene, polypropylene or layers of both, typically 10–25 µm thick and around 40 % porous. It has to do two contradictory things at once: block every electronic contact between the electrodes, and let ions through as freely as possible. Its pores are far smaller than the electrode particles, so a particle cannot pass, but they fill with electrolyte, so ions can.
Many separators carry a thin ceramic coating, often aluminium oxide, which keeps them from shrinking when hot and resists puncture. Polyethylene also softens at around 130 °C and its pores close up, cutting off ion flow: a shutdown that can stop an overheating cell before it gets worse. Shutdown helps, but it is not a guarantee. If the temperature keeps rising, the film can shrink or melt away entirely, and a short through a hole in it is not stopped by shutdown at all.
06The electrolyte
A standard lithium-ion electrolyte is a lithium salt, usually lithium hexafluorophosphate (LiPF₆) at about one mole per litre, dissolved in a mixture of organic carbonate solvents. The mixture is a compromise between two kinds of solvent:
- Ethylene carbonate (EC), a cyclic carbonate, is very good at pulling the salt apart into free ions and forms a protective film on graphite. On its own, though, it is viscous, and it is solid at room temperature.
- Linear carbonates — dimethyl, ethyl methyl or diethyl carbonate (DMC, EMC, DEC) — are thin, runny liquids that let ions move quickly but dissolve the salt less well.
Mixed, they give a liquid with enough free ions and low enough viscosity, conducting on the order of 10 mS/cm at room temperature. Small amounts of additives, a few percent of compounds such as vinylene carbonate or fluoroethylene carbonate, are added to shape the protective film described next.
The electrolyte is also the weakest point for safety. The carbonate solvents are flammable, and LiPF₆ reacts with traces of water to form hydrofluoric acid, which attacks the electrodes. That is why cells are assembled in very dry rooms, which Lesson 3 describes, and why solid electrolytes, in Lesson 3B, are attractive.
07A layer nobody assembles
The electrolyte is not stable at the very low potential of a charged graphite electrode. During the first charges, a small part of it reacts on the graphite surface and leaves a film of solid products a few nanometres to tens of nanometres thick: the solid electrolyte interphase, or SEI. A good SEI lets lithium ions through but blocks further electrolyte from reaching the graphite, so the reaction almost stops.
Nobody coats the SEI on. It grows inside the finished cell during formation, the first controlled charges at the factory, and it keeps growing slowly for the rest of the cell’s life. That slow growth is one of the main reasons a cell loses capacity even while it sits unused, which is where Lesson 4 picks it up.
08Check yourself
Work each one out before opening the answer.
Electrolyte fills the pores of both electrodes. Why is it not a sixth layer?
Because a layer has its own place in the left-to-right stack and its own thickness. The electrolyte has neither: it is a liquid filling the pore space inside three layers that already exist — the negative electrode, the separator and the positive electrode — all at once. The metal foils contain none.
A manufacturer presses an electrode harder to lower its porosity from 35 % to 22 %. What does the cell gain, and what does it risk?
Gain: more active material per volume, so more energy per litre, and better particle-to-particle contact. Risk: less pore volume and narrower, more winding pores, so ions move more slowly; the cell delivers less at high current, takes longer to fill with electrolyte, and may plate lithium sooner when fast-charged. Pressed too hard, particles can also crack.
Why does the electrode recipe need conductive carbon if the active material is already a solid?
Because many active materials conduct electrons poorly, and particles only touch each other at small points. A few percent of fine carbon fills the gaps and makes a continuous electronic network from every particle to the foil. Without it, particles that are chemically fine would be electrically stranded and their lithium unusable.
Why mix ethylene carbonate with a linear carbonate instead of using one solvent?
Each one fixes the other’s weakness. Ethylene carbonate dissolves the salt well and protects graphite, but it is viscous and solid at room temperature. Linear carbonates are runny but dissolve the salt less well. The mixture gives both enough free ions and fast enough ion movement.
Separator shutdown sounds like a complete safety feature. Why is it not?
Shutdown only closes the pores, and only once the separator reaches its softening temperature. If heating continues, the film can shrink or melt, letting the electrodes touch over a large area. And a short through a puncture or a metal particle carries electrons directly, so closing the pores around it does not stop it.
09Practice quiz
Questions on this lesson. The multiple-choice ones are marked in your browser; the written one you check against a model answer.
10References and further reading
These sources go deeper. The explanation here is written independently.
- K. Xu, “Nonaqueous liquid electrolytes for lithium-based rechargeable batteries,” Chemical Reviews 104(10), 4303–4418 (2004). doi:10.1021/cr030203g
- P. Arora, Z. Zhang, “Battery separators,” Chemical Reviews 104(10), 4419–4462 (2004). doi:10.1021/cr020738u
- E. Peled, “The electrochemical behavior of alkali and alkaline earth metals in nonaqueous battery systems — the solid electrolyte interphase model,” Journal of The Electrochemical Society 126(12), 2047–2051 (1979). doi:10.1149/1.2128859
- A. Kwade et al., “Current status and challenges for automotive battery production technologies,” Nature Energy 3, 290–300 (2018). doi:10.1038/s41560-018-0130-3
- PEM of RWTH Aachen University and VDMA, Production Process of a Lithium-Ion Battery Cell, 5th ed., Frankfurt am Main, 2026. ISBN 978-3-947920-71-6.
- J. Newman, K. E. Thomas-Alyea, Electrochemical Systems, 3rd ed., Wiley-Interscience, 2004.