Inside a Cell: Electrodes, Separator, Electrolyte and the SEI

3.1 — The four functional parts

Strip away the can, the terminals and the label, and every rechargeable lithium cell in existence is the same four things in a sandwich: a cathode, an anode, a separator between them, and an electrolyte soaking through all of it.

3.2 — What the cathode is and what it decides

The cathode is a lithium-containing metal oxide or phosphate, coated as a powder-and-binder mixture onto aluminium foil. It is the cell’s lithium reservoir, and it sets three things almost single-handedly: nominal voltage, capacity per kilogram, and cost.

This is why chemistry names are cathode names. LFP, NMC, NCA, LMO and LCO all describe what is on the aluminium foil. The anode in all of them is usually the same graphite. When somebody tells you they have “an LFP battery”, they have told you about one of the four components — which happens to be the one that decides the most.

The chemistries are compared properly in Chapter 7. For now the point is structural: the cathode is where the design trade-off between energy, safety, life and cost is actually made.

3.3 — Graphite, silicon and the LTO exception

The anode is almost always graphite, coated onto copper foil. Graphite is a stack of carbon sheets, and lithium ions slot into the gaps between those sheets — a process called intercalation. Nothing reacts and nothing is consumed; the lithium is stored the way a book is stored on a shelf.

• •Graphite — the default. Cheap, stable, well understood, and it expands about 10 per cent on full lithiation, which the mechanical design has to accommodate.
• •Silicon-blended graphite — silicon holds far more lithium per gram, which is why it appears in high-energy cells. It also swells roughly 300 per cent, so it is used in small percentages and it shortens life. Every silicon cell is a negotiation between capacity and swelling.
• •Lithium titanate (LTO) — a completely different anode material. It gives extraordinary cycle life and fast charging but a much lower cell voltage, which is why LTO packs are expensive per kWh. Note that LTO is an anode substitution, not a cathode chemistry, even though it is always listed alongside LFP and NMC.

3.4 — Twenty microns between working and burning

The separator is a polyethylene or polypropylene film, typically 10 to 25 micrometres thick — a fraction of the thickness of a human hair. Its pores let lithium ions pass. Its solid material stops the electrodes ever touching each other.

Better separators carry a shutdown feature: the pores melt closed at around 130°C, stopping ion flow and shutting the cell down before it can run away. Many are also ceramic-coated, which raises the temperature at which the film itself shrinks. Both features cost money, and both are among the first things removed from a cheap cell.

When you are comparing two cells at very different prices, the separator is one of the places the difference is hiding. It does not appear on a datasheet as a line item, which is exactly why it is worth asking about.

3.5 — LiPF₆, solvents and additives

The electrolyte is usually lithium hexafluorophosphate (LiPF₆) dissolved in a mixture of organic carbonate solvents, with a proprietary additive package on top. It fills every pore in the electrodes and the separator, so ions have a continuous path.

Two consequences follow, and both are practical rather than academic.

• •It is flammable. The organic solvents burn. This is the root of thermal runaway severity in every lithium chemistry, LFP included. LFP’s advantage is that its cathode does not supply oxygen to that fire — it is not that the cell contains nothing that burns.
• •It hates water. LiPF₆ reacts with moisture to produce hydrofluoric acid, which destroys the cell from the inside. This is why electrolyte filling happens in a dry room below one per cent relative humidity, and why a cell with a compromised seal is finished even if it still works today.

The additive package is where much of a manufacturer’s real know-how sits. Additives control how the SEI forms, suppress gas generation, improve low-temperature performance and extend calendar life. They are typically under three per cent of the electrolyte by mass and they are never disclosed. Two cells with identical datasheets and different additive packages will not age the same way.

3.6 — Current collectors, the can, and the built-in safety devices

Two more components matter commercially even though they store no energy.

3.7 — What actually happens on charge and discharge

With the components in place, the operating principle is almost disappointingly simple.

In principle. What actually wears the cell out is not the main reaction at all — it is the slow side reactions happening alongside it. The most important of those has its own section, because it explains more field behaviour than anything else in this chapter.

3.8 — The SEI — the layer nobody mentions

On the very first charge a cell ever receives, the electrolyte decomposes on the graphite surface and forms a thin solid film. This is the SEI, the solid electrolyte interphase, and it is simultaneously essential and the leading cause of ageing.

It is essential because it passivates the anode. Once the film exists, further decomposition largely stops — the electrolyte is protected from the anode by a layer made out of decomposed electrolyte. Without it, the cell would consume itself in a handful of cycles.

The film never stops growing. It thickens slowly for the whole life of the cell, consuming a little more lithium each time and adding a little more internal resistance. Two things accelerate it sharply:

• •Heat. SEI growth follows Arrhenius kinetics, so the rate roughly doubles for every 10°C. A pack living at 45°C ages approximately four times faster than one at 25°C.
• •High state of charge. A fully charged anode sits at a potential where electrolyte decomposition is most favourable. Storing a pack at 100 per cent is storing it in the condition that damages it fastest.

Put those together and you have the explanation for the two storage rules that appear everywhere in this series and are almost always given without a reason: store at 40 to 60 per cent state of charge, and store cool. Both are SEI management. The full degradation picture, including what happens after SEI growth stops being the limiting mechanism, is in Chapter 5.

3.9 — Why any of this matters commercially

It is reasonable to ask why a pack integrator or a fleet operator should care about a film twenty nanometres thick. Three reasons, all of which cost money.

• •It tells you which claims are physically possible. A supplier offering NMC energy density with LFP safety and LFP pricing is describing a cell whose cathode cannot exist. Knowing where a property comes from is how you audit a claim without a laboratory.
• •It tells you what the invisible cost differences are. Separator quality, electrolyte additives and formation discipline do not appear on a datasheet. They are most of the gap between a cell that reaches its rated cycles and one that does not, and they are the first things a price-driven supplier removes.
• •It turns handling rules into reasoning. Do not store at 100 per cent. Do not charge below 0°C. Do not recharge a deeply over-discharged cell. Each of those is a rule until you know the mechanism, at which point it becomes obvious — and rules people understand are the ones a service network actually follows.