Section 1 · Topic 2
Anatomy of a Lithium-Ion Cell
A lithium-ion cell resolves into four components, each defined by a transport function rather than a material.
The cathode and anode are the two intercalation hosts; the electrolyte and separator together govern what crosses between them. Lithium ions must pass freely while electrons are forced through the external circuit — a separation of ionic and electronic paths that defines every component choice below.
The four components
Cathode — the lithium source
The cathode is the lithium reservoir in the as-built cell: a lithiated transition-metal compound — LiCoO₂, an NMC oxide, or LFP — whose crystal structure fixes the cell’s voltage, specific capacity, and thermal stability. The active material coats onto an aluminium current collector, stable at the high positive-electrode potential.
Anode — the lithium host
The anode hosts lithium during charge, conventionally graphite, whose layered structure accepts Li+ between its graphene planes at a potential close to that of lithium metal. It coats onto a copper current collector, stable at the low anode potential.
Electrolyte — the ion conductor
The electrolyte carries Li+ but not electrons: a lithium salt such as lithium hexafluorophosphate (LiPF₆) dissolved in a carbonate solvent blend. It wets the full electrode stack so ions can reach every active particle.
Separator — the gatekeeper
The separator is a porous polyolefin membrane that passes Li+ through its pores yet stays electronically insulating, holding the electrodes apart. A breach of this membrane is the direct path to an internal short, treated in Section 2.
The collector pairing cannot be reversed. Aluminium alloys with lithium at the low anode potential, so it fails as an anode collector; copper oxidises at the high cathode potential, so it fails at the cathode. This same constraint becomes a failure mode in over-discharge (Section 3), where a rising anode potential dissolves the copper.
The carbonate electrolyte is flammable and is the primary fuel in a cell fire. Much of the safety engineering in Section 2 exists to keep this solvent contained and below its decomposition temperature.
Why the architecture wins
The combination delivers the properties that displaced lead-acid and nickel-cadmium from most portable and traction roles. These figures erode with temperature, current, and age — the subject of Sections 3 to 5 — but they set the starting envelope.
- Specific energy
- 150–250 Wh kg⁻¹ at cell level
- Round-trip efficiency
- 90–95 %, little lost as heat
- Self-discharge
- 1.5–2 % / month at room temperature
- Nominal voltage
- 3.2–3.7 V per cell, chemistry-dependent
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