Li-ion Cell, 21700 Part

Overview

The 21700 lithium-ion cell is the cylindrical energy-storage building block of a modern electric-vehicle traction pack. The number is the form factor: 21 mm diameter × 70 mm tall. Compared with the older 18650 (18 × 65 mm), the 21700 holds roughly 50% more energy per cell while reducing the number of cells, welds, and electronic monitoring points a pack needs — which is why it became the workhorse format for automotive and premium power-tool packs.

A single cell stores about 18 watt-hours at a nominal 3.6 V and 5.0 Ah. On its own that is enough to run a laptop for a couple of hours; an electric car carries several thousand of them wired in series and parallel to reach the ~400 V and 60–80 kWh of a full pack.

How it is built

Internally the cell is a wound "jelly roll": three long, thin layers — a coated cathode foil, a separator, and a coated anode foil — are stacked and spiral-wound into a cylinder, then dropped into a nickel-plated steel can.

• Cathode: aluminium foil coated with a nickel-rich NMC 811 oxide (80% nickel, 10% manganese, 10% cobalt). High nickel content raises energy density and cuts expensive cobalt, at the cost of more demanding thermal management.
• Anode: copper foil coated with graphite, increasingly blended with a few percent silicon oxide to push capacity higher.
• Separator: a microporous polyethylene/polypropylene film, often ceramic-coated, that keeps the electrodes apart while letting lithium ions through. If it fails, the cell shorts internally — the root cause of most thermal-runaway events.
• Electrolyte: a lithium-salt (LiPF₆) solution in organic carbonates that carries the ions between electrodes.
• Safety hardware: a CID (current-interrupt device) and a vent in the cap disconnect and relieve pressure if the cell is overcharged or overheats.

During charge, lithium ions move from cathode to anode and intercalate between the graphite layers; during discharge they move back, pushing electrons through the external circuit. That reversible shuttle is what makes the cell rechargeable for ~1,000 full cycles before capacity fades to about 80%.

Key specifications, explained

• Capacity (5 Ah) is how much charge it holds; multiplied by voltage it gives energy (~18 Wh).
• C-rate expresses current relative to capacity: this cell's 30 A continuous rating is 6C, enough for brisk acceleration without overheating.
• Specific energy (~260 Wh/kg) sets how far a car goes per kilogram of battery — the single number that most defines EV range and cost.
• Cycle life and calendar life determine warranty; both degrade faster at high state-of-charge, high temperature, and high charge rates, which is why the pack's Battery Module and BMS keep cells cool and avoid the voltage extremes.

Manufacturing

Cells are made on highly automated lines: electrode slurries are coated onto foil and dried, calendered (compressed) to the right density, slit to width, wound, inserted into cans, filled with electrolyte under vacuum, sealed, and then formed — a first, carefully controlled charge that builds the solid-electrolyte interphase (SEI) layer on the anode. Each cell is then graded by capacity and internal resistance so that only well-matched cells go into the same module.

Where it is used

In this graph the 21700 appears wherever a high-energy cylindrical cell is needed — most prominently inside the EV Battery Module, where dozens are welded in parallel groups and series strings, monitored by a module BMS slave board, and cooled by a plate. The same format (or its 18650 cousin) shows up in scooters, power tools, and home storage. Because it is one shared node here, its "Used in" list is a quick map of everything in the catalogue that depends on this one component.

Variants and alternatives

Cell makers tune the same can to different priorities: high-energy cells (more silicon, thinner foils) maximise range; high-power cells (lower-resistance chemistry) maximise discharge current. Alternative chemistries trade off differently — LFP (lithium iron phosphate) gives up energy density for lower cost, better cycle life, and far better thermal safety, and is increasingly used in standard-range vehicles, while solid-state designs aim to replace the liquid electrolyte entirely.