Reference · Battery Cell Foundations
Lithium-ion cell cheat sheet
The formulas, rules and typical numbers from the whole course on one page. Numbers are indicative ranges for orientation, not design values; each card links to the lesson that explains it.
Inside a cell
Cu foilnegative electrode
graphiteseparatorpositive electrode
NMC, LFP…Al foil
Electrolyte — LiPF₆ in carbonate solvents — fills the pores of both electrodes and the separator. It is not a layer.
| Discharge | Charge | |
|---|---|---|
| e⁻, outside | − → load → + | + → charger → − |
| Li⁺, inside | − → + | + → − |
| Anode (oxidation) | negative | positive |
Industry calls the negative electrode “the anode” at all times. Copper on the negative side, aluminium on the positive: each survives only there.
Formulas
| Current | I = C × Q | A = h⁻¹ × Ah |
|---|---|---|
| C-rate | C = I / Q | 1C on 20 Ah = 20 A |
| Ideal duration | t = 1 / C | 2C → 30 min; a definition, not a runtime |
| Energy | E = Q × Vnom | 20 Ah × 3.65 V = 73 Wh |
| Packs | Series adds voltage; parallel adds capacity; energy adds either way. | |
| Arrhenius | k(T)/k(Tref) = exp[(Ea/R)(1/Tref − 1/T)] | kelvin; about ×2 per +10 °C near 50 kJ/mol |
| SEI film | δ ∝ √t | 4 × the time → 2 × the film |
| Fick’s first law | J = −D dc/dx | flux runs down the gradient |
Typical numbers
| Nominal cell voltage | graphite/NMC 3.6–3.7 V · graphite/LFP 3.2–3.3 V |
|---|---|
| Electrode potential vs Li | graphite ≈ 0.1 V · layered oxide ≈ 3.7–3.9 V · LFP ≈ 3.4 V |
| Foils | copper 6–10 µm · aluminium 10–15 µm |
| Separator | 10–25 µm thick, about 40 % porous |
| Electrode porosity | about 20–35 % after calendering |
| Electrolyte | about 1 mol/L LiPF₆ · about 10 mS/cm |
| Specific capacity | graphite 372 mAh/g · lithium metal 3,860 mAh/g (theoretical) |
| End of life | 80 % of rated capacity, by convention |
| Air travel | spare batteries up to 100 Wh in hand luggage |
How cells are made
- Electrode manufacturing — mixing → coating → drying → calendering → slitting → vacuum drying
- Cell assembly, in a dry room at a dew point of about −30 to −60 °C — separation → stacking or winding → tab welding → packaging → electrolyte filling → wetting
- Cell finishing — formation (first charge at about 0.1–0.5C; builds the SEI) → degassing → aging (up to about three weeks; screens for internal shorts) → end-of-line testing and grading
Calendering sets porosity. A burr from slitting can short a cell. Cells shipped alone by air: at most 30 % state of charge.
Solid-state electrolytes
| Family | Conductivity | Made dense by | Watch out for |
|---|---|---|---|
| Sulfide | ≈ 1–10 mS/cm | pressing | H₂S with moisture |
| Oxide | ≈ 0.1–1 mS/cm | sintering | brittle; contact |
| Halide | ≈ 1 mS/cm | pressing | reduced by Li metal |
| Polymer | ≈ 0.1 mS/cm warm | extrusion | needs 60–80 °C |
Disappears: electrolyte filling and wetting. Appears: electrolyte layer forming, densification, lithium-metal anode handling, stack pressure.
Why cells age
| Mechanism | Feeds mainly | Accelerated by |
|---|---|---|
| Corrosion | impedance ↑ | temperature |
| SEI growth | capacity ↓ | temperature, time, high state of charge |
| Gas | capacity ↓, safety | temperature, overcharge |
| Surface deposits | impedance ↑ (power fade) | cycling |
| Lithium plating | capacity ↓; self-discharge ↑ and shorts if dendrites grow | cold charging, high current |
| Cracking | capacity ↓ and impedance ↑ | deep cycling |
Three signals: capacity fade, impedance rise, self-discharge rise. Heat speeds most aging; cold charging adds plating — so the best temperature window is moderate.