Lithium-Ion Battery

Why the lithium-ion battery matters

The lithium-ion cell is the enabling technology of the mobile and electrified era. Nothing before it combined high voltage, high energy density, low self-discharge, and hundreds to thousands of rechargeable cycles in one package. That combination is why a phone the size of a deck of cards runs all day, why laptops shed their power bricks, and why a 500 kg battery pack can move a two-ton car 400 km. The reason is fundamentally electrochemical: lithium is the lightest metal and the most electropositive element, so per kilogram of active material it offers the largest possible charge storage at the highest possible voltage.

• Electric vehicles. A modern EV pack stores 50–100 kWh across thousands of cells, delivering the ~3.7 V-per-cell chemistry that makes 400 V and 800 V drivetrains practical.
• Consumer electronics. Phones, laptops, earbuds, and watches all run on single Li-ion cells because of the flat, high 3.6–3.7 V discharge plateau.
• Grid storage. Utility-scale LiFePO4 installations buffer solar and wind, shifting midday generation to evening demand.
• Aerospace and tools. Cordless power tools, drones, and satellites exploit the high specific power and energy.
• Longevity. Well-managed cells retain 80% capacity after 1,000–3,000 cycles, making the cost-per-cycle low enough for daily use.

How it works, step by step

A lithium-ion cell has five essential parts: a graphite anode coated on copper foil, a layered metal-oxide cathode coated on aluminum foil, a separator (a porous polyolefin membrane, often ceramic-coated), a liquid electrolyte (a lithium salt such as LiPF₆ dissolved in organic carbonates like EC/DMC), and the two current collectors. The genius of the design is that both electrodes are host lattices — cages that lithium ions reversibly slide into and out of — so no metallic lithium is deposited during normal cycling. This is the "rocking-chair" or intercalation principle.

1. Discharge (delivering power). At the anode, intercalated lithium gives up an electron: the graphite oxidizes and releases Li+ into the electrolyte. The half-reaction is Li_xC₆ → 6C + xLi⁺ + xe⁻.
2. Ion transport. The Li+ ion diffuses through the liquid electrolyte and crosses the micron-thin porous separator, which physically prevents the electrodes from touching while letting ions pass.
3. Cathode insertion. At the cathode, Li+ inserts into the metal-oxide lattice and a transition-metal cation is reduced: for cobalt oxide, Li_1−xCoO₂ + xLi⁺ + xe⁻ → LiCoO₂.
4. Electron path. The electrons the anode released cannot cross the separator, so they travel the external circuit — through your phone, motor, or load — which is the useful current.
5. Charge (storing power). An external voltage above the cell's open-circuit value reverses everything: Li+ is pulled out of the cathode and forced back into the graphite, converting electrical work into stored chemical potential.

Overall, the reversible cell reaction is written LiC₆ + CoO₂ ⇌ C₆ + LiCoO₂, with discharge running left-to-right. The fully lithiated graphite composition is LiC₆, meaning one lithium atom is stored per six carbon atoms — this stoichiometry sets graphite's theoretical capacity at 372 mAh/g.

The SEI: the film that makes it work

On the very first charge, the graphite anode sits at roughly 0.1 V versus Li/Li+, a potential far below the electrochemical stability window of the carbonate electrolyte. The electrolyte therefore reduces on the graphite surface and deposits a thin passivating film — the solid-electrolyte interphase (SEI), typically a few to tens of nanometers thick. A well-formed SEI is the paradox that makes the whole chemistry viable: it must conduct Li+ ions while blocking electrons, so it stops further electrolyte decomposition yet still lets the battery cycle. Forming it consumes a few percent of the cell's initial capacity (the "formation loss"). Over years, the SEI slowly thickens, cracks, and self-repairs, permanently trapping lithium and raising internal resistance — the dominant mechanism of calendar and cycle-life fade.

Worked example: sizing a cell by energy and C-rate

The stored energy of a cell follows directly from its capacity and average voltage:

E = Q · V_avg

where E is energy in watt-hours (Wh), Q is charge capacity in amp-hours (Ah), and V_avg is the average discharge voltage in volts (V). Take a common 21700 cylindrical cell rated Q = 5.0 Ah with V_avg = 3.6 V:

E = 5.0 Ah × 3.6 V = 18.0 Wh

If the cell masses 70 g, its specific energy is E/m = 18.0 Wh / 0.070 kg = 257 Wh/kg, right at the state-of-the-art figure. Now suppose we discharge it at 2C. C-rate ties current to capacity:

I = C_rate · Q   and   t = 1 / C_rate

with I the current (A), C_rate the multiple of capacity per hour (1/h), and t the nominal discharge time (h). At 2C the current is I = 2 × 5.0 = 10 A and the cell empties in t = 1/2 = 0.5 h (30 minutes). That 10 A flowing through an internal resistance of, say, R_int = 15 mΩ dissipates P = I²R = (10)²(0.015) = 1.5 W of waste heat inside the cell, which is exactly why fast charging and high-power discharge demand active cooling.

Common Li-ion cathode chemistries at a glance

Chemistry	Nominal V	Specific energy (cell)	Cycle life	Best for
LCO (LiCoO2)	3.7 V	150–200 Wh/kg	500–1,000	Phones, laptops
NMC (LiNiMnCoO2)	3.6–3.7 V	200–270 Wh/kg	1,000–2,000	EVs, tools
NCA (LiNiCoAlO2)	3.6 V	200–260 Wh/kg	1,000–1,500	EVs, high energy
LFP (LiFePO4)	3.2 V	90–160 Wh/kg	2,000–5,000+	Grid, safe EVs
LMO (LiMn2O4)	3.7 V	100–150 Wh/kg	300–700	Power tools, medical

Common misconceptions and failure modes

• "There's lithium metal inside." In normal operation there isn't — the lithium lives as ions inside two host lattices. Metallic lithium plating is a failure mode, triggered by fast charging in the cold, and it grows dendrites that can pierce the separator.
• "Fully charging and fully draining is best." The opposite. Holding a cell at 4.2 V (100%) or dropping it to near 0% stresses the electrodes; cycling within roughly 20–80% state of charge can multiply cycle life several-fold.
• "Bigger cells make higher voltage." Voltage is fixed by the electrode chemistry (~3.7 V), not size. Capacity (Ah) scales with size; voltage scales only by wiring cells in series.
• "Memory effect." That was a NiCd problem. Li-ion has essentially no memory effect.
• Thermal runaway. Overcharge, crush, internal short, or overheating can start a self-accelerating exothermic cascade — SEI breakdown, then electrolyte and cathode decomposition releasing oxygen — reaching 400–800 °C and propagating cell to cell. Battery management systems, current-interrupt devices, and ceramic separators exist to stop it.
• Capacity fade. The slow, permanent loss from SEI growth, lithium inventory loss, and active-material cracking — distinct from the reversible capacity you get back by warming a cold cell.