Undergraduate · 30 min

Inside a lithium-ion cell: the Li⁺ shuttle

Trace the path of Li⁺ through anode, electrolyte and cathode during discharge, vary the rate, and connect the shuttle picture to capacity fade and fast-charge limits.

Goal

Map the discharge half-reactions $\ce{LiC6 -> C6 + Li+ + e-}$ and $\ce{Li1-xCoO2 + xLi+ + xe- -> LiCoO2}$ , and explain why the separator must pass Li⁺ but block electrons.

Apparatus and reagents

Virtual cell: three-stage flow diagram with a rate control; the readout idea — count Li⁺ crossing the separator per second.

Procedure

Set rate to minimum and watch a single Li⁺ leave the anode: write the deintercalation half-reaction it represents.
Follow it through the separator: electrons cannot take this path — where do they go instead?
At the cathode, the ion is inserted between oxide layers: connect that to why layered $\ce{LiCoO2}$ or NMC hosts are chosen.
Raise the rate to maximum: picture which stage becomes the bottleneck (solid-state diffusion in the particles) and what that implies for fast charging.

What to observe

Li⁺ and electrons split at the anode and reunite at the cathode — the ion goes through the cell, the electron through the wire doing work.
Only ions move inside; the dashed separator passes them but its polymer wall stops any electron shortcut (which would short the cell).
Faster discharge means more ions in transit per second — the same picture explains heat build-up and diffusion limits at high C-rate.

Explanation

A Li-ion cell works because Li⁺ can leave and re-enter host lattices without destroying them: graphite holds Li between graphene sheets (max $\ce{LiC6}$ , theoretical 372 mAh/g) and layered oxides like $\ce{LiCoO2}$ accept it back, releasing ~3.7 V. The electrolyte must be a Li⁺ conductor and electron insulator — a porous polyethylene separator provides the physical barrier. Degradation (SEI growth, lithium plating at high charge rate, cathode cracking) all show up as the shuttle losing passengers.

History of the experiment

Whittingham built the first Li-ion electrode (

\ce{TiS2}

, 1976), Goodenough found the 4 V

\ce{LiCoO2}

cathode (1980), and Yoshino engineered the carbon-anode commercial cell (Sony, 1991) — Nobel 2019.

Chemists behind it

John B. Goodenough M. Stanley Whittingham Akira Yoshino