Chemistry

Electrochemistry

Zinc and copper half-cells joined by a wire and a salt bridge. Click the switch inside the scene to close the circuit and watch the reaction run. Drag the concentration sliders below it to watch the Nernst voltage update live.

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Two reactions run simultaneously. At the zinc anode, atoms lose electrons and dissolve: Zn → Zn²⁺ + 2e⁻. At the copper cathode, those electrons arrive through the wire and plate out as solid metal: Cu²⁺ + 2e⁻ → Cu.

The wire carries electrons but not ions. Without a path for ions, charge would build up in each beaker and the reaction would stall. The salt bridge — a KNO₃ gel tube — lets K⁺ drift toward the anode and NO₃⁻ toward the cathode, maintaining electrical neutrality.

The standard cell potential is E° = +1.10 V, the difference of the two standard half-cell potentials: +0.34 − (−0.76) = 1.10 V.

The Nernst equation

Cell voltage depends on concentration. This follows from thermodynamics in three lines.

Lithium intercalation

In a Li-ion cell, lithium ions shuttle between two host lattices — never forming metallic lithium. During charging, Li⁺ deintercalates from the LiCoO₂ cathode, crosses the electrolyte, and intercalates between graphite layers at the anode. This is reversible insertion into vacant lattice sites, not a chemical reaction or metal plating.

Graphite fills in four distinct stages as lithium content increases — each stage a distinct ordered phase with its own voltage plateau. Stage 4 (0–15% SOC, ~0.20 V) has Li in every 4th graphene gallery; Stage 1 (65–100% SOC, ~0.07 V) has every gallery filled, giving the maximum capacity composition LiC₆. Each stage transition leaves a sharp feature in the dQ/dV plot — a key diagnostic fingerprint that blurs as a cell degrades.