FOUNDATION · INSIDE A LITHIUM-ION CELL
How a battery works
Where does a battery’s energy come from, and what changes when you charge it?
The idea
A lithium-ion cell has two hosts for lithium. Lithium is held more loosely in graphite than in cobalt oxide, so letting it move from one to the other releases energy. The catch is that the ion and its electron have to travel separately: the ion through the liquid inside the cell, the electron out through your device.
What you’re looking at
A cell cut open. On the left, graphite: stacked carbon sheets that hold lithium between them. On the right, grains of cobalt oxide. Between them sits a porous separator soaked in electrolyte, and above runs the wire through the load. Mint beads are lithium ions; violet beads are electrons.
Try it
Switch between Discharge and Charge to reverse both journeys. The slider sets how much lithium is in the graphite, and the graph and reactions below follow it. Inside graphite zooms in to the carbon sheets. The cell is idealised: it shows directions and amounts of lithium, not voltages or speeds.
Choose the lithium distribution. Motion shows the direction at this fixed composition.
The cell, crystal and graph share this state.
THE SAME LITHIUM, A DIFFERENT HOST
Nothing is used up
The distribution changes
Lithium leaves graphite and enters cobalt oxide. One electron moves through the circuit for every lithium ion transferred.
x = lithium per host formula unit
THE REACTIONS, LINKED
GRAPHITE · OXIDATION
COBALT OXIDE · REDUCTION
Cumulative change since the selected mode’s starting point: discharge starts at graphite x = 0.60, charge at x = 0.30. Both half-reactions advance together.
Sources, geometry & model assumptions
One consistent reaction state
This ideal insertion cell transfers lithium reversibly between equal amounts of C6 and CoO2 host units. Discharge changes graphite from Li0.60C6 to Li0.30C6 and cobalt oxide from Li0.60CoO2 to Li0.90CoO2. Charge retraces that partial window. These are illustrative compositions, not a full state-of-charge scale.
Switching modes preserves the current composition. Direction markers move continuously while composition stays fixed; the slider alone changes the lithium inventory. Electrode polarity stays fixed; electrochemical roles reverse: oxidation defines the anode, reduction defines the cathode. Battery engineering often names electrodes by their discharge roles; this visualization labels the active reaction explicitly.
The electrolyte has no net lithium accumulation in this bookkeeping model. Counterions, solvent, SEI, gradients, charge layers and side reactions are omitted. No voltage, current, physical rate or performance is calculated. The 3D cell is a cutaway diagram, not a manufactured cell design.
Inside graphite
Carbon atoms form hexagonal sheets. Lithium can occupy the galleries between them, entering or leaving through exposed edges. The aligned carbon sheets illustrate a local intercalated graphite motif. Real graphite changes layer registry and staging with composition; those structural changes are not solved here.
Site brightness encodes fractional mean occupancy from the same x as the graph. The highlighted moving sphere explains the direction through a gallery; it is not an additional counted atom. Sizes and spacing are selected for legibility. The motion is illustrative, not molecular dynamics or a computed minimum-energy path.
Sources behind the model
- U.S. Department of Energy — Batteries
Coupled ionic and electronic transport; chemical and electrical energy.
- Ohzuku, Iwakoshi & Sawai (1993)
Primary research on lithium–graphite intercalation.
- Mizushima et al. (1980)
Reversible lithium insertion in LixCoO2.
- Hazrati, de Wijs & Brocks (2014)
Carbon-layer registry, intercalation and graphite staging. Open author text.
- IUPAC: anode · cathode
Reaction-based electrode definitions.
Checks, not empirical validation
Independent equations check both modes, lithium and charge conservation, reactions, reversal continuity, path domains, carbon-ring geometry and gallery motion. These are software and bookkeeping checks; they do not validate battery performance.
Read the complete model record1Two paths1 of 2
Two paths, one reaction
Lithium ions cross the electrolyte. Electrons take the external circuit. The electrode reactions connect their journeys.
Run the cell
The cell is idealised: it shows directions and amounts of lithium, not voltages or speeds.
1Two paths2 of 2
Charging swaps the roles
The charging source reverses the coupled reactions. Lithium returns to graphite as electrons arrive through the external circuit. Polarity stays fixed; the roles reverse: oxidation defines the anode, reduction the cathode.
Run the cell
Battery engineering often names electrodes by their discharge roles; this visualization labels the active reaction explicitly.
2Nothing used up1 of 2
Nothing is used up: the distribution changes
Lithium leaves graphite and enters cobalt oxide. One electron moves through the circuit for every lithium ion transferred.
Direction markers move continuously while composition stays fixed; the slider alone changes the lithium inventory.
2Nothing used up2 of 2
The reactions, linked
Both half-reactions advance together: one electron through the circuit for every lithium ion across the electrolyte.
Run the cell
Cumulative change since the selected mode’s starting point: discharge starts at graphite x = 0.60, charge at x = 0.30.
3Inside graphite1 of 2
Between the sheets, out through an edge
Graphite releases lithium from its interlayer galleries. Its carbon framework remains in place.
Sizes and spacing are selected for legibility. The motion is illustrative, not molecular dynamics or a computed minimum-energy path.
3Inside graphite2 of 2
Through an edge, between the sheets
Graphite accepts lithium into its interlayer galleries. This reversible insertion is called intercalation.
Site brightness encodes fractional mean occupancy from the same x as the graph. Real graphite changes layer registry and staging with composition; those changes are not solved here.