the lithium-ion battery
The lithium-ion battery is the rechargeable cell in your phone, laptop, and electric car. It stores energy by shuttling lithium ions back and forth between two electrodes — imagine a rocking chair that moves lithium from one seat to the other and back again.
Here is how it works. Charging pushes lithium ions (Li+) out of the cathode — a layered oxide such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LFP) — through the electrolyte, and into a graphite anode, where they slot in between the carbon layers (this is called intercalation). Discharging reverses the flow and drives the matching electrons out through your device to do work. A typical cell stores about 150 to 250 Wh per kg. From a materials point of view the whole story is electrode materials, intercalation, and the electrolyte.
This matters because it made portable electronics and electric vehicles possible (its inventors won the 2019 Nobel Prize in Chemistry). The honest caveats: capacity fades as the electrodes crack and the surface (SEI) film thickens with cycling; a damaged cell can thermally run away and catch fire; lithium and cobalt raise supply and ethics concerns; and lithium-ion still stores far less energy per kilogram than a liquid fuel like gasoline.
Charging your phone drives lithium ions out of the cathode's layered lithium cobalt oxide, through the electrolyte, and into the graphite anode, where they slot in between carbon layers; the matching electrons travel the outside wire. Discharge reverses it, and those returning electrons power the screen. At about 150 to 250 Wh/kg it stores several times more energy per kilogram than the old nickel-cadmium cells it replaced.
A rechargeable cell is really a story about electrode materials and where lithium ions like to sit.
Capacity fades because electrodes crack and the surface (SEI) layer thickens with cycling; a damaged cell can also thermally run away and catch fire, and lithium-ion is still well below fuels in energy per kilogram.