Step outside on a freezing January morning, pull out your phone, and watch the battery icon plummet from 40% to dead in minutes. Nothing changed inside the phone. No app started draining power. The cold did something to the battery itself, something that reveals what a battery actually is and what it is not.
A battery does not store electricity. It stores chemical energy and manufactures electricity on demand, one ion at a time, through continuous chemical reactions at two electrodes.
Most people picture a battery as a tank of electricity that slowly drains, like gasoline in a car. Fill it up, use it, refill. That mental model is completely wrong. There is no pool of electrons sitting inside your phone waiting to be used. Instead, a battery is a chemical factory. It stores energy in the atomic bonds of lithium compounds, and it generates electrical current only when those compounds react. When your phone "charges," it is not filling a tank. It is physically driving lithium ions backward into a carbon lattice, storing energy in the arrangement of atoms.
A lithium-ion cell has four essential layers. The anode (negative side) is made of graphite: sheets of carbon atoms arranged in layers with tiny gaps between them. The cathode (positive side) is a lithium metal oxide, typically NMC (nickel-manganese-cobalt) or LFP (lithium iron phosphate), with a crystal structure that can accept and release lithium ions. Between them sits the separator, a microporous plastic membrane only 25 microns thick, soaked in electrolyte, a lithium salt dissolved in organic solvent.
When you use your phone, the battery discharges. At the anode, lithium atoms tucked between the graphite layers undergo oxidation: each one releases an electron and becomes a positively charged lithium ion (Li+). The ion is small enough to pass through the separator's microscopic pores and travel through the electrolyte to the cathode. But the electron cannot pass through the electrolyte. It is forced to take the long way around, through the external circuit, through your phone's processor, screen, and radios, doing useful work along the way. At the cathode, the lithium ion and the electron reunite, slotting into the metal oxide crystal through reduction.
Charging reverses the entire process. The charger applies a voltage higher than the battery's natural voltage, forcing electrons back through the circuit and driving lithium ions from the cathode back through the electrolyte to the anode. The ions re-intercalate (wedge themselves back between the graphite layers), storing energy in their position. This is why "charging" is the right word: you are literally charging the anode with lithium, not filling anything with electricity.
Why batteries degrade and what temperature does to them
Every time lithium ions shuttle back and forth, a tiny fraction of them get trapped. On the very first charge, some lithium reacts with the electrolyte at the anode surface, forming a thin film called the SEI layer (solid electrolyte interphase). This film is actually necessary: it protects the graphite from further electrolyte decomposition. But it keeps growing, slowly, over hundreds of cycles. Each cycle traps a few more lithium ions permanently in this film. Those ions can never participate in the electrochemical reaction again. After 500 to 1,000 full cycles, enough lithium is trapped that usable capacity drops to about 80% of original.
Temperature accelerates everything. Heat speeds up the parasitic reactions that thicken the SEI layer. Storing a fully charged phone at 40 degrees Celsius degrades the battery twice as fast as storing it at 25 degrees. Cold does the opposite: it slows ion migration by increasing the viscosity of the electrolyte. At 0 degrees Celsius, lithium ions move so sluggishly that the battery can only deliver 70 to 80% of its rated capacity. At -20 degrees, that drops to 50 to 60%. The chemistry still works; it just runs in slow motion. That is why your phone dies in the cold: the battery is not empty; the ions simply cannot move fast enough to sustain the current your phone demands.
Drag the temperature slider below freezing to see usable capacity plummet as ion migration slows. Increase the C-rate to see voltage sag under heavy load.
The price of all that energy in a small package
Lithium-ion batteries pack enormous energy into tiny volumes. That same density is what makes them dangerous when something goes wrong: a punctured separator can trigger thermal runaway above 130 degrees Celsius.
This tradeoff shapes the entire industry. Lithium iron phosphate (LFP) cells sacrifice 30 to 40% of the energy density of NMC cells but are dramatically safer: their cathode structure does not release oxygen during decomposition, making thermal runaway far less likely. Tesla uses NCA cells (high density, needs aggressive thermal management) in its performance vehicles and LFP cells (lower density, inherently safer) in its standard-range models. Every battery design is a negotiation between how much energy you want per kilogram and how much safety engineering you can afford around it.
Every lithium-ion battery in every phone, laptop, and electric car is doing the same thing: shuttling lithium ions back and forth between two electrodes, storing energy in the arrangement of atoms, not in a pool of electricity. The anode oxidizes; the cathode reduces; the electrons are forced through your device because the electrolyte will not let them take the shortcut. Charging reverses the chemistry. Cold slows the ions. Heat degrades the film. And every cycle traps a few more lithium atoms permanently. Understanding this changes how you treat every battery you own: charge to 80%, keep it cool, and avoid deep discharges. You are not managing a fuel tank. You are managing a chemical factory that degrades a little more every time it runs.