Batteries store energy by separating chemical substances that want to react, then controlling when electrons are allowed to flow through a circuit. In a lithium-ion battery, energy is stored in the positions of lithium ions inside solid electrode materials. This matters because batteries power phones, laptops, electric vehicles, medical devices, and renewable energy systems.
Engineering a good battery means balancing energy storage, power output, lifetime, safety, cost, and mass.
During discharge, lithium ions move through the electrolyte from the negative electrode to the positive electrode, while electrons travel through the outside circuit to do useful work. During charging, an external power source forces the ions and electrons back in the opposite direction. The separator keeps the electrodes from touching while still allowing ions to pass, preventing a short circuit.
The voltage of a cell comes from the difference in chemical potential between the two electrodes.
Key Facts
- Stored electrical energy can be estimated by E = VIt, where V is voltage, I is current, and t is time.
- Battery capacity is often measured in ampere-hours, with charge Q = It.
- Energy in watt-hours is E = V × Ah for an ideal battery at constant voltage.
- During discharge in a lithium-ion cell, Li+ ions move through the electrolyte and electrons move through the external circuit.
- Power output is P = VI, so higher current or higher voltage gives higher power.
- Internal resistance causes heat loss according to P_loss = I^2R.
Vocabulary
- Anode
- The electrode where oxidation occurs, which releases electrons during battery discharge.
- Cathode
- The electrode where reduction occurs, which accepts electrons during battery discharge.
- Electrolyte
- The ion-conducting material that lets ions move between electrodes while blocking most electron flow.
- Separator
- A thin porous layer that keeps the electrodes from touching while allowing ions to pass through.
- State of charge
- The fraction of a battery's usable capacity that remains available at a given time.
Common Mistakes to Avoid
- Saying that electrons flow through the electrolyte is wrong because electrons mainly travel through the external circuit, while ions move through the electrolyte.
- Confusing energy with power is wrong because energy is the total stored or delivered amount, while power is the rate of energy transfer.
- Assuming a battery has constant voltage at all times is wrong because real cell voltage changes with state of charge, current, temperature, and aging.
- Ignoring internal resistance is wrong because it reduces useful voltage, wastes energy as heat, and can limit safe current.
Practice Questions
- 1 A lithium-ion cell has an average voltage of 3.7 V and a capacity of 2.5 Ah. Estimate its stored energy in watt-hours.
- 2 A battery delivers 4.0 A at 12 V for 30 minutes. How much energy does it deliver in joules?
- 3 Explain why a separator is essential in a lithium-ion battery even though it does not store much energy itself.