A battery is an electrochemical device that converts stored chemical energy directly into electrical energy. This conversion relies on the movement of charged particles—electrons through an external circuit and ions internally—to generate an electric current. The central components facilitating this transfer are the electrodes, the cathode and the anode. The distinction between these electrodes is based on the chemical reactions occurring at their surfaces, which determine the direction of electron flow and the resulting electrical polarity.
Understanding Polarity in Discharge Mode
When a battery is actively discharging, meaning it is powering a device, it functions as a galvanic cell, spontaneously generating electricity. In this standard mode of operation, the cathode is the positive terminal of the battery. The anode is the negative terminal and the source of the electrons.
Electrons are released from the anode and flow through the external circuit, providing power to the connected device. These electrons eventually arrive at the positive cathode, completing the external circuit. This flow establishes the electrical polarity most people associate with a functioning battery: the cathode is positive, and the anode is negative.
Defining Electrodes by Chemical Process
The most accurate way to define the cathode and anode is by the type of chemical reaction occurring at their surface, which remains constant regardless of the battery’s operational state. The cathode is defined as the electrode where reduction occurs (the process of gaining electrons). Conversely, the anode is where oxidation occurs (the process of losing electrons).
A simple way to remember these processes is the mnemonic “OIL RIG”: Oxidation Is Loss, Reduction Is Gain. During discharge, the material at the anode undergoes oxidation, releasing electrons that travel through the external circuit. These electrons are accepted by the material at the cathode, which undergoes reduction, completing the chemical reaction.
In a lithium-ion battery, for example, lithium ions and electrons are released at the graphite anode during discharge. The ions travel through the internal electrolyte, while the electrons travel externally to the cathode, typically a metal oxide. This consistent chemical role—reduction at the cathode and oxidation at the anode—is the fundamental scientific definition that supersedes the electrical sign.
Why Polarity Reverses During Charging
Confusion surrounding electrode polarity often arises with rechargeable batteries, which operate in two modes. When plugged into a charger, the battery switches from being a galvanic cell (energy source) to an electrolytic cell (energy receiver). The external power source forces the electrochemical reaction to run in reverse, pushing the current against its natural flow.
During charging, the cathode (site of reduction) is connected to the negative terminal of the charger. The anode (site of oxidation) is connected to the positive terminal of the charger. The external voltage reverses the flow, forcing electrons back into the electrode that was the positive cathode during discharge.
While the chemical definitions of the electrodes stay the same—the cathode is always the site of reduction—the electrical sign convention relative to the external circuit appears to flip. The cathode now receives electrons from the charger’s negative terminal, and the anode loses electrons to the positive terminal. This apparent reversal highlights the importance of using the chemical reaction to define the electrode, rather than its momentary electrical polarity.
Essential Internal Components of a Battery Cell
Beyond the cathode and anode, a battery cell requires two other components to function: the electrolyte and the separator. The electrolyte is a medium (often liquid, gel, or solid) that facilitates the movement of ions between the two electrodes. It acts as an internal highway for charged particles, balancing the charge as electrons flow through the external circuit.
The electrolyte must possess high ionic conductivity, allowing ions to pass freely, but it must also be non-conductive to electrons. This non-conductivity is crucial because it forces the electrons to take the external circuit route, providing usable electrical energy. Without this internal ion transfer, a significant charge buildup would occur at the electrodes, causing the electron flow to stop immediately.
The separator is a physical barrier placed between the cathode and the anode to prevent them from touching. If the two electrodes made contact, the battery would short-circuit, leading to a rapid and uncontrolled release of energy. This thin, porous membrane electrically isolates the two sides while allowing ions to pass through the electrolyte held within its pores.