How do batteries work?
Batteries power our modern world, from the smartphone in our pockets to the electric vehicles we drive. But have you ever wondered what's actually happening inside these batteries? The science behind it is surprisingly clever, involving fundamental chemical reactions that convert stored chemical energy into the electrical current that runs our devices. Let’s dive a little deeper!
The basics: how electrons create electricity
The magic of batteries lies in their ability to create and control a flow of electrons. Chemical reactions (known as redox reactions) within the battery cause a buildup of electrons at one end of the battery: the anode terminal.
Like most particles, electrons don’t like to be concentrated together and will automatically spread out toward areas with a lack of electrons, otherwise known as positively charged areas — such as the cathode at the other end of the battery.
However, the electrons can’t move directly from the anode to the cathode as the battery has a barrier to prevent this: the electrolyte. So, instead they have to pass through the circuitry of whatever device the battery is connected to.
The process of oxidation and reduction
A battery supplies current only when its positive and negative poles are connected in a closed circuit. This activates a redox reaction: oxidation at the anode releases electrons, while reduction at the cathode absorbs them. An easy way to remember this is with the mnemonic “LEO the lion goes GER” – Loss of Electrons is Oxidation, Gain in Electrons is Reduction.
A separator inside the battery blocks electrons but allows ions to pass, so electrons can reach the cathode only by flowing through the external circuit, where they power a device such as a lamp.
Inside the battery, ions move through the electrolyte to maintain charge balance while the electrode reactions continue. This process lasts until the reactive materials are depleted, at which point the battery can no longer deliver useful current and is considered empty.
Redox reactions are all around us in various forms, such as the gradual formation of rust on a garden gate, or the quick and enormous release of energy when a rocket engine is ignited. Electrons move from high concentration to low concentration just like a ball rolls down a hill - to transition toward a lower and more stable energy state.
Which chemical reactions take place to make batteries work?
Oxidation and reduction are also called ‘half-reactions’. They occur at the anode and cathode terminals respectively, facilitated by the electrolyte between them, which acts as a concentration gradient.
Using alkaline batteries as an example, the zinc (anode) loses electrons through oxidation, which are then gained by the ions in the electrolyte potassium hydroxide (cathode) through reduction. This oxidation reduction cycle creates a flow of electrons (electric current) from the anode, through the device’s circuitry, to the cathode.
In lithium cobalt oxide batteries, reduction at the cathode consists of cobalt oxide combining with lithium-ions to form lithium-cobalt oxide (LiCoO2). The half-reaction here is:
CoO2 + Li+ + e- → LiCoO2
Almost simultaneous to this event, oxidation at the anode occurs as the graphite intercalation compound LiC6 forms graphite (C6) and lithium-ions. The half-reaction is:
LiC6 → C6 + Li+ + e-
These two half reactions combine to form a redox reaction outlined in the chemical equation below. Read from left to right, this describes what happens during discharging of the cells. Read from right to left, we see what happens during the charging of the lithium-ion battery:
LiC6 + CoO2 ⇄ C6 + LiCoO2
What about rechargeable batteries?
In primary batteries, electrons always move from anode to cathode. The chemicals in the battery only react once to produce electricity and are then spent.
However, in secondary (also known as rechargeable) batteries, this reaction is reversible as the chemicals are able to receive charge once again by reversing the flow of electrons and thus building up an electric concentration at the anode terminal to repeat the process.
In a battery, electrical energy (power) is generated through a chemical process. You need different raw materials to facilitate this reaction. The raw materials in rechargeable batteries are somewhat scarcer and require more processing. Also, the actual production process is often more expensive and slower. This partly explains why rechargeable batteries are significantly more expensive than their non-rechargeable counterparts.
NiMH = NoRM
Although rechargeable batteries may seem like the new normal today, the concept has existed for much longer. In 1859, the Frenchman Gaston Planté assembled the first rechargeable battery: a lead-acid battery. Today, there are about ten types of rechargeable batteries. A large part of these are not found in stores, but are used, for example, in satellite probes or for industrial applications. Most rechargeable batteries - we mean the familiar AA and AAA types - are primarily nickel-metal hydride (NiMH) batteries. Previously, nickel-cadmium (NiCd) was the standard, but this type has almost completely been phased out today. This is partly because they lose their capacity more quickly.
When do I choose rechargeable and when not?
That depends on what you want to use the batteries for.
Non-rechargeable batteries
For energy-efficient devices (calculator, clock, or remote control) that require relatively little power, it is more evident to use a primary battery. For devices that must always be ready, such as smoke detectors and flashlights, it is also better to use primary batteries. These lose, unlike rechargeable batteries, little power over time (the 'self-discharge').
Rechargeable batteries
For devices that require a lot of power in a short time or that are used very often and intensively, you are better off with rechargeable batteries. Consider devices like gaming consoles, music players, your child's favorite toy, walkie-talkies, and digital cameras.
