Why aren't all batteries rechargeable?

Listener Michael got in touch to ask "Why can't batteries, such as AA or AAA size, be recharged? What's the difference between regular batteries and rechargeables, especially lithium ones? Is this a 'big battery' conspiracy to sell more batteries, or are there valid reasons?" 

Katie - A big battery conspiracy huh? Well here are 3 scientists who can route out this recharging riddle: Gareth Hinds from the National Physical Laboratory, and David Hall and Didi Rinkel from Cambridge University.

Gareth - There are two types of battery – primary cells, which are designed for single use, and secondary cells, which are rechargeable.

David - Every single battery in the world consists of a positive electrode, a negative electrode, and some sort of electrolyte solution – that is a salt dissolved in a solvent.

Gareth -  When a battery is discharged, electrochemical reactions occur at each electrode, converting chemical reactants to products and generating electricity.

David - However, there are many choices for the electrodes and for the salts and solvents. Only some of these can be recharged, which scientists call “secondary cells” – but for others, like most AA and AAA batteries, using the stored energy is a one-way street.

Didi - Whether a battery is rechargeable or not depends on what the positive and negative electrodes are made of. The most common AA and AAA batteries are called alkaline batteries, and these have zinc metal and manganese dioxide electrodes. When you use the battery, the zinc metal is eaten up and you form zinc oxide. Unfortunately this reaction is irreversible, which means that you cannot get the zinc metal back if you recharge the battery.

Gareth - In a secondary cell the electrochemical reactions are reversible. For example, in a Li-ion battery the very small Li-ions can easily insert into both electrode materials  - usually graphite and a mixed metal oxide - so the electrochemical reactions work equally well in both directions. This means that the battery can be charged and discharged many times; usually for thousands of cycles.

Didi - There are also rechargeable AA or AAA batteries, such as the nickel metal hydride battery. The reactions in this type of battery are reversible which means that you can recharge the battery and use it again.

Gareth - So this is not a conspiracy by battery manufacturers – primary cells are inherently limited to a single discharge, while secondary cells are not.

David - Ultimately, when choosing a battery to use, one has to consider the energy needs of the device. For example, a cordless drill needs lots of power in short bursts, which takes a higher voltage battery. But a smoke detector uses very small amounts of energy over a long period of time, and so needs low-voltage batteries, such as your standard double or triple As.

Katie -  I just need to remember to check which batteries are the rechargeable ones before I put them in the charging station! Thanks Gareth, Didi and David. From batteries to boredom, because next time we’re boring into this question from Douglas…

Douglas - Do animals get bored eating the same thing all the time? Like sheep, do they get bored of one type of grass?