Lithium-ion batteries power the lives of millions of people each day. From … Contact online >>
Lithium-ion batteries power the lives of millions of people each day. From
Batteries have changed a lot in the past century, but there is still work to do.
Image source: Andy Simmons / Flickr.
How do batteries power our phones, computers and other devices?
Research Team Leader, Advanced Energy Storage Technologies
Professor Maria Forsyth FAA
Chair, Electromaterials and Corrosion Sciences
Professor Ray Withers FAA
Research School of Chemistry
The Australian National University
Director, Centre for Clean Energy Technology
University of Technology Sydney
Imagine a world without batteries. All those portable devices we''re so dependent on would be so limited! We''d only be able to take our laptops and phones as far as the reach of their cables, making that new running app you just downloaded onto your phone fairly useless.
Luckily, we do have batteries. Back in 150 BC in Mesopotamia, the Parthian culture used a device known as the Baghdad battery, made of copper and iron electrodes with vinegar or citric acid. Archaeologists believe these were not actually batteries but were used primarily for religious ceremonies.
The invention of the battery as we know it is credited to the Italian scientist Alessandro Volta, who put together the first battery to prove a point to another Italian scientist, Luigi Galvani. In 1780, Galvani had shown that the legs of frogs hanging on iron or brass hooks would twitch when touched with a probe of some other type of metal. He believed that this was caused by electricity from within the frogs'' tissues, and called it ''animal electricity''.
Volta, while initially impressed with Galvani''s findings, came to believe that the electric current came from the two different types of metal (the hooks on which the frogs were hanging and the different metal of the probe) and was merely being transmitted through, not from, the frogs'' tissues. He experimented with stacks of layers of silver and zinc interspersed with layers of cloth or paper soaked in saltwater, and found that an electric current did in fact flow through a wire applied to both ends of the pile.
Volta also found that by using different metals in the pile, the amount of voltage could be increased. He described his findings in a letter to Joseph Banks, then president of the Royal Society of London, in 1800. It was a pretty big deal (Napoleon was fairly impressed!) and his invention earned him sustainedrecognition in the honour of the ''volt'' (a measure of electric potential) being named after him.
So what exactly was happening with those layers of zinc and silver, and indeed, the twitchingfrogs'' legs?
A battery is a device that stores chemical energy, and converts it to electricity. This is known as electrochemistry and the system that underpins a battery is called an electrochemical cell. A battery can be made up of one or several (like in Volta''s original pile)electrochemical cells. Each electrochemical cell consists of two electrodes separated by an electrolyte.
So where does an electrochemical cell get its electricity from? To answer this question, we need to know what electricity is. Most simply, electricity is a type of energy produced by the flow of electrons. In an electrochemical cell, electrons are produced by achemical reaction that happens at one electrode (more about electrodes below!)and then they flow over to the other electrode where they are used up. To understand this properly, we need to have a closer look at the cell''s components, and how they are put together.
To produce a flow of electrons, you need to have somewhere for the electrons to flow from, and somewhere for the electrons to flow to. These are the cell''s electrodes. The electrons flow from one electrode called the anode (or negative electrode) to another electrode called the cathode (the positive electrode). These are generally different types of metals or other chemical compounds.
In Volta''s pile, the anode was the zinc, from which electrons flowed through the wire (when connected) to the silver, which was the battery''s cathode. He stacked lots of these cells together to make the total pile and crank up the voltage.
There are a couple of chemical reactions going on that we need to understand. At the anode, the electrode reacts with the electrolyte in a reaction that produces electrons. These electrons accumulate at the anode. Meanwhile, at the cathode, another chemical reaction occurs simultaneously that enables that electrode to accept electrons.
The technical chemical term for a reaction that involves the exchange of electrons is a reduction-oxidation reaction, more commonly called a redox reaction. The entire reaction can be split into two half-reactions, and in the case of an electrochemical cell, one half-reaction occurs at the anode, the other at the cathode. Reduction is the gain of electrons, and is what occurs at the cathode; we say that the cathode is reduced during the reaction. Oxidation is the loss of electrons, so we say that the anode is oxidised.
Each of these reactions has a particular standard potential. Think of this characteristic as the reaction''s ability/efficiency to either produce or suck up electrons—its strength in an electron tug-of-war.
Standard potentials for half-reactions
Below is a list of half reactions that involve the release of electrons from either a pure element or chemical compound. Listed next to the reaction is a number (E0) that compares the strength of the reaction''s electrochemical potential to that of hydrogen''s willingness to part with its electron (if you look down the list, you will see that the hydrogen half-reactionhas an E0 of zero). E0is measured in volts.
The reason this list is so interesting is that if you pick two reactions from the list, and combine them to make an electrochemical cell, the E0 values tell you which way the overall reaction will proceed: the reaction with the more negative E0 value will donate its electrons to the other reaction and this determines your cell''s anode and cathode. The difference between the two E0 values tells you your cell''s electrochemical potential, which is basically the voltage of the cell.
So, if you take lithium and fluoride, and manage to combine them to make a battery cell, you will have the highest voltage theoreticallyattainable for an electrochemical cell. This list also explains why in Volta''s pile, the zinc was the anode, and silver the cathode: the zinc half-reaction has a lower (more negative) E0 value (-0.7618) than the silver half-reaction (0.7996).
Standard potentials for reduction half-reactions
Source: UC Davis ChemWiki
Any two conducting materials that have reactions with different standard potentials can form an electrochemical cell, because the stronger one will be able to take electrons from the weaker one. But the ideal choice for an anode would be a material that produces a reaction with a significantly lower (more negative) standard potential than the material you choose for your cathode. What we end up with is electrons being attracted to the cathode from the anode (and the anode not trying to fight very much), and when provided with an easy pathway to get there—a conducting wire—we can harness their energy to provide electrical power to our torch, phone, or whatever.
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