What is the difference between a primary and secondary battery, and what aspects of the battery performance are most important? Keen to learn more about this interesting topic and to get som answers, we reached out to an expert in the field, Prof. Erik Berg at Uppsala University.
Batteries have been developed since the beginning of the 19th century when we started to make more and more use of electric energy to power different things, Prof. Berg explains. Before the entrance of the generator, a very simple battery was what we had access to, he says. Over time, the need for different types of batteries have increased and, as a result, the battery technology has evolved. The development of different kinds of batteries are guided by the application where they should be used. As different needs have been identified, batteries have been designed to fit those needs, so every battery is intimately linked to a certain application. This is still an ongoing process. In the 20th century, there was a rich spectra of different battery chemistries being developed Prof. Berg says
All batteries, regardless of construction and usage, contain three components; a positive electrode, a negative electrode, and an electrolyte in between, Prof. Berg explains. The composition of the two electrodes can be widely different, and so can the composition of the electrolyte. In the first batteries that were developed, zinc and copper were used as electrode materials, and zinc- or copper sulphate solution was used in between to mediate the reactions. One can think of it as one electrode being the oxidant, i.e., able to deliver charge, while the other electrode is the reductant, able to accept charge, Prof. Berg explains. Since there is a chemical reaction between the two electrodes, there is a current flowing from one electrode to the other, and this can then be used to power different devices, such as a torch or a cell phone.
There are two categories of batteries, so called ‘primary’ batteries and ‘secondary’ ones. The primary batteries can only be discharged. The battery cell is assembled in a factory, and then you use it until there is no energy left, whereafter it must be disposed or recycled, Prof. Berg explains. The other category of batteries, the so-called secondary batteries, can be recharged several times.
The first batteries were developed in the 1800s, and they were primary batteries, i.e., they could only be discharged once. They were based on an invention by Alessandro Volta. He assembled copper and zinc electrodes, i.e., two metal plates, with an electrolyte in between, containing brine, i.e., salt water. The design of the cell resulted in a spontaneous reaction that was driving the battery, Prof. Berg says.
After Voltas invention, came another guy called Daniel, an English chemist, Prof. Berg says. He came up with a way to separate the two electrodes and make the whole process reversible. So, he was able to, not only make the current go from one end to the other, but he was also able to charge the battery so that the current could go back to the original electrode. So, this is when the rechargeable version was introduced, in the middle of the 19th century. In the meantime, scientists and engineers have been experimenting with replacing the copper, replacing the zinc, and replacing the electrolyte, and there has been a lot of different battery inventions. Both primary cells and secondary cells have been developed in parallel to each other, Prof. Berg says.
There are five key characteristics that are important when it comes to batteries, Prof. Berg says. The first one is energy density, i.e., how much energy that can be stored per volume or per weight. The second one is power density, i.e., how much energy that can be delivered per time unit. A third characteristic that is important is battery lifetime. For example, if you take a pacemaker battery, it cannot be recharged, it can only be used once and then it must be replaced. In this situation, we need a primary battery which can be discharged for a very long time, ideally as long as the lifetime of a human being, Prof. Berg explains. So, this battery must have a high energy density and a long lifetime just for discharging. A fourth important aspect is cost. And finally, safety is important. Essentially, we are storing energy in a small ‘box’, and when you store energy somewhere, for example in the tank of your car, there is always a risk of a quick, sudden release of energy, which would lead to failure and safety issues, and which therefore must be avoided. So, these are the five key aspects of a battery. Nowadays, we could also add sustainability as a sixth important aspect, Prof. Berg says.
Each battery application has different requirements with respect to these five, or six, aspects. If you take a lead-acid battery, for instance, it does not have a high energy density compared to for example a Li-ion cell, but is has quite a long lifetime, and it is very cheap, Prof. Berg says
Over the years, many types of battery chemistries have been introduced. One example is lead-acid. This is the technology that we use in cars, as start-up batteries, Prof. Berg says. The lead-acid battery can be used for a long time. It is being charged as you drive, and then when you stop the car and need to start it again, you use the power that you have in your lead-acid battery to start the car, Prof. Berg explains. The battery is only used for a very short time, and the use of energy is very rapid. Then, it is immediately re-charged. This is a very small amount of energy compared to what you can fit into for example a li-ion battery. Other chemistries in use are, for example, nickel - cadmium batteries where Ni and Cd are the two metals used inside the battery. We have Nickel - hydride batteries where you are working with nickel and hydrogen. Then there are different kinds of primary cells. The pacemaker battery which is based on lithium-iodine cells. There are a range of different chemistries, and all of them differ with respect to the five key aspects, as it just depends on the application where you decide to use them so to say, Prof. Berg says.
Listen to the full interview with Prof. Berg to learn more about batteries, how they work, and how the Li-ion battery research was initiated and finally resulted in a Nobel prize award.
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