Battery research with a SEM: inspecting one layer at a time
Batteries revolutionized the world of electronics by enabling us to carry an energy reserve in our pockets. Miniaturization and efficiency are the two key words when it comes to new developments in this field, impacting with the battery materials’ properties and stretching their limits. Let’s take a look at how researchers characterize materials and gather relevant information about batteries using scanning electron microscopy (SEM).
The structure of a battery consists of three main components: two electrodes made of different materials and an insulation membrane in between them. The different chemical composition of the electrodes makes them available for chemical interaction and during the reduction-oxidation processes that subsequently takes place, energy is released. The chemical energy stored in the electrodes is therefore converted into electrical energy and can be employed to power up our electronic devices.
To go from the original battery concept (which would fit on a table) to the small and long-lasting battery of a smartwatch, some improvements were made. These mostly affected the materials used for the battery construction, rather than the working principle, which remained conceptually unchanged.
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Engineering batteries: what matters?
When designing a new battery structure, the specification of the product that will be powered by it are crucial to achieve a good match in terms of size and capacity. There are some parameters that are commonly found in the battery research and development process:
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Nominal voltage
This is an index of what voltage the battery can supply. A car and a watch require different amounts of energy and these values are obtained using different types of electrodes.
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Self-discharge rate
Batteries cannot keep their charge forever and sometimes they just lose it. This can be tolerable for some applications, but can become extremely annoying if, for example, it happens with the battery of a remote controller, which requires very small amounts of energy with long time intervals in between. Temperature typically plays a dominant role in this context (ever wondered why your phone battery dies faster when it’s cold?).
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Charging cycles
IF the battery can be recharged, it is very likely that it must be done quite often. Charge and discharge cycle will damage the battery components over time (specifically the electrodes) and the total amount of energy that can be accumulated will decrease over time. Optimizing the material shape and composition helps to produce batteries that can withstand thousands of charging cycles and lose less than 10% of their nominal capacity.
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Energy density
As its name suggests, this defines the amount of energy that can be accumulated per volume unit. This is improved not just by engineering the composition of the electrodes, but also their shape, to optimize the use of space with regard to the available reaction surface. In addition, the components’ size has been drastically reduced.
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Safety
Reducing the size of components raises an important safety issue: the proper insulation of the electrodes. It is not a mystery that batteries can explode (you probably recall how some smartphone producers have actually struggled with this issue). This can happen, for example, when the insulating membranes that separate the electrodes break due to a mechanical stress (in other words, if the battery is bent too much).
Improving battery quality with SEM
All these parameters have, as mentioned, a strong dependency on the material composition and structure. These parameters can be easily monitored, but appropriate analysis instrumentation is required.
Figure 1: left and right: SEM images of raw powders used in the production of cathodes. SEMs are ideal tools for investigating small particles in the range of micrometers or nanometers.
SEM gives you the opportunity to improve battery research by enabling you to magnify your sample hundreds of thousands of times, making features of a few nanometers clearly visible. In this way it is possible to measure the cross section of layers, as well as the size of the small features on the electrode’s surface that improve the contact surface.
In addition, it is possible to apply both thermal and mechanical stress to a membrane and observe its behavior on a microscopic level, thus allowing battery researchers to understand the cause of an eventual rupture.
Energy-dispersive X-ray microanalysis is often combined with SEM to locally identify the chemical composition of the sample accurately and with an outstanding, sub-micron, spatial resolution. And the analysis only takes few seconds!
Figure 2: An example of how EDS can be used to trace how the sample composition changes along a line. Spot analysis, line scan or area map can be used to monitor the distribution of different phases in a specific region of the sample.
You want to find out more on how SEM is used to investigate batteries? This application note provides you with insights into exactly what can be revealed with SEM. Find out more about what kind of research is ongoing on batteries and the results that a SEM can produce by downloading this application note:
