To build better batteries, you need to catch them in the act
Two scanning electron microscope images that illustrate how a traditional electrolyte can cause dendrite growth (left), while a new electrolyte instead causes the growth of smooth nodules that don’t short-circuit batteries (right).
Emerging industries, from large-scale energy storage to electric cars, will need longer lasting batteries. But to build them, we need to know a lot more about what is limiting battery life.
New tools let researchers examine, at the nanometer scale, batteries while they’re in operation. This helps them identify internal faults that can trigger battery failure.
Advanced tools, such as electron microscopes and synchrotrons – a very powerful light source – let us look at batteries while they’re in use. High speed cameras and detectors, chip technology and the ability to process large amounts of data also play a role.
This emerging field still has its obstacles: the high energy x-rays or electron beams used by these tools can interfere with battery operation, and typically the sample size is limited because it needs to fit into a relatively small instrument space.
Despite the technical challenges, these tools can provide us with important insights into the current limitations of battery technology.
How can we look at batteries while they are in operation?
To understand how we look at batteries in action, it’s important to first understand their parts.
Each lithium-ion battery, for example, has a positive and negative electrode, and an electrolyte that separates them. This electrolyte, typically a liquid chemical mixture, allows an electrical charge (in the form of lithium ions) to flow. Lithium ions diffuse through the electrolyte between the electrodes depending on whether the cell is being charged or discharged.
When imaging batteries that are operating, it’s possible to see these nanoscale processes and pinpoint problems with the materials used. In the lab, a coin cell battery is often used for testing.
A range of tools can be used to look at batteries in this way, but x-ray and electron microscopy techniques are particularly promising.
For researchers to be able to see what’s inside a battery, the imaging beam, whether light, x-ray or electron beam, needs to pass through the sample. Just think about light hitting a wall rather than a window: if the battery is too thick, the x-ray or electron beam cannot penetrate.
Conventional lab x-rays have a low energy and intensity, and so cannot penetrate very deeply into a material. However, an x-ray beam from a synchrotron has a considerably higher energy and allows for deeper penetration.
However synchrotrons are typically very large facilities that are difficult to operate and access.
A more common instrument is the transmission electron microscope (TEM). A TEM is a microscope that uses an electron beam instead a light beam, unlike a conventional microscope. The electron beam can allow for magnification of more than one million times.
However, if an electron beam was passed through air, it would scatter considerably and you would not be able to see anything. For this reason, operation of a TEM requires a very high vacuum which allows the electron beam to easily pass.
Unfortunately, this presents another challenge for researchers: the vacuum makes the inclusion of a liquid electrolyte (present in many standard batteries) impossible, as the liquid would likely evaporate.
Recently, new TEM holders have been designed that allow the battery material and the liquid electrolyte to be encased between two electron transparent windows, as well as the current to be passed through the battery material.
This makes it possible to create an image at very high magnifications while operating the battery.
What battery problems are we looking for?
This emerging type of battery research is needed to address the faults in batteries.
Of particular importance are the conditions that allow for lithium dendrite growth.
Lithium dendrites are microscopic tree-like structures that can grow from a lithium electrode, potentially short-circuiting the cell. This process can even cause a battery fire, and the issue is hampering the use of powerful lithium electrodes.
Preliminary work has shown that it is possible to image the dynamic growth of lithium dendrites in a TEM.