Method Tracks What Happens Inside Batteries

The method could be key to designing more efficient batteries for specific uses such as electric cars and airplanes.

This illustration shows ions in a fully charged lithium-ion battery. A team of researchers discovered a new method to precisely measure the movement of these ions through a battery. (Image: Department of Energy)

February 1, 2021

Argonne National Laboratory, Lemont, Illinois

The future of mobility is electric cars, trucks, and airplanes. But there is no way a single battery design can power that future. Even cellphone and laptop batteries have different requirements and different designs. The batteries required over the next few decades will have to be tailored to their specific uses. And that means understanding exactly what happens, as precisely as possible, inside each type of battery.

Every battery works on the same principle: ions, which are atoms or molecules with an electrical charge, carry a current from the anode to the cathode through material called the electrolyte, and then back again. But their precise movement through that material, whether liquid or solid, has puzzled scientists for decades. Knowing exactly how different types of ions move through different types of electrolytes will help researchers figure out how to affect that movement to create batteries that charge and discharge in ways most befitting their specific uses.

A team of scientists has demonstrated a combination of techniques that allows for the precise measurement of ions moving through a battery. The combination of different experimental methods measures velocity and concentration and then compares them both to theory. Those methods include using ultra-bright X-rays to measure the velocity of the ions moving through the battery and to simultaneously measure the concentration of ions within the electrolyte while a model battery discharged. The research team then compared their results with mathematical models. Their result is an extremely accurate figure representing the current carried by ions — what is called the transport number.

The transport number is essentially the amount of current carried by positively charged ions in relation to the overall electric current; the team’s calculations put that number at approximately 0.2. This conclusion differs from those derived by other methods due to the sensitivity of this new way of measuring ion movement.

For this experiment, the research team used a solid polymer electrolyte instead of the liquid ones in wide use for lithium-ion batteries. The polymers are safer since they avoid the flammability issues of some liquid electrolytes.

In the past, the best way to research the inner workings of batteries was to send a current through them and then analyze what happened afterward. The ability to trace the ions moving in real time offers scientists a chance to change that movement to suit their battery design needs.

The next step is to analyze more complex polymers and other materials, such as calcium and zinc, and eventually into liquid electrolytes.

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