So far, we have only been able to indirectly guess why the temperatures are high. battery Efficiency and durability decrease during use. A team from Dresden-Rossendorf-Helmholtzzentrum (HZDR) used so-called operando X-rays for the first time. X-ray photography Looking directly at sodium,zinc The temperature of a molten salt battery is approximately 600℃. The images reveal a previously hidden process and show that under real-world operating conditions, separation layers within a battery can be problematic. The findings provide important information regarding a new simplified concept of large cells. energy storage.
Liquid Metal Battery Design – Project Solstice
HZDR / Browrock
Sodium-zinc molten salt batteries are considered a very promising option for stationary energy storage. But they age too quickly. “These systems have great potential because sodium and zinc are cheap and easily available,” explains Dr. Norbert Weber. As coordinators of the EU project SOLSTICE, HZDR scientists have systematically studied various sodium-zinc storage concepts. “At the same time, we did not have a clear understanding of why the efficiency of the cell decreases significantly during use.” One of the advantages of high-temperature technology is that the metal is liquid at several hundred degrees Celsius and can be transported particularly quickly. But it is precisely this dynamic that makes the system difficult to control.
For a long time, it was only possible to speculate indirectly why sodium-zinc molten salt batteries deteriorate prematurely. Traditional electrochemical measurements record current and voltage, but do not provide a complete picture of the processes inside the cell. “Our batteries are completely liquid; what happens there is very dynamic,” explains Martins Sarma, lead author of the study. “But you can’t open the battery and see what’s inside while it’s operating. And when you let the battery cool, the structure changes fundamentally.”
Direct observation of charging cycles using X-rays
Nevertheless, to visualize these processes, the team used operando X-ray radiography. This is the first imaging method that allows charging and discharging to be directly tracked under real operating conditions. This revealed the movements of sodium, zinc, and electrolytes that determine battery efficiency and lifespan. These images unexpectedly revealed separators, a component thought to be essential to many cellular concepts. This is a porous separation layer between the electrodes that is thought to prevent direct contact between sodium and zinc, preventing unwanted side reactions.
However, X-ray examination revealed that zinc can accumulate in the area of the separator during operation. There, it loses electrical contact with the electrodes and cannot continue to power the battery. “It’s like the material is stuck in a sieve,” says Dr. Natalia Shevchenko, who works on electrochemical energy storage and its analysis at HZDR. “Over time, more active zinc is lost. This is a mechanism that helps explain cellular aging.”
These results demonstrate one important point above all else. That is, the separator in a sodium-zinc molten salt battery is not a passive component. They have a major impact on behavior and cellular aging. In supplementary experiments without the separator, the scientists observed that the zinc did not attach to the solid barrier and was permanently lost. However, at the same time, sodium and zinc come into contact more easily, which increases self-discharge. This comparison clearly shows that separators need to be re-evaluated under high temperature conditions.
Based on this, the team is currently working on dedicated improvements to the cell concept. The aim is to better control the transport of substances between liquid phases without relying on complex or costly components. In the long term, this should create a robust, simple and economical solution that facilitates the use of sodium-zinc molten salt batteries even in large-scale energy storage outside the laboratory.