“Normally electrolytic fluids and electrodes are in a permanent electrochemical exchange,” says Metzger. “Thus far it has not been possible to investigate the reactions at the anode and cathode independently of each other. We are the first to manage this successfully.”
The team’s battery test cell, which comprises an anode, a cathode and electrolytes, is not completely sealed, but rather is fitted with a fine capillary. This allows gasses that are released during charging and discharging to be sampled and investigated using a mass spectrometer.
To study the processes at anodes and cathodes independently of each other, the engineers also modified the membrane – a thin glass ceramic platelet coated with aluminium and synthetics – to make it permeable not only by lithium ions, but also by all other components of the electrolytic fluid.
The results demonstrate that the stability of electrodes and electrolytes depends on several factors, including charging voltage, operating temperature and chemical impurities.
The higher the applied voltage and temperature, the faster the electrolytic fluid decomposes. The gasses released in the process, mainly carbon monoxide and carbon dioxide, can cause the battery enclosure to balloon.
Even smallest traces of water that intrude into the cell are said to release hydrogen at the anode and acts as an oxidising agent on the carbon in the cathode. This impairs the conductivity of the electrode.
The chemical reactions that take place at the anode and cathode lead to interactions. This crosstalk reduces the overall cell performance.
One research result, in particular, bears a direct consequence in practice: The higher the desired voltage, the less residual moisture the materials may contain.
Manufacturers could extend the lifetime of future cells by replacing the electrolyte ethylene carbonate with more stable solution components. A small amount of ethylene carbonate, however, is currently required in systems to passivate the anode.