Cleaner lithium, better batteries

Abstract

Lithium metal is widely considered an ideal anode for next-generation solid-state batteries due to its high theoretical capacity and low electrochemical potential. However, most lithium foils used in laboratories and industry are produced via molten salt electrolysis and can contain up to 1% impurities, especially sodium, which are challenging to remove during fabrication. Now, Juri Becker, Jürgen Janek and colleagues at Justus-Liebig-University Giessen reveal that even trace levels of impurities in lithium foils can accumulate at the interphase with solid electrolytes during cycling, impeding lithium transport and degrading battery performance.

The researchers compare commercial lithium foils with electrodeposited lithium, which is used in ‘anode-free’ cell designs where the battery initially contains only a bare current collector (for example, copper foil) and no lithium on the anode side. Using advanced surface characterization techniques such as time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy, the team shows that sodium and other impurities accumulate at the electrolyte–anode interphase during lithium stripping, reducing the amount of lithium that can be effectively utilized. In contrast, lithium electrodeposited in situ — directly onto the current collector inside the battery — exhibits significantly higher purity. This in situ ‘electrochemical purification’ process enhances interfacial stability and improves lithium utilization.