BTU-Publication reaches rank 25 in Scientific Reports Top 100 in Chemistry in 2021

Spent lithium-ion batteries are increasingly becoming an important secondary raw material source for nickel, cobalt, manganese and lithium salts, which can be used for the production of new batteries. The approach being pursued by Prof. Dr. Jörg Acker's research group at the Department of Physical Chemistry is much more far-reaching. The aim of the research work is to recover the still functional cathode material from spent lithium-ion batteries for immediate reuse. In the paper, "Studies on the deposition of copper in lithium-ion batteries during the deep discharge process," published in Nature Scientific Reports, researchers from the Department of Physical Chemistry address the mechanisms of deep discharge of lithium-ion batteries, which is the first step of any recycling process and thus the fundamental prerequisite for safe opening and disassembly of the batteries.

A particular challenge of any recycling strategy is the minimization of impurities in the recycled material. Copper impurities in particular have a negative impact on the performance of the material or hinder subsequent recycling steps. It is well known that copper enters the recyclate primarily through the destruction or incomplete separation of the anodes in the battery. A new finding was that significant copper contents occur in the recyclate, where elemental copper appears to have grown directly on the material. This is an unintended side reaction during deep discharge of the batteries, which has been incompletely understood and interpreted based on conjecture.

During the scientific investigation, it was elucidated which boundary conditions lead to the electrochemical dissolution of the anode and the subsequent deposition of elemental copper on the cathode to be recycled. In this process, these deposits not only contaminate the material, but also lead to internal short circuits, which cause an increased hazard potential for the deep discharge process. In this work, the most important stages of the process were elucidated and, for the first time, the copper concentration in the battery electrolyte under different states of charge was analyzed with pinpoint accuracy. This made it possible to identify the point of origin at which irreversible destruction of the battery begins. A local depletion of lithium causes a change in the charge transport from lithium to copper ions, which leads to irreversible copper deposition on the cathode.

The work thus not only makes an important contribution to the understanding of the processes taking place in the battery, but also provides guidance for any battery user on safe and non-destructive deep discharge.