The lithium-ion-battery (LIB) recycling business is a worldwide growing market. With a volume up to
250,000 tons per year of batteries from electronic vehicles exceeding their lifetime, which is referred to as end-of-life (EoL) batteries, the used metals are valuable resources for further processing. Therefore, the extraction of metals from EoL batteries is an urgent topic to counteract the scarcity issue concerning cobalt, nickel and lithium and to minimize the mining of the corresponding ores.
LIB specialized recycling companies follow two different approaches. In pyrometallurgy LIBs are
smelted while in hydrometallurgy they are leached in aqueous solutions. While pyrometallurgy is
energy consuming and non-selective, the hydrometallurgic method is preferred to recover different
metals with high selectivity and high purity. However, the drawback of this method is that large
amounts of inorganic acid and reductive agents are required for leaching. Typically, leaching solution
contains sulfuric acid and hydrogen peroxide.

Many researchers focus on the optimization of the recycling process to achieve higher leaching rates,
higher concentrations or to be more sustainable by using mild conditions. However, most of them are
only varying in the leaching conditions and examine the leaching kinetics. Still, the leaching
mechanism and limitations are not well understood and explanations why a chosen medium works or
not are elusive.
For example, using 1 M sulfuric acid as leaching reagent is not sufficient to leach lithium cobalt oxide
(LCO), even though it should be theoretically, taking the Pourbaix diagram
as reference. In praxis, leaching stops and only the addition of reducing agents improves the leaching process. For a better understanding, we present an analysis of the solid phase of the leached cathode material during the process. A systematic examination of the composition and structure of the leached material after a
certain leaching duration is performed with ICP-OES, XPS, XRD and SEM measurements.
We discovered with XRD that the limitation of the sulfuric acid leaching progress of LCO is induced by
structural phase transitions during leaching. Furthermore, SEM and XPS analysis revealed that the
leached particles exfoliate due to strong delithiation. These significant findings are improving future
leaching and recycling processes to obtain high extracted amounts of metals.