Protonation of Lix(Co,Ni,Mn)O2 compounds in acidic conditions

The easy intercalation/deintercalation of Li+ ions is the key factor for the functionality of the layered LixCoO2 compound as a Li-ion battery cathode material. However, the hexagonal CoO2-layer piling in the LixCoO2 structure is prone to incorporate not only monopositively-charged lithium ions but also monopositive protons. Here we employ a hydrothermal acid treatment for a gradual (controlled by the treatment time) de-intercalation of Li+ ions and a subsequent intercalation of protons between the CoO2 layers in LixCoO2. We demonstrate that the lithium-for-proton exchange reaction can be monitored by AAS for the Li-content, by XRD for the proton-containing phase formation, and by FTIR and TG for the appearance of specific features indicative of the incorporated protons. With XPS we verify that upon the lithium-for-proton exchange the valence of cobalt remains the same, i.e. the proton incorporation compensates the positive charge loss due to the Li+-ion extraction. Finally, we show that essentially parallel lithium-for-proton exchange reactions occur also for Ni- and Mn-substituted Li(Co,Ni,Mn)O-2 samples, that is, for compounds that are highly relevant in current Li-ion battery technology.