Raman spectroscopy and laser-induced degradation of groutellite and ramsdellite, two cathode materials of technological interest

Abstract

Manganese oxides are important geomaterials, used in a large number of applications. For instance, as pigments in art works or in the treatment and removal of heavy metals from drinking water. Particularly, ramsdellite [Mn⁴⁺O₂] and groutellite [(Mn_0.5⁴⁺,Mn_0.5³⁺)O_1.5(OH)_0.5], because of their 2 × 1 frameworks that enable proton diffusion, are very important cathode materials. Manganese oxides commonly occur as crypto-crystalline and very fine mixtures of different Mn-phases, iron oxides, silicates and carbonates. Thus, proper characterization can be a difficult task using XRPD. The lack of Raman data on groutellite and the little and conflicting data on ramsdellite do not allow their proper identification by Raman spectroscopy. In this work we characterize natural mixtures of ramsdellite and groutellite by combining SEM-EDS, XRPD, FT-IR and Raman spectroscopy, to provide reference Raman spectra. Our data show that they have a typical and unmistakable spectra, allowing clear recognition. Moreover, we have investigated their laser-induced degradation. Our data show that groutellite transforms into ramsdellite, by the loss of H⁺ and the oxidation of Mn³⁺ to Mn⁴⁺, already at a very low laser power. Further increasing the laser power the formation of hausmannite [Mn²⁺Mn₂³⁺O₄] occurs via the reduction of Mn cations. Our data can be used to study the discharge mechanism in cathodic battery materials, by monitoring the Mn reduction from ramsdellite to groutellite, and finally to groutite [α-Mn³⁺OOH]. Moreover, Raman mapping allows the study of their distribution in all the investigated samples and, indirectly, those of H⁺ and Mn³⁺, which plays a key-role in electrochemical activity of these compounds.