XRD/Raman spectroscopy studies of the mechanism of (de)intercalation of Na+ from/into highly crystalline birnessite

Abstract

Due to its low-cost and environmental friendliness, birnessite-type manganese oxide has attracted wide interest for use as a cathode material in electrochemical energy storage applications. The mechanisms of energy storage and release have been studied in some detail during the last decade, but despite some agreement, some aspects of the storage and release mechanisms are still under debate. The main reason for this, we argue, is the varying interpretations of Raman spectroscopy data in the literature. Therefore, we undertook a detailed correlative Raman spectroscopy/XRD study in combination with cyclic voltammetry. Raman spectroscopy allowed for straightforward differentiation between symmetry changes during the (de)intercalation of Na-ions. More specifically, through the use of highly crystalline birnessite samples it is suggested that Raman spectra are sensitive to the lattice parameters β and d₀₀₁, which allowed us to derive unprecedented details of the changes in the birnessite structure that occur upon Na⁺ (de)intercalation. Furthermore, it is shown that the reversible hexagonal/monoclinic symmetry transition during the course of a charge/discharge cycle is a prerequisite for effective charge storage. Based on the results, a detailed mechanism describing the (de)intercalation of Na⁺ from/into birnessite is presented.