Structural–chemical disorder of manganese dioxides: 1. Influence on surface properties at the solid–electrolyte interface

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Abstract

Relationships between lattice parameters of manganese dioxides and their surface properties at the solid–aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; γ-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn3+/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure.

Introduction

Among the numerous forms of manganese oxides, close-packed structure dioxides are of particular interest as lithium and proton insertion materials of battery electrodes for electrochemical energy storage [1]. The most active forms used in dry and alkaline batteries are synthetic γε manganese dioxides (MD) [2]. These materials display a great variety in X-ray diffraction (XRD) patterns, revealing structural disorder due to chemical defects induced during the synthesis process. Although the XRD patterns of most allotropic forms of γ/ε-MD are well identified, their interpretation remained an open question for a long time.

Basically, the structure of stoichiometric and ordered γ/ε-MD can be described by different distributions of manganese in an approximately hcp (hexagonal close-packing) oxygen network. These distributions are conveniently described by filled (MnO6) octahedra sharing opposite edges to form single, double, or wider chains defining tunnel structures along the c axis (Fig. 1).

Two end-members of MD are of interest in the present work: pyrolusite and ramsdellite. Pyrolusite (the natural mineral) or β-MnO2 (the synthetic tetragonal MnO2) is the most stable, stoichiometric, and ordered form, isotype with tetragonal rutile, made of single octahedra chains. Orthorhombic ramsdellite only exists as a natural mineral, still never synthesized as a pure mineral. It is made of double chains formed by edge-sharing octahedra [3]. Both minerals display well-identified XRD patterns, referred to in JCPDS files respectively as 43-1455 and 24-735.

In 1959 De Wolff [4] proposed that the great diversity in XRD patterns of γ/ε-MD is produced by a regular intergrowth of pyrolusite structural blocks in an initial ramsdellite network. However, the lack of powerful computing tools prevented a direct proof from being established. Thirty years later, Pannetier [5], starting from the De Wolff analysis, proposed a structural model which allowed the diffraction patterns to be analyzed by considering two types of defects: Pr, intergrowth of pyrolusite structural blocks in the ramsdellite network, and Tw, microtwinning of this interspersed structure. Rutile-type defects in a ramsdellite structure can be expressed quantitatively using Pr, defined as the probability that the “next” layer is rutile-type. Hence Pr=0 for ramsdellite, Pr=1 for β-MnO2, and 0<Pr<1 for the γ-MnO2 series. Although the original De Wolff model successfully accounted for many features in the XRD patterns of chemically synthesized manganese dioxides (CMD), it could not explain those of synthetic forms prepared by electrodeposition (EMD), which typically contain 6 broad lines, compared to the 14 in synthetic ramsdellite. Such broadening of specific reflections, as well as the occurrence of an apparently hexagonal structure, are explained by twinning of the orthorhombic ramsdellite lattice on the 021 and/or 061 planes (Fig. 2). Both parameters, Pr and Tw, are calculated from the analysis of diffraction patterns in the protocol defined by Pannetier [6].

It was later shown that the percentage of chemical defects, Mn3+ and manganese vacancies (both required for electrochemical activity), are respectively related to Pr and Tw [2], [7]. Although chemical synthesis is still in use, electrodeposition is the main process for preparing EMD active in alkaline batteries. It consists in anodically oxidizing an acidic Mn2+ sulfate solution into MnO2 at 97 °C. The first step of the reaction corresponds to the oxidation of Mn2+ into Mn3+, which is unstable in hot acidic solution and disproportionates into Mn2+ and Mn4+. This chemical step is slow. The Mn2+ ions remain in solution, whereas Mn4+ gives rise, at the potential value of water oxidation which generates oxygen, to a solid MnO2 deposit which traps Mn3+ ions. This electrochemical step is fast. The pH and the temperature of the Mn solution controls the percentage of Mn3+ and the percentage of Pr. The current density in the electrolysis cell controls the cell potential, the oxygen generation, which creates Mn vacancies, and the amount of MD deposit. Increase in current density is correlated to a decrease of the O/Mn ratio, and to a decrease of EMD density associated to an increase in microtwinning Tw [7]. One consequence of the above mechanisms is that the ranking of γ/ε-MD in terms of increasing Pr and Tw defects parallels that of electrochemical activity [7]. As a matter of fact, Mn3+ and manganese vacancies are compensated for by protons to balance the electrical neutrality of the chemical formula, as proposed by Ruetschi [3] in his vacancy model (V″″ represents a vacancy in the Schottky notation): Mn4+1−x−yMn3+yO2−2−4x−yV″″x(OH)4x+y.

Hydroxyl groups facilitate proton diffusion within the structure during the reduction process by insertion of the proton/electron couple, as well as electronic exchange interactions between Mn3+ and Mn4+. They must also influence surface acidity and the proton transfer at the interface during the first step of reduction in aqueous electrolytes.

The surface properties of MDs were far less studied than the bulk properties, despite their role in the reduction process by insertion of the proton–electron couple. This discrepancy is due to lack of knowledge of the surface composition and structure, which can be obtained for other metal oxides from crystallographic models. Therefore, the present investigation on MDs was focused on establishing experimental correlations between the structural disorder, characterized by Pr and Tw, and the surface properties at the solid–electrolyte interface, characterized by the point of zero charge (PZC). A companion paper [8] is devoted to the investigation of the primary energy distribution, using high-resolution gas adsorption.

Section snippets

Materials

Five MnO2 samples were examined. Two synthetic pyrolusite samples were selected for their different unit grain size, respectively coarse β-MnO2 (Touzart and Matignon, France), obtained from thermal decomposition of manganese nitrate, and nanocrystalline β-MnO2, obtained by spray-vapor deposition [9]. A synthetic ramsdellite (named S-Rams) was prepared by a modified hot acid treatment of LiMn2O4 adapted from [10]. A commercial chemical manganese dioxide, “WSA” grade (named CMD WSA), was

Characterization of the samples

The XRD patterns of the studied samples are presented on Fig. 3. Their analysis after the protocol defined by Pannetier [6] leads to the determination of the parameters Pr and Tw characterizing the structural disorder.

Table 1 shows these parameters for the present samples, along with those of a natural and synthetic ramsdellite.

It can be seen that synthetic ramsdellites are not really pure materials, since they contain a significant proportion of pyrolusite intergrowth defects (0.20<Pr<0.27)

Conclusion

Experimental relations between bulk structural properties of disordered MDs and surface properties at the solid–water and solid–electrolyte interface were established. The end-members ramsdellite and pyrolusite always display the most distant surface characteristics: the area per adsorbed water molecule ranges from 6.3 to 13.1, and the point of zero charge ranges from 2 to 7.3. The disordered samples displayed intermediate values.

In order to better understand these relations, a global approach,

Acknowledgements

The EBC Company is acknowledged for funding this work, and Dr. Elisabeth Djurado (LEPMI Grenoble) for her help in preparing nanocrystalline β-MnO2.

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