Layered and Tunneled Manganese Oxides As Battery Cathode Materials - the Role of Electrochemically Active and Inert Metal Cations

Abstract

Fundamental studies involving the impact of physical and chemical properties of materials on their electrochemical properties are critical to the rational development of battery materials which may address the present and future requirements for stationary and portable power. For large scale batteries, the use of environmentally benign and low cost energy storage materials is desired. Manganese-based materials are desirable in this regard as they are environmentally friendly, inexpensive, and can be prepared using scalable, aqueous based synthesis methods. Preparation, characterization, and battery electrochemistry of two promising structural motifs of manganese oxides will be described, specifically tunneled (hollandite) and layered (birnessite) crystallographic structures. The use of complimentary bulk (synchrotron based diffraction, thermogravimetric analysis, optical emission spectroscopy) and local (transmission electron microscopy based atomic imaging, nanodiffraction and electron energy loss spectroscopy) to elucidate structures of as-prepared, electrochemically reduced and oxidized materials will be highlighted. The impact of crystallite size will also be discussed. Hollandites (Mx’Mn8O16) are one-dimensionally (1D) structured materials consisting of octahedral units of edge-sharing manganese oxide octahedra which interlink to form tunnels of 2 x 2 dimensions. Bimetallic hollandite type materials have been prepared where cations with 1+ or 2+ charge partially occupy locations within the manganese oxide tunnels, and the Mn cations possess mixed 3+ and 4+ oxidation states. Typically, the ion located within the tunnel (M’) is electrochemically inert (i.e. K+); however, silver hollandite is an interesting case as the Ag+ cationic center is potentially electrochemically active. The electrochemistry of potassium and silver based hollandites in lithium-ion and beyond lithium-ion battery systems will be described. The impact of the structural M’ cation and the role of oxygen vacancies on ion transport will be discussed. A novel recyclable battery electrode will also be described, which provides full restoration of initial capacity after a facile regeneration process. Birnessite (Mx’MnO2), a layered manganese oxide, exists as a monoclinic crystal comprised of layers of edge-sharing manganese-oxygen octahedra (MnO6) with cations positioned between the layers. Although birnessite can be formed without any interlayer cations, monovalent (i.e. Na+, K+, Li+, Cs+) or divalent (Mg2+ or Cu2+) cations can also be incorporated between the layers. The impact of an electrochemically active interlayer cation, copper (Cu2+), on capacity and structural integrity of the birnessite structure upon discharge-charge cycling will be described, including a detailed view of the Mn and Cu oxidation state changes along with variations of the local neighboring atom environments around the Mn and Cu centers.