Page 148 - Handbook of Battery Materials

116 3 Structural Chemistry of Manganese Dioxide and Related Compounds

3.4
Reduced Manganese Oxides
The reduced manganese oxides and oxide – hydroxides will also be considered
briefly in this section, since many of them are of interest for the performance of
aqueous manganese dioxide electrodes (e.g., in primary cells), but also in recharge-
able alkaline manganese dioxide/zinc cells (RAM cells; e.g., Ref. [97]). Compounds
of the composition MnOOH are the reaction product of the electrochemical
reduction of manganese dioxides, the electrochemically inactive spinel-type com-
pounds (e.g., Mn 3 O 4 or Mn 2 O 3 ) are formed during cycling of a MnO 2 cathode,
and the manganese hydroxide Mn(OH) 2 is the manganese compound with
the lowest oxidation state occurring in aqueous systems. The lattice constants
and symmetry data of several reduced manganese oxides are summarized in
Table 3.4.
3.4.1
Compounds of Composition MnOOH

3.4.1.1 Manganite (γ -MnOOH)
The crystal structure of manganite is closely related to that of pyrolusite in that it
consists of single chains of edge- and corner-sharing Mn(O,OH) 6 octahedra. The
coordination polyhedra of the Mn 3+ ions in the structure are strongly distorted due
to the Jahn–Teller effect of the trivalent manganese ions and the substitution of
O 2− counter-ions by OH ions. This results in the formation of four short Mn–O
−
bonds (with Mn–O distances ranging from 188 to 198 pm) and two longer apical
Mn–OH bonds (220–233 pm). The formation of hydrogen bridging bonds leads to a
pseudo-orthorhombic (or monoclinic) superlattice of the tetragonal β-MnO 2 parent
structure. Manganite is the thermodynamically stable modification of MnOOH;
therefore it can found in natural deposits as well as being easily prepared in the
laboratory. It also occurs as the reaction product during electrochemical reduction
of β-MnO 2 in batteries.

3.4.1.2 Groutite (α-MnOOH)
In the same way as manganite may be regarded as the structurally closely related
reduction product of β-MnO 2 , groutite or α-MnOOH was found to be isostructural
with ramsdellite. The arrangement of the Mn(O,OH) 6 octahedra in α-MnOOH is
very similar to that of the ramsdellite modification of MnO 2 . The structure consists
of double chains of octahedra. As has already been described for manganite, the
protons occupy positions in the crystal structure which allow them to build up
a hydrogen-bound network within the [2 × 1] tunnels. The situation for MnOOH
is comparable with that of the compounds Li x MnO 2 , in which the lithium ions
occupy sites in the tunnel of a ramsdellite host lattice. In both compounds the
Mn–O octahedra are strongly distorted because of the insertion of foreign cations
3+
into the lattice and the resulting reduction from Mn 4+ to Mn . A schematic
drawing of the protonated ramsdellite or α-MnOOH structure (diaspore type)