Rechargeability of MnO2 in KOH media produced by decomposition of dissolved KMnO4 and Bi(NO3)(3) mixtures .2. A reaction viewpoint on the role of Bi

The beneficial effect of the presence of Bi atoms on the rechargeability of MnO2 can be explained through a reaction viewpoint. Bi(III) and Mn(X) atoms (X = IV, III, or II) give stable complexes. Furthermore, the formation of Mn3O4 can be viewed as a stepwise 3D polymerization, i.e., as a reaction chain composed of basic addition reactions. In this way, we can easily understand that the presence of Bi(III) disturbs the polymerization progress because these complex-formation reactions short-circuit the addition reactions. When a Bi atom replaces a Mn(II) or Mn(LII) atom, the building of spinel lattice is disturbed. As the size of Bi(III) ions (0.96 Angstrom) are much larger than Mn(II) (0.67 Angstrom) or Mn(III) (0.73 Angstrom) ions, they cannot insert themselves into the spinel lattice. This prevents the formation of an extensive spinel lattice, which is well known to be a poor electroactive compound. The reaction viewpoint allows us to define two criteria for searching a beneficial effect on the rechargeability of MnO2 material: (i) a strong interaction between the metal cations and Mn atoms is required, (ii) the radius of the cation must be sufficiently large to prevent its insertion into the spinel lattice.