The influence of bismuth oxide doping on the rechargeability of aqueous cells using MnO2 cathode and LiOH electrolyte
Bi-doped manganese dioxide (MnO2) has been prepared from γ-MnO2 by physical admixture of bismuthoxide (Bi2O3). The doping improved the cycling ability of the aqueous cell. These results are discussed and compared with the electrochemical behavior of bismuth-free MnO2. Batteries using the traditional potassium hydroxide (KOH) electrolyte are non-rechargeable. However, with lithium hydroxide (LiOH) as an electrolyte, the cell becomes rechargeable. Furthermore, the incorporation of bismuth into MnO2 in the LiOHcell was found to result in significantly longer cycle life, compared with cells using undoped MnO2. The Bi-doped cell exhibited a greater capacity after 100 discharge cycles, than the undoped cell after just 40 cycles. X-ray diffraction and the microscopic analysis suggest that the presence of Bi3+ ions reduces the magnitude of structural changes occurring in MnO2 during cycling. Comparison with additives assessed in our previous studies (titanium disulfide (TiS2); titanium boride (TiB2)) shows that the best rechargeability behavior is obtained for the current Bi-doped MnO2. As the size of Bi3+ ions (0.96 Å) is much larger than Mn3+ (0.73 Å) or Mn2+ (0.67 Å) they have effectively prevented the formation of non-rechargeable products.