Physical and electrochemical basis of induction of reversibility in manganese dioxide reduction and reoxidation

"Chemically Modified" (CM) rechargeable MnO$\sb2$ cathode materials have been prepared, based on patents issued to Ford. The reaction mechanisms of the discharge and recharge of these materials have been extensively studied in the present work and compared with behaviour at Blank MnO$\sb2$ (without Bi) and regular $\gamma$-MnO$\sb2,$ by various experimental techniques including classical electrochemical methods, as well as a newly developed in-situ uv-visible spectro-electrochemical method and a new in-situ X-ray absorption (XAS) electrochemical procedure. The roles of the soluble Mn(+III) species and Bi dopants in the discharge and recharge processes have been investigated in some detail. Firstly, the presence of Bi(III) favours the formation of soluble Mn(OH)$\sb6\sp{3-}$ species in both the processes of discharge and recharge and, secondly, it appears to work as a "catalyst" for the steps involved in further reduction of Mn(OH)$\sb6\sp{3-}$ species to Mn(OH)$\sb2$ in discharge, and re-oxidation of Mn(OH)$\sb2$ to MnO$\sb2$ on recharge. Based on the optical absorbance measurements, an heterogenous mechanism in which the soluble Mn(OH)$\sb6\sp{3-}$ intermediate is involved, is directly indicated as being the preferred pathway at CM MnO$\sb2$ and operates in parallel with the so-called homogenous mechanism in both the processes of reduction of MnO$\sb2$ and re-oxidation of reduced MnO$\sb2$ materials. Thus, from the soluble Mn(OH)$\sb6\sp{3-}$ species, either Mn(OH)$\sb2$ can be deposited, on reduction, or MnO$\sb2$ re-formed on re-oxidation, on the porous electrode matrix at favourable and practically significant current-densities. The CM MnO$\sb2$ is then rechargeable over a practically useable voltage range and, importantly, over the 2-electron charge capacity. On account of the preferred heterogeneous pathway at CM MnO$\sb2,$ high (up to 6C) discharge rates can be realized at a remarkably constant plateau voltage, independent of discharge rate. In the light of the mechanism studies, the electrochemical performance of the CM MnO$\sb2$ has been studied. The CM MnO$\sb2$ cathodes were characterized by (a) good multiple rechargeability, (b) high energy and power densities, (c) achievement of high charging/discharging rates at a surprisingly constant working voltage and (d) low cost. Use of an ion-selective separator enables the CM MnO$\sb2$ to be used in a secondary alkaline MnO$\sb2$ battery system with a Zn anode, and an appreciable cycle-life to be realized which is not possible if ZnO$\sb2\sp{2-}$ ions diffuse to the MnO$\sb2$ cathode. These characteristics make the secondary CM MnO$\sb2$/Zn battery system a promising candidate for electric vehicle applications and also for many other market niches currently occupied by Ni-Cd secondary and MnO$\sb2$/Zn primary cells in the consumer market.