Page 250 - Lindens Handbook of Batteries

10.2 priMAry bATTerieS

TABLE 10.1 Major Advantages and Disadvantages of Magnesium batteries
Advantages Disadvantages
Good capacity retention, even under high-temperature storage Delayed action (voltage delay)
Twice the capacity of corresponding Leclanché batteries evolution of hydrogen during discharge
Higher battery voltage than zinc-carbon batteries Heat generated during use
Competitive cost poor storage after partial discharge

10.2 CHEMISTRY

The magnesium primary battery uses a magnesium alloy for the anode, manganese dioxide as the
active cathode material but mixed with acetylene black to provide conductivity, and an aqueous
electrolyte consisting of magnesium perchlorate, with barium and lithium chromate as corrosion
inhibitors and magnesium hydroxide as a buffering agent to improve storability (pH of about 8.5).
The amount of water is critical, as water participates in the anode reaction and is consumed during
the discharge. 1
The discharge reactions of the magnesium/manganese dioxide battery are
-
Anode Mg + 2OH = Mg(OH) + 2e
2
Cathode 2MnO + H O + 2e = Mn O + 2OH -
2
2
2
3
Overall Mg + 2MnO + H O = Mn O + Mg(OH) 2
3
2
2
2
The theoretical potential of the battery is greater than 2.8 V, but this voltage is not realized in practice.
The observed values are decreased by about 1.1 V, giving an open-circuit voltage of 1.9 to 2.0 V, still
higher than for the zinc-carbon battery.
The rest potential of magnesium in neutral and alkaline electrolytes is a mixed potential, deter-
mined by the anodic oxidation of magnesium and the cathodic evolution of hydrogen. The kinetics
of both of these reactions are strongly modified by the properties of the passive film, its history of
formation, prior anodic (and to a limited extent cathodic) reactions, the electrolyte environment, and
magnesium alloying additions. The key to a full appreciation of the magnesium electrode lies in an
2
understanding of the predominantly Mg(OH) film, the factors that govern its formation and dis-
2
solution, as well as the physical and chemical properties of the film.
The corrosion of magnesium under storage conditions is slight. A film of Mg(OH) that forms on
2
the magnesium provides good protection, and treatment with chromate inhibitors increases this pro-
tection. As a result of the formation of this tightly adherent and passivating oxide or hydroxide film
on the electrode surface, magnesium is one of the most electropositive metals to find use in aqueous
primary batteries. However, when the protective film is broken or removed during discharge, corro-
sion occurs with the generation of hydrogen
Mg + 2H O → Mg(OH) + H 2
2
2
During the anodic oxidation of magnesium, the rate of hydrogen evolution increases with increas-
ing current density due to destruction of the passive film, which exposes more (cathodic) sites on
the bared magnesium surface. This phenomenon has often been referred to as the “negative differ-
ence effect.” An appreciable rate of anodic oxidation of magnesium can only take place on the bare
metal surface. Magnesium salts generally exhibit low levels of anion conductivity, and one could
-
theoretically invoke a mechanism wherein OH ions migrate through the film to form reaction prod-
uct Mg(OH) at the magnesium-film interface. in practice this does not occur at a sufficiently rapid
2
rate and instead the film becomes disrupted, in all likelihood mechanically, as the result of anodic
3
current flow. A theoretical model for the breakdown of the passive film has been proposed. 4–7 This