The role of an electrolyte (a solution containing ions) is to close the internal electrical circuit of the cell. Ions serve as charge carriers in the battery, enabling electrons (i.e., current) to flow through the external circuit, where they can power a load such as a light bulb, sensor, or remote control. For details, see: “What is the role of the battery electrolyte?”
In aqueous batteries the electrolyte has a second role; water from the electrolyte is consumed in the cell reaction. This article will focus on aqueous electrolytes.
Without providing extensive details on various battery chemistries, we should examine two equations for the cathode processes relevant to zinc-carbon, alkaline-manganese dioxide batteries, and nickel-zinc batteries, respectively.
Zinc-carbon and alkaline-manganese dioxide batteries use MnO2 as their cathode reactive material, which is reduced according to Equation 1:
MnO2 + H2O + e– MnOOH + OH– (1)
For nickel-zinc cells, the cathode-active material is nickel oxyhydroxide, NiOOH, which reduces according to Equation 2:
NiOOH + H2O + e– Ni(OH)2 + OH– (2)
Water is consumed in both of these reactions.
Theoretical calculations show that about 2 g of water is consumed for every 10 g of MnO2 (Eq. 1), and approximately the same amount of water is needed for every 10 g of NiOOH (Eq. 2). If one limits the quantity of electrolyte to contain only the theoretical amount of water, the cell would have a very limited lifetime; its operational life would be significantly reduced as some of the water is not available for the discharge of the cathode.
In order to maintain the stream of ions in the cell in spite of water consumption from the cathode reaction, one needs an excess of water. For a given mass of electroactive materials, there is a direct relationship between the quantity of energy stored in the cell and the amount of electrolyte incorporated in the cell (and implicitly, the available water). This means that not only the cathode and anode materials, but the amount of electrolyte impacts the specific energy (Wh/kg) of a cell, as well. The general rule of thumb is that roughly 4x the theoretical amount of water is required.
High salt (ions) content in the electrolyte will offer high conductivity (and, consequently a negligible voltage drop in the battery due to internal resistance) due to higher concentration in conductive ions. This makes it possible to source high current from the cell (in scientific terms: to operate the cell at high C-rate). Let us recall that the cell theoretical voltage is determined thermodynamically, which means that it has a well-defined value for chosen battery chemistry. Therefore, the maximum cell voltage is set by the battery materials and corresponds to a standard value. The operating voltage, at which the cell can source power, is less than the standard value. It is the result of the standard voltage minus the voltage drops caused by battery internal resistance and polarization. Both internal resistance and polarization are impacted by the choice of ion concentration in the electrolyte. A low concentration will increase electrode reactivity (lower polarization), but will reduce ionic conductivity, thus increasing internal resistance.
Another parameter, which can be tuned for a given cell, is the current density, which results from the rate at which the electrode reactions proceed. A more conductive electrolyte will enable greater current, and implicitly, increased power and improved power density of the cell. An important trade-off is that highly concentrated electrolytes contain less water to react prior to drying out. This negative effect on the battery performance has to be balanced with the positive effect of using a higher concentration electrolyte.
An advantage of slight excess of electrolyte solution is that it compensates for losses by evaporation and corrosion; this prevents any dry-out of the cell, and extends the shelf life. Nonetheless, there is a trade-off between assembling cells with too little or, conversely, too much electrolyte. Hence, an overwhelming excess of electrolyte would correspond to “flooding” the electrodes, which may end up with unnecessary weight and volume, mechanical decay of the electrode(s) and would decrease the specific energy and energy density of the cell.
As mentioned in an earlier blog, ultrathin batteries operate exclusively with the electrolyte solution stored in the separator and the porous electrode materials. For example, thin film batteries can be made with microporous polyolefin separators, which have their pores filled with the ionically conductive liquid electrolyte. Indeed, this corresponds to a small volume of electrolyte, made available to the process.
Both the amount and concentration of the electrolyte incorporated in a battery represent a trade-off. Cells “starved” of electrolyte may dry out quickly, while cells “flooded” with electrolyte have worse specific energy and energy density parameters than cells containing just enough electrolyte. High salt concentration of the electrolyte enables faster discharge, which increases power density. Electrolytes with high salt concentration provide, however, less water to the cathode reaction reducing the practical utilization of the reactive chemicals.