The electrolyte concentration of a zinc manganese battery is one of the core factors affecting its internal resistance characteristics. Changes in electrolyte concentration directly alter the battery's ohmic and polarization resistance through mechanisms such as ion migration rate, electrode reaction efficiency, and side reaction pathways, thus affecting overall performance. As the medium for ion conduction, the electrolyte concentration determines the number and migration ability of positive and negative ions in the solution, and the ion migration rate is closely related to the battery's internal resistance. When the electrolyte concentration is within a suitable range, ion conduction efficiency is high and internal resistance is low; if the concentration deviates from the optimal value, whether too high or too low, it will lead to an increase in internal resistance and trigger a series of chain reactions.
In a zinc manganese battery, the electrolyte is typically composed of ammonium chloride, zinc chloride, and a small amount of zinc oxide or ammonia. Its concentration directly affects the ionic strength and conductivity of the solution. When the electrolyte concentration is too low, the number of freely moving ions in the solution decreases, the interionic spacing increases, and the resistance to be overcome during migration increases, leading to a significant increase in ohmic internal resistance. At this point, the ion conduction speed slows down during battery discharge, leading to increased charge accumulation on the electrode surface and enhanced electrochemical and concentration polarization, further increasing the total internal resistance. Furthermore, low-concentration electrolytes may result in insufficient electrolyte absorption, causing poor contact between the electrodes and the separator, forming localized resistance hotspots and accelerating the deterioration of internal resistance.
Conversely, if the electrolyte concentration is too high, although the number of ions increases, the interaction forces between ions (such as van der Waals forces and electrostatic repulsion) also increase, leading to a decrease in ion migration rate. In high-concentration solutions, ions must traverse denser "ion clouds," increasing frictional resistance and thus raising the ohmic internal resistance. Simultaneously, excessively high concentrations may trigger side reactions; for example, the zinc anode reacts with chloride ions in the electrolyte to form insoluble zinc chloride precipitate, which covers the electrode surface, hindering contact between the active material and the electrolyte, forming a "passivation layer." This physical barrier significantly increases charge transfer resistance, causing a surge in polarization resistance and a sharp decline in battery performance.
Changes in electrolyte concentration can also indirectly alter internal resistance characteristics by affecting the electrode reaction equilibrium. In a zinc-manganese battery, manganese dioxide (MnO₂) at the positive electrode and zinc (Zn) at the negative electrode complete a redox reaction via ion conduction in the electrolyte. When the electrolyte concentration is appropriate, ion conduction matches the electrode reaction rate, resulting in stable internal resistance. However, if the concentration is imbalanced, the reaction equilibrium is disrupted. For example, a low-concentration electrolyte may lead to insufficient depolarization of the positive electrode, increased hydrogen evolution, and the formation of bubbles covering the electrode surface, increasing contact resistance. Conversely, a high-concentration electrolyte may inhibit the oxidation reaction at the negative electrode, reducing the zinc dissolution rate and increasing the reaction interface resistance. These effects all influence the overall internal resistance performance of the battery by altering the polarization resistance.
Furthermore, electrolyte concentration significantly affects the battery's low-temperature performance and self-discharge characteristics. At low temperatures, the ion migration rate of low-concentration electrolytes is already low; further reduction in concentration will exacerbate the increase in internal resistance, leading to difficulty in battery startup. While high-concentration electrolytes have lower internal resistance at room temperature, they may experience a surge in internal resistance at low temperatures due to increased viscosity and decreased ion activity. Regarding self-discharge, excessively high concentrations may accelerate side reactions between the electrodes and the electrolyte, consuming active materials and indirectly increasing internal resistance; excessively low concentrations may cause the electrolyte to dry out, leading to direct electrode contact and short circuits, resulting in abnormal internal resistance.
In practical applications, the electrolyte concentration of a zinc-manganese battery needs to be experimentally optimized to determine the optimal range. For example, alkaline zinc-manganese batteries, by using potassium hydroxide (KOH) instead of traditional ammonium chloride electrolyte, not only improve ionic conductivity but also reduce internal resistance. Their electrolyte concentration is typically controlled at a specific ratio to balance ion migration rate and the risk of side reactions, achieving minimum internal resistance and maximum performance. This optimization strategy provides an important reference for the improvement of neutral zinc-manganese batteries; that is, by adjusting the electrolyte composition and concentration, internal resistance characteristics can be significantly improved and battery life extended.
