D42.4 Primary Batteries

Primary batteries are single-use batteries that cannot be recharged.

Zinc-Carbon Battery

A common primary battery is the dry cell, which is a zinc-carbon battery. The zinc serves as both a container and the negative electrode (anode). The positive electrode (cathode) is a rod made of carbon that is surrounded by a paste of manganese(IV) oxide, zinc chloride, ammonium chloride, carbon powder, and a small quantity of water.

A diagram of a cross section of a dry cell battery is shown. The overall shape of the cell is cylindrical. The lateral surface of the cylinder, indicated as a thin red line, is labeled “zinc can (electrode).” Just beneath this is a slightly thicker dark grey surface that covers the lateral surface, top, and bottom of the battery, which is labeled “Porous separator.” Inside is a purple region with many evenly spaced small darker purple dots, labeled “Paste of M n O subscript 2, N H subscript 4 C l, Z n C l subscript 2, water (cathode).” A dark grey rod, labeled “Carbon rod (electrode),” extends from the top of the battery, leaving a gap of less than one-fifth the height of the battery below the rod to the bottom of the cylinder. A thin grey line segment at the very bottom of the cylinder is labeled “Metal bottom cover (negative).” The very top of the cylinder has a thin grey surface that curves upward at the center over the top of the carbon electrode at the center of the cylinder. This upper surface is labeled “Metal top cover (positive).” A thin dark grey line just below this surface is labeled “Insulator.” Below this, above the purple region, and outside of the carbon electrode at the center is an orange region that is labeled “Seal.”
Figure: Zinc-carbon battery. A cross section of a flashlight battery, a zinc-carbon dry cell.

The reaction at the anode can be represented as the oxidation of zinc:

Zn(s) ⟶ Zn2+(aq) + 2 e–          E°anode = -0.763 V

The reaction at the cathode is more complicated, in part because more than one reduction reaction is occurring. The series of reactions that occurs at the cathode is approximately:

2 MnO2(s) + 2 NH4Cl(aq) + 2 e– ⟶ Mn2O3(s) + 2 NH3(aq) + H2O(ℓ) + 2 Cl–(aq)

The overall reaction for the zinc–carbon battery can be represented as:

2 MnO2(s) + 2 NH4Cl(aq) + Zn(s) ⟶ Mn2O3(s) + 2 NH3(aq) + H2O(ℓ) + 2 Cl–(aq) + Zn2+(aq)

The cell potential is about 1.5 V initially, and decreases as the battery is used. As the zinc container oxidizes, its contents eventually leak out, so this type of battery should not be left in any electrical device for extended periods.

The voltage delivered by a battery is the same regardless of the size of a battery. For this reason, D, C, A, AA, and AAA batteries all have the same voltage. However, larger batteries can deliver more moles of electrons and will therefore last longer if powering the same device.

Alkaline Battery

Alkaline batteries were developed in the 1950s partly to address some of the performance issues with zinc–carbon dry cells, and are manufactured to be their exact replacements. As their name suggests, these types of batteries use alkaline electrolytes, often potassium hydroxide.

A diagram of a cross section of an alkaline battery is shown. The overall shape of the cell is cylindrical. The lateral surface of the cylinder, indicated as a thin red line, is labeled “Outer casing.” Just beneath this is a thin, light grey surface that covers the lateral surface and top of the battery. Inside is a blue region with many evenly spaced small darker dots, labeled “M n O subscript 2 (cathode).” A thin dark grey layer is just inside, which is labeled “Ion conducting separator.” A purple region with many evenly spaced small darker dots fills the center of the battery and is labeled “ zinc (anode).” The very top of the battery has a thin grey curved surface over the central purple region. The curved surface above is labeled “Positive connection (plus).” At the base of the battery, an orange structure, labeled “Protective cap,” is located beneath the purple and blue central regions. This structure holds a grey structure that looks like a nail with its head at the bottom and pointed end extending upward into the center of the battery. This nail-like structure is labeled “Current pick up.” At the very bottom of the battery is a thin grey surface that is held by the protective cap. This surface is labeled “Negative terminal (negative).”
Figure: Alkaline battery. Alkaline batteries were designed as direct replacements for zinc-carbon batteries.

The reactions are:

Oxidation (anode): Zn(s) + 2 OH–(aq) ⟶ ZnO(s) + H2O(ℓ) + 2 e– E°anode = -1.28 V
Reduction (cathode): 2 MnO2(s) + H2O(ℓ) + 2 e– ⟶ Mn2O3(s) + 2 OH–(aq) E°cathode = +0.15 V
overall: Zn(s) + 2 MnO2(s) ⟶ ZnO(s) + Mn2O3(s) E°cell = +1.43 V

An alkaline battery can deliver about three to five times the energy of a zinc-carbon dry cell of similar size. Alkaline batteries sometimes leak potassium hydroxide, so these should also be removed from devices for long-term storage. While some alkaline batteries are rechargeable, most are not. Attempts to recharge an alkaline battery that is not rechargeable often leads to rupture of the battery and leakage of the potassium hydroxide electrolyte.

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