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We've been exploring the applications of voltaic cells. We want to manipulate redox reactions to supply electricity. We do that by separating the reactants into two half-cells. The half-cells connect via a wire and electrolyte solution. What's the result? We generate a current! The redox reaction's free energy produces electrical work. We can use the flowing electrons to power devices. What's the most prominent application? Batteries! We studied the two broad categories of batteries: wet-cell and dry-cell. Today, we'll look at single-use batteries. In our next lesson, we'll look at rechargeable batteries.
Primary Batteries
What do you think of when you think of batteries? The first thing that comes to my mind is the Energizer bunny. But, more seriously, I think of the tiny cylindrical AA batteries. We call these primary batteries. Primary batteries are single-use dry-cells. After discharging them, we throw them away or recycle them. We use them all the time! Why? Because they're portable. I did a quick look around my house for primary battery devices. What did I find? The coffee scale, my car's key fob, the television remote, and my wife's watch use primary batteries. Our whole world constantly uses primary batteries. In 2021, the global primary battery market was 9.87 billion dollars. And that was a down year because of the pandemic! By 2026, experts estimate the market will reach about 16 billion dollars!
Benefits of Primary Batteries
Are you wondering why we don't use rechargeable batteries for everything? We have the technology. For example, our computers and phones use them. It turns out that primary batteries have several benefits.
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Size and Cost: We can make small rechargeable batteries. For example, my wife has a Fitbit. It's tiny and charges via USB. But often, it's easier to make smaller primary batteries. Why? Think about the cost of a Fitbit. It's a glorified watch that counts your steps and pulse. But, even the knock-off brands cost fifty dollars or more. Part of the cost comes from the battery. It would be cheaper if it used primary batteries. But, most people don't want to constantly buy batteries for their everyday devices.
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Shelf-life: While planning this explanation, I dug out some batteries from my kitchen drawer. They had been lying around since early 2020. They're a couple of years old. It seems I used two batteries from a pack of twenty. I guess my coffee scale doesn't use much energy. When its batteries finally die, the ones in the drawer will be ready to go. They last forever! Before, we talked about internal resistance. A primary battery's internal resistance is minuscule. So, they'll only be a tiny bit weaker after a couple of years.
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Charging can be Impractical: Sometimes charging a battery is impossible. For example, a heart pacemaker. We can't plug in a person. So, we have to use a primary battery with a slow discharge rate. A pacemaker's battery often lasts five to seven years.
Types of Primary Batteries
There are a few cathode-anode combinations for primary batteries.
Zinc-Carbon:
The name zinc-carbon battery has a misleading name. In reality, the electrodes are zinc and manganese (IV) oxide. They're combined with an ammonium chloride electrolyte. So, why do we need carbon? The carbon is part of the cathode and increases conductivity. Most often, we use the allotrope graphite. Remember, graphite is made of sheets of graphene. It's hybridized. So, each carbon has three single bonds and one double bond. Our VSEPR geometry is planar. But, the pi-bond generates a series of delocalized electrons. Hence, its conductivity.
Zinc-carbon batteries were first developed in the 1870s. By the early 1900s, they were commercially available. What were the benefits of zinc-carbon batteries? All the components are cheap! They also produce a steady voltage of around . They are great for applications with small power draws. So, most AA and AAA batteries were zinc-carbon. They were most often used in flashlights, remotes, smoke detectors, and small displays.
Zinc-Manganese (IV) Oxide Alkaline Batteries:
The zinc-manganese alkaline battery is an improvement of the zinc-carbon battery. What's the problem with the zinc-carbon battery? The electrolyte corrodes the zinc electrode. So, over time the battery loses charge and has a limited shelf-life. Notice that ammonium is a weak acid. To avoid corrosion, we need to increase the pH. So, we replaced the ammonium chloride electrolyte with potassium hydroxide. But, that created a new problem. Sometimes the batteries leaked. If you see a furry white coating, be careful. The salt is potassium carbonate. It's an irritant and can cause a burning sensation.
Lithium Based Primary Batteries:
We often talk about lithium in rechargeable devices. But, there are primary lithium-based batteries. The benefit of lithium is that it can produce a massive voltage. Think back to our standard electrode potentials. Lithium's was . The most common primary battery uses lithium and manganese dioxide. So, its voltage is around !
The benefit of lithium-based batteries is that they have a high energy density. So, we can use them in applications that need extended time frames. One example is pacemakers. We don't want to have to replace someone's battery every year. But, lithium batteries can last five to ten years. But, lithium batteries are expensive compared to zinc batteries. So, they don't make sense for short-term applications. For example, many children's toys get discarded before the lithium battery discharges. So, we stick with zinc for short-term applications.
Are you wondering how we can use lithium? It's an alkali metal. So, it spontaneously reacts with any moisture. It seems challenging to handle, construct and transport. It turns out we don't have to use pure lithium metal. We can use lithium ions dissolved in an organic solvent. Organic solvents are hydrophobic. So, we don't have to worry about water. The solvent also helps stabilize the charge.
