Video Transcript
In this video, we will learn how to
describe secondary galvanic cells and explain how they can be recharged. You may be familiar with galvanic
cells. Galvanic cells are a type of
electrochemical cell in which a spontaneous redox reaction occurs, generating a
potential difference between the electrodes of two half cells and this potential
difference drives current through the wire.
There are two categories of
galvanic cell, primary galvanic cells and secondary galvanic cells. A primary galvanic cell follows
this definition very closely. Chemical energy is converted to
electrical energy. The same occurs in a secondary
galvanic cell. But in a secondary galvanic cell,
the opposite can also occur. Electrical energy can be converted
to chemical energy. We can define a secondary galvanic
cell as a type of electrochemical cell that can be run as both a galvanic cell and
as an electrolytic cell. Primary galvanic cells are
single-use cells. They run flat when all the chemical
energy has been converted to electrical energy. Primary cells cannot be
recharged. Secondary galvanic cells, however,
are rechargeable.
Let’s focus on secondary galvanic
cells. During discharge, they behave as a
galvanic cell and chemical energy is converted to electrical energy. The process of discharging is when
a battery powers or supplies electrical current to an external device as it
generates electrons through a redox reaction. During charging, also called
recharging, a secondary cell behaves as an electrolytic cell. Electrical energy is converted to
chemical energy. Recharging is when an external
current is applied to reverse discharging and convert electrical energy into
chemical energy. During discharge, a cell loses its
energy store. And during recharge it regains its
energy store.
Let’s now look at some examples of
secondary galvanic cells. A common example of a secondary
galvanic cell is a car battery, also known as a lead-acid accumulator battery. When starting a car, the battery
acts as a galvanic cell to give power to the starter motor during ignition. It also powers the car lights and
other electrical systems, but whilst driving the alternator of the car recharges the
battery by an electrolytic reaction. So, we have galvanic discharge and
electrolytic recharge. Another example of a secondary
galvanic cell is the rechargeable lithium-ion battery. These types of batteries are used
in laptops, smartphones, and in cordless power tools such as a cordless drill.
Let’s now investigate these two
types of batteries, starting with the lead-acid battery. The diagram shows the construction
of a lead-acid accumulator battery. Although car batteries differ
slightly, most have a similar structure. The anode and cathode are made of
plates of lead metal or lead alloys. Each plate is designed like a grid
and resemble a waffle. And the grids are filled with
special lead materials. The negative anode plates are
filled with spongy lead. Spongy lead is a form of finely
divided and compressed lead metal, while the positively charged cathode plates are
filled with a lead oxide, PbO2, which is lead(IV) oxide.
Usually there are six cells in
series, forming a battery with a total cell potential or an emf of 12 volts, so two
volts per cell. The cathode and anode plates are
immersed in a dilute sulfuric acid electrolyte. The plates are prevented from
touching by separators. And the entire system of six cells
are encased in an insulating material like rubber or plastic.
Now that we know the construction
of a lead-acid battery, let’s have a look at the chemistry which occurs during
discharge and recharge. During discharge — that is, while
starting the car or running electrical systems such as lights or the radio — the
following two half-equations occur. At the anode, spongy lead with an
oxidation state of zero is oxidized to a plus two state. At the cathode, lead(IV) is reduced
to lead(II). Now, these half-equations are
sometimes written slightly differently, but they all result in the same final or
overall reaction equation. Lead, from spongy lead, reacts with
lead(IV) oxide and dilute sulfuric acid to produce lead sulfate, which is a solid,
and water.
The reduction half potential at the
anode is negative 0.36 volts and at the cathode positive 1.69 volts. Using these values, we can work out
the cell potential, using 𝐸 cell is equal to 𝐸 cathode minus 𝐸 anode. Substituting in these values, we
get positive 2.05 volts per cell in the battery. And remember, there are six
cells. So usually, a car battery has an
overall cell potential of about 12 volts.
We’ve seen that this overall
equation is for discharge. During recharging, the opposite or
reverse reaction will occur. And the anode and cathode swap
polarity. Over a long period of time, the
lead sulfate is not all converted back into the initial reactance. During recharging, over time, the
ability of the reverse reaction to occur diminishes. Some of the lead product is
converted to a very stable crystalline form. And the amount of lead sulfate
available for the reverse reaction decreases. Eventually, batteries need to be
recycled or replaced.
Now that we know about lead-acid
batteries, let’s have a look at the lithium-ion battery. We mentioned earlier that
lithium-ion batteries are found in many portable electronic devices, for example,
laptops, mobile phones, and cordless power tools. There are many different types of
lithium-ion batteries. The diagram shows a simplified
lithium-ion battery. The cathode is composed of lithium
cobalt oxide. The anode is made from lithium
graphite, where the pink dots in both the anode and cathode represent lithium
particles. A plastic separator between the two
electrodes separates the two materials but does allow lithium ions to move through
it, hence the name lithium-ion battery. The electrolyte, which coats both
electrodes, is lithium hexafluorophosphate.
A variety of transition metals can
be used depending on the type of lithium-ion battery. This particular lithium-ion battery
uses cobalt. When the lithium-ion battery is
being used to supply electrical energy to a device, in other words, when it is
discharging, the following two half-reactions occur. At the anode, oxidation occurs and
lithium ions and electrons are liberated. The anode is negatively charged,
and electrons entering it flow into the external circuit through the electrical
device and into the cathode, which is positively charged. At the same time, liberated lithium
ions move through the plastic separator towards the cathode. At the cathode, reduction
occurs.
Incoming electrons, lithium ions,
and the cobalt oxide can react to form the product lithium cobalt oxide. And the overall reaction is LiC6
solid plus CoO2 solid reacting to give C6 solid plus LiCoO2 solid. A typical lithium-ion battery has
an emf of three volts, although voltage varies depending on the type of lithium-ion
battery. The reaction shown here represent
galvanic action. Remember, the opposite will occur
during recharging. Let’s have a look. I have erased some information on
the screen. Let’s now fill in the correct
information for a recharging process. This will be electrolytic action,
where the battery acts like an electrolytic cell. This time, the anode is the lithium
cobalt oxide compound, and the reaction which occurs goes from right to left.
Oxidation is still occurring with
LiCoO2 being broken down into cobalt oxide, lithium ions, and electrons. The anode now has a positive
polarity. Negatively charged electrons enter
the anode and leave through the wire and enter the negatively charged cathode. At the same time, lithium ions move
from the anode to the cathode through the plastic separator. At the cathode, electrons coming
into the wire combine with incoming lithium ions from the anode. And these react with the graphite
structure, performing the reverse reaction to reform lithium graphite. Reduction has occurred at the
cathode. As a result, the overall reaction
will occur from right to left this time.
We’ve learned a lot about lead-acid
batteries and lithium-ion batteries. Now, let’s compare the two. An advantage of the lead-acid
battery is its low internal resistance, which results in it providing high current
to the starter motor. However, lead-acid batteries need
to be kept upright. This is a disadvantage. They are large and bulky. And they are rather heavy, so they
have a low power-to-weight ratio. Lithium-ion batteries, however, are
lightweight. Hence, they’re used in many mobile
devices. They are relatively small compared
to lead-acid batteries or even very small. They are sealed and so do not need
to be kept upright. They provide a large voltage for
their size compared to regular alkaline batteries. Incorrect or overcharging of these
batteries can shorten their lifespan.
Now, let’s summarize the key points
of this video. Secondary galvanic cells can be
discharged and recharged. During discharge, they act like a
galvanic cell and during recharge, like an electrolytic cell. Lead-acid batteries are an example
of a secondary galvanic cell. They are used in cars and consist
of spongy lead, which is finely divided lead, and lead(IV) oxide electrodes in a
sulfuric acid electrolyte. These batteries are relatively
heavy and large. The second type of secondary
galvanic cell we looked at are the lithium-ion batteries. They are often used in mobile
devices such as laptops, cellular phones, and power tools. Lithium graphite and a lithium
cobalt oxide are commonly used as electrode materials. And these batteries are generally
lightweight, small, and generate a large voltage relative to their size.
