In this explainer, we will learn how to describe secondary cells and explain how they can be recharged.
Galvanic cells are types of electrochemical cells that generate a potential difference through a spontaneous redox reaction. They can be classified into two varieties, primary galvanic cells and secondary galvanic cells. In this explainer, we will focus on secondary galvanic cells.
Definition: Secondary Galvanic Cell
A secondary galvanic cell is a type of electrochemical cell that can be run as both a galvanic cell and as an electrolytic cell.
While primary cells are single-use, secondary cells can be recharged. This means that the cell can be run as both a galvanic cell during discharge and as an electrolytic cell when being recharged. A common example is an automobile battery that can act as a galvanic cell when it powers a starter motor and behave as an electrolytic cell when it is being recharged by the car’s alternator.
Discharging is when a galvanic cell or battery powers an external device as it generates electrons through a redox reaction.
This dual use of the cell is the principle behind rechargeable batteries, which require an external dc current to be applied to the cell for recharging to occur. The direct current (dc) is applied in the opposite direction to the current flow that occurs during the discharge process. Consequently, the redox reaction of the chemicals in the cell proceeds in the opposite direction, recreating the chemicals used in the initial reaction.
Recharging is when an external current is applied to reverse discharging and convert electrical energy into chemical energy.
We will now develop our understanding of secondary cells by studying two examples: the lead–acid battery and the lithium-ion battery.
The lead–acid accumulator battery can be found under most car hoods, and it is used to power lights and the ignition system. It is also used as a backup emergency power source when the engine is not running properly. The lead–acid accumulator battery is commonly referred to simply as the “car battery,” and it usually consists of six cells of +2.05 V each that are connected in series to form a battery with a total cell potential of approximately 12 V.
Many car manufacturers have adjusted the design of car batteries to optimize their performance, but all car batteries have essentially the same structure. The anode and cathode are usually made out of lead metal plates. These plates have a grid construction, which allows either spongy lead in the case of the anode or lead(IV) dioxide in the case of the cathode to fill the grids.
Definition: Spongy Lead
It is a form of finely divided and compressed lead metal that is used in the electrodes of certain secondary cells.
Plate separators are often included to prevent the anode and cathode from touching. The electrolyte in this cell is a dilute sulfuric acid, . There are six cells in total, and they are all encased in rubber and/or a plastic, such as polystyrene, to insulate the battery from the rest of the car. An illustration of a typical lead–acid battery can be seen below.
We can understand the chemistry of electrochemical cells by examining the half-equations that occur at each electrode. Different versions of the half-equations exist, but they all result in the same final overall equation for the lead–acid battery as it discharges.
At the anode, oxidation occurs during discharge: with a standard electrode potential of +0.36 V.
At the cathode, reduction of the lead(IV) dioxide occurs during discharge: with a standard electrode potential of +1.69 V.
If we combine these two half-equations, the overall cell equation for a lead–acid battery when it is acting as a galvanic cell is
We are able to calculate the standard cell potential in the following manner:
Example 1: Explaining Why a Salt Bridge or Porous Separator Is Not Necessary In a Typical Lead-Acid Battery
Why is no salt bridge, porous separator, or an equivalent necessary for lead–acid batteries?
- Secondary galvanic cells do not need salt bridges.
- The battery casing acts as a salt bridge, completing the circuit.
- The lead atoms and ions are too large to desorb from the electrodes.
- A porous separator would prevent the recharging of the battery.
- Both half-cells use the same electrolyte.
A lead-acid battery is a common secondary galvanic cell used in many cars to power electrical systems.
Although the construction of these batteries may differ between manufacturers, they all have a similar set of components. In a lead-acid battery, the anode and cathode are usually made from lead. The lead is shaped in a waffle type pattern which allows another form of lead, known as spongy lead, to fill the spaces.
At the anode, we see an oxidation reaction between lead and sulfuric acid.
At the cathode, a reduction reaction takes place, which again requires sulfuric acid.
Both of these reactions proceed with the same electrolyte of sulfuric acid. The electrodes do not need to be completely separated from each other, and the electrolyte flows around both sets of electrodes.
From this information, we can see that the correct answer is E; both half cells use the same electrolyte.
Although we consider lead–acid batteries to be rechargeable for most practical purposes, their ability to accept charge is reduced over time and they must eventually be replaced.
During the discharge reaction, as we saw in the overall chemical equation above, lead sulfate is produced. This lead sulfate first forms in an amorphous state and is easily reverted back into lead, lead dioxide, and sulfuric acid when the reaction is reversed by an external current. However, over time, small amounts of this lead sulfate turn permanently into a more stable crystalline form reducing the amount of material that can take part in the redox reaction.
The extent to which this has happened in a used car battery can be measured indirectly using a device known as a hydrometer.
A hydrometer measures the relative density of liquids and it typically has a scale that can measure the specific gravity of a substance such as sulfuric acid.
As previously stated, different batteries will have different values; but as an example, a completely charged lead–acid battery may have a density of 1.28 g/cm3 to 1.30 g/cm3. If the density of the acid in that battery were to drop lower than 1.20 g/cm3, then that would indicate that the battery would need to be recharged.
The following picture shows how a hydrometer can be used to measure the relative density of sulfuric acid in a car battery.
Example 2: Identifying When and How to Measure the Charge in a Lead–Acid Battery
Two slightly used replacement lead–acid batteries are compared before being installed into a vintage automobile. Battery A is found to have a specific gravity of 1.25 and battery B has a specific gravity of 1.19.
- What is the name of the scientific instrument
used to measure the specific gravity of the sulfuric acid in the
lead–acid batteries used in cars?
- Which battery is closest to being fully charged?
- Battery A
- Battery B
- In which battery is the concentration of sulfuric acid lower?
- Battery B
- Battery A
We need a specific instrument to measure the specific gravity of the sulfuric acid in a lead–acid battery. A hydrometer measures the relative density of liquids and has a specific gravity scale for this particular use.
For the sake of completeness, eudiometers measure gas volume, gravimeters measure gravity, manometers measure air pressure, and dynamometers measure torque or force.
When a battery is closer to being fully charged, it will have a greater specific gravity as there is a higher concentration of sulfuric acid. There is a high concentration of sulfuric acid because the discharge reaction of a lead–acid battery involves lead, lead(IV) oxide, and sulfuric acid reacting together. As such, battery A, and answer A, is the correct choice.
As discussed, if a battery has lower specific gravity, more sulfuric acid will have been used; and as such, the concentration will be less, and so battery B, and answer A, is the correct choice for this part of the question.
Direct electric current is applied across the two poles of a lead–acid battery when it is recharged by an external battery charger. The voltage applied must be slightly higher than the voltage that the battery is capable of producing when operating as a galvanic cell. This has the effect of forcing the chemical reaction to take place in the opposite direction, as can be seen in the following equation:
The lead at the anode, the lead(IV) oxide at the cathode, and the aqueous sulfuric acid electrolyte are renewed. The same overall effect of recharging is achieved by the alternator, also known in some countries as the dynamo, which recharges the battery little by little during motion through the same mechanisms.
The lithium-ion battery is the next secondary cell that we will consider in this explainer.
This common rechargeable battery is found in many portable electronics such as cell phones and laptop computers.
There are many different types of lithium-ion batteries. Most of these lithium-ion batteries are relatively light, have long life spans, and can store large amounts of energy.
The cathodes in lithium-ion batteries are often made from the lithium cobalt oxide () compound. The anode is made from lithium graphite (). A plastic isolator separates the two electrodes, but it does not restrict the movement of the lithium ions. The electrolyte is liquid lithium hexafluorophosphate (), which coats the two electrodes and the separator.
As with all secondary cells, the polarity of the electrodes reverses depending on whether the cell is operating as a galvanic cell and discharging or acting as an electrolytic cell and recharging.
The structure of a typical lithium-ion battery is shown in the following figure. The figure can also be used to understand the direction of ion and electron movement during discharge.
The following reactions take place during discharge. Lithium graphite separates out into graphite, lithium ions, and electrons at the anode:
Lithium cobalt oxide is formed at the cathode:
This gives an overall reaction for the discharging of a lithium-ion battery of
The opposite reactions occur during the recharging process.
Graphite and lithium ions reform the lithium graphite compound at the cathode:
Lithium cobalt oxide is broken down into at the anode and this reaction liberates ions and electrons:
The overall chemical reaction for the recharging of a lithium-ion battery is
Although voltages will vary depending on the exact manufacture of the cell, a typical electromotive force (emf) for a lithium-ion battery would be +3 V.
Example 3: Describing the Movement of Lithium Ions When Lithium-Ion Batteries Discharge
Which of the following statements correctly describes the movement of lithium ions during discharge of a lithium-ion battery?
- From the positive graphite electrode to the negative electrode where they form a lithium compound
- From the negative electrode as part of a lithium compound to the positive graphite electrode
- From the positive electrode as part of a lithium compound to the negative graphite electrode
- From the negative graphite electrode to the positive electrode where they form a lithium compound
The reactions that occur inside a lithium-ion battery are reversible reactions whose direction depends upon whether or not the battery is discharging or being recharged.
During the discharge process, lithium graphite is separated into graphite, lithium ions, and electrons:
The electrons travel through the circuit to the cathode, where they combine with cobalt oxide and lithium ions to form lithium cobalt oxide:
During discharge, the battery acts as a galvanic cell with the lithium ions starting at the negative graphite anode and ending up at the positive cobalt oxide cathode.
Consequently, the correct answer is D.
Let us summarize what has been learned in this explainer.
- Secondary cells are different from primary cells due to the fact that they can be recharged.
- When a secondary cell is discharging, it is acting as a galvanic cell.
- When a secondary cell is recharging, it is acting as an electrolytic cell.
- A lead–acid accumulator battery consists of metal electrodes containing spongy lead in the case of the anode and lead(IV) dioxide in the case of the cathode.
- The electrolyte used in the lead–acid battery is sulfuric acid.
- The overall reaction that occurs during the discharge of a lead–acid battery is however, this reaction is reversed during recharging.
- Hydrometers are used to measure the specific gravity of the sulfuric acid inside lead–acid batteries to determine the remaining charge.
- Lithium-ion batteries are a common form of secondary cells found in cell phones and laptops.
- During discharge, lithium graphite separates into lithium ions and graphite to provide a source of ions.
- The cathode in a lithium-ion battery is often lithium cobalt oxide, .
- The overall reaction for a typical lithium-ion battery during discharge is this reaction is reversed during recharging.