Video Transcript
The following reactions occur in at
least one type of mass-manufactured battery. Which equation is related to the
cathodic reaction of a lithium-ion battery? (A) LiC6 solid reacts to form C6
solid plus Li1+ aqueous plus one electron. (B) LiC6 solid plus CoO2 solid
react to form C6 solid plus LiCoO2 solid. (C) Pb solid plus H2SO4 aqueous
react to form PbSO4 solid plus two H+ aqueous plus two electrons. (D) CoO2 solid plus Li1+ aqueous
plus one electron react to form LiCoO2 solid. (E) PbO2 solid plus H2SO4 aqueous
plus two H+ aqueous plus two electrons react to form PbSO4 solid plus two H2O
liquid.
Galvanic cells are electrochemical
cells that produce electrical energy from spontaneous chemical reactions. There are two types of galvanic
cells: primary and secondary. Primary galvanic cells are only for
single use, but secondary galvanic cells can be recharged. Secondary galvanic cells can run as
both a galvanic and electrolytic cell. Examples of batteries that are
secondary galvanic cells include lithium-ion batteries and lead–acid car
batteries. In this question, we need to
determine the chemical reactions that occur inside a lithium-ion battery.
When looking at the reactants and
products in chemical equations (C) and (E), we can recognize that these reactions
occur in lead–acid car batteries, not lithium-ion batteries. So let’s go ahead and eliminate
these two answer choices and clear them from the screen.
Now, we need to determine which of
the remaining answer choices represents the cathodic reaction inside a lithium-ion
battery. Let’s take a look inside a
lithium-ion battery while it’s running as a galvanic cell, which means it is
discharging and powering an external device. In the galvanic cell, the anode is
made of lithium graphite, or LiC6. The cathode is made of lithium
cobalt oxide, or LiCoO2. The electrolyte liquid inside the
cell is lithium hexafluorophosphate, or LiPF6. Knowing this allows us to eliminate
answer choice (A), because the reaction shown involves the anode, not the
cathode.
At the anode inside the battery,
lithium graphite is oxidized, releasing lithium ions and electrons. Electrons flow through the circuit
to power an external device, and lithium ions flow from anode to cathode inside the
cell. At the cathode, the lithium ions
accept electrons and are therefore reduced, as they combine with cobalt oxide to
form lithium cobalt oxide. We can see that equation (D)
represents this reduction reaction at the cathode and must be the correct
answer. However, let’s take a quick look at
equation (B). Equation (B) represents the overall
reaction during discharging, when the galvanic cell is running.
So which equation is related to the
cathodic reaction of a lithium-ion battery? The answer is equation (D). CoO2 solid plus Li1+ aqueous plus
one electron react to form LiCoO2 solid.
