Lithium Cells (AQA A-Level Chemistry): Revision Notes

5.4.5 Lithium Cells

Introduction to Lithium Cells

Lithium cells, commonly found in devices like phones and laptops, offer high energy density and rechargeability, making them ideal for portable electronics. A typical lithium-ion cell consists of a lithium cobalt oxide ($LiCoO₂$) electrode as the positive electrode and a graphite electrode as the negative electrode, with a lithium salt electrolyte in an organic solvent.

How Lithium Cells Generate Current

In a lithium cell, the movement of lithium ions ($\text{Li}^+$) between the electrodes creates a flow of electrons through an external circuit, generating an electric current.

Negative Electrode (Anode):

During discharge, lithium atoms in the graphite electrode release electrons and are oxidised to lithium ions:

$$\text{Li} \rightarrow \text{Li}^+ + e^-$$

Positive Electrode (Cathode):

The lithium ions migrate through the electrolyte to the positive electrode, where they combine with electrons and cobalt oxide:

$$\text{Li}^+ + \text{CoO}_2 + e^- \rightarrow \text{Li}[\text{CoO}_2]^-$$

Rechargeable vs. Non-Rechargeable Cells

In rechargeable lithium-ion cells, the reactions are reversible:

• Discharge: Lithium moves from the graphite (negative electrode) to the cobalt oxide (positive electrode).
• Recharge: Applying an external current reverses the reaction, moving lithium ions back into the graphite electrode, effectively "storing" energy. In non-rechargeable lithium cells, such as lithium primary batteries, the electrode materials are consumed and cannot be regenerated, making them single-use.

Practical Use of Electrode Reactions to Predict Cell Behavior

In lithium-ion cells, examining the electrode potentials of each half-cell reaction allows us to determine the feasibility of the overall reaction and predict the direction in which the reaction will proceed.

Here's a breakdown of how to use electrode potentials to predict and understand the behaviour of a lithium-ion cell.

Example: Predicting the Behaviour of a Lithium-Ion Cell

A typical lithium-ion cell consists of:

• Negative Electrode (Anode): Graphite intercalated with lithium, where lithium is oxidised.
• Positive Electrode (Cathode): Lithium cobalt oxide ($\text{LiCoO}_2$), where lithium ions are reduced. The half-equations for the reactions at each electrode are as follows:

Anode (Oxidation):

$$\text{Li} \rightarrow \text{Li}^+ + e^- \quad \text{(E}^\theta = :highlight[-3.04 \, \text{V}])$$

Here, lithium is oxidised, releasing electrons into the external circuit and producing lithium ions.

Cathode (Reduction):

$$\text{Li}^+ + \text{CoO}_2 + e^- \rightarrow \text{Li}[\text{CoO}_2] \quad \text{(E}^\theta = :highlight[+0.50 \, \text{V}])$$

At the cathode, lithium ions combine with electrons and cobalt oxide to form lithium cobalt oxide.

Calculating the Cell Potential

To predict if the overall reaction is feasible, calculate the cell potential ($E^\theta_{\text{cell}}$) by subtracting the anode's $E^\theta$ from the cathode's $E^\theta$

$$E^\theta_{\text{cell}} = E^\theta_{\text{cathode}} - E^\theta_{\text{anode}}$$

Substituting in the values:

$$E^\theta_{\text{cell}} = (+0.50 \, \text{V}) - (-3.04 \, \text{V}) = :highlight[+3.54 \, \text{V}]$$

Since the overall cell potential is positive (+3.54 V), the reaction is thermodynamically favourable and will proceed spontaneously in the direction of lithium oxidation at the anode and lithium-ion reduction at the cathode. This positive cell potential means that the cell can produce an electric current as it discharges.

Interpretation of Cell Potential in Practice

• Discharge: During discharge, lithium is oxidised at the anode, releasing electrons to the external circuit. These electrons travel to the cathode, where they reduce lithium ions, forming lithium cobalt oxide.
• Recharge: When the cell is recharged, the external current forces the reverse reaction, moving lithium ions back to the anode. This reversibility is key to the functionality of lithium-ion cells as they can undergo multiple charge-discharge cycles. By examining the electrode potentials, you can predict that the lithium cell will produce a stable voltage of approximately 3.54 V during discharge. This high and stable voltage is why lithium-ion cells are ideal for portable electronic devices, where a reliable and efficient power source is essential.