Question

Describe the chemical nature of the electrodes in (a) a nickel-metal-hydride battery and (b) a lithium ion battery.

Short answer

NiMH batteries use a hydrogen-absorbing alloy for the anode and NiOOH for the cathode, whereas lithium-ion batteries use graphite for the anode and a lithium metal oxide for the cathode.

Step-by-step solution

Nickel-Metal-Hydride Battery - Anode Composition

In a nickel-metal-hydride (NiMH) battery, the anode (negative electrode) is typically made from a hydrogen-absorbing alloy. This alloy is capable of storing hydrogen ions, which is crucial for the battery's operation.

Nickel-Metal-Hydride Battery - Cathode Composition

The cathode (positive electrode) in a NiMH battery is typically composed of nickel oxyhydroxide (NiOOH). When the battery discharges, the nickel oxyhydroxide undergoes a reduction reaction.

Lithium-Ion Battery - Anode Composition

In a lithium-ion battery, the anode generally consists of a form of carbon, most commonly graphite. This material allows for the intercalation and deintercalation of lithium ions during charge and discharge cycles.

Lithium-Ion Battery - Cathode Composition

The cathode in a lithium-ion battery is often made from a lithium metal oxide, such as lithium cobalt oxide (LiCoO2), although other materials like lithium manganese oxide (LiMn2O4) and lithium iron phosphate (LiFePO4) can also be used.

Key concepts

Nickel-Metal-Hydride Battery

A nickel-metal-hydride (NiMH) battery is a type of rechargeable battery that uses a hydrogen-absorbing alloy as its anode and nickel oxyhydroxide as its cathode. These batteries are known for their ability to store and deliver a significant amount of energy, making them suitable for a variety of applications, from hybrid vehicles to portable electronics. - **Anode Composition**: The negative electrode, or anode, of a NiMH battery is typically made from a hydrogen-absorbing alloy. This alloy acts as the hydrogen storage medium. During the charging process, it absorbs hydrogen ions, which are then released during discharging to generate electrical power. - **Cathode Composition**: The positive electrode, or cathode, in a NiMH battery consists of nickel oxyhydroxide (NiOOH). During the discharge process, nickel oxyhydroxide accepts electrons through a reduction reaction, which is a key part of the battery's operation.

Lithium-Ion Battery

Lithium-ion batteries are among the most popular types of rechargeable batteries used in portable electronics and electric vehicles. They are well-regarded for their high energy density, lightweight design, and ability to retain charge over long cycles. - **Anode Composition**: The anode in a lithium-ion battery usually comprises a form of carbon, with graphite being the most common. This carbon structure allows lithium ions to intercalate—that is, insert themselves in the layers of graphite during charging—and deintercalate during discharging. - **Cathode Composition**: The cathode of a lithium-ion battery is made from lithium metal oxides. Common materials include lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4). These compounds enable the movement of lithium ions between electrodes, which is central to the battery's function.

Electrode Composition

The composition of electrodes in batteries is pivotal to their performance and characteristics. In electrochemistry, electrodes are the sites where chemical reactions take place, allowing for the conversion of chemical energy into electrical energy and vice versa. - **Anodes**: Typically, anodes are made from materials that can easily release or absorb charged particles. For NiMH batteries, hydrogen-absorbing alloys fulfill this role, while graphite is commonly used in lithium-ion batteries. - **Cathodes**: For a battery to function efficiently, its cathode must be able to accept electrons during discharge, facilitating a reduction reaction. This is why materials such as nickel oxyhydroxide and lithium metal oxides are common choices for cathodes in their respective battery types. The selection of materials for electrodes depends on desired characteristics like energy capacity, longevity, and safety.

Reduction Reaction

Reduction reactions are a fundamental concept in electrochemistry, playing a critical role in the operation of batteries. A reduction reaction involves the gain of electrons by a molecule, ion, or atom. In the context of batteries: - **Nickel-Metal-Hydride Batteries**: During discharge, the nickel oxyhydroxide cathode undergoes a reduction reaction. This involves the acceptance of electrons, which is essential for the flow of electricity and the conversion of chemical energy into usable power. - **Lithium-Ion Batteries**: The lithium-ion battery also utilizes reduction reactions at the cathode. As lithium ions move towards the cathode, they gain electrons, completing the circuit and enabling the battery to provide energy. Understanding reduction reactions helps in grasping how batteries operate at a chemical level, explaining how they can efficiently store and release energy.