Basic electro-chemical processes such as using redox reactions to create a flow of electrons are the basis for how batteries work. Most batteries or cells are based on the galvanic cell. Good examples of batteries based on galvanic cells are dry cell batteries commonly used in flashlights and transistor radios; lead storage batteries which are your car batteries; and lithium-ion batteries normally found in cell phones, digital cameras, laptops, and electric vehicles. Galvanic cells contain cathodes and anodes with some form of an electron salt bridge. The cathode is negatively charged, where reduction occurs, and where electrons are gained. The opposite end of the spectrum on the battery is the anode which is positively charged and where electrons are lost. It should be noted that this is when the cell is operating. Salt bridges help facilitate the longevity of the battery and complete the circuit for the flow of electrons. Without the salt bridge electrons would not flow from cation to anion since the circuit would not be complete and a buildup of residue that collects from using the battery would render it useless as no charge would be created.
It clearly outlines the key concepts of electrochemistry involved in their operation. Here’s a breakdown of the main points:
1. Electrochemical Reactions and Galvanic Cells:
- Batteries rely on redox reactions, where one element loses electrons (oxidation) and another gains them (reduction).
- This electron flow creates electricity in galvanic cells.
- Examples of batteries based on galvanic cells include dry cells, car batteries, and lithium-ion batteries.
2. Cathode, Anode, and Electrolyte:
- The cathode (negative) attracts electrons (reduction).
- The anode (positive) loses electrons (oxidation).
- The electrolyte conducts electricity within the battery, replacing the historical salt bridge.
3. Voltage and Standard Reduction Potentials:
- The voltage of a battery is determined by the difference in electrical potential between the anode and cathode, measured in volts.
- Standard reduction potentials indicate an element’s tendency to gain or lose electrons.
- Elements with high reduction potentials are good cathode materials, while those with low reduction potentials are good anode materials.
- The bigger the difference between the cathode and anode’s reduction potentials, the higher the voltage of the battery.
The Science Behind Batteries: Basics of Electrochemistry
Dry Cells
Dry cell batteries also known as the Leclanché cell are commonly used in devices such as flashlights. These batteries contain no fluid in them, hence the name dry cell. Dry cells have a graphite cathode rod in the center and a zinc anode. These are both in contact with a mixture of manganese dioxide MnO2 and carbon which contact the outside of the rod and zinc plate to help with the flow of electrons. However, as mentioned earlier batteries need to have some form of electron carrier, whether it be a salt bridge or electrolyte solution of some form. In these cells, a pasty-like substance is used which contains electrolytes, usually composed of zinc (II) chloride ZnCl2 and ammonium chloride NH4Cl. This substance is used instead of a typical aqueous, dissolved in water, electrolyte because it creates a safer and less likely to leak battery. The battery then has a thin paper spacer around the paste, cathode, and anode and is then finally enclosed in a protective coating composed of some form of steel. Generally, these cells produce around 1.5 volts of energy.
The Science Behind Batteries: Basics of Electrochemistry
The Science Behind Batteries: Basics of Electrochemistry
Lead Acid Storage Cells
Commonly seen in vehicles the lead storage battery is made up of six identical cells all joined together in series. Every one of the six cells is composed of a lead anode and a cathode made of lead dioxide PbO2 and both are on a metal plate that is in a sulfuric acid solution. The sulfuric acid acts as the electrolyte in this cell. If you look at your car battery these are enclosed in a plastic case. Lead storage cells output about 2.1 V per cell so in total the battery outputs around 12.6 V of energy used for starting your vehicle and running the other electronic components including but not limited to your radio. The interesting and useful part of lead cell batteries is that they can be recharged. Recharging batteries means that you run the battery through the reverse process that it normally goes through instead of the battery outputting energy, energy is inputted into the battery. This process is also called electrolysis.
The Science Behind Batteries: Basics of Electrochemistry
Lithium-Ion Cells
- Powering Everyday Devices: Lithium-ion batteries are dominant in portable electronics due to their small size, light weight, and efficient recharging capabilities.
- Components: They consist of a carbon-based anode (often graphite), a transition metal oxide cathode (like cobalt oxide), and a non-aqueous electrolyte in between.
- Electrolyte Function: The electrolyte solution, containing a lithium salt, allows lithium ions to flow freely.
- SEI Layer: A protective layer forms on the electrodes (SEI) due to the lithium salt, preventing uncontrolled electron flow but enabling lithium ion movement for the reaction.
- Cathode & Anode Roles: These depend on charging/discharging. During discharge, lithium ions flow from anode to cathode, generating electricity.
- Lithium Advantage: Lithium’s low reduction potential allows for high voltage output (around 3.4V) in these cells.
- Degradation: Over time, the SEI layer grows, slightly reducing battery capacity.
Here are some key takeaways:
Components and Materials:
- Anode: Made of graphite (carbon-based) which stores lithium ions (Li+) and lithium (Li) atoms.
- Cathode: Made of a transition metal oxide (like cobalt oxide CoO2) that can also store lithium ions.
- Electrolyte: Non-aqueous solution (no water) containing a lithium salt (LiClO4 or LiPF6) that allows lithium ions to flow.
The SEI Layer:
- A thin layer (Solid Electrolyte Interphase) forms on the electrodes during battery operation.
- This layer prevents uncontrolled electron flow but allows lithium ions to pass through, enabling the battery to function.
- Discharging: Lithium ions flow from the anode to the cathode, and electrons flow through the external circuit (powering your device).
- Charging: The process reverses, with lithium ions and electrons moving back to their original positions.
The Science Behind Batteries: Basics of Electrochemistry
Advantages of Lithium-ion Batteries:
- High energy density: Pack a lot of energy in a small and lightweight package.
- Rechargeable: Can be charged and discharged hundreds of times.
- High efficiency: Lose minimal energy during charge/discharge cycles.
The Science Behind Batteries: Basics of Electrochemistry
Limitations:
- SEI layer growth: Over time, the SEI layer can thicken, reducing battery capacity.
- Safety concerns: Lithium is highly reactive, requiring careful design and handling to prevent fires.
The Science Behind Batteries: Basics of Electrochemistry
Overall:
Lithium-ion batteries are a powerful technology due to their combination of high energy density, rechargeability, and efficiency. They are a key enabler for portable electronics in the 21st century.
