An Introduction to Batteries
Batteries store electricity and provide an easily accessible energy supply. Electricity storage can be achieved in a variety of ways. A simple example is when you rub a plastic rod with fur. The negative charge is transferred in the form of electrons to the rod, storing the charge. Yet, a more efficient method of electrical energy storage uses reduction-oxide reactions, also known as redox reactions. Battery chemistry is based on these redox reactions. A large amount of research has focused on energy storage with the aim of improving battery capacity to meet the ever-increasing need for accessible energy.
How Do Batteries Work?
Most batteries consist of the same components:
- The cathode and anode electrodes enable an output and input of electrical energy.
- The electrolyte facilitates controlled electron flow through the circuit.
- A separator creates mechanical stability and prevents immediate discharge.
- A battery management system (BMS) maintains a safe voltage region to prevent dangerous events such as thermal runaway (when cells rapidly oxidise releasing large amounts of thermal energy).
Storing Electricity
Batteries store electrical energy by transferring it into chemical potential. Typically, batteries work by a process known as intercalation. When a voltage is applied to a battery, the ions stored in the cathode are carried through an electrolyte medium to intercalate with the anode material. A separator moderates the ion flow while keeping the anode and cathode separated to prevent instantaneous discharging.
The type of ion depends on the type of battery, for example lithium-ion batteries have cathode materials that supply lithium ions.
Releasing Electricity
Connecting the battery to a circuit causes the battery to discharge. The circuit allows for a flow of electrons from the anode back towards the cathode, releasing the stored electricity.
The Discharge Rate
Typically referred to as C, the discharge rate affects how quickly charge flows through a circuit. A battery with a discharge rate of 1C can discharge its capacity in 1 hour. The higher the C value the faster the battery can discharge. If a battery holds 1000 mAh but can discharge with a current of 2000mA, then the battery has a C rate of 2C.
The ion mobility and overall capacity determines this value. If the lithium ions in a lithium-ion battery can move through the electrolyte faster, the electrons at the anode can discharge through a circuit faster. Similarly, during charging, the lithium ions can be stored in the anode faster.
Types of Battery
Batteries come in many different types with different chemistries offering different benefits and applications.
Primary and Secondary Batteries
Batteries can be organized into two main categories: primary and secondary. Primary batteries are single-use, while secondary batteries are rechargeable. Both can be further divided into subcategories depending on the battery material.
Primary batteries are typically assembled charged and the lifespan depends on the total chemically reactive material used. Secondary batteries still have a limited lifespan, known as the lifecycle but they have the benefit of being rechargeable. The lifecycle of a secondary battery is based on how many charge and discharge cycles that can occur until the capacity of the battery drops to below 80% of the original capacity.
While primary and secondary batteries have their differences, most consist of the same components.
Battery Applications
Battery applications depend on a range of characteristics, including electrical storage capacity, high power, and lifespan. Different batteries are known by their chemistries and research is highly focused on optimizing and discovering further applications.
Lithium-Ion
The most common type of secondary battery, often used in electrical goods and electrical vehicles (EV). Lithium-ion batteries offer high energy density, long cycle life, and are lightweight.
Alkaline
These are common primary batteries often used to power consumer electronics such as smoke detectors, flashlights, or games and toys. Alkaline batteries are reliable and have long shelf lives. Although, they cannot be recharged and have a low power output. They tend to be made of toxic materials and leakages are potentially dangerous if mishandled.
Lead-Acid
Commonly used in the automotive industry, these secondary batteries are reliable and provide large currents. The ability to offer a high and sudden charge is required for starting car engines, especially in colder climates.
Nickel Cadmium
Once popular, these secondary batteries have been replaced by safer battery technology due to environmental concerns. While they are not as common, nickel cadmium batteries are still used as emergency power as they deliver reliable power over a long lifecycle.
Battery Research
A vast amount of research is ongoing to determine the best chemistry to boost each aspect of a battery. Specifically, there is a large focus on lithium-ion batteries due to their use in EVs. While lithium-ion batteries are the dominant rechargeable batteries, there is potential for sodium-ion batteries. There is a natural abundance of sodium, making it easier and cheaper to obtain than lithium.
Improving Battery Storage
The specific capacity of a battery is defined as how much charge can be stored per unit mass: mAh/g. The theoretical capacity can be calculated:
Where Q is the specific charge, n is the number of electrons transferred per mole of reaction, F is Faraday's constant, and Mr is the molecular mass.
The experimentally produced storage capacities are far inferior to the theoretically calculated. This is due to imperfections and defects in the purity of the chemistry. For example, LiCoO2 has a theoretical capacity of 274 mAh/g but only an experimental capacity of 165 mAh/g.
Battery research has a strong focus on improving the capacity of batteries. This progress can be achieved by using different chemistries to take advantage of higher theoretical capacities, or by improving current technology to increase experimental values.
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