How Do Rechargeable Batteries Work?

Batteries can be recharged by plugging them into a power source, which initiates electrochemical reactions causing charged species within the battery to return to the anode. As a result, the battery’s potential energy is restored. It requires battery materials that are able to undergo redox reactions repeatedly. The rechargeable nature of a battery allows it to be used multiple times to power electronic devices and more. They are also referred to as secondary batteries, as opposed to primary batteries, which are only single use. The batteries found in phones, laptops and smart watches are all rechargeable, allowing us to reuse them even after they have completely discharged.

Working Principle of a Rechargeable Battery

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A rechargeable battery is able to charge and discharge multiple times. However, they still have a limited capacity to cycle between the two states, known as the lifecycle. The lifecycle of a rechargeable battery is based on how many times the battery can be fully recharged until the capacity of the battery drops to below 80% of its original capacity. This is when the battery capacity becomes noticeably bad to the consumer and may also lead to safety risks due to battery degradation. The rechargeability of a secondary battery stem from the reversibility of its redox reactions.

The components of a rechargeable battery are the same as a single use battery: electrodes, electrolyte, separator and battery management system. The cathode material is typically the source of positively charge particles such as ions or molecules.

Redox Reactions

Batteries take advantage of redox reactions which facilitates the transfer of electrons from one species to another. The movement of a negative electron through the external circuit is what allows us to access charge from a battery.

Let’s consider an example cathode active material such as lithium iron phosphate powder – LiFePO₄:

LiFePO₄ ↔ Li⁺ + e^- + FePO₄

Lithium iron phosphate exists in different states depending on whether it is charged or discharged. During charging, lithium ions leave the cathode via deintercalation. This causes the iron atoms to undergo oxidation where they release an electron and transition from Fe²⁺ to Fe³⁺:

LiFePO₄ → Li⁺ + e^- + FePO₄

During the discharge process when the battery is being used, lithium ions move back towards the cathode, causing the iron atoms to undergo a reduction reaction. As a result, the iron atoms transition from Fe³⁺ to Fe²⁺:

Li⁺ + e^- + FePO₄ → LiFePO₄

This cathode material and others repeat this chemical process which allows the battery to be recharged.

Example of a Rechargeable Battery

An LFP battery consists of an aluminum foil coated with lithium iron phosphate, which serves as the positive cathode. The negative anode is a copper foil coated with graphite. A polymer separator is placed between the electrodes. An electrolyte facilitates the flow of lithium ions between the cathode and anode. The figure shows lithium ions undergoing deintercalation from the cathode to the anode during the charging process.

Limitations to Rechargeable Battery Lifetime

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The capacity of a rechargeable battery can fade overtime. This results from the gradual loss of active lithium, loss of usable electrode surface area and increased internal resistance. A battery’s ability to storage and deliver charge is impacted by the following:

• Overtime the active cathode materials can degrade which can lead to a reduction in capacity to perform these redox reactions.
• The electrolyte inside the battery forms a solid electrolyte interphase (SEI) on the anode which can become unstable and decompose.
• Unwanted side reactions between the electrodes, electrolyte and impurities can produce byproducts that can lead to instability and block transport pathways.
• Exposure to extreme temperatures can also lead to the degradation of battery materials.

Types of Rechargeable Batteries

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Rechargeable batteries, come in a variety of chemistries, each with distinct characteristics, advantages, and limitations. The choice of battery type depends on factors such as energy density, weight, cost, lifespan, and performance under different environmental conditions. This section provides an overview of three major types of rechargeable batteries: lithium-ion batteries, lead-acid batteries, and nickel-based batteries. Each type operates based on different electrochemical principles, with unique active materials and electrolyte compositions that determine their applications and performance.

Lithium-ion Battery

Lithium-ion batteries are the latest rechargeable battery technology and are now widely used across different commercial applications. Typical cathode active materials include lithium containing, lithium cobalt oxide powder, lithium manganese oxide powder and lithium nickel manganese cobalt oxide powder. The lithium ions typically undergo deintercalation from the cathode active material and act as the charged species which allows the external flow of electrons. The anode material is usually graphite or another carbon based conductive material.

Advantages of lithium-ion batteries include high energy density, low self-discharge rate, and lightweight design, making them ideal for compact, portable applications. However, disadvantages include higher cost compared to other battery types, sensitivity to overcharging and overheating, and degradation over repeated charge-discharge cycles, particularly if exposed to high temperatures or deep discharges.

Lead-acid Battery

Lead-acid batteries are well-established and are commonly used in applications requiring large storage capacity, such as vehicle starter batteries and backup power systems. The electrochemical reactions occurring in a lead-acid cell are:

Cathode (reduction): PbO₂ + HSO₄⁻ + 3H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Anode (oxidation): Pb + HSO₄⁻ → PbSO₄ + H⁺ + 2e⁻

Its advantages include low cost, high voltage, and a large capacity for storing potential energy. However, its disadvantages include a relatively high mass, poor performance at low temperatures, and an inability to retain its potential over extended periods of disuse.

Nickel-based Battery

Nickel based batteries include other metals such as zinc, iron or cadmium. Nickel oxide hydroxide (NiO(OH)) is the cathode active material and the other metal is the anode active material. Potassium oxide (KOH) is used as the electrolyte to allow the movement of charged species. Nickel-cadmium batteries rely on the movement on negative OH^- ions, unlike most other batteries that rely on positive cation movement.

Example Nickel-Cadmium electrochemical reactions:

Cathode (reduction): 2NiO(OH) + 2H₂O + 2e⁻ → 2Ni(OH)₂ +2OH⁻

Anode (oxidation): Cd + 2OH^- → Cd(OH)₂ +2e⁻

Advantages of nickel-based batteries include robust performance over a wide temperature range, good cycle life, and high discharge rates, making them suitable for high-power applications. However, disadvantages include relatively low energy density, memory effect issues (especially in older Ni-Cd types), and the environmental concerns associated with toxic materials such as cadmium.

Cathode Active Materials

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