What Is The Energy Transformation Of A Rechargeable Battery?

A rechargeable battery is a type of energy storage device that can be charged, discharged into a load, and recharged multiple times. Rechargeable batteries are commonly used in a wide range of applications including consumer electronics, electric vehicles, grid energy storage, and more. Compared to single-use disposable batteries, rechargeable batteries offer economic and environmental benefits such as cost savings and reduced waste. Some of the most common types of rechargeable batteries include lithium-ion, nickel-cadmium, and lead-acid batteries. The ability to reuse rechargeable batteries hundreds or thousands of times makes them a practical and sustainable way to power portable devices and major systems. Their reusability offers convenience and reliability for powering anything from small gadgets to industrial equipment. With the growth of renewable energy, electric transportation, and energy storage technology, rechargeable batteries are becoming increasingly important in enabling an energy transition away from fossil fuels.

Table of Contents

Chemical Reactions

Batteries store and release energy through chemical reactions. Rechargeable batteries rely on reversible chemical reactions to generate electricity.

During discharge, the chemical reactions inside the battery produce electrons that can flow through an external circuit and power devices. The battery acts as an energy source.

During recharging, an external power source applies a voltage that drives the chemical reactions in reverse. This restores the battery to its original charged state so it can provide power again.

The key components that drive these chemical reactions are the anode, cathode, and electrolyte. The materials used for these components determine the amount of energy that can be stored.

Anode

The anode is the negative electrode of a rechargeable battery. During discharge when the battery is providing power, oxidation reactions occur at the anode. This means the anode gives up electrons, causing the anode material to be oxidized.

For example, in a lithium-ion battery the anode is typically made of graphite or silicon. When the battery discharges, lithium ions intercalate into layers of the graphite or silicon, giving up electrons. The anode reactions can be summarized as:

Anode material + Li+ + e- ⇌ Li-intercalated anode material

During charging when an external power source is applied, the reverse happens – the lithium ions deintercalate from the anode and the electrons are returned. This reduction reaction restores the anode back to its original oxidized state.

Cathode

The cathode is the positive electrode in a rechargeable battery. It is typically made of a metal oxide material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4).

During discharge of the battery, the cathode goes through an oxidation reaction. Positively charged lithium ions migrate from the cathode through the electrolyte and to the anode. This releases electrons which travel through the external circuit, generating electricity to power the device. The cathode material becomes reduced as it loses lithium ions in this process.

On charging, the reaction is reversed. Lithium ions get reduced as they receive electrons from the external power supply and move back into the crystal structure of the cathode. This causes the cathode material to become oxidized again as it regains lithium ions.

Electrolyte

The electrolyte is a chemical medium that allows the flow of ions between the anode and cathode. Inside a rechargeable battery, the electrolyte is a liquid or gel-like substance that contains free ions. These free ions enable the transfer of electrical charge within the battery.

During discharge, the electrolyte provides a pathway for the positive lithium ions to flow from the anode to the cathode. When charging, the electrolyte allows the lithium ions to flow back to the anode. Without the presence of an electrolyte, the battery would not be able to generate electricity through electrochemical reactions.

Common electrolytes used in lithium-ion batteries include ethylene carbonate, dimethyl carbonate, and lithium salts such as lithium hexafluorophosphate (LiPF6). The choice of electrolyte impacts properties like conductivity, stability, and safety. Electrolyte research aims to find solutions that promote optimal ionic mobility while minimizing flammability and toxicity.

Discharging

Discharging a rechargeable battery refers to the process of the battery releasing stored chemical energy as electrical energy. This occurs when a load or external circuit is connected to the battery terminals, allowing electrons to flow from the anode to the cathode through the external circuit.

During discharging, the anode undergoes oxidation, releasing electrons. These electrons flow from the anode, through the external circuit where they do useful work, and back to the cathode. At the cathode, the electrons are accepted by the cathode material, undergoing reduction. To maintain charge neutrality inside the battery, positively charged ions also flow from the anode to the cathode through the electrolyte.

As the battery discharges, the anode and cathode materials are converted to their discharged states. The chemical energy stored in the battery during charging is released as electrical energy to power the connected device. Discharging stops when the anode and cathode are fully converted, and the battery must be recharged before further use.

Charging

Charging a rechargeable battery involves applying an external electrical current that reverses the chemical reaction that occurred during discharge. The lithium ions flow from the cathode back to the anode, re-inserting themselves into the anode material. Meanwhile, electrons are supplied from the charger, flowing from the positive current collector, through the external circuit, and back to the negative current collector. This flow of lithium ions and electrons from the cathode to the anode re-establishes the chemical equilibrium that existed prior to the battery being discharged. The charging process requires energy input equal to the energy that was taken out during discharge, plus a bit more to account for inevitable energy losses during the charging cycle. Rechargeable batteries use different chemical couples that allow this charging/discharging cycle to occur hundreds or thousands of times before significant battery degradation occurs.

Energy Efficiency

Rechargeable batteries undergo energy losses during both discharging and charging, which limits their overall energy efficiency. The main sources of energy loss are:

Internal resistance – As current flows through the battery, internal resistance causes some energy to be dissipated as heat. Minimizing internal resistance improves efficiency.

Self-discharge – When not in use, rechargeable batteries slowly discharge over time due to side chemical reactions. Keeping batteries cool and avoiding deep discharge reduces self-discharge.
Overpotential – Extra voltage beyond the battery’s nominal voltage is required to overcome kinetic barriers during charging and discharging reactions. This overpotential represents an energy loss.
Incomplete reactions – Not all active materials fully participate in the reactions, leaving some material unused. Improving electrode design helps maximize material utilization.

Cycling degradation – Repeated charge/discharge cycles degrade the battery over time, causing lower capacity and energy efficiency.

While improvements in battery chemistry and engineering continue to increase efficiency, losses remain inherent to the electrochemical system. Practical energy efficiency for commercial rechargeable batteries tends to fall in the range of 60-90%.

Battery Materials

The materials used for the electrodes and electrolyte have a major impact on the performance of a rechargeable battery. Here are some of the most common materials used:

Anode Materials

Graphite – This is the most common material used for the anode. It has a low cost and provides good energy density.

Lithium Titanate (LTO) – LTO has a faster charging speed than graphite but lower energy density. It’s more stable and safer.

Silicon – Silicon has a super high capacity for lithium ions, allowing it to store much more energy. However, it suffers from swelling and cracking.

Cathode Materials

Lithium Cobalt Oxide (LCO) – This is a traditional cathode material with good energy density but safety risks at high voltages.

Lithium Iron Phosphate (LFP) – LFP cathodes have lower energy density but better power capability, stability and safety.

Lithium Nickel Manganese Cobalt Oxide (NMC) – NMC offers high energy density and is common in electric vehicles, but has safety concerns.

Lithium Nickel Cobalt Aluminum Oxide (NCA) – NCA provides very high capacity and energy density but requires safety circuits to prevent overheating.

Conclusion

In summary, the energy transformation that takes place in a rechargeable battery involves chemical reactions between the anode, cathode, and electrolyte. During discharge, the anode is oxidized, releasing electrons that travel through the external circuit to generate electricity. The electrons are accepted by the cathode, which undergoes reduction. The electrolyte allows ions to flow between the electrodes to balance the charge. This process is reversed during charging, using electrical energy to drive the chemical reaction back to the original state.

Looking forward, improvements in rechargeable battery technology will focus on boosting energy density to increase runtime, improving safety, lowering cost, and extending cycle life. Potential advances include new battery chemistries, nanostructured materials, solid electrolytes, and integrated battery management systems. With ongoing research and innovation, rechargeable batteries will continue playing a key role in energy storage for portable electronics, electric vehicles, grid storage, and other applications.