Conversion

There are three main ways battery electrode materials store energy: intercalation, conversion, and alloying. Intercalation is the most widely used mechanism today as it forms the basis of most conventional lithium-ion batteries. Conversion stores energy in a fundamentally different way and has different implications on cycle life, energy density, and rate capability. 

Unlike intercalation where the working ion reversibly inserts into available spaces in the electrode structure without significant change in the host material, conversion involves the active material chemically transforming into new compounds during charge and discharge. 

Where Does the Energy Come From?

Every chemical compound has a certain amount of stored chemical energy. Some compounds are more stable than others and naturally prefer to exist in a lower-energy state. A conversion battery works by taking advantage of the energy difference between these states.

During discharge, the reactants are converted into new products that are more thermodynamically stable. Because the products are at a lower energy state than the original materials, energy must be released as the reaction proceeds. The battery captures this energy and delivers it as electrical work to an external circuit.

During charge, electrical energy is supplied and reverses the reaction. This converts the stable discharge products back into the original materials, effectively pushing the system to a higher-energy state. The energy supplied during charging is thus stored as chemical potential energy within the battery.

To achieve this, conversion reactions involve breaking existing chemical bonds, forming new ones, and often creating entirely new phases. The amount of energy that can be stored or released is determined by the difference in Gibbs free energy between the charged and discharged states. A larger free-energy difference generally means more electrical energy can be delivered by the battery.

Real Life Examples 

Lithium-Sulfur Battery

A well known example of conversion chemistry is the lithium-sulfur battery. The cathode is made of elemental sulfur, and the anode is typically lithium metal. During discharge, the lithium metal at the anode is oxidized, producing electrons and lithium ions. The electrons travel through the external circuit and the lithium ions travel through the electrolyte towards the cathode.

At the cathode, unlike in intercalation where the lithium ions would insert into available sites in the electrode material, sulfur chemically reacts with the lithium ions and electrons. It goes through a series of intermediate compounds called lithium polysulfides, and eventually forms lithium sulfide (Li₂S). Lithium sulfide is a more thermodynamically stable state than the original sulfur and lithium metal. The battery converts this decrease in free energy into useful electrical work.

During charge, lithium sulfide goes through various intermediate states and is transformed back to elemental sulfur and lithium metal which are higher free energy states. 

Metal-Air Battery

Another example of where conversion is used is in metal-air batteries. These batteries use a metal anode (such as iron, aluminium, or zinc), and use oxygen from the air as a reactant. During discharge, the metal reacts with oxygen to form metal oxides or metal hydroxides, which are more thermodynamically stable than the pure metal and oxygen on their own. As the system moves to this lower-energy state, energy is released. 

During charge, the metal oxide is reduced back to pure metal which is at a higher free energy state. Just like in lithium-sulfur batteries, the stored energy comes from the free-energy difference between the charged and discharged materials.

Why Conversion?

Conversion chemistries are attractive because they can achieve higher theoretical energy densities than most intercalation-based chemistries. Energy density is determined by two factors:

Where: E = energy density, C = capacity, V = voltage

While conversion-based batteries do not necessarily have higher voltages than intercalation-based batteries, they often have higher capacities. Battery capacity is fundamentally determined by how much charge can be stored, which depends on the number of electrons transferred during the reaction. Theoretical capacity is given by:

Where: n = number of electrons transferred per formula unit, M = molecular weight of the active material

Many conversion reactions involve large changes in oxidation state, which allows multiple electrons to be transferred per active atom, meaning more charge can be stored per unit mass. This results in higher theoretical capacities. 

Why Not Conversion?

1. Volume expansion: Because new compounds are constantly being formed and broken apart, conversion materials undergo large volume changes during cycling. Volume changes can:

   • Crack the active material causing loss of active material 

   • Disconnect conductive pathways which reduces electrical conductivity

   • Damage the electrode structure which accelerates capacity loss over time

As a result, cycle life is typically significantly worse than in intercalation-based batteries. 

2. Slow kinetics: Conversion reactions involve bond breaking, bond formation, and phase transformations. These processes generally have slower kinetics than intercalation, which limits how quickly the battery can charge and discharge. The slow kinetics often results in poorer rate capability than intercalation-based batteries. 

3. Voltage Hysteresis: The forward and reverse pathways in conversion are often not identical. Breaking and reforming compounds requires overcoming significant kinetic barriers associated with charge transfer, mass transport, and phase transformations. As a result, the battery often charges at a higher voltage than it discharges. The difference between the charge and discharge voltage is known as voltage hysteresis. Large voltage hysteresis reduces round-trip efficiency because energy supplied during charging is lost as heat rather than being recovered during discharge.

In Conclusion

Conversion-based batteries store energy by chemically transforming the active material into entirely new compounds, and have the attractive quality of higher energy densities. However, this mechanism also introduces practical challenges that come with repeatedly breaking and reforming chemical bonds like volume expansion, slow kinetics, voltage hysteresis, and shorter cycle life. Conversion materials continue to be an active area of battery research as they promise enormous energy storage potential. 

Read more about intercalation here