'Intercalation-type electrodes' are the most commonly used electrodes in Li-ion batteries. They function via a process known as intercalation, where lithium ions are inserted or removed from the layers of the electrode materials during charging or discharging cycles. For instance, a popular anode material, graphite, can intercalate lithium ions into its layered structure, forming lithium intercalated graphite (LiC6) when the battery is charged.
While these electrodes are excellent in terms of longevity and efficiency, their energy storage capacity, or theoretical capacity, unfortunately hits a limit. This is where the 'Conversion-type electrodes' come into play.
✔️Conversion-Type Electrodes:
Conversion-type electrodes do not store lithium ions in a layered crystal structure. Instead, they house a much more dynamic process called conversion reactions. In these reactions, lithium ions react with the electrode material to form entirely new compounds. During a conversion reaction, electrode materials, such as metal fluorides (like FeF3) and metal oxides (like Fe3O4), react with lithium ions to create entirely new compounds.
One of the greatest advantages of conversion-type electrodes is their incredibly high theoretical capacity. The Fe3O4 electrode, as an example, has a theoretical capacity of about 926 mAh/g—almost 2.5 times greater than that of graphite.
However, like all innovative technologies, conversion-type electrodes have their share of challenges. The conversion reactions often lead to substantial volume changes in the electrode material, causing mechanical stress that may degrade the electrode over time, shortening its life cycle. Plus, conversion reactions are typically slower than intercalation reactions, which can limit the battery's charge and discharge rates. Moreover, the formation of a solid-electrolyte interphase (SEI) layer can be a significant issue for conversion-type electrodes. This layer forms on the electrode's surface and can grow thicker with each cycle, thereby increasing the battery's internal resistance and reducing its efficiency.
Despite these challenges, conversion-type electrodes are an exciting area of research, holding great potential for boosting the energy density of Li-ion batteries. By using nanostructured materials, innovative coating techniques, and new electrolyte compositions, researchers are working tirelessly to improve the life cycle, rate capability, and efficiency of these electrodes.