Intercalation

Intercalation is a process in which an ion or molecule is reversibly inserted into an interstitial site of a host. This is one of the three main ways batteries store energy, and its reversibility is what makes intercalation-based batteries rechargeable.

How does it work?

When an intercalation-type battery is charged, electrical energy oxidizes the cathode material, driving the release of cations which move through the electrolyte towards the anode. The movement of the ions occurs in tandem with the movement of electrons through an external circuit. 

At the anode, the cations are simultaneously reduced at the electrode-electrolyte surface and inserted into vacant sites in the electrode crystal lattice. The lattice structure undergoes a slight change to accommodate the ions.

As a cation moves into the host lattice, its free energy decreases, and the energy difference is stored as chemical energy in the electrode. This energy is associated with the interactions between the ion and the host, which hold the ion securely in place.

On discharge, spontaneous redox reaction at the anode surface releases cations, and the stored chemical energy is converted to electrical energy through the movement of electrons in the external circuit. 

Key considerations for Intercalation (using Li ion batteries as a reference)

• Electrode potential: If the electrode potential drops too close to 0 V vs Li/Li⁺, lithium plating becomes more favorable than intercalation. In this case,  ions are reduced directly to metallic lithium on the electrode surface instead of inserting into the lattice.

• Available intercalation sites: If there aren’t enough vacant sites, ions accumulate at the electrode-electrolyte interface. This causes a drop in local potential and increases the risk of Li plating.

• Li ion flux: If the rate at which Li ions arrive at the electrode-electrolyte interface is too high, intercalation kinetics will be unable to keep up with the Li ion arrival. This (similar to the case of inadequate intercalation sites) results in an increase in the local Li ion concentration, dropping the electrode potential, and increasing the likelihood of Li plating. If the rate is too low, intercalation becomes limited by ion transport, leading to poor rate performance.

---