Intercalation

Intercalation is one of the three main ways batteries reversibly store energy. The other two mechanisms are conversion reactions and alloying reactions, which rely on fundamentally different electrochemical processes. In intercalation-based batteries (like lithium-ion), electrode materials are arranged in crystal lattices that naturally contain spaces called interstitial sites. Intercalation is simply the reversible insertion of ions into these interstitial sites of the electrode crystal lattice.

How Does it Work?

When an intercalation-based battery is charged, electrical energy drives an oxidation reaction at  the cathode. This releases electrons and positively charged ions (cations) from the cathode material. The electrons move through the external circuit towards the anode while the cations travel through the electrolyte towards the anode to maintain charge-balance from the electron movement. 

At the anode, the arriving cations are reduced at the electrode-electrolyte interface and inserted into vacant interstitial sites in the electrode’s crystal lattice. To accommodate the inserted ions, the lattice structure expands slightly. This change is ideally small and reversible to enable long cycle life.

When the cations insert into the electrode, they move to a lower-energy state within the host material. The energy difference is stored as chemical energy in the electrode material, specifically, in the interactions between the ion and the host structure which stabilize the ion securely in place.

During discharge, the process reverses: oxidation reactions occur at the anode surface, releasing cations from the electrode lattice. The stored chemical energy is then converted back into electrical energy as the electrons travel through the external circuit to the cathode. 

Key Considerations for Intercalation 

(Using Lithium-ion batteries as a reference point)

• Electrode potential: During charge, if the electrode potential at the anode drops too close to 0V vs Li/Li⁺, lithium plating can become more likely than intercalation. In this case, instead of inserting into the electrode lattice, lithium ions are reduced directly to metallic lithium on the electrode surface. This can create safety risks and cause long term capacity loss.

  
  

  On the cathode side, if the electrode potential gets too high from excessive de-lithiation, it can drive electrolyte oxidation, and accelerate cathode degradation. Essentially, there’s a safe voltage window for intercalation, outside of which side reactions are more likely.

  
  

• Available intercalation sites: Intercalation can only occur if there are vacant sites in the host structure. As the electrode fills up, fewer sites are available, and it becomes harder for incoming lithium ions to insert. When this happens, lithium starts to accumulate at the electrode-electrolyte interface, increasing the local overpotential of the system, and effectively making lithium plating an easier reaction to proceed with.

• Solid-state diffusion: Even if there are available sites, lithium still needs to move from the electrode surface into the bulk of the electrode. This process is called solid-state diffusion. If diffusion is too slow, lithium can’t move deeper into the electrode fast enough, creating concentration gradients within the structure. This effectively makes deeper sites inaccessible on a practical charging timescale. Lithium accumulates at the electrode surface, again, increasing overpotential and making plating more likely.

• Lithium ion flux: Lithium ions also need to travel through the electrolyte to reach the electrode. If they arrive at the electrode-electrolyte interface too quickly, the intercalation reaction kinetics may not be able to keep up. This, once more, leads to a buildup of lithium ions near the interface, making lithium plating more likely. On the other hand, if the ion arrival rate is too slow, intercalation becomes limited by ion transport through the electrolyte, resulting in poor rate performance.