Intercalation

A breakdown of how batteries reversibly store energy inside electrode crystal structures

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Intercalation is a process where an ion or molecule is reversibly inserted into the spaces within an electrode (host) material’s crystal structure. These spaces are called interstitial sites. Intercalation is one of the three main ways batteries store energy, and its reversibility is what allows intercalation-based batteries to be rechargeable. The other two mechanisms are conversion reactions and alloying reactions, which store energy through different electrochemical processes.

How does it work?

When an intercalation-type battery is charged, electrical energy drives an oxidation reaction at  the cathode. This releases positively charged ions (cations), which travel through the electrolyte towards the anode. At the same time, electrons move through the external circuit towards the anode, balancing the charge from the ion movement through the electrolyte. 

At the anode, the arriving cations are reduced at the electrode-electrolyte intreface 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 allow the repeated host and release of ions during cycling. 

As a cation moves into the host lattice, it is reduced and its free energy decreases. The energy difference is stored as chemical energy in the electrode material, specifically, in the interactions between the ion and the host structure that 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 Li-ion batteries as an example):

• Electrode potential: If the electrode potential drops too close to 0 V vs Li/Li⁺, lithium plating becomes more favourable 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

  
  

• Available intercalation sites: If there aren’t enough vacant sites in the host structure for intercalation, arriving ions will accumulate at the electrode-electrolyte interface. This build up lowers the local potential of the system according to the Nernst equation, and increases the likelihood of Li plating.

  
  

• Li-ion flux: If Li-ions arrive at the electrode-electrolyte interface too quickly, the intercalation reaction kinetics may not be able to keep up. This leads to a buildup of lithium ions near the interface, again, lowering the local potential and increasing the likelihood of Li plating. 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.