See into the SEI

Understand what the Solid Electrolyte Interface (SEI) is, it's function, characteristics, and the role it plays in achieving high performance in Li-ion batteries.

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The solid electrolyte interphase (SEI) is a thin passivation layer that forms on the anode during the first few charge–discharge cycles of a battery. Structurally, the SEI typically consists of two sub-layers: an outer porous organic layer, and an inner dense inorganic layer.

While SEI formation can be tricky, a well-formed SEI is essential for long-term cycling stability and reduced calendar aging of lithium-ion batteries.

How is the SEI formed?

SEI is created from decomposed electrolyte components. During initial battery cycles, when the anode potential drops below the electrolyte reduction potential (usually <1.0 V vs. Li/Li⁺), electrons from the anode reduce electrolyte molecules at the interface. Lithium ions from the electrode or electrolyte then combine with these reduced species, forming stable compounds such as Li₂CO₃, LiF, and RCO₂Li [2].

Growth mechanism

The SEI grows in two stages:

1. Kinetics-limited stage: This is the initial growth stage where the rate of formation depends on how quickly the electrolyte reacts with electrons from the anode.

2. Diffusion-limited stage: In this stage, SEI growth is dependent on how fast the electrons, electrolyte components, and Li ions can diffuse from the electrode and electrolyte to react. As the SEI gets thicker over time and diffusion gets more and more impeded, its growth rate reduces. 

   
   

The initial quick growth followed by the slow down of diffusion with increasing SEI thickness results in SEI growth following a logarithmic time dependence.

What Makes an Ideal SEI?

A properly formed SEI is crucial for battery performance and safety. The most desirable characteristics are:

1. Ionically conductive and electrically insulating

   Ionic conductivity in the SEI is necessary for lithium ions to migrate freely during charge and discharge. Poor ionic conductivity increases impedance, reduces power capability, and worsens rate performance.

   
   

   Electrical resistivity is paramount in preventing further electrolyte decomposition. If electrons leak through the SEI from the anode, continuous electrolyte reduction occurs, causing SEI thickening, loss of lithium inventory, electrolyte depletion, reduction in coulombic efficiency, and gas generation.

   
   

2. Thin and porous

   Thinness minimizes ionic impedance. The SEI is inherently less conductive than the bulk electrolyte so thicker SEI increases impedance.

   Porosity is needed for the SEI to facilitate ion movement. Increasingly dense SEI creates a tortuous path that impedes ion movement to the electrode. 

Increased impedance raises the cell’s overpotential, resulting in higher charge voltages and lower discharge voltages. This manifests in lower power output and reduced battery efficiency.

3. Uniform

Non-uniform SEI results in lower resistance spots which Li-ions would preferentially travel through. When this happens, Li-ion flux along the length of the electrode is uneven and there is a buildup of ions at particular spots on the electrode. At these localized spots, the surface potential drops and results in lithium plating since at lower potentials, it is more kinetically favorable to plate metallic lithium than intercalate lithium. This could result in dendrite formation and affect the safety and lifespan of the battery. 

4. Mechanically and chemically stable

A good SEI should remain intact over repeated cycling. When SEI breaks down or cracks, pathways open up to reveal new un-passivated areas of the electrode. Without the electrically insulating layer, electrons  are able to move through and reduce more electrolyte which causes further loss of lithium inventory, further electrolyte depletion, and thicker and non-uniform SEI growth. 

Conclusion

The SEI is a double-edged sword in lithium-ion batteries. When well-formed, it reduces calendar ageing, and ensures stable, long-term cycling. However, poorly formed SEI layers contribute to lithium loss, impedance growth, and even catastrophic battery failure from dendrite formation.

Continued research on electrolyte additives, optimized formation protocols, and advanced anode materials is key to engineering the “ideal” SEI - thin, uniform, stable, ionically conductive, and electrically insulating.

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References

[2] S. K. Heiskanen, J. Kim, and B. L. Lucht, “Generation and Evolution of the Solid Electrolyte Interphase of Lithium-Ion Batteries,” Joule, vol. 3, no. 10, pp. 2322–2333, Oct. 2019, doi: 10.1016/j.joule.2019.08.018.