Electrolyte decomposition

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Cathode–electrolyte interphase

The counterpart of the SEI on the positive electrode: a thin layer of electrolyte oxidation products on the oxide surface. It is thinner and less stable than the SEI, and its growth contributes to impedance rise, especially at high voltage.

Electrochemical stability window

The potential range within which an electrolyte is neither reduced nor oxidised. Carbonate electrolytes are only stable from about 1 V to 4.3 V vs Li/Li⁺; graphite operates below that, which is why the SEI, not thermodynamics, keeps the cell alive.

Electrolyte dry-out

Loss of liquid electrolyte to decomposition and film growth until parts of the electrode are no longer wetted. It appears late in life as a sudden rise in resistance, and is one cause of the knee in a capacity curve.

Gas generation

Formation of gases — ethylene, hydrogen, carbon monoxide and carbon dioxide — by electrolyte decomposition, mostly during formation and later under high voltage or temperature. In a pouch cell it shows as swelling; in a can it raises pressure until the vent opens.

Solid–electrolyte interphase

A film of electrolyte reaction products on the negative-electrode particles — graphite, and also silicon or lithium metal. It protects the electrode from further reaction but grows slowly for the whole life of the cell, consuming cyclable lithium.

References & further reading

Technical references

1. Lithium ion battery degradation: what you need to know

   Physical Chemistry Chemical Physics · 2021

   Technical reference · Degradation mechanisms, modes, operational effects and their interactions.