Author: Sarah B.
One of the most persistent misconceptions in the battery community is the belief that the HOMO–LUMO gap of a solvent defines the electrochemical stability window of an electrolyte. It doesn’t. And this misunderstanding is costing us — scientifically and financially.
The energy diagram [1] illustrates the electrochemical stability window of an electrolyte in a battery cell, relative to the energy levels of the anode and cathode electrodes.
The overall cell voltage (𝐄𝐜𝐞𝐥𝐥) is related to the difference in the electrochemical potentials of the cathode and anode.
The diagram shows the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the electrolyte.
For an electrolyte to be stable, the anode and cathode electrochemical potentials must fall within this stability window.
In reality, HOMO and LUMO are electronic-structure concepts for isolated molecules, not indicators of oxidation or reduction events in real electrochemical environments. Electrochemical stability is governed by redox potentials, which depend on:
- Gibbs free energy differences,
- solvation effects,
- ion–solvent interactions,
- concentration and activity,
- reactive intermediates,
- and even electrode surface chemistry.
Ignoring these factors leads to:
- wrong predictions of solvent stability
- misleading screening of electrolyte components
- wasted computational and experimental effort
- premature investment in chemistries that fail in practice
A striking example:
Water has a ~9 eV HOMO–LUMO gap — which would make it a “perfect” electrolyte by that standard — yet its real stability window is only 1.23 V.
This is a textbook demonstration that HOMO–LUMO has nothing to do with actual electrochemical stability. For electrolyte design, we should stop using HOMO/LUMO as a shortcut and instead focus on:
- oxidation potential
- reduction potential
- the Fermi level of electrons in solution
Moving away from the HOMO–LUMO myth is not just a theoretical correction — it’s a necessary step to prevent misdirected research, wasted resources, and missed opportunities in developing next-generation batteries. Let’s design electrolytes based on real electrochemistry, not orbital diagrams.
References
- Pekka Peljo and Hubert H. Girault, Electrochemical potential window of battery electrolytes: the HOMO–LUMO misconception, Energy & Environmental Science, Issue 9, 2018
See also: Standard Electrode Potentials