Basics about Batteries: Link 1
Battery components
Battery physics
Open circuit voltage (OCV)
OCV is formed by the difference between chemical potentials of anode (μa) and cathode (μc).
SEI and CEI formation
As shown in above figure, if electrodes and the solid electrolyte have a mismatch of chemical potential, spontaneous chemical reaction(s) may occur once these two materials are in contact.
- Solid electrolyte interphases (SEI) forms if anode chemical potential (μa) is above the LUMO of the electrolyte.
- Cathode electrolyte interphases (CEI) forms, if the μc is below the HOMO of the electrolyte
A beneficial SEI/CEI should be a passivating layer that is conductive to Li ions but not electrons, and extend the working voltage window of electrolytes, as shown in the above Figure. However, if the SEI/CEI is a mixed ionic and electronic conductor (MIEC), the SEI/CEI will continue to grow, deteriorating the cell performances
Factors affecting HOMO and LUMO
The onset oxidation potential (HOMO) of an electrolyte depends on the anionic framework in the electrolyte structure. The stronger electronegativity of the anion (the strong electron-withdrawing), the harder for the electrolyte to lose electron (to be oxidized). For example, the oxidation potential or HOMO of electrolytes follows a general trend of chlorides > oxides > sulfides >nitrides (Li3YCl6 > LLZO > LPS > LiPON), which matches the electronegativity of Cl>O>S>N.
Similarly, the reduction potential (LUMO) of an electrolyte depends on its cation framework, especially the accessible lower oxidation states of the cation and its thermodynamic reduction potential.