The electrolyte is the medium that allows ionic transport between the electrodes during charging and discharging of a cell.
Electrolytes in lithium ion batteries may either be a liquid, gel or a solid. Lithium batteries use non-aqueous electrolytes because of reactivity of lithium with aqueous electrolytes and the inherent stability of non-aqueous electrolytes at higher voltages.
Liquid electrolytes are a combination of a solution of solvents, salts and additives. The liquid electrolyte in Li-ion cells is typically lithium hexafluorophosphate (LiPF6) dissolved in a mixture of organic solvents.
When discharge begins the lithiated carbon releases a Li+ ion and a free electron.
Electrolyte, that can readily transports ions, contains a lithium salt that is dissolved in an organic solvent. The Li+ ion, which moves towards the electrolyte, replaces another Li + ion from the electrolyte, which moves towards the cathode. At the cathode/electrolyte interface, Li+ ions then become intercalated into the cathode and the associated electron is used by the external device.
The role of the electrolyte  is to act as a medium for ionic conduction and a barrier for electronic conduction to avoid self-discharge of the cell. Other essential requirements are:
- viscosity of electrolytes, which influences ionic mobility, should be low (<2 cP);(2)
- Chemical and thermal stability in a wide range of voltage and temperatures
- A wide electrochemical stability window (0.01–5 V vs. Li/Li+) of operation
- stable against oxidation and reduction reactions
- able to withstand the potential window of the electrochemical reaction
- good Li+ conductivity
- Ionic conductivity should be greater than 1 mS cm−1
- adequate diffusion of Li-ions
- does not dissolve the SEI
- Environment friendliness, cost-effectiveness, and safety in operating conditions.
- low toxicity
- low cost
- high dielectric constant (>20) of solvents helps to dissociate salt
- Inert behavior towards other battery components such as separator, current collector, and packaging materials
Additives to the electrolyte are the “secret sauce” that can extend the life or improve certain aspects of the cell. Hence these have become a ground for patents and differentiation.
In the cell manufacturing process the filling of the cell with electrolyte and ensuring it fully encompasses all areas of the active materials is critical for the cell performance and lifetime.
- Christina Sauter, Raphael Zahn and Vanessa Wood, Understanding Electrolyte Infilling of Lithium Ion Batteries, Journal of The Electrochemical Society, 2020 167 100546
- Yuliya Preger, Loraine Torres-Castro, Jim McDowall, Chapter 3 Lithium-ion Batteries, Sandia National Laboratories and Saft America Inc.
- Das, Dhrubajyoti, Sanchita Manna, and Sreeraj Puravankara. 2023. Electrolytes, Additives and Binders for NMC Cathodes in Li-Ion Batteries—A Review Batteries 9, no. 4: 193