Electrolyte

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.

Lithium ion cell

Lithium Ion Cell

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 [2] 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

Cold Temperature Performance

The electrolyte plays a key part in the Cold Temperature Charge / Discharge performance of the Lithium-Ion cell.

Below 0 °C, the viscosity of the electrolyte increases while the Li+ conductivity decreases, limiting the process of Li+ diffusion.

The low-temperature restriction of Li+ transport in the electrolyte is much larger than that in the electrode.

At the electrode–electrolyte interface the wettability and compatibility of the viscous electrolyte with respect to the electrode and separator become worse, increasing the resistance of LIBs, while a thicker SEI on the anode side hinders transport on the interface, as well as the desolvation process.

electrolyte conductivity versus temperature

Evan M. Erickson et al [4] show how the conductivity of the electrolyte increases with temperature.

The difference in conductivity of the electrolyte between -20°C and 20°C can be 4x higher.

Electrolyte Additives

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.

Cell Manufacturing

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.

filling process

What are the Electrolyte Fill Requirements for a cell versus chemistry, capacity, format, lifetime and other parameters?

The calculation is based on the porosity of the cathode, anode and separator. Added to this is the free volume and then a multiplier to account for losses in the filling process.

Image: Thomas Knoche, Florian Surek, Gunter Reinhart, A process model for the electrolyte filling of lithium-ion batteries, 48th CIRP Conference on MANUFACTURING SYSTEMS – CIRP CMS 2015, Procedia CIRP 41 ( 2016 ) 405 – 410

References

  1. Christina Sauter, Raphael Zahn and Vanessa Wood, Understanding Electrolyte Infilling of Lithium Ion Batteries, Journal of The Electrochemical Society, 2020 167 100546
  2. Yuliya Preger, Loraine Torres-Castro, Jim McDowall, Chapter 3 Lithium-ion Batteries, Sandia National Laboratories and Saft America Inc.
  3. 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
  4. Evan M. Erickson, Elena Markevich, Gregory Salitra, Daniel Sharon, Daniel Hirshberg1, Ezequiel de la Llave, Ivgeni Shterenberg, Ariel Rosenman, Aryeh Frimer and Doron Aurbach, Review—Development of Advanced Rechargeable Batteries: A Continuous Challenge in the Choice of Suitable Electrolyte Solutions, Journal of The Electrochemical Society, Volume 162, Number 14