Electrolyte Fill Requirements

What are the electrolyte fill requirements for a cell versus chemistry, capacity, format, lifetime and other parameters?

Lithium ion cell

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.

The crux of this volume calculation lies in the porosity of both electrodes and separators. However, the distinguishing factor lies in determining the additional volume of electrolyte required beyond the porosity for each cell format. The four steps in this calculation are:

  1. Electrolyte volume = positive electrode pore volume + negative electrode pore volume + separator pore volume
  2. Pore volume of the electrode = Area of the active material × Thickness × Porosity
  3. Pore volume of the seperator = Area of the seperator × Thickness × Porosity
  4. Actual injected electrolyte volume = (volume of all pores + form factor free volume) x injection coefficient

This calculated value is for the active material and the free volume within the cell needs to be added. Finally an injection coefficient takes into account losses in the assembly process, plus safety factors.

Typical Values

  • in a commercial Li-ion cell, electrolyte ~1.3 g/Ah
  • coin cells typically ∼100 g/Ah

Cell Format

The free volume inside the cell, space not occupied by components, is different for each cell format. At a high level the smallest to largets % free to active volume is:

  • Pouch Format
  • Cylindrical Cell ~9 to 11%
  • Prismatic Cell with Stacked Layers
  • Prismatic Cell with Jelly Roll(s)

This is part of the requirement for the free electrolyte, a 40Ah prismatic cell has around 19 to 30g of free electrolyte and this can be as high as 70g [1].

This post has been built based on the support and sponsorship from: MAHLE Powertrain LtdThermo Fisher ScientificEatron TechnologiesAbout:Energy and Quarto Technical Services.

Injection Coefficient

The injection coefficient is a multiplier for the assembly process. The value appears to be given by references as around 1.4x the calculated fill volume after the free volume based on cell format has been added.

Thomas Knoche et al [5] look at the filling process and show the process phenomena.

References

  1. Natalia P. Lebedeva, Franco Di Persio, Theodora Kosmidou, Denis Dams, Andreas Pfrang, Algirdas Kersys, and Lois Boon-Brett, Amount of Free Liquid Electrolyte in Commercial Large Format Prismatic Li-Ion Battery Cells, Journal of The Electrochemical Society, 166 (4) A779-A786 (2019)
  2. Shuru Chen, Chaojiang Niu, Hongkyung Lee, Qiuyan Li, Lu Yu, Wu Xu, Ji-Guang Zhang, Eric J. Dufek, M. Stanley Whittingham, Shirley Meng, Jie Xiao, Jun Liu, Critical Parameters for Evaluating Coin Cells and Pouch Cells of Rechargeable Li-Metal Batteries, Joule, Volume 3, Issue 4, 2019
  3. Stacked battery electrolyte dosage Excel calculator, zhihu.com
  4. Dr. Martin P. Lautenschlaeger, Benedikt Prifling, Benjamin Kellers, Julius Weinmiller, Dr. Timo Danner, Prof. Volker Schmidt, Prof. Arnulf Latz, Understanding Electrolyte Filling of Lithium-Ion Battery Electrodes on the Pore Scale Using the Lattice Boltzmann Method, Batteries and Supercaps, Volume 5, Issue 7, July 2022
  5. 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
electrolyte fill calculation

The calculation is now available as part of the Battery Calculations Workbook.

The worksheet allows you to select the Anode and Cathode from drop-down lists or specify your own.

Enter the other parameter values, cathode and anode thickness, area and even cell format.

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