Electrochemical Approach to Investigate the Electrolyte Wetting Process of Lithium-Ion Cells
Authors: Reinke Christian , Nikolowski Kristian , Wolter Mareike , Michaelis Alexander 
Published: February 2020

Expanding life time as well as lowering production costs of a lithium-ion cell are two important subjects, which are addressed in the current research of lithium-ion batteries. One approach to reach these aims is an optimization of the electrolyte filling and formation process of the lithium-ion cells. This process steps have a major contribution to the production costs, since it requires high investments to provide a sufficient amount of formation equipment. Furthermore, since this processes can take up to 3 weeks, quite a lot of stock capacity is necessary [1]. After filling the cell with electrolyte, the formation process starts with different aging and charging steps with different durations, charging conditions (e.g. c-rate, voltage cut-off) and atmosphere conditions (e.g. pressure and temperature profiles). The aging steps during the formation process are applied to ensure a proper saturation of all the pores of the electrodes with electrolyte. This is of particular importance for graphite based anodes since a SEI film is built on the graphite particles during the charging steps, which greatly influences the cell’s specific values (e.g. lifetime and safety) [2]. To ensure a good and homogenous quality of the SEI film, there should be no significant amount of electrolyte-dry graphite particles, when the first charging step is applied to the cell. So it’s of relevant importance, to know when this state is reached.

Commonly, the optimization of the formation process is an empirical process. For a systematical optimization of the formation process of lithium ion cells, amongst others, a detailed knowledge of the electrolyte wetting process is necessary. First of all, the wetting process is of course greatly influenced by the choice of the materials of the cell components (primarily electrodes and separator) and material parameters (e.g. porosity and torosity). The wettability of single components can be evaluated by tracking the wetting progress of individual components visually or with gravimetric techniques, but there is no generally established test [3]. Beside, since the interface between electrodes and separator has a strong influence on the electrolyte wetting behavior, findings about the wettability of the individual components are not easily transferable to the wetting behavior in a full cell geometry. Furthermore the cell geometry itself has a great impact to the electrolyte wetting behavior, since it can favor or limit the electrolyte supply to the edges of the cell and as a consequence the wetting speed within the cell. This is of particular importance for bigger cell designs (e.g. PHEV 1).

 Based on electrochemical measurements we propose a novel method, which is performed in situ, to investigate the electrolyte wetting process in lithium-ion full cells. The experiments were executed with a modified pouch-bag cell design enabling us to initiate electrical measurement simultaneous to the start of the filling process under controlled conditions in a climate chamber.

The reliability of the method is demonstrated by investigating the electrolyte wetting behavior of cells with different commercially available separators, including polyolefin, ceramic and ceramic coated polyolefin separators. Measurements on the separator by visual methods were executed to validate our novel electrochemical method. To demonstrate the potential of the method, the influence of temperature conditions during the wetting process was investigated. By using the method the wetting speed and degree can be measured in situ on full cell level for the first time.

 In this presentation, the method’s principals will be presented and the results will be discussed. Furthermore the potential of this method will be illustrated, for example, how and which modification of the components can be realized and how they can help to optimize the electrolyte wetting behavior of lithium-ion cells.

References

[1] D.L. Wood, J. Li, C. Daniel, Journal of Power Sources 275 (2015) 234–242.

[2] P. Verma, P. Maire, P. Novák, Electrochimica Acta 55 (2010) 6332–6341.

[3] S.S. Zhang, Journal of Power Sources 164 (2007) 351–364.