A Numerical Study on Effects of Electrolyte Dry-out in Separators of Lithium-Ion Batteries
Authors: Yu Seong-Taek , Choi Jin Hyeok , Kim Charn-Jung , Lee Kyu-Jin 
Published: May 2020

A variety of strategies to deal with the Lithium-ion batteries (LIBs) reaching the end-of-life (EOL) is being proposed, one of which is the secondary use applications such as the grid distribution system or the off-grid system. [1] It is reported that within the EOL range, the capacity fade generally represents linear dependence on charge throughput and that SEI layer growth is the dominant factor. After prolonged cycling, however, the transition point to the nonlinearity of aging is observed and the LIBs capacity dropped suddenly, indicating that some other aging mechanisms. [2] This rapid capacity drop phenomenon has been reported by experimental tests of other groups. [3-5] Among many phenomena related to the rapid drops, electrolyte dry-out including decomposition of additives has been considered as one of the major factors.

In this work, we employ an electrochemical physical model for 20Ah NCM pouch-type cell, which describes both solid and liquid phase behavior through a concentrated solution and porous electrodes. A pseudo-two-dimensional (P2D) electrochemical physical model calculates lithium concentrations and electric potentials in the electrolyte and the active materials and allows them to expand to additional physical phenomena. Saturation in the porous media is defined to account for the dry-out of the electrolyte. Because electrodes have higher wettability than separators, if the decreases of saturation occur in the LIBs, the problem area is more likely to be a porous separator than a composite electrode. [6] For this reason, we assumed that saturation reduction occurs only at separators as the electrolytes are decreased in the cell. Reduced saturation in a separator can be followed by a block of ionic transport through separator pores, and result in the sudden death of LIBs battery.

The P2D model cannot solve non-uniform phenomena appearing along the electrode area because of a one-dimensional calculation in the electrode thickness direction. Therefore, an expanded pseudo-three-dimensional (P3D) model was used to examine how dry-out in the separator affects the physics of the LIBs quantitatively. The P3D model is possible to simulate the imbalance in LIBs cells as well as the end-of-life of LIBs. Also, parametric studies about area and saturation of the dry-out we conducted by the model. The model results show that there is a critical saturation of the dry-out leading a sharp performance drop. This model can account for the sudden degradation of LIBs and the loss of ionic path and the internal imbalance caused by electrolyte dry-out.

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