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Li-ions transport across electrolytes and SEI like beads passing through a Galton Board

Science China Press

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IMAGE: Schematic of the Galton Board model. When the small balls accumulate in the various bins at the bottom, their distribution in each of the bins will approximate the normal distribution... view more 

Credit: ©Science China Press

The kinetics of Li-ions transport across the electrolyte and the SEI is usually the rate-determining step in the Li plating-stripping process. Before electroplating on the anode surface, Li-ions migrate from a cathode to an anode through pores in a separator that is filled with electrolytes. The Li metal surface facing the pores in the separator becomes enriched with Li-ions due to their accumulation inside the pores. Thus, the fresh Li metal selectively deposits on these regions with a higher concentration of Li-ions, forming an uneven Li metal surface, as well as dendritic Li growth.

Recently, the research group of Professor Hengxing Ji of the University of Science and Technology of China presents a coating layer formed by COF-LZU1 particles on a commercial polypropylene (PP) separator to redistribute the Li-ions passing through the PP pores.

The interlayer comprises closely packed particles of a Schiff-base covalent organic framework (COF) COF-LZU1, which is an electronic insulator with a high chemical and thermal stability containing well-aligned channels of ~1.8 nm in diameter. The nanochannels in the COF-LZU1 particles in the coating layer hinder the migration of anions between the electrodes, leading to a high Li-ion transference number of 0.77±0.01, which has long been considered to increase the energy efficiency of Li batteries.

In addition, Li-ions move through the COF-LZU1 layer, analogous to beads passing through a Galton Board. This process, for a large number of beads, statistically approximates the normal distribution. In this regard, the COF-LZU1 layer serves to effectively redistribute the Li-ions that pass through the pores of the commercial separator so as to yield a uniform distribution, which can transform the mossy or dendritic Li to a smooth Li deposition, rendering improved battery performances. The mechanism of the behaviors of the COF-LZU1 layer can be extended to different types of porous materials to regulate ion distributions in different energy storage or conversion systems.

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See the article: Xie H, Hao Q, Jin H, Xie S, Sun Z, Ye Y, Zhang C, Wang D, Ji H, Wan L-J. Redistribution of Li-ions using covalent organic frameworks towards dendrite-free lithium anodes: a mechanism based on a Galton Board. Sci. China Chem., 2020, DOI:10.1007/s11426-020-9796-9.

https://doi.org/10.1007/s11426-020-9796-9

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