Redistribution of Li-ions using covalent organic frameworks towards dendrite-free lithium anodes: a mechanism based on a Galton Board
Science China Chemistry 63(9):1306-1314 Sep 2020 

Xie, Huanyu; Hao, Qing; Jin, Hongchang; Xie, Shuai; Sun, Zhaowei; Ye, Yadong; Zhang, Chaohui; Wang, Dong; Ji, Hengxing; Wan, Li-Jun


Because of its high theoretical specific capacity and low reduction potential, Li metal is considered to be key to reaching high energy density in rechargeable batteries. In this context, most of the research has focused on suppressing dendrite formation during Li deposition to improve the cycling reversibility and safety of the batteries. Here, covalent organic framework (COF) film coating on a commercial polypropylene separator is applied as an ion redistributor to eliminate Li dendrites. The COF crystallites consist of ordered nanochannels that hinder the movement of anions while allowing Li-ions to transport across, leading to a high Li-ion transference number of 0.77±0.01. The transport of Li-ions across the COF film can be considered to be analogous to beads passing through a Galton Board, a model that demonstrates a statistical concept of a normal distribution. Thus, an even distribution of Li-ions is obtained at the COF Li metal interface. The controlled Li-ion flux yields a smooth Li metal surface after 1,000 h (500 times) of cycling, leading to a significantly improved cycling stability and reversibility, as demonstrated by Cu||Li half cells, Li||Li symmetric cells, and LiFePO4||Li full cells. These results suggest that, following the principle of a Galton Board, nanopore insulators such as COF-based materials are effective ion distributors for the different energy storage or conversion systems. 

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