Multifunctional covalent organic frameworks for high capacity and dendrite-free lithium metal batteries

For its high specific capacity of 3860 mAh g−1 and low redox potential of −3.04 V (vs. SHE), lithium (Li) metal has been regarded as one of the most promising anode materials for the next-generation batteries. However, the limited Li utilization and the detrimental dendrite growth severely impede the practical application of Li metal batteries. Herein, the covalent organic frameworks LZU1 (COF-LZU1) is introduced to serve as a protective layer in-between the Li anode and separator. Because of the interaction between bis(trifluoromethanesulonyl)imide (TFSI) anions and aldehyde functional groups in COF-LZU1, TFSI− are immobilized on the COF-LZU1, alleviating the impact of space charge and hence suppressing the dendrite growth. Furthermore, the imine-linked COF-LZU1 is lithiophilic and electrolyte wettable, enabling a homogenous Li ions flux distribution and transport and hence resulting in a low voltage hysteresis. With the dendrite-free anode, the utilization of Li achieves >99% in Li–Cu cell and the life-span is over 2000 h in Li–Li symmetric cell with a low voltage hysteresis of ≈35 mV. Moreover, Li-Sulfur batteries with COF-LZU1 layer present significantly better stability and rate properties than the ones without COF-LZU1. Therefore, utilization of the COF-LZU1 protective layer has been successfully demonstrated to be an effective approach to enable safe and high power/energy density Li metal batteries.