Highly uniform lithiated nafion thin coating on separator as an artificial SEI layer of lithium metal anode toward suppressed dendrite growth

A lithiated Nafion (Li⁺-Nafion) thin coating on lithium (Li) metal anode has been evidenced to be capable of inhibiting Li dendrite growth. However, shortcomings of Li metal such as poor stability in air and high reactivity with water bring inconvenience in the coating preparation. To address this issue, a highly uniform Li⁺-Nafion thin layer coated commercial polypropylene (PP) separator is fabricated via an ultrasonic spray method. In cells, the Li⁺-Nafion thin coating faces and contacts with Li metal anode to act as an artificial solid electrolyte interphase (SEI) layer of the anode. The addition of N,N-Dimethylformamide in sprayed aqueous Li⁺-Nafion solution is crucial for the highly uniform deposition of Li⁺-Nafion thin coating with a thickness of 0.8 μm due to the improved wettability of the sprayed Li⁺-Niafion solution with the PP separator. The uniform Li⁺-Nafion thin coating significantly increases the Li⁺ transference number (t_Li+) of the PP separator from 0.34 to 0.76. The improved t_Li+ and the high uniformity of the Li⁺-Nafion thin coating enable the coated PP separator to suppress Li dendrite growth, and consequently to promote the cycle performance of lithium-sulfur cells. This work proposes an implantation approach for artificial SEI layers employed in Li metal batteries.