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The metallic lithium anode provides unparalleled opportunities for rechargeable batteries with very high energy density. A main problem hindering the development of cells using metallic lithium anodes stems from the electrochemical instability of the interface between metallic lithium and organic liquid electrolytes. This paper reports an approach rationally designing the surface characteristic of separator for stable, dendrite-free operation of lithium–metal batteries. A unique polymer multilayer PEI­(PAA/PEO)3 was fabricated on the microporous polyethylene (PE) separator by a simple layer-by-layer (LbL) assembly process, which maintains the pore structure and thickness of PE separator but remarkably enhances the ionic conductivity (from 0.36 mS cm–1 to 0.45 mS cm–1) and Li+ transference number (from 0.37 to 0.48), as well as stabilizes lithium metal anodes against the reaction with liquid electrolytes during storage and repeated charge/discharge cycles, which is responsible for restraining the electrode polarization and the formation of lithium dendrites, and therefore endows lithium metal batteries with long-term cycling at high columbic efficiency and excellent rate capability, as well as the improved safety.

Sodium-ion batteries (SIBs) are emerging power sources for the replacement of lithium-ion batteries. Recent studies have focused on the development of electrodes and electrolytes, with thick glass fiber separators (∼380 µm) generally adopted. In this work, we introduce a new thin (∼50 µm) cellulose-polyacrylonitrile-alumina composite as a separator for SIBs. The separator exhibits excellent thermal stability with no shrinkage up to 300 • C and electrolyte uptake with a contact angle of 0 •. The sodium ion transference number, t + Na , of the separator is measured to be 0.78, which is higher than that of bare cellulose (t + Na : 0.31). These outstanding physical properties of the separator enable the long-term operation of NaCrO 2 cathode/hard carbon anode full cells in a conventional carbonate electrolyte, with capacity retention of 82% for 500 cycles. Time-of-flight secondary-ion mass spectroscopy analysis reveals the additional role of the Al 2 O 3 coating, which is transformed into AlF 3 upon long-term cycling owing to HF scavenging. Our findings will open the door to the use of cellulose-based functional separators for high-performance SIBs.

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