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Metal–metal battery bears great potential for next‐generation large‐scale energy storage system because of its simple manufacture process and low production cost. However, the cross‐over of metal cations from the cathode to the anode causes a loss in capacity and influences battery stability. Herein, a coating of poly (ionic liquid) (PIL) with poly(diallyldimethylammonium bis(trifluoromethanesulfonyl)imide) (PDADMA+TFSI−) on a commercial polypropylene (PP) separator serves as an anion exchange membrane for a 3.3 V copper–lithium battery. The PIL has a positively charged polymer backbone that can block the migration of copper ions, thus improving Coulombic efficiency, long‐term cycling stability and inhibiting self‐discharge of the battery. It can also facilitate the conduction of anions through the membrane and reduce polarization, especially for fast charging/discharging. Bruce‐Vincent method gives the transport number in the electrolyte to be 0.25 and 0.04 for PP separator without and with PIL coating, respectively. This suggests that the PIL layer reduces the contribution of the internal current due to cation transport. The use of PIL as a coating layer for commercial PP separator is a cost‐effective way to improve overall electrochemical performance of copper–lithium batteries. Compared to PP and polyacrylic acid(PAA)/PP separators, the PIL/PP membrane raises the Coulombic efficiency to 99% and decreases the average discharge voltage drop to about 0.09 V when the current density is increased from 0.1 to 1 mA cm−2. [ABSTRACT FROM AUTHOR]

The dual‐ion battery (DIB) is a promising energy storage system that demonstrates high‐power characteristics and fast‐charging capability. However, conventional electrolytes are not compatible with the high‐voltage graphite cathode and the reactive Li metal anode, thus leading to the poor cycle stability and low Coulombic efficiency of the DIB. Here, an all‐fluorinated electrolyte is reported that can enable a highly stable operation of the graphite||Li DIB up to 5.2 V by forming robust and less‐resistive passivation films on both electrodes to reduce side reactions. The electrolyte allows reversible PF6– anion insertion/extraction and Li+ cation plating/stripping in the graphite||Li battery, achieving stable cycling with 94.5% capacity retention over 5000 cycles at 500 mA g–1, high capacity utilization of 91.8% of the available charge capacity at 50 C (5000 mA g–1), and also minimal self‐discharge. At a low temperature of 0 °C, this all‐fluorinated electrolyte exhibits 97.8% of the room temperature reversible capacity, along with ≈100% capacity retention after more than 3000 cycles, at 5 C. This work sheds a new light on the development of fluorinated electrolytes for high voltage and long‐lasting DIBs. [ABSTRACT FROM AUTHOR]