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.

Air-stable layered structured cathodes with high voltage and good cycling stability are highly desired for the practical application of Na-ion batteries. Herein, we report a P2-Na_2/3Ni_2/3Te_1/3O₂ cathode that is stable in ambient air with an average operating voltage of ~3.8 V, demonstrating excellent cycling stability with a capacity retention of more than 92.7% after 500 cycles at 20 mA g⁻¹ and good rate capability with 91.9% capacity utilization at 500 mA g⁻¹ with respect to capacity at 5 mA g⁻¹ between 2.0 and 4.0 V. When the upper cutoff voltage is increased to 4.4 V, P2-Na_2/3Ni_2/3Te_1/3O₂ delivers a reversible capacity of 71.9 mAh g⁻¹ and retains 91.8% of the capacity after 100 cycles at 20 mA g⁻¹. The charge compensation during charge/discharge is mainly due to the redox couple of Ni²⁺/Ni³⁺ in the host with a small amount of contribution from oxygen. The stable structure of the material without phase transformation and with small volume change during charge-discharge allows it to give excellent cycle performance especially when the upper cutoff voltage is not higher than 4.2 V.