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Despite a recent increase in the attention given to sodium rechargeable battery systems, they should be further advanced in terms of their energy density and reliability to successfully penetrate the rechargeable battery market. Here, a new room temperature ZEBRA-type Na-CuCl 2 rechargeable battery is demonstrated that employs CuCl 2 cathode material and nonfl ammable inorganic liquid electrolyte. The cathode delivers a high energy density of ≈580 Wh kg −1 with superior capacity retention over 1000 cycles as well as a high round-trip effi ciency of ≈97%, which has never been obtained in an organic electrolyte system and high-temperature ZEBRA-type battery. These excellent electrochemical performances are mainly attributed to the use of the SO 2 -based inorganic electrolyte, which guarantees a reversible conversion reaction between CuCl 2 and CuCl with NaCl. It is also demonstrated that the proposed battery chemistry can be extended to other copper halide materials including CuBr 2 and CuF 2 , which also show highly promising battery performances as cathode materials for the Na-Cu halide battery system.

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