New conversion chemistry of CuSO₄ as ultra-high-energy cathode material for rechargeable sodium battery

We report the nano-sized CuSO₄–carbon nanotube composite (nano-CuSO₄/C) as a novel conversion-based cathode material for Na-ion batteries (NIBs). The nano-CuSO₄/C undergoes a conversion reaction during the charge/discharge process with a high redox potential of ~2.7 V (vs. Na⁺/Na) and the highest reported energy density for NIB cathode materials. Nano-CuSO₄/C exhibits excellent electrochemical performance, with a specific capacity of ~335 mAh g⁻¹ at a rate of C/30 (1C = 335 mA g⁻¹), and even at 5C, its capacity is maintained up to ~204 mAh g⁻¹, corresponding to ~61% of the theoretical capacity. Furthermore, nano-CuSO₄/C delivers outstanding capacity retention of ~72% over 300 cycles at 2C with high coulombic efficiency of more than 99%. We confirm the reversible sodium storage mechanism on nano-CuSO₄/C under Na-ion battery system using various analyses, such as operando/ex situ X-ray diffraction, X-ray absorption near edge structure spectroscopy, extended X-ray absorption fine structure spectroscopy, transmission electron microscopy, and time-of-flight secondary-ion mass spectroscopy. CuSO₄ is transformed into Cu⁰ and Na₂SO₄ during the discharge (reduction) process, and the original CuSO₄ is recovered during the charge (oxidation) process. This fundamental understanding of CuSO₄ provides insight for the use of high-capacity conversion-based cathode materials in NIBs.