Non-aqueous rechargeable calcium-ion batteries based on high voltage zirconium-doped ammonium vanadium oxide cathode

Calcium-ion batteries heed the research attention as a energy storage system due to the merits of natural abundance, low-cost, reduction potential (−2.87 V vs. SHE) similar to lithium (−3.04 V vs. SHE) and high theoretical working potential of calcium. However, the practical realization experiences a barrier due to the lack of suitable high-voltage positive electrodes to accommodate the large Ca ²⁺ with sufficiently fast ionic conduction. Herein, we investigate the zirconium-doped ammonium vanadium oxide (Zr–NH ₄ V ₄ O ₁₀) as a high-voltage cathode for the rechargeable calcium-ion batteries for the first time at room temperature. It exhibited an initial discharge capacity of 78 mAh g ⁻¹ with an average discharge voltage of ∼3.0 V vs. Ca ²⁺ |Ca and delivered an initial energy density of 231 W h kg ⁻¹ in calcium triflate [Ca(CF ₃ SO ₃) ₂ ] added in propylene carbonate electrolyte. Moreover, the Zr–NH ₄ V ₄ O ₁₀ host presents a high-rate capability, excellent cycling stability (0.021% capacity decay per cycle) for 500 cycles with ∼89% capacity retention, maintaining ∼100% Coulombic efficiency. This is one of the first demonstrations of reversible Ca-ion storage with a high voltage (∼3.0 V) using a conventional electrolyte.