High-stability aqueous Ca-ion batteries featuring hydrated eutectic electrolytes and small-molecule conjugated anodes

Abstract

Aqueous Ca-ion batteries represent a promising avenue for safe, sustainable, and cost-effective energy storage. However, they encounter challenges such as electrolyte freezing, narrow voltage windows, and electrode compatibility. To overcome these issues, we introduce novel hydrated eutectic electrolytes (HEEs) based on Ca(ClO₄)₂·4H₂O and sulfolane. This marks the first application of HEEs in aqueous calcium-ion batteries. The optimized HEEs achieve an ultra-low freezing point of −81.8 °C and expand the electrochemical stability window beyond 2.5 V. Molecular dynamics simulations and density functional theory calculations confirm that these improvements stem from reconstruction of the Ca²⁺ solvation sheath and suppression of water activity via modified hydrogen bonding. When paired with a copper hexacyanoferrate (CuHCF) cathode and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) anode, these electrolytes deliver exceptional performance. Half-cells exhibit unprecedented cycling stability (>10 000 cycles) and high-rate capability. CuHCF‖PTCDI full cells deliver a capacity of 26.6 mAh g⁻¹ at 0.2 A g⁻¹ (based on total electrode mass), along with outstanding longevity, retaining 75.0% capacity after 5150 cycles at 0.5 A g⁻¹. Crucially, the HEEs enable reliable low-temperature operation, with full cells maintaining 131.3% capacity retention over 2400 cycles at 0 °C. This work positions HEEs as a versatile platform for durable, low-temperature aqueous Ca-ion batteries.