Moisture-Controlled Electrolyte Engineering Enables Durable Calcium-Ion Batteries

Calcium-ion batteries (CIBs) offer several advantages. CIBs are viable alternatives to lithium-based battery systems owing to the natural abundance, low cost, and high volumetric capacity of calcium. However, their development has been severely constrained by electrolyte instability and water sensitivity. We conducted a systematic examination of Ca(ClO₄)₂ and Ca(PF₆)₂ electrolytes, focusing on low-cost salt production, solvent selection, and stringent dehydration procedures. Acetonitrile (ACN) was the ideal solvent for high salt solubility and reversible Ca²⁺ electrochemistry, while carbonate solvents failed rapidly. We found that even a small amount of moisture in the electrolyte significantly affected the electrochemical performance. This study improved the dehydration process by using 3 Å molecular sieve (MS3A) and vacuum drying to reduce moisture to ppm levels, stabilizing the electrolyte. Prussian blue (PB) half cells exhibited reversible capacities of up to ≈95 mAh g^-1, whereas PB-hard carbon full cells utilizing dried Ca(ClO₄)₂ showed stable cycling over 240 cycles with a Coulombic efficiency of ≈99% and capacity loss of only ≈17%. This study establishes a moisture-controlled electrolyte as a critical enabler for practical CIBs.