High-solvation electrolytes for ultra-stable calcium-ion storage

Abstract

Calcium-ion batteries (CIBs) have potential as electrochemical energy storage devices due to the low redox potential of Ca²⁺/Ca and the abundant reserves of Ca. However, the unsatisfactory calcium storage performance of electrode materials limits the development of CIBs. Here, we propose a design principle of high-solvation electrolytes to achieve ultra-stable calcium-ion storage. In high-solvation electrolytes, the decomposition of TFSI⁻ ions and the formation of a CaF₂-rich cathode electrolyte interface with Ca²⁺ insulation can be suppressed. With this electrolyte, Na₂V₆O₁₆·2.9H₂O shows a high discharge capacity of 240.7 mA h g⁻¹ at 20 mA g⁻¹ and an ultra-long life of 60 000 cycles (over 600 days) at 1000 mA g⁻¹. A three-dimensionally reduced graphene oxide aerogel and (NH₄)₂V₆O₁₆·1.5H₂O also exhibit a long life of 6000 cycles and 9000 cycles, respectively. These materials have the longest cycle life among reported materials so far in CIBs. This work endows the electrode materials with ultra-stable calcium-ion storage and provides a design principle of electrolytes for cathode materials in CIBs.