Phase Stability and Kinetics of Topotactic Dual Ca²⁺–Na⁺ Ion Electrochemistry in NaSICON NaV₂(PO₄)₃

Recent reports of reversible calcium plating and stripping have rekindled interest in the development of Ca-ion batteries (CIBs) as next-generation energy storage devices. This technology has the potential to overcome the limitations of conventional Li-ion batteries, but CIBs are plagued by a paucity of suitable cathode materials. To date, NaSICON-structured NaV2(PO4)3 has been demonstrated as a successful cathode candidate, exhibiting reversible (de)intercalation of 0.6 mol Ca2+ along with stable cycling performance. However, a complex multiphase mixture forms on discharge so the Ca-ion charge storage mechanism in the NaSICON framework is poorly understood. In this work, we report on an investigation of the structure and/or Na+/Ca2+ environment(s) of a variety of chemically prepared NaSICON CaxNayV2(PO4)3 phases which were characterized using synchrotron XRD, SEM-EDS, 23Na NMR, and TEM. Highly calciated CaV2(PO4)3, Ca1.5V2(PO4)3, and CaNaV2(PO4)3 phases can be prepared at high temperature, but─unlike Ca0.6NaV2(PO4)3─these materials are electrochemically inactive. To better understand the fundamental factors impacting successful Ca2+ electrochemistry in this system, DFT was employed to examine the CaxNayV2(PO4)3 phase diagram and Ca2+ diffusion mechanism. Theoretical insights show that phase separation into Na-rich and Ca-rich phases is a reason for the capacity limitation and demonstrate that Na+ ions in the host materials assist the migration of neighboring Ca2+ ions, enabling reversible electrochemistry in CaxNayV2(PO4)3. This investigation of fundamental principles affecting reversible Ca2+ (de)intercalation in CaxNayV2(PO4)3 allows for the development of design principles to enable the discovery of a variety of successful cathodes for CIBs.