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 NaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> has been demonstrated as a successful cathode candidate, exhibiting reversible (de)intercalation of 0.6 mol Ca<sup>2+</sup> 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. Here in this work, we report on an investigation of the structure and/or Na<sup>+</sup>/Ca<sup>2+</sup> environment(s) of a variety of chemically prepared NaSICON Ca<sub>x</sub>Na<sub>y</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> phases which were characterized using synchrotron XRD, SEM-EDS, <sup>23</sup>Na NMR, and TEM. Highly calciated CaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, Ca<sub>1.5</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>, and CaNaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> phases can be prepared at high temperature, but -unlike Ca<sub>0.6</sub>NaV<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>-these materials are electrochemically inactive. To better understand the fundamental factors impacting successful Ca<sup>2+</sup> electrochemistry in this system, DFT was employed to examine the Ca<sub>x</sub>Na<sub>y</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> phase diagram and Ca<sup>2+</sup> 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<sup>+</sup> ions in the host materials assist the migration of neighboring Ca<sup>2+</sup> ions, enabling reversible electrochemistry in Ca<sub>x</sub>Na<sub>y</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub>. This investigation of fundamental principles affecting reversible Ca<sup>2+</sup> (de)intercalation in Ca<sub>x</sub>Na<sub>y</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> allows for the development of design principles to enable the discovery of a variety of successful cathodes for CIBs.