Electrochemical and Chemical Na<sup>+</sup>/Li<sup>+</sup>Ion Exchange in Na-Based Cathode Materials: Na<sub>1.56</sub>Fe<sub>1.22</sub>P<sub>2</sub>O<sub>7</sub>and Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>2</sub>F<sub>3</sub>
Journal of the Electrochemical Society