NASICON Na<sub>3</sub>V<sub>2</sub>(PO<sub>4</sub>)<sub>3</sub> Enables Quasi-Two-Stage Na<sup>+</sup> and Zn<sup>2+</sup> Intercalation for Multivalent Zinc Batteries
Jesse S. Ko, Partha P. Paul, Gang Wan, Natalie Seitzman, Ryan H. DeBlock, Bruce Dunn, Michael F. Toney, Johanna Nelson Weker
Abstract
Identifying positive electrode materials capable of reversible multivalent electrochemistry in electrolytes containing divalent ions such as Mg2+, Ca2+, and Zn2+ at high operating potentials remains an ongoing challenge in “beyond lithium-ion” research. Herein, we explore the Zn2+ charge-storage mechanism of a vanadium-based Na+ superionic conductor (NASICON), Na3V2(PO4)3. By using X-ray synchrotron techniques to unravel potential-dependent structure–property relationships, we ascribe the reversible electrochemical behavior of Na3V2(PO4)3 to a quasi-two-stage intercalation process that involves both Na+ and Zn2+. Initial charging of Na3V2(PO4)3 leads to a Na+-extracted phase corresponding to NaV2(PO4)3, whereas subsequent discharge results predominantly in Na+ intercalation followed by Zn2+ intercalation. Operando X-ray diffraction of Na3V2(PO4)3 was used to study the phase changes associated with the first charge/discharge process, and ex situ measurements were used to precisely link the changes in the crystal structure to a quasi-two-stage intercalation of Na+ and Zn2+. The corresponding changes in the V-oxidation state, V-O coordination, and the presence of Zn2+ were confirmed by X-ray absorption spectroscopy. The results of this work present a comprehensive understanding of the charge-storage properties for a well-established NASICON structure that confers both the high capacity (∼100 mA h g–1) and high potential (1.35 and 1.1 V vs Zn/Zn2+).