NASICON Na3V2(PO4)3 Enables Quasi-Two-Stage Na+ and Zn2+ Intercalation for Multivalent Zinc Batteries

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+).