Stanford Synchrotron Radiation Lightsource

Widespread applications for electrochemical energy storage based on lithium-ion batteries now demand higher performance in terms of energy and power density, coupled with robust cycle life.  Despite major advancements in lithium-ion batteries, a shortage of precious metals, such as cobalt, which is typically used in the cathode in lithium-ion batteries and is sourced from only a few countries globally, is predicted. This shortage has driven the energy storage community to develop “beyond lithium-ion” technology, based on earth-abundant metals, such as monovalent sodium and potassium or divalent calcium, magnesium, and zinc.  Divalent metals, if used directly as a metal anode, can provide high capacity density compared to lithium because each ion carries twice the charge. They are also relatively cost-effective. However, challenges remain in identifying a cathode material capable of high capacity, stable cycling, and balancing the capacity of multivalent metal anodes. Moreover, a critical challenge in developing cathode materials for multivalent charge storage lies in addressing sufficient multivalent cation mobility required for reversible intercalation.

Figure. Schematic of the potential-dependent crystal structures associated with the charge and discharge cycle of Na₃V₂(PO₄)₃ determined from x-ray diffraction.

In a recent study, researchers from SSRL investigated the potential-dependent structure–property relationships, along with chemical changes, of Na₃V₂(PO₄)₃, a vanadium phosphate-based cathode for a Zn-metal battery (Figure). The Na⁺ superionic conductor (NASICON) structure has Na⁺ occupying two different crystal lattice sites: the Wyckoff 6b (Na1) and 18e (Na2) sites. It has been used earlier as a cathode material for sodium-ion batteries and showed promise in accommodating Zn-ions in the 18e sites after Na-ions are extracted. Researchers investigate the potential-dependent structure–property relationships, along with chemical changes, of Na₃V₂(PO₄)₃ using x-ray synchrotron-based methods to correlate crystal structure changes with the charge/discharge operation.

With a combination of operando and higher resolution ex situ x-ray diffraction measured at Beam Lines 11-3 and 2-1 at SSRL, researchers revealed a quasi-two-step insertion  process with both Na⁺ and Zn²⁺ reversibly filling the 18e sites of the NASICON structure (Figure). The oxidation-reduction reactions associated with this quasi-two-stage electrochemical process and the presence of ionic Zn²⁺ was verified by x-ray absorption spectroscopy at the Zn and V K-edges. The results provide an exciting direction for utilizing a polyanionic framework for multivalent ion insertion and may be applicable to other divalent ions such as Mg²⁺ and Ca²⁺.