Electrochemical Ion Exchange in Nasicon Based Solid Electrolytes

Abstract

Solid electrolytes form the crux of the all-solid-state batteries. Garnets, sulfides, and NASICONs are some of the most promising solid electrolyte material families. An ideal solid electrolyte should have higher ionic conductivity, higher mechanical strength, chemically and electrochemically stable with Li-anode and cathode, and environmentally benign. In the search for a reliable solid electrolyte, researchers are revisiting NASICON material families which have a wide compositional variety [1-2]. Properties of these materials can be easily tuned with substitution and replacement of cations and anions. In this work, we are exploring AM1M2(XO4)3 (where M1=Mn, M2=Zr, X=P) NASICONs for Li-ion based ASSBs. Rhombohedral polymorph of this composition provides the highest ionic conductivity. However, the synthesis of a Li-based NASICON in this composition is not a straightforward through conventional methods.In this work, we have investigated the capability of Na1+xMnx/2Zr2-x/2(PO4)3 (NMZP), which crystallizes in R-3C space group, being able to conduct Li-ions. NMZP was synthesized from x=0.5 to x=2. Neutron diffraction revealed that there are two main sites for Na-ions in this structure denoted by M1 and M2. Rietveld refinement analysis shows a monotonic increase of Na occupancy and Na-O bond length increase in the M2 site with an increase in x suggesting higher concentration of mobile Na in the M2 site (Figure 1a and b). Electrochemical impedance spectroscopy (EIS) analysis revealed that there is an increase in ionic conductivity from x=0.5 to x=1.5. The ionic conductivity of compositions with x=1.5 and x=2 is approximately the same which can be attributed to vacancy-charge-imbalance. Galvanostatic plating and stripping were carried out on symmetric Na | NMZP | Na, and Li | NMZP | Li cells at 70 ⁰C. A stable polarization profile was observed for both cells at various current densities (20, 40, and 80 µA cm-2 for Na | NMZP | Na and 40, 60, and 80 µA cm-2 for Li | NMZP | Li cell) (Figure 1c). Stable polarization observed for Li | NMZP | Li suggests that Na-ions in M2 sites can be readily exchanged with Li-ion from electrodes. Additionally, Na-ions should be pushed out of the structure and platted on the Li-counter electrode. Post-mortem analysis Li-foil from Li|NMZP|Li cell was carried out with SEM after 120 hours of cycling. Li-metal foil showed the presence of Na-metal plating on Li-electrode validating this phenomenon [3]. To further corroborate our findings that NMZP can conduct Li-ions readily, post-mortem XPS analysis of Li-foil and NMZP pellet from Li | NMZP | Li cell was carried out after 120 hours of cycling. Linear scanning map of Li-metal shows the presence of Na metal from 0.15 to 2.07 at% showcasing electroplated Na out of NMZP structure. Furthermore, XPS depth profiling of the cycled NMZP pellet shows the presence of Li within the structure (225 nm depth). These experiments prove the dual-ion conduction capability of NMZP type of NASICON materials. The proof-of-concept work showcased here opens exciting avenues of material combinations which can be leveraged for both Na and Li all-solid-state batteries. References Zhou et al., Small Methods 2020, 4, 2000764.Gao et al., J. Am. Chem. Soc. 2018, 140, 51, 18192–18199Parejiya et al., ACS Energy Lett. 2021, 6, 429−436 Figure 1