(87e) Stabilizing Sodium Ion Transfer at the Nasicon Solid-State Electrolyte and Metallic Na Anode Interface By Nanoscale Metal Oxide Coating

Sodium (Na) ion batteries (SIBs) are widely regarded as a promising solution to large-scale electrical energy storage demands due to the natural abundance of metal and its high energy density of 1166 mAh/g as a metallic anode [1]. In order to advance the practicality of SIBs, we must harness the highly reactive nature of Na metal anode, and by employing solid-state electrolytes we can eliminate the safety risk caused by the possible reactions between Na metal and conventional liquid electrolytes [2].

Na⁺ superionic conductor (Na₃Zr₂Si₂PO₁₂, NASICON) is a ceramic-type solid-state electrolyte discovered by Goodenough et. al. in 1970s [3]. Its relatively high ionic conductivity at room temperature and exceptional thermal and chemical stability allows it to become one of the most studied solid-state electrolytes for Na batteries. However, the inherent poor wettability between NASICON and Na metal anode leads to high interfacial impedance and localized Na⁺ flux, which are detrimental to the long-term cyclability of solid-state batteries. Therefore, various approaches have been attempted to reduce interfacial resistance and promote uniform plating and stripping of Na+, with one example being the introduction of an artificial interlayer [4].

In this work, we aim to investigate nanoscale metal oxide coatings’ effect on stabilizing Na⁺ transfer in a systematic manner. Metal oxide coatings have great compatibility with NASICON, and once proven effective, their deposition can be a scalable and cost-effective method in the production of solid-state Na metal batteries. Atomic layer deposition (ALD) is employed to deposit various metal oxides of tunable thicknesses onto the NASICON surface, and after optimizing the composition and thickness of the coating layer, symmetric cells can demonstrate critical current densities (CCD) up to 12 mA/cm², which is 15 times higher than the CCD of 0.8 mA/cm² seen with the unmodified NASICON. In addition, the metal oxide coated NASICON allows stable plating/stripping cycling performance at a current density of 1.6 mA/cm² for over 300 cycles and 600 hours. In order to reveal the mechanism behind the performance improvement, the crystallinities and chemical compositions of uncoated and coated NASICON samples were studied using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) both as fabricated and after an extended period of cycling. The grain size and surface morphology are studied with scanning electron microscopy (SEM), and cross-sectional samples for transmission electron microscopy (TEM) are obtained using focused-ion beam (FIB) in order to investigate the microstructures formed at the interface of the metal oxide coating and NASICON. In-situ electrochemical cells compatible with X-ray micro-computed tomography (micro-CT) are fabricated for direct observation of the morphological evolution along with localized Na+ transfer in uncoated NASICON and uniform Na+ transfer in NASICON with nanoscale metal oxide coating.

References

[1] Tang, B.; Jaschin, P. W.; Li, X.; Bo, S.-H.; Zhou, Z. Critical interface between inorganic solid-state electrolyte and sodium metal. Mater. Today 2020, 41, 200– 218, DOI: 10.1016/j.mattod.2020.08.016

[2] Manthiram, A.; Yu, X.; Wang, S. Lithium Battery Chemistries Enabled by Solid-State Electrolytes. Nat. Rev. Mater. 2017, 2, 16103, DOI: 10.1038/natrevmats.2016.103

[3] Goodenough, J. B.; Hong, H.Y.-P.; Kafalas, J. A. Fast Na+ -ion transport in skeleton structures. Mater. Res. Bull. 1976, 11, 203, DOI:10.1016/0025-5408(76)90077-5.

[4] Matios, E.; Wang, H.; Wang, C. L.; Hu, X. F.; Lu, X.; Luo, J. M.; Li, W. Y. Graphene Regulated Ceramic Electrolyte for Solid-State Sodium Metal Battery with Superior Electrochemical Stability. ACS Appl. Mater. Interfaces 2019, 11, 5064– 5072, DOI: 10.1021/acsami.8b19519