Development of Multi-Analysis By Operando Spectroscopy / Elemental Measurement in Oxide-Type All-Solid-State Na Batteries
Authors: Hiraoka Koji , Yamamoto Kazuo , Kobayashi Takeshi , Sakamoto Tetsuo , Seki Shiro 
Published: February 2024

Introduction

Oxide-type all-solid-state Na batteries (ASSBs) are attracted attention owing to their high safety, resource abundant and intrinsic chemical stability, toward electric storage system for renewable energies, such as photovoltaics and wind generations, using electrochemical devices in urban locations. Although the ASSBs shows low output performances and high internal resistances due to their low sinter density in general, the improvement of these properties have been strenuously studied by using the spark-plasma-sintering (SPS) and sintering additives of low melting point material such as B2O3. However, charge-discharge process is complex reactions extremely, which includes redox reactions of electrode, ionic transport of solid electrolyte at macro / micro-scales every component material. Therefore, to perspective understanding of charge-discharge reactions in the ASSBs, the construction of analysis technique is strongly desired by combination of evaluation methods with suitable selection for each scale. Herein, the multi-scale analysis was demonstrated by using Operando scanning electron microscopy / energy dispersive X-ray spectroscopy (SEM / EDS) (µm-scale) and Raman spectroscopy (atomic-order) for observing changes of Na concentration, bonding states during charge-discharge processes, respectively, in addition evaluation of elemental distributions through the time-of-fright secondary ion mass spectrometry (TOF-SIMS) (nm-scale).

Experimental

The ASSBs were fabricated by multi-step-sintering method using SPS1 from Na3V2(PO4)3 (NVP) as positive (PE) and negative electrode (NE) active materials, Na3Zr2Si2PO12 (NZSP) as solid electrolyte (SE), B2O3 as adhesive between PE/SE and SE/NE layers. The electrode composition were NVP : NZSP : carbon = 25 : 60 : 15 by weight ratio. The sintered ASSB was physically split for arbitrary size, and then obtained piece loaded into a sample holder of Ar-ion milling system in Ar atmosphere to prepare smooth surface of cross-sectional direction. After the Ar-ion milled sample was transferred to holder enabled to apply voltage, Operando SEM-EDS was performed by cyclic voltammetry (CV) with 0.2 mV s-1 of scan rate, 0.5 - 2.5 V of voltage range at room temperature, and acquired elemental mapping images every 0.5 V for 3 min of recording time. As evaluation at 2, 3 cycle, Operando Raman spectroscopy using the same sample was performed by CV with same conditions at room temperature (at 2 cycle) and 60oC (at 3 cycle), using electrochemical measurement cell having observation window of quartz glass. After 3 cycles, the TOF-SIMS mappings were acquired from all area of the cross-sectional ASSB by using plus mode.

Results & Discussion

From the Operando SEM / EDS, the distributions of Na element clear changed in PE and NE layers under charge-discharge processes, the 2D-dimensional profile on Na counts normalized by P, which is estimated to be not contribute electrochemical reactions, is shown Figure 1 (a). The Na concentration were confirmed decrease / increase trends at charge process, even though there were decreased / increased at discharge process in the PE and NE layers, respectively. These behaviors of concentration change mean to Na transport between the PE and NE layers with deintercalation and intercalation reactions of NVP during charge-discharge processes. On other hand, Figure 1 (b), (c) shows the Raman spectra changes for PE and NE layers at 2 cycle. Although the D- (1380 cm-1) and G- (1520 cm-1) bands related to carbon material did not change, the integrated intensities at about 780 cm-1 clear increased / decreased at PE layer in charge-discharge processes, respectively. These peaks were assigned to PO4 bonding of NVP, and suggested to reversible structural change owing to intercalation and deintercalation reactions of Na. In addition, the calculation result of Na pathway in the NVP was reported by migration between PO4 tetrahedral along the x direction with low activation energy, and should affect bonding states owing to change of backbone structure during charge-discharge processes.2 Therefore, the peak of PO4 bonding is considered as estimation index on the state-of-charge with electrode redox reactions in ASSBs. In other words, intercalation / deintercalation of Na affects not only structural change of NVP, but also concentration change in all areas of PE and NE layers and these reactions could be directly observed by combination of Operando SEM / EDS and Ram.an spectroscopy. In this presentation, we will also report the results of elemental distributions after 3 cycles by acquired from the TOF-SIMS analysis and correlationship between each evaluation technique for ASSB.

D. Kutsuzawa, et al., ACS Appl. Energy Mater., 5, 4025–4028 (2022)

W. Song et al., J. Mater. Chem. A, 2, 5358 (2014).

Figure 1