Degradation Control of All-Solid-State Na Batteries by Inorganic Oxide Coated Positive Electrode Active Material
Authors: Ichikawa Takaaki , Hiraoka Koji , Kobayashi Takeshi , Seki Shiro 
Published: February 2024

All-solid-state Na batteries (ASSBs) are attracted attention owing to their high safety and resource properties for next-generation batteries. Na has the advantage of abundant resources compared to Li. In addition, polymer electrolyte is expected to improve the safety of the battery by suppressing the risk of ignition with liquid leakage. In general, the degradation of batteries using polymer electrolyte mainly occurs at the positive electrode (PE)/electrolyte interface during charge process. In case of batteries using active material having layered structures such as NaCoO2 and polyether electrolyte, the degradation of CoO2 structure and decomposition of polyether during the deintercalation of Na+ in the active material. These PE / electrolyte interface degradations contribute to increasing resistance of the battery. To suppress interfacial degradation, inorganic oxides coating for active materials were reported in the Li-based battery[1]. In this study, Na3PO4 or Na3Zr2Si2PO12 (NZSP) coated PEs were applied for ASSBs to suppress degradation. The degradation behavior of ASSBs was investigated by charge / discharge test and XRD analysis using extracted PE sheet from cell. Furthermore, Raman spectroscopy was applied on the cell before and after charge / discharge test to investigate structural changes in active material and polymer electrolyte.
NaTFSA as Na salt, P(EO/PO) as polyether-based macromonomer, and DMPA as photoinitiator were mixed into Ar-filled glovebox. NaTFSA was added by ratio of [Na]/[O]=0.1 per molar of oxygen units of P(EO/PO). Polymer electrolyte films were prepared by UV irradiation for obtained homogeneous solution. NaCoO2 (NCO) as active material, acetylene black (AB) as conductive additive, and P(EO/MEEGE)-NaTFSA as binder were mixed by 82:5:13 weight ratio. The Na3PO4 and NZSP as inorganic oxides were added 5 wt.% and 10 wt.% for NCO, respectively. Applied slurry (thick. 50 µm) onto Al foil was cut to Φ16 mm after vacuum dry. The PE sheet, Na metal negative electrode, and polymer electrolyte were assembled into coin-cell to fabricate ASSBs. The charge / discharge tests for more than 10 cycles were performed at 333K after heat-treated at 363K for 48h. After charge / discharge test, XRD analysis was applied to extracted PE sheet from the cell under a low dew point for structural evaluation. Moreover, PE, polymer electrolyte, and Na metal were initially cut to 9 mm × 9 mm and introduced into the Raman cell with observation window to observe the cross-section of the cell. Raman spectroscopy was performed to the cross-section of the cell before and after charge / discharge test in order to investigate the degradation behavior of PE and polymer electrolyte.
Fig. 1 shows charge / discharge profiles of non-coated and 10 wt.% NZSP coated PE in the cells. The 1st discharge capacity was 36 mAhg-1 in the non-coated system, and was also the insufficient capacity of about 30% from the theoretical capacity (ca. 118 mAhg-1) of NCO. In contrast, the NZSP-coated PE improved the 1st discharge capacity to 60 mAhg-1, and clear plateau was observed owing to their crystal structure change of NCO. These results suggest that inorganic oxide served as sufficient ionic conduction path between PE and electrolyte, reducing the interfacial resistance. Fig. 2 shows the cycle number dependencies of capacity and Coulombic efficiency. ASSB with inorganic oxides coated PE showed high discharge capacity retention and Coulombic efficiency in comparison with non-coated system. These results are suggesting that inorganic oxides coating active material surface suppress increase of interfacial resistance and degradation of the PE/electrolyte by prevent the formation of side products such as Co3O4
[2]. NZSP has higher Na ionic conductivity than Na3PO4, and can cover more apparent area of NCO surface than Na3PO4 because of their smaller particle size. Therefore, from comparing Na3PO4 and NZSP coating, NZSP more contributes to suppressing degradation of the cell. Fig. 3 shows the results of XRD analysis for PEs after charge / discharge test. Peaks attributed to NCO were confirmed in the PE before degradation, however, peak intensity decreased after degradation, especially for (003) and (006) of NCO. The peak derived from Co3O4 was confirmed only after degradation. These results suggest that structural degradation of NCO occurs during charging and discharging of the cell, resulting in the formation of Co3O4. In addition, the inorganic oxide coating should be contributed to maintaining the structure of the active material. In this presentation, the analysis results of structural changes of PE and electrolyte before and after charge / discharge test by Raman spectroscopic measurement will be also reported.
References: [1] S. Seki, et.al ., J. Electrochem. Soc., 153 (6) A1073-A1076 (2006). [2] J. Qiu, et.al ., Adv. Funct. Mater., 30(22) 1909302 (2020).

Figure 1