Degradation Analysis of Oxide-Based All-Solid-State Na Batteries by Operando Raman Spectroscopy

Oxide-based all-solid-state Na batteries (ASSBs) show promise as alternative systems of conventional Li-ion batteries for large-scaled renewable energy storage owing to the availability of the required resources and the safety of these devices. ASSBs are usually prepared by a sintering method and have several performance disadvantages, such as capacity degradation with cycling. In this study, we prepared [NaTi 2 (PO 4 ) 3 (NTP) negative electrode|Na 3 Zr 2 Si 2 PO 12 (NZSP) electrolyte|Na 4 Ni 3 (PO 4 ) 2 P 2 O 7 (NNPP) positive electrode] ASSBs by using a suitable sintering agent and analyzed the product by a combination of electrochemical and operando Raman spectroscopic methods. Significant irreversible capacity after the first cycle was confirmed by cyclic voltammetry. Microscopic operando Raman spectra were observed at the positive and negative electrodes and the electrolyte layer. Although the chemically stable NZSP layer did not show structural changes with a charge–discharge cycle, the spectra of the electrode active materials NNPP and NTP exhibited irreversible structural changes, which contributed to their large capacity losses. Operando spectroscopic techniques are an effective analytical tool for ASSBs owing to their physically separated sandwich structure and clear cross-section.