Multilayer Cell System with Reference/Carrier-Ion Source Electrodes Elucidates the Degradation Mechanism Governing All-Oxide Solid-State Batteries

Oxide-based solid-state batteries are highly promising as next-generation batteries owing to their outstanding chemical stability. However, there is limited understanding regarding their electrochemical properties and degradation mechanisms because it is difficult to carry out detailed electrochemical analyses and track the full cell degradation process. This report describes the development of a multilayer cell system for multiple electrochemical analyses in oxide-based solid-state batteries, which enables monitoring of each electrode’s potential during charge/discharge cycles. The shifting of electrode operation windows caused by active carrier ion loss can also be determined, and symmetric cells can be constructed at various states of charge to measure the electrochemical impedance of a single electrode. The developed multilayer cell system is characterized by pairs of a solid electrolyte layer and a reference/carrier-ion source electrode layer, which together sandwich the conventional three-layer full cell. The multilayer cell system investigated in this study comprises a targeted all-oxide solid-state sodium-ion battery with Na3Ti2(PO4)3 as the bipolar active material and Na3Zr2(SiO4)2(PO4) (NZSP) as the solid electrolyte. Na3V2(PO4)3 is used as the active material of reference/carrier-ion source electrodes. Evaluations of this targeted battery with a multilayer cell system elucidate the degradation mechanism governing the all-oxide solid-state Na3Ti2(PO4)3 battery. The observed capacity fade is attributed to the loss of active carrier ions, mainly due to the decomposition of the solid electrolyte NZSP exposed to low potential at the anode. Notably, the developed multilayer cell system can be applied to other targeted oxide-based solid-state batteries, thereby facilitating broader electrochemical and degradation analyses.