Synthesis and Characterization of Von Alpen-Type Nasicon Ceramic and Its Application to Rechargeable Seawater Battery

Rechargeable seawater battery (SWB), which utilizes naturally abundant seawater as its cathode, is having attention for a next-generation large-scale energy storage system (ESS) due to its low cost and eco-friendliness. For an effective Na+ transfer solid electrolyte for SWB, Hong-type NASICON (Na3Zr2Si2PO12) ceramic has been used because of its known high Na-ion conductivity (~3x10-4 S/cm). However, the SWB coin cell test showed that the Hong-type NASICON ceramic accounts for 62% (132Ω) of the total cell resistance (214 Ω). This has been considered as one of major drawbacks for developments of SWB technology. Hence, it has been requested to achieve higher ionic conductivity of Na-ion conducting ceramics, increasing power of the current SWB by reducing its cell ohmic resistance. In this talk, von Alpen-type NASICON (Na3.3Zr1.55Si0.7P1.3O11) ceramic was synthesized by solid-state reaction and was observed to have ~1.0x10-3S/cm of Na-ion conductivity, which three times higher than that of Hong-type one. It was found that the von Alpen-type NASICON has more rhombohedral phases with a bigger grain size (2~6μm) compared to the major phases of Hong-type one, and additionally less secondary phases of ZrO2 were observed in the von Alpen-type NASICON. These differences were believed to be major reasons for higher Na-ion conductivity observed in von Alpen-type NASICON ceramic. These structural characteristics were also found to led to a stronger mechanical strength (50MPa) of the von Alpen-type NASICON rather than that (20MPa) of Hong-type one, which enables us to have thinner thickness (~0.5mm) of the ceramic solid electrolyte at the present work. With the use of von Alpen-type NASICON having a higher Na-ion conductivity and thinner thickness in SWB coin cell, it shows 96% of the power increased (to 9.8 mW/cm2 from 5.1 mW/cm2 of the cell using Hong-type ceramic). More details will be reported in the talk.