Enhancing Na ion conductivity of Na chloride cathode material NaFeCl ₄ through polyatomic anion substitution

Na-ion batteries are anticipated for large-scale energy-storage applications due to their resource advantages over Li-ion batteries. Chloride-based cathode materials exhibiting high voltage, offers the possibility to achieve high energy density. While chloride electrodes suffer from leaching into liquid electrolytes, employing all-solid-state batteries with solid electrolytes can mitigate this issue. A sodium chloride cathode material NaFeCl₄ has been reported to operate at a high voltage derived from chloride (3.45 V vs. Na/Na+) and demonstrates a high energy density of 281 Wh (kg-cathode)−1. However, the ionic conductivity of NaFeCl4 is relatively low (5.9 × 10−6S cm−1 at 30 ºC). In this study, we conducted anion substitution to improve the ionic conductivity of NaFeCl4 and found that substituting the chloride ion Cl− with the nitrite ion NO₂−, which has the same valence and a similar ionic radius, increased the ionic conductivity by approximately fourfold (2.7 × 10−5S cm−1 at 30 ºC). Raman spectroscopy and XRD measurements confirmed the incorporation of nitrite ion (NO₂−) units into the lattice, leading to lattice expansion. These findings demonstrate that the substitution of polyatomic anions is a promising approach for expanding the compositional space of chloride-based solid solutions and enhancing their ionic conductivity.