Extending Electrochemical Window of Na⁺ Halide Solid Electrolyte for All-Solid-State Na Ion Battery

All-solid-state Na ion batteries (ASNBs) using inorganic solid electrolytes (SEs) have attracted much interest owing to their combined potential advantages of improved safety and cost, compared to conventional lithium-ion batteries with flammable liquid electrolyte. Most studies on Na⁺ SEs have focused on increasing ionic conductivity. Meanwhile, sulfide Na⁺ SEs are highly attractive because they can be easily integrated into ASNB cells by simple cold-pressing-based fabrication. However, sulfide SEs are limited in operating low-voltage batteries due to their electrochemical instability over 2.5 V region (<i>vs.</i> Na/Na⁺), which thus hinders achieving high energy density of ASNBs.<br/>To alleviate this shortcoming, Na⁺ halide SEs for 3 V class ASNB cells with NaCrO₂ have also been reported, but the number of the experimental reports is only a few: NaAlCl₄, Na₂ZrCl₆, Na_3-xEr_1-xZr_xCl₆, and Na_3-xY_1-xZr_xCl₆.<br/>In this presentation, our recent results on the development of new Na halide SE using interfacial Na⁺ ion conduction with wide electrochemical window. Application to all-solid-state Na⁺ ion batteries with operating voltage of 1.5-5.0 V (vs. Na/Na+) will be also presented.<br/><br/>[1] Hiram Kwak, et al., <i>Energy Storage Materials</i>, <b>37</b>, 47 (2021).<br/>[2] Hiram Kwak, et al., <i>ACS Energy letters</i>, <b>7</b>, 1776 (2022).<br/>[3] Juhyoun Park, et al. <i>ACS Energy Letters</i>, <b>7</b>, 3293 (2022)