A Highly Reversible Electroconductive Liquid–Solid Electrolyte System for Na Metal Plating/Stripping with High Areal Capacity

Anode-free sodium metal batteries (AFSMB) are promising candidates for maximizing energy density and minimizing cost and safety hazards in the absence of metallic sodium during cell assembly. The practical implementation of AFSMBs is hindered by the low cycling stability of Na metal plating and stripping, particularly under high areal capacities. Despite the high energy density of AFSMBs, most of their reported areal capacities are less than 5 mAh/cm², which is a similar level to those of conventional lithium-ion batteries, owing to the irreversibility of Na metal plating and stripping. The main causes of irreversible metal plating/stripping can be classified into two categories: 1) electrolyte decomposition and the formation of a solid electrolyte interphase (SEI) layer, and 2) the formation of an electronically inactive dead metal.<br/>Here, we proposed an electroconductive liquid/solid dual electrolyte system that simultaneously suppresses the side reactions of electrolytes and confers electronic conductivity to connect the inactive dead Na. The electroconductive liquid electrolyte accepts electrons at a certain energy level and forms electronic conductivity, and the solid electrolyte can prevent internal short-circuit through a low electronic conductivity. Each function is verified using sodium biphenyl (NaBP) liquid electrolyte and NASICON (Na Super Ionic CONductor, Na_1+<i>x</i>Zr₂Si<i>_x</i>P_3-<i>x</i>O₁₂) solid electrolyte through a seawater battery (SWB) system, which consists of an infinite seawater sodium source at the cathode. Compared with conventional 1 M NaPF₆ in dimethoxyethane (DME) electrolyte, the NaBP electrolyte suppresses side reactions such as gas evolution according to the redox-active properties. In addition, the electronic conductivity of NaBP enables the full stripping of dead Na in the absence of direct contact with the current collector, and the stripping efficiency is quantitatively confirmed through the designed experiments. Finally, the anode-free SWB cells have achieved a high coulombic efficiency of &gt;99.9% for over 60 cycles at a high areal capacity of ~24 mAh/cm². This study provides insight into the Na plating/stripping properties in anode-free systems and proposes a significant strategy for improving the reversibility of metal anodes for various battery systems with solid electrolytes.