can act as a physical resistance inhibiting dendrite plating through the electrolyte avoiding potential safety hazards such as thermal runaway. Furthermore, the nonflammable SSEs are not a fire hazard, thus, the cell can fail safely if short circuit occurs. Among the inorganic SSEs, Na super ionic conductor (or commonly known as NASICON) electrolyte, with a stoichiometric formula of Na 1+x Zr 2 Si x P 3-x O 12 (or NZSP), has a strong commercial potential due to its high room temperature ionic conductivity, large electrochemical stable window, ease of synthesis, and high stability in ambient condition. [4] Besides realizing an electrochemically stable and a low interfacial resistance, the physical contact interface between the metallic Na and SSE has to remain intact during the repeated (de)sodiation. However, pore formation can occur at the interface during the desodiation. [5] This deteriorates the physical contact area and an increase in the local current density. Eventually, premature electrolyte failure due to the rapid formation of Na dendrites is inevitable. The pores formation at the interface can be addressed to the sluggish diffusion kinetics in the anode layer during the desodiation. [5b,c,6] Every Na + ion desodiated at the interface introduces a vacancy in the metallic Na and this vacancy can be annihilated by vacancy diffusion or adatom diffusion. [5a,7] However, if the dissolution current is higher than the Na + diffusion kinetics in the anode, the vacancy would accumulate rapidly forming pores at the interface. To address the sluggish diffusion kinetics, a high stacking pressure was applied and the metal would creep ensuring the intimate contact during repeated (de)sodiation. [5b,c] However, this is not a practical solution as an enormous stacking force is required for a practical battery pack and the thin SSE may suffer from mechanical failure due to the pressure. Recently, it was reported that a high stacking pressure can cause metallic Na to creep through the SSE and short-circuit the cell. [8] Alternatively, a fast diffusion kinetics in the anode layer will promote replenishment of the vacancy before pores begin to form at the interface preserving the intimate physical contact. In retrospect, Na alloys present to has a high diffusivity kinetic that can effectively reduce the pores formation. [9] Though many have reported good surface wetting by surface coating or alloying, little has emphasized on the significance of Na + diffusion kinetics that leads to better galvanostatic cycling All-solid-state alkaline metal batteries are perceived as the "holy-grail" high energy density storage system. A robust physical contact between the anode and the solid-state electrolyte is paramount for a stable cycling. However, the sluggish Na + diffusion kinetic in the metallic sodium results in loss of physical contact during desodiation and promotes rapid sodium penetration. Herein, instead of applying high stacking pressure, a composite anode consisting of Na and Na 15 Sn 4 is proposed to be the anode...
