Gassing in Sn-Anode Sodium-Ion Batteries and Its Remedy by Metallurgically Prealloying Na

Despite the high theoretical energy densities of tin as the anode of sodium-ion batteries, its electrochemical performance has been plagued by the inadequate initial Coulombic efficiency (ICE) and poor cycle life. While it is generally believed that mechanical degradation, namely, pulverization and subsequent loss of electrical contact, is the underlying cause, here, we show that gassing is an essential problem in sodium-ion batteries with Sn anode. Since the gas generation appears at a certain voltage, reducing the voltage by metallurgically prealloying Sn with Na could be a solution, with an additional benefit of compensating for "live" Na loss in future cycles. When the metallurgically alloyed foil is used as the anode in Na₃V₂(PO₄)₂F₃ (NVPF)//alloy full cells, the ICE is significantly improved from 24.68 to 75%, and from the 2nd cycle to the 100th cycle, the average Coulombic efficiency can be maintained up to 99.44%. The full cell can run for 100 cycles, with acceptable capacity decay to 81.4 mAh/g(NVPF) from the initial 112.5 mAh/g(NVPF).