Machinable, high‐conductivity NaSICON through mitigation of humidity effects during solid‐state synthesis

Abstract The Na + super ion conductor (NaSICON, Na 1+ x Zr 2 Si x P 3‐ x O 12 ) is a solid electrolyte well‐known for fast, selective Na + transport at low temperatures, uniquely enabling sodium‐based batteries. Producing high‐quality NaSICON from solid‐state methods, especially when cost‐effective, potentially hygroscopic precursors are used, is not trivial. To understand and eliminate the influence of humidity during processing, a scheme was developed to reproducibly yield a high Na + conductivity (3.75 mS/cm at 25°C, 81.7 mS/cm at 150°C), high density (97%), and machinable NaSICON without the use of binders, sintering aids, or dopants. Controlled humidity studies over 20%–50% RH coupled with thermal, structural, and electrical analysis reveal that calcination temperatures < 1000°C leave NaSICON processing susceptible to water absorption at > 20% RH due to the presence of hygroscopic Na 3 PO 4 and Na 2 CO 3 during shaping, pressing, and sintering. Water absorption results in NaSICON with lower densities, machinability, and Na + conductivity, due to impaired intergranular Na + transport. At the other extreme, fully converting precursor to the NaSICON phase at 1230°C before pressing and sintering leads to poor conductivity and density. By calcining at 1000°C, excellent quality NaSICON may be produced under a range of laboratory environments, enabling low‐cost production of high‐conductivity, machinable NaSICON necessary the ever‐growing energy storage market.