Pressureless Sintered Dense, Free Standing, Li+ Conducting Garnet Thin Films (< 50 µm) from Flame Made Nanoparticles

Abstract

Li7La3Zr2O12 (LLZO) garnet electrolytes show the highest potential utility in future solid state Li batteries due to a wide electrochemical stability window, stability in contact with lithium metal, and high ionic conductivity. The literature is replete with methods of optimizing compositions targeting optimal conducting properties, however, no examples of thin films (< 50 µm) with ionic conductivities rivaling bulk counterparts (0.1-0.3 mS cm-1) have been reported. To date, LLZO has been pressed into pellets (1-2 mm) and either pressureless sintered at 1100-1200 ºC for 10-40 h covered in mother powder or at 40-60 MPa at 1000-1100 ºC for 1-4 h. Both approaches are energy intensive and the scalability is questionable. Although LiTi2(PO4)3 solid electrolytes have been studied extensively and processed by tape-casting, a low cost approach, no similar LLZO examples have been reported. Some challenges in processing LLZO thin films include high lithium loss rates, reaction of LLZO with common ceramic crucibles, and energy and equipment intensive sintering processes. In this work, LLZO nano-powders produced by LF-FSP (liquid-feed flame spray pyrolysis) are used to formulate ball-milled suspensions that are then cast. Dried green films can be made in a process that offers potential for continuous casting. Films are cut and sintered at selected sintering schedules providing dense thin films. 30-50 wt. % excess lithium is used to compensate for lithium loss during sintering. Green films were sintered above 900 ºC for selected dwell times to show that even a difference of 0.2 h at peak temperature undergo phase composition changes due to lithium loss. Sintered films show trans-granular fracture mode with ionic conductivities of 0.1-0.2 mS cm-1 and >95 wt. % LLZO. The very first example of LLZO thin films with similar properties to bulk counterpart is reported. Furthermore, shorter processing steps and lower energy input for sintering compared to other reported methods make usage of LF-FSP synthesized nano-powders more desirable.