Stability of garnet-type solid electrolyte LixLa3A2 -yByO12 (A = Nb or Ta, B = Sc or Zr)

•Garnet-like Li6.25La3A2 -xBxO12 (A = Nb5 + or Ta5 +, B = Sc3 + or Zr4 +) were synthesized.•The samples composed of Ta showed high stability to lithium metal.•The samples were easily proton-exchanged in water.•The samples were stable against an electrolyte for aqueous Li-air battery system.

Garnet-type solid electrolytes LixLa3A2 -yByO12 (A = Nb5 + or Ta5 +, B = Sc3 + or Zr4 +) were examined with respect to their stability in contact with lithium metal, water, and a liquid electrolyte for application in an aqueous lithium-air secondary battery. Electrolyte samples were synthesized by a solid state reaction method using a magnesia rather than alumina crucible to avoid aluminum contamination. The garnet-type electrolytes including Zr4 + ions exhibited higher ionic conductivity than those with Sc3 + ions. The electrolytes including Ta5 + were stable in contact with lithium metal, whereas those with Nb5 + were not. Lithium ions in all the samples were partly proton-exchanged in distilled water; however, the garnet-like structure remained unchanged as reported in many papers. In contrast, all the electrolyte samples were stable in the aqueous solution of 10 M LiCl and 2 M LiOH (suitable electrolyte for aqueous Li-air battery system). The garnet-type lithium ion conductors comprising Zr4 + and Ta5 + ions were revealed to be suitable for use as the protective membrane between the lithium metal electrode and aqueous electrolyte in rechargeable lithium-air batteries.