High Li ion conductivity in a garnet-type solid electrolyte via unusual site occupation of the doping Ca ions

•Mechanochemical doping of garnet-type solid Li-ion conductors with Ca places it at octahedral 6-coordination sites.•High ionic conductivity of 5 × 10− 4 S cm− 1 and wide electrochemical stability window of 9 V are obtained.•A generic strategy is identified for doping garnet structure with rare earth oxides to improve electrochemical properties.

The search for solid electrolytes with high stability and ionic conductivity is a long sought-after goal in the development of safe and high energy density Li-ion batteries. Garnet-type lithium conductors form a most promising family of materials due to their good chemical stability. However, the low conductivity at room temperature prevents wider application of these materials. It has long been recognized that alkaline earth metals are confined exclusively to dodecahedral 8-coordinated sites within the garnet framework. In contrast to this dominant viewpoint, we show that Ca2 + cations can occupy the octahedral 6-coordinated sites, leading to an enhanced room temperature conductivity of 5.2 × 10− 4 S cm− 1 and reduced activation energy of 0.27 eV, together with a 0 to 9 V electrochemical stability window. This finding opens up a new opportunity for the design of ceramic electrolytes with higher conductivities, providing added impetus for further exploration of oxide electrolyte chemistry.

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