Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte

•LLZTO solid electrolyte can be short-circuited by lithium dendrites.•The period before short-circuit positively correlates to the relative density.•Lithium dendrites are observed directly inside short-circuited LLZTO electrolyte.•Lithium dendrites can grow through grain boundaries and interconnected pores.

This work investigated the resistance of Al-contained Li6.75La3Zr1.75Ta0.25O12 (LLZTO) sintered pellets to lithium dendrites. The short-circuiting period for Li/LLZTO/Li cells under a high applied direct current showed positive correlation to the variation in the relative densities of the LLZTO pellets sintered at different temperatures. Lithium dendrites were observed directly inside short-circuited LLZTO electrolytes. Our observations demonstrate that lithium dendrites can grow through grain boundaries and interconnected pores in the ceramic electrolytes.