We report on a new ultrafast solid electrolyte of the composition Li11Si2PS12, which exhibits a higher room-temperature Li ion diffusivity than the present record holder Li10GeP2S12. We discuss the high-pressure synthesis and ion dynamics of tetragonal Li11Si2PS12, and comparison is made with our investigations of related members of the LMePS family, i.e. electrolytes of the general formula Li11-xMe2-xP1+xS12 with Me = Ge, Sn : Li10GeP2S12, Li7GePS8, Li10SnP2S12. The structure and dynamics were studied with multiple complementary techniques and the macroscopic diffusion could be traced back to fast Li ion hopping in the crystalline lattice. A clear correlation between the diffusivity and the unit cell volume of the LGPS-type electrolytes was observed.

Bulk Li2O2 is shown to exhibit ionic conductivity via lithium vacancies and electronic conductivity via electron holes (localized as superoxide ions). This is the first systematic study on the charge carrier chemistry of peroxides with high relevance for the performance kinetics of Li-oxygen batteries.

Conductivity, ionic transference number, and chemical diffusion coeffi cients are determined for KO 2 , RbO 2 , and CsO 2 . Based on such results, a defectchemical model is constructed. These superoxides are found to exhibit a total conductivity in the range of 3 × 10 -7 to 5 × 10 -5 S cm -1 at 200 °C with contributions from ionic and electronic carriers. The ionic conductivity is caused by alkali interstitials and superoxide vacancies as mobile defects, and is found to exceed the n-type electronic conductivity. 18 O isotope exchange on powder samples (monitoring the gas phase composition) shows that essentially all oxygen can be exchanged. At high p O 2 this largely occurs without breaking of the O-O bond-indicating a suffi cient mobility of molecular superoxide species in the solid-and with an effective rate constant that is much higher than for other large-bandgap mixed conducting materials such as SrTiO 3 .

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