Electronic states calculated by the DV-Xα cluster method for lithium ion conductive Li2S–SiS2–Li4SiO4 oxysulfide glasses

The electronic states of the Li2S–SiS2–Li4SiO4 oxysulfide glasses were investigated by using the DV-Xα cluster method as a non-empirical molecular orbital calculation. The calculation was carried out for the dimer-framed Li6Si2OmSn (m+n=7) cluster models expected to be present in the oxysulfide glasses. The bond states around lithium ions changed from covalent to ionic with an increase in the number of non-bridging oxygens. The increase of covalency of the Si–S and Si–O bonds in the frame structure brought about the increase of covalency between the frame and lithium ions. The bond states around lithium ions of the cluster with one bridging oxygen were similar to those of the cluster in the pure sulfide glasses. This similarity seems to be one of the reasons for high ionic conductivities, over 10−4 S cm−1, in the oxysulfide glasses with small amounts of lithium ortho-oxosalts.