Ionic Conductivity and Phase Transition of the Bromide Spinels, Li2 − 2x M 1 + x Br4 ( M = Mg , Mn )

Abstract

A new bromide spinel Li2MgBr4has been found in the LiBr-MgBr2system. The orthorhombic cell dimensions are a = 7.771, b = 11.04, and c = 7:735A at room temperature. The electrical conductivities of the new bromide spinel and the related spinel Li2MnBr4have been measured. Li2MgBr4and Li2MnBr4have been found to have high ionic conductivities of 4.5 x 10-2-Scm-1and 7.3 x 10-2-Scm-1at 400°C, respectively. High temperature x-ray diffraction analysis indicates a phase transition from a distorted structure to a cubic structure around 300°C for Li2MgBr4and around 270°C for Li2MnBr4. Electrical conductivity measurements suggest that the transitions correspond to those from the low to high ionic conductivity phase. A solid solution is observed on substitution of lithium ions for Mg2+or Mn2+ions. The substitution leads to higher ionic conductivity only for the cubic spinel. No significant composition dependence of the conductivity or the activation energy for conduction has been observed in the distorted structures. © 1986, The Electrochemical Society, Inc. All rights reserved.