By re-evaluating the outcome of conductivity measurements on carbonate-based composites recently reported in the literature, it is demonstrated that the disregard of fundamental relationships of electrochemistry makes the interpretation provided in the literature a pure speculation. Therefore, the supposed evidence of multi-ionic conduction and ionic conductivity enhancement must be regarded as completely unfounded. Despite the non-existence of these exceptional conduction properties, composite electrolytes comprising an alkali ion and an oxygen ion conductor may prove to be more advantageous than single-phase electrolytes due to a substantial difference in the nature of the electrode polarization during ion flow. (C) 2017 Elsevier Ltd. All rights reserved.