Comment on "A K2Fe4O7 superionic conductor for all-solid-state potassium metal batteries", by H. Yuan et al., J. Mater. Chem. A, 2018, <bold>6</bold>, 8413-8418

We comment on the article J. Mater. Chem. A, 2018, 6, 8413-8418 by Yuan et al., which describes the synthesis of a novel ionic conductor for potassium with excellent performance. We tried to replicate the synthesis, but unfortunately we obtained hematite instead, which is not a conductor of ions. We suggest that the material analyzed by the authors of the article is not a novel material but rather a mixture of hematite and potassium hydroxide, a residue of the synthesis. The observed conductivity is explained with the presence of an aqueous solution of potassium hydroxide, which forms due to air moisture.