Non-aqueous potassium-ion batteries: a review

•Like lithium but unlike sodium, potassium intercalates into graphite.•Potassium ions show an increased mobility in electrolytes and selected materials.•Hexacyanometalates can achieve energy densities higher than 500 Wh kg−1 vs. K metal.•Polyanionic KVPO4F shows a high discharge potential of 4.1 V vs. K metal.•Selected hard carbon materials were cycled for 4000 cycles with about 175 mAh g−1 vs. K metal.