Design of Stoichiometric Layered Potassium Transition Metal Oxide for K-Ion Batteries

The layered transition metal oxides (TMOs) which have been investigated as cathode materials for K-ion batteries (KIBs) have so far exhibited moderate specific capacity and rate capability. [1-6] However, all the layered K-TMOs reported to date are K-deficient phases ( x ≤ 0.7 in K x TMO 2 ), [1-6] which limits their use in practical rocking-chair batteries because a pre-potassiation process of the electrodes would be required to insert enough K in the cells. In this respect, it is important to understand the factors that destabilize (or stabilize) the layered structure of K x TMO 2 ( x = 1) and then design a stoichiometric K x TMO 2 ( x = 1) cathode material. In this work, we find that the strong electrostatic repulsion between K ions due to the short K + -K + distance destabilizes the layered structure in a stoichiometric composition of KTMO 2 . However, a stoichiometric KCrO 2 is thermodynamically stable in the layered structure despite short K + -K + distance unlike other KTMO 2 compounds that form non-layered structures. The unique stability of layered KCrO 2 is attributable to the unusual ligand field preference of Cr 3+ in octahedral sites that can compensate for the energy penalty from the short K + -K + distance. Therefore, we develop the stoichiometric layered KCrO 2 cathode material for KIBs and investigate its K-storage properties. In K-half cells, the KCrO 2 cathode delivers a reversible specific capacity of ~90 mAh/g with an average voltage of ~2.73 V ( vs. K/K + ). In-situ diffraction and electrochemical characterization further demonstrate multiple phase transitions via reversible topotatic reactions occurring as the K content changes. References Vaalma, C., et al. Non-aqueous K-ion battery based on layered K 0.3 MnO 2 and hard carbon/carbon black. J. Electrochem. Soc. 163, A1295 (2016) Kim, H. et al. K-ion batteries based on a P2-type K 0.6 CoO 2 cathode. Adv. Energy Mater. 7, 1700098 (2017) Hironaka Y. et al. P2- and P3-K x CoO 2 as an electrochemical potassium intercalation host. Chem. Commun. 53, 3693 (2017) Kim, H. et al. Investigation of potassium storage in layered P3-type K 0.5 MnO 2 cathode. Adv. Mater. 29, 1702480 (2017) Wang, X. et al. Earth Abundant Fe/Mn-based layered oxide interconnected nanowires for advanced K-ion full batteries. Nano Lett. 17, 544 (2017) Liu, C. et al. K 0.67 Ni 0.17 C 0.17 Mn 0.66 O 2 : A cathode material for potassium-ion battery. Electrochem. Commun. 82, 150 (2017)