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

The layered transition metal oxides (TMOs) have been investigated as cathode materials for Li-and Na-ion batteries because of their high specific capacity and rate capability. [1-3] In this respect, researchers have recently studied the layered TMOs as cathode materials for K-ion batteries, and they have so far exhibited only moderate specific capacity and rate capability. [4-9] However, all the layered K-TMOs reported to date are K-deficient phases ( x ≤ 0.7 in K x TMO 2 ), [4-9] which limits their use in practical rocking-chair batteries because in a typical alkali-intercalation battery system all the alkali is brought in through the cathode. The use of K-deficient phases in cathodes requires a pre-potassiation process of the electrodes in order to insert enough K in the cells. Therefore, it is vital 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 for K-ion batteries. 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 . [10] The stoichiometric KCrO 2 is thermodynamically stable in the layered structure despite a 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 + ). The practical feasibility of a KCrO 2 cathode is confirmed in a full-cell system using a graphite anode. In-situ diffraction and electrochemical characterization further demonstrate multiple phase transitions via reversible topotatic reactions occurring as the K content changes. References Blomgren, G. E. The Development and Future of Lithium Ion Batteries. J. Electrochem. Soc. 164, A5019 (2017) Nitta, N. et al. Li-ion battery materials: present and future. Nano Today 18, 252 (2015) Clement, J. R. et al. Manganese-Based P2-Type Transition Metal Oxides as Sodium-Ion Battery Cathode Materials. J. Electrochem. Soc. 162, A2589 (2015) 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) Kim, H et al. Stoichiometric Layered Potassium Transition Metal Oxide for Rechargeable Potassium Batteries. Chem. Mater. DOI: 10.1021/acs.chemmater.8b03228 (2018)