K₃MnO₄ a New Cathode Material for K-Ion Batteries

To meet future energy demands, developing beyond-lithium energy storage technologies have become a major part of global research activities. Potassium-ion batteries (KIBs) represent a promising technology thanks to their chemical and economic advantages 1–3 . To succeed in developing commercial KIBs, it is necessary to develop new cathode materials 4 which must be non-toxic, inexpensive, easy to prepare and have a reasonable K + diffusion pathway to improve their electrochemical performance. Here we have chosen to explore the inexpensive and non-toxic K-Mn-O system, where only two-dimensional materials of the K x MnO 2 type have been studied so far for their electrochemical properties 5,6 . In this system, a composition attracted our attention: K 3 MnO 4 . In principle, up to two potassium ions can be removed from the structure, leading to KMnO 4 with a theoretical capacity of 227 mAh/g. Hagenmuller and co-workers succeeded in preparing two polymorphs of K 3 MnO 4 as single crystals 7 . The high temperature polymorph γ-K 3 MnO 4 has a cubic symmetry (P213) with a disordered distribution of K and Mn in similar tetrahedral sites. The low temperature polymorph β-K 3 MnO 4 , can be prepared by annealing γ-K 3 MnO 4 under argon. This polymorph has a tetragonal symmetry (I-42m) with ordered MnO 4 tetrahedra. Mn is reported at the edge and at the center of the tetragonal unit cell, surrounded by four oxygens. These phases could be described as a 0D-type structure built of isolated MnO 4 tetrahedra surround by potassium ions. We recently succeed in the synthesis of a new polymorph of K 3 MnO 4 , labelled α-K 3 MnO 4 , 8 prepared by a conventional solid-state route by annealing a mixture of KO 2 and MnO under vacuum at 500°C in a Al 2 O 3 boat. The structure of this polymorph could be described with a “zig-zag” distribution of the MnO 4 tetrahedra (Fig. 1a) and is isostructural to the iron-based K 3 FeO 4 9,10 with an orthorhombic symmetry (Pnma). The blue colour of the powder is classically associated to the Mn +V oxidation state, as confirmed by magnetic and EDS analyses. K 3 MnO 4 exhibits a reversible capacity of 70 mAh/g in the 1.6-3.5 V vs. K + /K potential window at C/20 (Fig. 1b). In our presentation, we will detail the electrochemical behaviour of K 3 MnO 4 and discuss the structure/properties relationship of this family of materials, showing the interest in the exploration of such a system. Xu, Y. et al. 2023 roadmap for potassium-ion batteries. J. Phys. Energy 5 , 021502 (2023). Tian, Y. et al. Promises and Challenges of Next-Generation “Beyond Li-ion” Batteries for Electric Vehicles and Grid Decarbonization. Chem. Rev. 121 , 1623–1669 (2021). Zhang, W., Liu, Y. & Guo, Z. Approaching high-performance potassium-ion batteries via advanced design strategies and engineering. Sci. Adv. 5 , eaav7412 (2019). Nathan, M. G. T. et al. Recent Advances in Layered Metal-Oxide Cathodes for Application in Potassium-Ion Batteries. Adv. Sci. n/a , 2105882. Liu, T. et al. Insight of K-deficient layered KxMnO2 cathode for potassium-ions batteries. J. Energy Chem. 64 , 335–343 (2022). Liu, C., Luo, S., Huang, H., Zhai, Y. & Wang, Z. Layered potassium-deficient P2- and P3-type cathode materials KxMnO2 for K-ion batteries. Chem. Eng. J. 356 , 53–59 (2019). Olazcuaga, R., Reau, J.-M., Leflem, G. & Hagenmuller, P. Préparation, Proprietés Cristallographiques et Magnétiques des Phases K3XO4 (X=V, Cr, Mn). Z. Für Anorg. Allg. Chem. 412 , 271–280 (1975). Sagot, A., Stievano, L. & Pralong, V. K3MnO4: A New Cathode Material for K-Ion Batteries. ACS Appl. Energy Mater. 6 , 7785–7789 (2023). Kokarovtseva, I. G., Belyaev, I. N. & Semenyakova, L. V. Oxygen Compounds of Iron(VI, V, IV). Russ. Chem. Rev. 41 , 929–937 (1972). Hoppe, R. & Mader, K. Zur Konstitution von K3[FeO4]. Z. F ür Anorg. Allg. Chem. 586 , 115–124 (1990). Figure 1