K3MnO4 a New Cathode Material for K-Ion Batteries
Authors: Sagot Armance , Stievano Lorenzo , Pralong Valerie 
Published: August 2024

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 advantages1–3. To succeed in developing commercial KIBs, it is necessary to develop new cathode materials4 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 KxMnO2 type have been studied so far for their electrochemical properties5,6.
In this system, a composition attracted our attention: K3MnO4. In principle, up to two potassium ions can be removed from the structure, leading to KMnO4 with a theoretical capacity of 227 mAh/g.
Hagenmuller and co-workers succeeded in preparing two polymorphs of K3MnO4 as single crystals7. The high temperature polymorph γ-K3MnO4 has a cubic symmetry (P213) with a disordered distribution of K and Mn in similar tetrahedral sites. The low temperature polymorph β-K3MnO4, can be prepared by annealing γ-K3MnO4 under argon. This polymorph has a tetragonal symmetry (I-42m) with ordered MnO4 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 MnO4 tetrahedra surround by potassium ions.
We recently succeed in the synthesis of a new polymorph of K3MnO4, labelled α-K3MnO4,8 prepared by a conventional solid-state route by annealing a mixture of KO2 and MnO under vacuum at 500°C in a Al2O3 boat. The structure of this polymorph could be described with a “zig-zag” distribution of the MnO4 tetrahedra (Fig. 1a) and is isostructural to the iron-based K3FeO4
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. K3MnO4 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 K3MnO4 and discuss the structure/properties relationship of this family of materials, showing the interest in the exploration of such a system.

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Figure 1