Manganese Hexacyanomanganate as a Positive Electrode for Nonaqueous Li-, Na-, and K-Ion Batteries

Abstract

K2Mn­[Mn­(CN)6] is synthesized, characterized, and evaluated as possible positive electrode material in nonaqueous Li-, Na-, and K-ion batteries. This compound belongs to the rich and versatile family of hexacyanometallates displaying distinctive structural properties, which makes it interesting for ion insertion purposes. It can be viewed as a perovskite-like compound in which CN-bridged Mn­(CN)6 octahedra form an open framework structure with sufficiently large diffusion channels able to accommodate a variety of insertion cations. By means of galvanostatic cycling and cyclic voltammetry tests in nonaqueous alkali metal half-cells, it is demonstrated that this material is able to reversibly host Li+, Na+, and K+ ions via electrochemical insertion/deinsertion within a wide voltage range. The general electrochemical features are similar for all of these three ion insertion chemistries. An in operando X-ray diffraction investigation indicates that the original monoclinic structure is transformed into a cubic one during charging (i.e., removal of cations from the host framework) and that such a process is reversible upon subsequent cell discharge and cation reuptake.

Resolving the Seeming Contradiction between the Superior Rate Capability of Prussian Blue Analogues and the Extremely Slow Ionic Diffusion

“…The studied PBAs follow different ion insertion mechanisms, as reflected in the shape of the cyclic voltammograms (CVs) in Figure (the potential interval in aqueous solution corresponds to the reversible extraction of one K + per Fe in MHCF). As previously reported, for K-rich K-FeHCF and K-NiHCF samples, a monoclinic-to-cubic phase transformation occurs upon K + deinsertion. ,− Phase transition potentials correspond to sharp maxima in the differential capacitance versus potential plots (Figure B,D). In this case, diffusion coefficients can be estimated by PITT and EIS methods in more narrow potential intervals compared to K-poor CuHCF and NiHCF materials, where a solid solution pathway is dominant and no lattice rearrangements take place upon Fe oxidation and/or reduction. , The single-phase nature of Fe redox in CuHCF and NiHCF is corroborated by a smooth change in the differential capacitance with potential (Figure A,C).…”

Challenges and Strategies toward Cathode Materials for Rechargeable Potassium‐Ion Batteries

“…[89,90] Owing to the open-framework structures, two possible redox centers, and tunable chemical compositions, PBAs have received considerable attention as cathode materials of PIBs. [91,92] Among the various subgroups of PBAs, hexacyanoferrates (M′ = Fe) are currently the most important because of their relatively high redox potential, low-cost precursors, and ecofriendly synthesis. [21] The crystal structure of a typical hexacyanoferrate is shown in Figure 3a,b.…”

Octahedral tilting in Prussian blue analogues

“…The distortion in Na 2 MM 0 (CN) 6 switches from monoclinic a À a À b + to rhombohedral a À a À a À upon solvent removal accompanied by increased amplitude of the distortion. 11,41 Whether K-based PBAs also show hydration-switchable tilts is unknown: monoclinic symmetry is reported for several compounds with high concentration of K and low hydration states, 3,67 and the larger radius of K leaves less free space for water molecules. Furthermore, water alters the pressure-induced phase transition of MnPt(CN) 6 , which-at least in the hydrated case-is driven by tilts.…”

“…Porous, defective systems of formula M II [M 0III (CN) 6 ] 0.67 can be used for catalysis and adsorption of gases and toxic metals, 1,2 whereas defect-free, alkalicontaining PBAs, e.g. A 2 M 0II M 0II (CN) 6 , are suitable electrode materials 3 and/or may show interesting magnetic properties. 4 The stoichiometry and associated functionality is stipulated by the charges of the transition metals, where lower charges increase the scope for inclusion of vacancies and/or A-site cations [Fig.…”