From krohnkite- to alluaudite-type of structure: novel method of synthesis of sodium manganese sulfates with electrochemical properties in alkali-metal ion batteries

The alluaudite-type of structure is of huge research interest as an open matrix ensuring fast alkali-metal ion mobility, a property that could contribute to the development of novel electrode materials for rechargeable alkali-metal ion batteries. In this contribution, we provide new data on the formation of well-crystallized sodium manganese sulfates Na2+delta Mn2-delta/2(SO4)(3) with an alluaudite-type of structure by simple dehydratation of the corresponding dihydrate Na2Mn(SO4)(3)center dot 2H(2)O with a krohnkite-type of structure. The structure of Na2+delta Mn2-delta/2(SO4)(3) is determined on the basis of Rietveld refinement of powder XRD patterns, infrared (IR) and Raman spectroscopy and electron paramagnetic resonance at X- and Q-band frequencies (EPR). From a structural point of view, the release of two H2O molecule from the krohnkite phase takes place by a transformation of the infinite [Mn(SO4)(2)(H2O)(2)] chains into Mn2O10 dimers bounded by distorted Na(1)O-polyhedra. The anhydrous sulfates are able to participate in the electrochemical reaction delivering a reversible capacity of 135 mA h g(-1), when they are used as cathode materials in lithium ion cells. The stability of the alluaudite phase Na2+delta Mn2-delta/2(SO4)(3) in the lithium electrolyte solution and the mechanism of the electrochemical reaction are discussed on the basis of ex situ EPR, IR and Raman spectroscopy. This is a first report on electrochemical activity of manganese-based sulfate with an alluaudite-type of structure.