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

Abstract

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 Na_2+δMn_2−δ/2(SO₄)₃ with an alluaudite-type of structure by simple dehydratation of the corresponding dihydrate Na₂Mn(SO₄)₃·2H₂O with a kröhnkite-type of structure. The structure of Na_2+δMn_2−δ/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 kröhnkite 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 Na_2+δMn_2−δ/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.