Knowledge on the formation of mixed transition metal layers on lithium and sodium transition metal oxides, Li/Na(Co,Ni,Mn,)O, determines the ability to control their electrochemical properties as electrode materials in alkaline ion batteries. Taking this into account, herein we combine the EPR and Na MAS NMR spectroscopic techniques to gain insights into the structural peculiarities of the mixed cobalt-nickel-manganese layers of NaCoNiMnO with a three-layer stacking (P3-type) structure. Two types of compositions are examined where diamagnetic Co and paramagnetic Ni and Mn are stabilized: NaCoNiMnO and NaNiMnO. EPR spectroscopy operating in the X- and Q-band region is applied with an aim to improve the spectra resolution and, on the other hand, to provide straightforward information on the coordination of the transition metal ions inside the layers. The analysis of EPR spectra is based on the reference for the Mn and Ni ions occurring simultaneously in oxides with two layer stacking, P2-NaNiMnO. Complementary to EPR, Na MAS NMR spectroscopy at high spinning rates is undertaken to assess the local structure of the Na nucleus in the layered P3-NaCoNiMnO oxides. All results are discussed taking into account the EPR and NMR data for the well-known lithium analogues O3-LiCoNiMnO and O3-LiNiMnO. Finally, the structure peculiarities of the transition metal layers extracted from the EPR and NMR methods are demonstrated by electrochemical intercalation of Li ions into P3-NaCoNiMnO.
The development of lithium and sodium ion batteries without using lithium and sodium metal as anodes gives the impetus for elaboration of low-cost and environmentally friendly energy storage devices. In this contribution we demonstrate the design and construction of a new type of hybrid sodium-lithium ion cell by using unique electrode combination (Li4Ti5O12 spinel as a negative electrode and layered Na3/4Co1/3Ni1/3Mn1/3O2 as a positive electrode) and conventional lithium electrolyte (LiPF6 salt dissolved in EC/DMC). The cell operates at an average potential of 2.35 V by delivering a reversible capacity of about 100 mAh/g. The mechanism of the electrochemical reaction in the full sodium-lithium ion cell is studied by means of postmortem analysis, as well as ex situ X-ray diffraction analysis, HR-TEM, and electron paramagnetic resonance spectroscopy (EPR). The changes in the surface composition of electrodes are examined by ex situ X-ray photoelectron spectroscopy (XPS).
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+d Mn 2Àd/2 (SO 4 ) 3 with an alluaudite-type of structure by simple dehydratation of the corresponding dihydrate Na 2 Mn(SO 4 ) 3 $2H 2 O with a kröhnkite-type of structure. The structure of Na 2+d Mn 2Àd/2 (SO 4 ) 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 H 2 O molecule from the kröhnkite phase takes place by a transformation of the infinite [Mn(SO 4 ) 2 (H 2 O) 2 ] chains into Mn 2 O 10 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 À1 , when they are used as cathode materials in lithium ion cells. The stability of the alluaudite phase Na 2+d Mn 2Àd/2 (SO 4 ) 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.
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