New electrode materials of layered oxides, Na2/3Ni1/3Mn2/3-xTixO2 (0 ≤ x ≤ 2/3), are successfully synthesized, and their electrochemical performance is examined in aprotic Na cells. A Na//Na2/3Ni1/3Mn1/2Ti1/6O2 cell delivers 127 mA h g(-1) of reversible capacity and the average voltage reaches 3.7 V at first discharge with good capacity retention.

Electrochemical properties and structural changes during charge for NaCrO 2 , whose structure is classified as α-NaFeO 2 type layered polymorph (also O3type following the Delmas' notation), are examined as a positive electrode material for nonaqueous Na-ion batteries. NaCrO 2 delivers initial discharge capacity of 110 mAh g −1 at 1/20C rate in the voltage range of 2.5−3.6 V based on reversible Cr 3+ /Cr 4+ redox without oxidation to hexavalent chromium ions, while the initial discharge capacity is only 9 mAh g −1 when cutoff voltage is set to 4.5 V. Results from exsitu X-ray diffraction, X-ray absorption spectroscopy, and DFT calculations reveal that the irreversible phase transition occurs after sodium extraction by charging over a voltage plateau at 3.8 V associated with the lattice shrinkage along the c-axis in the case of x > 0.5 in Na 1−x CrO 2 , which originates from the migration of chromium ions from octahedral sites in CrO 2 slabs to both tetrahedral and octahedral sites in interslab layer. The irreversible structural change would disturb sodium insertion into the damaged layer structure during discharge, resulting in the loss of reversibility as electrode materials. Reversible cycle range with stable capacity retention is, therefore, limited to the compositional range of 0.0 ≤ x ≤ 0.5 in Na 1−x CrO 2 .

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A titanium-substituted lithium-excess molybdenum oxyfluoride, Li2.1–y Ti0.2Mo0.7O2F, is synthesized by mechanical milling and tested as a positive electrode material in a conventional carbonate-based electrolyte or concentrated electrolyte. Reversibility as the electrode material is significantly improved by suppression of dissolution of the molybdenum oxyfluoride on electrochemical cycles. Li2.1–y Ti0.2Mo0.7O2F delivers a reversible capacity of 265 mA h g–1 after 30 cycles at a rate of 50 mA g–1 with concentrated electrolyte. This finding contributes to the development of high-energy and long-cycle-life rechargeable lithium batteries with lithium-excess metal oxyfluorides in the future.

Thermal stability and phase transition processes of NaCrO and NaCrO are carefully examined by high-temperature synchrotron X-ray diffraction method. O3-type NaCrO shows anisotropic thermal expansion on heating, which is a common character as layered materials, without phase transition in the temperature range of 27-527 °C. In contrast, for the desodiated phase, in-plane distorted P3-type layered oxide (P'3 NaCrO), phase transition occurs in the following order. Monoclinic distortion associated with Na/vacancy ordering is gradually lost on heating, and its symmetry increases and changes to a rhombohedral lattice at 207 °C. On further heating, phase segregation to two P3 layered metastable phases, which have different interlayer distances (17.0 and 13.5 Å, presumably sodium-rich and sodium-free P3 phases, respectively) are observed on heating to 287-477 °C, but oxygen loss is not observed. Oxygen loss is observed at temperatures only above 500 °C, resulting in the formation of corundum-type CrO and O3 NaCrO as thermodynamically stable phases. From these results, possibility of NaCrO as a positive electrode material for safe rechargeable sodium batteries is also discussed.