New Insight into Structural Evolution in Layered NaCrO₂ during Electrochemical Sodium Extraction

Electrochemical properties and structural changes during charge for NaCrO₂, whose structure is classified as α-NaFeO₂ type layered polymorph (also O3-type following the Delmas’ notation), are examined as a positive electrode material for nonaqueous Na-ion batteries. NaCrO₂ 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 Cr³⁺/Cr⁴⁺ 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 ex-situ 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–xCrO₂, which originates from the migration of chromium ions from octahedral sites in CrO₂ 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–xCrO₂.