Suppressing the chromium disproportionation reaction in O3-type layered cathode materials for high capacity sodium-ion batteries

Chromium-based layered cathode materials suffer from the irreversible disproportionation reaction of Cr⁴⁺ to Cr³⁺ and Cr⁶⁺, which hinders the reversible multi-electron redox of Cr ions in layered cathodes, and limits their capacity and reversibility. To address this problem, a novel O3-type layer-structured transition metal oxide of NaCr_1/3Fe_1/3Mn_1/3O₂ (NCFM) was designed and studied as a cathode material. A high reversible capacity of 186 mA h g⁻¹ was achieved at a current rate of 0.05C in a voltage range of 1.5 to 4.2 V. X-ray diffraction revealed an O3 → (O3 + P3) → (P3 + O3′′) → O3′′ phase-transition pathway for NCFM during charge. X-ray absorption, X-ray photoelectron and electron energy-loss spectroscopy measurements revealed the electronic structure changes of NCFM during Na⁺ deintercalation/intercalation processes. It is confirmed that the disproportionation reaction of Cr⁴⁺ to Cr³⁺ and Cr⁶⁺ can be effectively suppressed by Fe³⁺ and Mn⁴⁺ substitution. These results demonstrated that the reversible multi-electron oxidation/reduction of Cr ions can be achieved in NCFM during charge and discharge accompanied by CrO₆ octahedral distortion and recovery.