Sodium ion batteries are attractive alternative energy storage technology to lithium-ion batteries due to its low-cost. There has been growing attention in developing new cathode materials for sodium ion batteries. The Iron-based layered oxide cathode is of significant interest due to the low-cost, abundant, environmentally-friendly material selection. O3 type α-NaFeO₂ cathode material was first introduced by Okada and coworkers.¹ It exhibited a capacity of 83 mAh g^-1 and excellent reversibility with the potential window of 1.5-3.6V. However, the capacity significantly decreased when charged beyond 3.5V, resulting from the irreversible phase formed at high voltage. In order to stabilize the structure at high cutoff voltage and thus to achieve high energy density, nickel and manganese were introduced to substitute Fe in the α-NaFeO₂ cathode to form Na(Ni_xFe_1-xMn_x)O₂ (NFM) structure with enhanced electrochemical performance. Layered Na(Ni_1/3Fe_1/3Mn_1/3)O₂ cathode was first reported by Kim et al. with a reversible capacity of 100 mA h g⁻¹ for 150 cycles (1.5–4.0 V).² The systematic study with various Fe composition were conducted by Yabuuchi et al.³ and Ding et al.⁴, both claiming ~130 mAh g^-1 reversible capacity from Fe=0.4 and Fe=0.2, respectively. Besides, the electrochemical performance of O3 type layered oxide cathode materials can be enhanced by lithium substitution.^5-8 Oh et al. reported O3-type Na[Li_0.05(Ni_0.25Fe_0.25Mn_0.5)_0.95]O₂ cathode with improved capacity retention and structural stability. The XRD data suggested that the phase transition from hexagonal O3 to monoclinic P′3 was delayed in Li substituted NFM cathode (Li-NFM), leading to an enhanced stability.⁶

Herein we demonstrate a new perspective that Li substitution in NFM cathode can facilitate Na diffusion due to the Jahn Teller (JT) effect resulted from high spin Mn ions. The ex situ soft X-ray adsorption spectroscopy (SXAS) qualitatively suggests that the amount of high spin Mn ions on the surface is larger than that in the bulk. The high spin Mn³⁺ ion has a half occupied majority spin e_g orbital favoring a JT distortion.⁹ Previously, Li et al. has discussed that Jahn Teller effect can assist Na diffusion through integrated experimental and simulation study.¹⁰ In our study, the galvanostatic intermittent titration technique (GITT) result indeed indicated that Na diffusion in Li-NFM is larger than that in NFM. In addition, structural stability of Li-NFM cathode was confirmed by ex situ selected area electron diffraction (SAED) and high resolution transmission electron microscopy (HRTEM). When fully charged, the well-defined crystalline structure of Li-NFM cathode was partially damaged due to the stacking fault, resulting in diffusive ring on SAED pattern and disordered HRTEM image. However, after discharged, the diffusive ring disappears and long-term ordering appears on the SAED and HRTEM, respectively, suggesting reversible reconstruction of crystal structure and great structural stability.

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