Читать книгу Na-ion Batteries - Laure Monconduit - Страница 27

Another elementally attractive material in terms of abundant resources is O3-NaFeO₂. Although O3-NaFeO₂ is well known as a typical structural type of α-NaFeO₂, the electrochemical properties were first reported by Takeda et al. in 1994 (Takeda et al.1994) and O3-NaFeO₂ delivers a discharge capacity of ca. 120 mAh g⁻¹ based on Fe^3+/4+ redox in the Li cell. The electrochemical Fe^3+/4+ redox activity was a surprising fact because O3-LiFeO₂ delivers a reversible capacity of <10 mAh g⁻¹ and is electrochemically inactive unlike O3-NaFeO₂. Moreover, O3-LiFeO₂ cannot be directly synthesized by a solid-state reaction and is prepared by Li⁺/Na⁺ ion-exchange from O3-NaFeO₂ (Ado et al. 1997) due to the relatively large ionic radius of Fe³⁺ as shown in Figure 1.3. As is the case in the Li cell, O3-NaFeO₂ is electrochemically active in a Na cell and delivers a reversible capacity of ca. 80 mAh g⁻¹ based on Fe^3+/4+ redox with a flat potential plateau at ca. 3.3 V versus Na⁺/Na in the voltage range of 2.5–3.4 V (Figure 1.8) (Takahashi et al. 2004; Okada et al. 2006; Yabuuchi et al. 2012b; Zhao et al. 2013; Li et al. 2018). Fe³⁺/⁴⁺ redox was confirmed with ⁵⁷Fe Mössbauer spectroscopy (Takeda et al. 1994; Zhao et al. 2013; Lee et al. 2015) and oxidation of oxide ions was recently revealed with soft XAS at O K-edge (Li et al. 2018). The mean working voltage of 3.3 V is the highest among single 3d transition metal O3 and O’3 systems. When Na⁺ ions are extracted by >0.5 mole from Na_xFeO₂ upon a charging process, the discharge capacity significantly deteriorates due to an irreversible structural change accompanied by Fe migration into the interslab space (Yabuuchi et al. 2012b) as seen in NaTiO₂, NaVO₂ and NaCrO₂. Detailed phase transitions during charge/discharge were studied with ex situ ⁵⁷Fe Mössbauer spectroscopy and operando synchrotron XRD by Lee et al. and the results revealed non-equilibrium phase-transition behavior among original O3, secondary O3 and O’3 phases in which iron ions simultaneously migrate into interslab space even in x ≤ 0.5 in Na_xFeO₂ (Lee et al. 2015). Furthermore, reduction of Fe⁴⁺ accompanied by electrolyte decomposition was also observed during storage for 2 days after charging to 3.6 V. Suppression of the electrolyte decomposition and of the migration of iron ions is required to utilize Fe^3+/4+ redox in layered oxides for Na-ion batteries.