LITMIR.BIZ

Скачать книгу

gray box highlights the high voltage region, where O is the active redox process. (b) Galvanostatic charge/discharge curves in 3.0–4.7 V versus Na/Na⁺ at a rate of C/20. The inset shows a cyclic voltammetry curve at the second cycle at a scan rate of 0.1 mV s⁻¹. Reprinted with permission from Mortemard de Boisse et al. (2018). Copyright 2018, Wiley-VCH. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip

      1.3.1.4. O3-NaFeO₂

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.

      1.3.1.5. O3-NaCoO₂

      Electrochemical properties of O3-NaCoO₂ in Na cells were first reported by Braconnier et al. in 1980 (Braconnier et al. 1980) at almost the same time when those of O3-LiCoO₂ in Li cells were first reported by (Mizushima et al. 1980). O3-NaCoO₂ delivers a reversible capacity of ca. 140 mAh g⁻¹ in the voltage range of 2.5–4.0 V (Figure 1.8) (Yoshida et al. 2013; Lei et al. 2014). As discussed above on O’3-NaMnO₂, O3-NaCoO₂ represents a stepwise voltage profile that is attributed to multiple phase transitions associated with CoO₂-slab gliding and in-plane Na⁺/vacancy ordering in the interslab spacing as described in section 1.2.2.

      1.3.1.6. O’3-NaNiO₂

      Electrochemical properties and phase transitions of O’3-NaNiO₂ in Na cells were first reported by Braconnier et al. in 1982 (Braconnier et al. 1982) and re-investigated by Vassilaras et al. in 2013 (Vassilaras et al. 2013). O’3-NaNiO₂ delivers a reversible capacity of ca. 100 mAh g⁻¹ with stepwise voltage profile in the voltage range of 1.25–3.75 V (Figure 1.8). Detailed phase transitions during charge-discharge were reported by Han et al. (2014) and Wang et al. (2017a) with operando XRD measurements. The results revealed that O’3-NaNiO₂ transforms into at least six phases of O’3 and P’3 during a charging process. Both the authors found that no original O’3 phase was observed on the discharging process and the irreversible phase transition was thought to cause capacity degradation during charge–discharge cycles. Unlike Ti, V, Cr, Mn and Fe, but like Co system, O’3-NaNiO₂ delivers a large discharge capacity of ca. 130 mAh g⁻¹ even after charging to the high voltage of 4.5 V versus Na (Vassilaras et al. 2013; Wang et al. 2017a). Wang et al. revealed from in situ synchrotron XRD that the Na-extracted phase of Na_0.17NiO₂ is irreversibly formed upon charging to 4.5 V and is partly remained in the core of the particles even after discharging to 2.0 V versus Na. Migration of nickel ions into interslab space was often observed for O3-LiNiO₂ at the end of charging to 4.45 V versus Li (Croguennec et al. 2001). However, Li et al. mentioned no migration of nickel ions for O’3-NaNiO₂ after charging to 4.5 V versus

Na from the unpublished HRTEM image in the literature (Li et al. 2016). The reaction mechanism of O’3-NaNiO₂ might be different to that of O3-LiNiO₂.

      These fundamental studies are very helpful to understand the electrochemical properties of the complicated multiple transition metal systems. Transition metals usually dominate redox potential and phase transitions of O3-type layered materials. However, those of multiple 3d transition metal systems are not simple and are influenced by difference in oxygen orbital contribution to the redox reaction (Nanba et al. 2016). Not only nominal valence of transition metals but also hybridization between oxygen 2p and transition metal 3d orbitals plays a critical role in determining the redox potential of layered transition metal oxides in Na batteries. Redox potential and electrochemical activity of nickel ions vary on the other transition metal elements in NaMO₂ (Nanba et al. 2016). Selection of the transition metals is, therefore, important to synthesize ternary and quaternary 3d transition metal systems exhibiting excellent electrochemical performance.

      1.3.2. O3-Na[M,M’]O₂ (M, M’ = transition metals)

      Cobalt-containing sodium layered oxides often exhibit good rate performance in Na cells. However, the less abundant elemental resources do not meet the demand for Na-ion batteries, and iron and/or manganese-based oxides have been extensively studied as positive electrode materials for Na-ion batteries in the past decade (Yabuuchi and Komaba 2014).

      1.3.2.1. O3-Na[Fe,M]O₂

Electrochemical properties of iron-based multiple transition metal oxides in a Na cell were first reported by Okada and colleagues (2006). They compared electrochemical properties of O3-NaFeO₂, NaFe_1/2Ni_1/2O₂ and NaNi_1/2Ti_1/2O₂ in Na cells (Okada et al. 2006). O3-NaFe_1/2Ni_1/2O₂ delivers a relatively larger reversible capacity of ca. 110 mAh g⁻¹ corresponding to 0.4 Na extraction from NaFe_1/2Ni_1/2O₂ in the voltage range of 2.0–3.8 V (Wang et al. 2014) compared to ca. 80 mAh g⁻¹ for O3-NaFeO₂ (Yabuuchi et al. 2012b). Electronic states of Fe and Ni in O3-NaFe_1/2Ni_1/2O₂ are high spin (HS) Fe³⁺ (t_2g³e_g²) and LS Ni³⁺ (t_2g⁶e_g¹) configurations respectively. Vassilaras et al. revealed by density functional theory (DFT) calculations and Electron Energy Loss Spectroscopy (EELS) that Fe³⁺ oxidation is responsible for most of the electrochemical charging reaction in 2.0–3.9 V versus

Скачать книгу