another issue of the practical use of Na-ion is the low working potential of the positive electrode materials. The voltage of NaCoO2, isostructural to α-NaFeO2, in a Na cell at the voltage plateau region close to the end of discharge is ca. 2.5 V and much lower than ca. 3.9 V for α-NaFeO2 type LiCoO2 in a Li cell as shown in Figure 1.1. The large difference cannot be explained only by the standard redox potential of Na metal, which is lower than that of Li metal by ca. 0.3 V (Marcus 1985; Komaba et al. 2015). The voltage difference is much smaller than that between NaxCoO2 and LiCoO2 at the end of discharge (ΔV = ca. 1.5 V) (Kubota et al. 2014). The large voltage difference is probably due to larger ionic size and lower Lewis acidity of Na+ in comparison to Li+ as discussed by Goodenough et al. (1980).
Figure 1.1. Comparison of charge/discharge curves for Li//LiCoO2 and Na//NaCoO2 cells. A schematic illustration of the crystal structure of isostructural LiCoO2 and NaCoO2 is also shown. Reproduced with permission from Yabuuchi et al. (2014). Copyright 2014, American Chemical Society. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip
Since our group first demonstrated hard carbon//NaNi1/2Mn1/2O2 full cells exhibiting acceptable cycle stability in 2009 (Komaba et al. 2009) and the results were published in Advanced Functional Materials in 2011 (Komaba et al. 2011), yearly numbers of scientific papers on sodium batteries have rapidly increased (Kubota et al. 2018a) and significant efforts have been devoted to attaining high-voltage and high-capacity positive electrode materials for Na-ion batteries (Delmas et al. 1982; Pan et al. 2013; Slater et al. 2013; Xu et al. 2013; Yabuuchi et al. 2014; Clement et al. 2015; Kundu et al. 2015; Fang et al. 2016; Kim et al. 2016; Deng et al. 2017; Hwang et al. 2017b; Ortiz-Vitoriano et al. 2017).
Figure 1.2. Average voltage (V) and energy density (Wh kg−1) versus gravimetric capacity (mAh g−1) for selected positive electrode materials for Na-ion batteries. Energy density was calculated with the hard carbon (reversible capacity of 350 mAh g−1 with Eave = 0.3 V vs. Na metal) as negative electrode materials. Reproduced with permission from Kubota et al. (2018b). Copyright 2018, Wiley-VCH. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip
Figure 1.2 shows a comparison of reversible capacity and working voltage among the positive electrode materials. The positive electrode materials for Na-ion batteries can be basically categorized into layered oxides, polyanionic compounds, and Prussian blue analogues. Polyanion materials seem to exhibit relatively higher working voltage and smaller capacities compared to the others. However, the high-voltage polyanion materials typically contain costly cobalt (Nose et al. 2013) or vanadium (Jian et al. 2012; Kang et al. 2012; Lim et al. 2014; Zhang et al. 2016) except for Na2Fe2(SO4)3 (Barpanda et al. 2014). On the other hand, Na-containing layered transition metal oxides exhibit relatively lower working voltage but larger capacities on the basis of redox activity of Fe3+/Fe4+ or manganese Mn3+/Mn4+. Because of the high-performance redox of iron and manganese, abundant mineral resources of iron and manganese are available, and the resultant cost-reduction will be thus advantageous for stationary applications of Na-ion batteries. We emphasize that the layered transition metal oxides meet this important demand for future application. Of course, higher energy density (i.e. large capacities and high working potential), long-term cycle life, high coulombic and energetic efficiencies, thermal-stability, and ease in handling and production, toxic-free chemistry and so on are desired for the practical use as positive electrode materials for Na-ion batteries.
In this chapter, developments of Na-containing layered 3d transition metal oxides are reviewed for the application as active materials of Na-ion batteries based upon the authors’ experience since 2003 (Komaba 2019). The electrochemical performances, phase transitions during the charge/discharge, surface chemistry in the batteries, key factors influencing the battery performances and future prospective are discussed mainly based on our leading studies on the layered oxides since 2005.
1.2. Crystal structures of layered materials
1.2.1. Crystal structures of synthesizable NaxMO2
An iron based oxide, α-NaFeO2, is one of the most well-known Na-containing layered transition metal oxides in inorganic chemistry and in the battery research community. This is because LiCoO2, LiNi0.8Co0.15Al0.05O2 and LiNi1/3Mn1/3Co1/3O2 used as positive electrode in commercialized Li-ion batteries are isostructural to α-NaFeO2. That is, the layered rock salt–type structure with space group of R-3m is called α-NaFeO2 type. α-NaFeO2-type materials are found in NaMO2 (M = Co, Cr, Fe, Ti, Sc, etc.), as shown in Figure 1.3. In contrast, α-NaFeO2-type LiM’O2 ones are crystallized only for M’ = Co, Ni, Cr and V, which can be explained by the difference in ionic radii between Li+ and Na+ ions (Shirane et al. 1995; Kanno et al. 1997). The ionic radius of a Li+ ion at an octahedral site of sixfold coordination is 0.76 Å and is quite close to those of transition metal ions (Shannon 1976). Because of the similar ion size, lithium ion is often mixed with transition metal ions during a solid-state synthesis reaction of LiMO2, resulting in the formation of a cationordered rock salt–type (γ-LiFeO2 type) or a cation-disordered rock salt–type (NaCl type) phase as seen in Figure 1.3 (Hoffmann Hoppe 1977; Shirane et al. 1995; Kanno et al. 1997). An ionic radius of Na+ (1.02 Å) is larger than those of Li+ and transition metal ions, leading to the obvious separation of the Na+ and transition metal layers. A variety of transition metals can be accommodated in α-NaFeO2 type. This fact implies that cation mixing between Na+ and transition metal ions is suppressed and avoided in the synthesis of NaMO2 and various transition metals are simultaneously adopted to form α-NaFeO2-type solid solutions, which is advantageous for optimizing composition of NaxMO2 representing good electrochemical properties for Na-ion batteries.
Figure 1.3. Structure field map of ABO2 compounds. Modified with permission from Kanno et al. (1997). Copyright 1997 Elsevier. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip
A systematic notation system for layered transition metal oxides containing alkali metal was proposed by Delmas et al. (1977, 1980). Layered oxides of α-NaFeO2 and α-NaCoO2 are categorized into O3-type materials, and β- and γ-NaxCoO2 are P’3- and P2-type materials, respectively. Schematic illustrations of typical layered structures for sodium transition metal oxides were drawn using the program VESTA (Momma and Izumi 2011) and are shown in Figure 1.4 (Kubota et al. 2014).
In O3-type (α-NaFeO2-type) structure, MO2 slabs consisting of edge-shared MO6 octahedra stack along c-axis with cubic close-packed oxygen as
