Читать книгу Na-ion Batteries - Laure Monconduit - Страница 20

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, LiNi_0.8Co_0.15Al_0.05O₂ and LiNi_1/3Mn_1/3Co_1/3O₂ 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 Na_xMO₂ 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-Na_xCoO₂ 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 AB CA BC array and alkali metal ions are accommodated at octahedral sites in the interslab space. The number of MO₂ slabs is three in the hexagonal unit cell. Namely, O in O3-type means the octahedral site accommodating alkali metal ions and following 3 is the number of MO₂ slabs included in a hexagonal unit cell. When the hexagonal lattice is distorted into monoclinic or orthorhombic lattice, a prime symbol is added between the alphabet and number, but the number of MO₂ layers is counted in pseudohexagonal unit cells, such as O’3-type NaMnO₂ with a monoclinic lattice (space group [S.G.], C2/m) (Parant et al. 1971), P’3-type Na_xCoO₂ (S.G., C2/m) (Fouassier et al. 1973) and P’2-type Na_xMnO₂ with an orthorhombic lattice (S.G., Cmcm) (Parant et al. 1971). O3-type NaCoO₂ reversibly transforms into P3-type Na_xCoO₂ by electrochemical Na extraction during the charging process (Braconnier et al. 1980).

Figure 1.4. Schematic illustrations of the crystal structures of O3-, P3-, and P2-type A_xMO₂. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip

P3-type Na_xCoO₂ can also be obtained as a Na deficient (0.5 ≤ x < 1) and low-temperature phase by a solid state reaction (Fouassier et al. 1973). In the P3-type layered structure, alkali metal ions occupy prismatic sites in the interslab space between MO₂ slabs stacking with AB BC CA array of oxygen packing along the c-axis and the number of MO₂ slabs is three in the hexagonal unit cell with space group of R3m. The P3 type is generally formed by electrochemical Na extraction from O3-type accompanied by the gliding of MO₂ slabs without breaking the M-O bonds. P3-type materials are often observed as Na-deficient intermediate phases transform from O3-type ones by Na extraction. Compared to the P3-type phase, the P2-type (β-RbScO₂-type) phase is recognized as a high-temperature one, and P2-type (γ-)Na_xCoO₂ is actually obtained by heating at a higher temperature than that for P’3-Na_xCoO₂ (Fouassier et al. 1973). The phase transition from P3- to P2-type phase is accompanied by breaking Co-O bonds by heating at high temperature and does not occur in a Na (de)intercalation reaction at room temperature. In the P2-type structure, alkali metal ions occupy prismatic sites in interslab space between MO₂ slabs stacking with an AB BA array of oxygen packing along the c-axis and the number of MO₂ slabs is two in the hexagonal unit cell with space group of P6₃/mmc. The P2-type phase can electrochemically transform into an O2-type phase by the gliding of MO₂ slabs without breaking the M-O bonds (Lu and Dahn 2001a) as explained in detail in section 1.2.3.

The average valence of transition metals in O3-NaMO₂ (M = Ti, V, Cr, Mn, Fe, Co, Ni) and P2-Na_2/3MO₂ (M = V, Mn, Co) is +3 and +3.33, respectively. Tetravalent or higher valent metals are necessary for the crystallization of P2-type materials, and only V, Mn and Co oxides have been reported as single 3d transition metal P2 systems so far due to difficulty in formation of trivalent Ti and tetravalent Cr, Fe and Ni under ambient air or oxygen pressure. In contrast to the single 3d transition metal P2 system, multiple transition metal P2 systems can be stabilized by combining divalent Ni, trivalent Cr and Fe, tetravalent Ti and Mn. Moreover, monovalent alkali metal ions, pentavalent Bi and Sb, and hexavalent Te are also includable for stabilization of O3- and P2-type layered oxides. These transition metal elements dominate the charge/discharge capacity and redox potential of O3-NaMO₂ and P2-Na_2/3MO₂ in Na cells as well as the structural changes accompanied by Na-extraction/insertion with the formation of Na/vacancy and charge orderings (or formation of dimers or trimers of transition metals).