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Review Article Chem Soc Rev (coordination number 6: 0.76 Å). Also, sodiated transition metal materials are highly hygroscopic, even with brief expo- sure to air,32 and caution is necessary to avoid hydration of the material, particularly the surface, which results in formation of NaOH that degrades electrode performance due to its insulating properties. Thus, the preparation of sodiated cathode materials and batteries requires meticulous handling and moisture-free conditions. 2.1. Two-dimensional layer transition metal oxides The above-mentioned prototype compounds are composed of two- and three-dimensional crystal structures. Early investigation of two-dimensional layer oxides was performed by Delmas and Hagenmuller in the early 1980s.13,14,33 They defined the crystal structure of layered compounds depending on the stacking sequence of alkali ions between layers. Sodiated transition metal oxides, Na1xMO2 (M: transition metal), were representatively classified into two main groups, O3 type and P2 type (Fig. 2), by Delmas et al.33 Those crystal structures comprise sheets of edge- sharing MO6 octahedral layers sandwiched between Na ion layers into which ionic species are inserted in an octahedral (O) or a prismatic (P) environment. The number (O2, O3, P2, and P3) indicates the packing number of Na ion octahedral or prismatic layers within each unit cell. In addition, the prime symbol (0) indicates monoclinic distortion, such that O03 and P03 represent monoclinic distortion of the O3 and P3 phases, respectively. The O3 type is stable when the x value is high in Na1xMO2 (x is close to 0), in which the average oxidation state of M is close to 3+. Electrochemical de-/sodiation of the O3 structure Fig. 2 progresses with reversible structural transformation of O3 2 O03 2 P3 2 P03. Na+ ions energetically favor a prismatic environment when Na+ ions are partly extracted from the crystal structure, which creates vacancies. At the same time, this extraction induces strong repulsion of oxygen in the Na layers, and the interlayer distance thus expands. Na+ diffusion occurs faster in the P03 phase due to the greater interlayer distance compared to O3. These transitions are followed by gliding of the MO2 slab without breakage of M–O bonds, as proposed by Delmas et al.33 The P2 type has a different optimal environment for Na compared to the O3 type. In particular, the structure is stable View Article Online Fig. 1 Recent research progress in sodium ion batteries: (a) cathode, (b) anode, (c) electrolyte and (d) binder. Crystal structures; (a) perspective view (top) and top view of the (00l) layer of the O3 structure, (b) perspective view (top) and top view of the (00l) layer of the P2 structure in which yellow sphere is face shared Na1 and blue one is edge sharing Na2 atoms, and (c) perspective view (top) and top view of the (00l) layer of the P3 structure. 3532 | Chem. Soc. Rev., 2017, 46, 3529--3614 This journal is © The Royal Society of Chemistry 2017 Open Access Article. Published on 28 March 2017. Downloaded on 7/1/2019 3:41:21 AM. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.PDF Image | Sodium-ion batteries present and future
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