Na-ion Batteries. Laure Monconduit

Na-ion Batteries - Laure Monconduit


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on the particle surface. The accumulated species on the surface are insulating and lead to an increase in interfacial resistance and deteriorated electrochemical performance (You et al. 2019). Thus, surface coating is very effective and some researchers applied to O3-type materials. Carbon-coated O3-NaCrO2 (Yu et al. 2015), Al2O3-coated O3-Na[Ni0.6Co0.2Mn0.2]O2 (Hwang et al. 2017a) and ZrO2-coated O3-NaNi0.7Mn0.15Co0.15O2 (You et al. 2019) have demonstrated superior rate performance and suppression of the surface degradation owing to the inert carbon, Al2O3 and ZrO2 layers. Hwang et al. (2017a) revealed that ca. 20-nm-thick Al2O3 coating on the periphery of the primary O3-Na[Ni0.6Co0.2Mn0.2]O2 particles remained intact and served as a protective layer against HF attack in the cell. NaF is deposited on bared O3-Na[Ni0.6Co0.2Mn0.2]O2 particles during charge–discharge cycles, resulting in thick layer on the surface evidenced with X-ray photoelectron spectroscopy (XPS). Al2O3 coating successfully suppresses formation of a thick NaF deposition layer on the surface of the O3 materials due to HF scavenging in the electrolyte to form AlF3 by the proposed following reaction:

      [1.1]images

      Formation of an AlF3 layer on the outermost surface of the Al2O3 coating protects the O3 materials and improves rate performance, coulombic efficiency and capacity retention of the O3-type materials. In order to obtain a large reversible capacity by charging to high voltage, surface protection of O3-type materials is necessary to capture HF generated from the electrolyte as well as to supress the reaction with moist air, leading to superior electrode performance in Na cells.

      1.4.1. Practical issues of P2-type materials for Na-ion batteries

      Similar to the O3-type layered sodium 3d transition metal oxides (including O’3-type ones), P2-type sodium 3d transition metal oxides have been developed for achieving high-capacity, high-power, long-life and safe Na-ion batteries: (1) increase in a reversible capacity and operation voltage, (2) suppression of significant interslab shrinkage and irreversible phase transitions, (3) smoothing voltage profiles with suppressed Na+/vacancy ordering, (4) suppressing insertion of water molecules to interslab spacing and (5) surface protection against moist air and HF attack. However, there is a significant difference between P2- and O3-type layered oxides with single 3d transition metal systems. In contrast to O3-type layered oxides in which most of 3d transition metals are able to be accommodated in the slab, P2 type phases crystallize with only V, Mn and Co as single 3d transition metal systems. P2-type materials are generally Na deficient and crystallize in the Na content of 0.5 ≤ x ≤ 0.8 in NaxMO2. Many researchers synthesize P2-type materials in the typical composition of Na2/3MO2 (Na2/3M3+2/3M4+1/3O2). The average valence of transition metals in Na2/3MO2 is +3.33 and tetravalent or higher valent transition metals are contained in the slab of a P2-type structure. Thus, P2-type materials are also able to contain various 3d and 4d transition metals, alkali metals and alkaline earth metals in the binary, ternary and quaternary systems. Furthermore, the influence of the different oxygen stacking of O3 and P2 types on the charge–discharge voltage is very small, as shown in Figure 1.13 (Kubota et al. 2018a), except for the high Na content region of 0.0 ≤ x ≤ 1/3 in Na1-xMO2. Thus, the strategy and knowledge of the multiple transition metal systems with inert-metal doping or surface coating, which are developed in O3-type materials, can be applied to P2-type materials and also from P2- to O3-type materials. Because of the specific oxygen stacking of the P2-type materials and unstable occupation of transition metal ions at the octahedral interslab sites, migration of transition metal ions is suppressed even in the high voltage region above 4 V. Thus, P2-type materials are usually charged to >4.0 V in order to increase the working voltage.

      Figure 1.13. Charge/discharge curves of P2, P’3 and O3 type NaxCoO2 in Na cells. Reprinted with permission from Kubota et al. (2018a). Copyright 2018, Wiley-VCH. For a color version of this figure, see www.iste.co.uk/monconduit/batteries.zip


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