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Review Article Chem Soc Rev comparison with P2 type NaxCoO2 (B140 mA h g1 Delmas et al.).62 Recent calculations revealed that direct hopping from one octahedral to an adjacent octahedral site requires high activation energy to overcome the barriers. In contrast, the P2 type framework has an open path for Na+ diffusion that is expected to have a lower diffusion barrier, such that Na+ diffusion occurs readily in the P2 structure relative to the O3 structure. As mentioned in Section 2.1.1 and Fig. 3a and b, NaFeO2 exhibits poor electrochemical performance. Again, NaCoO2 is active in a very narrow range. However, once a solid solution of NaCoO2–NaFeO2 was formed, a high discharge capacity was obtained at high rates up to a 30C-rate, with Na[Fe0.5Co0.5]O2 in particular.47 2.1.4. Na1xCoO2 (x r 0.3) and derivatives. Although sodium cobaltites can be used in thermoelectric and superconductor applications,63–68 Na1xCoO2 is a Na+ insertion host material according to Delmas et al.33 Due to the ease of synthesis, this material can be synthesized via a solid state reaction in the temperature range of 500–800 1C under oxygen pressures of 0.4rxr0.45(P03),0.26rxr0.36(P2),x=0.23(O03),and x = 0 (O3) in Na1xCoO2. They suggested that oxygen-deficient NaxCoO2y was a stabilized form due to the instability of Co4+ when Co3+ and Co4+ are mixed. Early work found that the P2 structure was maintained over a wide range 0.46 r x r 0.83 in NaxCoO2, although two-phase domains were observed in the charge–discharge curves (Fig. 5a).33 Molenda et al. correlated discharge capacity and the oxygen content using Na0.7CoO2y (y = 0.004 and 0.073; Fig. 5b).69 The less-oxygen-deficient phase, Na0.7CoO2y (y = 0.004), resulted in more Na+ intercalation than Na0.7CoO2y (y = 0.073) with a higher operating voltage. The original electron holes arising from oxygen nonstoichiometry lower the electric conductivity via formation of unstable Co4+. Hence, more Na should be added to equalize the Co oxidation state. This was proven experimentally by Chou et al. using the oxygen nonstoichiometric single crystal Na0.7CoO2y (y B 0.073) in air and Na0.75Co2y (y B 0.08) in oxygen.70 For this reason, oxygen deficiency in Na0.7CoO2 induces a lower electrochemical capacity and operating voltage such that an oxygen atmosphere is required to minimize oxygen deficiency and improve electrode performance in terms of capacity and operating voltage. Delmas suggested the presence of several NaxCoO2 struc- tures and Shacklette confirmed that it has four phases.71 In particular, O3, O 0 3, and P3 layer structures were formed in a temperature range of 400–600 1C, whereas the P2 structure was stable only when the heating temperature was 4700 1C in an oxygen atmosphere via the conversion of the P3 phase, which involved rotation of CoO6 octahedra and Co–O bond breakage. The voltage profile, operating voltage, and phase transition are very similar for the O3 and P3 phases during de-/sodiation (Fig. 5c). For the P2 phase, although similar behavior was observed in a voltage range of 2.7–3.5 V, phase transition appeared more complicated below 2.7 V compared with O3 and P3 (Fig. 5c). The difference in Na+ ordering for the P2 and P3 phases could account for the behavior variation below 2.7 V, at which considerable Na+ is included at trigonal sites. Capacity retention was over 80% for 300 cycles for P2-Na0.7CoO2, Mn4+/3+ oxidation below 3.5 V. This coincides with the results of Yabuuchi and Komaba. Jung et al.58 explored the effects of Co on P2-Na0.7[(Fe0.5Mn0.5)1xCox]O2 (x = 0–0.2). Note that the partial replacement of Fe by Co in O3-Na[Fe1xCox]O2 dramatically enhanced capacity and performance.47 Similarly, the addition of Co into the transition metal layers of Na0.7[(Fe0.5Mn0.5)1xCox]O2 resulted in higher capacity and contributed to stable cycling behavior during cycling, in particular for Na0.7[(Fe0.5Mn0.5)0.8Co0.2]O2. In contrast to P2-Nax[Fe0.5Mn0.5]O2, P2-Na0.7[(Fe0.5Mn0.5)0.8Co0.2]O2 underwent a phase transition toward the O2 phase when desodiated and the O2 phase was transformed into P2 upon sodiation. The P2 to O2 transition was reversibly achieved via the gliding of slabs due to prismatic site instability without Na+ ions. This simple phase transition is related to the addition of Co in the transition metal layers, as a result of structural stabilization. The phase transition from P2 to OP4 is supposed to occur (Fig. 4e); however, the added Co that stabilizes the crystal structure is likely to suppress phase transition towards the OP4 structure because of the suppression of Fe3+ migration to a tetrahedral or octahedral interspace. Therefore, the simple phase transition from P2 to O2 is responsible for the better capacity retention of Co-doped Na0.7[(Fe0.5Mn0.5)1xCox]O2. In comparison with the O3 layer structure, the simple phase transition during the de-/sodiation process of P2 layer compounds is obviously advantageous in preserving the original structure during cycling. However, an important issue regarding the Na-deficient P2 structure is that such high capacities can be obtained only after the first cycle with a Na metal counter electrode. Compensation of Na in the synthetic state causes formation of an O3 and/or O 0 3 structure with a further increase in the Na content of Nax[Fe1/2Mn1/2]O2 (x Z 0.8).59 Therefore, the irreversible capa- city of the first cycle is an intrinsic issue with P2 type materials. Singh et al. suggested using a NaN3 additive, sacrificial salt, which acts as follows: 2NaN3 - 3N2 + 2Na+ + 2e.60 This oxidative decomposition was effective in circumventing the irreversible capacity loss in Na2/3[Fe1/2Mn1/2]O2 in the first cycle. NaN3 was used as a source of extra Na+ ions added to the cathode. Hence, NaN3-added P2 Na2/3[Fe1/2Mn1/2]O2 could successfully reduce the irreversible first charge capacity from 58 mA h g1 to 27 mA h g1 (Fig. 4f). Taking into account N2 release after oxidative decom- position at high potentials, an appropriate amount of NaN3 is recommended because decomposition can cause swelling of cells. However, degassing is possible in pouch-type cells, such that the addition of sacrificial salts is likely to facilitate full cell configuration of the P2 cathode materials. Sensitivity in air, in particular uptake of CO2 in air, is a serious problem because of the formation of electrochemical- inactive Mn4+ on the surface of active materials. Ni-doping of the transition metal sites in Nax[Fe0.5Mn0.5]O2 is less prone to react in air.61 2.1.3. Na1xCoO2 and derivatives. Delmas et al. pioneered Na+ intercalation properties using O3 type NaCoO2. The structure underwent reversible structural transitions (O3 2 O03 2 P03) in the range of NaxCoO2 (x = 0–0.2; Fig. 5a).33 Apart from the excellent reversibility, the resulting capacity was very small in View Article Online 3536 | 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. 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