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Materials 2020, 13, 3453 4 of 58 of 87% after 100 cycles in the voltage region of 1.5–4.2 V [51]. Note that the substitution of Ni alone for Mn does not give good results because of the P2–O2 transition at low voltage (below 2.3 V) which damages the structural stability and thus the capacity retention. One solution to avoid this effect is the limitation of the operating voltage range to 2.3–4.1 V [52,53], but this is detrimental to the capacity and thus the energy density. The other solution is the additional substitution of Mg [51] or a transition element, among them Ti [54] but the capacity retention was tested at 0.1C on 20 cycles only, or over 50 cycles at very slow rate (C/50) [55]. The role of the Mg substitution in P2-Na2/3Ni1/3-xMgxMn2/3O2 (0 ≤ x ≤ 0.2) has been investigated by Tapia-Ruiz et al. [56]. In particular, they showed that the Mg substitution increases the diffusion coefficient of Na. In P2-Na0.67MnxCo1-xO2, Co-rich phases exhibit a high structural stability and superior capacity retention, whereas Mn-rich phases discharge higher capacities [57]. Improved results were obtained with Fe [58]. The Na0.5[Ni0.23Fe0.13Mn0.63]O2 cathode delivered 200 and 150 mA·h·g−1 at 15 mA·g−1 (C/10) and 100 mA·g−1, respectively. At current density 100 mA·g−1, the capacity was retained at 125 mA·h·g−1 after 100 cycles [59]. Almost the same initial capacities were obtained in P2-Na2/3[Fe0.5Mn0.5]O2 but with a much poorer cycle ability, due to a structural phase transition [60] and the fact that the migration of Fe3+ into tetrahedral sites in the interlayer space is avoided by the Ni substitution [61]. P2-Na2/3[Fe1/4Co1/4Mn0.5]O2 exhibited a high rate performance of 130 mA·h·g−1 at 30C [62], but the cycle life was poor, due to a P2–O2 transition was accompanied by a large lattice volumetric contraction at 4.2 V [63]. The sol-gel synthesis of this material improved the cycle ability, with a first discharge capacity of 157 mA·h·g−1 and a capacity retention of 91 mA·h·g−1 after 100 cycles at 130 mA·g−1 [64]. The substitution of other elements that are different from transition metals, namely Li, Cu, was also investigated. In particular, P2-Na0.85Li0.17Ni0.21Mn0.64O2 delivers a capacity of 95–100 mA·h·g−1 between 2.0 and 4.2 V with a capacity retention of 98% over 50 cycles at C/10 [65]. The Li+ ions remain fixed in the transition layer [65,66], allowing more Na ions to reside in the prismatic sites at high voltage, the reason why the P2-structure is stabilized [67]. The Cu substitution improves the capacity retention with respect to Mg or Ni doping, and also improves the rate capability [68]. Even at a current rate of 1000 mA·g−1, the capacity retention of Na0.67CuxMn1-xO2 is raised to 76.6% after 500 cycles for x = 0.33 [69]. Indeed, an important progress has been made recently by divalent Zn-doping of P2-type Mn-based cathodes, since Zn-doping reduces the amount of the JT distorted Mn3+ centers, and thus improves the structural stability. For instance, Zn-doped Na0.833[Li0.25Mn0.75]O2 (NLMO) delivered a capacity 162 mA·h·g−1 very stable over 200 cycles at 0.2C [70]. This important improvement with respect to the electrochemical properties of the undoped samples has been attributed to the localization of Zn2+ in the Na-layer, which stabilizes the diffusion channels during charge/discharge processes. Mg-doped Na[Li0.25 Mn0.75 ]O2 has the same Mn electronic structure as Zn-doped Na[Li0.25 Mn0.75 ]O2 and thus experiences the same reduction of the JT distortion. Indeed, Na2/3 Mn1-y Mgy O2 (y = 0.05 and 0.1) has an improved structural stability with respect to the undoped material [71]. For the same reason the Mg-doping suppressed the P2-O2 phase transition in Na0.67 Mn0.67 Ni0.33 O2 [72]. Another example of the stabilization of the P2 structure by the Na-site Mg substitution was demonstrated with the superior electrochemical performance of Na0.7 Mg0.05 [Mn0.6 Ni0.2 Mg0.15 ]O2 [73] (see Figure 2). Due to Na+ /vacancy-order superstructures, undoped P2-layered oxides suffer from multiple voltage plateaus. Such is the case of pristine Na[Li0.25Mn0.75]O2 charge–discharge profile. On the other hand, the profile of the Mg-doped material in Figure 2a shows that Mg-doping was effective to smooth the charge–discharge profile, which is beneficial for the capacity retention. After first charging, the six redox peak couples (see Figure 2b) are well overlapped, indicating the high reversibility. The charge/discharge profiles in the second cycle within a narrow voltage window between 2.5 and 4.2 V are reported in Figure 2c. The polarization of the plateau at higher voltage is 0.10 V, smaller than that of the other plateau, demonstrating that the Na-poor phase has enhanced kinetics for Na-ion and electron transportation. When cycled between 2.5 and 4.2 V at 1C rate, this cathode demonstrated a capacity of 70 mA·h·g−1, with 79% capacity retentionPDF Image | Electrode Materials for Sodium-Ion Batteries
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