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and negative electrodes in the NaPF6-based electrolyte delivered a capacity of 80 mA h g−1 at 1C (76 mA g−1), retained at 65 mA h g−1 after 100 cycles. The capacity at 12C was still 75% of the capacity at 1C. In the same spirit, an increase of the diffusion coefficient can also be obtained by mixing two structures. In particular, P2-O3 composites were found efficient to improve the rate capability owing Materials 2020, 13, 3453 5 of 58 to a better sodium diffusion. Among them, Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+d delivered a capacity of 134 mA h g−1 at 1C [76]. Another example is the P2-P3 composite Na0.66Co0.5Mn0.5O2, which demonstrated after 1000 cycles. Na Ni Mn Cu Mg O cathode material consisting of multiple-layer oriented much better electro2/c3hem1/6ical2p/3rop1e/9rties1/1t8han2 the P2-sample with the same composition. The stackingnanoflakeswasproposedbyXiaoetal.[74].Thiscathod−e1materialdemonstratedago−o1drate composite delivered an initial discharge capacity of 86.5 mA h g maintained at 78.9 mA h g at the capability (64.0 mA·h·g−1 and 11.4 kW·kg−1 at 30C). 100th cycle at 10C [77]. Figure 2. (a) Charge and discharge profiles of P2-layered Na0.7[Mn0.6Ni0.4-xMgx]O2 with x = 0.02 Figure 2. (a) Charge and discharge profiles of P2-layered Na0.7[Mn0.6Ni0.4-xMgx]O2 with x = 0.02 (MNM- (MNM-2) between 1.5 and 4.2 V. (b) CV curves of MNM-2 between 1.5 and 4.2 V (scan rate: 0.1 mV·s−1). 2) between 1.5 and 4.2 V. (b) CV curves of MNM-2 between 1.5 and 4.2 V (scan rate: 0.1 mV s−1). (c) (c) Charge and discharge profiles of MNM-2 in the second cycle between 2.5 and 4.2 V. (d) Charge and Charge and discharge profiles of MNM-2 in the second cycle between 2.5 and 4.2 V. (d) Charge and discharge profiles of MNM-2 at different current densities from 0.2 to 10C. (e) Cycle performance at 1C discharge profiles of MNM-2 at different current densities from 0.2 to 10C. (e) Cycle performance at for 1000 cycles. (f) Rate capability of MNM-2 from 0.2 to 25C between 2.5 and 4.2 V. Reproduced with 1C for 1000 cycles. (f) Rate capability of MNM-2 from 0.2 to 25C between 2.5 and 4.2 V. Reproduced permission from [73]. Copyright 2019 The American Chemical Society. with permission from [73]. Copyright 2019 The American Chemical Society. at 1C. Wang et al. noticed that smaller Na+ diffusion coefficient is observed in P2-type layered oxides Another +strategy to stabilize the P2 phase was the coating of the+P2-p+articles with a protective exhibiting Na /vacancy-ordered superstructures because of strong Na -Na interaction in the alkali layer. Without coating, the P2-O2 crystal phase transition and the large volume change of the O2 metal layer and charge ordering in the transition metal layer [75]. They showed that such Na vacancy phase (more than 20%) is difficult to avoid, since the O2 structure has a lower formation energy ordering can be avoided by choosing the transition metal ions with similar ionic radii and different density than the P2 stru3+cture at 4h+igh voltage. The coating aims to mitigate the volume change during redox potentials like Cr and Ti . The full symmetric cell with P2-Na0.6[Cr0.6Ti0.4]O2 as both positive cycling, and suppress the side reaction during long cycling within the high voltage windo−w1 . This and negative electrodes in the NaPF6-based electrolyte delivered a capacity of 80 mA·h·g at 1C strategy −h1as been used in particu−l1ar with P2-Na2/3[Ni1/3Mn2/3]O2, which is a high-voltage cathode (76 mA·g ), retained at 65 mA·h·g after 100 cycles. The capacity at 12C was still 75% of the capacity material for Na-ion batteries with a theoretical capacity of 173 mA h g−1 and a long operation voltage plateau of 4.2 V. Therefore, this cathode is attractive, but has a very poor cycle ability. A remarkable In the same spirit, an increase of the diffusion coefficient can also be obtained by mixing two improvement was achieved by coating the particles with Al2O3 [78]. This coating was able to suppress structures. In particular, P2-O3 composites were found efficient to improve the rate capability owing to unfavorable side reactions with the electrolyte at high voltage, as well as exfoliation of the metal oxide a better sodium diffusion. Among them, Na0.66Li0.18Mn0.71Ni0.21Co0.08O2+d delivered a capacity of 134 layers, −le1ading to 73.2% capacity retention over 300 cycles. During cycling, the coating formed mA·h·g at 1C [76]. Another example is the P2-P3 composite Na0.66Co0.5Mn0.5O2, which demonstrated polymeric species such as poly(ethylene oxide), which provide flexibility in the SEI, and this much better electrochemical properties than the P2-sample with the same composition. The composite suppressed exfoliation of the P2 layered materia−l1 [79]. More recently, Na2/−3[1Ni1/3Mn2/3]O2 was delivered an initial discharge capacity of 86.5 mA·h·g maintained at 78.9 mA·h·g at the 100th cycle modified with an ionic conducting NaPO3 coating layer via melt-impregnation [80]. The at 10C [77]. Another strategy to stabilize the P2 phase was the coating of the P2-particles with a protective layer. Without coating, the P2-O2 crystal phase transition and the large volume change of the O2 phase (more than 20%) is difficult to avoid, since the O2 structure has a lower formation energy density than the P2 structure at high voltage. The coating aims to mitigate the volume change during cycling, and suppress the side reaction during long cycling within the high voltage window. This strategy has been used in particular with P2-Na2/3[Ni1/3Mn2/3]O2, which is a high-voltage cathode material for Na-ion batteries with a theoretical capacity of 173 mA·h·g−1 and a long operation voltage plateau of 4.2 V. Therefore, this cathode is attractive, but has a very poor cycle ability. A remarkable improvement was achieved by coating the particles with Al2O3 [78]. This coating was able to suppress unfavorable side reactions with the electrolyte at high voltage, as well as exfoliation of the metal oxide layers, leading to 73.2% capacity retention over 300 cycles. During cycling, the coating formed polymeric species suchPDF Image | Electrode Materials for Sodium-Ion Batteries
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