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Materials 2020, 13, 3453 3 of 58 Materials 2020, 13, x FOR PEER REVIEW 3 of 53 2. Cathode Materials 2. Cathode Materials Na-based layered oxide materials are a viable Na-ion battery cathode [40]. Their theoretical Na-based layered oxide materials are a viable Na-ion battery cathode [40]. Their theoretical capacity is high (up to 244 mA·h·g−1 for O3-NaMnO2) and the synthesis process is rather simple. capacity is high (up to 244 mA h g−1 for O3-NaMnO2) and the synthesis process is rather simple. They They are classified O3-, P2-, and P3-types depending on the stacking sequence of oxygen layers are classified O3-, P2-, and P3-types depending on the stacking sequence of oxygen layers (O3: (O3: ABCABC stacking; P2: ABBA stacking; P3: ABBCCA stacking) (see Figure 1) [41]. A review ABCABC stacking; P2: ABBA stacking; P3: ABBCCA stacking) (see Figure 1) [41]. A review on the on the structure–function–property relationship of layered oxides for SIBs has been published by structure–function–property relationship of layered oxides for SIBs has been published by Wang et Wang et al. [42]. al. [42]. Figure 1. Schematic illustrations of definitions of layered NaTMO2 phases of (a) O3 type, (b) P3 type, Figure 1. Schematic illustrations of definitions of layered NaTMO2 phases of (a) O3 type, (b) P3 type, (c) O2 type, and (d) P2 type. Illustrations of Na+ ion diffusion pathways through (e) indirect tetrahedral (c) O2 type, and (d) P2 type. Illustrations of Na+ ion diffusion pathways through (e) indirect sites in the O-type stacking sequence and (f) direct prismatic sites in the P-type stacking sequence. tetrahedral sites in the O-type stacking sequence and (f) direct prismatic sites in the P-type stacking Reproduced with permission from [41] Copyright 2016 Royal Society of Chemistry. sequence. Reproduced with permission from [41] Copyright 2016 Royal Society of Chemistry. 2.1. P2-Layered Oxide Materials 2.1. P2-Layered Oxide Materials To stabilize the P2-sructure, the concentration of Na ions must be reduced and attention in recent To stabilize the P2-sructure, the concentration of Na ions must be reduced and attention in recent years has been focused on NaxMO2 (M stands for a transition metal ion) with x = 2/3 (the P2 phase years has been focused on NaxMO2 (M stands for a transition metal ion) with x = 2/3 (the P2 phase cannot be maintained at larger x). P2-NaxCoO2 is known for its poor cycle ability, but recent progress cannot be maintained at larger x). P2-NaxCoO2 is known for its poor cycle ability, but recent progress has been achieved by controlling the morphology. Na0.7CoO2 spheres 5 μm in diameter demonstrated has been achieved by con−tr1olling the morphology. N−a10.7CoO2 spheres 5 μm in diameter demonst+rated a capacity of 125 mA·h·g at 0.04C (about 5 mA·g ) in the voltage range 2.0–3.8 V vs. Na /Na, a capacity of 125 mAh g−1 at 0.04C (about 5 mA g−1) in the voltage range 2.0–3.8 V vs. Na+/Na, and the and the capacity retention at 0.4 C was 86% after 300 cycles [43]. Na0.7CoO2 nanosheets orderly grown capacity retention at 0.4 C was 86% after 300 cycles [43].−N2 a0.7CoO2 nanosheets orderly grown on Ni on Ni foam delivered an areal capacity of 1.16 mA·h·cm at 1C rate, with a prolonged cycling life. foam delivered an are−a1l capacity of 1.16 mAh cm−2 at 1C rate, with a prolonged cycling life. More than More than 50 mA·h·g capacity was preserved after 1100 cycles at 6C, and even at a larger rate of 50 mAh g−1 capacity was prese−r1ved after 1100 cycles at 6C, and even at a larger rate of 15C, a capacity 15C, a capacity of 57.8 mA·h·g maintained. [44]. Among P2-NaxMO2, a partial substitution of the of 57.8 mAh g−1 maintained. [44]. Among P2-NaxMO2, a partial substitution of the cation has been cation has been used in recent works to stabilize the P2-structure (and thus increase the capacity used in recent works to stabilize the P2-structure (and thus increase the capacity retention). This effect retention). This effect has been evidenced in Na0.7CoO2, where the Mg substitution also increases has been evidenced in Na0.7CoO2, where the Mg +substitution also increases the rate capability owing the rate capability owing to an increase of the Na diffusion coefficient [45]. In addition, the cationic to an increase of the Na+ diffusion coefficient [45]. In addition, the catio+nic substitution improves the substitutionimprovesthecyclinglifeduetothesuppressionoftheNa /vacancyorderingcondition, cycling life due to the suppression of the Na+/vacancy ordering condition, experienced in P2-NaxCo1- experienced in P2-NaxCo1-y(Mn2/3Ni1/3)yO2 [46], and also Ca-substitution [45,47]. This is also true with y(Mn2/3Ni1/3)yO2 [46], and also Ca-substitution [45,47]. This is also true with Ni substitution, but only Ni substitution, but only below a solubility limit [48], and Ti substitution, but only for Ti concentration below a solubility limit [48], and Ti substitution, but only for Ti concentration smaller than 10% [49]. smaller than 10% [49]. Nevertheless, the sodium ion battery based on P2-NaxCoO2 cathode still has an Nevertheless, the sodium ion battery based on P2-NaxCoO2 cathode still has an energy density that energy density that is too small for practical applications [49,50]. is too small for practical applications [49,50]. The substitution effect is even more important in the case M = Mn because the substitution The substitution effect is even more important in the case M = Mn be3c+ause the substitution is is mandatory to suppress the Jahn–Teller (JT) distortion due to Mn ions. As a result, mandatory to suppress the Jahn–Teller (JT) distortion due t−o1 Mn3+ ions. As a result, P2– P2–Na0.67Ni0.25Mg0.1Mn0.65O2 delivers a capacity of 100 mA·h·g at C/10 rate with a retention Na0.67Ni0.25Mg0.1Mn0.65O2 delivers a capacity of 100 mA h g−1 at C/10 rate with a retention 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 doesPDF Image | Electrode Materials for Sodium-Ion Batteries
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