Effect of nickel and iron on structural and electrochemical properties of O3 type layer cathode materials for sodium-ion batteries

Journal of Power Sources 324 (2016) 106e112

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Effect of nickel and iron on structural and electrochemical properties of O3 type layer cathode materials for sodium-ion batteries Jang-Yeon Hwang a, 1, Seung-Taek Myung b, 1, Doron Aurbach c, Yang-Kook Sun a, * a

Department of Energy Engineering, Hanyang University, Seoul 133-791, South Korea Department of Nano Engineering, Sejong University, Seoul 143-747, South Korea c Department of Chemistry, Bar-Ilan University, Ramat-Gan 5290002, Israel b

h i g h l i g h t s
We investigate the effect of Ni and Fe contents on O3-type NFM cathode.
The Ni contents contribute to a higher discharge capacity.
An appropriate amount of Fe contents improves the electrochemical properties.
The resulting demonstrates an appropriate balancing of the Ni and Fe contents.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 January 2016 Received in revised form 21 April 2016 Accepted 16 May 2016

We investigate that the effect of Ni and Fe contents on structural and electrochemical properties of O3type layered Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) in which Mn is fixed at 25%. As increasing the Ni contents, the capacities are gradually higher while the capacity retention and thermal properties are inferior. When Fe contents are increased, by contrast, the electrode exhibits stable capacity retention and satisfactory thermal stability although the resulting capacity slightly decreases. Structural investigation of post cycled electrodes indicate that lattice variation is greatly suppressed from x ¼ 0.5 in Na[Ni0.75xFexMn0.25]O2. This indicates that an appropriate amount of Fe into the Na[Ni0.75xFexMn0.25] O2 stabilizes the crystal structure and this leads to the good cycling performances. Also, the better structural stability obtained by Fe addition is responsible for the less heat generation at elevated temperature for the desodiated Na1d[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) caused by less evaporation of oxygen from the crystal structure. © 2016 Elsevier B.V. All rights reserved.

Keywords: O3-type Layered structure Cathode material Sodium ion battery Oxalate

1. Introduction To address the depletion of fossil fuels and air pollution problems many green energy systems have been explored over the past few decades. Along these lines, research in the area of energy storage using lithium-ion batteries has intensively progressed over the past two decades to meet high energy density, long cycle life, and safety demands [1,2]. For mid to large scale applications, however, presently available lithium-ion batteries face some barriers including the potential global shortage of lithium resources, toxicity, and the cost of Co. Rechargeable sodium-ion batteries can

* Corresponding author. E-mail address: [email protected] (Y.-K. Sun). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2016.05.064 0378-7753/© 2016 Elsevier B.V. All rights reserved.

be a possible alternative for lithium-ion batteries for mid to large scale battery systems because of their cost effectiveness and the abundance of sodium [3e11]. Various types of cathode materials have been introduced for use in sodium-ion battery systems since 1980s [12e40]. Among them, layer structure cathode materials with sodiated forms, P2-type [15e25] and O3-type [26e40], in which the sodium ions are accommodated at prismatic and octahedral sites, respectively, have been studied. P2-type materials show a higher specific capacity than O3-type materials. They usually exhibit high discharge capacities above 150 mAh g1 in the voltage range of 1.5e4.3 V vs. Na/ Naþ. The Na occupancy is usually below x ¼ 0.7 in NaxMO2 (M: transition metal), which results in unbalanced coulombic efficiency at the first cycle. Namely, a low discharge capacity is obtained from filling the empty prismatic sites with 0.3 mol of Naþ ions, finally forming NaMO2 in the fully discharged (reduced) state. In terms of

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capacity retention, we recently elaborated to improve the capacity retention of O3-type compounds [35]. For O3-type materials, the crystal structure is basically the same as the well-known LiCoO2 [41], LiNiO2 [42], Li[Ni1/3Co1/3Mn1/3]O2 [43], and Li[Ni0.5Mn0.5]O2 [44]. The difference is that the Fe3þ/4þ redox is highly active for O3type NaFeO2 system in nonaqueous Na electrolytes [45], which is particularly interesting due to the cost and eco-friendliness of Fe compared to Co [27,31e33]. Although the O3-type NaFeO2 cathode material has several advantages, it suffer from the intrinsic problem of iron migration at the desodiated state above ~3.8 V vs. Na/Naþ, which limits the cell operating voltage [27]. Therefore, it is interpreted that some works designed to suppress the irreversible iron migration in the O3-type layered structure that have improved electrochemical properties by Ni and Mn substitution into Fe sites [29,31]. We recently demonstrated a high capacity delivery in a sodiated Na-Ni-Fe-Mn-O compound, which crystallized into an O3 layer structure, Na [Ni0.25Fe0.50Mn0.25]O2 [32]. Despite of many efforts from above reviews, the effect of Ni and Fe in Na-Ni-Fe-Mn-O compounds and its optimal composition to deliver a high capacity with good capacity retention have not been suggested yet. Therefore, in this study, we explored the electrochemical and related structural properties of O3-type Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) materials in the limited voltage range of 1.5e3.9 V. Through this investigation, we suggest the effect of Ni and Fe with an optimized ratio of Ni/Fe in Na[Ni0.75xFexMn0.25]O2 cathode materials. 2. Experimental section 2.1. Material synthesis [Ni0.75xFexMn0.25]C2O4 (x ¼ 0.4, 0.45, 0.5, and 0.55) precursors were synthesized via a co-precipitation method. For the preparation of the [Ni0.75xFexMn0.25]C2O4 (x ¼ 0.4, 0.45, 0.5, and 0.55) powders, NiSO4$6H2O, FeSO4$7H2O, and MnSO4$H2O were used as the starting materials. Stoichiometric amounts of high-purity NiSO4$6H2O (0.5 mol dm3), FeSO4$7H2O (0.5 mol dm3), and MnSO4$H2O (0.5 mol dm3) were dissolved in distilled water. The solution mixture was slowly added into 0.5 mol dm3 ammonium oxalate ((NH4)C2O4), which is equimolar of the transition metals to coprecipitate in oxalate form. At the same time, an appropriate amount of a NH4OH solution was added into the reactor to adjust the pH 7. The co-precipitation solution was continuously stirred for 3 h and the temperature was kept constant at 70  C. After the reaction, the precursor powders were filtered, washed, and dried in a vacuum oven overnight at 110  C to remove absorbed water. Na [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) samples were prepared by thoroughly mixing the precursor [Ni0.75xFexMn0.25] C2O4 with Na2CO3 at various temperatures for 24 h in air at a heating rate of 3  C min1 to 950  C for Na[Ni0.20Fe0.55Mn0.25]O2, 900  C for Na[Ni0.25Fe0.50Mn0.25]O2, 900  C for Na [Ni0.30Fe0.45Mn0.25]O2, and 870  C for Na[Ni0.35Fe0.40Mn0.25]O2. The samples were then quenched to room temperature. Slow cooling usually induces formation of NaOH and Na2CO3 as byproducts. We believe that those were produced during the cooling process. In order to obtain single phase, therefore, we quenched all the samples used in this experiment. 2.2. Material characterization The crystalline phases of the synthesized materials were characterized by powder X-ray diffraction (Rint-2000, Rigaku, Japan) using Cu-Ka radiation. The FULLPROF Rietveld program was used to analyze the powder diffraction patterns [46]. The particle morphologies of the precursor and as-synthesized powders were

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observed by scanning electron microscopy (JSM 6400, JEOL Ltd., Japan). The chemical compositions of the produced powders were analyzed by atomic absorption spectroscopy (Vario 6, Analyticjena, Germany). For the differential scanning calorimetry (DSC) experiments, the cells were charged to 3.9 V and opened in an Ar-filled dry box. Typically, 3e5 mg of sample was collected in a stainless steel sealed pan with a capacity of 30 mL and a gold-plated copper seal capable of withstanding a pressure of 150 atm. The thermal stability was determined using a differential scanning calorimeter (DSC, 200PC, Netzsch) with scan rate of 1  C min1. Chemical extraction of Naþ ions from the as-synthesized Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) was carried out in a 0.5 mol dm3 (NH4)2S2O4 solution at room temperature. After extraction for a given time, the solid was filtered, washed with deionized water, and dried at 80  C. The chemically desodiated powders were subjected to thermogravimetric analysis (loaded sample amount: 10 mg, DTG-60, Shimadzu, Japan). The direct current (DC) electrical conductivity was measured by a direct volteampere method (CMT-SR1000, AIT Co.) in which disk samples were contacted with a four-point probe. 2.3. Electrochemical test Electrochemical testing was performed in R2032 coin-type cells adopting Na metal (Alfa Aesar, USA) as the anode for the half-cell tests. The cathodes were fabricated by blending the prepared Na [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) powders (85 wt %), carbon black (7.5 wt%), and polyvinylidene fluoride (7.5 wt%) in N-methylpyrrolidinone. The slurry was then applied onto Al foil and dried at 110  C for 12 h in a vacuum oven. Then, disks were punched out of the foil (electrode size: 14 4, average active materials loading amount: 2.85 mg cm2). After the electrode preparation, they were stored in a glove box. The electrolyte solution was 1.0 M NaClO4 in a 98:2 volumetric mixture of propylene carbonate and fluoroethylene carbonate [47] (PANAX ETEC Co., Ltd., Korea). All cells were prepared in an Ar-filled dry box. The cells were typically cycled within the voltage range of 1.5e3.9 V versus Na/Naþ where 1 C ¼ 130 mA g1 at 30  C. 3. Results and discussion [Ni0.75xFexMn0.25]C2O4 (x ¼ 0.4, 0.45, 0.5, and 0.55) oxalate was synthesized via a precipitation method (Fig. S1). The chemical composition of the prepared oxalate was analyzed by atomic absorption spectroscopy (AAS), as shown in Table S1, which demonstrates that the analyzed data agree with the as-designated compositions. The SEM images of calcined Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) show submicron-sized angulated primary particles (300e500 nm in size) in Fig. 1. Thermal sodiation of [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) oxalate with Na2CO3 led to the formation of highly crystalline layered compounds consisting of a rhombohedral structure. Rietveld refinement of the XRD data, hence, was performed assuming a R-3m space group (Fig. 2). Since undesired impurity, b-NaFeO2, was noticed, the thermal sodiation temperature varied with composition (Fig. S2). Due to the significant difference in ionic radii between Naþ and transition metal ions, cation mixing or exchange in the Na layer was not considered for the refinement. The refinement patterns indicate good agreement between the calculated and simulated results, indicating that the produced Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) was crystallized into the O3-type layer structure. As a result of the refinement, the a-axis, c-axis, and the resulting unit volume increased with increasing Ni content (Table S2) because of the larger ionic radius of Ni2þ (0.69 Å) compared to Fe3þ (0.645 Å) [48].

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Fig. 1. SEM images of the as-prepared Na[Ni0.75xFexMn0.25]O2 powders: (a) x ¼ 0.4, (b) x ¼ 0.45, (c) x ¼ 0.5, and (d) x ¼ 0.55.

This linear variation in the lattice parameter indicates the formation of a solid solution, obeying Vegard’s law [49]. In addition, the DC electric conductivity was also dependent on the amount of Fe in Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55). Namely, the value for Na[Ni0.2Fe0.55Mn0.25]O2 was approximately six-fold greater than that for Na[Ni0.35Fe0.40Mn0.25]O2, as shown in Table S3. This implies that the Fe content plays an important role in determining the conductivity. Fig. 3 shows electrochemical properties of the Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) electrodes in the voltage range of 1.5e3.9 V vs. Na/Naþ measured. It is obvious that the delivered capacity decreases with increasing Fe content (decreasing Ni content) at 0.1C-rate (13 mA g1): 144 mAh g1 for x ¼ 0.4, 138 mAh g1 for x ¼ 0.45, 135 mAh g1 for x ¼ 0.5, and 133 mAh g1 for x ¼ 0.55, although the resulting initial coulombic efficiencies were approximately 95% for all of the electrodes (Fig. 3a). According to our prior report regarding Na[Ni0.25Fe0.50Mn0.25]O2, the average oxidation states of Ni, Fe, and Mn were 2þ, 3þ, and 4þ, respectively, as confirmed by X-ray absorption near edge spectroscopy [32]. In that case, divalent Ni was first oxidized to trivalent or tetravalent Ni and Fe was subsequently oxidized from trivalent to tetravalent upon charging. Because of the similarity of the structure and chemical composition of the present Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55), we assumed that the oxidation state of Ni would first vary from Ni2þ to Ni4þ in the lower voltage plateau and the trivalent Fe would then oxidize to tetravalent Fe in the higher voltage region. The dQ dV1 profiles of Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) exhibit a clear pair of oxidation and reduction peaks (Fig. 3b). Although a higher capacity was delivered with the higher Ni content due to the electrochemical activity of Ni2þ/4þ, the capacity retention demonstrates that a Fe-rich composition exhibits better capacity retention (Fig. 3c). The Na[Ni0.35Fe0.40Mn0.25]O2 electrode showed a poor capacity retention of 86.8% (121.6 mAh g_1) after 20

cycles at a 0.2 C-rate (26 mA g1) even though it delivered the highest first discharge capacity of 140.0 mAh g1. The Fe-rich electrodes obviously had improved capacity retentions of 92.8% (126.5 mAh g1 for x ¼ 0.45), 93.4% (124.1 mAh g1 for x ¼ 0.5), and 95.4% (125.4 mAh g1 for x ¼ 0.55) during the first 20 cycles at a 0.2 C-rate. The retention was more dominant at the higher 0.5 Crate (65 mA g1) with values of 81.4% (89.0 mAh g1 for x ¼ 0.4) and 96.0% (116.3 mAh g1 for x ¼ 0.55). In addition, the voltage polarization (DV) linearly decreased between the charge and discharge redox potential from DV ¼ 0.11 for x ¼ 0.40 to DV ¼ 0.04 for x ¼ 0.55 (cyclic voltammogram results in Fig. S3). The lower polarization can be ascribed to the high electric conductivity of Na[Ni0.2Fe0.55Mn0.25] O2 compared that of the Na[Ni0.35Fe0.40Mn0.25]O2. These results demonstrate two findings: i) an increment of the Ni content is effective to deliver a high capacity and ii) Fe contributes to high electronic conductivity and stable capacity retention in Na [Ni0.75xFexMn0.25]O2. Strikingly, higher Fe content greater than 55% is not favored (Fig. S4) because of dramatic capacity fading. It is most likely that the iron migration is accelerated in the Na [Ni0.75xFexMn0.25]O2. As is clearly shown in Fig. S3, the Na [Ni0.15Fe0.6Mn0.25]O2 electrode exhibited the low initial coulombic efficiency of 83%, fast capacity decay during the cycling, and poor rate capability. Therefore, we suggest that Na[Ni0.2Fe0.55Mn0.25]O2 is the best composition in terms of stable electrochemical properties. As previously mentioned, the discharge capacities of the Na [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) electrodes are associated with the electrochemical reaction related to the Ni2þ/4þ and Fe3þ/4þ redox couples in the voltage range of 1.5e3.9 V. To clarify this issue, we evaluated the crystal structure and electrochemical properties of Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) before and after the cycling test (Fig. 4). Although all of the electrodes showed the same XRD patterns of the O3-type layer structure, the original crystal structures were not maintained after

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the extensive cycling test after 40 cycles. The XRD results demonstrated that the hexagonal 003 peak was shifted towards lower angle and hexagonal 104 peak was broadened and shifted towards a higher angle as a result of the coexistence of two phases (hexagonal P3 and monoclinic O3 phases) after the cycling of Na [Ni0.35Fe0.40Mn0.25]O2 (Fig. 4a). By contrast, the appearance of two phases was less dominant when the composition was Na [Ni0.3Fe0.45Mn0.25]O2 (Fig. 4b). Although a hexagonal P3 phase is notable for Na[Ni0.25Fe0.50Mn0.25]O2 (Fig. 4c), the original hexagonal O3 phase was completely recovered after the extensive cycling in Na[Ni0.20Fe0.55Mn0.25]O2 (Fig. 4d), where negligible broadenings of the XRD peaks are observed. In addition, minimal variation of the crystal structure was observed in the lattice parameters calculated by a least square method (Fig. 4e). The Rietveld refinement data indicated that the a-axis, c-axis, and cell volume linearly increased as the Ni content increased (Table S2). After the cycling tests, for instance, the difference of the lattice parameters dramatically increased with increasing Ni content. It is believed that these differences are caused by structural instability triggered by a higher amount of Ni in the composition, which affects the poor cyclability comparison with a lower Ni content. Although Fe is known to migrate in O3-NaFeO2, which usually leads to poor cyclability, appropriate amount of Ni (x) and Fe (0.75x) and 25% Mn in transition metal layer is helpful to maintain the structural stability. Therefore, we suggest that a higher amount of Fe of approximately

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Fig. 3. (a) First charge/discharge curves at a 0.1 C-rate (13 mA g1), (b) dQ dV1 profiels of Na[Ni0.75xFexMn0.25]O2 electrodes and (c) cycling data measured at a 0.2 Crate (26 mA g1) and 0.5 C-rate (65 mA g1) in the voltage range of 1.5e3.9 V.

55% in the transition metal layer is required to stabilize the crystal structure upon cycling. As a result of the XRD analysis, we found that the introduction of appropriate amounts of Ni and Fe is helpful to maintain the structural stability, which results in a good cycle retention. The rate capabilities of the Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) electrodes were tested (Fig. 5). All cells were charged to 3.9 V at a constant 0.1 C-rate (13 mA g1) and then discharged to 1.5 V at different C-rates ranging from 0.1 to 10 C-rate (13 mA g1e1300 mA g1). In particular, the drastic difference of the discharge capacity was shown above a 5 C-rate with a

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Fig. 4. XRD patterns of pristine and cycled Na[Ni0.75xFexMn0.25]O2 electrodes: (a) x ¼ 0.4, (b) x ¼ 0.45, (c) x ¼ 0.5, and (d) x ¼ 0.55. (e) Variation of the lattice parameters of before and after cycled Na[Ni0.75xFexMn0.25]O2 electrodes.

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98 mAh g1 for Na[Ni0.2Fe0.55Mn0.25]O2 and 56 mAh g1 for Na [Ni0.35Fe0.40Mn0.25]O2 at a 10 C-rate. According to our prior works [25,26], Na[Ni0.25Fe0.50Mn0.25]O2 led to higher electronic conductivities of 6.6  107 S cm1 relative to Na[Ni0.25Fe0.25Mn0.5]O2 (4.3  107 S cm1). Similarly, in the present study, the electric conductivity monotonously increased from 1.4  107 S cm1 to 8.8  107 S cm1 with increasing Fe content in Na[Ni0.75xFexMn0.25]O2 (Table S3). Hence, the increment of the Fe content is beneficial in improving the electric conductivity, which is associated with a low band gap energy (~2.5 eV for Fe2O3) [50]. Therefore, the enhanced rate performance with increasing Fe content results from the improvement of the electric conductivity derived from Fe in the compound.

Finally, we evaluated the safety characteristics of the Na [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) materials using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The oxygen release was measured from chemically desodiated Na1d[Ni0.75xFexMn0.25]O2 powders by using TGA, as shown in Fig. 6a. Chemical desodiation was carried out using 0.5 mol dm3 (NH4)2S2O4 oxidant at room temperature [51]. All of the Na [Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) materials showed weight loss in three steps attributed to oxygen release between 50  C and 600  C. With lowering Fe content, the phase transition temperature was shifted to a lower temperature from 273  C for Na0.39[Ni0.2Fe0.55Mn0.25]O2 to 250  C for Na0.32[Ni0.35Fe0.4Mn0.25]O2 while the total amount of oxygen release increased from 11.9% for Na0.39[Ni0.2Fe0.55Mn0.25]O2 to 18.5% for Na0.32[Ni0.35Fe0.4Mn0.25]O2. The thermal stabilities of the electrochemically desodiated wet Na1d[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) electrodes were determined by using DSC (Fig. 6b). With increasing Ni contents, the exothermic reaction peak temperature gradually shifted to lower temperatures, accompanied by a higher heat generation. The values were 289.8  C and 314 J g1 for Na0.43[Ni0.2Fe0.55Mn0.25]O2, 283.8  C and 324 J g1 for Na0.41[Ni0.25Fe0.50Mn0.25] O2, 276.3  C and 356 J g1 for Na0.39[Ni0.30Fe0.45Mn0.25]O2, and 267.5  C and 442 J g1 for Na0.36[Ni0.35Fe0.40Mn0.25]O2. These results reveal that unstable and reactive Ni4þ ions in the desodiated state caused oxygen removal from the crystal structure, and the oxygen evolution can be suppressed as the amount of Fe increased in the crystal structure. Based on the above results, the structural and thermal stabilities, which lead to a good cycle retention and rate capability, were dependent on the Fe content in the Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) materials. Although the discharge capacity of the higher Fe content materials slightly decreased, the improved cycling and rate capability can overcome this small issue. Therefore, balancing the transition metal layer with optimized amount of Fe and Mn is significantly important to

J.-Y. Hwang et al. / Journal of Power Sources 324 (2016) 106e112

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(HIM) funded by the Ministry of Science, ICT and Future Planning and by the Human Resources Development program (No. 20154010200840) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

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Fig. 6. (a) TGA curves of chemically desodiated Na1d[Ni0.75xFexMn0.25]O2 and (b) DSC traces of desodiated Na1d[Ni0.75xFexMn0.25]O2 obtained after charging to 3.9 V.

ensure the structural, electrochemical, and thermal stability in O3 Na[Ni0.75xFexMn0.25]O2 layer compounds. 4. Conclusion We investigate the effect of the Ni and Fe contents in Ni-Fe-Mnbased O3-type layered Na[Ni0.75xFexMn0.25]O2 (x ¼ 0.4, 0.45, 0.5, and 0.55) cathode materials. The Ni composition contributes to a higher discharge capacity but leads to fast capacity fading due to structural instability. An appropriately high amount of Fe contents obviously improves the electrochemical properties (cycle retention and rate capability), structural stability, and safety properties, even though these materials showed slightly lower discharge capacities. Therefore, these results demonstrate that appropriate balancing of the Ni and Fe composition ratio is important to provide good electrochemical properties and to address safety issues in O3-type sodiated Ni-Fe-Mn-based layered cathodes. Acknowledgements This work was supported by the Global Frontier R&D Program (2013M3A6B1078875) on Center for Hybrid Interface Materials

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