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Unveiling the Role of Co in Improving the Highrate Capability and Cycling Performance of Layered Na Mn Ni CoO Cathode Materials for Sodium Ion Batteries 0.7

0.7

0.3-x

x

2

Zheng-Yao Li, Jicheng Zhang, Rui Gao, Heng Zhang, Zhongbo Hu, and Xiangfeng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b04073 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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ACS Applied Materials & Interfaces

Unveiling the Role of Co in Improving the High-rate Capability and Cycling Performance of Layered Na0.7Mn0.7Ni0.3-xCoxO2 Cathode Materials for Sodium Ion Batteries

Zheng-Yao Li, Jicheng Zhang, Rui Gao, Heng Zhang, Zhongbo Hu and Xiangfeng Liu*

College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Abstract Co substitution has been extensively used to improve the electrochemical performances of cathode materials for sodium-ion battery (SIB) but the role of Co has not been well understood. Herein, we have comprehensively investigated the effects of Co substitution for Ni on the structure and electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 (x = 0, 0.1, 0.3) as cathode materials for SIB. In comparison with the Co-free sample, the high-rate capability and cycle performance have been greatly improved by the substitution of Co and some new insights into the role of Co have been proposed for the first time. With the substitution of Co3+ for Ni2+ the lattice parameter a decreases but c increases and the d-spacing of sodium-ion diffusion layer has been enlarged, which enhances the diffusion coefficient of sodium-ion and the high-rate capability of cathode materials. In addition, Co substitution shortens the bond lengths of TM-O (TM = transition metal) and O-O due to the smaller size of Co3+ than Ni2+, which accounts for the decreased thickness and volume of TMO6 1

ACS Paragon Plus Environment


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octahedron. The contraction of TM-O and O-O bond lengths and the shrinkage of TMO6 octahedron improve the structure stability and the cycle performance. Last but not least, the aliovalent substitution of Co3+ for Ni2+ can improve the electronic conductivity during the electrochemical reaction, which is also favorable to enhance the high-rate performance. This study not only unveils the role of Co in improving the high-rate capability and the cycle stability of layered Na0.7Mn0.7Ni0.3-xCoxO2 cathode materials but also offers some new insights into designing high performance cathode materials for SIBs.

KEYWORDS：Sodium-ion batteries; cathode materials; layered oxides, P2-structure; Cobalt substitution

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Introduction The demand of powering the electronic devices and electrical vehicles has promoted the development of portable energy system. Lithium ion batteries (LIBs) and sodium ion batteries (SIBs), as the promising electrochemical energy storage systems with clearness and convenience, have attracted great attention1-6. Based on the insertion and de-insertion of Li-/Na-ion between the cathode and anode materials, the energy undergoes the storage and conversion in LIBs and SIBs. Thus, the structural and physical properties of a given material, such as the structure type, structure stability and the layered structures with more open space to accommodate the ion motion, can directly and extremely affect the final electrochemical performance of rechargeable batteries. The transition metal oxides associated with the electrochemical active ions of Ni, Mn and Co elements have aroused great interest over many years in LIBs since the earlier reports of the initial addition of Co into coexisted oxide cathode of Mn and Ni for LIBs7-9. Previous studies in LIBs have demonstrated that the substitution of Co element can largely enhance the electrochemical performance of the typical LiNi1-xMnxO2 (0≤x≤1) oxide cathode and the different element exhibits various functions during electrochemical process based on phase diagram analysis2,

10, 11

. Thus, this exciting advancement has made the

LiNi1-x-yMnxCoyO2 (0≤x, y≤1, 1-x-y ﹤ 1) promising electrode materials for the next-generation lithium ion battery to drive electrical vehicles, in terms of the high energy density2, 3, 12. Although the energy-storage mechanism between the LIBs and SIBs is similar, the single element Na-based analogues associated with the Li-based 3



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compounds always undergo some significant differences in the electrochemical behaviors, which include the multiple voltage plateaus in the charge and discharge profiles, the poor rate capability and the limited cycling performances related to the complex phase transformation and/or Na+-vacancy ordering upon the sodium insertion and extraction13-20. Some modification strategies for LIBs cathode materials have been also extensively used to develop advanced transition metal oxides based cathode materials for SIBs21-27. It has been shown that the multi-component oxides of Ni, Mn and Co exhibit promising electrochemical performance as cathode materials for SIBs28-37. However, to the best of our knowledge, the role of Co in improving the electrochemical performances of cathode materials for SIBs has not been well understood. Herein we systematically investigated the mechanism of Co in enhancing the electrochemical properties of Na0.7Mn0.7Ni0.3-xCoxO2. In this study, we have prepared a series of layered Na0.7Mn0.7Ni0.3-xCoxO2 (x = 0, 0.1, 0.3) cathode materials for SIBs and tried to unveil the role of Co on the improvement of the electrochemical performances. Through the combination of the structural analysis, electrochemical tests and electronic conductivity measurement the role of Co in improving the high-rate capability and cyclic performance for Na0.7Mn0.7Ni0.3-xCoxO2 has been proposed as follows: (1) The substitution of Co3+ for Ni2+ enlarges the d-space of the interslab thickness, which facilitates Na-ion diffusion and enhances the high-rate capability; (2) The substitution of Co induces the shrinkage of TM-O and O-O bond lengths and the contraction of TMO6 octahedron, which improves the structure stability and the cycling performance; (3) The aliovalent 4


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substitution of Co3+ for Ni2+ further enhances the electronic conductivity, which is favorable to the high-rate capability. This study not only unveils the role of Co substitution on the improvement of the electrochemical performances but also offers some new insights into the design and exploration of high performance cathode materials for SIBs. Experimental section Synthesis of Cathode Materials The sol-gel route was used to synthesize the designed Na0.7Mn0.7Ni0.3O2 (MN), Na0.7Mn0.8Ni0.2Co0.1O2 (MNC), and Na0.7Mn0.7Co0.3O2 (MC) oxides. Firstly, the citric acid (CA) and ethylene glycol (EG) were dissolved to form the mixed solution. Secondly, the Sodium nitrate (3% excess), Manganese nitrate, Nickel nitrate and/or Cobalt nitrate with stoichiometric ratio were added into the above solution to obtain the wet sol. Then, the wet sol was aged at 80 °C for many hours and dried at 150 °C overnight. Finally, the resulting gels were grounded and calcined at 500 °C for 5 h to decompose the organic component and nitrate, following the 900 °C for 12 h in air to obtain the final samples, respectively. All the chemicals above were purchased from China National Medicines Corp. Ltd and were used without any further purification. Material Characterization and Analysis Powder X-ray diffraction (XRD) patterns were collected by a Persee instrument with Cu(Kα) radiation at 0.01° per step between 2θ=10 and 70°. The unit cell parameters were refined using the Fullprof software based on Rietveld method. Scanning electron microscopy (SEM) measurements were performed on a Hitachi 5



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S-4800 apparatus (2 kV).X-ray photoelectron spectroscopy (XPS) analysis was collected on an ESCALAB MK II X-ray photoelectron spectrometer (Mg exciting source). The electrical resistivity measurements were made on disc-shaped pellets by the two-point dc method at room temperature. Electrochemical Measurements Electrochemical studies were carried on by coin cells (R2025) and metal sodium plate was used as the counter electrode, 1.0 M NaClO4 in propylene carbonate (PC)/ ethylene

carbonate (EC) mixed solvents (volume 1:1) was used as electrolyte and

the glass fiber membrane GF/D (Whatman) was used as the separator. The active material was mixed with super P carbon and poly(vinylidene fluoride) (PVDF) (mass ratio is 75:15:10)in N-methylpyrrolidinone (NMP) solvent to form the composite electrode. The slurry was uniformly pasted on an Al foil then dried overnight at 120°C in a vacuum oven. The sodium-ion batteries were made in glove-box of Ar-filled gas. Galvanostatic charge-discharge tests were performed in the voltage range of 1.5-4.2V versus Na+/Na by an automatic galvanostat (NEWARE) at various current densities. The cyclic voltammogram (CV) analysis was measured by the electrochemical workstation (PGSTAT302N, Autolab). Results and discussion Structural characterization The Figure 1(a)-(d) shows the X-ray diffraction (XRD) patterns and the Rietveld refinement results of the as-prepared Na0.7Mn0.7Ni0.3O2 (MN), Na0.7Mn0.7Ni0.2Co0.1O2 (MNC), and Na0.7Mn0.7Co0.3O2 (MC). The detectable diffraction peaks in each pattern 6


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of the compounds can be well indexed to the pure P2-structure with a space group of P63/mmc (No. 194). Fig. 1(e) shows the left-shift of (002) peak with the addition of Co. Further analysis of XRD data was carried out by Rietveld refinement method. The refined lattice parameters of each sample and the relationship are depicted in Fig. 1(f), Table 1 and Table S1-S3. With the substitution of Co, the lattice parameter a decreases, but c increases. The similar changes of lattice constants caused by replacing the larger Ni2+ (0.69Ǻ) by smaller size ion has also been found in previous literatures38-40. As shown in Fig. 1(e), the shift of (002) peak toward a low angle with the increase of Co content indicates the expanded d-spacing in the layered transition metal oxides. The expanded d-spacing is equivalent to the thickness of Na ion diffusion layer since the (002) peak is related to the layer-to-layer distance between two adjacent TMO6 layers41, 42.

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Figure 1.(a) XRD patterns of MN, MNC and MC samples; (b)-(d) The Rietveld refinement results of XRD profiles for MN (b), MNC (c) and MC(d); (e)The shift of (002) peak; (f) The relationship of the lattice parameters as a function of Co content. The (002) peak shifts to a low angle with increasing Co content indicating the expansion of d-spacing. The lattice parameter a decreases with the increase of Co substitution but c increases.

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Table 1 The refined crystallographic lattice parameters by Rietveld method of as-prepared materials. Samples

a /Å

c /Å

Cell volume /Å3

Fitting factor Rp (%)

MN

2.8747(1)

11.1868(3)

80.062(3)

8.35

MNC

2.8611(3)

11.2252(3)

79.581(4)

9.95

MC

2.8565(2)

11.2324(1)

79.374(3)

7.29

The property is significantly determined by the micro-structure of materials, especially for lithium-ion batteries and sodium-ion batteries, where the materials experience the insertion and de-insertion of Li-ion or Na-ion. Thus, to further probe the structural evolution induced by incorporating Co, the crystal structure was constructed based on the refined results. The local environment analysis of inter-atomic distances, the slab thickness (TMO6 octahedron, TM = transition metal) and the d-spacing of interslab thickness (Na layer) are presented in Figure 2(a)-(d) and Table 2-3. The interslab layer (Na ion diffusion layer) and slab layer (TMO6 octahedron layer) alternatively arrange along the c-axis direction to form the P2-type layered structure, where two adjacent TMO6 slab and one Na interslab in the middle constitute the “sandwich” configuration. The Na-ion with oxygen ions in-plane forms the triangular prism (Na-ion diffusion layer) in ab plane. From the viewpoint of material structural design, it can be inferred that as for the layered structure the larger space of the Na layer is easier for the migration of Na-ion. The structural data demonstrates that the average bond lengths of TM-O and O-O, 9



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and the slab thickness of TMO6 octahedron all decrease with increasing Co content, which may originate from the smaller ionic size of Co3+ (0.545Ǻ) than Ni2+ (0.69Ǻ). The shorter bond length usually means the higher binding energy

43

, which can be

indicative of the enhanced stability of the crystal structure. Further local atom environment analysis of the adjacent TMO6 octahedron in-plane of each material indicates that the bond angle TM-O3-TM gradually (in Figure 2(d) and Table 3) increases with the introduction of Co. Manthiram44 has declared that the increased V-O-V bond angle between two adjacent VO6 polyhedra along a direction in NaVOPO4 can accommodate the bigger sodium ion compared to LiVOPO4. Interestingly, though the ion radius of Co3+ (0.545Ǻ) is smaller than that of Ni2+ (0.69Ǻ) the d-spacing of Na ion diffusion layer (interslab thickness) gradually increases from 3.6577Å of MN to 3.7207Å of MNC and 3.8976Å of MC with the increase of Co content, which may be derived from the expansion of c and the contraction of the TMO6 octahedron. This is also in good agreement with the evolution of (002) peak in Figure 1(e). The volume shrinkage of the TMO6 octahedron can arise from the reduced TM-O and O-O atomic distances as shown in Figure 2 (d) and Table 3. For the P2-type layered structure, the decrease of lattice constant a and the increase of c can lead to the “slenderness” of the structure along c-axis direction. On the other hand, the total layer numbers of the slab and interslab are constant, thus it can be estimated that the compression of the slab thickness can produce more open space of the interslab without largely deteriorating the structural stability. The enhancement of the structure stability of cathode materials is favorable for improving 10


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the prolonged cyclic performance. The expanded d-spacing of Na-ion diffusion layer is indicative of the acceleration of diffusion kinetics for sodium-ion, which can improve the high-rate capability during the insertion and de-insertion of sodium-ion45-48. In addition, the SEM images of as-prepared materials are shown in Figure S1. The substitution of Co has little effect on both the morphology and particle size.

Figure 2 The refined crystal structure of each material. Grey sphere, red sphere and green sphere are Na, O and TM (TM = Mn, Ni, Co). (a) MN, (b) MNC, (c) MC, (d) the O-O atomic distance and bond angle of TMO6 octahedron. The average bond lengths of TM-O and O-O, and the slab thickness of TMO6 octahedron all decrease with increasing Co content but the d-spacing of Na ion diffusion layer (interslab thickness) and the bond angle TM-O3-TM gradually increases with the substitution of Co.

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Table 2 The atomic distances, slab thickness (TMO6) and the d-spacing of sodium ion layer for as-prepared materials. Samples

MN

MNC

MC

TM-O(Å)

1.9213

1.9036

1.8596

TMO2(Å)

1.9357

1.8919

1.7186

d-spacing(Å)

3.6577

3.7207

3.8976

Table 3 The evolution of O-O atomic distance in each TMO6 octahedron and the bond angle. Samples

MN

MNC

MC

O1-O3(Å)

2.8747

2.8612

2.8565

O1-O4(Å)

2.8747

2.8612

2.8565

O1-O5(Å)

2.5499

2.5116

2.3819

O1-O6(Å)

2.5499

2.5116

2.3819

O3-O4(Å)

2.8747

2.8612

2.8565

O3-O5(Å)

3.8426

3.8071

3.7193

O3-O6(Å)

2.5499

2.5116

2.3819

TM-O3-TM(°)

96.854

97.446

100.355

Oxidation State Analysis by XPS In general, in the LiNi1-x-yMnxCoyO2 (0≤x, y≤1, 1-x-y＜1) for LIBs, the Ni-ion, Co-ion and Mn-ion adopt +2, +3 and +4 oxidation state, respectively. Thus, the Mn4+ shows electrochemical inert in such oxide cathodes for LIBs, but it is helpful for the structure stability of cathode materials upon intercalation and de-intercalation of 12


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Li-ion2,3,10,11. The oxidation states of transition metals of the as-prepared cathode materials for SIB were analyzed by the X-ray photoelectron spectroscopy (XPS) and shown in Figure 3(a)-(c).The main peak on 642.17eV of Mn 2P3/2 indicates the existence of Mn4+ in the oxides. The existence of Ni2+ can be certified by the Ni 2P3/2 main peak at around 854.5eV. The Co 2P3/2 peak located at 780.1eV demonstrates the oxidation state of Co is +3 in the as-synthesized cathode oxides. The valence results of the transition metal ions are also consistent with the previous reports23,

49, 50

.

NaxMnO2 is a +3 and +4 oxidation state coexisting oxide and can be denoted as another format NaxMn3+xMn4+1-xO2, which can give a clear picture that the Ni2+and Mn4+ can eliminate the Mn3+ component in MN and MNC materials based on the possibility of replacing 2Mn3+ by Ni2+ and Mn4+, the Co3+ can totally substitute the Mn3+ component in MC material21. In general, the Jahn-Teller effect is related to the Mn3+ ions, which can seriously degrade the structural stability and the repeated cyclic performance of composite electrode upon electrochemical cycling21, 51. Similar to the LIBs, the Ni2+ and Co3+ in this work should compensate the charge mechanism while the Mn4+ seems to be an inactive component. However, the Mn4+ indeed can be activated during insertion and de-insertion of sodium-ion while be discharged below 2V49, 52, 53.

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Figure 3 The XPS spectra and fitting results of transition metal element in powder samples:(a) Mn4+, (b) Ni2+ and (c) Co3+. The fitting results of XPS spectra indicate the existence of Mn4+ and Ni2+ and Co3+.

Electrochemical Performance Measurement 14


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Figure 4(a)-(c) shows the CV curves of the cathodes at a scan rate of 0.2mV/s. The numbers of oxidation and reduction peaks for the Co-containing samples are less than the MN. According to the previous studies about the electrochemical activation of Ni, Co and Mn-ion49, 52-54 and the vacancy-dominated character of sodium-deficient oxides4, the multiple peaks of MN between 3.2V and 3.9V can be associated with the Ni-ion48, 53-55 and the Na+/vacancy ordering process18, 56. The peaks of all the cathodes at about 2.0V can be related to the Mn4+/Mn3+ redox process48, 53, 55. Other weak peaks between about 2.0V and 3.0V may be attributed to the Na+/vacancy ordering process18, 56

. Interestingly, the peak numbers of the samples with Co substitution are decreased.

The additional peaks in the range of 3.0-4.0V of MC cathode can be involved with the Co-ion redox reaction52, 53 and the Na+/vacancy ordering process18, 56 Considering the electrochemical active Ni2+ and Co3+ ions, the peaks above 3.0V for the MNC cathode might be related to the Ni and Co-ion. Although the clear explanation is not obtained of the CV plots for all the cathodes here the effect of Co on such cathode materials can be revealed.

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Figure 4 The cyclic voltammogram (CV) curves at a scan rate of 0.2 mV s-1 in voltage range of 1.5-4.2V: (a) MN, (b) MNC, (c) MC. The peak numbers of the samples decrease with Co substitution.

The shape of charge and discharge profiles is also critical to assess battery property and step-like curves are difficulty to be directly used in practical applications21, 49. The 16


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charge and discharge profiles with multiple voltage plateaus always appear in the transition metal oxide cathodes for SIBs, which are largely related to the complex phase

transformation

and/or

the

Na+-vacancy

ordering13-20

due

to

the

vacancy-dominated character of sodium-deficient transition metal oxide cathodes4. To evaluate the effect of Co substitution on the electrochemical behavior, the charge/discharge curves at the initial two cycles of the as-synthesized MN, MNC and MC cathodes in the range of 1.5 - 4.2V at 12mA g-1 (0.05C, 1C = 240mAh g-1) are shown in Figure 5(a)-(c). The NaxCoO2 oxide can exhibit a series of voltage plateaus upon charging and discharging13. The NaNiO2 cathode also shows the multiple voltage plateaus during intercalation and de-intercalation of sodium-ion19. However, what’s the interesting is that the MN here also displays a few obvious plateaus in charge/discharge profiles while the MC exhibits very smooth curves in spite of the same Mn content in the two oxides. On one hand, the shape evolution of charge and discharge profiles becomes much smoother with the increase of Co content, which indicates the impact of Co on smoothing the curves. On the other hand, it can be inferred that the Ni element always shows the extremely negative impact on the smoothness of curves though the Mn content is very high, which is also in agreement with the reported results17, 19, 57, 58. Further analysis declares that the MN cathode shows an obvious charge and discharge plateau at around 4.1V, which may derive from the P2-O2 phase transformation17. The similar plateaus disappear with the introduction of Co demonstrating that Co can effectively hinder the phase transformation. However, this similar phenomenon also occurs in other P2-type 17



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materials, where the possible mechanism might arise from the oxygen removal in the crystal structure32, 59.

Figure 5 The charge/discharge profiles at first and second cycle of (a) MN, (c) MNC, (b) MC at the current density of 12 mA g-1 and 1.5-4.2V. The charge and discharge profiles become much smoother with the substitution of Co. 18


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The initial discharge capacity of MN is close to 155mAh g-1 and shows little change with the increase of Co content. Previous studies have proved that Ni, Co and Mn elements can contribute to the charge compensation and electrochemical capacity in the range 1.5-4.2V49, 52, 53, while Mn does not participate in the redox reaction in the range of 2.0-4.2V during charge and discharge process17, 33, 54, 55, 58. However, this differs from the analogues in LIBs where the different element exhibits diverse effect upon electrochemical process and Mn4+ is electrochemical inert. But Mn4+ can play an important role in stabilizing the structure during insertion and de-insertion of Li-ion. The reversible capacity, in particular at an enhanced high rate, is one of the significant factors to determine the entire properties for rechargeable LIBs and SIBs. High capacity means the more available energy-stored in material and high energy density, such as Ni-rich based material for LIBs2, 3, 12.The ability of charging and discharging at a high current density is crucial for the battery. To detect the effect of Co substitution on the rate performance, the rate capability tests were performed in the voltage range of 1.5-4.2V at various current densities as shown in Figure 6(a). The charge and discharge profiles at different current densities are presented in Figure 6(b)-(d). The MN cathode material can display discharge capacity of ca.157, 147, 130, 113 and 99 mAh g-1 at 12, 24, 48, 120, 240 mA g-1, which are equal to 0.05C, 0.1C, 0.2C, 0.5C and 1C, respectively. The MNC cathode material offers the discharge capacity of about 154, 148, 143, 136 and 127 mAh g-1 while the MC cathode material delivers the discharge capacity of around 155, 149, 144, 136 and 130 mAh g-1 at the same rates, respectively. However, when the current density was increased to 480(2C), 19



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1200(5C), 1920(8C) and 2400mA g-1 (10C), respectively, the significant differences can be observed clearly. Compared to the reversible capacities of ca.83, 60, 45 and 36mAh g-1 for the MN cathode, respectively, the MNC cathode exhibits the reversible capacity of about 115, 95, 80 and 70mAh g-1 and the MC cathode can deliver the discharge capacity of around 122, 106, 95 and 87mAh g-1 at the same current densities, respectively. In addition, the rate capability of MC in this work have also been significantly enhanced compared to most of P2-type cathodes reported in the previous publications. 33, 34, 48, 49, 53, 55, 60, 61 The results prove that the substitution of Co can largely improve the rate performance of such layered cathode materials. The enhancement of the high-rate capability in part derives from the enlarged d-space of the sodium-ion diffusion layer, which can reduce the migration barrier and improve the diffusion coefficient of sodium-ion. The computational calculation by Ceder45, 46 has proved that the energy barrier of Li-ion diffusion is inversely proportional to the enlargement of the Li-ion interslab. And the experiment observations of the improved high-rate performance associate with the expanded space of Li-ion layer have also been reported by Zhou47.

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Figure 6 (a) Rate capabilities of cathodes at various current densities (1C = 240 mAh g-1) and corresponding rate curves of each cathode: (b) MN, (c) MNC, (d) MC. The rate performance of Na0.7Mn0.7Ni0.3-xCoxO2 has been largely enhanced with the substitution of Co.

The structure stability is critical for the long-term cycling performance of an electrode material, which can partly affect the real working life of a rechargeable battery. Therefore, to clearly probe the impact of Co substitution on the cycling stability of the cathode material, the electrodes were carried out at 24mA g-1 (0.1C) and 240mA g-1 (1C), as shown in Figure 7(a) and (e), respectively. The charge and discharge profiles of the 1st, 10th, 20th, 30th, 40thand 50th cycle at 0.1C are presented in Figure 7(b)-(d). The activation at 0.05C for the initial three cycles was applied before the cyclic measurements. The corresponding charge and discharge profiles of the MC 21



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at 1C are presented in Figure 7(f). The discharge capacity of MN decreases from the about 147 mAh g-1 to ca.106 mAh g-1 at 50th cycle, which is equal to about 72% capacity retention at 0.1C. In contrast, the Co-containing samples show largely enhanced cyclic performance. MNC and MC both deliver a similar initial capacity of about 148mAh g-1 but still offer about 137 and 141mAh g-1 capacity after 50 cycles, which correspond to the capacity retention of about 92% and 95%, respectively. Even at a high rate of 240 mA g-1 (1C) and the long-cyclic test of 100 cycles, as shown in Fig.7(e) and (f), the MNC and MC cathodes can maintain the reversible capacity of 94 and 111mAh g-1, respectively, which offer the capacity retention of 74% and 85%, respectively. In contrast, the MN only has a capacity of 61mAh g-1with a much lower capacity retention of about 60% at 1 C and 100th cycle. In addition, it should be noted that the voltage slightly decays with increasing cycles, which might be largely related to the phase transformations during Na+ insertion/desertion. But Co substitution can mitigate the voltage decay during the charge-discharge cycles, which further indicates that the substitution of Co for Ni can suppress the phase transitions and subsequently improve the structure stability.

22


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Figure 7(a) The cyclic performance of the cathodes at 0.1C and the corresponding cyclic curves of the cathodes at 0.1C, (b) MN, (c) MNC, (d) MC. (e) The cyclic performance of the cathodes at 1C, (f) Cyclic curves of MC electrode at 1C. The cycling stability of Na0.7Mn0.7Ni0.3-xCoxO2 has been improved with the substitution of Co.

There are some reports about the deteriorated cyclic performance beyond the voltage range of 2.0-4.0V of P2-materials62. However, the excellent cyclic stability 23



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occurs by adding Co here even in the extended voltage range of 1.5-4.2V. Yamada et al reported that the passivating layer around the surface can be formed by replacing Mn by Co, which is favorable for the cycling performance63. In addition, the long-cycle stability of Co-containing cathodes, in particular the MC, can be as great as some reported cathodes with excellent stability in literatures22, 23, 49, 53, 55, 64-66. Thus, the results have effectively proved that the substitution of Co can significantly enhance the cycling property during the insertion and de-insertion of sodium ion, which is largely associated with the enhanced structural stability induced by the shrinkage of TM-O and O-O bond lengths and the contraction of TMO6 octahedron upon the substitution of Co for Ni. Diffusion Coefficient and Electronic Conductivity Tests An electrochemical reaction, such as the redox reaction in Li-/Na ion battery, includes the simultaneous participation of both Li-/Na ions and the electrons. Therefore, to guarantee the high-rate capability of battery, both the diffusion of Li-/Na ions and the transportation of electrons should be enhanced. The apparent chemical diffusion coefficient of sodium-ion was measured by cyclic voltammogram (CV) technique. The CV analysis was carried out at different scan rate of 0.2, 0.4, 0.6 and 0.8mV s-1, as shown in Fig. S2(a)-(c). Fig.S3(a)-(c) show the data point and the linear fitting results between peak current (Ip) of main peak at around 2.0V and the square root of scan rate. Fig.S3(a)-(c) exhibits the great linear relationship between the oxidation peak current and the square root of scan rate (v1/2) indicating the diffusion-limited process. Therefore, the estimated apparent chemical diffusion 24


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coefficient can be calculated by the following equation: Ip = 2.69×105ACD1/2z3/2v1/2， where Ip is the main peak current (A), n is the number of electron, F is the Faraday constant, A is the electrode area (cm2), C is the concentration of sodium-ion, v is the scan rate (V/s), R is the gas constant, T is the absolute temperature and D is the estimated chemical diffusion coefficient of sodium-ion (cm2/s). The slope of the curves between peak current (Ip) and the square root of scan rate (v1/2) is shown in Table S4 and the calculated apparent chemical diffusion coefficient of sodium-ion is shown in Fig.8(a). As shown in Fig.8(a), the estimated apparent chemical diffusion coefficient is about 10-12-10-11 cm2/s and the diffusion coefficient can be largely increased by adding Co into the oxides, which are associated with the gradually enlarged sodium-ion diffusion layer. The electronic conductivity of each power material was analyzed and shown in Fig.8(b). The electronic conductivity of Co-containing samples is much higher than that of the MN, and the results are very close to the values reported by Ceder61. The enhanced electronic conductivity might be largely derived from the aliovalent substitution of Co3+for Ni2+. In addition, according to Dahn’s studies39, 41 the superlattice ordering exist in the Co-free material and the ordering can affect the water insertion. However, it is unclear here that whether the ordering has impact on the electron conduction. Besides, according to the solid-solution phase diagram analysis2, 11, 12, Co component plays an important part in rate performance in solid-solution phase LiNi1-x-yMnxCoyO2 (0≤x, y≤1, 1-x-y＜1) for LIBs, where the reason might partly originate from the great electron conduction in LiCoO2, which adopts a lower band gap according to electronic structure67-69. 25



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Figure 8 (a) The estimated apparent chemical diffusion coefficient of each cathode, (b) the electronic conductivity of each cathode material. Both the apparent chemical diffusion coefficient of sodium-ion and electronic conductivity of Na0.7Mn0.7Ni0.3-xCoxO2 have been enhanced with the substitution of Co.

In combination with the material structural analysis, the electrochemical tests and the electronic conductivity measurements, the possible mechanism of the improvement of electrochemical performances by adding Co element in transition metal oxide cathodes for SIBs can be clearly unveiled. On one hand, the introduction of Co into the Na0.7Mn0.7Ni0.3-xCoxO2(x = 0, 0.1, 0.3) enlarges the d-spacing of sodium-ion diffusion layer which can reduce the barrier of Na-ion migration and 26


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enhance the high-rate performance. At the same time, the introduction of Co shortens the bond lengths of TM-O and O-O which leads to the decreased thickness and volume of TMO6 octahedron and increases the structure stability. On the other hand, the addition of Co also improves the electronic conductivity which is also favorable to the enhancement of the high-rate performance. The unveiling of the role of Co in improving the electrochemical performances in this work will also offer some new insights into designing advanced electrode material for SIBs. Conclusions In summary, we have unveiled the role of Co on the enhancement of the electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 as cathode materials for sodium ion batteries and proposed some insights for the first time. With the substitution of Co the capacity of MNC and MC at high current density has been significantly

improved.

The

high-rate

performance

of

Co-substituted

Na0.7Mn0.7Ni0.3-xCoxO2 can be largely attributed to the enlargement of sodium ion diffusion layer and the enhancement of the electronic conductivity induced by the substitution of Co3+ for Ni2+. In addition, the capacity retention has also been largely enhanced with the substitution of Co for Ni. The improvement of the cyclic performance for the Co-substituted Na0.7Mn0.7Ni0.3-xCoxO2 can be largely attributed to the enhanced structure stability which may originate from the contraction of TM-O and O-O bond lengths and the shrinkage of TMO6 octahedron. The new insights into the role of Co in the improvement of the electrochemical performances of Na0.7Mn0.7Ni0.3-xCoxO2 as cathode materials for sodium ion batteries might be helpful 27



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to design the high performance cathode materials for SIBs.

ASSOCIATED CONTENT

Supporting Information: The refined XRD results by Rietveld method of MN, MNC and MC; the SEM images of as-prepared materials; the cyclic voltammogram (CV) curves of MN, MNC and MC at various scan rate; the data point of peak current (Ip)

vs. scan rate and the fitting results for MN, MNC, and MC; The slope of the curves between peak current and the square root of scan rate. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION *Corresponding

Author: [email protected] University of Chinese Academy of

Sciences, Beijing 100049, China. Tel. +86 10 8825 6840

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Grant 11575192), the State Key Project of Fundamental Research (Grant 2012CB932504) of Ministry of Science and Technology of the People's Republic of China, and “Hundred Talents Project” of the Chinese Academy of Sciences.

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Metal

Oxide

of

NaNi1/4Co1/4Fe1/4Mn1/8Ti1/8O2

as

a

High-Rate-Capability

and

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(60) Wua, X.; Guo, J.; Wang, D.; Zhong, G.; McDonald, M. J.; Yang, Y. P2-type Na0.66Ni0.33-xZnxMn0.67O2 as New High-Voltage Cathode Materials for Sodium-Ion Batteries. J. Power Sources 2015, 281, 18-26. (61) Liu, L.; Liu, X.; Bo, S. -H.; Wang, Y.; Chen, H.; Twu, N.; Wu, D.; Ceder, G. High-Performance P2-Type Na2/3(Mn1/2Fe1/4Co1/4)O2 Cathode Material with Superior Rate Capability for Na-Ion Batteries Adv. Energy Mater. 2015, 5, 1500944. (62) Wang, H.; Yang, B.; Liao, X.-Z.; Xu, J.; Yang, D.; He, Y.-S.; Ma, Z.-F. Electrochemical Properties of P2-Na2/3Ni1/3Mn2/3O2 Cathode Material for Sodium Ion Batteries when Cycled in Different Voltage Ranges. Electrochim. Acta 2013, 113, 200-204. (63) Wang, X.; Tamaru, M.; Okubo, M.; Yamada, A. Electrode Properties of P2-Na2/3MnyCo1-yO2 as Cathode Materials for Sodium-Ion Batteries. J. Phys. Chem. C 2013, 117, 15545-15551. (64) Li, Y.; Yang, Z.; Xu, S.; Mu, L.; Gu, L.; Hu, Y. -S.; Li, H.; Chen, L. Air-Stable Copper-Based P2-Na7/9Cu2/9Fe1/9Mn2/3O2 as a New Positive Electrode Material for Sodium-Ion Batteries Adv. Sci. 2015, 2, 1500031. (65) Buchholz, D.; Vaalma, C.; Chagas, L. G.; Passerini, S. Mg-Doping for Improved Long-Term Cyclability of Layered Na-Ion Cathode Materials - The Example of P2-type NaxMg0.11Mn0.89O2. J. Power Sources 2015, 282, 581-585. (66) Talaie, E.; Duffort, V.; Smith, H. L.; Fultz, B.; Nazar, L. F. Structure of the High Voltage Phase of Layered P2-Na2/3-z[Mn1/2Fe1/2]O2 and the Positive Effect of Ni Substitution on its Stability. Energy Environ. Sci. 2015, 8, 2512-2523. (67) Van der Ven, A.; Aydinol, M. K.; Ceder, G.; Kresse, G.; Hafner, J. First-Principles Investigation of Phase Stability in LixCoO2. Phys. Rev. B 1998, 58, 2975-2987. 36


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(68) Antolini, E. LiCoO2: Formation, Structure, Lithium and Oxygen Nonstoichiometry, Electrochemical Behaviour and Transport Properties. Solid State Ionics 2004, 170, 159-171. (69) Ménétrier, M.; Saadoune, I.; Levasseur, S.; Delmas, C. The Insulator-Metal Transition upon Lithium Deintercalation from LiCoO2 Electronic Properties and Li-7 NMR Study. J. Mater. Chem. 1999, 9, 1135-1140.

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Unveiling the Role of Co in Improving the High ... - ACS Publications

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