New Insights into the Roles of Mg in Improving the Rate Capability and

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New Insights into the Roles of Mg in Improving the Rate Capability and Cycling Stability of O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2 for Sodium-ion Batteries Cheng Zhang, Rui Gao, Lirong Zheng, Yongmei Hao, and Xiangfeng Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18226 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Applied Materials & Interfaces

New Insights into the Roles of Mg in Improving the Rate Capability and Cycling Stability of O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2 for Sodium-ion Batteries Cheng Zhang,†,§ Rui Gao,† Lirong Zheng,‡ Yongmei Hao*,§ and Xiangfeng Liu*,† †College

of Materials Science and Opto-Electronic Technology, University of

Chinese Academy of Sciences, Beijing 100049, P. R. China ‡Institute

of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P.

R. China §

School of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Beijing 100049, P. R. China

KEYWORDS: sodium-ion battery; layered oxide cathode; O3 phase; Mg doping; improvement mechanism

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ABSTRACT: Elements doping has been used to improve the electrochemical performances of O3-type layered transition metal oxide cathodes for sodium-ion batteries. But their roles and the improvement mechanism have not been clearly understood. Herein, the effects of Mg substitution for Mn on the structure and electrochemical

performances

of

NaMn0.48Ni0.2Fe0.3Mg0.02O2

have

been

comprehensively investigated and some new insights into the roles of Mg in improving the rate capability and cycling stability have been presented. 1) The substitution of Mg for Mn enlarges the interlayer spacing, which not only enhances Na+ diffusion and the rate capability but also alleviates the lattice strains induced by Na+ intercalation/deintercalation. 2) the substitution of Mg for Mn also shrinks TM-O bond and TMO2 slabs which enhances the layered structure stability. 3) Mg substitution also mitigates the structure distortion or volume change of the crystal lattices and suppresses the irreversible phase transitions. 4) the substitution of low valence Mg2+ for Mn3+ reduces Mn3+ and minimizes Jahn–Teller effect, which also further alleviates the irreversible phase transformations and improves the layered structure stability. This study not only unveils the roles of Mg but also presents some insights into designing the cathode materials with both high rate capability and high cycling stability through the lattice structure regulation.


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INTRODUCTION

With the widespread use and the increasing cost of lithium-ion batteries (LIBs), sodium-ion batteries (SIBs) have attracted much attention as promising candidates for large-scale energy storage due to the low cost and similar energy storage mechanism between Li and Na.1-6 However, the practical applications of SIBs still face some critical challenges. Unlike LIBs, the larger ionic radius of Na+ (1.02Å v.s. 0.76Å) , the sluggish migration kinetics of massive Na+ and the complicated phase transitions induced by Na+ intercalation/deintercalation usually result in the poor cycling stability, inferior rate capability and low energy density of SIBs.4, 7-14 Therefore, it is quite essential to explore cathode materials with high rate capability and high cycling stability in order to boost the development and application of SIBs. Some cathode materials for SIBs, such as NaxCoO2, NaFePO4, Na0.44MnO2, NaMnO2, NaVPO4F and Na2CoPO4F have been developed.3-4,

15-20

Among the available cathode materials,

layered transition metal oxides NaxMO2 (M = Co, Ni, Mn, Ti, Fe, etc.) have attracted particular attention owing to their great potential in rate capability and theoretical capacity.21-22 Layered transition metal oxides can be grouped into two basic categories: P2-type (ABBA stacking, Na+ in prismatic sites) and O3-type (ABCABC stacking, Na+ in octahedral sites). Sodium-sufficient O3-type layered transition metal oxides cathodes have a similar structure with the widely used LiCoO2 cathode for LIBs and receive much attention. But O3-structure cathodes usually suffer from poor rate capability and inferior cycling stability. The fading of the reversible capacity may be due to the structural transitions during charge/discharge.17,

23-24

The poor rate 3

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capability is mainly caused by the slow Na+ migration kinetics. Foreign elements doping has been considered as one of the effective strategies to overcome these shortcomings.25-32 Guo et al.

28

reported that the electrochemical performances of

NaNi0.5Mn0.5O2 were largely improved through the substitution of Ti4+ for Mn4+. Sun et al.25 stabilized the layered structure of NaNi0.25Fe0.25Mn0.5O2 by appropriate Li substitution and they attributed the stabilization to the stronger bonding of Li-O than Ni-O and Mn-O. Just recently, Guo et al.29 designed high performance cathode material NaFe0.45Co0.5Mg0.05O2 and attributed its excellent properties to the synergetic effect of Fe3+ (high redox potential), Co3+ (good kinetics), and inactive Mg2+ (stabilizing structure).

Foreign elements doping has been extensively applied to improve the electrochemical performances of the layered transition metal oxides cathode materials but the roles of the incorporated doping elements and the improvement mechanism are still not very clear owing to the differences in the size, valence, electrochemical activity, the substituted atoms types and even the atoms coordination environments. However, understanding how the introduced foreign elements modulate the crystal and electronic structure to improve the electrochemical performances is of great importance for unrevelling the structure-performance relationship and rationally designing high performance cathode materials.

In this study, we have comprehensively investigated the effects of the substitution of Mg for Mn on the crystal structure, electronic structure and electrochemical 4 ACS Paragon Plus Environment

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performances of O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2 by means of ex-situ x-ray diffraction, x-ray photoelectron, ex-situ x-ray absorption spectroscopy and electrochemical techniques. The roles of Mg on the modulation of the crystal or electronic structure and the improvement of both the rate capability and the cycling performance have been unraveled and some new insights into the roles of Mg in improving the rate capability and cycling stability of O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2 for sodium-ion batteries have been proposed besides the well-known pinning effect (The stabilization of Mg on the layered structure comes from its inactivity and the distortion of MeO6 centered by inactive Mg ions is negligible29). Our present study indicates that the substitution of Mg for Mn in O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2 not only stabilizes the layered structure but also enhances the rate capability. The substitution of Mg for Mn enlarges the interlayer spacing which decreases the Na+ diffusion barrier and mainly contributes to the enhanced rate capability. The expansion of the interlayer spacing can also mitigate the lattice strain or the volume change induced by sodiation/desodiation and alleviate the irreversible phase transitions because of the facilitation of Na+ intercalation/deintercalation. In addition, both the bond length of TM-O and the thickness of TMO2 slabs are shortened by the substitution of Mg for Mn which are favorable to the stabilization of the layered structure. Moreover, the substitution of low valence Mg2+ for Mn3+ can also minimize the possible formation of high spin Mn3+ owing to the charge compensation and mitigate the related Jahn-Teller effect.

EXPERIMENTAL SECTION 5 ACS Paragon Plus Environment


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Synthesis of Cathode Materials. NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) material was synthesized by a conventional solid-state reaction in air atmosphere using precursors of CH3COONa·3H2O, CH3COOMn·3H2O, CH3COONi·3H2O, Fe2O3, and MgO. An excess of 5% CH3COONa·3H2O was added to compensate the loss of sodium at high temperature. The powder precursor was ground and calcined at 500 °C for 5 h and then calcined at 900 °C for 12 h in air to obtain the final materials. All the above chemicals were purchased from China National Medicines Corp. Ltd. and were used without any further purification.

Electrochemical tests. The active material, acetylene black and poly (vinylidene fluoride) binder (PVDF) were combined in a ratio of 75: 15: 10 in a N-methyl-2-pyrrolidene (NMP) solvent to produce a uniform slurry. Then the slurry was pasted uniformly onto an Al foil and dried overnight at 120 °C in a vacuum oven. CR2025 coin-type cells were assembled in a glove box, using Na discs as the negative electrode and 1.0 M NaClO4 in propylene carbonate (PC) and glass fiber GF/D (Whatman)

were

used

as

the

electrolyte

and

separator.

Galvanostatic

charge−discharge tests were carried out in the voltage range of 1.5−4.2V versus Na+/Na using an automatic galvanostat (NEWARE) at different current densities at room temperature. Cyclic voltammetry (CV) was performed with an electrochemical workstation (PRINCETON, PMC-2000).

Material characterization. Powder X-ray diffraction (XRD) patterns were tested by the Rigaku Smartlab using Cu (Kα) radiation in steps of 0.01° and a 2θ range of 6 ACS Paragon Plus Environment

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10−80°. The unit cell parameters were refined using Fullprof software based on the Rietveld method. For ex situ XRD studies, cells were prepared as described above but were charged to a particular voltage once and then stopped. X-ray-absorption fine structure (XAFS) spectra were collected on the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF Beijing, China). Electrodes for ex situ XRD measurements were recovered from these cells in an argon-filled glovebox, and the electrode materials were washed with dimethyl carbonate (DMC, BASF).

Scanning

electron microscopy (SEM) measurements were carried out on Hitachi SU8010and Energy dispersive spectroscopy (EDS) elementary mapping was collected on Bruker XFlash. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB MK II X-ray photoelectron spectrometer (Al exciting source). Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was performed on IRIS Intrepid II XSP (Thermo Electron). The magnetic measurements of Samples were performed using Physical Properties Measurement System (PPMS) -vibrating sample magnetometer (VSM) manufactured by Quantum Design.

RESULTS AND DISSCUSSION

The detailed compositions of the two samples of NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) were analyzed by ICP-AES and summarized in Table S1. As shown in Table S1, the measured compositions of Mg-free or Mg-doped samples are very close to the chemical

stoichiometry.

Powder

X-ray

diffraction

(XRD)

patterns

of

NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) are shown in Figure 1a. All the diffraction 7 ACS Paragon Plus Environment


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peaks can be well indexed to a rhombohedral lattice with a space group of R3m , indicating the purity of α-NaFeO2 phase. Figure 1b indicates that the (003) peak moves to the small angel region when Mn is substituted by Mg. Through Bragg equation 2dsinθ=nλ we can infer that the doping of Mg for Mn expands the interplanar spacing, which has also been further proved by the subsequently refined crystallographic data.33 Ceder et al reported that the expansion of the interlayer spacing could enhance Li ions mobility in layered LiMn0.5Ni0.5O2.34 Given the similarity between Na-ion batteries and Li-ion batteries, the Na ions may follow the similar pattern during insertion and deinsertion. Therefore, the larger interplanar space between (003) slabs will be beneficial to the Na+ diffusion. Figure 1c and d show Rietveld refinement profiles of the two samples based on XRD data. The calculated patterns are in well agreement with the experimental data with a small Rwp. The refined crystallographic data of NaMn0.5-xNi0.2Fe0.3MgxO2 (x= 0, 0.02) are summarized in Table S2-S4. As shown in Table S2, the lattice parameters a and c both increase with the substitution of Mg for Mn, which can be attributed to the bigger ionic radius of Mg2+ (0.72 Å) than Mn3+ (0.645 Å). As shown in Table S4, Mg occupies 3b sites. The deviation from an ideal rhombohedral phase is the result of Jahn–Teller activity which is caused by Mn3+.35 It is obvious that the substitution of Mn by Mg in O3-NaMn0.5Ni0.2Fe0.3O2 can suppress the orthorhombic distortion. With the introduction of low valence Mg2+, the average Mn oxidation state is much closer to tetravalence and Mn3+ content declines to maintain the electric neutrality. The d-spacing of the Na layer also largely increases from 3.3288 Å to 3.3979 Å with the 8 ACS Paragon Plus Environment

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substitution of Mg for Mn, which may not only enhance the Na+ diffusion ability during the sodiation/desodiation process but also can mitigate the lattice strain or irreversible phase transitions induced by Na+ intercalation/extraction.28, 36-38 Figure S1 shows the schematic structure of the refinment results. O3-NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) denotes a layered structure where the layers of oxide-ion stack in the way of ABCABC and Na+ is located in the octahedral sites. Symbols A, B, and C represent the different oxygen layers. The yellow bullets represent Na+ ions and blue layers refer to the TMO2 layer. The distance (d-spacing) between two blue layers is represented by the letter d. In order to further confirm the substitution of Mg for Mn the magnetization curves of NaMn0.5-xNi0.2Fe0.3MgxO2 (x=0, 0.02) at room temperature are exhibited as shown in Figure S2. The results show that both of the two samples were paramagnetic at room temperature. By comparing the slope of straight line, we can conclude that the magnetic susceptibility of NaMn0.48Ni0.2Fe0.3Mg0.02O2 is a little lower than that of NaMn0.5Ni0.2Fe0.3O2 due to the magnetic dilution effect of Mg, which also further proves the substitution of Mg for Mn.

The size and morphology of the as-prepared samples were investigated by scanning electron microscopy (SEM). As shown in Figure 2a and b, the distribution of the particle size is in the range of 1–3 µm. It is apparent that almost all the particles of the two samples are polyhedral and plate-like, which means the substitution of Mg for Mn has little effect on the size and morphology. Energy dispersive spectroscopy (EDS) elementary mapping reveals that

Na, Mn, Ni, Fe, Mg and O are distributed

uniformly throughout the material, as displayed in Figure 2c. 9 ACS Paragon Plus Environment


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The X-ray photoelectron (XPS) measurements were performed to investigate the oxidation states of the Mn, Ni, Fe, Mg ions of the as-prepared samples and the results are shown in Figure 3. The binding energy of Mn 2p3/2,1/2 peaks at about 642.1eV and 653.9 eV indicate the existence of Mn4+.39 In NaxMnO2, the Mn-ion is trivalent and tetravalence mixed oxidation state which can be rewritten as NaxMn3+xMn4+1-xO2. And the Jahn−Teller effect which is related to the Mn3+ ions could seriously damage the structural stability and the cyclic performance.25, 40-41 The Ni 2p3/2,1/2 peaks at 854.5eV and 872.5 eV with a spin–orbit separation of 18 eV correspond to Ni2+.33 The peaks of Ni 2p3/2,1/2 at 855.6 eV, 872.7 eV are assigned to Ni3+ species.42 The Fe 2p3/2 peak located at 710.8eV demonstrates that the oxidation state of Fe is +3 in this oxides.43-44 And the Mg 1s peak of about 1303eV indicates the existence of Mg2+.

Figure 4 shows the contrast of Mn 2p3/2 of NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02). The Mn 2p3/2 peak can be separated into two peaks at 641.0eV and 642.3 eV, which correspond to Mn3+ and Mn4+, respectively. Comparing Figure 4a and b, the ratios of Mn4+ to Mn3+ (noted as Mn4+/Mn3+) increase after Mg-doping. The relative content of Mn3+ decreases from 66.2% to 61.5%, while the relative content of Mn4+ increases from 33.8% to 38.5%, as shown in Table S5. To analyze the valences of the inner elements Ar+ etching is used as shown in Figure S3 and Table S6. The results indicate that the ratio of Mn4+/Mn3+ changes very slightly from the outer to the inner for both Mg-free or Mg-doped sample. The ratio of Mn4+/Mn3+ for Mg-doped sample is always higher than that of Mg-free sample. The results indicate that the substitution of


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low valence Mg2+ for Mn3+ can mitigate the formation of high spin Mn3+ owing to the charge compensation and alleviate the related Jahn-Teller effect.

The electrochemical performances of the O3-NaMn0.5−xNi0.2Fe0.3MgxO2(x = 0, 0.02) electrode were evaluated in half cells at room temperature. Figure 5a and b display the first three galvanostatic charge/discharge profiles of NaMn0.5Ni0.2Fe0.3O2 and NaMn0.48Ni0.2Fe0.3Mg0.02O2 in the range of 1.5-4.2 V at 12 mA g−1. The NaxMnO2 and NaNiO2 oxides both exhibit some voltage plateaus during charge and discharge.45 However, it is interesting that although the O3-NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02) also displays a few obvious plateaus during charge and discharge, the galvanostatic charge/discharge profiles are quite smooth despite Mn and Ni content are in the two oxides. The smooth curves indicate that Mg-doping has little damage on the electrochemical reaction process. The charge and discharge capacity of the first cycle is about 130 mA h g-1 (x = 0) and 160 mA h g-1 (x= 0.02).

In the field of large-scale energy storage, the rate capability is an important factor for the application of the batteries.33,

46

To further evaluate the electrochemical

properties of the as prepared cathodes, the rate capabilities were tested in the range of 1.5-4.2 V at various current densities and the curves are shown in Figure 5c. The reversible capacities of Mg-free NaMn0.5Ni0.2Fe0.3O2 cathode are 130, 117, 105, 85, 66, 40, 5, 2 and 1.5 mA h g-1 at 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 8C and 10C, respectively.

In

contrary,

the

rate

capabilities

of

Mg-containing

NaMn0.48Ni0.2Fe0.3Mg0.02O2 cathodes are enhanced to 160, 136, 118, 100, 87, 72, 40, 11 ACS Paragon Plus Environment


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19 and 10 mA h g-1, respectively, indicating the Mg substitution can largely enhance the rate capability.

The cycling performance is shown in Figure 5d. The O3-NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02) cathodes demonstrate a significant improvement in the cycling stability at 1.5–4.2V. Mg substitution for Mn effectively enhances the capacity retention of the electrodes. The capacity retention for NaMn0.48Ni0.2Fe0.3Mg0.02O2 after 100 cycles at 0.1C is greatly increased to 99% in compared to 81% of NaMn0.5Ni0.2Fe0.3O2, which suggests that NaMn0.48Ni0.2Fe0.3Mg0.02O2 is a superior stable and reversible cathode material for sodium-ion batteries. This can be attributed to the enhanced stability of the layered structure by the substitution of Mg for Mn, which will be further confirmed by ex situ XRD. The Coulombic efficiency for NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) cathode are shown in Figure S4.The results indicate that Mg doping also slightly enhanced the Coulombic efficiency.

In order to confirm the substitution of Mg for Mn on the layered structure stability, we analyze the lattice parameters change before and after 50 cycles in the range of 1.5−4.2V. XRD patterns of the pristine and the electrodes after cycles for NaMn0.5Ni0.2Fe0.3O2 and NaMn0.48Ni0.2Fe0.3Mg0.02O2 are shown in Figure S5. The refined crystallographic data based on the diffraction data are summarized in Table 1. In order to compare the change degree of the lattice parameters before and after 50 cycles, we also calculate the variation of the lattice parameters (Table 1). After 50 cycles, ∆a, ∆c and ∆V of NaMn0.50Ni0.2Fe0.3O2 are 0.87, -2.42 and -0.73%, 12 ACS Paragon Plus Environment

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respectively.

In

contrast,

after

50

cycles,

∆a,

∆c

and

∆V

of

NaMn0.48Ni0.2Fe0.3Mg0.02O2 are largely decreased to 0.010, -0.5 and -0.48%, respectively. With Mg substitution, the variation of the lattice constants a, c, V of NaMn0.48Ni0.2Fe0.3Mg0.02O2 is only 1.1, 20.5, and 66% of NaMn0.5Ni0.2Fe0.3O2, which further indicates that the doping of Mg remarkably alleviates the irreversible distortion of the crystal lattices during Na+ insertion/deinsertion and greatly improves the layered structure stability.

Figure S6 shows the cyclic voltammograms (CV) curves of the layered NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02) samples at a scan rate of 0.1 mV s-1 in the voltage range of 1.5~4.2 V. The CV curves present an obvious oxidation peak at 3.5V along with a reduction hump at 2.7 V and a reduction peak at 3.7 V, which is possibly related to the redox couples of Ni2+/Ni4+.38 Compared with NaMn0.5Ni0.2Fe0.3O2, the NaMn0.48Ni0.2Fe0.3Mg0.02O2 exhibits weaker and broader CV curves, implying a temperate transformation between the lattice phases. Furthermore,the smooth curves indicate that doping Mg may suppress the multiphase transformation.37

In order to further investigate the reaction mechanism of Fe, Mn and Ni upon Na ions extraction/insertion, ex-situ X-ray absorption spectroscopy (XAS) analysis was performed on the cathode of NaMn0.48Ni0.2Fe0.3Mg0.02O2 at different states. Figure 6 shows the normalized X-ray absorption near-edge structure (XANES) spectra at Fe, Mn, Ni-K edges. The XANES spectra at the Ni K-edge display an obvious shift to the higher energy region as the voltage increases to 4.2V, indicating that the valence of Ni 13 ACS Paragon Plus Environment


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was increased from Ni2+ to Ni4+. And when the voltage decreases to 1.5 V, the valence of Ni reduces again to Ni2+.28, 30 The Fe K-edge shows a slight chemical shift at 4.2 V, which means that iron ions take part in the electrochemical oxidation by increasing the valence state during the charge process.47 In another aspect, no obvious shift of Mn K-edge can be observed. The K-edge curves of electrode correspond to those of MnO2, indicating that most of the Mn ions are electrochemically inactive as +4 valence during charge/discharge processes. The above results show that within the potential window of 1.5–4.2 V, the measured capacity for the electrodes mainly come from the Ni2+/Ni3+ and Fe3+/Fe4+ redox couples while Mn stabilizes the structure with unchanged oxidation state.48

Based on Randles– Sevcik equation, the Na-ion diffusion coefficients Dapp which is used to evaluate the kinetic performance of NaMn0.5-xNi0.2Fe0.3MgxO2 (x = 0, 0.02) cathodes is calculated by cyclic voltammogram (CV) technique. The CV analyses at different scanning rates are shown in Figure S7a and b, and the peak 1 was used for calculation. It is obvious that the current peak Ip is linear correlation with the square root of scanning rate ν1/2 (Figure S7c, d), indicating the diffusion-limited process.49 From the slope of dIp/dν1/2 we calculate the Dapp of both cathodes. With the doping of Mg, the Na-ion diffusion coefficient has increased from 1.63×10-12 cm2 s-1 (NaMn0.5Ni0.2Fe0.3O2) to 5.48×10-12 cm2 s-1 (NaMn0.48Ni0.2Fe0.3Mg0.02O2), which is in well agreement with the enlargement of the interlayer spacing and contributes to the high rate performance.


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To monitor and uncover the reaction mechanism and changes in the crystal structure of NaMn0.5Ni0.2Fe0.3O2 and NaMn0.48Ni0.2Fe0.3Mg0.02O2, ex-situ XRD patterns were collected at different charge and discharge states of the first cycle and the results are displayed in Figure 7. Figure 7 a and b show ex-situ XRD patterns of the first cycle of NaMn0.5Ni0.2Fe0.3O2 and NaMn0.48Ni0.2Fe0.3Mg0.02O2, respectively. The ex-situ XRD patterns illustrate that a major phase transformation happens during the desodiation process from O3 to P3 phase. The (003) peaks of the O3 phase transforms into a hexagonal P3 phase which appears at a lower angle. The changes in the crystal structure mainly caused by Na+ extraction/insertion which happens during charge and discharge reveal the coexistence of O3 and P3 phases, corresponding to 0.10–0.15 Na deintercalation. According to Bragg equation (2dsinθ=nλ), the left shift of (003) and (006) diffraction peaks indicates an enlarged interlayer spacing for the new P3 phase. As further extraction of Na ions, the (104) peak gradually disappears and the diffraction pattern can be indexed to a hexagonal P3 phase. And no appearance of new peaks indicates that the insertion/deinsertion reaction in this region happens in single-phase (P3) condition which is in accordance with the sloping changing curve.28 The material reverts to the O3 phase after being discharged to 2.0 V, indicating that the phase transformation of the O3 material is highly reversible during the electrochemical Na insertion/deinsertion reaction in the initial cycle. In addition, it should be noted that when the cathode is discharged to 3.88V a much larger fraction of O3-phase forms from P3-phase in Mg-doped sample than Mg-free sample. When the cathode is discharged to 3.2V, only a very small amount of P3-phase is left in 15 ACS Paragon Plus Environment


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Mg-doped sample. In contrast, there is still a large amount of P3-phase left in Mg-free sample when the cathode is discharged to 3.2V. The results indicate that Mg doping could facilitate the reversible phase transition, which is also in well agreement with the enlargement of the interlayer spacing and the Na+ diffusion coefficient. The cycling performance profile of NaMn0.48Ni0.2Fe0.3Mg0.02O2 (Figure 5) becomes much smoother in compared with that of Mn0.5Ni0.2Fe0.3O2 (Figure 5), despite the occurrence of the phase transformation. We can infer that the substitution of Mg2+ for Mn3+ causes the phase transformation in the NaMn0.48Ni0.2Fe0.3Mg0.02O2 to be less destructive and mitigates the irreversible phase transitions. The doping of Mg2+ for Mn makes the cathode material more stable by facilitating the reversible transition from the O3 to the P3 phase.

CONCLUSIONS

The effects of the substitution of Mg for Mn on the structure and electrochemical performances

of

O3-NaMn0.48Ni0.2Fe0.3Mg0.02O2

have

been

comprehensively

investigated. The reversible capacity, the cycling stability and the rate capability all have been largely enhanced by Mg substitution. The roles of Mg on the modulation of the crystal or electronic structure and the improvement of both the rate capability and the cycling performance have been unraveled. In addition to the well-known pinning effect of inactive element doping on the structure stability the substitution of Mg for Mn enlarges the interlayer spacing which improves the Na+ diffusion coefficient and contributes to the high rate capability. The lattice strain or the volume change and the 16 ACS Paragon Plus Environment

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irreversible phase transitions induced by sodiation/desodiation have been mitigated because of the facilitation of Na+ intercalation/deintercalation. In addition, the substitution of Mg for Mn shortens the bond length of TM-O and the thickness of TMO2 slabs which are favorable to the stabilization of the layered structure. The substitution of low valence Mg2+ for Mn3+ can also minimize Mn3+ and the related Jahn–Teller effect, which further mitigates the irreversible phase transitions and benefits the layered structure stability. This study gives some new insights into improving the structural stability and the electrochemical performances of O3-type layered oxide materials for Na-ion batteries.

Figure 1. (a) Powder X-ray diffraction patterns of NaMn0.5Ni0.2Fe0.3O2 and NaMn0.48Ni0.2Fe0.3Mg0.02O2. (b) Shift of the (003) peak. (c) Rietveld refinement results of XRD



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profile for NaMn0.5Ni0.2Fe0.3O2. (d) Rietveld refinement results of XRD profile for NaMn0.48Ni0.2Fe0.3Mg0.02O2.

Figure 2. SEM image of as-prepared (a) NaMn0.5Ni0.2Fe0.3O2 and (b) NaMn0.48Ni0.2Fe0.3Mg0.02O2. (c) EDS mapping images of NaMn0.48Ni0.2Fe0.3Mg0.02O2.


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Figure 3. XPS spectra and fitting results of NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02): (a) Mn4+, (b) Ni2+, and (c) Fe3+, (d) Mg2+.

Figure 4. Mn 2p3/2 spectra of as-prepared (a) NaMn0.5Ni0.2Fe0.3O2 and (b) NaMn0.48Ni0.2Fe0.3Mg0.02O2.



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Figure 5. The first three galvanostatic charge/discharge curves of as prepared cathodes in the voltage range of 1.5-4.2 V at 0.05C for (a) NaMn0.5Ni0.2Fe0.3O2 and (b) NaMn0.48Ni0.2Fe0.3Mg0.02O2. (c) Rate capabilities of NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02) at various current densities. (d) Cycling performance of NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02) at a current density of 0.1C in the voltage range of 1.5-4.2 V.


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Figure 6. Ex-situ XANES spectra at (a) Fe K-edge and (b) Mn K-edge (c) Ni K-edge of NaMn0.48Ni0.2Fe0.3Mg0.02O2 electrode collected at different charge/discharge states.



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Figure 7. Ex-situ XRD patterns of the first cycle of (a) NaMn0.5Ni0.2Fe0.3O2 and (b) NaMn0.48Ni0.2Fe0.3Mg0.02O2.


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Table 1. Crystallographic lattice parameters Refined by Rietveld method of thin slice electrodes.

Phase Symmetry Space group

NaMn0.5Ni0.2Fe0.3O2

NaMn0.48Ni0.2Fe0.3Mg0.02O2

α-NaFeO2

α-NaFeO2

Rhombohedral

Rhombohedral

R3m

R3m

a/Å

2.9588(3)

2.9370(2)

c/ Å

16.0274(3)

16.3597(3)

121.52(4)

122.21(3)

∆a (%)

0.87

0.0089

∆c (%)

-2.42

-0.50

∆V (%)

-0.73

-0.48

Rwp (%)

4.64

3.92

Rp (%)

6.32

5.17

3

V/Å

ASSOCIATED CONTENT

Supporting Information. Further details about structure and electrochemical performance of O3-NaMn0.5−xNi0.2Fe0.3MgxO2 (x = 0, 0.02). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author * E-mail: Xiangfeng Liu, [email protected]

* E-mail: Yongmei Hao, [email protected]

Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENT

This work was supported by National Natural Science Foundation of China (Grant 11575192),

and

the

211211KYSB20170060),

International the

Partnership

Scientific

Instrument

Program

(Grant

Developing

No. Project

(ZDKYYQ20170001) and “Hundred Talents Project ”of the Chinese Academy of Sciences.

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Table of Contents


New Insights into the Roles of Mg in Improving the Rate Capability and

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