vacancy Disordering of P2

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Modulating the Interlayer Spacing and Na+/ vacancy Disordering of P2-Na0.67MnO2 for Fast Diffusion and High Rate Sodium Storage Da Tie, Guofeng Gao, Fang Xia, Ruyun Yue, QIngjie Wang, Ruijuan Qi, Bo Wang, and Yufeng Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Applied Materials & Interfaces

Modulating the Interlayer Spacing and Na+/vacancy Disordering of P2Na0.67MnO2 for Fast Diffusion and High Rate Sodium Storage Da Tie1, Guofeng Gao1, Fang Xia1, Ruyun Yue1, Qingjie Wang3, Ruijuan Qi4, Bo Wang1, Yufeng Zhao1,2* 1State

Key Laboratory of Metastable Materials Science and Technology, Key Laboratory of Applied Chemistry, Yanshan University, Qinhuangdao 066004, China 2Institute

of sustainable Energy, Shanghai University, Shanghai 200444, China

3State

Key Laboratory of Advanced Chemical Power Sources, Guizhou Meiling Power Sources Co. Ltd. Zunyi, Guizhou 563003, China 4Key

Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University, Shanghai, 200062, China Corresponding Email: [email protected]

Abstract Modulating the interlayer spacing and Na+/vacancy disordering can significantly affect the electrochemical behavior of P2-type cathode materials. In this work, we prepare a series of P2-Na0.67MnO2 cathodes (Na0.67Ni0.2-xMn0.8MgxO2) with varied doping amount of Mg and Ni to realize the maximization of the interlayer spacing within the experimental range, as well as optimized the Na+/vacancy ordering. Consequently, the as-prepared Na0.67Ni0.1Mn0.8Mg0.1O2 illustrates an excellent rate performance of 193 mAh g-1 discharge capacity at 0.1C (1C=180 mA g-1), even at a high rate of 8C, the battery can also deliver a capacity of 70 mAh g-1. The kinetics analysis indicates the raising of Na+ mobility, which could due to the reduced Na+/vacancy ordering and the enhanced Na interlayer spacing. The co-doping of Ni and Mg also enhances the stability of the layered structure, leading to improved cycling performance of 74.7% capacity retention after 100 cycles. Keywords: sodium-ion battery, manganese-based cathode, Mg doping, high rate

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capability, Na+/vacancy disordering, Introduction

Lithium-ion batteries(LIBs) have attracted increasing interest in energy storage due to the high energy density, which are suffering insufficient reserves of lithium sources. The demand to develop new alternatives to substitute some of LIBs is imminent. Sodium-ion batteries (SIBs) represent a potential candidate addressing the issues of cost and sources.1-5 Among cathode materials for SIBs, the P2-manganese-based cathode material (Na0.67MnO2) demonstrates merely structural damage during charge and discharge, whereby the triangular prism facilitates the removal and embedding of Na+. However, these materials usually undergo Jahn-Teller distortion of Mn3+ and relatively low average voltage platform in their charge/discharge process, leading to poor electrochemical performance.6,7 Ni-doped Na0.67NixMn1-xO2 with high theoretical capacity has shown promise among these P2-manganese oxides, in which the presence of active Ni2+/Ni4+ redox couples can provide a high voltage of 3.5 V, and the existence of Ni2+ can overcome the Jahn-Teller distortion of Mn3+. Nevertheless, this kind of material usually suffers from poor rate performance due to the plateaux occurring during charging/discharging process due to the specific Na+/vacancy ordering patterns and the formation of the O2 phase.8 Rational structural design is believed can efficiently improve the rate performance of such materials. The main efforts have been devoted to electron-conductive materials coating,9,10 foreign ions doping11,12, constructing mixed phases material13-16 or other techniques. By doping with proper heteroatoms in TM layers could effectively regulate the space of Na-ion motion, thus facilitating the transportation of Na-ions during charge and discharge, and decrease the activation barrier of Na


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transportation.17,18 By cosubsitution of Ni and Mn with Cu and Ti in NaNi0.5Mn0.5O2, the interlayer spacing of Na interlayer can be decreased from 3.17 Å to 3.13 Å, which prove that the interlayer spacing can be controlled by doping.19 Otherwise, Na+/vacancy ordering is tightly connected with the diffusion of Na+ by affecting Na diffusion path, which often occurs while the Fermi levels (ca. redox potentials) between the different transition metal (TM) species are similar.20-22 The occurrence of phase change causes rearrangement of Na+/vacancy during charge and discharge, which is the reason of Na+/vacancy ordering and further hinders the transportation of Na+ in the Na interlayer, leading to poor rate performance. Chen et al.23 selected the TM ions with closely ionic radii and substantially different Fermi levels versus Na, here Cr3+ and Ti4+ in expectation to build the P2-layered oxide free of Na+/vacancy ordering, which results in a very high ionic conductivity. In these manners, by regulating the proportion of different TMs, one can change the Na interlayer spacing and Na+/vacancy ordering in the system to improve the rate performance of the material. Couple of researches have been reported in Ni, Mg co-doping P2-NaMnO2 and revealed the synergism of the heteroatoms.24–27 In this article, a more Mn-rich system has been chosen to prove the structural changes in the different proportion of Ni and Mg. Both bivalent transition metal elements are doped in Na0.67MnO2 to overcome the Jahn-Teller distortion of Mn3+, Ni is doped to raise up the voltage platform.28 In previous reports, Na0.67Ni1-xMnxO2 have two voltage plateaus of 3.3 and 3.7 V, which might lead to the rearrangement of different Na+/vacancy ordering patterns at particular Na stoichiometries. Considering the similar ionic radius, and substantial difference in the Fermi levels between Ni2+ and Mg2+, substituting Ni2+ with Mg2+ in TMO2 slabs can suppress the Na+/vacancy ordering of the system.29 The co-doping of



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Ni and Mg enlarge the Na interlayer spacing offers fast transfer dynamics for Na+ diffusion, obtaining a high electron and ion conductivity, the charge storage capability is also enhanced. The as-prepared Na0.67Ni0.1Mn0.8Mg0.1O2(NMM0.1) sample demonstrates a remarkably high specific discharge capacity of 193 mAh g-1 at 0.1C, 74.7% capacity retention after 100 cycles and excellent rate performance of 70 mAh g-1 at 8C. An improved diffusion coefficient (D) of co-doped Na0.67MnO2 compared to Ni or Mg-doped samples is indicated by kinetics analysis. Rietveld methods suggest the enlarged Na interlayer spacing and Na+/vacancy disordering of NMM0.1, which favors the excellent rate performance. Experimental Methods Material synthesis: A series of P2-type Na0.67Ni0.2-xMn0.8Mg0.1Ox(NMM) were prepared by a sol-gel method. 0.017 mol citric acid (≥99.5%, Macklin) was dissolved into

50ml

de-ionized

water,

then

stoichiometric

ratio

of

CH3COONa(>98.0%,Aldrich), Mn(CH3COO)2(98.0%, Aldrich), and Ni(CH3COO)2 (98.0%，Aldrich), Mg(CH3COO)2 (98.0%，Aldrich) with a total amount of 0.017 mol were added according to the proportion of different amount of substances. Having been stirring for 30 min, the water in the solution was evaporated at 80 °C, and then the precursors dried at 70 °C for 24 h. For the NMM, the dried mixture was calcined at 500 °C for 12 h and 900 °C for another 10 h in air. Finally, after natural cooling, the NMM samples were obtained. Materials characterization: The powder data were collected by the powder X-ray diffraction (PXRD, Rigaku P/max 2200VPC) using the Cu-Kα (λ1 = 0.15406 nm, λ2 = 0.154439 nm) radiation source in the range from 5° to 75°.SEM and TEM images in this work were collected by Zeiss SUPRA 55 and Hitachi HT-7700（120kV）. X-


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ray photoelectron spectroscopy (XPS) performed on a Thermo Fisher Multi-element (K-alpha) high-transmission spectrometer input lens, with energy ranging from 200 eV to 3 keV. Electrode preparation and electrochemical measurements: To perform the electrochemical properties of our materials, 2032 coin cells were used. A slurry composed of active material, acetylene black, poly(vinyl difluoride (PVDF) with a mass ratio of 8:1:1, dissolved in N-Methylpyrrolidone (NMP), which was then pasted onto aluminum to prepare the working cathode. The coin cells were assembled in an argon-filled glove box. Sodium foil was used as the counter electrode, and the electrolyte was 1 mol L−1 NaClO4 dissolved in EC/DMC(volume 1:1) with 2 vol% FEC as the electrolyte additive, and Celgard 2400 was employed as a separator. Before testing the electrochemical performance, the assembled cells were submerged for several hours. The cells were subjected to cycling in the voltage range of 1.5-4.4V (vs Na+/Na) at 25°C. For the EIS test, the Na+ diffusion coefficient (D) is evaluated based on the slope of the low-frequency region, and the calculation formula is:

D=

R2T2 2A2n4F4C2σ2

Here R is the gas constant (8.314 J K-1 mol-1), T is the absolute temperature (298 K), A is the surface area of the electrode, n is the number of electrons per molecule during oxidation, F is the Faraday constant (96485 C mol-1), C can be calculated by the density and the molecular weight, σ is Warburg factor associated with the line of Z’∼ω−1/2.



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For the GITT tests, the half-cells are charged at 0.05C for 30 min, followed by opencircuit relaxation for 2 hours. The Na+ diffusion coefficient (D) calculation formula is: 4 𝑚𝐵𝑉𝑚 2 𝑑𝐸𝑠 2

( )( )

D = 𝜋τ

𝑀𝐵 𝑆

𝑑𝐸τ

Here τ is the time for an applied galvanostatic current, MB and mB are the molecular weight and the mass of active material, respectively. Vm is the molar volume, S is the surface area of the electrode, ΔEs and ΔEτ are steady-state (equilibrium) voltage and the total change of the cell voltage E during the current pulse. Results and discussion In order to reveal the effect of different proportions of nickel-magnesium co-doping on material structure, a series of P2-type Na0.67Ni0.2-xMn0.8MgxO2 (NMMx, x corresponding to Mg content of 0, 0.05, 0.1, 0.15 and 0.2) have been synthesized through the adjusting of Mg and Ni contents. No peaks of impurities are detected from the XRD patterns, which indicates the Mg2+ and Ni2+ have entered into the crystal structure. P2-type NMM structure composes of oxygen layers stacked in an ABBA arrangement, in which Mg, Mn and Ni occupy the octahedral sites between the AB oxide layers and Na occupies the trigonal prismatic sites between AA and BB layers.30 The crystallographic parameters are presented in Figure 2(c). The Rietveld method is used to analyze and refine XRD data of NMM using the P63/mmc space group. (Figure 1(a)-(e)) The analogue refined parameters observed in all the NMM samples indicate the long-range structure is unchanged. The P2 phase NNM0.1 is fitted with lattice parameters and structural information of a=b=2.8863Å, c=11.3301Å, V=81.742Å3. All of the refined structural information of NMM is shown in Table S1S5. We observed that with the increase of Mg stoichiometries, the corresponding


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NMM lattice volume increase steadily, which may be due to that the ionic radius of Mg2+ (0.72Å) is larger than that of Ni2+ (0.69Å). However, with the increase of Mg content, the Na interlayer spacing of Na0.67Ni0.2-xMn0.8MgxO2 firstly increases and then decreases with corresponding values of 3.760Å, 3.817Å, 3.827Å, 3.789Å, 3.759Å, respectively, whereby NMM0.1 has the largest Na interlayer spacing as illustrated in Figure 1(f). Generally, the enlarged Na interlayer spacing can decrease the resistance of the Na+ diffusion, which may have a positive impact on improving the diffusion rate of Na+ in the bulk phase during the charging/discharging process.31 Mg substitution causes the contraction of TMO2 layers and enlargement of Na interlayer spacing associated with the increase in interaction between TM and O and reduction of the binding energy between Na and O (Table 1). Except for the interlayer spacing, the

phase changes during charge/discharge could

cause the rearrangement of Na+/vacancy, which results in Na+/vacancy ordering and further hinders the transportation of Na+ in the Na interlayer. This can also lead to poor rate performance. The Na+/vacancy ordering usually occurs when the Fermi levels of the different type TMs are similar. Thus, considering the same valence but apparent difference in the Fermi level between Ni2+and Mg2+, substituting Ni2+ with Mg2+ in TMO2 slabs can effectively suppress the disadvantages

such as TMs

ordering, charge ordering, and Na+/vacancy ordering. These kinds of ordering affect each other. For example, Na+/vacancy ordering and TMs ordering are coupled to each other. There is less possible Na+/vacancy ordering without TMs ordering and vice versa. To investigate whether the orderings are suppressed in the Ni, Mg co-doping system, the XRD data are also refined using the Rietveld method by the P63/mcm space group crystallographic model. The P63/mcm model considers a larger hexagonal unit cell than that of P63/mmc.32 In this model, we have considered that



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Ni2+ and Mg2+ are located at the same octahedral 2b sites due to their identical pristine charge and relatively similar sizes, whereas Mn4+ atoms are located in both 2a and 4d sites in the TMO2 layers. In contrast, in the P63/mmc space group, all the TMs are located at only one of the 2a positions, which indicates that the s space group P63/mcm can describe a more ordered structure and more disordered for P63/mmc. All refined structural information of NMM are shown in Figure S1 and Table S6-S10. Comparing with the refinement results of these two space groups, NMM0 and NMM0.2 can be more fitted to P63/mcm space group, NMM0.05 and NMM0.15 are similarly fitted to both two space groups, and NMM0.1 is more fitting P63/mmc, which illustrate that NMM0.1 is more TMs structural disordering, and Na+/vacancy ordering can be suppressed in NMM0.1. The morphological features and particle size of the NMM0.1 are investigated by SEM and TEM (Figure 2 (a-b)), and that of the other samples are shown in Figure S2. SEM micrographs showed irregular particles with hexagon bulk type ranging from 1-3 μm. The outline of bulk is clear, indicating that the sample prepared in the experiment has good crystallinity. From the TEM images, it can also be seen that the material is a bulk structure with sharp edges and corners, and the structure is consistent with the results obtained by SEM. The HRTEM images and selected area electron diffraction (SAED) patterns confirm the P2 phase within the as-prepared NMM which also shown in Figure S3. Lattice images of NMM0.1 in Figure 2(d) illustrate a d-spacing of 2.57Å, corresponding to [100] lattice plane of P2 phase material. The corresponding selected area electron diffraction (SAED) patterns illustrate the diffraction correspond to [100] and [101] zone axis, (Figure 2(d)). The EDS mappings (Figure 2(b)) illustrate that the Na, Ni, Mn, Mg and O are uniformly distributed in the


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bulk phase, indicating the Ni and Mg has participated in the crystallization throughout the component. To further explore the synergistic effect of nickel and magnesium in the material, Xray photoelectron spectroscopy (XPS) is used to investigate the oxidation states of all elements in NMM0.1 (Figure 3). It can be seen that the major peaks at 854.68 eV and 872.08 eV are attributed to the peak of the Ni 2p line, while the 859.89 eV and 848.64 eV is the satellite peak, which indicates that the dominant nickel ion state is +2.33 It can be seen that the characteristic Mg 1s of Mg2+ appears at 1302.96 eV, which indicates that the valence of magnesium ion in the material is +2. In addition, the presence of two peaks of Mn 2p3/2 and Mn 2p1/2 at 641.64 eV and 653.19 eV, where the main peak of 2p3/2 belongs to Mn at 641.64 eV, also indicates the Mn4+ in the material. The existence of the two main peaks indicates that the dominant valences of manganese in the material are +3 and +4 states.34 In general, the oxidation states of Mn-ion are mixed in the P2–NaxMnO2 material. The divalent TM2+ doping can effectively alleviate the Jahn–Teller effect with the Mn3+ ion by the possible replacement of two Mn3+ with a TM2+ and a Mn4+. However, previous studies have demonstrated that the presence of Mn4+ in the electrode material is a relatively more stable state which is difficult to be oxidized to a higher oxidation state, so Mn4+ ions generally do not undergo redox within the voltage range of 1.5-4.4V. The introduction of the Ni2+ element in the material can not only inhibit the Jahn-Teller disproportionation effect of manganese to a certain extent, but the Ni2+/Ni4+ redox can provide a certain contribution for the electrochemical performance of the material.35 However, with the addition of Ni2+, the P2-O2 phase change of the material is also a factor that cannot be ignored. The P2-O2 phase transition is due to the dramatic volumetric change of the material with phase transition to the O2 phase caused by the



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shift of oxygen atoms in the structure when the charge voltage of the material reaches 4.2 V during charge and discharge of the material.21,36 Bruce et al.23 have proved the Mg substitution suppress Na+/vacancy ordering effectively, which causes a new gradual solid solution phase change, the P2-O2 transition at voltage of 4.2V can be hindered by switching to an OP4 phase which is the most thermodynamically stable phase at high voltages in the Mg-substituted P2 phase materials. The electrochemical performance of NMM cathodes in SIBs are tested in the voltage range of 1.5–4.4 V. Figure 4(a) and Figure S5 demonstrate the CV curves of NNM materials tested at 0.1mV s-1. When the Mg doping amount x=0, the CV curve of NNM0 displays three couple of obvious redox peaks in the voltage range of 1.5-4.4 V. The redox peaks at 2V/2.6V and 3.6V/3.7V correspond to the Mn3+/Mn4+ and Ni2+/Ni4+redox reaction, respectively. The 3.9V/4.2V shows the P2-O2 phase change of the material.37,38 The latter two peaks both lead to Na+/vacancy ordering in sodium layers. With the addition of Mg2+ in the material, it can be seen in Figure 4(a) that when x = 0.1, the redox peak intensities of the material at 3.6/3.7 V, 3.9/4.2V are significantly reduced, and two voltage plateaus disappeared completely without decreasing the capacity, the Na+/vacancy ordering superstructure in the NMM0 is substituted by a solid-solution reaction, and only one broad redox peak is observed in the CV test of NMM0.1, demonstrating that when more Mg2+ mixed into the phase, the Na+/vacancy ordering is restrained effectively during the charging/discharging process. The extended solid-solution phase over a wider range of composition replace the original two phase reaction, which can lead to a faster ion diffusion in the Na interlayer, this phenomenon agrees well with the electrochemical observations of smoother charge/ discharge curves. With the continued raising in the amount of Mg in NMM, when x = 0.2, the material has only one redox peak at 2V/2.6V. The charge-


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discharge curve is smoother and the material discharge specific capacity is drastically reduced, which due to the electrochemical inert of Mg2+. The cycle performance of Na0.67Ni0.2-xMn0.8MgxO2 with different nickel and magnesium contents is shown in Figure 4(b) in the voltage range of 2–4.2V, and Figure S4 in the voltage range of 1.5–4.4V. The replacement of Ni2+ by Mg2+ favors the initial capacity retention on cycling. In the narrow voltage cycle test, with the content of Mg2+ increases, the discharge specific capacity of the materials is 125 mAh g-1 and 128.1 mAh g-1, 133.9 mAh g-1, 108.1 mAh g-1, 71.6 mAh g-1, respectively. After 100 cycles, the discharge specific capacity of the materials is 59.7 mAh g-1, 81.3 mAh g-1, and 100 mAh g-1, 86.1 mAh g-1, 56.1 mAh g-1, respectively. capacity retention rate is 49.3%, 63.5%, 74.7%, 79.6%, 78.3%, respectively. In the wider voltage cycle test, the initial discharge capacity of each material increases and the capacity retention rate decreases. capacity retention rate is 37.1%, 57.8%, 69.1%, 70.1%, 58.2%, respectively. Since Mg2+ and Ni2+ have the same valence state, Mg2+ will preferentially replace the Ni2+ in the material during doping, which will also reduce the electrochemical activity of the material. Although theoretical capacity loss occurs due to the introduction of inactive Mg, the existence of Mg enhances capacity retention. Earlier works confirmed when Mg is substituted in the material, a wider steady voltage range of P2 phase is obtained, otherwise, a new OP4 phase emerges, which delays the occurrence of oxygen layer glides and obtains a more stable structure during charging and discharging process.38 For NMM0.2, the cycle performance in wide voltage test is far worse than narrow voltage, which might due to the oxygen removal in the high voltage of the structure. Therefore, an appropriate amount of Mg2+ could be introduced in the experiment to improve the structural stability of the material during the charging/discharging process.



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The rate performances are tested in 1.5–4.4V. The initial charge capacity decreases from 215.8 mAh g-1 of NMM0 to 163.7 mAh g-1 of NMM0.2, which should be due to the replacement of Ni2+ with electrochemically inactive Mg2+. The NMM0.1 cathode displays discharge capacity of ca.193, 170, 146, 138, 124, 97, 70 mAh g-1, corresponding to 0.1C, 0.2C, 0.5C, 1C, 2C, 5C and 8C, respectively. The initial discharge capacity of NMM0.1 is 193 mAh g-1 in 0.1C, (Figure 4(b)) which is lower than that of NMM0, but the rate performance of NMM0.1 are largely improved comparing with almost other materials (Figure 4(c)). With the amount of Mg doping increases, the material's rate performance tends to increase firstly and then decrease, which consists with the change of Na interlayer spacing. The enlarged interlayer spacing of the Na+ diffusion path and the inhibition of Na+/vacancy ordering via the synergetic effect of Ni2+ and Mg2+ co-doping favor charge/discharge capability at high current density. Otherwise, NMM0.2 exhibits the lowest discharge capacities at all rates, which might due to more severe polarization phenomenon which observed in the CV test. Bruce et al.39 investigated the phase change of Mg-doped P2-NaxMnO2 compounds by in-situ XRD, demonstrating that more Cmcm phase occurred in a low current density, which is beneficial to obtain a high capacity. However, only a small amount of Cmcm phase can be obtained because of the lower utilization of electrode at a high rate resulting in a low capacity during discharge, which is also a potential cause of the poor rate performance of NMM0.2. Furthermore, single phase reaction caused by inhibiting Na+/vacancy ordering which illustrated in charge and discharge curves are related to high rate electrochemical performance. It has been reported that Na+/vacancy ordering occurs at particular Na stoichiometries in NaxMnO2 compounds, which is a significant obstacle to Na+ diffusion.40,41 The previous works confirmed that low concentrations of heteroatoms in the TMO2 slab, here Mg2+, can prevent the occurrence of Na+/vacancy ordering in NaxMnO2 compounds, which has been


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observed in some other works.7,42 The material's excellent rate performance and cycle performance are attributed to the co-doping of Mg2+ and Ni2+ in the crystal structure of the material to enhance the structural stability. It has been proved that Na+ diffusion in P-type NaxMnO2 is faster than in O-type compounds. Strong in-plane Na-Na interactions make Na-ion diffusion highly correlated.43 The apparent diffusion coefficients of Na+ of NMM samples are calculated through GITT test. (Figure S6) Figure 5(b) compares Na+ diffusion coefficient (D) of NMM samples. The Na+ mobility in NMM0.1 is increased with diffusion coefficients ranging from 10−11 to 10−10 cm2 s−1 in the Na range of 0.6 ≤ δ ≤ 0, whereas NMM0 and NMM0.2 showed low diffusion coefficients. Otherwise, the D vs δ curve of NMM0.1 demonstrates much fewer jumps as compared to other samples, which could be due to the less Na+/vacancy ordering in the NMM0.1 and consistent with the sloping characteristic of the charging-discharging curves.29 Figure 5(c) is electrochemical impedance spectroscopy (EIS) results of NMM at open circuit voltage (OCV), which provides another evidence to the enhancement of structural design for Na+ migration. Table S11 illustrates the D values calculated from EIS. The larger D value from EIS for NMM material confirms the hypothesis which Ni, Mg codoping has a positive promotion for the Na+ diffusion. Figure 5(c) and (d) illustrates that the diffusion rate of NMM0.1 cathode is faster than all other NMM cathodes. The spacing of the Na interlayer in NMM cathode can explain the difference which also confirmed by Rietveld methods, which is shown in Figure 5(a). As declared by Clement et al,44 TMs charge disordering in TMO2 slabs and Na+/vacancy disordering in the Na interlayers are fostered by Mg2+. At the end of charging, a P2’ phase with high conductivity usually appears in the Mg doping Na0.67MnO2. When the Mg doping amount is 0.1, the proportion of the P2’ phase with a lower Na content is expected to



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increase with increasing discharge rate. Therefore, the rational allocation of nickelmagnesium doping ratio offers promising structural advantages of fast diffusion kinetics. Conclusion In summary, we have successfully synthesized a novel Na0.67Ni0.1Mn0.8Mg0.1O2 as a positive electrode material of SIBs, which has an excellent high rate performance, high specific capacitance, and good cyclic stability. The enhanced electrochemical performance is due to the synergetic effect of Ni2+ and Mg2+. Especially, we explained the enhancement in rate performance is attributed to the enlarged Na interlayer spacing and diminishing of Na+/vacancy ordering. Our findings open strategies for designing high rate performance cathode materials for sodium-ion batteries. Supporting Information Supplementary figures of control samples, including SEM, TEM, HRTEM and SAED images, CV and charge/discharge curves, Refinement results. ACKNOWLEDGMENT We thank the financial supports from the National Natural Science Foundation of China (51774251), Hebei Natural Science Foundation for Distinguished Young Scholars (B2017203313), Hundred Excellent Innovative Talents Support Program in Hebei Province (SLRC2017057), opening project of state key laboratory of advanced chemical power sources ( SKL-ACPS-C-11). AUTHOR INFORMATION


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Figure 1. Rietveld refinements of (a) NMM0; (b) NMM0.05; (c) NMM0.1; and (d)NMM0.15 and (e) NMM0.2. (f) Interlayer spacing of the Na0.67Ni0.2-xMn0.8MgxO2 by Rietveld refinements as a function of the Mg content.


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Table 1. Atomic distances, interlayer spacing of the Na layer and for as-prepared materials. Samples

NMM0

NMM0.05

NMM0.1

NMM0.1

NMM0.2

5 TM–O (Å)

1.937

1.936

1.931

1.933

1.936

TM–TM (Å)

2.850

2.851

2.852

2.854

2.856

TMO2 (Å)

1.956

1.930

1.857

1.910

1.976

Interlayer

3.615

3.644

3.720

3.670

3.605

spacing (Å)



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Figure 2. (a) SEM images of the Na0.67Ni0.1Mn0.8Mg0.1O2 sample. (b) TEM images and corresponding EDS elemental mapping of Na, Mn, Ni, Mg and O of the of the Na0.67Ni0.1Mn0.8Mg0.1O2 sample. (c) HRTEM images of and the corresponding SAED of Na0.67Ni0.1Mn0.8Mg0.1O2 sample. (d) The P2-type crystal structure of Na0.67Ni0.2xMn0.8MgxO2

sample


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Figure 3. XPS analysis of the NMM0.1: (a) XPS spectrum of Mn 2p 1s; (b) O 1s; (c) Ni 2p; and (d) Mg 1s



Figure

4.

(a)

Cyclic

voltammograms

of

selected

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cyclic

sweeps

of

Na0.67Ni0.1Mn0.8Mg0.1O2, obtained at 0.1 mV s-1 within the potential range of 1.5–4.4V versus Na/Na+. (b) charge/discharge voltage profiles of Na0.67Ni0.1Mn0.8Mg0.1O2 material between 1.5 and 4.4 V at 0.1C. (c) The C-rate performance of the Na0.67Ni0.2xMn0.8MgxO2

material (1C corresponding to a specific current of 180mA g−1). (d) The


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discharge profiles in different rate of 0.1C, 0.2C, 0.5C,1C, 2C, 5C and 8C. (e) Specific discharge capacities versus cycle number for 100 cycles of Na0.67Ni0.2-xMn0.8MgxO2.

Figure 5. (a) The effect of different Mg contents on Na+ diffusion pathway. (b) Calculated Na chemical diffusion coefficients from GITT. (c) EIS for the materials at open circuit potential before charge and discharge tests. (d) Dependence of Zre on the reciprocal square root of the frequency (ω−1/2) in the low-frequency region for the materials.



TOC：


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vacancy Disordering of P2

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