P3 Na0.9Ni0.5Mn0.5O2 Cathode Material for Sodium Ion Batteries

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A P3 Na0.9Ni0.5Mn0.5O2 cathode material for sodium ion batteries Tim Risthaus, Lifang Chen, Jun Wang, Jinke Li, Dong Zhou, Li Zhang, De Ning, Xia Cao, Xin Zhang, Gerhard Schumacher, Martin Winter, Elie Paillard, and Jie Li Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b03270 • Publication Date (Web): 19 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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Chemistry of Materials

A P3 Na0.9Ni0.5Mn0.5O2 cathode material for sodium ion batteries Tim Risthaus,†,‡ Lifang Chen,§,‡ Jun Wang,*,†,‡ Jinke Li,∥ Dong Zhou,⊥ Li Zhang,⊥ De Ning,⊥ Xia Cao,† Xin Zhang,*,§ Gerhard Schumacher,⊥ Martin Winter,†,∥ Elie Paillard,∥ and Jie Li*,† †MEET

Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, Germany §State

Key Laboratory of Chemical Resource Engineering, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China ∥Helmholtz-Institute ⊥Helmholtz-Center

Münster (IEK 12), Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany

Berlin for Materials and Energy, Hahn-Meitner-Platz 1, 14109 Berlin, Germany

ABSTRACT: NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) layered oxides have been prepared and studied as cathode materials in sodium metal cells. The influence of sodium content on the structure and electrochemical performance of NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) have been investigated. When x is between 0.5 and 0.8, the materials crystallize in P2 phase. For x in the range of 0.9 to 1.2, novel P3-type materials have been obtained. Of great interest is the P3-type material with the specific composition Na0.9Ni0.5Mn0.5O2 since that it can deliver high discharge capacities (141 and 102 mAh g-1 at 10 and 100 mA g-1, respectively). Compared to P2 NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 0.8) materials, it exhibits much better cycling stability (78% capacity retention after 500 cycles in the voltage range of 1.5-4.5 V) and an initial Coulombic efficiency of ~ 100%, which is more desirable for practical use. In addition, ex situ X-ray absorption near edge spectra (XANES) reveal that the redox reaction of nickel ions predominantly contributes to the capacity. Operando X-ray diffraction (XRD) demonstrates reversible phase changes during the charge/discharge. Density functional theory (DFT) calculations indicate that P3 NaNi0.5Mn0.5O2 shows a low Na+diffusion barrier of 237 meV. This unexplored class of P3 cathode materials induces new perspectives for the development of layered cathode materials and more energy-dense sodium ion batteries.

1. Introduction As the applications for lithium ion batteries (LIBs) keep increasing, concerns regarding the availability of lithium and other metals that play an essential role in LIBs, such as Co and Cu, are growing. Especially for large-scale energy storage facilities, which will play an important role in fully exploiting fluctuating renewable energy sources and hence reducing the dependency on fossil fuels, it is desirable to find a low cost alternative to LIBs.1-3 Sodium ion batteries (SIBs) are considered a promising candidate as they offer cost advantages over LIBs due to the abundance of sodium and the possibility of replacing the copper current collector by much cheaper aluminum, since sodium does not alloy with aluminum.4-8 Unlike for LIBs, high capacity layered cathode materials for SIBs do not need Co for structural stabilization.9-20 However, it is questionable, whether simple replacement of Li by Na along with the exchange of current collector will bring cost savings per Watt-hour (Wh), due to the lower energy density of SIBs.4-7 Hence, enhancing the energy density, especially through finding suitable electrode materials is one of the key challenges to enable the commercialization of SIBs. Layered sodium transition metal oxides with the general chemical formula of NaTMO2, where TM denotes either a single transition metal (Mn, Ni, Fe, Ti, V etc.) or a mixture, are known to offer high capacities, energy densities and fast two-dimensional Na+-ions diffusion.9-20 Among the

vast variety of chemical compositions, the compounds employing Mn and Ni simultaneously are of particular interest as they combine the high working potential of Ni2+/Ni4+ redox couple and the environmental friendliness and low cost of Mn.11, 21 These materials are commonly found to adopt either a P2 structure or an O3 structure according to Delmas’ notation.9 Whether a prismatic (P) or an octahedral (O) coordination of Na+ ions by oxygen ions is obtained, depends primarily on the ratio of sodium to transition metal (Na/TM) and the synthesis condition.13, 17, 20, 22 Two well-studied compounds in this family are P2 Na0.67Ni0.33Mn0.67O211, 19, 23-26 and O3 NaNi0.5Mn0.5O2.21, 27-29 It is well known that P2 phase outperforms O3 phase in terms of achievable discharge capacity and rate capability, owing to a more open framework.30, 31 However, since that the cathode serves as Na source in the practical use, the utilization of P2-type materials is seen problematic due to their Na deficiency. O3-type NaNi0.5Mn0.5O2 material in turn is suitable for practical use, but exhibits slow kinetics for Na+-ions diffusion and unsatisfied reversibility above 4.0 V.29, 32 Thus, combining the advantages of the fast Na+ ions diffusion in the P2-type material with the appropriate ratio of Na/TM in the O3-type material shows particular interest. In this work, we report the synthesis and characterization of layered NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) cathode materials for SIBs. The materials consist of a P2

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phase when x is ≤ 0.8, and a novel P3 phase as x is ≥ 0.9. Especially, the as-obtained P3 Na0.9Ni0.5Mn0.5O2 material exhibits outstanding electrochemical performance, which overcomes the specific drawbacks associated with P2- and O3- type materials. Furthermore, the electrochemical performance of the as-obtained P3 Na0.9Ni0.5Mn0.5O2 material is investigated in different voltage ranges and with different electrolytes. Moreover, the reaction mechanisms are examined by ex situ XANES and operando XRD measurements. Additionally, first-principles calculation reveals the Na+-diffusion barrier of P3 NaNi0.5Mn0.5O2. 2. Experimental section Synthesis. NaxNi0.5Mn0.5O2 active materials were synthesized by a solid-state reaction using mixed transition metal carbonate precursors and sodium carbonate. Stoichiometric amounts of nickel sulphate and manganese sulphate were dissolved in distilled water. The (Ni0.5Mn0.5)CO3 precursor was then precipitated by addition of an aqueous solution of sodium carbonate (Na2CO3) and ammonium. During the precipitation, the temperature was fixed at 60 °C and the pH value was kept constant at 8. The dispersion was stirred overnight. After thoroughly washing the precursor with distilled water, the precursor was dried and intensely mixed with different amounts of Na2CO3, according to the desired sodium content in the final product. The mixed powder materials were calcined at 500 °C for 5 h and finally heated to 800 °C for 18 h to obtain the final products. Physical characterization. X-ray powder diffraction (XRD) was carried out on a Bruker D8 Advance (Germany) using Cu Kα radiation in the 2θ range from 10° to 70°. The particle morphology of the active materials was examined by scanning electron microscopy (SEM, Zeiss Auriga Crossbeam Workstation). High-resolution transmission electron microscopy (HRTEM) image was taken by a FEI Tecnai G2 F20 TEM at an accelerating voltage of 200 kV. Inductively coupled plasma optical emission spectrometry (ICP-OES) was applied on the spectro ARCOS instrument for element determination. Electrochemical measurements. In order to prepare electrodes, the obtained material together with conductive carbon (TIMCAL Super P) and polyvinylidene difluoride (PVdF, ARKEMA KYNAR®) binder (weight ratio 80:10:10) was mixed in 1-methyl-2-pyrrolidinone (NMP, ACROS Organics) and cast onto Al foil. After being dried, electrode tapes were punched into Ø12 mm discs, pressed by a hydraulic press, and dried at 120 °C under vacuum overnight. 2032-type coin cells were assembled in glove box using 1 M NaPF6 in 1:1 weight ratio of ethylene carbonate to dimethyl carbonate (EC:DMC) as electrolyte. The mass loading of the active material was approximately 3 mg cm-2. Metallic sodium foil served as the counter electrode. The cells were galvanostatically charged and discharged on Maccor series 4000 battery testers (USA) at different C rates (nominal current density 1 C = 100 mA g-1) between 1.5 and 4.5 V at 20 °C. X-ray absorption spectroscopy (XAS) measurements. XAS measurements of the electrodes at

pristine, charged (4.0 and 4.5 V) and discharged (1.5 V) states were performed at the KMC-2 beamline of the synchrotron BESSY at Helmholtz-Zentrum Berlin, Germany, using a graded Si–Ge (111) double-crystal monochromator. The electrodes were protected with adhesive Kapton tape. In order to eliminate the high order harmonics, about 65% of the maximum possible intensity of the beam is transmitted through the sample during the measurements. Pure Mn and Ni foil were measured simultaneously for energy calibration. Operando XRD measurement. To gain deeper insights in structural changes during sodiation and de-sodiation, operando XRD was performed. For this purpose, a thin beryllium plate, which is transparent for X ray, was used as current collector for the positive electrode material. To protect the current collector from side reactions with the electrolyte, Al was sputtered on the beryllium plate, forming a protective Al2O3 surface film. A highly concentrated slurry was coated onto the beryllium plate and the electrode was dried overnight. On the negative electrode side, sodium metal was employed. The cell was cycled using a low current (0.05 C; 1 C = 100 mAh g-1) and XRD scans in the 2θ range from 10° to 50° were continuously recorded every 45 minutes. Ex situ XRD measurement was performed on the cathode after three formation cycles at 0.1 C and charging to open circuit voltage (OCV, ~ 2.9 V). DFT calculation. All DFT calculations were performed with the Vienna Ab-initio Simulation Package (VASP) code.33 P3 structure was constructed according to its crystal data. The projector-augmented wave method and Perdew– Burke–Ernzerhof spin-polarized generalized-gradient approximation (GGA) functional were employed.34 Spinpolarized Hubbard U correction was added to address the self-interaction energy with the U value 4.9 eV for Mn and 6.4 eV for Ni.35-38 Na+-diffusion barrier was calculated by the climbing image nudged elastic band (CI-NEB) method.39 The plane-wave energy cut-off was set to 400 eV. 4×4×2 of the k-points was used in all the calculations and the Γ point was included. A 2×1×1 supercell was used to simulate Na+ migration process in the CI-NEB. According to the principle of maximum dispersion of Ni, we substituted a half of Mn for Ni (model 1). The convergence criteria were 10−5 eV for the electronic self-consistent iteration and 0.02 eV Å-1 for force. We also built different models, assuming three Ni in one column (model 2) or two Ni in one row (model 3). 3. Results and discussion The chemical composition of the Na0.9Ni0.5Mn0.5O2 was measured by ICP-OES, the result of which demonstrates that the experimental Na:Ni:Mn is 0.92:0.50:0.50, close to the nominal value of the designed sample. The crystallographic structure of the Na0.9Ni0.5Mn0.5O2 sample was analyzed by XRD and the corresponding diffraction pattern is shown in Figure 1a. P3 and O3 phases can be distinguished by the ratio of (104) to (015) in the peak intensity, though no difference can be viewed in the diffraction peak positions, since both crystallize in a


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Chemistry of Materials rhombohedral system. Several reports showed that for P3 phase, (015) peak is stronger than (104) peak, while the opposite phenomenon is observed for O3 phase.21, 32 In this case, the obtained XRD pattern shows much higher (015) peak than (104) peak, which indicates that the Na0.9Ni0.5Mn0.5O2 sample crystallizes in P3 structure. The Rietveld refinement was conducted using P3 structure in a trigonal lattice with R3m (No. 160) symmetry. The calculated pattern is in good agreement with the experimental data (Rwp = 7.1%). Trace amount of NiO (about 3.1 wt.%) as an impurity marked with asterisk in the pattern was included in the analysis. Hence, the results indicate that P3-type structure is appropriate for the asobtained Na0.9Ni0.5Mn0.5O2 material. Miller indices of the diffraction peaks are indicated in the pattern, and the parameters a and c are calculated to be 2.88 Å and 16.73 Å, respectively.

Figure 1. (a) XRD pattern and the Rietveld refinement profile, (b) SEM and (c) HRTEM images of the obtained P3Na0.9Ni0.5Mn0.5O2 material. The asterisk in the XRD pattern indicates NiO impurity. Inset: Schematic illustration of P3 crystal structure.

It should be noted that the typical conditions forming P3-type structure are low Na/TM ratio, low synthesis temperature, and the ratio of Mn to Ni > 1.0.40 In this work, the sodium content is relatively high as 0.9, and the ratio of Mn to Ni equals to 1.0. The obtained material still shows P3 structure, not O3 structure. It is worthy to mention that the formation of O3 or P3 phases correlates with the employed pH value in the co-precipitation process. If the pH value is too high, O3 phase is preferred, which is shown in Figure S1. In view of the literature on O3 materials prepared by co-precipitation method, the synthesis adopted hydroxide, with a high pH value.29, 41, 42 With the carbonate co-precipitation method,43, 44 the obtained (Ni0.5Mn0.5)CO3 precursor presents a hexagonal structure with a space group of R-3c, corresponding to MnCO3 (JCPDS No. 44-1472) or NiCO3 (JCPDS No. 12-0771), which have divalent transition metals in their formal charge, as shown in Figure S2. A schematic illustration of P3 structure is presented in the inset of Figure 1a. In this model, nickel and manganese ions are positioned in octahedral sites, while sodium ions are in prismatic sites. The morphology of the Na0.9Ni0.5Mn0.5O2 sample was characterized by SEM (Figure 1b) and HRTEM (Figure 1c). The sample is composed of mainly submicron particles. The HRTEM

image shows an interlayer spacing distance of 0.24 nm, which corresponds to (012) fringes of the P3 Na0.9Ni0.5Mn0.5O2 material.

Figure S3 shows the XRD patterns of the NaxNi0.5Mn0.5O2 samples with different Na contents (0.5 ≤ x ≤ 0.8 and 1.0 ≤ x ≤ 1.2). Depending on the Na content, two phases are identified: Starting with the lowest amount of Na, a hexagonal P2 phase (space group P63/mmc) along with a NiO rock salt impurity (space group R-3m) exists. By rising the sodium content, as mentioned above, a crossing point is reached at x = 0.9, where peaks of a rhombohedral P3 phase (space group R3m) show up. This goes along with a decrease in NiO impurity content, since that P3 phase can uptake more Ni than P2 phase. For Na content higher than 0.9 mol per formula unit, P3 phase is still predominant. However, unlike for lithiated compounds, it is of difficulty to prepare Na-rich layered oxides by direct solid-state synthesis with the excess amount of Na.45 When x = 1.1 or 1.2, NaTMOx and NiO impurities are observed, in addition to P3 structure.46 These impurities are expected to have a negative effect on the electrochemical performance. The morphology of the prepared NaxNi0.5Mn0.5O2 active materials (0.5 ≤ x ≤ 0.8 and 1.0 ≤ x ≤ 1.2) is shown in Figure S4. For the compounds having a P2-type structure, primary particles appear as well-defined thick platelets, decorated with a minor fraction of small and irregularly shaped particles, which are probably NiO impurity. These platelets are approximately 1 to 2 µm in length and around 0.5 µm in thickness. Primary particles of the compounds with P3-type structure are less clearly defined and strongly intergrown with each other. The average sizes are smaller compared to those of P2-type particles and decrease with Na content. Moreover, as mentioned above, the amount of NiO impurity is lower in P3 materials, on the surface of which the small particles are much less obvious. The electrochemical performance of the Na/Na0.9Ni0.5Mn0.5O2 cell was evaluated in coin cells in the voltage range of 1.5 and 4.5 V, as shown in Figure 2. The first charge/discharge profile at 0.1 C (1 C is defined as 100 mA g-1) is presented in Figure 2a. As can be seen, several plateaus are observed, which indicates the ongoing of rather complex electrochemical reactions. The initial charge and discharge capacities are 142 and 141 mAh g-1, respectively, resulting in a Coulombic efficiency of nearly 100%. Unlike Na-deficient P2 materials, the appropriate initial Coulombic efficiency of the Na/Na0.9Ni0.5Mn0.5O2 cell enables its promising practical use.47 The CV curves of the Na/Na0.9Ni0.5Mn0.5O2 cell from fourth to 10th cycles at a scan rate of 0.25 mV s-1 is shown in Figure 2b. Different sodium/vacancy ordering phenomena and phase transitions cause the splitting peaks in the curves.42 The peaks are reversible, but the intensity decreases as



cycling, which is likely due to the continuous side reactions between the electrode material and the electrolyte, resulting in the loss of activity.48 Despite of this, the Na/Na0.9Ni0.5Mn0.5O2 cell still exhibits very promising cycling performance at 1 C for 500 cycles, shown in Figure 2c. The cell delivers a discharge capacity of 102 mAh g-1 at 1C, which is 72% of the capacity at 0.1 C, indicating a good rate capability. And the capacity decreases to 79 mAh g-1 after 500 cycles, showing a good capacity retention of 78%. The Coulombic efficiency approaches to 100% during cycling, which is consistent with the good reversibility of the cell.

Na/NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) cells at 1 C in the voltage range of 1.5 – 4.5 V is presented in Figure S5b. The P3 Na0.9Ni0.5Mn0.5O2 cathode shows the best cycling stability (82% capacity retention) and the highest capacity value of 84 mAh g-1 after 400 cycles. Although it delivers lower capacities than the P2-type materials at the beginning, the higher capacities can be obtained only after several cycles. The fast capacity drop of the P2 cathodes, especially in the initial cycles, is probably due to some irreversible phase change occurring at high voltage of > 4.2 V.19

Figure 2. Electrochemical performance of the Na/Na0.9Ni0.5Mn0.5O2 cell with 1 M NaPF6 in EC:DMC (1:1 wt.%) as electrolyte and in the voltage range of 1.5 and 4.5 V. (a) Charge and discharge curves at 0.1 C. (b) CV plots at a scan rate of 0.25 mV s-1 from 4th to 100th cycles. (c) Cycling performance at 1 C for 500 cycles.

In comparison, the first charge and discharge capacities of the Na/NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) cells at 0.1 C between 1.5 and 4.5 V are given in Figure S5a. The P2 cathodes with 0.5 ≤ x ≤ 0.8 show initial Coulombic efficiencies higher than 1, indicating that more than stoichiometric Na+ ions are inserted in the discharge step than extracted in the previous charge step. This mismatch is attributed to the low cut-off voltage (< 2.0 V) and well known for Na-deficient cathode materials.23 The initial Coulombic efficiencies of the P2 materials depend slightly on the Na content, whereas, an initial Coulombic efficiency below 100% can be observed for the P3 cathodes of 0.9 ≤ x ≤ 1.2. The charge capacity continuously increases, while the discharge capacity decreases with the increase of Na content, which may arise from the impurity. The P3 Na0.9Ni0.5Mn0.5O2 cathode, which has the lowest amount of the impurity, delivers the highest discharge capacity among the materials when 0.9 ≤ x ≤ 1.2. To further display the advantage of the P3 Na0.9Ni0.5Mn0.5O2 cathode, the cycling performance of the

Figure 3. Normalized XANES spectra at (a) Ni and (b) Mn Kedge of the Na0.9Ni0.5Mn0.5O2 electrodes at different states.

To get detailed information about the redox reactions, XAS studies were conducted on the Na0.9Ni0.5Mn0.5O2 electrodes at different states (pristine, charged to 4.0 V, charged to 4.5 V and discharged to 1.5 V). Two main features are observed in Figures 3a and 3b from the X-ray near edge structure (XANES) at both Ni and Mn K-edge. The weak pre-edge peaks are at lower photon energies (the electric dipole-forbidden transition of 1s electrons to unoccupied 3d orbitals), and the prominent edge absorption peaks are at higher photon energies (the electron transition from 1s to 4p orbitals). The pre-edge and main edge of the pristine Na0.9Ni0.5Mn0.5O2 electrode are close to those of NiO and MnO2 reference compounds. Therefore, the oxidation states of Ni and Mn ions at pristine state are predominantly 2+ and 4+, respectively. As can be seen in Figure 3a, the Ni K-edge shifts to higher energy as the cell is charged to 4.0 V with an energy shift of ~ 1 eV,


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Chemistry of Materials and further proceeds to higher energy when the cell is charged to 4.5 V with another energy shift of ~ 1 eV. After discharged to 1.5 V, the Ni K-edge moves back to its original position without any changes, suggesting the highly reversible Ni2+/Ni4+ redox reaction in the first cycle. Unlike Ni ions, no clear changes can be observed at Mn K-edge in Figure 3b, manifesting that Mn ions are mainly inactive during the electrochemical process.

Figure 4. Operando XRD data recorded during galvanostatic cycling of the Na/Na0.9Ni0.5Mn0.5O2 cell at a rate of 0.05 C (left) along with illustration of the voltage profiles (right).

Studies on the structural changes of the P3 Na0.9Ni0.5Mn0.5O2 during cycling were carried out by ex situ and operando XRD measurements. As shown in Figure S6, the material still presents P3 structure at open circuit voltage of ~ 2.9 V after three cycles at 0.1 C, indicating that P3 structure is reversible upon cycling, which would be further examined by operando XRD studies. The waterfall diagram showing the evolution of XRD patterns upon charge/discharge (Scans 1-71) is displayed on the left side of Figure 4, and the corresponding voltage profile is depicted on the right side. The 2θ range is from 15° to 45°. Each point in this representation accounts for the mean voltage of a single XRD scan sweep. Figures S7–S14 further show the details by magnifying the XRD patterns. During the initial desodiation process (charged to ~ 3.80 V, Scans 1-8), as shown in Figures 4 and S7, all the characteristic peaks of P3 phase are still observed without forming new peaks, and the ratio of (104) to (015) is below 1, indicating that no phase change occurs during this process. As further charging the cell to ~ 4.15 V (Scans 9–16, Figures 4 and S8), the intensities of (101) and (104) peaks are increasing, while the intensities of (012) and (015) peaks are decreasing. The ratios of (101)/(012) and (104)/(015) evolve from < 1 to > 1, which are the strong indications of P3 to O3 phase transition.21 As the cell is charged from ~ 4.15 to ~ 4.25 V (Scans 17-24, Figures 4 and S9), O3 phase keeps unchanged since the position of all diffraction peaks and the ratios of (101)/(012) and (104)/(015) exhibit only negligible change. When the cell is charged above 4.25 V (Scans 25–36, Figures 4 and S10),

a new diffraction peak at 20.5° is observed, and the peak intensity is increasing. The new peak is attributed to Z phase, which is reported in recent reference.49 Meanwhile, the (003) and (104) peaks of O3 phase are decreasing, but do not completely disappear, demonstrates the coexistence of O3 and Z phases between 4.25 and 4.5 V. In the discharge process to ~ 2.40 V (Scans 37-43 and 44-52, Figures 4, S11 and S12), the phase changes from the mixture of O3 and Z phases back to P3 phase via O3 phase at ~ 3.60 V. When the cell is discharged to 1.5 V, as shown in Figures 4 and S13 (Scans from 53 to 64), P3 phase transforms into O3’ phase with the characteristic peak (003) moving from 16.3° to 17.0° (lattice parameter c is decreasing during the sodiation process), while the peaks (101) and (012) toward lower 2θ angles (lattice parameters a and b are increasing during the sodiation process), and the intensity ratio of (104)/(015) increasing to > 1. P3 phase can be recovered from O3’ phase when the cell is further charged to ~ 2.9 V (Scans 65-71, Figures 4 and S14). Overall, the electrochemical reactions during the first charge/discharge process are highly reversible, which could contribute to the impressive cycling stability.

Figure 5. (a) Cycling stabilities of the Na/Na0.9Ni0.5Mn0.5O2 cells in different voltage ranges, and charge/discharge curves in the voltage ranges of (b) 1.5 - 4.5 V, (c) 1.5 - 4.0 V and (d) 2.0 - 4.0 V from 4th to 50th cycles.

The cycling stabilities of the Na/Na0.9Ni0.5Mn0.5O2 cells were further studied in different voltage ranges (1.5 - 4.5 V, 1.5 - 4.0 V and 2.0 - 4.0 V), as shown in Figure 5. In 47 cycles between 1.5 and 4.5 V, the cell exhibits a capacity retention of 95%, with the initial capacity of 102 mAh g-1 and 97 mAh g-1 at 50th cycle. In the narrowed voltage ranges of 1.5 - 4.0 V and 2.0 -4.0 V, the capacity retentions are improved to 99% and 100%, respectively. However, the capacity values are lowered to 92 and 83 mAh g-1, respectively. Figures 5b - 5d show the corresponding charge/discharge curves of the Na/Na0.9Ni0.5Mn0.5O2 cells. Regarding the specific



energy, it can be seen that the mean voltage of the cell cycled in the voltage range of 1.5 - 4.5 V keeps decreasing during cycling, resulting in the specific energy values decrease from 290 to 263 mWh g-1. While the voltage curves of the cells cycled in the voltage ranges of 1.5 - 4.0 V and 2.0 - 4.0 V are stable against cycling, and the specific energy maintain around 260 and 250 mWh g-1, respectively. Nevertheless, unlike P2 Na0.67Ni0.33Mn0.67O2 cathodes, the P3 Na0.9Ni0.5Mn0.5O2 cathode delivers high capacity and relatively good cycling performance in the voltage range of 1.5 - 4.5 V. To understand the impact of different electrolyte systems on the performance of the cells, long cycling behaviors of the Na/Na0.9Ni0.5Mn0.5O2 cells based on three different electrolytes are presented in Figure S15. These electrolytes are 1M NaPF6 in 1:1 wt.% EC/DMC (E1), 1M NaPF6 in propylene carbonate (PC, E2) and 1M NaClO4 in 1:1 wt.% of diethylene carbonate (DEC)/PC (E3). The Na/Na0.9Ni0.5Mn0.5O2 cell using E2 as the electrolyte shows an initial discharge capacity of 116 mAh g-1 with capacity retention of only 21% after 500 cycles at 1 C; the cell employing E3 as the electrolyte exhibits an initial discharge capacity of 105 mAh g-1 with even worse capacity retention of 19%. The cell using E1 as the electrolyte delivers the best electrochemical performance. The deteriorated cycling performance of the cells using E2 or E3 electrolytes is because of the instabilities of PC solvent and NaClO4 salt at charging voltage higher than 4 V. The key, for improving the performance of this highly interesting class of P3 cathode materials for SIBs, and particularly their cycling stability, lies in the choice of stable electrolyte formulations.

of Mn atoms are substituted by Ni (model 1). The lattice parameters of P3 NaNi0.5Mn0.5O2 are a = b = 2.859 Å, and c = 15.968 Å. In the structure, one Na+ ion transits from Ni top site to nearby hollow site then to Mn top site (Figure 6b). From Figure 6c, P3 NaNi0.5Mn0.5O2 shows a low Na+-ion migration barrier of 237 meV, which is similar with the value of P3 Na0.6TiO2 material (220 meV).32 In addition, the Na+-ion diffusion barrier of P3 NaNi0.5Mn0.5O2 is calculated to be 191 meV if assuming three Ni in one column (model 2), as shown in Figure S16a. In the case of two Ni in one row (model 3), the Na+diffusion barrier of P3 structure is 197 meV (Figure S16b). These values are slightly lower than that of the patterning establishing in terms of the principle of maximum dispersion of Ni (model 1). Despite of different models, the P3 structure exhibits low Na+diffusion barrier. 4. Conclusions In summary, P2 (0.5 ≤ x ≤ 0.8) and P3 (0.9 ≤ x ≤ 1.2) NaxNi0.5Mn0.5O2 materials are prepared by a simple and easily up-scalable solid-state reaction method. The novel P3-type cathode material with the composition of Na0.9Ni0.5Mn0.5O2 is of great interest since it exhibits an appropriate initial Coulombic efficiency, high capacities and excellent cycling performance. It would address the Na-deficiency issue of P2 structure and the slow Na+ions kinetics of O3 structure. Unlike P2 sodiumdeficient cathodes, the P3 Na0.9Ni0.5Mn0.5O2 is more practical in the full-cell application. The charge/discharge capacity is attributed to Ni2+/Ni4+ redox reaction. P3 phase undergoes reversible phase changes during the charge/discharge process, which results in good cycling stability. The unexplored P3 cathodes may have the chance to be used in the energy storage devices when they are further improved.

ASSOCIATED CONTENT

Figure 6. (a) The optimized primitive cell, (b) the Na+diffusion path and (c) the Na+-diffusion barrier of P3 NaNi0.5Mn0.5O2.

To further explore the advantage of the P3 Na0.9Ni0.5Mn0.5O2 cathode material, the Na+-diffusion barrier of P3 NaNi0.5Mn0.5O2 was calculated using the DFT calculation by CI-NEB method. The optimized primitive structure is shown in Figure 6a, in which half

Supporting Information. XRD patterns of the materials prepared by co-precipitation methods at pH = 8.0 and > = 10.0; XRD pattern of the (Ni0.5Mn0.5)CO3 precursor; XRD patterns of the NaxNi0.5Mn0.5O2 (x = 0.5-0.8 and 1.0-1.2) materials; SEM pictures of the NaxNi0.5Mn0.5O2 (x = 0.5 - 0.8 and 1.0 - 1.2) materials; Initial charge/discharge capacities of the Na/NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) cells at 0.1 C in the voltage range of 1.5 - 4.5 V; Cycling performance of the Na/NaxNi0.5Mn0.5O2 (0.5 ≤ x ≤ 1.2) cells at 1 C for 400 cycles in the voltage range of 1.5 - 4.5 V; XRD pattern of the P3 cathode after three formation cycles and charging to open circuit voltage (OCV, ~ 2.9 V); Operando XRD analysis of the (de)sodiation mechanism of the Na0.9Ni0.5Mn0.5O2 cathode material; Cycling performance of the Na/Na0.9Ni0.5Mn0.5O2 cells with different electrolyte systems at 1 C for 500 cycles in the voltage range of 1.5 - 4.5 V; Na+-diffusion barriers of the P3 NaNi0.5Mn0.5O2 calculated by model 2 and model 3. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


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Chemistry of Materials Corresponding Author *Jun Wang. [email protected]; *Xin Zhang. [email protected]; *Jie Li. [email protected]. ‡Tim Risthaus, Lifang Chen and Jun Wang contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors kindly acknowledge the financial support of Federal Ministry of Education and Research (BMBF), Federal Ministry of Economic and Technology and Federal Ministry for the Environment (BMWi), Nature Conservation and Nuclear Safety (BMU) of Germany within the project KaLiPat (03EK3008), the Fundamental Research Funds for the Central Universities of China (XK1802-6 and 12060093063). Allocation of beam time at KMC-2 beamline, BESSY-II, Berlin, Germany, is gratefully acknowledged.

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Chemistry of Materials

A P3 Na0.9Ni0.5Mn0.5O2 cathode material exhibits a high discharge capacity of 141 mAh g-1 with an initial Coulombic efficiency of ~100% at 10 mA g-1, and good capacity retention of 78% after 500 cycles at 100 mA g-1, in the voltage range of 1.5 - 4.5 V.


P3 Na0.9Ni0.5Mn0.5O2 Cathode Material for Sodium Ion Batteries

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