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Insights into the Dual-electrode Characteristics of Layered Na0.5Ni0.25Mn0.75O2 Materials for Sodium-Ion Batteries Manikandan Palanisamy, Hyun Woo Kim, Seongwoo Heo, Eungje Lee, and Youngsik Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15355 • Publication Date (Web): 09 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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ACS Applied Materials & Interfaces

Insights into the Dual-electrode Characteristics of Layered Na0.5Ni0.25Mn0.75O2 Materials for SodiumIon Batteries Manikandan Palanisamy,† Hyun Woo Kim,† Seongwoo Heo,† Eungje Lee*,‡ and Youngsik Kim*,† †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea ‡

Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA

KEYWORDS: Mixed hydroxy-carbonate; Layered P2-type structure; Dual-electrode; Chargedischarge cycling; Symmetric sodium-ion batteries.

ABSTRACT: Sodium-ion batteries are now close to replacing lithium-ion batteries because they provide superior alternative energy storage solutions that are in great demand, particularly for large-scale applications. To that end, the present study is focused on the properties of a new type of dual-electrode material, Na0.5Ni0.25Mn0.75O2, synthesized using a mixed hydroxy-carbonate route. Cyclic voltammetry confirms that redox couples, at high and low voltage ranges, are facilitated by the unique features and properties of this dual-electrode, through sodium ion deintercalation/intercalation into the layered Na0.5Ni0.25Mn0.75O2 material. This material provides superior performance for Na-ion batteries, as evidenced by the fabricated sodium cell that

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yielded initial charge-discharge capacities of 125/218 mAh g–1 in the voltage range of 1.5 – 4.4 V at 0.5 C. At a low voltage range (1.5 – 2.6V), the anode cell delivered discharge-charge capacities of 100/99 mAh g–1 with 99% capacity retention, which corresponds to highly reversible redox reaction of the Mn4+/3+ reduction and the Mn3+/4+ oxidation observed at 1.85 and 2.06 V, respectively. The symmetric Na-ion cell, fabricated using Na0.5Ni0.25Mn0.75O2, yielded initial charge-discharge capacities of 196/187 µAh at 107 µA. These results encourage the further development of new types of futuristic sodium-ion-battery based energy storage systems.

1. INTRODUCTION Due to the seemingly limitless global supply of sodium, sodium-ion batteries (SIBs) are targeted to replace lithium-ion batteries (LIBs) for electrical energy storage, and this objective is becoming close to being realized. To that end, research and development into sodium ion intercalation materials began to be explored with preliminary studies in the 1980s when electrochemically active systems were developed that were capable of attaining highly reversible sodium ion intercalation/deintercalation suitable for SIBs.1-7 Furthermore, great effort has been devoted to developing superior electrodes lined with materials such as NaCoO2,8 NaxMnO2,9 NaNi0.5Mn0.5O2,10 NaCrO2,10 NaVO2,11 NaxFe1/2Mn1/2O2,12 Na[Ni1/3Fe1/3Mn1/3]O2,13 NaTi0.5Ni0.5O2,14 Na1.0Li0.2Ni0.25Mn0.75Oδ,15 NaxMnyNizFe0.1Mg0.1O2,16 as well as polyanionic systems like NaFePO4,17 Na3V2(PO4)3,18 NaVPO4F,19 Na2FePO4F,20,21 NaFeSO4F22 as cathodic materials for SIBs. Subsequently, attention turned to Mn-based cathodic materials (NaxMnO2) that have become prime systems because of advantages that include the use of inexpensive elements, coupled with high voltages, among others. 9


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AxMO2-layered structures (A = alkali, M = transition metal) involving sodium are found most commonly to be O3, P3, or P2-type (3, 2 indicates the repeating unit cell in the transitionmetal layer).23-28 This layered P2-type structure is considered to be superior for SIBs compared with the O3 or P3-type structures.23,28-30 In particular, sodium ion intercalation/deintercalation processes possibly occur in these layered materials, as reported by Berthelot et al.,8 based on two different sodium ions sites: (i) 2b sites that share two faces with MO6 octahedra, and (ii) 2d sites that share their edges with MO6 octahedra.23,28-30 For the layered P2-type phase, the condition that Na/M < 1 needs to be satisfied for sodium-ion-deficient materials, which form during high temperature calcination (> 850 °C).27,30,31 In order to improve intercalation/deintercalation stability during charge-discharge cycling in SIBs, layered P2-type structures have been further developed through the active introduction of additional transition metals into MO2 sheets.14,15,32 Significant reports on P2-type Na2/3Ni1/3Mn2/3O2 and Na2/3Co2/3Mn1/3O2 materials reveal improvements in AxMO2 for application to SIBs.33,34 Accordingly, AxMO2 formulas with intermixed transition metals have been reported to serve as anodic materials for SIBs.35,36 Hence, Na-ion full cells have been designed using similar anodic materials that have shown dual-electrode characteristics for symmetric Na-ion batteries (Na-ion full cells). By employing the same electrode, it is possible to impart enhanced safety during Na+ ion insertion into the anodic material over sodium plating, because it requires less voltage and involves a simplified, low-cost manufacturing process due to the absence of a second material coated on the Cu current collector (as anode) during large-scale production. For this reason, the search for materials exhibiting dual-electrode characteristics is warranted for the further development of symmetric Na-ion batteries. In the literature, Na-ion cathodes with dualelectrode characteristics are very few and limited to Na3V2(PO4)3,18,37 Na3Ti2(PO4)3,38 α-



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Na2Ni2Fe(PO4)3,39 Na2V6O16,40 and Na0.8Ni0.4Ti0.6O2.36 However, it is clear that existing symmetric Na-ion batteries are limited by their anode performance that hinders cycling because of severe capacity fading.39-41 Recently, pioneering work on symmetric Na-ion batteries using a P2-type Na0.66Ni0.17Co0.17Ti0.66O2 material has been reported.35 This material exhibited prolonged cycling stability and is a potential cathode and anode (dual-electrode) candidate for use in symmetric Na-ion batteries.35 Despite this, alternative, superior, dual-electrodes need to be developed for symmetric Na-ion batteries, in order to replace the P2-type Na0.66Ni0.17Co0.17Ti0.66O2 material, which is desirable due to the toxicity of Co, and the substantial cost associated with Ti for large-scale production. With these concerns in mind, the progress of symmetric Na-ion batteries has focused on the layered P2-type family using Ni and Mn species, because they are non-toxic and inexpensive, desirable properties for the large-scale production of dual-electrodes. In particular, the dissolution of manganese at high voltages (> 4 V) can be minimized by its substitution with active Ni2+ ions in the transition layer. Hence, Na0.5Ni0.25Mn0.75O2 (hereafter, NNMO) is a potential anodic material for use in symmetric Naion batteries. Indeed, the unique features and dual-electrode characteristics of the NNMO material are induced due to the presence of two redox species based on Ni2+ and Mn4+ ions, coupled with cathodic and anodic electrochemical processes, making this material as a superior candidate for application in symmetric Na-ion batteries. Na0.5Ni0.25Mn0.75O2 has been synthesized by the mixed hydroxy-carbonate (MHC) method using (Ni0.25Mn0.75)2(OH)2CO3 as precursor. The MHC solid-state reaction is carried out under homogeneous, wet-chemical co-precipitation conditions, by ensuring atomic-scale mixing of the Ni and Mn ion constituents to give highly dispersed electrode materials. The synthesized layered-P2-type Na0.5Ni0.25Mn0.75O2 material delivers high initial charge-discharge capacities of


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125/218 mAh g–1 at 0.5 C. Subsequently, the same layered-P2-type NNMO material serves as an anode, yielding discharge-charge capacities of 100/99 mAh g–1, with 99% capacity retention, ideal for the fabrication of symmetric Na-ion batteries, and exhibiting excellent electrochemical performance with prolonged cycling stability. 2. EXPERIMENTAL ASPECTS The dual-electrode material, NNMO, was prepared in two steps involving the synthesis of the MHC precursor followed by calcination with a stoichiometric amount of Na2CO3, to yield the oxidic product, as shown schematically in Figure 1a. A solution of Ni(NO3)2.6H2O and Mn(NO3)2.4H2O (Sigma-Aldrich) was slowly added a solution of NaOH (2 mol) and Na2CO3 (1 mol) at 40 °C as shown in Figure 1a. Filtration, washing (distilled water and ethanol), and drying at 60 °C (24 h), afforded the (Ni0.25Mn0.75)2(OH)2CO3 precursor. The (Ni0.25Mn0.75)2(OH)2CO3 precursor and Na2CO3 were thoroughly milled for 2 h (FRITSCH pulverisette) and the milled blend was calcined at 900 °C for 12 h to produce the oxidic Na0.5Ni0.25Mn0.75O2 phase. Upon calcination, the thermal decomposition of the milled blend of MHC and Na2CO3 can be expressed by Equation (1).

The NNMO material was analyzed for its physical and electro-chemical properties. The presence of Na, Ni, and Mn was determined by inductively coupled plasma–optical emission spectrometry (ICP–OES, VARIAN 700–ES). The crystalline phase was examined by powder Xray diffraction (XRD) using a Bruker D8 Advance X–ray diffractometer with a Cu Kα X–ray source, recorded over the 2θ range of 10 to 80°. The probable lattice parameters were calculation by Rietveld refinement (GSAS program) and the schematic of the layered P2-type structure was created using the Diamond program. Morphological investigations were carried out using field-



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emission scanning electron microscopy (FESEM, Hitachi, S-4800) with energy dispersive X-ray analysis (EDX elemental mapping) and high-resolution transmission electron microscopy (HRTEM, JEOL, JEM–2100F). The synthesized material was examined by X-ray photoelectron spectroscopy using a ThermoFisher Multi-element (K-alpha) high-transmission spectrometer input lens, with energy ranging from 200 eV to 3 keV. Electrochemical studies were conducted for Na half-cells such as Na vs. NNMO (CR2032, coin cell) with 1 M NaPF6 in 1:1 EC–PC electrolyte with a glass fiber separator (electrochemical workstations: Biologic Science Instruments VSP 300 and WonATech). The NNMO electrodes (4.1 mg cm-2) were fabricated under vacuum drying at 120 °C for 12 h, with a ratio of 80% active material, 10% Super-P carbon (TIMCAL) and 10% polyvinylidene fluoride (PVdF) in N-methyl-2-pyrrolidone (NMP), using an Al current collector (15-µm thick). The coin cells were assembled using Na metal foil and NNMO as the cathode. Impedance analyses (EIS) were carried out between 100 kHz and 5 mHz. Cyclic voltammetric studies were performed from 1.5 to 4.4 V vs. Na+/Na at 0.1 mV s–1. Galvanostatic charge–discharge cycling studies were performed between 1.5 – 4.4 V vs. Na+/Na at 0.5 C (cathode) and 0.1 C (anode). Notably, a symmetric Na-ion cell was fabricated using the same electrode as both cathode and anode (weight ratio of 1:1) and subjected to galvanostatic charge–discharge cycling studies. 3. RESULTS AND DISCUSSION Herein, the facile mixed hydroxy-carbonate synthesis method holds the key to achieving higher electrochemical performance of SIBs. Accordingly, a layered P2-type NNMO material was synthesized using the non-toxic (Ni0.25Mn0.75)2(OH)2CO3 precursor as shown in Figure 1a. This method leads to a highly dispersed electrode material, in a facile route over shorter milling


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times (~2 h) than the conventional solid-state route. The upcoming section discusses the features of this material, such as size tunability, homogeneity and performance.

Figure 1. Synthesis and structural details of NNMO: (a) schematic depicting the synthesis of the NNMO material using the mixed hydroxy-carbonate method; (b) powder X–ray diffraction pattern of NNMO obtained at 900 °C for 12 h, with Rietveld refinement analysis; and (c) schematic illustration of the layered structure depicting MO6 (octahedra) and Na layers. 3. 1. Structural characterization of Na0.5Ni0.25Mn0.75O2 material The composition of the synthesized NNMO, obtained at 900 °C for 12 h, was confirmed by ICP-OES, and closely matches the designed product, with relative metal ratios of 2.10:1.02:3.03 for Na, Ni, and Mn elements, respectively. The powder X-ray diffraction pattern of NNMO (900 °C for 12 h) is shown with Rietveld refinement analysis in Figure 1b. All diffraction peaks are well-indexed with a hexagonal layered structure under the P63/mmc space



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group (JCPDS# 00-054-0894).28-30,42,43 Rietveld refinement provides lattice parameters and unit a cell volume of a = 2.8780 Å, c = 11.2794 Å and V = 80.91 Å3 for the hexagonal structure.2830,42,43

The XRD pattern clearly shows high crystallinity devoid of an impurity phase(s). In

addition there are no diffraction peaks at 2θ = 38°, 46°, 53° and 58°, which indicates the absence of a P3-type phase in the synthesized material. Accordingly, the calculated patterns (red, Figure 1b) are in good agreement with those obtained experimentally, as indicated by black asterisks (Figure 1b) that correspond to the layered P2-type structure.28-30,42,43 Other refined structural parameters are summarized in Table S1 in the Supporting Information. The obtained goodnessof-fit and R-factor for synthesized NNMO (900 °C for 12 h) are χ2 = 6.5 and Rwp = 4.6% by Rietveld refinement analysis. In addition to this, the layered P2-type structure, with hexagonal symmetry, determined using the refined lattice parameters, is depicted schematically in Figure 1c. Clearly, the P2-type structure consists of layers of MO6 (M = Ni, Mn) octahedra and Na layers (Figure 1c). The morphology of NNMO (900 °C for 12 h) was investigated by FESEM and this material exhibited a flaky bundle morphology with fine spherical and flaky particles, as shown in Figures S1a-c.28,29 In addition, the presence of Na, Ni, Mn and O in NNMO are confirmed by energy dispersive X-ray analysis, as shown in Figure S1d. Significantly, the morphology of the cycled material has been investigated, the results of which are depicted in Figures S1e-f. From this analysis, it can be seen that the flaky bundle morphology of the layered NNMO material (collected from the cathode of the full-voltage range cell) is retained, but with significant differences in bundle/particle shape observed (Figure S1e-f). Furthermore, the corresponding FESEM elemental mapping confirms the presence of the same elements observed by ICP–OES, viz., Na, Ni, Mn and O, in the NNMO material, as shown in Figure S2. More importantly, the


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particle shapes and sizes of this NNMO material were determined by HRTEM, the results of which are depicted in Figures 2a-d. Figures 2a-b reveals that the hexagonal flaky bundle morphologies have flake sizes of ~25 nm and sharp edges. These HRTEM images corroborate the high crystallinity of the sample and are consistent with the sharp, intense peaks observed in the powder X-ray diffraction pattern of NNMO, as discussed previously. Furthermore, Figure 2c corroborates the high crystalline phase of NNMO by the presence of bright concentric hexagonal SAED spots, consistent with hexagonal symmetry.42,44 Finally, the high crystallinity of the NNMO material is confirmed by the presence of lattice fringes in its HRTEM image (Figure 2d).

Figure 2. Morphological investigation using HRTEM and SAED techniques for the NNMO material (900 °C, 12 h). (a) The flaky bundle morphology; (b) layer thickness of ~25 nm; (c) SAED pattern depicting bright concentric hexagonal spots for the layered structure; and (d) HRTEM image showing the lattice fringes, confirming the high crystallinity of the synthesized material. 9 Environment ACS Paragon Plus


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XPS studies can furnish information about the oxidation states of the metal species present in the sodium ion compounds of transition metal oxides.45-49 For this reason, XPS analyses were carried out to confirm the oxidation states of Mn and Ni in the synthesized NNMO. In this context, the XPS core level spectra of Na 1s, Ni 2p, Mn 2p and O 1s are provided in Figures 3a-d, with deconvolution (the best-fitted profile is indicated in red, and the peak deconvolution depicted by green shading), for the prepared NNMO sample. The binding energy of the Na 1s emission peak is positioned at 1070.68 eV and appears as a broad signal with the best fitted profile indicated in red, corresponding to the deconvoluted peak marked in green (Figure 3a).45 The Ni 2p XPS spectrum in Figure 3b shows two characteristic peaks, which can be assigned to the broad Ni 2p1/2 (871.68 eV) and Ni 2p3/2 (854.48 eV) peaks that are related to the Ni2+ oxidation state. The binding energy corresponding to the center of Ni 2p3/2 peak of NiTiO3 has been reported to be 855.8 eV.45-47 The binding energy values of the Mn 2p1/2 and Mn 2p3/2 peaks are 653.98 eV and 642.08 eV, respectively, and indicate the presence of a typical Mn4+ oxidation state, as depicted in Figure 3c.45,47-49 Moreover, the binding energy value of the O 1s component is 529.68 eV,46,49 originating from Na–O, Ni–O, and Mn–O in the synthesized NNMO material (Figure 3d). From these XPS analyses, the binding energies observed at 871.68/854.48 and 653.98/642.08 eV are due to the presence of Ni and Mn in their +2 and +4 states, respectively.


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Figure 3. XPS analyses of the NNMO material (900 °C, 12 h): (a) XPS spectrum of Na 1s; (b) Ni 2p; (c) Mn 2p; and (d) O 1s. The experiment results are indicated in blue, fitted peak areas in green and the best-fitted profiles in red. 3. 2. Insights into the dual-electrode characteristics of the layered Na0.5Ni0.25Mn0.75O2 material To scrutinize the dual-electrode characteristics of NNMO, the electrochemical performance of this material was characterized by electrochemical impedance measurements, cyclic voltammetry and galvanostatic charge-discharge experiments. Accordingly, the kinetic characteristics of sodium ion intercalation/deintercalation into the electrodes was investigated by EIS analysis of the fabricated Na half cells before (inset Figure 4a) and after charge-discharge cycling studies as shown in blue/green. The fitted circuit components are marked in red/brown in Figure 4a and depict the complex plane of the Nyquist plot over the frequency range: 100 kHz to 5 mHz. The EIS profiles before and after cycling exhibit the high-frequency semicircles 11 Environment ACS Paragon Plus


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accredited to complete interfacial resistance (Rct = ~ 70/620 Ω) of the system, as well as the lowfrequency sloped line related to the Warburg process (Zw), which describes the diffusion of Na+ ions between the active material and electrolyte (interface).50,51 Subsequently, the entire observed impedance profiles were reasonably fitted, with goodnesses-of-fit χ2 = 1.49 × 10–2 for before cycling (blue/red) and χ2 = 2.41 × 10–2 for after cycling (green/brown), using the complex equivalent circuits (R, C and Q) as listed in Table S2. In addition to this, it can be seen that the electrolyte offers solution resistance at the high frequency end, as shown in the inset of Figure 4a (RSB = before cycling and RSA = after cycling).50,51 In order to further probe sodium ion intercalation/deintercalation into NNMO, CV studies were investigated using Na cells (Na vs. NNMO) between 1.5 – 4.4 V at 0.1 mV s–1, for 5 cycles (CR-2032), as depicted in Figure 4b. From the cyclic voltammograms, redox couples are observed at 4.20/3.97 V, 3.72/3.51 V and 2.15/1.76 V (Figure 4b) and can be attributed to the P2/O2 phase transitions associated with nickel and manganese redox processes, as described in previous studies.28-30,52,53 Over the first cycle, the quasi-reversible redox peak for the phase transition (prismatic P2-type to octahedral O2-type coordination) is observed at 4.20/3.97 V, while the second redox peak (3.72/3.51 V) can be attributed to the Ni2+/4+ redox process. Furthermore, a third redox peak (2.15/1.76 V) that is related to Mn3+/4+ oxidation and Mn4+/3+ reduction during the cathodic scan (first cycle) is also observed.28-30,52,53 During subsequent cycling, reversible redox peaks at 2.15/1.94 V, 3.72/3.63 V and 4.20/3.97 V are observed for the corresponding manganese redox process (Mn3+/4+), the simultaneous dynamic nickel redox process (Ni2+/4+), and the P2/O2 phase transition, respectively. Hence, after a few cycles, the material exhibits highly reversible and stable redox peaks over a wide cathodic potential range.


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Curiously, the redox couples observed over high and low voltage ranges highlight the merits of the dual-electrode properties of this material.36 Accordingly, the redox peaks corresponding to nickel and manganese in the same material indicate that both redox processes proceed during the electrochemical reaction. In this context, the CV studies were extended to further investigate the dual-electrode characteristics by varying the potential range over 3.0 – 4.4 V for the cathode, and 1.5 – 2.5 V for the anode, at 0.1 mV s–1 for 5 cycles, as depicted in Figures 4c-d. It can be seen that the redox couple related to the cathode (P2/O2 phase transition, Ni2+/4+) and anode (Mn4+/3+) are obtained at almost identical potentials over the wide CV scan (Figure 4b) as indicated in Figures 4c-d. More importantly, it is pertinent to note that the cyclic voltammograms of the anode show high reversibility in comparison to those of the high-voltage cathode studies. Hence, unique, dual-electrode features are displayed by the NNMO material. Individual cathode (Figure 4c) and anode (Figure 4d) CV studies reveal the dual-electrode facets of NNMO, and provides encouragement for the development a new type of symmetric Na-ion battery using this material. From these CV results, galvanostatic charge-discharge measurements were carried out for fabricated Na cells (Na vs. NNMO), based on the full cathode voltage range of 1.5 – 4.4 V at 0.5 C, the high cathode voltage range of 3.0 – 4.4 V at 0.5 C, and the anode low voltage range of 1.5 – 2.5 V at 0.1 C (theoretical capacity, 1 C = 135 mAh g–1) in a CR-2032 coin-cell configuration, as shown in Figure 5. The charge-discharge cycling profiles of NNMO (Figure 5a) are composed of two regions. The first is a sloping profile region that is related to Mn3+/4+ and Ni2+/4+ redox processes (below 3.9 V), while the second is a flat charge-discharge region associated with phase transition (prismatic P2-type to octahedral O2-type coordination, above 3.9 V), during cycling.2830,52-54

The two regions are principally attributed to the overall capacity of NNMO during charge-



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discharge cycling. The sodium cells, viz., Na vs. NNMO, delivered initial charge-discharge capacities of 125/218 mAh g–1 in the voltage range of 1.5 to 4.4 V, even at 0.5 C (Figure 5a), which is higher than the previous report related to the same composition at the low current density of 0.1 C (120/210 mAh g–1)55 and all other Na-ion cathodes in the literature.28,29,53,56

Figure 4. Electrochemical studies of the NNMO material (900 °C for 12 h): (a) Impedance study of the sodium cell (Na vs. NNMO) measured before (inset) and after charge-discharge cycling studies, as indicated in blue/green, with the fitted equivalent circuits marked in red/brown; the frequency range from 100 kHz to 5 mHz. Cyclic voltammograms with different voltage ranges corresponding to (b) the cathode over the full voltage range of 1.5 – 4.4 V; (c) the cathode over the high voltage range of 3.0 – 4.4 V; and (d) the anode over the low voltage range of 1.5 – 2.5 V; experiments were conducted at 0.1 mV s–1 for 1 – 5 cycles using 1 M NaPF6 in 1:1 EC–PC as electrolyte.


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The observed capacity of the flat charge-discharge profile is superior than that in literature reports.28,29,52,53,56 The NNMO material exhibits a flat profile in the high voltage (4.1 V) range that accounts for the delivered capacity of ~60 mAh g–1, even at 0.5 C, which is significantly higher than other cathode materials reported in the literature over the flat profile region.8-22,28,29,53,56-58 From the CV studies, it is clear that the phase transition of this material at high voltage is more reversible for prismatic P2-type to octahedral O2-type coordination when compared with materials described in other reports.28,29,52,53,56 The NNMO material delivers charge-discharge capacities of 125/218 mAh g–1 and 175/170 mAh g–1 for the first and 25th cycles, respectively, with a capacity retention of 78% at 0.5 C, as depicted in Figure 5d (full cathode voltage range of 1.5 – 4.4 V at 0.5 C). Subsequently, sodium cells were tested at higher voltage ranges (3.0 – 4.4 V) which yielded initial charge-discharge capacities of 135/112 mAh g– 1

at 0.5 C (Figure 5b,d); this charge capacity is close to the theoretical capacity of 135 mAh g–1.

Higher charge-discharge capacities and flat profiles are obtained at ~4.1 V, even at 0.5 C, which is a significant signature of the layered NNMO structure when compared to that previously reported at 0.1 C.55 The cycled charge-discharge voltage curve of the cell is observed to be slightly different from the initial cell. This could be due to the morphology change of the NNMO sample during cycling, which is shown in Figure S1. Although, the fade in capacity observed for the cathode at high voltage is inferred to be due to the P2-type phase not completely retaining its state at 0.5 C, as indicated by the irreversible phase transitions in Figures 5a-b, cell resistance (quantified high resistance of R1, R2, R3 and R5 in Figure 4a and Table S2) is also observed to increase during cycling (full cathode high voltage range), as shown in Figure 5a.28-30,52-54



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The stability of the anode was confirmed through CV studies. The fabricated Na cells (made using same material) were subjected to galvanostatic discharge-charge measurements, over the voltage anode range of 1.5 to 2.5 V at 0.1 C, as depicted in Figure 5c. In this context, the anode studies reveal that the layered P2-type structure, composed of Ni and Mn species in an AxMO2 formula, exhibits highly stable charge-discharge curves. To attain maximum capacity retention, the anode cell was tested at the typical current density rate of 0.1 C and delivered discharge-charge capacities of 121/100 mAh g–1 for the first cycle (Figure 5c). In subsequent cycles the cell discharge-charge capacities were stable at 100/99 mAh g–1, with 99% capacity retention as revealed in Figure 5d (low voltage anode range of 1.5 – 2.5 V at 0.1 C). Further, the anode cycling studies of NNMO material is extended to 100 cycles as described in Figure S3 voltage vs capacity (a) and capacity vs cycle number performance (b) for fabricated coin type Na-ion half-cell (Na vs. NNMO) in between 1.5 – 2.5 V at 0.1 C (low voltage anode range). From this obtained results, the anode cycle stability and capacity retention of NNMO is superior when compared to materials described in previous reports.39-41 Therefore, the synthesized NNMO material, prepared via the MHC method59 using (Ni0.25Mn0.75)2(OH)2CO3 as precursor, exhibits the dual-electrode characteristics desired for the development of a new type of symmetric Na-ion battery.


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Figure 5. Galvanostatic charge-discharge studies of fabricated Na cells (Na vs. NNMO): (a) voltage vs. capacity profiles for the cathode over the full voltage range of 1.5 – 4.4 V at 0.5 C; (b) voltage vs. capacity profiles for the cathode over the high voltage range of 3.0 – 4.4 V at 0.5 C; (c) voltage vs. capacity profiles for the anode over the low voltage range of 1.5 – 2.5 V at 0.1 C; and (d) capacity vs. cycle number over 1.5 – 4.4 V at 0.5 C (full cathode voltage range), 3.0 – 4.4 V at 0.5 C (high cathode voltage range), and 1.5 – 2.5 V at 0.1 C (low voltage anode range), for 1 – 25 cycles using 1 M NaPF6 in 1:1 EC–PC as electrolyte.



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3. 3. Symmetric Na-ion batteries: Features and performance In view of the unique dual-electrode features of the NNMO material, coin-type symmetric Na-ion cells (NNMO vs. NNMO) were fabricated with the CR-2032 cell configuration, as shown schematically in Figure 6a. It is inferred that the stabilized high voltage cathode discharge capacity is 73 mAh g–1 at 25th cycle as shown in Figure 5b. Then, the anode half-cell is delivered the discharge capacity of 100 mAh g–1 as depicted in Figure 5c. Accordingly, the weight ratio of 1:1 for anode and cathode is used to fabricate the symmetric Naion batteries. After aging for 24 h, this symmetric Na-ion cell was tested through formation and cell-stabilization cycles, followed by further electrochemical cycling studies, as depicted in Figures 6b-c. This fabricated cell yielded initial charge-discharge capacities of 196/187 µAh (cell capacity) at a constant current density of 107 µA, as shown in Figures 6b-c. The output cell voltage of this cell was greater than 2 V as depicted in Figure 6b. Furthermore, the present study was extended to investigate cell performance and cycling stability of the symmetric Na-ion cell over the voltage range of 1.0 – 2.5 V, and up to 100 cycles (107 µA), the results of which are displayed in Figure 6c. In particular, this fabricated cell delivered charge-discharge capacities of 176/170 µAh at the 50th cycle, with a capacity retention of 91%. Furthermore, capacity fade is reduced over prolong cycling, attaining charge-discharge capacities of 171/165 µAh at the 100th cycle, with a retained coulombic efficiency of 96% in lined the capacity retention of 88 %. (Figure 6c) in comparison with other reported symmetric Na-ion batteries.18,35-41 From the obtained coulombic efficiency (96%) of symmetric Na-ion, it is credited that the NNMO material appropriated to dual-electrode characteristics with prolong cycling stability. In general, batteries are tested with cut-off voltage based on anode and cathode potential. Hence, the symmetric Naion cell was tested in between 1.0 to 2.5 V related to high voltage cathode range and low voltage


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anode range. This is first report for the fabrication of a symmetric Na-ion battery using this NNMO material that exhibits dual-electrode features. Hence, this study validates the development of new types of futuristic energy storage systems using layered P2-type NNMO materials.

Figure 6. Dual-electrode features of NNMO: (a) Schematic diagram of the proposed symmetric Na-ion cell (NNMO vs. NNMO); (b) voltage vs. capacity; and (c) capacity vs. cycle number performance of the fabricated symmetric Na-ion cell over the 1.0 – 2.5 V range, at a constant current density of 107 µA, for 100 cycles (electrolyte: 1 M NaPF6 in 1:1 EC–PC).



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4. CONCLUSIONS In summary, the Na0.5Ni0.25Mn0.75O2 (NNMO) material has been synthesized using a nontoxic, mixed hydroxy-carbonate, (Ni0.25Mn0.75)2(OH)2CO3, precursor. The obtained material has a hexagonal layered P2-type structure with hexagon flaky bundle morphology, as determined by powder X–ray diffraction, FESEM and HRTEM techniques. Fortuitously, the redox peaks obtained at high and low voltages are related to the unique dual-electrode properties of the NNMO material. The highly reversible redox peak at 1.85/2.06 V is confirmed to correspond to Mn4+/3+ reduction and of Mn3+/4+ oxidation processes, suggesting NNMO is good anode behavior for Na-ion batteries. The fabricated sodium cell delivers high initial charge-discharge capacities of 125/218 mAh g–1 over 1.5 – 4.4 V at 0.5 C. Furthermore, the anode cell (made using the same NNMO material) yielded discharge-charge capacities of 100/99 mAh g–1 with 99% capacity retention at a typical current density rate of 0.1 C over 1.5 – 2.5 V, and proved to be a stable anode candidate. With these favorable features, symmetric Na-ion cells were fabricated using NNMO, yielding initial charge-discharge capacities of 196/187 µAh at 107 µA that are sustained with minimal fading over prolong cycling (1 – 100 cycles). These results validate the development of new types of futuristic energy storage systems using sodium-ion technologies. ASSOCIATED CONTENT Supporting Information. Supporting information is available from the ACS Publications website. FESEM images, crystallographic atomic parameters, equivalent circuit components and Na-ion anode cell performance of NNMO material.


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AUTHOR INFORMATION Corresponding Author *E–mail: [email protected] (Y. Kim) *E–mail: [email protected] (E. Lee) †

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, South Korea ‡

Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA Author Contributions The manuscript was written with the contribution of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the 2017 Research Fund (1.170012.01) of UNIST (Ulsan National Institute of Science and Technology) and National Research Foundation of Korea (NRF2014R1A2A1A11052110). Argonne National Laboratory’s work was supported under U.S. Department of Energy, contract DE-AC02-06CH11357.

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Table of Contents graphic Graphical abstract The present study is focused on new type of dual electrode characteristics of Na0.5Ni0.25Mn0.75O2 material and facilitating to Na+ ion (de)intercalation for symmetric Na-ion batteries. Accordingly, the fabricated symmetric Na-ion cell yielded an initial charge-discharge capacities of 196/187 µAh at 107 µA with prolong cycling stability.

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