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Structural characterization of layered Na0.5Co0.5Mn0.5O2 material as a promising cathode for sodium-ion batteries
Journal of Power Sources 363 (2017) 442e449
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Structural characterization of layered Na0.5Co0.5Mn0.5O2 material as a promising cathode for sodium-ion batteries Palanisamy Manikandan a, Seongwoo Heo a, Hyun Woo Kim a, Hu Young Jeong b, Eungje Lee c, Youngsik Kim a, d, * a
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA d Energy Materials and Devices Lab, 4TOONE Corporation, UNIST-gil 50, Ulsan 44919, Republic of Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Layered Na0.5Co0.5Mn0.5O2 material synthesized by mixed hydroxycarbonate route. HAAD-STEM and ABF-STEM images revealed the local structure. Sodium cell delivered an initial charge-discharge capacity 53/ 144 mAh g1 at 0.5 C. Stabilized charge-discharge capacity 118/118 mAh g1 obtained at 3 C. Na-ion full-cell fabricated and enabled to attain high voltage region at 0.212 mA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2017 Received in revised form 27 July 2017 Accepted 29 July 2017
Layered Na0.5Co0.5Mn0.5O2 material is synthesized through a facile mixed hydroxy-carbonate route using (Co0.5Mn0.5)2(OH)2CO3 precursor and well characterized as a hexagonal layered structure under P63/mmc space group. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) are calculated by Rietveld refinement analysis. A flaky-bundle morphology is obtained to the layered Na0.5Co0.5Mn0.5O2 material with the hexagonal flake size ~30 nm. Advanced transmission electron microscopic images are revealed the local structure of the layered Na0.5Co0.5Mn0.5O2 material with contrasting bright dots and faint dark dots corresponding to the Co/Mn and Na atoms. Two oxidation and reduction peaks are occurred in a cyclic voltammetric analysis corresponding to Co3þ/Co4þ and Mn3þ/ Mn4þ redox processes. These reversible processes are attributed to the intercalation/de-intercalation of Naþ ions into the host structure of layered Na0.5Co0.5Mn0.5O2 material. Accordingly, the sodium cell is delivered the initial charge-discharge capacity 53/144 mAh g1 at 0.5 C, which cycling studies are extended to rate capability test at 1 C, 3 C and 5C. Eventually, the Na-ion full-cell is yielded cathode charge-discharge capacity 55/52 mAh g1 at 0.212 mA and exhibited as a high voltage cathode for Na-ion batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: Mixed hydroxy-carbonate Layered structure Charge-discharge cycling Cathode performance Sodium-ion battery
* Corresponding author. School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.jpowsour.2017.07.116 0378-7753/© 2017 Elsevier B.V. All rights reserved.
1. Introduction Sustainable energy sources are in high demand to overcome the
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fossil fuel crisis and for use in such as portable devices, electric vehicle and grid storage applications. In recent years, Na-ion batteries (SIBs) have emerged as potentially superior storage devices of energy from renewable sources because the vast natural resources of sodium worldwide are sufficient for large-scale production of Na-ion materials [1e6]. For this attention, SIBs are mainly focused on hybrid electric vehicles and smart grid applications [7]. Hence, the energy density and power density have been associated with the intercalation/de-intercalation performance of the cathode materials [8e10]. This performance is a prime factor for the development of renewable energy sources, now being directed towards a green environment with wireless communication, electric powered transportation and static grid energy storage [8e10]. For this reason, researchers have begun searching for suitable Naþ ion compounds and performing structural studies to determine, which sodium materials are best for storage of electrical energy [11e13]. Accordingly, it is challenging and highly motivating to investigate advanced cathode materials for SIBs with superior rate performance. In the 1980s, preliminary sodium studies examined the intercalation/de-intercalation electrochemical reactions [14e20]. Subsequently, the development of Na-ion cathode materials was intensively explored; in particular, these included layered structures with the general formula of AxMO2 (A e alkali, M transition metal) such as NaxCoO2 [21], NaxMnO2 [22], NaCrO2 [23] and NaVO2 [24]. These early choices exhibited multiple phase transition during Naþ ion intercalation/de-intercalation, and offered limited capacity with poor rate capability. More recently, the investigation of Na-ion cathode materials has shifted to materials with multiple transition metals such as NaxFe1/2Mn1/2O2 [25], Na(Ni1/3Fe1/3Mn1/3) O2 [26], NaTi0.5Ni0.5O2 [27], NaxMnyNizFe0.1Mg0.1O2 [28], Na0.67Ni0.23Mg0.1Mn0.67O2 [29], Na0.66Ni0.33exZnxMn0.67O2 [30], Na0.8Ni0.4Ti0.6O2 [31], Na0.66Ni0.17Co0.17Ti0.66O2 [32], Na3V2(PO4)3 [33,34], Na3V2(PO4)2F3 [35], NaVPO4F [36] and Na2FePO4F [37] to attain higher rate capability and good cycle performance. Nevertheless, the cathode materials listed are still limited in terms of rate capability, cell voltage and full-cell performance. Therefore, research is still needed to provide better cathodes for high voltage SIBs. Indeed, introducing Co3þ ions instead of Mn4þ into the NaxMnO2 structure, accelerated progress toward high voltage and excellent specific capacity. This was achieved by minimizing Jahn-Teller distortion and resisting manganese-ion dissolution by the presence of high voltage Co3þ ion species [38,39]. In view of this, the studies of Na0.67Mn0.65Ni0.2Co0.15O2 [38] and NaxNi0.22Co0.11Mn0.66O2 [39] cathode materials showed remarkable cycling potential for SIBs. Furthermore, research on the NaxCo0.7Mn0.3O2 cathode material indicated not only superior performance, but also less charge-discharge capacity (95 mAh g1 at 1 C) in cycling studies [40]. In terms of specific capacity, the investigation of Na0.66Co0.5Mn0.5O2 material delivered specific capacity of 156 mAh g1 at 1 C [41], which is higher than that of NaxCo0.7Mn0.3O2 cathode material [40]. Unfortunately, the cycling performance of the Na0.66Co0.5Mn0.5O2 material showed capacity fading during cycling, which was attributed to manganese dissolution followed by instability of the structure during cycling. It is also pertinent to question whether the sodium-deficient, layered NaxCo1/3Ni1/3Mn1/ 3O2 [42] and NaxNi0.5Mn0.5O2 [43,44] materials have revealed multi-phase formation and structural stability, based on the sodium content in the AxMO2. Such materials exhibit poor rate capability and cycling performance for SIBs. Scrutinizing these results, the key point appears to be need to fix the stoichiometric sodium content (x ¼ 0.5) in the NaxCoyMnzO2 composition. This should play a vital role in improving the structural stability during high-voltage charge-discharge cycling of SIBs.
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In this work, the stoichiometric layered Na0.5Co0.5Mn0.5O2 material was prepared by a mixed hydroxy-carbonate method using (Co0.5Mn0.5)2(OH)2CO3 precursor. The obtained layered Na0.5Co0.5Mn0.5O2 material is well characterized and facilitates intercalation/de-intercalation, providing the characteristics of a promising cathode for SIBs. The synthesized material yielded charge-discharge capacity of 53/144 mAh g1 at 0.5 C for SIBs. Ultimately, the study of cell performance was extended to full-cell fabrication using conventional a hard carbon (HC) anode (i.e., HC vs Na0.5Co0.5Mn0.5O2) and yielded charge-discharge capacity 55/ 52 mAh g1 at 0.212 mA during 1st cycle of high voltage SIBs.
2. Experimental aspects Layered Na0.5Co0.5Mn0.5O2 material was prepared by mixed hydroxy-carbonate (MHC) method, which MHC route described in our earlier report [45]. In this synthesis, a nitrate solution of Co(NO3)2.6H2O and Mn(NO3)2.4H2O (Sigma-Aldrich) were mixed together and added to a solution containing 2 mol NaOH and 1 mol Na2CO3 at 40 C with vigorous stirring. The obtained precipitate of (Co0.5Mn0.5)2(OH)2CO3 precursor was washed with distilled water and ethanol, followed by drying at 60 C for 24 h. Finally, the (Co0.5Mn0.5)2(OH)2CO3 precursor was homogenized with Na2CO3 powder for 2 h at 400 rpm using a FRITSCH pulverisette. The homogenized powder was calcined at 900 C for 12 h and yielded the layered Na0.5Co0.5Mn0.5O2 material as a final product. During calcination, the thermal decomposition of the homogenized powder of Na2CO3 and (Co0.5Mn0.5)2(OH)2CO3 precursor can be explained in relation to the formation of layered Na0.5Co0.5Mn0.5O2 product by Equation (1).
(1) Next, the material composition of the newly synthesized, layered Na0.5Co0.5Mn0.5O2 material was examined using inductively coupled plasmaeoptical emission spectrometry (ICPeOES, VARIAN 700eES) for the presence of elements Na, Co and Mn based on the stoichiometric proportion. The powder X-ray diffraction pattern (XRD) was recorded using a Bruker D8 Advance Xeray diffractometer with a Cu Ka Xeray source within the 2W range from 10 to 80 , to examine the crystal structure of the synthesized material. The lattice parameters were obtained by Rietveld refinement analysis with best-fitted goodness of fit using the GSAS program and the crystallographic structure was schematized using the Diamond program. The morphology of the flaky-bundles was investigated by high-resolution transmission electron microscopy (HRTEM, JEOL, JEMe2100F). High-angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) and annular bright field-scanning transmission electron microscopy (ABF-STEM) images were recorded using an advanced transmission electron microscope (Titan microscope, Titan G2(60e300)) with energy dispersive X-ray analysis (EDX, elemental mapping). X-ray photoelectron spectroscopy (XPS) was carried out to determine the oxidation states of the transition metals in the synthesized Na0.5Co0.5Mn0.5O2 material using a ThermoFisher Multi-element (K-alpha) high-transmission spectrometer (energy range of input lens 200 eV to 3 keV). The electrochemical characterization of the layered Na0.5Co0.5Mn0.5O2 material was provided by CR-2032-type half and full-cells for SIBs using VMP3 Biologic and WonATech. For the fabrication of cathodes, the slurry contained 80% active material, 10% SP-carbon (Timcal) and 10% poly(vinylidene fluoride) in N-
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methyl-2-pyrrolidone. The slurry was coated on Al current collector (15 mm thick) and vacuum dried at 120 C for 12 h. The cathode was roll pressed and punched as disc shape with average diameter of 14 mm and active material loading of ~3.2 mg. Metallic sodium was used as anode for the fabrication of sodium half-cells. The test cells were assembled in an argon-filled glove box (KIYON). The electrolyte for the SIBs was 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive and glass fiber was used a separator. Cyclic voltammetry analysis were carried out from 1.5 to 4.4 V vs Naþ/Na at 0.5 mV s1 with metallic sodium as a reference electrode for the SIBs. Galvanostatic chargeedischarge cycling studies were performed between 1.5 and 4.4 V vs Naþ/Na at different rates 0.5, 1, 3 and 5 C up to 50 cycles. Furthermore, the Na-ion full-cells were fabricated using HC (80% active material, 10% SP-carbon (Timcal) and 10% PVdF in N-methyl-2-pyrrolidone) coated on Cu current collector (9 m) and cycled between 2 and 4.4 V at 0.212 mA for SIBs. 3. Results and discussion In this study, layered Na0.5Co0.5Mn0.5O2 material was obtained through a facile MHC route involving mixed hydroxy-carbonate of (Co0.5Mn0.5)2(OH)2CO3 precursor. It is pertinent to note that this method results in atomic scale-mixing of Co and Mn ion constituents and facilitates shorter milling time (~2 h) than the conventional solid state method. As a result, this route enabled synthesis of energy transfer materials with superior electrochemical performance for use in SIBs, as described in the following electrochemical section. 3.1. Structural investigation of Na0.5Co0.5Mn0.5O2 material Using ICP-OES analysis, the design ratio of Na, Co and Mn constituents in the elemental composition of the as-prepared Na0.5Co0.5Mn0.5O2 material was confirmed. The powder X-ray diffraction pattern and Rietveld refinement analysis of layered Na0.5Co0.5Mn0.5O2 material (900 C for 12 h) are shown in Fig. 1a, and the corresponding schematized layered structure is described in Fig. 1b. The XRD pattern clearly shows that greater crystallinity and all the diffraction planes can be well indexed with P2-type phase (JCPDS# 00-054-0894), and that the hexagonal layered structure belongs to the P63/mmc space group [40e42,46e48]. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) were calculated by Rietveld refinement analysis and the listed its crystallographic atomic parameters are listed in Table S1. The calculated pattern (red) had the best goodness of fit with the experimental data and the lattice parameters showed good agreement with the reported literature on the P2-type phase corresponding to a hexagonal layered structure [39e42,46e49]. From the Rietveld refinement analysis, the goodness of fit (c2) and R-factor (Rwp) were obtained and found to be c2 ¼ 1 and Rwp ¼ 2.7% for the layered Na0.5Co0.5Mn0.5O2 material (prepared at 900 C for 12 h via the MHC route). The goodness of fit c2 ¼ 1 exhibited the best fitted profile for the powder pattern of layered Na0.5Co0.5Mn0.5O2 material. The layered structure was schematized using the obtained crystallographic parameters, as shown in Fig. 1b. Each structure comprised two: an octahedral of Co/MnO6 and a trigonal prism of Na (Fig. 1b) [41,46e48]. The obtained layered Na0.5Co0.5Mn0.5O2 material was subjected to morphological investigation using the HRTEM technique. From this, it can be seen that the flaky-bundle morphology presented with a flake size of ~30 nm, as shown in Fig. 2a and b [40,46,47]. The hexagonal shape is present in the flaky-bundle morphology, as shown in Fig. S1 [40,46,47]. Also, the detailed local structure of the layered Na0.5Co0.5Mn0.5O2 material was investigated using highangle annular dark field (HAADF) and annular bright field (ABF)
Fig. 1. (a) Powder X-ray diffraction pattern with Rietveld refinement analysis of layered Na0.5Co0.5Mn0.5O2 material (900 C for 12 h) and (b) schematic layered structure with octahedral of Co/MnO6 and trigonal prism of Na, using crystallographic parameters.
scanning transmission electron microscopy (STEM) techniques (Fig. 2cel) [31,32,50,51]. The HAADF-STEM images revealed flakybundle particles (Fig. 2c), and atomic resolution images of the local structure of layered Na0.5Co0.5Mn0.5O2 material revealed the column positions of the transition-metal atoms (Co/Mn), as shown in Fig. 2d. In [010] direction, the layered Na0.5Co0.5Mn0.5O2 material exhibited bright spots in the energy dispersive X-ray analysis, which bright spots were attributed to higher crystalline characteristics of layered Na0.5Co0.5Mn0.5O2 material, as shown in inset the (#) [31,32,49,52] of Fig. 2d. For more in detail, the local structure of the layered Na0.5Co0.5Mn0.5O2 material was analyzed using HAADF-STEM and ABF-STEM images as shown in Fig. 2eeh. It appears that the contrasting bright dots in the HAADF-STEM images (Fig. 2eef) and dark dots in the ABF-STEM images (Fig. 2geh) indicate the column positions of the Co and Mn atoms, respectively [31,32]. Further, the faint dark dots in the interlayer positions located in the ABF-STEM images correspond to the column positions of Na and O atoms in its layered structure, as shown in Fig. 2geh [31,32]. The HAAD-STEM and ABF-STEM images corroborated the schematized structure of the layered Na0.5Co0.5Mn0.5O2 material, indicating the presence of Na, Co/Mn and O; as given in
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Fig. 2. Morphology and structural characteristics of the layered Na0.5Co0.5Mn0.5O2 material: (a), (b) the layered Na0.5Co0.5Mn0.5O2 particles revealed flaky-bundle morphology with the flaky size of ~30 nm by HRTEM; (c) HAADF-STEM image of flaky-bundle layered Na0.5Co0.5Mn0.5O2 particles; (d) atomic resolution of the local structure of transition-metal atoms (Co/Mn) and (#) exhibits bright spots in the energy dispersive X-ray analysis in [010] direction; (e) HAAD-STEM image of contrasting bright dots corresponding to the Co/Mn atoms with (f) enlarged view and schematized layered structure; (g) ABF-STEM image of faint contrasting dark dots corresponding to the Na atom with (h) enlarged view and schematized layered structure; finally, advanced TEM elemental mapping: (i) presence of Na in red, (j) Co in orange, (k) Mn in green and (l) O in yellow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2f,h [31,32]. The HAADF-STEM image (enlarged from Fig. 2e) specifies the detailed atomic arrangement of Co/Mn with O in the Co/MnO6 eoctahedral unit, as given in Fig. 2f. In addition to this, the enlarged image of HAADF-ABF (from Fig. 2g) reveal the atomic arrangement of Co/Mn with O atoms in the Co/MnO6 eoctahedral, and of Na with O atoms in the Na etrigonal prism, which corroborated the layered stacking described in Fig. 2h [31,32]. In particular, the obtained ABF-STEM images confirmed that the Na atoms of the P2-type phase are clamped between two layers in the Co/MnO6 eoctahedral unit, as schematized in Fig. 1b. As a result, the presence of the elements Na, Co, Mn and O in the Na0.5Co0.5Mn0.5O2 particles was confirmed by advanced transmission electron microscope (TEM) elemental mapping, as shown in Fig. 2iel. The oxidation state of the transition metals in the layered Na0.5Co0.5Mn0.5O2 material was determined by the XPS technique [38,41e43,53e56]. Thus, the information about the oxidation state has a vital role in electrochemical performance and establishes the intercalation/de-intercalation process of the electrode material during charge-discharge cycling, in line with the redox reaction [38,41e43,53e55]. Accordingly, the presence of oxidized states of
Na, Co, Mn and O was confirmed based on the binding energy value shown in Fig. 3aed for the layered Na0.5Co0.5Mn0.5O2 material. In the Na 1s spectrum, one peak was obtained at 1071.08 eV and appears as a broad peak, which peak fit well (as indicated in red) with the corresponding peak deconvolution (gray) shown in Fig. 3a [38,42]. Further, two major peaks were observed in the Co 2p spectrum, which peaks were assigned to Co 2p1/2 at 795.48 eV and Co 2p3/2 at 780.38 eV. This is attributed to Co3þ in LiCoO2, indicating the 3 þ oxidation state of Co [38,41]. In Fig. 3c, the two core peaks at 654.28 eV and 642.88 eV are associated with Mn 2p1/2 and Mn 2p3/2 spectra for tetravalent Mn [38,41,43,54,55]. The determined 4 þ oxidation state of Mn with fitted profile (red) and peak deconvolution (gray) of the experiment result (cyan), are in good agreement with the previous literature [38,41,43,54,55]. Subsequently, the O 1s spectrum contained a broad peak located at the binding energy value of 529.88 eV. This spectrum includes three peaks, described in its peak deconvolution in Fig. 3d [53,55,56]. In this context, the peak located at 529.88 eV for O 1s corresponds to O2 2 anions in the structure of the layered Na0.5Co0.5Mn0.5O2 material [56]. The two remaining faint peaks of the O 1s spectrum are
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Fig. 3. XPS studies on oxidation state of transition metals for the layered Na0.5Co0.5Mn0.5O2 material: (a) XPS spectrum of Na 1s, (b) Co 2p, (c) Mn 2p and (d) O 1s with the experimental result marked in cyan, fitted peak area in gray and the maximum fitted profile in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
associated with the presence of adsorbed surface species and coordination deficiency of lattice oxygen [56]. Therefore, the confirmed oxidation states of Na, Co, Mn and O are well matched with those of reported cathode materials for use in Na-ion battery applications. 3.2. Stable layered Na0.5Co0.5Mn0.5O2 cathode material for sodiumion batteries Cyclic voltammetry studies were performed on the layered Na0.5Co0.5Mn0.5O2 material and revealed that the material exhibited a highly reversible system with Naþ intercalation/deintercalation processes. Accordingly, the fabricated sodium cells (Na vs layered Na0.5Co0.5Mn0.5O2) were investigated using CV analysis between 1.5 and 4.4 V vs Naþ/Na at 0.5 mV s1 for 5 cycles. It can be seen that there are two oxidation and reduction peaks obtained in its CV scan, as marked in Fig. 4a. During the initial cycle, the high voltage oxidation peak at 4.19 V is credited to the oxidation of Co3þ to Co4þ state in line with the reduction peak at 3.75 V (Fig. 4a) [38,57,58]. More importantly, the second reduction peak at 1.90 V is obtained with the reduction of Mn4þ to Mn3þ; with a pronounced oxidation peak at 2.30 V (Fig. 4a). The redox peaks are attributed to the enhanced electrochemical performance of layered Na0.5Co0.5Mn0.5O2 material [38,46e48,57,58]. Subsequent cycles onward, two obvious reversible redox peaks appeared and exhibited highly reversible, stable redox peaks for the Na-ion reversible system. Signature of its CV studies on the Na-ion reversible system,
galvanostatic charge-discharge measurements were tested to fabricate sodium cells of Na vs layered Na0.5Co0.5Mn0.5O2 material between 1.5 and 4.4 V vs Naþ/Na at 0.5 C for 50 cycles, as shown in Fig. 4b. The sodium cells delivered the initial charge-discharge capacity of 53/144 mAh g1 at 0.5 C. During the initial cycle, the charge voltage plateau was associated with Co3þ to Co4þ oxidation and simultaneous de-intercalation of Naþ ions from the layered Na0.5Co0.5Mn0.5O2 structure. The sodium half cells are delivered the less initial charge capacity 53 mAh g1 at 0.5 C due to OCV around 3.0 V as reported in the literature for the NaxMO2 materials [40,41,59]. The initial discharge capacity was 144 mAh g1 at 0.5 C, and this capacity was contributed by the reduction of Mn4þ to Mn3þ and by the Naþ ion intercalation into the host structure of the layered Na0.5Co0.5Mn0.5O2 material, as described in previous CV studies. After its initial cycle, the redox pairs of Co3þ/Co4þ and Mn4þ/Mn3þ are involved in the charge-discharge capacity with complete intercalation/de-intercalation processes. Hence, the delivered charge-discharge capacities at the 2nd and 50th cycles were 144/140 mAh g1 and 127/127 mAh g1, with discharge capacity retention of 88% at 0.5 C (Fig. 4b). The delivered capacities are significantly higher than in previous reports related to Na0.67Mn0.65Ni0.2Co0.15O2 [38], NaxNi0.22Co0.11Mn0.66O2 [39], NaxCo0.7Mn0.3O2 [40] and Na0.66Co0.5Mn0.5O2 [41] cathode material for SIBs. Notably, capacity-fade was not observed in the Na-ion cycling performance of the layered Na0.5Co0.5Mn0.5O2 material and facilitated extension of its rate capability studies. The rate performance of voltage vs capacity and capacity vs cycle number are shown in Fig. 4cef, and delivered initial charge-discharge capacity of 50/
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Fig. 4. Na-ion electrochemical performance of the layered Na0.5Co0.5Mn0.5O2 material: (a) cyclic voltammetric analysis of fabricated sodium cells (Na vs layered Na0.5Co0.5Mn0.5O2) in between 1.5 and 4.4 V vs Naþ/Na at 0.5 mV s1 for 5 cycles; galvanostatic charge-discharge studies of layered Na0.5Co0.5Mn0.5O2 material between 1.5 and 4.4 V vs Naþ/Na related to voltage vs capacity profiles at (b) 0.5 C; (c) 1 C; (d) 3 C; (e) 5 C and (f) capacity vs cycle number performance at different rates for 50 cycles using 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive electrolyte.
141 mAh g1 (1 C), 50/100 mAh g1 (3 C), and 47/80 mAh g1 (5 C) with good cycling stability. In particular, the same voltage plateaus observed at the 0.5 C rate were retained in all high-rate cycling studies and yielded 50th cycle charge-discharge capacity of 80/ 80 mAh g1 at 5 C, with maximum capacity retention of 100%. Further, the decreased capacity is attributed to the less Na-ion (de) intercalation process with respect to high current rate due to increasing Ohmic drop and polarization resistance at the electrodeelectrolyte interface of layered Na0.5Co0.5Mn0.5O2 material. From the obtained Na-ion performance, it is concluded that the layered Na0.5Co0.5Mn0.5O2 material is retained its capacity stability during charge-discharge cycling at high current and exhibited a superior cathode (141 mAh g1 at 1 C) for SIBs than other significant Na-ion cathode materials such as Na(Ni1/3Fe1/3Mn1/3)O2 (90 mAh g1) [26] NaTi0.5Ni0.5O2 (90 mAh g1) [27], Na0.67Mn0.65Ni0.2Co0.15O2 (125 mAh g1) [38] and NaxCo0.7Mn0.3O2 (95 mAh g1) [40].
studies at the constant current density of 0.212 mA, as shown in Fig. 5. The full-cell delivered the chargeedischarge cathode capacity 55/52 mAh g1 at the high current rate of 0.212 mA (1st cycle) and exhibited high voltage region between 2.0 and 4.4 V (Fig. 5b). Further, the full-cell performance revealed its stable cycling performance with the cathode capacity retention 90% (50th cycle) corresponding to charge-discharge capacity 48/47 mAh g1 at 50th cycle (Fig. 5c). The obtained coulombic efficiency is 97% (50th cycle) for the full-cell cycling study at the high current rate of 0.212 mA for the layered Na0.5Co0.5Mn0.5O2 material. The Na-ion full-cell exhibited a high voltage region (Fig. 5b) in the stable cycling studies of layered Na0.5Co0.5Mn0.5O2 material (Fig. 5c) in comparison with other reported Na-ion full-cell voltage profile [26]. From the obtained results, it is clear that the layered Na0.5Co0.5Mn0.5O2 material has promised as a superior cathode material for high voltage Na-ion batteries/full-cells.
3.3. Sodium-ion full-cell performance using pre-sodiated layered Na0.5Co0.5Mn0.5O2 cathode
4. Conclusion
As pointed out superior half-cell cycling studies, the coin type Na-ion full-cells (HC vs layered Na0.5Co0.5Mn0.5O2 material) were fabricated as schematized in Fig. 5a using a HC anode and cycled in the voltage range from 2.0 to 4.4 V at the constant current density of 0.212 mA, as shown in Fig. 5bec. In the full-cell studies, the layered Na0.5Co0.5Mn0.5O2 material was pre-sodiated by its half-cell cycling at 0.5 C for 3 cycles as given in Fig. S2. The half-cell was opened in its discharged state in Ar atmosphere and the presodiated cathode was used for fabrication of full-cells with HC anode. After 24 h aging, the formation cycling and cell stabilization cycle steps were carried out and used to further full-cell cycling
The cathode material Na0.5Co0.5Mn0.5O2 was prepared by a facile MHC route using (Co0.5Mn0.5)2(OH)2CO3 precursor. This ensured atomic scale-mixing of Co and Mn ion constituents and production of a hexagonal layered structure that belongs to the P63/mmc space group. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) were calculated by Rietveld refinement analysis. The layered Na0.5Co0.5Mn0.5O2 particle exhibited flaky-bundle morphology with hexagonal flakes size of ~30 nm. The local structure for Na, Co, Mn and O constituents (at atomic level resolution) was established based on contrasting bright dots and faint dark dots in the HAAD-STEM and ABF-STEM images. From the XPS spectrum, Co and Mn elements present in
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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). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.116. References
Fig. 5. Na-ion full-cell performance of the layered Na0.5Co0.5Mn0.5O2 material: (a) Schematic illustrates the Na-ion full-cell (HC vs. Na0.5Co0.5Mn0.5O2); (b) voltage vs capacity and (c) capacity vs cycle number performance of fabricated coin type Na-ion full-cell (HC vs layered Na0.5Co0.5Mn0.5O2 material) cycled in between 2.0 and 4.4 V at the high current rate of 0.212 mA for 50 cycles using 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive electrolyte.
the layered Na0.5Co0.5Mn0.5O2 material were confirmed as being in 3þ and 4þ oxidation states. Significantly, the CV scan demonstrated two oxidation and reduction peaks corresponding to Co3þ/Co4þ and Mn3þ/Mn4þ redox process for the Naþ ion intercalation/deintercalation mechanism in the host structure of the layered Na0.5Co0.5Mn0.5O2 material. Accordingly, the sodium cells fabricated for SIBs and delivered an initial charge-discharge capacity of 53/144 mAh g1 at 0.5 C with good capacity retention. Moreover, rate capability cycling studies at 1 C, 3 C and 5 C showed good performance and yielded similar charge-discharge capacity of 80/ 80 mAh g1 (5 C) at the 50th cycle for SIBs. The Na-ion full-cells were assembled using HC anode, and delivered initial chargedischarge capacity 55/52 mAh g1 at 0.212 mA with high voltage region. From the obtained cycling performance of the Na-ion fullcells, the layered Na0.5Co0.5Mn0.5O2 material has demonstrated its potential as a promising cathode for high voltage SIBs.
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Structural characterization of layered Na0.5Co0.5Mn0.5O2 material as a promising cathode for sodium-ion batteries Palanisamy Manikandan a, Seongwoo Heo a, Hyun Woo Kim a, Hu Young Jeong b, Eungje Lee c, Youngsik Kim a, d, * a
School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea UNIST Central Research Facilities (UCRF), Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea Chemical Science and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA d Energy Materials and Devices Lab, 4TOONE Corporation, UNIST-gil 50, Ulsan 44919, Republic of Korea b c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Layered Na0.5Co0.5Mn0.5O2 material synthesized by mixed hydroxycarbonate route. HAAD-STEM and ABF-STEM images revealed the local structure. Sodium cell delivered an initial charge-discharge capacity 53/ 144 mAh g1 at 0.5 C. Stabilized charge-discharge capacity 118/118 mAh g1 obtained at 3 C. Na-ion full-cell fabricated and enabled to attain high voltage region at 0.212 mA.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 May 2017 Received in revised form 27 July 2017 Accepted 29 July 2017
Layered Na0.5Co0.5Mn0.5O2 material is synthesized through a facile mixed hydroxy-carbonate route using (Co0.5Mn0.5)2(OH)2CO3 precursor and well characterized as a hexagonal layered structure under P63/mmc space group. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) are calculated by Rietveld refinement analysis. A flaky-bundle morphology is obtained to the layered Na0.5Co0.5Mn0.5O2 material with the hexagonal flake size ~30 nm. Advanced transmission electron microscopic images are revealed the local structure of the layered Na0.5Co0.5Mn0.5O2 material with contrasting bright dots and faint dark dots corresponding to the Co/Mn and Na atoms. Two oxidation and reduction peaks are occurred in a cyclic voltammetric analysis corresponding to Co3þ/Co4þ and Mn3þ/ Mn4þ redox processes. These reversible processes are attributed to the intercalation/de-intercalation of Naþ ions into the host structure of layered Na0.5Co0.5Mn0.5O2 material. Accordingly, the sodium cell is delivered the initial charge-discharge capacity 53/144 mAh g1 at 0.5 C, which cycling studies are extended to rate capability test at 1 C, 3 C and 5C. Eventually, the Na-ion full-cell is yielded cathode charge-discharge capacity 55/52 mAh g1 at 0.212 mA and exhibited as a high voltage cathode for Na-ion batteries. © 2017 Elsevier B.V. All rights reserved.
Keywords: Mixed hydroxy-carbonate Layered structure Charge-discharge cycling Cathode performance Sodium-ion battery
* Corresponding author. School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 44919, Republic of Korea. E-mail address: [email protected] (Y. Kim). http://dx.doi.org/10.1016/j.jpowsour.2017.07.116 0378-7753/© 2017 Elsevier B.V. All rights reserved.
1. Introduction Sustainable energy sources are in high demand to overcome the
P. Manikandan et al. / Journal of Power Sources 363 (2017) 442e449
fossil fuel crisis and for use in such as portable devices, electric vehicle and grid storage applications. In recent years, Na-ion batteries (SIBs) have emerged as potentially superior storage devices of energy from renewable sources because the vast natural resources of sodium worldwide are sufficient for large-scale production of Na-ion materials [1e6]. For this attention, SIBs are mainly focused on hybrid electric vehicles and smart grid applications [7]. Hence, the energy density and power density have been associated with the intercalation/de-intercalation performance of the cathode materials [8e10]. This performance is a prime factor for the development of renewable energy sources, now being directed towards a green environment with wireless communication, electric powered transportation and static grid energy storage [8e10]. For this reason, researchers have begun searching for suitable Naþ ion compounds and performing structural studies to determine, which sodium materials are best for storage of electrical energy [11e13]. Accordingly, it is challenging and highly motivating to investigate advanced cathode materials for SIBs with superior rate performance. In the 1980s, preliminary sodium studies examined the intercalation/de-intercalation electrochemical reactions [14e20]. Subsequently, the development of Na-ion cathode materials was intensively explored; in particular, these included layered structures with the general formula of AxMO2 (A e alkali, M transition metal) such as NaxCoO2 [21], NaxMnO2 [22], NaCrO2 [23] and NaVO2 [24]. These early choices exhibited multiple phase transition during Naþ ion intercalation/de-intercalation, and offered limited capacity with poor rate capability. More recently, the investigation of Na-ion cathode materials has shifted to materials with multiple transition metals such as NaxFe1/2Mn1/2O2 [25], Na(Ni1/3Fe1/3Mn1/3) O2 [26], NaTi0.5Ni0.5O2 [27], NaxMnyNizFe0.1Mg0.1O2 [28], Na0.67Ni0.23Mg0.1Mn0.67O2 [29], Na0.66Ni0.33exZnxMn0.67O2 [30], Na0.8Ni0.4Ti0.6O2 [31], Na0.66Ni0.17Co0.17Ti0.66O2 [32], Na3V2(PO4)3 [33,34], Na3V2(PO4)2F3 [35], NaVPO4F [36] and Na2FePO4F [37] to attain higher rate capability and good cycle performance. Nevertheless, the cathode materials listed are still limited in terms of rate capability, cell voltage and full-cell performance. Therefore, research is still needed to provide better cathodes for high voltage SIBs. Indeed, introducing Co3þ ions instead of Mn4þ into the NaxMnO2 structure, accelerated progress toward high voltage and excellent specific capacity. This was achieved by minimizing Jahn-Teller distortion and resisting manganese-ion dissolution by the presence of high voltage Co3þ ion species [38,39]. In view of this, the studies of Na0.67Mn0.65Ni0.2Co0.15O2 [38] and NaxNi0.22Co0.11Mn0.66O2 [39] cathode materials showed remarkable cycling potential for SIBs. Furthermore, research on the NaxCo0.7Mn0.3O2 cathode material indicated not only superior performance, but also less charge-discharge capacity (95 mAh g1 at 1 C) in cycling studies [40]. In terms of specific capacity, the investigation of Na0.66Co0.5Mn0.5O2 material delivered specific capacity of 156 mAh g1 at 1 C [41], which is higher than that of NaxCo0.7Mn0.3O2 cathode material [40]. Unfortunately, the cycling performance of the Na0.66Co0.5Mn0.5O2 material showed capacity fading during cycling, which was attributed to manganese dissolution followed by instability of the structure during cycling. It is also pertinent to question whether the sodium-deficient, layered NaxCo1/3Ni1/3Mn1/ 3O2 [42] and NaxNi0.5Mn0.5O2 [43,44] materials have revealed multi-phase formation and structural stability, based on the sodium content in the AxMO2. Such materials exhibit poor rate capability and cycling performance for SIBs. Scrutinizing these results, the key point appears to be need to fix the stoichiometric sodium content (x ¼ 0.5) in the NaxCoyMnzO2 composition. This should play a vital role in improving the structural stability during high-voltage charge-discharge cycling of SIBs.
443
In this work, the stoichiometric layered Na0.5Co0.5Mn0.5O2 material was prepared by a mixed hydroxy-carbonate method using (Co0.5Mn0.5)2(OH)2CO3 precursor. The obtained layered Na0.5Co0.5Mn0.5O2 material is well characterized and facilitates intercalation/de-intercalation, providing the characteristics of a promising cathode for SIBs. The synthesized material yielded charge-discharge capacity of 53/144 mAh g1 at 0.5 C for SIBs. Ultimately, the study of cell performance was extended to full-cell fabrication using conventional a hard carbon (HC) anode (i.e., HC vs Na0.5Co0.5Mn0.5O2) and yielded charge-discharge capacity 55/ 52 mAh g1 at 0.212 mA during 1st cycle of high voltage SIBs.
2. Experimental aspects Layered Na0.5Co0.5Mn0.5O2 material was prepared by mixed hydroxy-carbonate (MHC) method, which MHC route described in our earlier report [45]. In this synthesis, a nitrate solution of Co(NO3)2.6H2O and Mn(NO3)2.4H2O (Sigma-Aldrich) were mixed together and added to a solution containing 2 mol NaOH and 1 mol Na2CO3 at 40 C with vigorous stirring. The obtained precipitate of (Co0.5Mn0.5)2(OH)2CO3 precursor was washed with distilled water and ethanol, followed by drying at 60 C for 24 h. Finally, the (Co0.5Mn0.5)2(OH)2CO3 precursor was homogenized with Na2CO3 powder for 2 h at 400 rpm using a FRITSCH pulverisette. The homogenized powder was calcined at 900 C for 12 h and yielded the layered Na0.5Co0.5Mn0.5O2 material as a final product. During calcination, the thermal decomposition of the homogenized powder of Na2CO3 and (Co0.5Mn0.5)2(OH)2CO3 precursor can be explained in relation to the formation of layered Na0.5Co0.5Mn0.5O2 product by Equation (1).
(1) Next, the material composition of the newly synthesized, layered Na0.5Co0.5Mn0.5O2 material was examined using inductively coupled plasmaeoptical emission spectrometry (ICPeOES, VARIAN 700eES) for the presence of elements Na, Co and Mn based on the stoichiometric proportion. The powder X-ray diffraction pattern (XRD) was recorded using a Bruker D8 Advance Xeray diffractometer with a Cu Ka Xeray source within the 2W range from 10 to 80 , to examine the crystal structure of the synthesized material. The lattice parameters were obtained by Rietveld refinement analysis with best-fitted goodness of fit using the GSAS program and the crystallographic structure was schematized using the Diamond program. The morphology of the flaky-bundles was investigated by high-resolution transmission electron microscopy (HRTEM, JEOL, JEMe2100F). High-angle annular dark fieldscanning transmission electron microscopy (HAADF-STEM) and annular bright field-scanning transmission electron microscopy (ABF-STEM) images were recorded using an advanced transmission electron microscope (Titan microscope, Titan G2(60e300)) with energy dispersive X-ray analysis (EDX, elemental mapping). X-ray photoelectron spectroscopy (XPS) was carried out to determine the oxidation states of the transition metals in the synthesized Na0.5Co0.5Mn0.5O2 material using a ThermoFisher Multi-element (K-alpha) high-transmission spectrometer (energy range of input lens 200 eV to 3 keV). The electrochemical characterization of the layered Na0.5Co0.5Mn0.5O2 material was provided by CR-2032-type half and full-cells for SIBs using VMP3 Biologic and WonATech. For the fabrication of cathodes, the slurry contained 80% active material, 10% SP-carbon (Timcal) and 10% poly(vinylidene fluoride) in N-
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methyl-2-pyrrolidone. The slurry was coated on Al current collector (15 mm thick) and vacuum dried at 120 C for 12 h. The cathode was roll pressed and punched as disc shape with average diameter of 14 mm and active material loading of ~3.2 mg. Metallic sodium was used as anode for the fabrication of sodium half-cells. The test cells were assembled in an argon-filled glove box (KIYON). The electrolyte for the SIBs was 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive and glass fiber was used a separator. Cyclic voltammetry analysis were carried out from 1.5 to 4.4 V vs Naþ/Na at 0.5 mV s1 with metallic sodium as a reference electrode for the SIBs. Galvanostatic chargeedischarge cycling studies were performed between 1.5 and 4.4 V vs Naþ/Na at different rates 0.5, 1, 3 and 5 C up to 50 cycles. Furthermore, the Na-ion full-cells were fabricated using HC (80% active material, 10% SP-carbon (Timcal) and 10% PVdF in N-methyl-2-pyrrolidone) coated on Cu current collector (9 m) and cycled between 2 and 4.4 V at 0.212 mA for SIBs. 3. Results and discussion In this study, layered Na0.5Co0.5Mn0.5O2 material was obtained through a facile MHC route involving mixed hydroxy-carbonate of (Co0.5Mn0.5)2(OH)2CO3 precursor. It is pertinent to note that this method results in atomic scale-mixing of Co and Mn ion constituents and facilitates shorter milling time (~2 h) than the conventional solid state method. As a result, this route enabled synthesis of energy transfer materials with superior electrochemical performance for use in SIBs, as described in the following electrochemical section. 3.1. Structural investigation of Na0.5Co0.5Mn0.5O2 material Using ICP-OES analysis, the design ratio of Na, Co and Mn constituents in the elemental composition of the as-prepared Na0.5Co0.5Mn0.5O2 material was confirmed. The powder X-ray diffraction pattern and Rietveld refinement analysis of layered Na0.5Co0.5Mn0.5O2 material (900 C for 12 h) are shown in Fig. 1a, and the corresponding schematized layered structure is described in Fig. 1b. The XRD pattern clearly shows that greater crystallinity and all the diffraction planes can be well indexed with P2-type phase (JCPDS# 00-054-0894), and that the hexagonal layered structure belongs to the P63/mmc space group [40e42,46e48]. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) were calculated by Rietveld refinement analysis and the listed its crystallographic atomic parameters are listed in Table S1. The calculated pattern (red) had the best goodness of fit with the experimental data and the lattice parameters showed good agreement with the reported literature on the P2-type phase corresponding to a hexagonal layered structure [39e42,46e49]. From the Rietveld refinement analysis, the goodness of fit (c2) and R-factor (Rwp) were obtained and found to be c2 ¼ 1 and Rwp ¼ 2.7% for the layered Na0.5Co0.5Mn0.5O2 material (prepared at 900 C for 12 h via the MHC route). The goodness of fit c2 ¼ 1 exhibited the best fitted profile for the powder pattern of layered Na0.5Co0.5Mn0.5O2 material. The layered structure was schematized using the obtained crystallographic parameters, as shown in Fig. 1b. Each structure comprised two: an octahedral of Co/MnO6 and a trigonal prism of Na (Fig. 1b) [41,46e48]. The obtained layered Na0.5Co0.5Mn0.5O2 material was subjected to morphological investigation using the HRTEM technique. From this, it can be seen that the flaky-bundle morphology presented with a flake size of ~30 nm, as shown in Fig. 2a and b [40,46,47]. The hexagonal shape is present in the flaky-bundle morphology, as shown in Fig. S1 [40,46,47]. Also, the detailed local structure of the layered Na0.5Co0.5Mn0.5O2 material was investigated using highangle annular dark field (HAADF) and annular bright field (ABF)
Fig. 1. (a) Powder X-ray diffraction pattern with Rietveld refinement analysis of layered Na0.5Co0.5Mn0.5O2 material (900 C for 12 h) and (b) schematic layered structure with octahedral of Co/MnO6 and trigonal prism of Na, using crystallographic parameters.
scanning transmission electron microscopy (STEM) techniques (Fig. 2cel) [31,32,50,51]. The HAADF-STEM images revealed flakybundle particles (Fig. 2c), and atomic resolution images of the local structure of layered Na0.5Co0.5Mn0.5O2 material revealed the column positions of the transition-metal atoms (Co/Mn), as shown in Fig. 2d. In [010] direction, the layered Na0.5Co0.5Mn0.5O2 material exhibited bright spots in the energy dispersive X-ray analysis, which bright spots were attributed to higher crystalline characteristics of layered Na0.5Co0.5Mn0.5O2 material, as shown in inset the (#) [31,32,49,52] of Fig. 2d. For more in detail, the local structure of the layered Na0.5Co0.5Mn0.5O2 material was analyzed using HAADF-STEM and ABF-STEM images as shown in Fig. 2eeh. It appears that the contrasting bright dots in the HAADF-STEM images (Fig. 2eef) and dark dots in the ABF-STEM images (Fig. 2geh) indicate the column positions of the Co and Mn atoms, respectively [31,32]. Further, the faint dark dots in the interlayer positions located in the ABF-STEM images correspond to the column positions of Na and O atoms in its layered structure, as shown in Fig. 2geh [31,32]. The HAAD-STEM and ABF-STEM images corroborated the schematized structure of the layered Na0.5Co0.5Mn0.5O2 material, indicating the presence of Na, Co/Mn and O; as given in
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Fig. 2. Morphology and structural characteristics of the layered Na0.5Co0.5Mn0.5O2 material: (a), (b) the layered Na0.5Co0.5Mn0.5O2 particles revealed flaky-bundle morphology with the flaky size of ~30 nm by HRTEM; (c) HAADF-STEM image of flaky-bundle layered Na0.5Co0.5Mn0.5O2 particles; (d) atomic resolution of the local structure of transition-metal atoms (Co/Mn) and (#) exhibits bright spots in the energy dispersive X-ray analysis in [010] direction; (e) HAAD-STEM image of contrasting bright dots corresponding to the Co/Mn atoms with (f) enlarged view and schematized layered structure; (g) ABF-STEM image of faint contrasting dark dots corresponding to the Na atom with (h) enlarged view and schematized layered structure; finally, advanced TEM elemental mapping: (i) presence of Na in red, (j) Co in orange, (k) Mn in green and (l) O in yellow. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 2f,h [31,32]. The HAADF-STEM image (enlarged from Fig. 2e) specifies the detailed atomic arrangement of Co/Mn with O in the Co/MnO6 eoctahedral unit, as given in Fig. 2f. In addition to this, the enlarged image of HAADF-ABF (from Fig. 2g) reveal the atomic arrangement of Co/Mn with O atoms in the Co/MnO6 eoctahedral, and of Na with O atoms in the Na etrigonal prism, which corroborated the layered stacking described in Fig. 2h [31,32]. In particular, the obtained ABF-STEM images confirmed that the Na atoms of the P2-type phase are clamped between two layers in the Co/MnO6 eoctahedral unit, as schematized in Fig. 1b. As a result, the presence of the elements Na, Co, Mn and O in the Na0.5Co0.5Mn0.5O2 particles was confirmed by advanced transmission electron microscope (TEM) elemental mapping, as shown in Fig. 2iel. The oxidation state of the transition metals in the layered Na0.5Co0.5Mn0.5O2 material was determined by the XPS technique [38,41e43,53e56]. Thus, the information about the oxidation state has a vital role in electrochemical performance and establishes the intercalation/de-intercalation process of the electrode material during charge-discharge cycling, in line with the redox reaction [38,41e43,53e55]. Accordingly, the presence of oxidized states of
Na, Co, Mn and O was confirmed based on the binding energy value shown in Fig. 3aed for the layered Na0.5Co0.5Mn0.5O2 material. In the Na 1s spectrum, one peak was obtained at 1071.08 eV and appears as a broad peak, which peak fit well (as indicated in red) with the corresponding peak deconvolution (gray) shown in Fig. 3a [38,42]. Further, two major peaks were observed in the Co 2p spectrum, which peaks were assigned to Co 2p1/2 at 795.48 eV and Co 2p3/2 at 780.38 eV. This is attributed to Co3þ in LiCoO2, indicating the 3 þ oxidation state of Co [38,41]. In Fig. 3c, the two core peaks at 654.28 eV and 642.88 eV are associated with Mn 2p1/2 and Mn 2p3/2 spectra for tetravalent Mn [38,41,43,54,55]. The determined 4 þ oxidation state of Mn with fitted profile (red) and peak deconvolution (gray) of the experiment result (cyan), are in good agreement with the previous literature [38,41,43,54,55]. Subsequently, the O 1s spectrum contained a broad peak located at the binding energy value of 529.88 eV. This spectrum includes three peaks, described in its peak deconvolution in Fig. 3d [53,55,56]. In this context, the peak located at 529.88 eV for O 1s corresponds to O2 2 anions in the structure of the layered Na0.5Co0.5Mn0.5O2 material [56]. The two remaining faint peaks of the O 1s spectrum are
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Fig. 3. XPS studies on oxidation state of transition metals for the layered Na0.5Co0.5Mn0.5O2 material: (a) XPS spectrum of Na 1s, (b) Co 2p, (c) Mn 2p and (d) O 1s with the experimental result marked in cyan, fitted peak area in gray and the maximum fitted profile in red. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
associated with the presence of adsorbed surface species and coordination deficiency of lattice oxygen [56]. Therefore, the confirmed oxidation states of Na, Co, Mn and O are well matched with those of reported cathode materials for use in Na-ion battery applications. 3.2. Stable layered Na0.5Co0.5Mn0.5O2 cathode material for sodiumion batteries Cyclic voltammetry studies were performed on the layered Na0.5Co0.5Mn0.5O2 material and revealed that the material exhibited a highly reversible system with Naþ intercalation/deintercalation processes. Accordingly, the fabricated sodium cells (Na vs layered Na0.5Co0.5Mn0.5O2) were investigated using CV analysis between 1.5 and 4.4 V vs Naþ/Na at 0.5 mV s1 for 5 cycles. It can be seen that there are two oxidation and reduction peaks obtained in its CV scan, as marked in Fig. 4a. During the initial cycle, the high voltage oxidation peak at 4.19 V is credited to the oxidation of Co3þ to Co4þ state in line with the reduction peak at 3.75 V (Fig. 4a) [38,57,58]. More importantly, the second reduction peak at 1.90 V is obtained with the reduction of Mn4þ to Mn3þ; with a pronounced oxidation peak at 2.30 V (Fig. 4a). The redox peaks are attributed to the enhanced electrochemical performance of layered Na0.5Co0.5Mn0.5O2 material [38,46e48,57,58]. Subsequent cycles onward, two obvious reversible redox peaks appeared and exhibited highly reversible, stable redox peaks for the Na-ion reversible system. Signature of its CV studies on the Na-ion reversible system,
galvanostatic charge-discharge measurements were tested to fabricate sodium cells of Na vs layered Na0.5Co0.5Mn0.5O2 material between 1.5 and 4.4 V vs Naþ/Na at 0.5 C for 50 cycles, as shown in Fig. 4b. The sodium cells delivered the initial charge-discharge capacity of 53/144 mAh g1 at 0.5 C. During the initial cycle, the charge voltage plateau was associated with Co3þ to Co4þ oxidation and simultaneous de-intercalation of Naþ ions from the layered Na0.5Co0.5Mn0.5O2 structure. The sodium half cells are delivered the less initial charge capacity 53 mAh g1 at 0.5 C due to OCV around 3.0 V as reported in the literature for the NaxMO2 materials [40,41,59]. The initial discharge capacity was 144 mAh g1 at 0.5 C, and this capacity was contributed by the reduction of Mn4þ to Mn3þ and by the Naþ ion intercalation into the host structure of the layered Na0.5Co0.5Mn0.5O2 material, as described in previous CV studies. After its initial cycle, the redox pairs of Co3þ/Co4þ and Mn4þ/Mn3þ are involved in the charge-discharge capacity with complete intercalation/de-intercalation processes. Hence, the delivered charge-discharge capacities at the 2nd and 50th cycles were 144/140 mAh g1 and 127/127 mAh g1, with discharge capacity retention of 88% at 0.5 C (Fig. 4b). The delivered capacities are significantly higher than in previous reports related to Na0.67Mn0.65Ni0.2Co0.15O2 [38], NaxNi0.22Co0.11Mn0.66O2 [39], NaxCo0.7Mn0.3O2 [40] and Na0.66Co0.5Mn0.5O2 [41] cathode material for SIBs. Notably, capacity-fade was not observed in the Na-ion cycling performance of the layered Na0.5Co0.5Mn0.5O2 material and facilitated extension of its rate capability studies. The rate performance of voltage vs capacity and capacity vs cycle number are shown in Fig. 4cef, and delivered initial charge-discharge capacity of 50/
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Fig. 4. Na-ion electrochemical performance of the layered Na0.5Co0.5Mn0.5O2 material: (a) cyclic voltammetric analysis of fabricated sodium cells (Na vs layered Na0.5Co0.5Mn0.5O2) in between 1.5 and 4.4 V vs Naþ/Na at 0.5 mV s1 for 5 cycles; galvanostatic charge-discharge studies of layered Na0.5Co0.5Mn0.5O2 material between 1.5 and 4.4 V vs Naþ/Na related to voltage vs capacity profiles at (b) 0.5 C; (c) 1 C; (d) 3 C; (e) 5 C and (f) capacity vs cycle number performance at different rates for 50 cycles using 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive electrolyte.
141 mAh g1 (1 C), 50/100 mAh g1 (3 C), and 47/80 mAh g1 (5 C) with good cycling stability. In particular, the same voltage plateaus observed at the 0.5 C rate were retained in all high-rate cycling studies and yielded 50th cycle charge-discharge capacity of 80/ 80 mAh g1 at 5 C, with maximum capacity retention of 100%. Further, the decreased capacity is attributed to the less Na-ion (de) intercalation process with respect to high current rate due to increasing Ohmic drop and polarization resistance at the electrodeelectrolyte interface of layered Na0.5Co0.5Mn0.5O2 material. From the obtained Na-ion performance, it is concluded that the layered Na0.5Co0.5Mn0.5O2 material is retained its capacity stability during charge-discharge cycling at high current and exhibited a superior cathode (141 mAh g1 at 1 C) for SIBs than other significant Na-ion cathode materials such as Na(Ni1/3Fe1/3Mn1/3)O2 (90 mAh g1) [26] NaTi0.5Ni0.5O2 (90 mAh g1) [27], Na0.67Mn0.65Ni0.2Co0.15O2 (125 mAh g1) [38] and NaxCo0.7Mn0.3O2 (95 mAh g1) [40].
studies at the constant current density of 0.212 mA, as shown in Fig. 5. The full-cell delivered the chargeedischarge cathode capacity 55/52 mAh g1 at the high current rate of 0.212 mA (1st cycle) and exhibited high voltage region between 2.0 and 4.4 V (Fig. 5b). Further, the full-cell performance revealed its stable cycling performance with the cathode capacity retention 90% (50th cycle) corresponding to charge-discharge capacity 48/47 mAh g1 at 50th cycle (Fig. 5c). The obtained coulombic efficiency is 97% (50th cycle) for the full-cell cycling study at the high current rate of 0.212 mA for the layered Na0.5Co0.5Mn0.5O2 material. The Na-ion full-cell exhibited a high voltage region (Fig. 5b) in the stable cycling studies of layered Na0.5Co0.5Mn0.5O2 material (Fig. 5c) in comparison with other reported Na-ion full-cell voltage profile [26]. From the obtained results, it is clear that the layered Na0.5Co0.5Mn0.5O2 material has promised as a superior cathode material for high voltage Na-ion batteries/full-cells.
3.3. Sodium-ion full-cell performance using pre-sodiated layered Na0.5Co0.5Mn0.5O2 cathode
4. Conclusion
As pointed out superior half-cell cycling studies, the coin type Na-ion full-cells (HC vs layered Na0.5Co0.5Mn0.5O2 material) were fabricated as schematized in Fig. 5a using a HC anode and cycled in the voltage range from 2.0 to 4.4 V at the constant current density of 0.212 mA, as shown in Fig. 5bec. In the full-cell studies, the layered Na0.5Co0.5Mn0.5O2 material was pre-sodiated by its half-cell cycling at 0.5 C for 3 cycles as given in Fig. S2. The half-cell was opened in its discharged state in Ar atmosphere and the presodiated cathode was used for fabrication of full-cells with HC anode. After 24 h aging, the formation cycling and cell stabilization cycle steps were carried out and used to further full-cell cycling
The cathode material Na0.5Co0.5Mn0.5O2 was prepared by a facile MHC route using (Co0.5Mn0.5)2(OH)2CO3 precursor. This ensured atomic scale-mixing of Co and Mn ion constituents and production of a hexagonal layered structure that belongs to the P63/mmc space group. The lattice parameters and unit cell volume (a ¼ 2.8363 Å, c ¼ 11.3152 Å and V ¼ 78.83 Å3) were calculated by Rietveld refinement analysis. The layered Na0.5Co0.5Mn0.5O2 particle exhibited flaky-bundle morphology with hexagonal flakes size of ~30 nm. The local structure for Na, Co, Mn and O constituents (at atomic level resolution) was established based on contrasting bright dots and faint dark dots in the HAAD-STEM and ABF-STEM images. From the XPS spectrum, Co and Mn elements present in
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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). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.116. References
Fig. 5. Na-ion full-cell performance of the layered Na0.5Co0.5Mn0.5O2 material: (a) Schematic illustrates the Na-ion full-cell (HC vs. Na0.5Co0.5Mn0.5O2); (b) voltage vs capacity and (c) capacity vs cycle number performance of fabricated coin type Na-ion full-cell (HC vs layered Na0.5Co0.5Mn0.5O2 material) cycled in between 2.0 and 4.4 V at the high current rate of 0.212 mA for 50 cycles using 1 M NaPF6 in 1:1 (v/v) EC-PC with FEC (5% wt.) additive electrolyte.
the layered Na0.5Co0.5Mn0.5O2 material were confirmed as being in 3þ and 4þ oxidation states. Significantly, the CV scan demonstrated two oxidation and reduction peaks corresponding to Co3þ/Co4þ and Mn3þ/Mn4þ redox process for the Naþ ion intercalation/deintercalation mechanism in the host structure of the layered Na0.5Co0.5Mn0.5O2 material. Accordingly, the sodium cells fabricated for SIBs and delivered an initial charge-discharge capacity of 53/144 mAh g1 at 0.5 C with good capacity retention. Moreover, rate capability cycling studies at 1 C, 3 C and 5 C showed good performance and yielded similar charge-discharge capacity of 80/ 80 mAh g1 (5 C) at the 50th cycle for SIBs. The Na-ion full-cells were assembled using HC anode, and delivered initial chargedischarge capacity 55/52 mAh g1 at 0.212 mA with high voltage region. From the obtained cycling performance of the Na-ion fullcells, the layered Na0.5Co0.5Mn0.5O2 material has demonstrated its potential as a promising cathode for high voltage SIBs.
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