Na2.67Mn1.67(MoO4)3: A 3.45 V Alluaudite-Type Cathode Candidate

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Na2.67Mn1.67(MoO4)3: A 3.45 V Alluaudite-Type Cathode Candidate for Sodium-Ion Batteries Jianhua Gao,*,† Pan Zhao,† and Kai Feng‡ †

School of Physics, Northwest University, Xi’an 710069, China Division of Energy Storage, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China

‡

S Supporting Information *

structure. To the best of our knowledge, this is the first time to report the compound. The pure phase of I can be easily prepared by the sol−gel method. It is surprising that I exhibits higher voltage (3.45 V vs Na+/Na) than alluaudite-type phosphates, despite the MoO4 group has a lower inductive effect than the PO4 group. The first discharge capacity of I reaches to ∼92 mA h g−1 at a C/10 rate, and it could be reversibly cycled for at least 5 cycles without structural decomposition at different rates. More interesting, the involvement of multiple redox processes occurring at both Mn and Mo sites is reflected in electrochemical plateaus around 3.5 V associated with the Mn3+/Mn2+ redox couple and 2.4 V associated with the Mo6+/Mo5+ redox couple, indicating that I would have a higher capacity in spite of the big molar mass of the Mo atom. Single crystals of I were grown through spontaneous crystallization from a high-temperature melt of the mixture of NaCl, MnCO3 and MoO3 in a molar ratio of 1:1:1 (the details can be found in the Supporting Information). The crystal structure of I was determined by single-crystal X-ray diffraction analysis (the details can be found in the Supporting Information, and detailed crystallographic data in Tables S1− S3). Analytically pure Na2CO3, MnCO3, (NH4)6Mo7O24·4H2O and citric acid were used as starting materials. Polycrystalline samples of I were synthesized according to the following methods. First, 0.02 mol of Na2CO3, 0.0167 mol of MnCO3 and 10 mL of distilled water were put into a clean beaker, and then HNO3 was slowly introduced into the above beaker dropwise under magnetic stirring. When a colorless solution was formed, 2 g of citric acid was added into the solution and stirred to form solution A. Second, 0.00429 mol of (NH4)6MO7O24·4H2O was dissolved in the distilled water to form solution B. Third, the mixed solution of A and B was kept at 70 °C to evaporate water until it turned into a wet gel. The wet gel was dried at 100 °C overnight to obtain the dry gel, which was preheated at 300 °C for 2 h in air atmosphere. Finally, the preheated material was ground and sintered at 650 °C for 24h in air atmosphere, the final products (polycrystalline samples of I) were obtained. As shown in Figure 1, the experimental powder X-ray diffraction (XRD) pattern agrees very well with the simulated one based on single-crystal XRD analysis, indicating that high-purity samples of I can be synthesized easily. The scanning electron microscopy (SEM)

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odium-ion batteries (SIBs) have been considered as a promising alternative to lithium-ion batteries (LIBs) because of the natural abundance, fast diffusion and facile interfacial kinetics of Na ions.1,2 Similar to LIBs, the research on cathode materials for SIBs also mainly centers on layered oxides,3−6 polyanionic phosphates,7−22 and sulfate-based polyanionic compounds.23−31 Among polyanionic cathodes, the alluaudite-type compounds are attracting much attention. The alluaudite-type minerals and synthetic compounds have the general formula A(1)A(2)M(1)M(2)2(XO4)3, where A sites and M(1) site could be mono- or divalent ions, e.g., Na+, Li+, Ca2+, Mn2+ etc.; M(2) site could be di- or trivalent ions, e.g., Mn2+, Fe2+, Mn3+, Fe3+, In3+ etc.; X could be P, As, S, Mo or W. In the alluaudite structure, along the c-axis direction there are two sets of tunnels, constructed by interconnection of MO6 octahedra and XO4 tetrahedra. The A(1) and A(2) sites are located in tunnels. When A sites were occupied by Na+ ions, due to the presence of tunnels, the alluaudite-type compounds exhibit the mobility of Na+ ions. The most common alluauditetype compounds are phosphates.32 It had been demonstrated that some alluaudite-type phosphates, such as NaMnIIFeIII2(PO4)320 and Na2FeIIxMnII2−x FeIII (PO4)3 (x = 0, 1 and 2)21,22 are electrochemically active, but they have lower voltage (below 3 V vs Na+/Na). Since 2014, Yamada’s group has reported that Na2+2xFe2−x(SO4)3 would be a very promising material as a cathode for SIBs to make SIBs competitive with the state-of-the-art LIBs, because it not only has the highest potential of 3.8 V vs Na+/Na for the Fe2+/Fe3+ redox couple in a Na ion cell system but also shows extremely high theoretical energy density (>540 W h kg−1 vs Na+/Na) and good cycle retention.25,33−35 Na2+2xFe2−x(SO4)3 also belongs to alluauditetype compound. In its structure, A(1), A(2) and M(1) sites are all occupied partially or fully by Na ions, whereas M(2) site is occupied partially by Fe2+ ions. Subsequently, an analogous alluaudite-type compound Na2+2xMn2−x(SO 4)3 was prepared36,37 and demonstrated to have a high potential of 4.4 or 4.1 V vs Na+/Na using density functional theory calculations,36,38 but until now there is still no electrochemical experimental data for SIBs due to the lack of safe electrolytes. Just recently, Na2.32Co1.84(SO4)3, a new member of the alluaudite family, was also synthesized by Barpanda’s group. And it may be work as a 5 V cathode material for sodium-ion batteries in the future.39 During our research on the system NaCl−MnCO3−MoO3, we serendipitously synthesized the crystals of Na2.67Mn1.67(MoO4)3 (I). Its structure was solved by single crystal X-ray diffraction and also belongs to alluaudite-type © XXXX American Chemical Society

Received: December 15, 2016 Revised: January 22, 2017 Published: January 25, 2017 A

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Figure 1. Experimental powder XRD pattern (down) and the calculated XRD pattern (up) based on single-crystal XRD analysis of Na2.67Mn1.67(MoO4)3.

Figure 3. (a) Sheet, parallel to bc plane, formed by Mn2O10 dimers and Mo(2)O4 tetrahedra. (b) Projected view along c direction showing the tunnels resided by Na(2) and Na(3) ions. MnO6 octahedra and MoO4 groups are shaded in blue and yellow, respectively.

and O are distributed uniformly in the particles. The elemental mapping confirms Na-rich and Mn-deficient, which are in accord with the structural analysis. No carbon was found in elemental mapping, indicating that citric acid was decomposed completely in the high temperature. Figure 2b shows the SEM image of the above as-synthesized samples ball-milled at 300 rpm for 24 h with carbon (super P). The ratio of samples to carbon is 80:20 wt %. The particle size is reduced to ∼0.2−3 μm after ball-milling, but the agglomeration of the particles becomes more significant than in the pristine sample. The elemental mappings of the sample before and after ball-milling are accordant except for the appearance of carbon signal (Figure S2 in the Supporting Information). Additionally, as shown in Figure S3 in the Supporting Information, the XRD pattern (Figure S3b) of the as-prepared samples aged for 2 months in the plastic valve bag is consistent with the simulated pattern (Figure S3a). Figure S3c,d shows the XRD patterns of samples that were exposed in a very moist environment for 1 week and 2 weeks, respectively. As can be seen, no degradation was observed for the 1 week aged samples, and after 2 weeks the appearance of new peaks in the XRD (marked with asterisks) was noticed, indicative of the material degradation. These indicate that I has a relatively good stability and need not to be kept in the inert atmosphere, unlike Na2+2xM2−x(SO4)3 (M = Fe and Mn), they are prone to ambient poisoning.36 I crystallizes in the monoclinic crystal system with space group of C2/c (No. 15). Its structure belongs to the typical alluaudite-type structure. In its structure, M(1) and A(1) sites are fully occupied by Na(1) and Na(2) respectively, A(2) sites are partially occupied by Na(3), and M(2) are partially occupied by Mn. Mn atom is coordinated with six O atoms

Figure 2. SEM images of Na2.67Mn1.67(MoO4)3: (a) as-synthesized and (b) after ball-milling.

image of the as-synthesized sample is shown in Figure 2a. Irregular particles ranging from ∼5 to 120 μm were observed in the SEM image. The big particle size could be rooted to the high-temperature and long-time annealing. The chemical composition was analyzed by energy-dispersive X-ray (EDX) spectroscopy in a selected area (Figure S1 in the Supporting Information). It can be seen that the elements of Na, Mn, Mo B

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Figure 5. (a) Charge/discharge curves of Na2.67Mn1.67(MoO4)3 at a C/10 rate; (b) capacity retention of the Na2.67Mn1.67(MoO4)3 electrode at different discharge current rates (1C = 100 m Ah g−1). Figure 4. Bond valence sum maps for sodium ions with an isosurface level of 0.4: (a) for Na2.67Mn1.67(MoO4)3; (b) for Na2.76Mn1.78(SO4)3.

contrast, under the same isovalue, there are two obvious differences in the BVS maps: (1) in I, the migration pathways along c-direction for Na(2) and Na(3) are wider; (2) in I, Na ions can migrate not only along c-axis direction (the tunnels) but also along b-axis direction, whereas in Na2.76Mn1.78(SO4)3, Na ions can only migrate along c-axis direction. These suggests that I has higher mobility of Na+ ions compared with Na2+2xM2−x(SO4)3. The reason maybe is that the bigger volume of MoO4 groups (compared with SO4 groups) makes the tunnels along c and b directions become wider, so that the energy barrier to motion of Na+ ions in the diffusional process become lower. The ball-milled samples of I were electrochemically tested at room temperature in coin cells with sodium metal as the reference and counter electrodes (detailed description of preparation of cell in the Supporting Information). Figure 5a shows the galvanostatic charge and discharge (GCD) curves of the cell between 1.5 and 4.3 V at a C/10 rate in the first and second cycles. Obviously, I can be very effectively cycled against a Na metal electrode with an average voltage of about 3.45 V vs Na+/Na. Two voltage plateaus in discharge curves are centered around 3.5 and 2.4 V. According to the previous report for Na3MnTi(PO4)344 and Na3MnPO4CO3,16 the voltage plateau of the Mn3+/Mn2+ redox couple locates at 3.6 V (Na3MnTi(PO4)3) or 3.5 V (Na3MnPO4CO3), so it can be judged that the 3.5 V voltage plateaus in our case should assign to Mn3+/ Mn2+ redox couple. As for 2.4 V voltage plateau, it may be associated with the Mo6+/Mo5+ redox couple, because the similar electrochemical plateau also appear in Li2Co2(MoO4)3.45 It is worthy of noting that the first and second discharge capacities in the GCD curves reach to 92 and

to form a distorted octahedron, which are coupled by edge sharing to form Mn2O10 dimer. A two-dimensional infinite sheet, parallel to bc plane, is consist of Mn2O10 dimers and Mo(2)O4 tetrahedra through sharing corners (Figure 3a). The sheets are linked together by Mo(1)O4 tetrahedra along a direction, to form a three-dimensional framework with cavities and tunnels. The Na(1) reside in the cavities, and the Na(2) and Na(3) are located in the open tunnels running along c direction (Figure 3b). As the matter of fact, the structure of I is isostructural with that of Na2+2xM2−x(SO4)3 (M = Fe and Mn). But there are still obvious differences centering at the coordination of Na ions and Na−O bond distance. The three Na are all coordinated to six oxygen atoms for I, whereas in Na2+2xM2−x(SO4)3, they are six- or eight-coordinated with O atoms25,36,37 (the details can be seen from Figure S4 in the Supporting Information). The coordination of Na and Na−O bond distance must have influence on the Na+ ions migration during the electrode reaction. Therefore, it is very necessary to figure out the difference of Na+ ions migration between MoO4and SO4-based alluaudite-type structures. It is demonstrated that the bond valence sum (BVS) map method is very simple and effective approach to obtain ions migration pathway.15,25,40,41 So, the BVS calculations were carried out using a 3DBVSMAPPER computer program42,43 to the crystal structure data of I and Na2.76Mn1.78(SO4)3.37 Figure 4 shows the BVS maps of Na ions for I and Na2.76Mn1.78(SO4)3 with an isosurface level of 0.4. As can be seen from Figure 4b, for Na2.76Mn1.78(SO4)3, its BVS map is almost same with that of Na2Fe2(SO4)3,25 but different from that of I (Figure 4a). By C

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Chemistry of Materials 86 mA h g−1, respectively. But the capacity fades quickly with the increase of current rates (Figure 5b) maybe due to the big particle sizes or the agglomeration of the particles. Even so, I could be reversibly cycled for at least 5 cycles without structural decomposition at different rates, indicating the good cycle retention. To improve the rate performance and investigate the sodium intercalation mechanism, more optimization would be necessary, such as synthesizing the nanosize particles of I using other advanced technology. In conclusion, a novel crystal, Na2.67Mn1.67(MoO4)3, was successfully obtained and the single crystal diffraction analysis demonstrated that it belongs to alluaudite-type structure, isostructural with Na2+2xM2−x(SO4)3 (M = Fe and Mn). The high-purity polycrystalline samples of Na2.67Mn1.67(MoO4)3 can be synthesized easily using the sol−gel method at low temperature (below 650 °C) in the air atmosphere. Compared with Na2.76Mn1.78(SO4)3, the BVS maps show that the title compound has higher mobility of Na+ ions. Our preliminary results show that Na2.67Mn1.67(MoO4)3 can effectively cycle Na ions reversibly at room temperature, and has a high average voltage (3.45 V), high capacity and good cyclability. These features make the title compound attractive for rechargeable Na-ion battery applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05308. Experimental details, Tables of crystallographic data, atomic coordinates and bond lengths, Figures S1−S4 (PDF) CIF data (Deposition No. CCDC 1522247) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*J. Gao. E-mail: [email protected]. ORCID

Jianhua Gao: 0000-0002-6232-7653 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Shaanxi Province (Grant No. 2016JM2006), the Fund of Education Committee of Shaanxi Province (Grant No. 16JS103).

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