Journal of Materials Science & Technology  2019 , 35 (7): 1250-1254 https://doi.org/10.1016/j.jmst.2019.01.010

Orginal Article

Triple effects of Sn-substitution on Na0.67Ni0.33Mn0.67O2

Xiaohui Rongabc, Fei Gaoa, Feixiang Dingbc, Yaxiang Lub*, Kai Yanga, Hong Lib, Xuejie Huangb, Liquan Chenb, Yong-Sheng Hub

aState Key Laboratory of Operation and Control of Renewable Energy and Storage Systems, China Electric Power Research Institute, Beijing 100192, China
bKey Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
cCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100190, China

Corresponding authors:   *Corresponding author.E-mail address: yxlu@iphy.ac.cn (Y. Lu).

Received: 2018-11-5

Revised:  2018-12-20

Accepted:  2018-12-30

Online:  2019-07-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

Layered oxides are one of the most promising cathode materials for sodium ion batteries (NIBs), however, the relatively low working voltage hinders the increase of energy density thus limiting the application scenarios of NIBs. Here we prepared and investigated a series of Sn4+ substituted Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0.10, 0.20, 0.30, 0.33) and found that Sn-substitution can induce three effects: promoting O3-stack formation, smoothing the voltage profile and increasing the working voltage to ∼3.6 V. This study would enrich the knowledge of Sn-substitution and give guide to the better design of high-voltage cathode materials for NIBs.

Keywords: Sodium-ion batteries ; Layered oxide cathode ; Tin-based ; Cathode

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Xiaohui Rong, Fei Gao, Feixiang Ding, Yaxiang Lu, Kai Yang, Hong Li, Xuejie Huang, Liquan Chen, Yong-Sheng Hu. Triple effects of Sn-substitution on Na0.67Ni0.33Mn0.67O2[J]. Journal of Materials Science & Technology, 2019, 35(7): 1250-1254 https://doi.org/10.1016/j.jmst.2019.01.010

1. Introduction

NIBs have attracted more and more interest in recent years due to their abundant and wide-distributed resources on earth crust, becoming a promising option for large-scale storage [[1], [2], [3]]. NIBs have the same working principles called “rocking-chair batteries” with that of lithium-ion batteries (LIBs), while the energy density of NIBs is lower than that of LIBs because Li (6.9 g/mol) is lighter than Na (23 g/mol) and E°(Li+/Li) is 0.3 V below E°(Na+/Na) (-2.71 V vs. standard hydrogen electrode, E° is standard electrode potential). Cathode materials play a critical role in improving the energy density of NIBs, and there have been numerous cathode materials reported in a broad range, mainly including transition-metal oxides (layered oxides, tunnel oxides), polyanionic compounds (phosphates, pyrophosphates, fluorophosphates, sulfates, mixed polyanions) and Prussian blue [[4], [5], [6]]. Among them, layered oxides are one of the most promising candidates towards practical application owing to their convenient synthesis and relatively high energy density [3,5,7]. Layered oxides with the formula of NaxMO2 (0 < x ≤ 1, M = Li, Mg, Ti, Mn, Fe, Co, Ni, Cu, etc.) could be classified into two main groups: O3 and P2 types, according to the definition of Delmas et al. [8], in terms of the Na site (O for octahedral sites and P for trigonal prismatic sites) and the number of MO2 layers in a repeated stacking unit (2 or 3). There are some differences in the electrochemical behaviors between the compounds with two kind structures. P2-type materials have relatively lower capacity as the Na content (Na < 0.8) is usually lower than that of O3-type materials (0.8 < Na ≤ 1.0) [1,5,[9], [10], [11]]. Furthermore, with similar composition the average working voltage of P2-type materials is usually higher than the O3 counterparts. For example, the average voltage of P2-Na0.67Ni0.33Mn0.33O2 is ∼3.25 V (charge/discharge between 2.0-4.0 V vs. Na+/Na) [12], while for O3-Na1.0Ni0.5Mn0.5O2 it is ∼2.8 V (2.2-3.8 V vs. Na+/Na) [13].

Recently, several Ni/Sn-based layered oxides with O3-type structure showing abnormal high voltage are reported. Sathiya et al. found that the Sn-substitution in O3-Na1.0Ni0.5Mn0.5O2 can effectively smooth the charge/discharge voltage profile and increase average voltage (3.1 V for O3-Na1.0Ni0.5Sn0.5O2 and 2.8 V for O3-Na1.0Ni0.5Mn0.5O2 in half-cell) [14]. Rong et al. reported O3-Na0.9Ni0.4Mn0.1Sn0.5O2 delivers average half-cell voltage of 3.45 V [15] and applied a patent in which Na0.6Ni0.3Mn0.2Sn0.5O2 shows a high working voltage ∼3.7 V [16]. Rong et al. [17] and Guo et al. [18] reported O3-Na0.7Ni0.35Sn0.65O2 has the working voltage of 3.7 V vs. Na+/Na, which is the highest voltage among reported O3-type materials based on Ni2+/Ni3+ redox. The origin of voltage increase is the increase of ionicity of Ni-O after Sn-substitution [14,17]. Furthermore, Sn-substitution can significantly suppress the O3-P3 transition and could improve the cycling capability [14,15,17]. However, the influence of Sn-substitution on P2-type Ni/Mn-based materials is still unknown, since O3-type materials have many differences with P2-type materials in many aspects, as mentioned previously.

Since Na0.67Ni0.33Mn0.67O2 is a typical P2-type Ni/Mn-based cathode material, which has been widely investigated, we prepared a series Sn-substituted samples with various substitution amounts (Na0.67Ni0.33Mn0.67-xSnxO2, x = 0.00, 0.10, 0.20, 0.30, 0.33). Then further investigations were carried out to find how Sn-substitution affects the structure and electrochemical behaviors of P2-type Na0.67Ni0.33Mn0.67O2.

2. Experimental

Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0.00, 0.10, 0.20, 0.30, 0.33) were synthesized via solid-state reaction. Na2CO3 (99%, Alfa), NiO (99%, Alfa), MnO2 (99%, Alfa) and SnO2 (99%, Alfa) were weighted according to the designed stoichiometric ratio following by manually mixed. The mixed powder was then heated at 950 °C in air atmosphere for 15 h and cooled to room temperature naturally.

The structure of the as-synthesized samples was characterized using X-ray diffractometer (D8 Bruker) with Cu radiation (wavelength = 1.5405 Å) in the scan range (2θ) from 10° to 80°. Particle morphology and elements distribution were investigated with scanning electron microscopy (SEM, Hitachi-S4800) with energy-dispersive X-ray spectroscopy (EDS). Elementary composition of the samples was confirmed via inductively coupled plasma atomic emission spectrometry (ICP-AES). High-resolution transmission electron microscope (HRTEM) was recorded on a FEI Tecnai F20 transmission electron microscope.

70 wt% active material, 20 wt% Super P and 10 wt% polyvinylidene fluoride (PVdF) were used to prepare working electrodes with Al foil as current collector. CR2032 coin cells with sodium metal as anode were assembled in argon filled glove box, using 1 mol/L NaClO4 in ethylene carbonate (EC): propylene carbonate (PC): dimethyl carbonate (DMC) (1: 1: 1 in volume) as electrolyte. The charge and discharge tests were performed on LAND CT2001A battery test system.

3. Results and discussion

Different amounts of Sn4+ are designed to add into P2-Na0.67Ni0.33Mn0.67O2, with the substitution amount from 0.1 to 0.33 and the XRD results are shown in Fig. 1(a). With the increase of substitution amount, diffraction peaks of O3-type phase arise and a pure O3 phase is obtained when Sn substitution amount is 0.33. Till now, all Ni-Sn-based layered cathode materials are reported to be O3-type [4,5,14,15,17], indicating that Sn-substitution is helpful to form O3 stacking even though Na content is designed at a relatively low amount (e.g., 0.67). This phenomenon could be attributed to the almost same ionic radius of Sn4+ and Ni2+ (0.69 Å), which lowers the solubility of Mn4+ (0.53 Å) and Ni2+ (0.69 Å). Furthermore, introducing Sn4+ could also suppress the Ni/Mn ordering. Fig. 1c shows the illustrations of P2-type and O3-type structure, which shows intuitionistic differences between these two structures. The SEM image in Fig. 1(b) shows that the primary particle size is about 2 μm with irregular shape. HRTEM results shown as Figs. S1 and S2 in supporting information also confirmed the particle size and layered structure.

Fig. 1.   Structure and morphology of as-synthesized samples: (a) XRD patterns of as-synthesized Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0, 0.10, 0.20, 0.30, 0.33); (b) SEM image of Na0.67Ni0.33Mn0.33Sn0.33O2 sample; (c) illustration of P2-type and O3-type structure.

Energy dispersive spectroscopy (EDS) mapping of the O3-Na0.67Ni0.33Mn0.33Sn0.33O2 particles demonstrates that the Na, Ni, Mn and Sn elements are homogeneously distributed, as shown in Fig. 2. ICP-AES was carried out to investigate the actual composition of the synthesized Na0.67Ni0.33Mn0.33Sn0.33O2 sample, and the results are shown in Table 1, which demonstrates that the ICP results agree well with the designed compositions.

Fig. 2.   EDS mapping of Na0.67Ni0.33Mn0.33Sn0.33O2 sample: (a) SEM image; (b) EDS analysis; (c) elemental mapping of Na, Ni, Mn, and Sn.

Table 1   ICP-AES results of Na0.67Ni0.33Mn0.33Sn0.33O2 (at.%).

NaNiMnSn
Stoichiometric0.670.330.330.33
Experimental0.6810.3330.3180.348

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Electrochemical behaviors of the as-prepared Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0, 0.1, 0.2, 0.3, 0.33) samples were evaluated in half cells with Na metal as both counter and reference electrodes. Fig. 3 shows charge/discharge profiles of the first two cycles in the voltage range of 2.5-4.1 V at 0.1 C (10 mA/g). The initial discharge specific capacity of the five electrodes (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5) are 82 mA h/g (0.32 Na), 68 mA h/g (0.28 Na), 65 mA h/g (0.28 Na), 63 mA h/g (0.29 Na) and 66 mA h/g (0.31 Na) respectively. It could be found that the de-intercalated/intercalated Na amount decreases slightly after Sn-substitution and the overall discharge capacity of the five electrodes reduces gradually with the increase of Sn-substitution amount, which could be accounted for structure defects (associated with the coexistence of P2 and O3 structures) and the increase of formula weight (103.6, 110.0, 116.3, 122.7, 124.1 for x = 0.0 to 0.5, respectively) except for the element nature difference between Mn4+ and Sn4+.

Fig. 3.   Charge and discharge profiles of first and second cycles of (a) Na0.67Ni0.33Mn0.67O2, (b) Na0.67Ni0.33Mn0.0.57Sn0.10O2, (c) Na0.67Ni0.33Mn0.47Sn0.20O2, (d) Na0.67Ni0.33Mn0.37Sn0.30O2, (e) Na0.67Ni0.33Mn0.37Sn0.33O2, and (f) comparison of normalized discharge profiles of Na0.67Ni0.33Mn0.67-xSnxO2 (x = 0, 0.10, 0.20, 0.30, 0.33).

Since the Na de-intercalation/intercalation amounts are limited to the 0.33 range for the five electrodes (the Ni contents are 0.33), the charge/discharge capacity is mainly contributed by Ni2+/Ni3+ redox. Although the redox reaction in the five electrodes is almost the same, the shapes of charge/discharge profiles show obvious differences. For the P2-Na0.67Ni0.33Mn0.67O2 in Fig. 3(a), the charge/discharge voltage profiles show two-plateau feature, which is caused by Na/vacancy superstructure ordering since there are two types of Na sites with different migration capabilities in P2-type structure. When substituting 0.1 Mn4+ with Sn4+, as shown in Fig. 3(b), the two-plateau feature dramatically becomes less apparent. With further increase of Sn-substitution, both charge and discharge voltage profiles become much smoother (Fig. 3(b)-(e)), which indicates that the Na/vacancy ordering and phase-transition are significantly suppressed by Sn4+. Furthermore, the average discharge voltage notably rises from 3.34 V to 3.60 V with higher Sn content, as clearly observed from the corresponding charge/discharge voltage profiles. It is of impressive to find that the average discharge voltage of O3-Na2/3Ni1/3Mn1/3Sn1/3O2 is appreciably higher than that of P2-Na2/3Ni1/3Mn2/3O2, which is barely found in traditional systems (without Sn4+). For direct comparison, the discharge voltage profiles of the two materials with normalized capacities are demonstrated in Fig. 3(f), presenting the evolution of smoothing and the rising of discharge voltage. The rising of the average voltage is caused by the increase of ionicity of Ni—O since the electronic density on oxygen is increased by the formation of Sn—O ionic bond [14,17].

The rate performance of Na0.67Ni0.33Mn0.33Sn0.33O2 is shown in Fig. 4. With the increase of current rate, the polarization becomes larger (Fig. 4(a)). The specific discharge capacity for 0.1 C, 0.2 C, 0.5 C, 1.0 C and 2.0 C are 65 mA h/g, 61 mA h/g, 56 mA h/g, 53 mA h/g and 50 mA h/g, and the capacity returns to 65 mA h/g after resetting to 0.1C, showing high reversibility (Fig. 4(b)). Differential capacity curves of Na0.67Ni0.33Mn0.33Sn0.33O2 at different C-rates based on Fig. 4(a) are shown in Fig. 4(c) and there are one pair of oxidation/reduction peaks at ∼3.7 V/3.6 V corresponding to Ni2+/Ni3+ redox. Since the Na intercalation/de-intercalation amount is limited (∼0.3), the structure revolution is a simple single-phase transition from P3 to O3 (the P3 structure is usually preferred when Na content is less than 0.5 triggered by Na-vacancy ordering, also confirmed by ex-situ XRD shown in Fig. S3 [14]). With the increase of the current rate, the oxidation peak shifts to higher voltage and the reduction peak shifts to lower voltage, showing larger polarization. For the curves at 1 C and 2 C, a new reduction peak at ∼3.2 V arises which could be caused by the kinetic difference of P3 and O3 structure [19]. Cycling performance of Na0.67Ni0.33Mn0.33Sn0.33O2 is shown in Fig. 4(d), showing outstanding capacity retention of 89.3% after 100 cycles without optimization of electrolyte. The comparison of the cycling capability and average discharge voltage of Na0.67Ni0.33Mn0.67-xSnxO2 with different Sn substitution content is shown in Figs. S4 and S5, showing Na0.67Ni0.33Mn0.33Sn0.33O2 with high working voltage and good cycling stability. After that, the electrode was charged to 4.3 V to improve the capacity, for Ni3+ could be possibly further oxidized to Ni4+. A new plateau appears at ∼4.2 V, contributing a capacity of only ∼25 mA h/g, could correspond to a phase transition from P3 to O’3 [20]. Nevertheless, the plateau fades quickly even at the beginning of the first discharge, indicating the structure is extraordinarily unstable with less Na content. The cycling performances of the same electrode material cycled in two different voltage ranges of 2.5-4.1 V and 2.5-4.3 V are compared in Fig. 4(f). It could be found that the discharge capacity in two different voltage ranges quickly gets close after only ∼5 cycles and the discharge capacity in the wider voltage range decreases obviously after only ∼30 cycles, indicating increasing the upper cut-off voltage is not helpful to improve either energy density or cycling performance.

Fig. 4.   Rate and cycling behaviors of Na0.67Ni0.33Mn0.33Sn0.33O2 electrode: (a) charge and discharge profiles at various C-rates and (c) corresponding differential capacity curves; (b, d) discharge capacity and coulombic efficiency vs. cycle number from 0.1 C to 2 C and of long-term cycling behavior at 0.1 C; (e) initial charge and discharge profiles at voltage range of 2.5-4.1 V and 2.5-4.3 V; (f) comparison of long-term cycling behavior at 0.1 C at voltage range of 2.5-4.1 V and 2.5-4.3 V.

4. Conclusion

In summary, we synthesized a series of Na0.67Ni0.33Mn0.33-xSnxO2 materials with different Sn-substitution amounts and systematically investigated the effects on bulk structure, phase transition and electrochemical behavior. It is demonstrated in this work the Sn-substitution leads to at least three effects: i) inducing formation of O3-stacking by tuning the radii distribution of atoms in MO2 layer; ii) smoothing the voltage profile by inhibiting complex phase transitions and Na+/vacancy ordering; iii) increasing the working voltage to ∼3.6 V by increasing the ionicity of Ni—O bond. The exploration substituting Sn into P2-type NIBs cathode enriches the understanding of Sn-substitution and guides the better design of high-voltage layered oxides cathode materials for NIBs.

Acknowledgment

This work was supported financially by the Funding from the Science and Technology Project of the State Grid Corporation of China (No. DG71-16-027, research on key technology of low-strain layered oxides for long-life Na-ion batteries).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.01.010.

The authors have declared that no competing interests exist.


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