Synthesis of a novel tunnel Na_0.5K_0.1MnO₂ composite as a cathode for sodium ion batteries

Abstract

A novel tunnel Na_0.5K_0.1MnO₂ composite assembled by two different tunnel structures of Na_0.44MnO₂ and KMn₈O₁₆ is synthesized by a co-precipitation method. Bundles of microrods and small nanorods could be observed in the Na_0.5K_0.1MnO₂ composite. The composite possesses high crystallinity and large stacking faults. When used as a cathode for sodium ion batteries, the composite exhibits a high specific capacity, excellent cyclability and superior rate capability. A high reversible discharge capacity of 142.3 mA h g⁻¹ could be delivered at 0.1C, with 94.7 mA h g⁻¹ retained after 100 cycles. Also 82.2 mA h g⁻¹ could be maintained after 300 cycles at 1.0C. More than 70 mA h g⁻¹ could be obtained at a high rate of 4.0C. The outstanding electrochemical performances may be attributed to the combined tunnel structures, one-dimensional rod-like morphology and massive structural stacking faults.

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1. Introduction

During the past decades, great development of lithium ion batteries (LIBs) has been achieved due to the huge efforts of numerous researchers. Lithium ion batteries have dominated the field of electronic equipment, as well as the successful application in hybrid electrical vehicles (HEVs), electrical vehicles (EVs) and energy storage stations.^1–3 However, the limited natural lithium reserves and relatively high cost may severely hinder the application of LIBs in large-scale renewable energy storage. Sodium ion batteries (SIBs), with the advantages of their chemical similarities with LIBs, natural abundance, even distribution of sodium reserves and low toxicity, have been regarded as a feasible alternative technology.^4–9

Various cathode materials for SIBs have been developed, including layered oxides,^10–12 polyanionic compounds^13–15 and other oxides.^16,17 Manganese (Mn) based oxides have been recognized as one of the prominent cathodes due to their attractive features of natural abundance, low-cost and non-toxicity.¹⁸ Among the Mn based oxides, tunnel Na_0.44MnO₂, with some outstanding properties such as relative high capacity, good cycling stability, reversible cyclability in aqueous electrolyte¹⁹ and excellent rate capability, has attracted much attentions.^20–26 Some pioneer works with encouraging results about tunnel Na_0.44MnO₂ have been done. Sauvage et al. synthesized pure Na_0.44MnO₂ via a solid state reaction method, with an initial capacity of 80 mA h g⁻¹ at 1/10C rate between 2.0 and 3.8 V (vs. Na/Na⁺).²² Cao and his coworkers prepared Na_0.44MnO₂ nanowires and demonstrated that the controlled structure could obviously enhance the electrochemical properties.²⁶ The Na_0.44MnO₂ nanowires electrode delivered 128 mA h g⁻¹ at 12 mA g⁻¹ with outstanding cyclability between 2.0 and 4.0 V (vs. Na/Na⁺). Nevertheless, the nanowire was obtained by a polymer-pyrolysis method, which is complicated and expensive. Some other researches had also proved the prominent application of Na_0.44MnO₂ material in SIBs.^20,27,28 But it's still challenging to produce Na_0.44MnO₂ material with high capacity and excellent cycling stability through a cost effective method. Meanwhile, it's noticeable that some previous reports have indicated that the cryptomelane-type KMn₈O₁₆ with tunnel structure could be a suitable cathode for LIBs.^29,30 The KMn₈O₁₆ microclusters assembled by nanofiber also showed reversible high reversible capacity of 360 mA h g⁻¹ at 100 mA g⁻¹ and good cycling stability.³⁰ What's more, the superior cyclability of KMn₈O₁₆ with wide voltage range of 1.5–4.2 V indicate a high structural stability. Considering the similarities between Na ion and Li ion, excellent electrochemical performances of KMn₈O₁₆ in SIBs could be expected. Recently, the great research results about layer-spinel cathode materials^31–33 in LIBs show the effective strategy to enhance the electrochemical performances by introducing the interaction between different structures. Based on these review, an attempt to combine the two tunnel structures could be carried out.

Here, we synthesized rod-like Na_0.5K_0.1MnO₂ composite by a simple co-precipitation method. The Na_0.5K_0.1MnO₂ composite is composed of two different tunnel structures (Na_0.44MnO₂ and KMn₈O₁₆). A high reversible capacity of 144.7 mA h g⁻¹ is delivered at 0.1C, with 94.7 mA h g⁻¹ retained after 100 cycles. And 82.2 mA h g⁻¹ could be maintained at 1.0C after 300 cycles. More than 70 mA h g⁻¹ could be obtained at a high current density of 4.0C. The results demonstrate that the composite possesses high capacity, excellent cycling performance and superior rate capability.

2. Experimental

2.1 Preparation of Na_0.5K_0.1MnO₂

Na_0.5K_0.1MnO₂ composite was synthesized by co-precipitation method. Typically, analytical reagent grade NaCH₃COO·3H₂O, KCH₃COO and Mn(CH₃COO)₂·3H₂O in a molar ratio of 0.5 : 0.1 : 1 and in quantities corresponding to 0.05 mol of the target Na_0.5K_0.1MnO₂ were dissolved in 100 ml of deionized water. 12.61 g of H₂C₂O₄·2H₂O, which was dissolved in another cup of 100 ml of deionized water, was added to the mixed solution of K, Na, Mn acetate. Then, the water was evaporated at 80 °C to afford a milky white precursor. After dried at 80 °C overnight, the precursor was pressed into pellets and preliminaries annealed at 450 °C for 6 h with subsequent heat treatment of 800 °C 15 h. Finally, the pellet was quenched to room temperature in liquid nitrogen and stored in the glove box with Ar atmosphere.

2.2 Characterization and electrochemical measurements

The Chemical composition of the sample was determined by using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Thermo Electron IRIS Intrepid II XSP). Thermogravimetric analysis was performed on a simultaneous thermal analysis apparatus (SDT Q600, TA instrument). The morphology and structure of the as-prepared sample were characterized by field emission scanning electron microscopy (SEM, HITACHI S-4800), transmission electron microscopy (TEM, JEM 2100), Raman system (Xplora, Horiba), and powder X-ray diffraction (XRD, Philips X'pert Pro Super X-ray diffract meter, Cu Kα radiation) measurements. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI QUANTUM 2000 instrument. Electrodes of sodium half-cell were made by spreading a mixture of 80 wt% Na_0.5K_0.1MnO₂ active material, 10 wt% acetylene black and 10 wt% PVDF onto aluminum foil current collectors. The mass loading of the active materials is 2.0 mg cm⁻². The as-prepared electrodes were dried at 80 °C in a vacuum oven for 12 h. Electrochemical properties of the electrodes were monitored by assembling them into coin cells (type CR2025) in an argon-filled glove box with water and oxygen contents less than 5 ppm. Metallic Na was used as the counter electrode and glass fiber (GF/A, Whatman) as separator. The electrolyte was made of NaClO₄ (1 mol l⁻¹) and a mixture of PC/EC in a volume ratio of 1 : 1 (purchased from Fosai New Materials Co., Ltd., Jiangsu, China). The cells were galvanostatically charged and discharged on a battery test system (LAND-2001A, Land Electronic Co., Ltd., Wuhan, China) with a cut-off voltage range of 1.5–4.3 V (vs. Na/Na⁺). Cyclic voltammetry (CV) was conducted on a CHI 660D electrochemical workstation (CH Instruments Co., Ltd., Shanghai, China) using coin cell at a scan rate of 0.2 mV s⁻¹ with the same cut-off voltage range for electrochemical tests were conducted at 30 °C. The cells charged/discharged to a certain voltage were disassembled in glove box and the active material electrodes were taken out, washed with dimethyl carbonate and dried. The ex situ XRD patterns were also collected on Philips X'pert Pro Super X-ray diffract meter.

3. Results and discussion

The crystal structure of tunnel Na_0.44MnO₂ with space group of Pbam is shown in Fig. 1a. Four MnO₆ octahedral sites and one MnO₅ square-pyramidal site construct the framework structure of Na_0.44MnO₂. And a double-tunnel structure is built up from double and triple chains of MnO₆ octahedral and single chains of MnO₅ square-pyramids by either edge or corner sharing. Four crystallographically distinct Na⁺ sites exist within the tunnel of Na_0.44MnO₂. Na1 locates in the six-sided tunnel, while the others (Na2, Na3 and Na4) reside in the large S-shaped tunnel (Fig. 1a). Sodium ions in large tunnels are highly mobile.^23,34 The unique large and double tunnel structure can also help to tolerate structural strains during charge–discharge processes, which can offer outperformed cyclability. The cryptomelane KMn₈O₁₆ shows an I4/m tetragonal structure with a distinct tunnel feature. The frame structure of KMn₈O₁₆ is built up by double chains of edge-sharing MnO₆ octahedral, forming tunnel structures, with K ions situated in the large tunnels (Fig. 1b). The K ions can not only maintain the structure, but also increase the ion diffusion rate.³⁵

Fig. 1 Schematic crystal structures of Na₄Mn₉O₁₈ (a) and KMn₈O₁₆ (b).

Fig. 2a shows TG/DTG curves of precursor at a heating rate of 10 °C min⁻¹ from room temperature to 1000 °C under air. The weight loss before 150 °C is attributed to the dehydration of the precursor. And further thermal decomposition occurred at around 300 °C. The final weight loss between 300 and 700 °C may be ascribed to the crystallization of the product. No obvious weight loss could be found after 700 °C. According to the results, the pre-calcination was carried out at 450 °C for 6 h to guarantee the complete thermal decomposition of the precursor. And further heat treatment at 800 °C for 15 h was applied to enhance the crystallinity of the final product. The molar ratio of Na, K, Mn is 0.498 : 1.003 : 1, which is close to the feed ratio of raw materials.

Fig. 2 TG and DTG curves of precursor (a) and Rietveld refinement of XRD patterns of Na_0.5K_0.1MnO₂ (b).

Rietveld refinement result of XRD pattern with PDXL program of Na_0.5K_0.1MnO₂ is given in Fig. 2b. Most of the diffraction peaks can be indexed to orthorhombic Na_0.44MnO₂ (PDF no. 00-027-0750) and tetragonal KMn₈O₁₆ (PDF no. 00-012-0706). And no obvious impurity could be found. The lattice constants of Na_0.44MnO₂ are a = 9.092 Å, b = 26.505 Å, c = 2.827 Å, which are in accord with previous reports.^20,36,37 the lattice parameter of KMn₈O₁₆ are a = b = 9.914 Å, c = 3.194 Å. The mass ratio of Na_0.44MnO₂ and KMn₈O₁₆ is 86.6% : 13.4%.

Fig. 3 displays the high resolution XPS spectra of Na_0.5K_0.1MnO₂. As shown in Fig. 3a, in addition to the C 1s level observed at 284.8 eV, two strong peaks appear at higher binding energy (>290 eV), which could be assigned to K 2p_3/2 and 2p_1/2. The observation of spin–orbital levels gives a direct evidence for the existence of potassium in Na_0.5K_0.1MnO₂ composite.³⁸ Mn 2p spectra with Gaussian fitting result indicates that energies centered at 641.9 eV, 640.5 eV, 653.7 eV and 652.4 eV could be ascribed to the presence of tetravalent Mn and trivalent Mn, which is agree with the crystal structure and some previous reports.^39–41

Fig. 3 High resolution XPS spectra with Gaussian fitting for C 1s and K 2p peaks (a) and Mn 2p (b).

Further structural information was obtained by Raman measurement with a 532 nm lasers as the excitation light (Fig. 4). The spectral shape and peaks locations are consistent with the previous reports, which confirm the existence of orthorhombic Na_0.44MnO₂.²⁰ Band at ∼640 cm⁻¹ maybe assigned to stretching vibrations of Mn–O, while the band at ∼360 cm⁻¹ is due to the bend vibrations of O–Mn–O.^20,42

Fig. 4 Raman spectra of Na_0.5K_0.1MnO₂.

Particles with two different morphologies could be observed in Na_0.5K_0.1MnO₂ composite (Fig. 5a). The large rod-like bundles assembled by microrods with a width of ∼200 nm and ∼10 μm in length could be identified (Fig. 5a and b), which could be attributed to sinter at high temperature.²⁰ Meanwhile, small nanorods with a diameter of 50 nm and a length of 500 nm were also clearly investigated (Fig. 5c). Large stacking faults could be detected in the microrods (Fig. 5d), which could be related to the introduction of KMn₈O₁₆ and maybe enhance the electrochemical performance.⁴³ Lattice fringes in the HRTEM image (Fig. 5e) of the microrods are ∼0.53 nm, corresponding to the interplanar distance of the (1 4 0) plane in Na_0.44MnO₂. The lattice fringes of ∼0.312 nm in small nanorods (Fig. 5f) can be ascribed to the (1 3 0) plane in KMn₈O₁₆. The Fast Fourier Transformation (FFT) results furtherly demonstrate the existence of Na₄Mn₉O₁₈ with orthorhombic lattice structure (Pbam space group) and KMn₈O₁₆ with tetragonal structure (I4/m space group). The characterization of morphology furtherly demonstrate that the tunnel Na_0.5K_0.1MnO₂ is composed of two kinds of particles with separately orthorhombic or tetragonal structure, which could be related to the big differences between the two structures.

Fig. 5 SEM images (a–c). TEM image (d), HRTEM images (e, f) and SAED patterns (inset e, f) of Na_0.5K_0.1MnO₂.

Fig. 6a shows cyclic voltammetry plots of Na_0.5K_0.1MnO₂ composite at a scan rate of 0.2 mV s⁻¹ between 1.50 and 4.30 V. Some differences could be found between the first cycle and the subsequent cycles. The oxidation peaks located at 3.16 V and 4.30 V are much stronger than that of the subsequent processes, which indicate some degree of irreversible reactions. The initial irreversible processes could be attributed to the multiatomic transition processes to relieve the structural strain for Na ion insertion–extraction and the surface reaction between the electrolyte and electrode. However, another two oxidation peaks between 3.24 and 3.60 V became stronger in the following cycles, which due to the activation process of KMn₈O₁₆ in the first cycle.²⁹ After the first scan, the consequent CV curves display several pairs of symmetrical redox peaks, denoting the complex biphasic transition mechanism during charge–discharge process that had not been clearly identified. It is considered that the presence of multi-phase states is strongly related with Na⁺/vacancy ordering, which is shown in Fig. 1.^20,44 It is interesting that the tunnel structure Na_0.5K_0.1MnO₂ material possesses relatively fewer redox peaks than normal Na_0.44MnO₂,²⁷ which could indicate that the introduction of KMn₈O₁₆ would smoothen the CV curves.

Fig. 6 Electrochemical characterization and battery performance of Na_0.5K_0.1MnO₂: cyclic voltammetry curves (a); cycling performances at 0.1C and 1C (b); typical charge–discharge curves between 1.5 and 4.3 V at 0.1C (c); rate capability (d); representative charge–discharge curves at different C rates (e); cycling performance at 1.0C between 2.0 and 4.0 V (f).

Fig. 6b demonstrated the excellent electrochemical performances of Na_0.5K_0.1MnO₂ with the cut-off voltage window of 1.5–4.3 V. A high reversible capacity of 142.3 mA h g⁻¹ could be delivered at 0.1C (20 mA g⁻¹), which is the highest result to the best of our knowledge. The high capacity could be ascribed to the existence of KMn₈O₁₆ and the stacking faults, which provide more accessible sites of Na ions.³⁰ The composite also performed outstanding cycling stability, with 94.7 mA h g⁻¹ maintained after 100 cycles. The coulombic efficiency reached 93% during the second cycle and further increased to >98% after 10 cycles. When cycled at 1C (200 mA g⁻¹), 104.2 mA h g⁻¹ was showed and 82.2 mA h g⁻¹ was retained after a long-term cycling of 300 cycles. The representative charge–discharge curves at 0.1C were arrayed in Fig. 6c. In the initial cycle, the discharge capacity is much higher than the charge capacity, which is consistent with the CV results, indicates more Na ions could be stored in the orthorhombic lattice,⁴⁴ and shows the activation process of KMn₈O₁₆.²⁹ Some short voltage plateaus were observed in the successive charge–discharge curves, which could be assigned to the symmetrical redox peaks as shown in CV profiles. It also can be observed that the Na_0.5K_0.1MnO₂ composite showed much smoother charge–discharge curves in comparison with previous reports,²⁰ which is also in accord with CV results. What's more, the Na_0.5K_0.1MnO₂ composite possesses not only high reversible capacity and excellent cycling stability, but also good rate capability (Fig. 6d). The test was started from 0.1C and successively increased to 0.5C, 1.0C, 2.0C and 4.0C. And the respectively average capacities are 129.4 mA h g⁻¹, 106.7 mA h g⁻¹, 98.9 mA h g⁻¹, 89.9 mA h g⁻¹ and 71.1 mA h g⁻¹. When the current density is back to 0.1C, more than 120 mA h g⁻¹ could be recovered, which demonstrated high reversibility. The selected charge–discharge curves at different C rates are shown in Fig. 6e. The voltage plateaus could be detected even at 4.0C, which demonstrate small electrode polarization. The outperformed rate capability is superior or at least comparable to the previous reports of Na_0.44MnO₂ and other tunnel structure composite,^20,26 which may be attributed to the fast diffusivity of Na ions in the novel tunnel structure of Na_0.44MnO₂ and KMn₈O₁₆, as well as the one-dimensional morphology including microrods and nanorods. The results could also demonstrate that the K ions in the tunnel structure could increase the Na ion diffusion rate. In order to compare with some previous reports, the cycling performance between 2.0 and 4.0 V was also conducted. As shown in Fig. 6f, a discharge capacity of 75 mA h g⁻¹ was observed after 500 cycles at 1.0C. The capacity increment phenomenon is maybe caused by the slow activation process in the narrow voltage range. And the cycling performance presented here is also at least comparable with previous reports.^20,26 The enhanced electrochemical performances could be related to the highly crystalline structure, suitable particles size and incorporation of KMn₈O₁₆. The highly crystalline structure and coexistence of KMn₈O₁₆ could lead to reversible phase transition during charge–discharge process and help maintain the structure stability in a relatively wide voltage range of 1.5–4.3 V. The rod-like morphology could effectively accommodate the structure strain during repeated Na ion insertion and extraction process. What's more, the one dimensional rods and K ions in the tunnel structure of KMn₈O₁₆ could also facilitate the diffusion of Na ions.

In order to analyze the structural evolution during the Na ion insertion and extraction process, the ex situ XRD measurement in the different charge–discharge stages at 0.1C were carried out as shown in Fig. 7. It is hard to clearly identify the appearance of a new phase, which is in accord with the results of Sauvage et al. and Cao et al. Unlike previous reports of Na_0.44MnO₂, the (3 5 0) reflection is split into (3 5 0) and (0 10 0) in the pristine XRD pattern, which is in accord with the study result of Chu et al. The portion of the (0 10 0) changed at different charge–discharge stages, which could be attributed to the evolution of the cell parameters. What's more, the location of diffraction lines belonged to KMn₈O₁₆ kept unchanged during the charge–discharge process, which indicates high structure stability. The ex situ XRD results indicate that different intermediate structures during complex phase transitions in the respectively charge–discharge stages are closely related, demonstrating a biphasic transition Na ion insertion–extraction mechanism. However, the detailed influence of incorporation of KMn₈O₁₆ still needs more investigation.

Fig. 7 Ex situ XRD patterns of Na_0.5K_0.1MnO₂ with the voltage range of 1.5–4.3 V in the first charge–discharge process.

Morphology change of the electrode after 300 cycles at 1.0C is presented in Fig. 8a. No cracks appeared on the electrode, which indicate excellent mechanical stability. The rod-like morphology could be preserved after 300 cycles at 1.0C, demonstrating high structural stability.

Fig. 8 Morphology of electrode after 300 cycles at 1.0C.

4. Conclusions

In summary, a novel composite Na_0.5K_0.1MnO₂ composed of orthorhombic Na_0.44MnO₂ and tetragonal KMn₈O₁₆ was prepared by co-precipitation method. Both of two phases contain large tunnel structure, which is beneficial for the insertion–extraction and diffusion of Na ions. The one-dimensional rod-like morphology and suitable particle size also help enhance the electrochemical performances in SIBs as cathode. The novel composite possesses high capacity, excellent capability and outperformed rate capability. A high reversible capacity of 142.3 mA h g⁻¹ could be delivered at 0.1C, with 94.7 mA h g⁻¹ retained after 100 cycles between 1.5 and 4.3 V. The material could still deliver more than 70 mA h g⁻¹ at 4.0C. When cycled between 2.0 and 4.0 V, a discharge capacity of 75 mA h g⁻¹ was observed after 500 cycles at 1.0C. The ex situ results demonstrate the high structure stability of Na_0.44MnO₂ and KMn₈O₁₆. The results indicate that the composite could be prominent cathode for SIBs used in large-scale energy storage. And the study also proved a possible strategy to enhance the electrochemical performances of SIBs' cathode material by combining different structures.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21373008, 21321062 and 21273184) and Natural Science Foundation of Fujian Province of China (No. 2015J01063).

Notes and references

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