Synthesis and application of ultra-long Na_0.44MnO₂ submicron slabs as a cathode material for Na-ion batteries

Abstract

Novel ultra-long Na_0.44MnO₂ submicron slabs were fabricated through the sol–gel method, followed by high-temperature calcination. The material has a thickness ranging from 100 to 250 nm and a length varying from 10 μm to 40 μm. Electrochemical characterization indicates that the material can deliver a high capacity, larger than 120 mA h g⁻¹ with stable cycling over 100 cycles in assembled non-aqueous Na-ion cells, the good performance of which is mainly attributed to the reduced sodium ion diffusion distance.

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Among various energy storage devices, Li-ion batteries have attracted most research attention mainly due to their high energy density and relatively high power density.¹ However, the high cost of lithium salts and safety issues have been barriers to the wide application of these batteries in electric vehicles and grid storage. Based on the abundant availability and low cost of sodium, ambient-temperature sodium-based batteries have shown great potential to replace the Li-ion devices for green energy storage needs.²

There are still great challenges for the performance of Na-ion batteries to catch up with Li-ion technology; thus, it is essential to explore new high-capacity sodium-based energy storage materials or to improve the known sodium compounds suitable for Na-ion cells.³ The sodium manganese oxides such as NaMnO₂, Na_0.67MnO₂, Na_0.60MnO₂, Na_0.44MnO₂ and other Na compounds (Na_xMn_yA_z, A = transition metal) have been considered as promising cathode materials owing to their high capacity and low cost.^4–9 In general, sodium manganese oxides mainly have two kinds of structure, layered, such as Na_0.67MnO₂, or tunnelled, e.g. Na_0.44MnO₂ structure, and both have large number of vacancies for the accommodation of Na ions.¹⁰ Among them Na_0.44MnO₂ is particularly attractive because of its unique 3D crystal structure, which greatly facilitates Na⁺ mobility while stabilizing sodium ions to prevent the crystal phase from transition to spinel phase.

Na_0.44MnO₂ materials can be synthesized via various approaches such as the solid-state route,^11–17 glycine–nitrate combustion,^11,14,15 hydrothermal synthesis,^18,19 thermo-chemical conversion,²⁰ polymer-pyrolysis²¹ and molten salt synthesis.²² Sauvage et al.¹³ prepared Na_0.44MnO₂ following the solid-state synthesis route, which showed a reversible insertion and extraction of Na ions in Na_0.44MnO₂ with an initial capacity of 80 mA h g⁻¹ at a 0.1 C rate; however, only half of the initial capacity was retained after 50 cycles, displaying poor cyclability, mainly due to failure of the Na_0.44MnO₂ lattice caused by extensive Na ion insertion and extraction. Na_0.44MnO₂ nanowires obtained by different approaches such as the solid-state method,⁵ polymer pyrolysis process,¹⁷ a hydrothermal approach¹⁸ and a chemical route exhibit very large surface area, single crystal powders and improved capacity cyclability. However, the solid-state reaction approach as discussed above has poor dispersion and large particle size, affecting battery performance. Both Na_0.44MnO₂ cathode material and the sol–gel method used for the fabrication are well known but efforts with sol–gel methods are still used frequently to create different unique structured Na_0.44MnO₂ materials.^20,22–24 In this work, we used a sol–gel method by using a different processing procedure, as well as a different chelating agent, to control the material morphology and structures for the synthesis of a slab-like Na_0.44MnO₂ material with good dispersity, ultra-long length and much lower thickness in the sodium ion deintercalation direction for a high capacity, over 120 mA h g⁻¹, and stable cycling performance of over 100 cycles. It is also revealed that the diffusion distance of sodium ions is reduced in the obtained material to effectively improve the performance of a sodium ion battery.

To synthesize the unique Na_0.44MnO₂ samples, a stoichiometric amount of NaNO₃ (with a 10% excess of sodium) and Mn(NO₃)₂ (50 wt% aq.) were dissolved in distilled water and stirred for 30 min to form a mixed solution. Then, the mixed solution was added dropwise into a 60 wt% citric acid solution. The resulting solution was heated to 90 °C and stirred for 4 h to obtain a clear and viscous gel. The resultant gel was dried at 120 °C for 12 h to produce the precursor. The solid precursor was then ground and heat-treated at 450 °C in air for 6 h to decompose the nitrate and eliminate the water. After cooling down to room temperature, the powdered precursor was ground again, pelletized and then calcined at 900 °C for 15 h in air to obtain the final product. The crystal structure of the obtained samples was characterized by powder X-ray diffraction (XRD, MAXima-X XRD-7000). Furthermore, the morphology and microstructure were examined by field-emission scanning electron microscopy (SEM, JSM-6700F) and transmission electron microscopy (TEM, JEM-2100). For the electrochemical test, the cathode materials were evaluated in 2032-type coin cells using a Na disk as counter electrode and 1 M NaClO₄ in ethylene carbonate–diethyl carbonate (EC–DEC, 50 : 50 vol%) solution as electrolyte. The cathode film was prepared by mixing the active material (80 wt%), acetylene black (10 wt%) and polyvinylidene fluoride (PVdF, 10 wt%) in N-methyl-pyrrolidinone (NMP) into an electrode slurry, and then casting the slurry on an aluminium foil collector and drying overnight in a vacuum at 120 °C. The assembly of the cells was carried out in a dry Ar-filled glove box. The galvanostatic charge–discharge tests were performed on a LAND cycler (Wuhan Kingnuo Electronic Co., China). Cyclic voltammetric measurements were carried out at scan rates of 0.25 mV s⁻¹ and 0.1 mV s⁻¹ using a CHI 660e electrochemical workstation (Chen Hua Instruments Co., China).

Na_0.44MnO₂ is isostructural to Na₄Mn₄Ti₅O₁₈ and crystallizes in an orthorhombic structure (space group: Pbam). Fig. 1a schematically shows that the framework of Na_0.44MnO₂ is built on MnO₅ pyramids and MnO₆ octahedra. These two kinds of polyhedra are interconnected by sharing corners to form two types of unique 1D tunnels. Thus, the prepared Na_0.44MnO₂ is a 3D crystal structure with interconnected unique 1D tunnels as reported in previous work.^10,25 Na1 sites are located in the smaller tunnels while Na2 and Na3 are situated in the large S-shaped tunnels. These tunnels serve as diffusion paths for free transport of Na⁺ ions, mainly along the c-axis direction. The XRD pattern in Fig. 1b shows that the prepared Na_0.44MnO₂ material crystallizes well in the orthorhombic Na₄Mn₉O₁₈ phase (PDF card no.: 27-0750).

Fig. 1 (a) Crystal structure model of orthorhombic Na_0.44MnO₂ (adapted from ref. 20). (b) XRD pattern of as-prepared Na_0.44MnO₂ materials.

The SEM image shows that pristine Na_0.44MnO₂ has a slab-like shape with scales of >20 μm (length) × ∼1 μm (width) × ∼100–250 nm (thickness) (Fig. 2a). The SEM and TEM images in Fig. 2b clearly show there is a wide range of width in Na_0.44MnO₂ varying from submicron to micron. The high-resolution TEM image (HRTEM) in Fig. 2c displays clear lattice fringes corresponding to the (200) plane of Na_0.44MnO₂ while indicating that the through-thickness orientation is along [001], which is consistent with Zhou's report.²⁰ As discussed above, the [001] plane is perpendicular to the c axis allowing sodium ion deintercalation along the thickness direction. Fig. 2d schematically represents the morphology and sodium ion deintercalation mechanism. In the synthesis conditions described in the experimental section, sodium manganese oxide tends to a growth direction of [010], which is along the b axis direction for formation of a flat topography. During the charge–discharge cycles, sodium ion intercalation and deintercalation should go through the large S-shaped channel along the c axis direction. This in turn tells us that reduction of the thickness in the c axis direction can reduce the diffusion distance of sodium ions, which is a known significant parameter for greatly enhancing the cycling and rate performance of an ion intercalation-based battery. The diffusion coefficient (D_Na⁺ = 2.64 × 10⁻¹⁴ cm² s⁻¹) was calculated from our measured impedance data based on the reported method.¹⁰

Fig. 2 (a) SEM images (the top right inset shows a high-magnification image), (b) TEM image, (c) high-resolution TEM image and (d) morphogenesis and sodium ion deintercalation mechanism of the Na_0.44MnO₂ submicron slabs.

Fig. 3a shows the galvanostatic charge–discharge capacities at a 0.1 C rate, indicating excellent cyclability with a capacity retention of 100% even after 100 cycles and a coulombic efficiency of more than 90%. It should be noted that the coulombic efficiency only reaches 90% after the 10^th prolonged cycle (Fig. 3a). This is possibly attributed to the slow electrode activation for the electrolyte to completely penetrate into the inner porous material surface. The different galvanostatic charge–discharge cycles (Fig. 3b) show that the specific capacity of the first charge cycle is higher than the theoretical capacity (121 mA h g⁻¹), whereas the closely following discharge cycle has one close to the theoretical value. The extremely high capacity in the first charge cycle could be ascribed to extraction of the excess Na ions from Na1 and Na2 sites in the as-prepared material as reported.²⁰ However, in the consecutive cycles the amount of Na ions deintercalated from the electrode should be decreased to the thermodynamic limit for a stable crystal structure.

Fig. 3 (a) Galvanostatic charge–discharge cyclability and coulombic efficiency, (b) different galvanostatic charge–discharge cycles, (c) cyclic voltammogram plots before and after cycles of the cathode samples against the Na electrode at a scan rate of 0.1 mV s⁻¹, (d) schematic illustration for the oxidation–reduction peaks shift before and after cycles, and (e) XRD pattern and SEM image (inset) of as-prepared Na_0.44MnO₂ materials after 100 cycles at 0.1 C.

Fig. 3b illustrates that each charge and discharge cycle has multiple steps, which is in agreement with the reported six biphasic transitions.¹³ For further examination of the electrochemical characteristics and cyclability of the prepared sample we conducted cyclic voltammetry (CV) studies before and after cycles. The CV curves in Fig. 3c also indicate reversibility of the six biphasic transitions during the anodic and cathodic sweeps, as observed from the galvanostatic charge–discharge cycles with their steps, and show very consistent reversibility of Na insertion and extraction before and after cycles. The distance between the discharge and charge curves is small, indicating a low electrochemical polarization.²⁶ As can be seen in Fig. 3d, the difference ΔE_{after cycles} between oxidation and reduction peaks increases after the cycles. This is caused mainly by the oxidation peak being more positive than that before cycling, indicating deintercalation polarization developing with the increased number of cycles while the intercalation process does not show significant polarization.

A rationally fabricated structure of the electrodes is able to play an essential role in the improvement of electrode kinetics for better performance of an ion intercalation-based battery, which requires a nanostructure to have not only more activation sites for reversible ion inter-/deintercalation processes but also enhanced mass transport by reduced diffusion distances for Na ion intercalation during the charge–discharge cycles. Although the length and width of the slabs of the Na_0.44MnO₂ crystals are in the submicron to micron range, their thickness direction along the [001] c axis could facilitate easy intercalation of Na ions to achieve the theoretical capacities while accommodating the strains from Na ion insertion into the material. Fig. 3e shows that the XRD pattern scarcely has any change, in addition to the intensity of the peaks, thus evidencing that the submicron slabs can retain their intact crystal structure for long charge–discharge cycles.

In summary, ultra-long Na_0.44MnO₂ submicron slabs were fabricated through the sol–gel method, followed by high-temperature calcination. Results show that a Na_0.44MnO₂ submicron slabs-based Na-ion battery achieves its theoretical capacity value over 100 cycles, which is attributed mainly to decreased sodium ion diffusion distance and the stable crystal structure.

Acknowledgements

This work is financially supported by Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energies under cstc2011pt-sy90001, the Start-up grant under SWU111071 from Southwest University and the Chongqing Science and Technology Commission under cstc2012gjhz90002. The work is also supported by grants from the National Natural Science Foundation of China (no. 21063014, 21163021 and 51101130), Fundamental Research Funds for the Central Universities (SWU113079, XDJK2014C051).

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Footnote

† These authors contributed equally to this work.

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