Modulating the kinetics of CoSe2 yolk–shell spheres via nitrogen doping with high pseudocapacitance toward ultra-high-rate capability and high-energy density sodium-ion half/full batteries

Jitao Geng a, Shiyu Zhang a, Edison Huixiang Ang b, Jia Guo a, Zhihua Jin a, Xiao Li a, Yafei Cheng *a, Huilong Dong *a and Hongbo Geng *a
aSchool of Materials Engineering, Changshu Institute of Technology, Changshu, Jiangsu 215500, China. E-mail: yafeicheng1990@163.com; huilong_dong@126.com; hbgeng@gdut.edu.cn
bNatural Sciences and Science Education, National Institute of Education, Nanyang Technological University, Singapore, Singapore

Received 5th May 2021 , Accepted 15th July 2021

First published on 20th July 2021


Abstract

Developing advanced anode materials with high capacity, and good rate and cycling performance for sodium-ion batteries still remains a major challenge at the moment. Herein, it is demonstrated that the introduction of nitrogen elements into CoSe2 yolk–shell spheres (N-CoSe2 yss) with rapid electrochemical kinetics contributes to excellent sodium storage performance. Specifically, the N-CoSe2 yss deliver a high capacity of 431 mA h g−1 at a high current density of 50 A g−1 and a high reversible capacity of 500 mA h g−1 after 1000 cycles at 10 A g−1 with nearly 100% coulombic efficiency. Experimental and density functional theory calculation results indicate that the superior electrochemical performance can be attributed to the synergistic effect of nitrogen doping and structure engineering, which can not only enhance the electronic/ion transport kinetics, but also improve the structural stability during cycling. When coupled with a high-voltage cathode, the full cell shows a high energy density of 129.38 W h kg−1 at a power density of 187.5 W kg−1, confirming the potential applications of N-CoSe2 yss in high-performance sodium-ion batteries.


1. Introduction

Transforming the intermittent energy into hydrogen through water splitting and developing advanced energy storage devices are the two effective strategies to make more usage of intermittent energy sources.1–4 Among the various energy storage devices, lithium-ion batteries (LIBs) have proven to be commercially successful, but the security issues and the poor lithium reserves seriously hinder their application in electric vehicles and large-scale equipment.5–8 To date, owing to the rich sodium resources and similar energy storage mechanism, sodium-ion batteries (SIBs) have become one of the best alternatives to LIBs for wholescale energy storage systems.9–11 However, the larger radius of sodium ions (0.112 nm) leads to adverse effects, such as the expansion of the host materials and slow diffusion kinetics during the process of sodium ion insertion/extraction.12,13 The resulting defects lead to a rapid capacity decline and poor cycling stability, which severely hinders the development of SIBs.14–16 To date, although a large amount of research has focused on developing cathode materials, the search for high-performance anode materials for SIBs is still an unsolved major challenge. Transition metal selenides (TMSes), such as FeSe2, NiSe2, CoSe2, MnSe2, MoSe2, Cu2Se and ZnSe, have recently attracted much attention as SIB anode materials due to their favorable physicochemical properties.17–23 Because of its superior sodium storage performance, CoSe2 has been widely investigated in sodium storage.24–27 Nevertheless, the huge volume change and the sluggish electrochemical kinetics of CoSe2 are inevitable during the sodium ion insertion/extraction process, resulting in poor cycling stability.28–30 In order to overcome the above problems, the hybridization of CoSe2 with carbonaceous materials is adopted. For example, using Co3[Co(CN)6]2 as the template, Liu et al. prepared a hierarchical CoSe2@nitrogen-doped carbon (CoSe2@NC) microcube composite.31 When used as the anode material of SIBs, it has an excellent reversible specific capacity of 384.3 mA h g−1 at a high current density of 2.0 A g−1 and a long cycle life. Yang et al. constructed necklace-like CNT/CoSe2@NC composites by a two-step MOF-engaged approach with a stable and reversible discharge capacity of 404 mA h g−1 after 120 cycles.32 However, the practical implementation is plagued by the intrinsically low specific capacity of carbon, which constrains the energy density of SIBs.

In this study, we report an efficient and scalable route to synthesize N-doping CoSe2 yolk–shell spheres (denoted as N-CoSe2 yss). Profiting from the nitrogen doping and yolk–shell structures, the as-prepared N-CoSe2 yss show long cycling stability, high specific capacity and good rate capability. Kinetics analysis indicates that the pseudocapacitive contribution dominates the electrochemical reaction process. When coupled with the Na3V2(PO4)2O2F cathode, the constructed full cell delivers a high energy density of 129.38 W h kg−1 at a power density of 187.5 W kg−1, which provides a promising anode candidate for SIBs.

2. Experimental section

Synthesis of cobalt precursor spheres

For the preparation of cobalt precursor spheres, 0.291 g of Co(NO3)2·6H2O was mixed with 12 mL of glycerin and 60 mL of isopropanol under evenly stirring and then the solution was introduced into a hydrothermal reactor and kept at 180 °C for 12 hours. After cooling to the indoor temperature, the cobalt precursor spheres were centrifuged and washed several times with water and ethanol, and then dried at 60 °C for 12 hours under vacuum.

Synthesis of the N–Co precursor

Cobalt precursor spheres of 50 mg were dispersed in 20 mL of ethanol under ultrasound treatment. After stirring for 30 min, 1.8 mL of ammonia hydroxide (25 wt%) was added and the obtained solution was transferred to a Teflon-lined stainless steel autoclave and kept at 150 °C for 2 hours to obtain the N–Co precursor.

Synthesis of the N-CoSe2 yolk–shell spheres

20 mg of the N–Co precursor and selenium powders were ground together. Then, the mixture was added into a tubular furnace and heated at 400 °C for 2 hours under a N2 atmosphere with a heating rate of 2 °C min−1. After cooling to room temperature, the N-CoSe2 yss were collected. For comparison, the CoSe2 spheres without nitrogen doping were obtained via the same annealing process by replacing the N–Co precursor with cobalt precursor spheres.

Materials characterization

X-ray diffraction (XRD, D/MAX-2000) was used to analyze the phase of the cobalt precursor, N–Co precursor, N-CoSe2 yss and CoSe2 yss with an acceleration voltage and an acceleration current of 40 kV and 30 mA, respectively. The scan range (2θ) is from 5 to 90 degree. The morphology and structure of the prepared samples were collected using a scanning electron microscope (SEM, SU-9010) and a transmission electron microscope (TEM, TecaniG-20). The high-resolution TEM (HRTEM), high-angle annular dark-field scanning TEM (HAADF-STEM) and energy-dispersive X-ray (EDX) spectrum elemental mappings were executed using a TEM. The surface element composition was analyzed using X-ray photoelectron spectrometers (XPS, ESCALAB).

Electrochemical measurements

The sodium storage performance was evaluated on a half-cell, in which the metallic sodium sheets and 1.0 M NaPF6 were used as the counter electrode and electrolyte, respectively. The working electrode was prepared by coating an active material slurry onto a copper foil, in which the slurry contains the active materials, single-walled carbon nanotubes and the binder (PVDF) with a ratio of 7[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]1. The uniform slurry was pasted onto a Cu foil and then was dried in a vacuum oven at 100 °C for 12 hours. Subsequently, the Cu foil coated with the active material slurry was compressed (∼10 MPa) and perforated to small discs. The cyclic voltammogram (CV), elecrochemical impedance spectrum (EIS) and charge/discharge tests were performed on the WaveDriver electrochemical workstation, Gamry electrochemical workstation and Neware BTS battery system, respectively. N-CoSe2 sym and Na3V2(PO4)2O2F with a weight ratio of 1[thin space (1/6-em)]:[thin space (1/6-em)](4.5–5) were coupled to assemble the sodium-ion full cell. Before assembling the full cell, the anode electrode was presodiated in a half-cell for five discharge/charge cycles at 0.1 A g−1 in the voltage range of 0.01–3 V to ensure the formation of a stable solid electrolyte interphase film. The electrolyte and separator were the same as that of the half cells.

Computational method

The density functional theory (DFT) calculations by the Vienna ab initio simulation package (VASP) were carried out to analyze the conductance of CoSe2 before and after doping.33 The ion-electron wave function was described by the projected augmented wave (PAW) method, and the electron exchange–correlation was expressed by the Perdew–Burke–Ernzerhof (PBE) functional.34,35 A cutoff energy of 400 eV was applied for the plane wave basis set. The convergence criteria for energy and force were set as 1 × 10−5 eV and 0.02 eV Å−1, respectively, during geometry optimization. The gamma-center 6 × 6 × 6 k-point meshes were chosen for lattice optimization, and the relaxed lattice parameter (5.83 Å) of the CoSe2 unit cell shows a good consistence with our measurement (5.86 Å). Then, we expanded the CoSe2 unit cell into the 2 × 2 × 2 supercell and obtained the N-CoSe2 yss with a doping concentration of 1.56%. A 3 × 3 × 3 k-point mesh was applied for the optimization of N-CoSe2 yss, while denser k-point meshes were chosen for the following property calculations on the projected density of states (PDOS), Bader charge population and charge density difference.36

3. Results and discussion

The SEM and TEM images displayed in Fig. S1 (ESI) show the morphology of the Co-precursor spheres. As illustrated in the SEM images (Fig. S1a and b, ESI), the as-prepared Co-precursor spheres possess a uniform size with a smooth surface and the average diameter is about 500 nm, which can be further presented in the TEM images (Fig. S1c and d, ESI). Fig. S2 (ESI) shows the XRD pattern of the Co-precursor, in which no peaks can be detected due to the poor crystallinity. Subsequently, the Co-precursor spheres are used as the sacrificial template for preparing the yolk–shell structures. After the nitrogen doping process, the surface of the N–Co precursors becomes much rougher compared with those of Co-precursor spheres (Fig. S3a and b, ESI) and the yolk–shell structure forms, which can be distinctly distinguished from the contrast between the darker outer shell and the shallower inner cavity (Fig. S3c and d, ESI).

After the selenation process, the sharp and clear XRD peaks can be observed in Fig. 1a, indicating its high crystallinity. All the diffraction peaks are consistent with the standard card (JCPDS No. 09-0234) of the pure cubic zeolite CoSe2 (Pa[3 with combining macron] (205) space group, a = b = c = 5.8593 Å), in which the peaks located at 30.48°, 34.20°, 37.62°, 43.69°, 51.75°, 56.48° and 58.85° were noticed, corresponding to (200), (210), (211), (220), (311), (230) and (321) lattice planes, respectively. The existence of the Co, C, O, Se and N elements in N-CoSe2 yss is also confirmed by the XPS survey spectrum (Fig. S4, ESI). The N 1s spectrum in Fig. 1b shows that there are three types of nitrogen species in the N-CoSe2 yss, namely pyridinic N (398.98 eV), pyrrolic N (400.28 eV) and graphitic N (405.68 eV).37,38 For the Se 3d spectrum (Fig. 1c), the two peaks at the location of 55.1 and 55.8 eV can be ascribed to Se 3d5/2 and Se 3d3/2, respectively. The peak of SeOx locating at 59.8 eV may be due to the surface oxidation of N-CoSe2 yss.39,40Fig. 1d illustrates the fine XPS spectrum of Co 2p, which contains Co 2p1/2, Co 2p3/2 and two satellite (Sat.) peaks. Considering the dual state characteristics of two spin orbits of Co2+ and Co3+, Co 2p3/2 is further divided into three components. The peak at 778.78 eV can be attributed to Co3+, and the peak at 780.88 eV can be attributed to Co2+ chemical species. Co 2p1/2 can also be divided into three components; the peak at 793.78 eV can be attributed to Co3+, and the peak at 796.68 eV can be attributed to Co2+ chemical species. Co2+ and Co3+ exist simultaneously in the Co 2p spectrum, which confirms that the cobalt on the surface of N-CoSe2 yss exists in mixed oxidation states.28,41


image file: d1qm00608h-f1.tif
Fig. 1 The structure of N-CoSe2 yss. (a) XRD pattern and XPS spectra of (b) N 1s, (c) Se 3d, and (d) Co 2p.

SEM images (Fig. 2a–c) clearly show the well maintained yolk–shell morphological characteristics of N-CoSe2 yss after selenation treatment at 400 °C in N2. TEM images show the detailed yolk–shell structure of the as-synthesized N-CoSe2 yss (Fig. 2d–f). The average diameter of the N-CoSe2 yss with the apparent internal gap between the yolk and shell is about 500 nm. The crystalline properties of N-CoSe2 yss are further revealed by HRTEM as shown in Fig. 2g. The observed lattice spacing of about 0.21 nm corresponds to the (220) plane spacing of CoSe2, which is consistent with the result of XRD. The element distribution diagrams in Fig. 2h show the uniform distribution of nitrogen, carbon, cobalt and selenium in cobalt selenide spheres. The CoSe2 spheres without N doping are also obtained as a comparison, whose crystal structure and morphology can be confirmed by the XRD pattern (Fig. S5, ESI) and the SEM images (Fig. S6, ESI).


image file: d1qm00608h-f2.tif
Fig. 2 The morphology of N-CoSe2 yss. (a–c) SEM images, (d–f) TEM images, (g) HRTEM image, and (h) HAADF-STEM image and the corresponding EDX elemental mapping.

To investigate the electrochemical sodium storage performance of N-CoSe2 yss, the coin-type half-cells with N-CoSe2 yss as the anode are assembled. Fig. 3a shows the initial five cycle CV curves in the voltage range of 0.01–3.0 V (relative to Na+/Na) at a scanning rate of 0.1 mV s−1. It can be observed that an obvious cathodic peak at 1.01 V in the first discharge process is corresponding to the activation and the generation of the solid electrolyte interphase (SEI) film.42,43 In the subsequent cycles, the cyclic voltammetric curves are basically superimposed, confirming the good electrochemical stability of N-CoSe2 yss. The peak at around 1.40 V reveals the generation of NaxCoSe2 during the insertion of Na+ and the two peaks (1.10 V and 0.78 V) are due to the transformation recation with the generation of CoSe and Co.42,44 In addition, the peak at ∼1.80 V can be attributed to the inverse conversion reaction of Co to CoSe2. Meanwhile, at a current density of 0.2 A g−1, the initial five discharge–charge curves of N-CoSe2 yss show a series of well-maintained platforms, which indicate the low electrochemical polarization of N-CoSe2 yss (Fig. 3b). The first discharge–charge capacities achieved by N-CoSe2 yss are 722.1 and 602.3 mA h g−1, respectively, showing an initial coulombic efficiency (CE) of 83.4%, which is much higher than that of CoSe2 yss (60.5%) as illustrated in Fig. S7 (ESI). In addition, the N-CoSe2 yss exhibit higher capacities in the following four cycles. As shown in Fig. 3c and Fig. S8 (ESI), the rate capabilities of the N-CoSe2 yss electrode are also tested at different current densities, in which the N-CoSe2 yss deliver the capacities of around 580, 567, 554, 534, 517 and 501 mA h g−1 at the current densities of 0.2, 0.5, 1.0, 2.0, 5.0 and 10.0 A g−1, respectively. Additionally, when the current density comes back to 0.2 A g−1, a specific capacity of about 535 mA h g−1 can still be achieved. Even at high magnification, the rate performance of the electrode material remains excellent, 483, 456 and 431 mA h g−1 at the current densities of 30, 40 and 50 A g−1 (Fig. 3d). To further reflect their excellent performance, a comparison of capacity between N-CoSe2 yss and other CoSe2-based materials is carried out. As displayed in Fig. S9 (ESI), the N-CoSe2 yss exhibits the highest capacity among all the current densities. The cycling performance of the N-CoSe2 yss is also tested at the current densities of 1.0 and 10.0 A g−1, respectively. Remarkably, as shown in Fig. S10 (ESI) and Fig. 3e, the N-CoSe2 yss can maintain a reversible capacity of around 530 mA h g−1 during the 300 cycles at 1.0 A g−1 and 500 mA h g−1 during the 1000 cycles at 10.0 A g−1 with both the CE of about 100%. It is obvious to see that the cycling performance of the N-CoSe2 yss is much better than that of CoSe2 yss, no matter the capacity or cycle numbers, which may be due to the nitrogen doping and the yolk–shell structure.


image file: d1qm00608h-f3.tif
Fig. 3 Electrochemical performance of N-CoSe2 yss. (a) CV curves at a sweep rate of 0.1 mV s−1 in the range of 0.01–3.0 V. (b) The initial five discharge/charge profiles of N-CoSe2 yss at a current density of 0.2 A g−1. (c and d) The rate performance of the N-CoSe2 yss and CoSe2 electrodes at different current densities from 0.2 to 10.0 A g−1 and 1.0 to 50.0 A g−1. (e) Cycling properties under a current density of 10.0 A g−1.

In addition, CV experiments based on dynamic analysis were carried out at different scanning rates from 0.1 to 3.0 mV s−1 to evaluate the contribution of the capacitance control in the charge storage. When the scanning rate increases from 0.1 to 3.0 mV s−1, the CV curves (Fig. 4a) show a similar shape, indicating the good stability of N-CoSe2 yss. In general, the relationship between current i and scanning rate v in the CV curve is expressed by the power law equation, where a and b are constants.45 If the b value is near to 0.5, the electrochemical reaction is considered to be mainly dominated by diffusion. When the b value reaches 1.0, quasi-capacitance controls the electrochemical process.46,47 As shown in Fig. 4b, according to the linear curve of log (i) vs. log (v), the peak fitting b values are 0.95, 0.99, 0.75 and 0.99, respectively. The results show that both the diffusion and the surface capacitance contribute to the storage of sodium in the N-CoSe2 yss anode. The contribution to the total charge generated by the capacitance control process can be analyzed by the following formula:48

i(V) = k1v + k2v1/2


image file: d1qm00608h-f4.tif
Fig. 4 Kinetic analysis of N-CoSe2 yss. (a) CV curves at different scan rates. (b) Determination of the b value using the relationship between the peak current and the scan rate. (c) Separation of the capacitive (shaded region) and diffusion currents at a scan rate of 1.0 mV s−1. (d) The contribution ratio of the capacitive and diffusion-controlled charges versus scan rate.

In the formula, k1v represents the capacitive-controlled interface sodium storage process and k2v1/2 reflects the contribution of the diffusion process. The contribution of diffusion control and capacitance control processes is drawn at different scanning speeds as shown in Fig. 4c and d. When the scanning rate increases from 0.1 to 3.0 mV s−1, the contribution of capacitive control increases and the capacitive contribution of the N-CoSe2 yss electrode at a scanning rate of 3.0 mV s−1 is 98.7% (Fig. 4c). Moreover, the ratios of pseudocapacitive contribution at different scan rates are illustrated in Fig. S11 (ESI), which can reach 95.6%, 96.3%, 98.0% and 98.7% at 0.6, 0.8, 2 and 3 mV s−1, respectively. This high capacitive contribution is mainly due to the good electrical conductivity and the unique yolk–shell structure of N-CoSe2 yss. The CV results show that the capacitive controlled storage has a significant contribution to the total capacity of N-CoSe2 yss at the high current density, which is consistent with the excellent rate performance of the observed N-CoSe2 yss electrode. The Nyquist plots of the N-CoSe2 yss and CoSe2 yss electrodes are shown in Fig. S12 (ESI), in which the charge transfer resistance (Rct) of N-CoSe2 yss (14.0 Ω) is much lower than that of CoSe2 yss (60.2 Ω), demonstrating a better transport/diffusion of electrons and sodium ions for N-CoSe2 yss.

Furthermore, the N-CoSe2 yss anode is coupled with the Na3V2(PO4)2O2F cathode to construct the sodium-ion full cell, in which Na3V2(PO4)2O2F was prepared according to the previously reported method.49 The corresponding XRD pattern and SEM image in Fig. S13 (ESI) reveal that the rod-like Na3V2(PO4)2O2F has been successfully prepared. Fig. 5a shows the rate performance of the full cell, in which the capacities of 378.53, 373.54, 339.59, 304.25, 278.91 and 259.91 mA h g−1 are achieved at the rates of 0.1, 0.2, 0.5, 1, 2, and 5 A g−1, respectively. The charge/discharge curves of the N-CoSe2 yss electrode at different rates can also exhibit a remarkable rate capability (Fig. 5b). Notably, the full cell can light up the LED panel (Fig. 5c), indicating its feasibility in practical applications. The Ragone plot in Fig. 5d compares the energy and power density of the prepared full cell with that of other sodium-ion full cells documented in the literature.50–54 The full cell delivers a high energy density of 129.38 W h kg−1 at a power density of 187.5 W kg−1. When the power density increases up to 8589 W kg−1, the energy density of the full cell still remains at 81.86 W h kg−1, further confirming the potential applications of our batteries.


image file: d1qm00608h-f5.tif
Fig. 5 The energy storage performance of the sodium ion full cell. (a) Rate capability. (b) Charge/discharge curves of the full cell at different rates. (c) Image of a small COSE-shaped LED panel lighted by the full cell. (d) Comparison of Ragone plots with other reported sodium-ion full cells.

Theoretical calculation was also performed to analyze the conductance of CoSe2 before and after N doping. The isosurface of the electron density difference of CoSe2 and N-CoSe2 yss (Fig. 6a and b) indicates that the doped N atom obtains electrons from the neighboring Co atoms and Se atom due to its strong electronegativity (Se2 in Fig. 6). Moreover, the large negative charge value on N (−0.90 eV), as well as the significant charge loss of the neighboring Co atoms (from +0.37 eV to +0.50 eV) and Se atom (from −0.18 eV to +0.12 eV) also support the analysis. Though the doping of N does not significantly change the metallic nature of CoSe2, the introduction of extra electrons by the doping of N could further improve the conductance of CoSe2. The PDOS diagrams in Fig. 6c and d indicate that both the CoSe2 and N-CoSe2 yss are good conductors with valence bands crossing through the Fermi level. The d-orbital in the Co atoms and the p-orbital in the Se atoms mainly contribute to the conductivity. However, the doping of the N atom makes the conductance of N-CoSe2 yss higher than that of CoSe2 by introducing the doping state in the conduction band at around 3 eV and the doping state at around −12 eV in the valence band. As a result, the energy gaps in the conduction and valence bands are decreased by 0.3 eV and 0.5 eV, respectively.


image file: d1qm00608h-f6.tif
Fig. 6 DFT calculations. The optimized structure of (a) 2 × 2 × 2 CoSe2 yss supercell and (b) N-CoSe2 yss. The charge density difference and Bader charge assigned on the corresponding atoms before and after doping are also provided, and the green isosurfaces represent the regions of negative charge accumulation. PDOS of (c) CoSe2 yss and (d) N-CoSe2 yss.

4. Conclusion

In summary, the uniform N-CoSe2 yss with an average diameter of 500 nm are synthesized. The nitrogen doping can enhance the electronic conductivity of CoSe2, leading to rapid charging and discharging processes. The unique yolk–shell structure can decrease the Na ion transport resistance and improve the structural stability, which endows the N-CoSe2 yss electrode with good cycling performance. When used as anodes for SIBs, the N-CoSe2 yss deliver a high reversible capacity of 530 mA h g−1 at 1 A g−1 over 300 cycles. Even at a high current density of 10 A g−1, a high capacity of 500 mA h g−1 can be maintained after 1000 cycles. Furthermore, the electrochemical mechanism can be revealed by kinetics tests and density functional theory calculations. Coupling with the high-voltage Na3V2(PO4)2O2F cathode, the as-fabricated full cell shows a high energy density of 129.38 W h kg−1, which will inspire the applications of N-CoSe2 yss in high-performance energy storage systems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

J. T. G. and S. Y. Z. contributed equally to this work. This work was financially supported by the National Natural Science Foundation of China (51801030 and 51902032), the Natural Science Foundation of Jiangsu Province (BK20191026), and the Natural Science Foundation of Guangdong Province (Grant No. 2018A030310571).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d1qm00608h

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