Composition Engineering Boosts Voltage Windows for Advanced

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Composition Engineering Boosts Voltage Windows for Advanced Sodium Ion Batteries Yunling Jiang, Guoqiang Zou, Hongshuai Hou, Jiayang Li, Cheng Liu, Xiaoqing Qiu, and Xiaobo Ji ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b05614 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 24, 2019

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ACS Nano

Composition Engineering Boosts Voltage Windows for Advanced Sodium Ion Batteries Yunling Jiang,† Guoqiang Zou,† Hongshuai Hou,† Jiayang Li, † Cheng Liu, † Xiaoqing Qiu† and Xiaobo Ji*.†,‡

† State Key Laboratory of Powder Metallurgy, College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China.

‡ College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083, Hunan, China.

*E-mail: [email protected].

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ABSTRACT Transition metal selenides have captured sustainable research attention in energy storage and conversion field as promising anodes for sodium-ion batteries. However, for the majority of transition metal selenides, the potential windows have to compress to 0.5-3.0 V for the maintenance of cycling and rate capability, which largely sacrifices the capacity under low voltage and impair energy density for sodium full batteries. Herein, through introducing diverse metal ions, transition metal selenides consisted of different compositions doping (CoM-Se2@NC, M=Ni, Cu, Zn) are prepared with more stable structures and higher conductivity, which exhibit superior cycling and rate properties than those of CoSe2@NC even at a wider voltage range for sodium ion batteries (SIBs). Particularly, Zn2+ doping demonstrates the most prominent sodium storage performance among series materials, delivering high capacity of 474 mAh g-1 after eighty cycles at 500 mA g-1 and rate capacities of 511.4, 382.7, 372.1, 339.2, 306.8 and 291.4 mAh g-1 at current densities of 0.1, 0.5, 1.0, 1.4, 1.8 and 2.0 A g-1, respectively. The composition adjusting strategy based on metal ions doping can optimize electrochemical performances of metal selenides, offer an avenue to expand stable voltage windows and provide a feasible approach for the construction of high specific energy sodium ion batteries. KEYWORDS bimetallic organic frameworks; transition metal diselenides; ions doping; voltage windows; sodium-ion batteries

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Recently, transition metal chalcogenides have captured tremendous attention as promising functional materials for energy storage and conversion owing to their high theoretical

capacity,

enhanced

security

and

straightforward

preparations.1-5

Nevertheless, when utilized as anodes of sodium ion batteries, transition metal oxides and sulfides will be separately restricted by the compact electrolyte interphase (SEI) films and the “shuttle effect”, which can impede ion transposition and break electrodes, respectively, further leading to unsatisfactory sodium storage capacity.6-9 Compared with the aforementioned materials, transition metal selenides with better conductivity and lower energy consumption of multi-electron conversion reaction have been regarded as advanced anodes for SIBs.10-12 Even so, the electrochemical performances of transition metal selenides are still limited by sizeable volume expansion effect during the charging/discharging processes evolved from the large radius and molar mass of sodium.10,

13

Consequently, excogitating effective

resolutions to modify the transition metal selenides is extremely pivotal. Normally, carbon-modification and particle-downsizing are employed to improve the property of electrode materials, which can reinforce the contacting area and the permeation of electrolyte, ultimately accommodating the volume change and enhancing the ion insertion/extraction kinetics.14-16 In particular, for transition metal diselenides, such as CoSe2, NiSe2 and FeSe2, the voltage windows are generally shortened to 0.5-3.0 V in order to obtain high-rate capability and long-term cyclability as a result of their poor properties stemmed from huge volume change during conversion-type reaction.8, 9, 17 And the reaction type can be controlled with this method.18 For instance, 3


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CoSe2/nitrogen-doped carbon nanofibers19 showed a reversible capacity of 371.8 mAh g-1 after 500 cycles at 0.2 A g-1. FeSe2 microspheres reported by Chen’s group20 delivered a Na storage capacity of 372 mAh g-1 after 2000 cycles at 1.0 A g-1. Moreover, Mai et al. presented an anode of NiSe2 nanooctahedra,21 exhibiting long-term cyclic stability of 313 mAh g-1 after 1000 cycles at 5 A g-1. The stable potential windows of the materials above are all within 0.5-3.0 V. Nevertheless, plentiful low-voltage capacity will be sacrificed in that case and the energy density will be deeply diminished when utilized for sodium full batteries, which might exert adverse effects on future applications. Hence, even though great improvements have been achieved, transition metal diselenides combining reasonable morphology and superior electrochemical performances under a more intact voltage window are still meaningful to investigate. Notably, it was suggested that doping a moderate amount of metal ions can promote the lifespan and rate capability of electrode materials. To be specific, with the introduction of the second metal ions, original metal ions can be activated, which facilitates fast diffusion of Na ions within the lattices and enhance the electrical conductivity of electrodes.22-27 Furthermore, the structural integrity and stability of electrode under high current densities will be slightly promoted by metal ions doping.28-31 For example, Cao et al. explored the effects of Ni doping on the PB cathode material, demonstrating that a higher diffusion rate of Na ions and a steadier PB lattice were acquired by the substitution with Ni ions, further resulting in an intensive electrochemical performance.28 Moreover, Cu-doped CoSe2 microboxes 4


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were utilized as anodes of SIBs, confirming that the electronic conductivity is improved by Cu doping, leading to superior sodium storage properties.24 Inspired by these previous works, bimetallic organic frameworks (BMOFs) are considered as optional precursors applied for metal ions doping and may have potentially advantages in various aspects below. In experimental aspect, there is no need to introduce the doping metal secondarily considering that BMOFs are comprised by two species of metal ions simultaneously, which can simplify the doping procedures. With respect to the introduction of carbon layers, BMOFs are consisted of special organic ligands which can be directly transformed into carbon after pyrolysis. In morphology, BMOFs are generally formed by regular structures, which are beneficial to provide distinctive morphology for products.32-36 Briefly, it is worthwhile to explore that without changing voltage windows, whether the electrochemical performance of transition metal diselenides templated by BMOFs can be optimized in the common effects of doping metal ions and introducing carbon layers. Herein, a series of N-rich carbon wrapped bimetallic cobalt-based diselenides with different metal ions doping (CoM-Se2@NC, M=Ni, Cu, Zn) have been prepared through the successive carbonization and selenylation templated by bimetallic zeolitic imidazolate frameworks (BM-ZIFs). Note that this design is advantageous to Na-storage in different aspects. Firstly, two conceptions of nitrogen and transition metal ions doping are simultaneously introduced to reinforce the intrinsic conductivity and cycling stability of CoSe2. Besides, more Na+ transport channels are introduced through utilizing BM-ZIFs as precursors, which can effectively accelerate the 5


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diffusion of Na+ and achieve excellent rate capability. Additionally, the external carbon layers coating can effectively facilitate the infiltration of electrolyte and alleviate volume changes during cycling processes. Moreover, it is well-known that the Co 3d electrons in CoSe2 possess a low-spin electronic structure t62ge1g, which can generate a strong Jahn-Teller effect and distortion in the crystal lattices.34 After introducing appropriate amount of foreign metal ions, the Jahn-Teller distortion of Co2+ will be partly alleviated, leading to more stable structures during cycling and a superior cycling stability.37-39 By virtue of the advantages mentioned above, the obtained materials show excellent electrochemical performances. As displayed before,24,

28

metal ions doping can obviously improve the electronic conductivity,

structural stability and integrity of CoSe2 during cycling, and then relieve the volume change during electrochemical conversion reactions, which further reinforce the cycling and rate property even under the voltage range of 0.01-3.0 V. This work systematically explore the influence of metal ions doping on transition metal diselenides anodes and prominent electrochemistry performances of CoSe2 can be obtained with the conjunct effects of introducing metal ions and N-rich carbon coating, which sheds light on the routes to facilitate sodium storage of diselenide anodes without shortening the voltage windows. RESULT AND DISCUSSION The formation and crystallographic structures of as-prepared BM-ZIFs and CoM-Se2@NC are illustrated by X-ray diffraction (XRD) and Raman spectra. The XRD patterns of BM-ZIFs series (Figure S1) are well consistent with ZIF-67, 6


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meanwhile, XRD results of the various CoM-Se2@NC products (Figure 1a) are in good accordance with the standard cubic CoSe2 (PDF#65-3327), of which the peaks are located at 34.38°, 37.74° and 51.86°correspond to (210), (211) and (311) lattice planes, respectively.33,

40-42

Additionally, Raman spectra of the as-acquired series

materials depicted in Figure 1b displays virtually similar Raman bands comprising of six peaks located at 188, 466, 515, 676, 1340, and 1582 cm-1. Concretely, the D peaks (1340 cm-1) and G peaks (1582 cm-1) are originated from sp3 and sp2 hybridized carbons with the intensity ratio (ID/IG) of around 0.96, confirming relative high graphitic feature of the nitrogen-doped carbon coating.43,

44

Moreover, the peaks

marked as A1g and Ag at 188 and 676 cm-1 are indexed to CoSe2, while the slight peaks appeared at 466 and 515 cm-1 are assigned to Eg and F2g characteristic peaks of CoO•Co3O4 due to the minor oxidation of Co by a bit of SeO2 in the Se source.40, 45, 46 The quite similar XRD and Raman characterizations of these products reveal that the crystal structure of cubic CoSe2 is hardly influenced by the introductions of a moderate amount of second metals ions (Zn, Cu, and Ni), verifying the homogeneous incorporation of transition metal ions and the in-situ substitution occurred on the sites of Co ions.33, 34 To further explore the content of N-doped carbon, thermogravimetric analysis (TGA) results of all final products at air atmosphere are shown in Figure 1c, demonstrating that the weight loss are all around 70% and the calculated N-doped carbon is 19% based on the equation 1. In details, it should be noted that the weight changes before 385 °C can be ascribed to the combustion of carbon and then CoSe2 is converted to CoSe and SeO2, eventually CoSe is transformed into Co3O4 and SeO2 at 7


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the temperature of 500 °C.8, 19 As presented in Figure 1d, the specific surface areas of these obtained materials are nearly identical, which are around 10.0 m2 g-1. Notably, the pore diameters distribution displayed in Figure S2 demonstrate that with the second transition metal ions doping, the pore diameter is reduced, which will be conducive to Na-storage. Furthermore, the process of metal ions doping is exhibited in Figure 1e, which can vividly explain the substitution between Co2+ and M2+.

N-doped carbon (wt%) = 1-

3 × molecular weight of CoSe2 × final weight of Co3O4 molecular weight of Co3O4

(equation1)

Figure 1: XRD patterns (a), Raman spectra (b), TGA curves (c), N2 adsorption-desorption isotherms (d) of series CoM-Se2@NC, (e) crystalline structures 8


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of samples and corresponding composition adjusting process. To analyze the elemental composition and valence bonds of CoM-Se2@NC products, CoZn-Se2@NC is chosen as a representative candidate for XPS characterization. The existences of Co, Zn, Se, C and N are affirmed in XPS survey spectrum (Figure 2a). As depicted in Figure 2b, high-resolution XPS spectrum of Co 2p is composed of six peaks. The peaks at 778.1 and 793.0 eV are attributed to the Co 2p3/2 and Co 2p1/2 of CoSe2, while the peaks of Co 2p3/2 and Co 2p1/2 located at 780.04 and 795.70 eV are indexed to the Co-O band due to native oxide layers on the surface.14,

40, 47

Moreover, two broad peaks at the end of each Co 2p signal are

ascribed to satellite peaks of Co 2p. Spectrum of Zn 2p in Figure 2c can be resolved into two peaks of Zn 2p3/2 and Zn 2p1/2 at 1020.93 and 1043.83 eV, respectively, which corresponds to Zn2+ in Zn-Se, confirming the introduction of Zn2+.23,

34, 48

Notably, Figure 2d demonstrates the 3d peaks of Se, of which the peaks presented at 54.3, 55.3, 57.4, 58.8 and 60.4 eV are associated with Se 3d5/2, Se 3d3/2, Co 3p3/2, Co 3p1/2, and SeO2, separately. The existences of Co 3p3/2 and SeO2 are ascribed to Co-Se bond and the sight oxidization of Se on the surface.49 C 2p spectrum in Figure 2e can be deconvoluted into four peaks corresponding to C-C (284.38 eV), C=N (285.93 eV), C-N (287.83 eV) and O-C=O (289.63 eV).50-53 Additionally, the high resolution N 1s spectrum of CoZn-Se2@NC in Figure 2f shows three types of nitrogen at 398.08, 399.83 and 401.68 eV, representing pyridinic, pyrrolic and graphitic N, respectively.54, 55 Furthermore, the XPS analysis of CoSe2@NC is provided in Figure S3, of which the high-resolution XPS spectrums of Co, Se, N, C are similar to 9


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CoZn-Se2@NC, further confirming the consistence on internal natures of the doped and non-doped products.

Figure 2: XPS survey (a) and Co 2p, Zn 2p, Se 3d, C1s and N 1s spectra (b-f) of CoZn-Se2@NC, respectively. Scanning electron microscopy (SEM) images of the as-prepared materials and their precursors are exhibited in Figure 3, S4 and S5. Visibly, the ZIF precursor series are composed of coincident rhombic dodecahedron structures with smooth surfaces and particle sizes of 500 nm. After the successive carbonization and selenylation, both of the polyhedral structures and particle size (500 nm) are well maintained, meanwhile, the polyhedrons are distributed homogeneously with much rougher surfaces. Due to their parallelism in morphology, transmission electron microscopy (TEM) is carried out on CoZn-Se2@NC as a representative to probe more insight regarding the microstructure of CoM-Se2@NC. As depicted in Figure 3g, the polyhedral morphology is further affirmed by the TEM images of CoZn-Se2@NC, moreover, it is observed that two characteristic lattice spacing marked in the high-resolution TEM 10


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image (Figure 3h) are consistent with the lattice distances of CoSe2 (210) plane (0.26 nm) and graphitic carbon (0.34 nm), respectively, which are in agreement with the Raman results above mentioned. In the selected area electron diffraction (SAED) image presented in Figure 3i, the diffraction rings of CoZn-Se2@NC are well corresponding to (200), (211), (220) and (311) lattice planes of CoSe2.8, 56 Moreover, the uniform distributions of Co, Zn, Se, N and C elements are demonstrated by TEM elemental mappings (Figure 3j), verifying the doping of Zn2+ again. In brief, the SEM and TEM images of the series products displayed in Figure 3, S4 and S5 confirm the successful introduction of the second transition metal ions, and further reveal that a handful of transition metal ions doping almost have no influence on the morphology of as-prepared materials. Based on the aforementioned discussion results, the fabrication process and morphology evolution of the obtained series materials are elucidated in Scheme 1. Initially, driven by the vacant orbitals from transition metal ions and the lone pair electrons from organic ligands, two different species of transition metal ions sources were complexed with the organic ligands (2-methylimidazole), which were assembled into BM-ZIF at room temperature in methanol solutions. Then, the carbonization and selenylation processes take place successively during pyrolysis under argon atmosphere. Concretely, with the increase of temperature, the imidazole organic chains were carbonized to porous N-doped carbon layer, and then selenium powder was reacted with Co2+ to acquire CoM-Se2@NC.

11


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Figure 3: SEM images of CoZn-ZIF (a-c) and CoZn-Se2@NC (d-f), TEM images (g), high-resolution TEM images (h), selected area electron diffractions (SAED) images (i) and mapping images (j) of CoZn-Se2@NC.

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Scheme 1: Illustration of the formation mechanism of CoM-Se2@NC polyhedrons. The sodium storage properties of CoM-Se2@NC and CoSe2@NC were investigated by cyclic voltammetry (CV) and galvanostatic charge/discharge measurements in half cells. According to the previous reports, the voltage ranges of transition metal diselenides were always chosen to be 0.5-3.0 V to maintain cycling stability.8, 10, 17 Consequently, in order to evaluate whether transition metal ions doping can facilitate cyclability of transition metal diselenides for anodes of SIBs without the voltage windows compressed and capacity sacrificed, the potential range of 0.01-3.0 V was defined. As depicted in Figure 4a, the cycling performances of prepared samples demonstrate that CoZn-Se2@NC, CoCu-Se2@NC, CoNi-Se2@NC and CoSe2@NC delivers initial discharge/charge capacities of 564.9/440.5, 544.9/398.5, 473.6/346.9 and 487.8/374.0 mAh g-1 at a current density of 500 mA g−1, with the coulombic 13


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efficiencies of 78%, 73.2%, 73.1% and 76.7%, respectively. The relatively low initial coulombic efficiencies can be mainly attributed to the irreversible process of the SEI film formation.27,

39, 53, 57

After eighty cycles, the capacities of CoZn-Se2@NC,

CoCu-Se2@NC, CoNi-Se2@NC still retain 470.5, 359.5 and 355.5 mAh g-1, however, it is obvious that the capacity of CoSe2@NC is sharply decreased to 231.4 mAh g-1. This phenomenon preliminarily clarifies that transition metal ions doping is conducive to the cycling performance of transition metal diselenides under wide voltage range, and the superior cyclability is likely due to the more stable structures of the doping samples.28 Meanwhile, Figure 4b exhibits the rate capacities of these series products at stepwise current densities of 0.1, 0.2, 0.5, 0.8, 1.0, 1.4, 1.8, 2.0 A g-1. Note that a superior rate property of CoZn-Se2@NC is acquired among the obtained materials, which delivers reversible capacities of 510, 470, 403, 380, 372, 340, 304 and 292 mAh g-1, respectively, and when the current density is restored to 0.1 A g-1, the capacity of CoZn-Se2@NC could recover back to 456.1 mAh g-1. To further explore the rate capabilities of these materials, the comparison of charge/discharge curves at different rates are presented in Figure 4c. Apparently, their charge/discharge platforms are shortened with the augment of current density, among which CoZn-Se2@NC can maintain best, especially the long charge plateaus at 1.85V. In order to deeply explore the cycling performance, the long-term cycling capability comparison of CoZn-Se2@NC and CoSe2@NC is shown in Figure S6, in which CoZn-Se2@NC deliver a high capacity of 295 mAh g-1 after 200 cycles at the current density of 2.0 A g-1, in contrast, the capacity of CoZn-Se2@NC is already reduced to 14


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145 mAh g-1, further indicating the excellent cycling ability of doped samples. Additionally, the SEM images of doped and non-doped samples after sufficient cycling are exhibited in Figure S7. It is clear that even though there are some deformations, the basic spheroid structures of CoZn-Se2@NC are well-maintained and these bind spheroids sticks on the super P layers, comparatively, the structures of CoSe2@NC are smashed absolutely and aggregated together completely. Hence, it is concluded that metal ions doping is beneficial to maintain the structural integrity of CoSe2 under the high rate cycling, which is a vital factors for cycling stability.

Figure 4: (a) The cycling performance, (b) rate capability, (c) discharge/charge platforms at different densities, (d) 35th platforms at 0.5 A g-1, (e) cyclic voltammetry (CV) curves at 0.1 mV s-1, (f) Nyquist plots, (g) corresponding linear relation of ɷ1/2 versus-Z″of series CoM-Se2@NC. 15


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As shown in Figure 4d, the 35th discharge/charge curves of acquired materials demonstrate

that

the

lengths

of

every

plateau

are

in

a

clear

order:

CoZn-Se2@NC>CoCu-Se2@NC >CoNi-Se2@NC>CoSe2@NC. And it is obvious that the charge platforms of CoZn-Se2@NC are at a lower voltage than others, meantime, the low-voltage capacity of CoZn-Se2@NC is higher as well. In details, the charge capacities of CoZn-Se2@NC and CoSe2@NC under 1.0 V are 60.6 and 50.3 mAh g-1, besides the discharge capacities under 0.5 V of these two samples are 123 and 93 mAh g-1, respectively. The higher low-voltage capacity and lower discharge platforms materials possess, the better electrochemistry performances of full battery they can deliver. Hence, it can be noted that metal doping samples are more beneficial for the application of sodium full cells. Moreover, the CV curves of four products at the scan rate of 0.1 mV s-1 are depicted in Figure 4e, showing that the positions of their redox peaks are almost accordant and well-corresponding to their discharge/charge profiles. Additionally, electrochemical impedance spectroscopy (EIS) was employed to analyze the resistances of the prepared anodic materials. The Nyquist plots in Figure S8 illustrate that the initial resistance of CoZn-Se2@NC is lower than others, confirming its higher conductivity. Besides, the specific resistivity and conductivity values of these samples without cycling displayed in Table S1 are clearly well-matched with the EIS results. Furthermore, in Figure 4f, the Nyquist plots after two cycles manifested the impedance of these samples are in a clear sequence of CoZn-Se2@NC

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