Formation of Nano-Dimensional NiCoO2 Encapsulated in Porous

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Formation of Nano-Dimensional NiCoO2 Encapsulated in Porous Nitrogen-Doped Carbon Sub-Microspheres from Bimetallic (Ni, Co) Organic Framework towards Efficient Lithium Storage Dienguila Kionga Denis, Zhengluo Wang, Xuan Sun, Fakhr uz Zaman, Jinyang Zhang, Linrui Hou, Jia Li, and Changzhou Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11822 • Publication Date (Web): 13 Aug 2019 Downloaded from pubs.acs.org on August 13, 2019

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ACS Applied Materials & Interfaces

Formation of Nano-Dimensional NiCoO2 Encapsulated in Porous Nitrogen-Doped Carbon Sub-Microspheres from Bimetallic (Ni, Co) Organic Framework towards Efficient Lithium Storage

Dienguila Kionga Denis,† Zhengluo Wang,† Xuan Sun, Fakhr uz Zaman, Jinyang Zhang, Linrui Hou,* Jia Li, Changzhou Yuan*

School of Materials Science and Engineering, University of Jinan, Jinan, 250022, P. R. China †

The authors contributed equally to this work.

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT Recently, rock-salt NiCoO2 (NCO) with desirable electronic conductivity has drawn enormous interest worldwide for energy-related applications. However, the intrinsically sluggish kinetics and electrode aggregation/volumetric change/pulverization during Liinsertion/extraction processes hugely limit its applications in Li-ion batteries (LIBs). In the contribution, we first devise a bottom-up method for scalable fabrication of the nanodimensional NCO particles encapsulated in porous nitrogen-doped carbon submicrospheres (NCS), which are derived from bi-metal (Ni, Co) metal-organic framework. The porous NCS, as a flexible conductive skeleton, can buffer distinct volume expansion as an efficient buffering phase, restrain agglomeration of nanoscaled NCO, and enhance electronic conductivity and wettability of the electrode. Benefiting from the synergistic functions between the nano-dimensional NCO and porous NCS, the obtained NCO@NCS anode (~74.5 wt.% NCO) is endowed with remarkable high-rate reversible capacity (~403.0 mAhg-1 at 1.0 A g-1) and cycling behaviors (~371.4 mAhg-1 after cycled for 1000 times at 1.0 A g-1) along with high lithium diffusion coefficient and remarkable pseudocapacitve contribution. Furthermore, the NCO@NCS-based full LIBs exhibit competitive lithium-storage properties in terms of energy density (~217.0 Wh kg-1) and cyclic stability. Furthermore, we believe that the methodology is highly promising in versatile design and construction of binary metal oxide/carbon hybrid anodes for advanced LIBs.

KEYWORDS: Nanoscaled NiCoO2; N-doped carbon sub-microspheres; Bimetallic metal-organic framework; Hybrid anode; Li-ion batteries


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INTRODUCTION The continuous increase in energy demand over the last decades has led to excessive consumption of fossil fuels, resulting in greenhouse gas emissions that are devastating and harmful to living beings. Given the constant industry development, it is more than imperative to develop researches based on sustainable electrochemical energy systems (EESs). Typically, Li-ion batteries (LIBs) become one of the most prospective candidates for EESs among many other types of energy-storage systems for various portable electronics, smart power grids, (hybrid) electric automobiles, and so on,1-4 thanks to their potential merits including high reversible capacities, long cyclic life, low-self-discharge capability and environmental efficiency.5, 6 Nevertheless, the theoretical capacity of the commercially available graphite anodes is as low as approximate 372 mAh g-1, severely limiting the extensive development of next-generation LIBs,7, 8 which thus stimulates the targeted exploitation of other promising anode materials.9 Motivated by the pioneering contributions from Tarascon s gourp,10, 11 transitional metal oxides (TMOs) have drawn numerous attention, and been applied in LIBs as alternative electrode candidates due to their high specific theoretical capacities than conventional carbon electrodes.11 Besides, the smart combination of two metallic species, i.e., the mixed binary TMOs, would especially favor for the enrichment in oxidationreduction reactions and enhanced electronic conductivity,12 which are indeed necessary for efficient electrochemical Li-storage applications. However, the inherent drawbacks of the TMOs electrodes, such as obvious volumetric change in continuous discharge/charge process, modest rate capability and fast capacity fading during cycling, seriously limit their extensive applications in LIBs.13 More appealingly, the rock-salt-type NiCoO2



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(NCO) appears as a promising anode owing to its higher electronic conductivity and redox activities than spinel NiCo2O4 and single-metal-phase NiOx and CoOx, unique compositions, and synergistic functions of bi-metal species.14-16,

3, 6

Nevertheless, the

cubic-phase NCO electrodes are still faced with the general issues of all the TMOs-based anodes during their applications in advanced LIBs. In recent years, metal-organic frameworks (MOFs) obtained through the supermolecular assembly of certain metal ions or aggregates with organic ligands have gained widespread attention in various applications, due to their adjustable metal ions and pore structure by using appropriate organic linkers.17, 18 From the perspective of energy storage technology, they can even be used directly as electrodes for LIBs and/or electrochemical capacitors, due to their large specific surface area (SSA), controlled pore structure, and ordered crystal structure.19-21 However, the low conductivity and poor stability of the MOFs themselves seriously limit their efficiency in energy-related applications. As a consequence, it would be even better to utilize the MOFs as the templates and/or precursors towards controllable fabrication of the TMOs/porous carbon composites,3, 22, 23 which will maximize the respective advantages of TMOs and porous carbon matrix with the synergistic effect of the two towards efficient Li-storage behaviors. Also, it remains greatly challengeable to make the ultrafine NCO NPs dispersed uniformly in the porous carbon framework towards advanced LIBs. Based on the aforementioned analysis, in this contribution, we innovatively explored a facile yet scalable synthetic methodology for purposeful preparation of NCO nanoparticles (NPs) encapsulated in the porous N-doped carbon sub-microspheres (NCS) (designed as NCO@NCS) by using the bimetal (Ni, Co) metal-organic framework


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(denoted as Ni-CO-MOF) as the precursor. Benefiting from its striking structural and component merits, as well as the synergy between the porous NCS and NCO nanoparticles

(NPs),

the

NCO@NCS

anode

exhibited

large

reversible

charging/discharging capacities, high-rate properties, and long-span cyclic stabilities as an anode material for high-performance LIBs. Experimental section Synthesis of the NCO@NCS. All the analytically pure chemicals were purphased from Sinopharm Chemical Reagent Co., Ltd, and utilized without any further treatments. The Ni(NO3)2·6H2O (0.74 mmol) and Co(NO3)2·6H2O (0.74 mmol) were well dissolved in the N, N-Dimethylformamide (DMF) solution (30 mL) with magnetic stirring at room temperature (RT). Then, the trimesic acid (150 mg) and polyvinylpyrrolidone (M.W. 40000, 1 g) were added in the above solution. After magnetically stirred for half an hour, the resulted solution was further put into a 50 mL Teflon lined stainless-steel autoclave, and kept for 6 h at 150 °C. The precipitate was obtained after washed with the DMF and absolute ethanol in order, and vacuum dried. Then, the as-resultant Ni-Co-MOF was put into 0.1 M KOH aqueous solution (100 mL). After stirred for 1 h at RT, washed and dried at 70 °C, the sample was achieved and designed as Ni-Co-hydroxide. Finally, the Ni-Cohydroxide was annealed at 200 °C for 2h with a ramp rate of 1 °C min-1, and further treated at 450 °C for 1 h in N2 with a ramp of 5 °C min-1. The final sample, named as NCO@NCS, was achieved. Material Characterizations. The crystalline phases were detected wity X-ray diffraction (XRD, Rigaku Ultima Ⅳ, Cu Kα). Morphologies and micro-structures of products were characterized by field-emission scanning electron microscope (FESEM, FEI QUANTA



FEG250, USA), transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) (JEOL, JEM-2100 system). Corresponding energy dispersive X-ray (EDX) analysis and element mapping images were taken by the X-ray spectroscopy, which was attached to the TEM instrument. Brunauer-Emmett-Teller (BET) SSA was determined by N2 adsorption/desoprtion isotherms with the surface area analyzer (Quantachrome, America), and pore diameter analysis was performed with the Barrett-Joyner-Halenda (BJH) method. The samples were calcinated in a Quartz tube furnace (GSL-1400X). Raman spectroscopy was conducted with a LabRAM HR with a laser excitation wavelength of 514.5 nm. Thermogravimetric (TG) (NETZSCH, America) along with differential scanning calorimeter (DSC) analysis was coducted with a NETZSCH system TG Analyzer (STA449 F5, Germany) in O2 atmosphere. X-ray photoelectron spectroscopy (XPS) was carried out by the Thermo ESCALAB 250Xi spectrometer with the Al Kα excitation source. The particle size distribution profile of the sample was derived with the software Nanomeasure1.2.5. Electrochemical Evaluations. Typically, a coin cell (CR2032) was used to evaluate electrochemical Li-storage behaviors of electrodes and full devices. The electroactive NCO@NCS, conductive acetylene black (AB) and sodium carboxymethyl cellulose (CMC) with a mass percentage of 70 wt.%, 20 wt.% and 10 wt.%, respectively, were blended in an agate morear with deionized water to render a homogeneous slurry. The Cu foil coated with the slurry was finally dried at 100 °C under vacuum as working electrode. Typical mass loading of the NCO@NCS is about 1.1 mg cm-2 for each cell. The Li foil


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and polypropylene microporous membrane were applied as the counter electrode and separator, respectively. The mixed ethylene carbonate/diethylene carbonate of volumetric ratio of 1: 1 with 1 M LiPF6 was used as the electrolyte for all electrochemical measurements. The Ar-filled glovebox (MBRAUN UNILAB PRO) with oxygen and water contents of < 0.5 ppm was used to assemble the cells. The charge and discharge tests were conducted in a potential range of 0.01 to 3.0 V (vs. Li/Li+) on the LAND CT2001A battery measuring system. Cyclic voltammetry (CV) was carried out in the electrochemical window from 0.01 to 3.0 V (vs. Li/Li+) at a sweep rate of 0.1 mV s-1. Electrochemical impedance spectroscopy (EIS) was tested over an impedance analysis frequency range of 105 Hz to 10-2 Hz with an amplitude of 5 mV. The CV and EIS tests were both performed on an electrochemical workstation (IVIUM Stat.h, the Netherlands) In regards to the positive electrode for assembling of full devices, the active material of commercial LiNi0.8Co0.15Mn0.05O2 (NCM), conductive CB, and polyvinylidene fluoride with a gravimetric ratio of 8 : 1 : 1, respectively, were mixed well in the dispersant of Nmethyl pyrrolidone in the agate morear. The resulting mixture was pasted on an aluminum foil, and further vacuum dried at 110 °C. The positive/negative capacity ratio matching was designed as 1.1 ‒ 1.5 : 1.0 in a full battery. For assembling full devices, the prelithiation process was necessary in the NCO@NCS anode to effectively compensate for the initial irreversible capacity loss (ICL). Specifically, the NCO@NCS anode was contacted with the lithium foil in the applied electrolyte under extra pressure for 2 h. The full devices were all assembled in the same condition as those for the half cells. The nominal capacity used in this section according to the cathode material (NCM) was ~200 mAh g-1,24, 25 and the calculation of specific energy density (E) of devices was performed



according to the Equation: E = C × V × (1 -δ),12 in which the V, C and δ represented average potential, specific discharge capacity, and penalty factor, respectively. All the devices were packaged by applying the same separators and electrolytes with those in half cells. The electrochemical window for the assembled full devices was ranged from 2.5 to 4.0 V. RESULTS AND DISCUSSION Structural and Physicochemical Characterizations. In the work, we innovatively explored a three-step synthetic strategy, that is, the bottom-up fabrication of bimetallic Ni-Co-MOF, phase transformation in KOH aqueous solution, and calcination in N2 atmosphere, to purposefully fabricate the NCO@NCS product, as schematically illustrated in Figure 1a. Figure 1b demonstrates the wide-angle X-ray diffraction (XRD) reflection of the resultant Ni-Co-MOF. Two sharp peaks at 2θ = 11.7 and 23.6 ° along with two weak reflections at 20.6 and 35.9 ° are detected, which is in agreement with those for other Ni/Co-based MOFs.26 After immersed in the aqueous KOH solution (0.1 M) for 12 h, the violet Ni-Co-MOF turns out to be blue-green, which indicates the occurrence of phase transformation. Further XRD observation confirms that the Ni-Cohydroxide sample is a solid solution of Ni(OH)2 (PDF no. 14-0117) and Co(OH)2 (PDF no. 30-0443) both with a hexagonal structure, and no other heterogeneous phase can be found. After final calcination in N2 at 300 °C, the as-obtained brownish-black product shows typical XRD reflections with three visible signals at 36.8 °, 42.8 ° and 61.8 °, corresponding to the (111), (200) and (220) crystal faces for the cubic phase NCO (PDF no. 10-0188), respectively, where the Ni2+ and Co2+ occupy the octahedral position of the center of the face O2-, and the position of the tetrahedral body is vacant (see Figure 1c).


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The recessed tetrahedral position of the NCO will render a three-dimensional (3D) porous channel for lithium-ion scattering and electron transport.1, 27 Along with the formation of rock-salt phase, the organic components in the Ni-Co-MOF would be in-situ converted into carbon materials, although we cannot detect the existence of the carbon from the XRD pattern of the NCO@NCS, maybe owing to its amorphous feature. According to the thermogravimetric-differential scanning calorimeter (TG-DSC) plots (Supporting Information (SI), Figure S1), the NCS in the hybrid can be calculated as ~23.7 wt.%. Raman spectroscopy of the NCO@NCS is shown in Figure 1d for further authenticating the specific micro-structures of the NCS. As fitted in Figure 1d, the ratio (i.e., IG/ID) of the G-band (1587.21 cm-1, disordered graphitic structure) to D-band (1373.13 cm-1, E2g phonon of sp2-C atoms) is ~1.16, indicating the desired electrical conductivity of the NCS.28-30 Besides, the I-band fitted at ~1199.79 cm-1 is related to the disorder and/or sp2sp3 bond in the graphitic lattices. And the D``-band centered at ~1493.80 cm-1 further verifies the existence of partial amorphous carbon in the NCS.29 X-ray photoelectron spectroscopy (XPS) technique was performed to investigate more specific compositions and detailed elemental oxidation states of the NCO@NCS sample. The XPS survey spectrum (SI Figure S2a) evidences the coexistence of these constituent elements including Ni, Co, O, C and N. The high-resolution Ni 2p and fitted plots are presented in Figure 1e. The peaks located at binding energies (BEs) of ~873 and ~855 eV belongs to the Ni 2p1/2 and Ni 2p3/2, respectively. The peak centered at ~854.6 eV is related to the Ni-O bond, and the other (~856.0 eV) corresponds to the Ni3+OH and typical multiplet splitting of Ni3+-O bond.1 Especially note that the peak of Ni 2p3/2 is much closer to ~854.9 eV (Ni2+), and far from the Ni3+ (~857.1 eV), indicative of



the main Ni(II) in the [email protected], 32 As for the Co 2p (Figure 1f), we can find two main peaks, i.e., Co 2p1/2 (~795.6 eV) and Co 2p3/2 (~780.0 eV), with an obvious peak separation of ~15 eV. Besides, the satellite (Sat.) peak (~786.3 eV) shows a peak separation of 6.3 eV with the Co 2p3/2, but the other (~802.6 eV) shows a difference of 7.0 eV with the Co 2p1/2. The BE separation observed here indicates that the Co2+ in the [email protected] As for the C 1s (SI Figure S2b), except the high-proportion C=C/C-C (~284.7 eV, ~0.57 at.%), typical C-OH (~286.1 eV) contributes to the improved wettability of carbonaceous materials,34 and the C=O bond (~287.7 eV) can improve electrode wettability, and restrain the co-intercalation of solvent molecules meanwhile, which is beneficial for improving reversible electrode capacity and Coulombic efficiency (CE). And the -COOH group (~288.3 eV) can participate in the Faraday reaction process, thus increasing the capacity to some extent.34 Besides the classic metal-oxygen (M-O) bonds, i.e., Ni-O (~529.2 eV) and Co-O between (~530.1 eV), other oxygen-based functionalities, including oxygen defects in NCO (O-I, ~530.6 eV),35 C-O bond in NCS (O-III, ~531.8 eV), and water molecules (O-II, ~531.2 eV) physically or chemically adsorbed on the surface of the NCO@NCS,33 can be discerned in typical O 1s (SI Figure S2c). As noted, the excess O species, to some extent, can reduce the electron conductivity of the NCO@NCS,36 but it will increase the wettability of the electrode and render more active sites for more efficient lithium storage meanwhile. Additionally, the N 1s (Figure 1g) exhibits three different N species including the pyridine N (N-6, ~398.2 eV), pyrrole N (N-5, ~399.5 eV) and graphitized N (N-Q, ~400.1 eV),3 and corresponding atomic ratio of the three are further collected (SI Table S1). The N-Q (~0.7 at.%), favoring for the electronic transport in the electrode,37 can compensate for the adverse effect of the


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functional oxygen-based groups upon the electronic conductivity of the NCS, to some extent. While the N-5 and N-6 with a total ratio of ~1.4 at.% can enhance the surfacecontrolled pseudo-capacitive process, and facilitate the smooth Li+ transportation,3 thus increasing the total capacity of the hybrid electrode to a certain extend. Figure 2a-c show FESEM images of the Ni-Co-MOF precursor. The Ni-Co-MOF possesses a non-uniform spherical shape of a size ranged from ~330 to ~1100 nm. The higher-magnification image (Figure 2c) of Ni-Co-MOF spheres visually evidences their smooth surface. Interestingly, after immersed in a KOH aqueous solution, the size of the obtained Ni-Co-hydroxide is almost unchanged (Figure 2d, e), but its surface turns out to be even rough (Figure 2f), which should be related to the accumulation of the Ni-Cohydroxide NPs during immersion in the KOH solution.38 And the final NCO@NCS product, as observed in Figure 2g-i, still keep the same morphology and size as those for the intermediate Ni-Co-hydroxide. To more detailed elucidate the micro-structures of the resulted samples, Figure 3a, b illustrate transmission electron microscopy (TEM) images of the Ni-Co-MOF. Typical solid spherical architecture with a smooth surface, similar to its FESEM observations (Figure 2a-c), is apparent for the precursor. As seen from the TEM images with different magnifications (Figure 3c-e), the Ni-Co-hydroxide essentially remains a solid feature, but with many discernable NPs of ~5 ‒ ~9 nm in size on the rough surface. Also, lots of inter-particle pores can be inspected in the Ni-Co-hydroxide, which renders the Ni-Cohydroxide with a large SSA of ~326.5 m2 g-1 and high pore volume (PV) of ~0.48 cm3 g-1 (SI Figure S3c, d; Table S2), much larger than those of the Ni-Co-MOF (SSA, ~6.6 m2 g-1; PV, ~0.04 cm3 g-1) (SI Figure S3a, b; Table S2). Figure 3f presents the TEM image



of the NCO@NCS. Solid sub-microspherical structure along with some partial hollow spheres is visible. The sharp color contrast in Figure 3g verifies the existence of the NCO NPs (black color) in the NCS (light color). Meanwhile, many void pores can be easily detectable in the NCO@NCS, which guarantees the fast immersion of the electrolyte and convenient Li+ diffusion. The interplanar spacing values in the high-resolution transmission electron microscopy (HRTEM) images of the nanoscaled NCO are ~0.15 and ~0.21 nm, corresponding to the interplanar distances of (220) and (200) crystalline planes of the NCO, respectively. Further selected area electron diffraction (SAED) image (Figure 3i) with the marked crystal planes visualizes the polycrystalline feature of the NCO phase. In addition, the discerned fringes of ~0.37 nm in the vesicle shell, ascribed to d(002) of the graphitic carbon,8 is apparently larger than typical graphite (0.34 nm) owing to the N doping in the NCS.13, 15 To figure out the compositional distributions in NCO@NCS specimen, scanning TEM (STEM) as well as elemental mapping was conducted (Figure 3j), and corresponding energy dispersive X-ray (EDX) mapping observations evidently verify the uniform distributions of the elemental Ni, Co, C, O and N in the composite sphere. The atomic Ni/Co/O ratio (SI Figure S4) is approximately 1 : 1 : 6, far from the stoichiometric ratio of the NCO. And the excess oxygen species should result from the adsorbed water and other oxygen-containing functional groups in the hybrid NCO@NCS. To visualize the NCO distribution in the NCS more clearly, we deliberately etch the NCO NPs with the HCl. As observed in Figure 3k, except the residual NCO NPs randomly dispersed in the NCS after etching, numerous hollow vesicles are presented in the NCS. And the size of NCO NPs lies mainly in the range of ~5.3 to ~17.8 nm, as examined form the statistical diagram (SI Figure S5), which is


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derived from the vesicle size in Figure 3k. The high-magnified TEM observation (Figure 3l) demonstrates that the shell of the vesicles is about 3 to 5 nm in thickness. It is worthy of noting that the shell of these carbon vesicles is highly graphitized with the cocatalysis of nickel-cobalt bimetals.39,

40

It is therefore easy to conclude that the nano-

dimensional NCO is uniformly encapsulated with a graphitic nano-carbon shell. Corresponding Brunauer-Emmett-Teller (BET) measurement (SI Figure S3e, f) derives a SSA of ~157.7 m2 g-1, a PV of ~0.27 cm3 g-1 and average pore diameter of ~3.7 nm (SI Table S2). The remarkable compositional and structural features would ensure more electroactive sites, convenient electron/ion transportation, extra space/physical buffer for accommodating

huge

volume

expansion,

flexible

matrix

towards

preventing

agglomeration of the NCO NPs during continuous lithiation/delithiation cycles and facilitate fast electrolyte immersion into the hybrid NCO@NCS for efficient Li-ion storage. Electrochemical Evaluation. As analyzed above, the remarkable structural and componential superiorities of the resulted NCO@NCS specimen promise its appealing application in LIBs as an anode. Figure 4a shows representative CV curves of the initial first, third and fifth cycles of the NCO@NCS electrode. In the first cycle, the cathodic peak (~0.30 V) corresponds to the reduction process of Ni2+ and Co2+ into Ni0 and Co0, and the peaks appearing at ~1.46 and ~2.28 V (vs. Li/Li+) in its anodic counterpart are related to the electrochemical oxidation of the Ni0/Co0 to bivalent Ni/Co.31 As for the 3rd and 5th CV cycles, the anode peaks coincide substantially with the first cycle, but the cathode ones move positively towards ~0.88 and ~1.35 V, which should result from the destruction and/or reconstruction of the [email protected] The good overlap of following



two CV profiles substantially verifies the excellent reversible lithiation/delithiation processes during the subsequent sweep cycles. Figure 4b illustrates the discharge-charge curves (200 mA g-1) of the NCO@NCS for different Li+-insertion/extraction cycles as indicated. During the 1st discharging cycle, a potential platform appears at ~0.65 V, which corresponds to the reduction process of the Ni(II)/Co(II). The two platforms located at 2.35 and 1.43 V during the initial charge cycle can be attributed to the oxidation process of the metallic Co/Ni into the Co(II)/Ni(II) species. Besides, the first discharge capacity of the NCO@NCS electrode is ~1146.6 mAh g-1 along with a charge capacity of~748.8 mAh g-1, resulting in an ICL of ~34.7%, i.e., the first CE of ~65.3%, mainly because of the electrolyte decomposition and formation of the solid electrolyte interphase (SEI) film.41 Also of note, the discharge capacity initially decreases till the 50th cycle, then increases by degrees up to ~816.2 mAh g-1 (the 410th cycle), and is finally stabilized at ~790.6 mAh g-1 (the 490th cycle), much larger than the theoretical capacity of the graphite anode.7,

8

The rate performance is an important index for practical

applications of LIBs. As plotted in Figure 4c, the NCO@NCS anode displays desirable Li-storage capability with average charging capacities of ~801.2, ~633.7, ~522.2, ~453.9, ~403.0, ~339.9 and ~282.8 mAh g-1 at various current rates of 100, 200, 500, 800, 1000, 1500 and 2000 mA g-1, respectively. Additionally, with the current density going back to 100 mA g-1, an average capacity still can reach ~656.4 mAh g-1, i.e., ~81.9% of the average value of the first ten cycles, indicating the remarkable rate properties of the NCO@NCS electrode. Encouragingly, the rate-dependent capacities of the NCO@NCS are far superior to other NCO-based electrodes reported recently (SI Table S3), such as NiCoO2 nanosheets@hollow carbon spheres (~631.7 mAh g-1 at 0.4 A g-1),42 mesoporous


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NiCo-NiCoO2/carbon xerogel (~861 mAh g-1 at 0.1 A g-1),32 and hollow NiCoO2 nanosheets@C (< 480 mAh g-1 at 0.4 A g-1).43 Figure 4d demonstrates cycling behaviors of the NCO@NCS at two different current densities of 200 and 500 mA g-1 over 500 consecutive discharge-charge cycles. In general, the progressive decrease in capacities, due to the structural damage in the electrode material and formation of SEI film.41,

23

are observed at the initial charging-

discharging process for two cases here. And the capacity then increases in subsequent cycles, as a result of the reversible formation of a polymeric layer, electrochemical activation process,27, 44 and synergistic interactions between the NCO and NCS.45 Finally, the capacities of the NCO@NCS remain as stable as ~674.1 mAh g-1 (500 mA g-1) and ~808.3 mAh g-1 (200 mA g-1). To further explore its striking power application, the longspan and high rate cycle performance of the NCO@NCS has been performed at a high current density of 1.0 A g-1, as illustrated in Figure 4e. Impressively, after 1000 uninterrupted cycles, a charging capacity of ~371.4 mAh g-1 is retained for the NCO@NCS anode, which fully authenticates its outstanding electrochemical stabilities, especially at high rates. Electrochemical impedance spectroscopy (EIS) has been further applied to provide necessary kinetic information related to the ohmic resistance (Rs), solid electrolyte interphase resistance (RSEI), charge-transfer resistance (Rct), and Warburg impedance.46 The Rs and RSEI values for the NCO @ NCS before and after the cycling test are very small and negligibly changed. More appealingly, the Rct value of the NCO@NCS electrode becomes even smaller with cycling (SI Figure S6), which decreases from 178 (the fresh) to 62 (the 500th) Ohm due to the activation process, followed by a small increase up to 88 (the 1000th) Ohm, attributed to the structure



damage and electrolyte decomposition during the cycling.47 The smaller Rct value with cycle undoubtedly favors for the long-term cyclic stability of the NCO@NCS anode at large current density.48 The physical properties of the NCO@NCS electrode after cycled for 1000 times at a current rate of 1.0 A g-1 were further investigated, as collected in Figure 5. As observed in Figure 5a ‒ c, the cycled NCO@NCS electrode with fully discharged state, similar to the fresh one (Figure 3g ‒ i), maintains a complete spherical morphology without any structural collapse. Higher-magnification TEM inspections (Figure 5d, e) visually evidence the good dispersion of the intact NCO NPs of < 10 nm in size in the carbon matrix, which is well confirmed by the STEM and EDX mapping observations (Figure 5d, e), confirming its superior structural retention capacity even over 1000 consecutive cycles at a high rate of 1.0 A g-1. The unique feature reveals that the flexible NCS matrix efficiently buffers volumetric change and serious aggregation of nanoscaled NCO particles over continuous lithiation/delithiation even at high rates.47 For in-depth understanding of the high-rate properties in aspects of reversible capacities and cycling behaviors, the Li+ transport process in the NCO@NCS anode is further investigated here. Typically, the Li+ diffusion coefficient (D) is derived from the CV curves at different scanning rates with the classical Randles-Sevcik equation: 49 3

1

1

i p  2.69 105 n 2 AD 2 v 2 C0

where the ip, n, A, v and ∆C0 are peak current, the number of electrons participating in reactions, electrode-electrolyte contact surface (geometric electrode area, ~1.13 cm2), sweep rate, and concentration change of Li+ ions in reactions, respectively. Typical CV curves of the NCO@NCS anode at various sweep rates as indicated are shown in Figure 16 ACS Paragon Plus Environment

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6a. Corresponding peak currents at the cathodic and anodic peaks, i.e., the Peakc and Peaka, as the function of v1/2 are plotted in Figure 6b. The Li+ diffusion coefficients for the anodic (Da) and cathodic (Dc) processes for the NCO@NCS electrode can be estimated as high as ~1.08 × 10-8 and ~2.97 × 10-8 cm2 s-1, respectively, which benefits from its characteristic compositional and porous structural merits. Corresponding kinetic analysis of the NCO@NCS anode is then conducted via CV tests. Figure 7a describes the CV curves at various sweeping rates ranged from 0.8 to 2.5 mV s-1. The relation between scanning rates and peak current is expressed by using the following Equations:50-52 i  av b

log i  log a +b log v where the a and b are a constant, and the adjustable parameter, respectively. As plotted in Figure 7b, the b value can be estimated as ~5.5 for the anodic process, suggesting the combination of battery (k2v1/2) and pseudocapacitive contribution (k1v) in the electrochemical Li-storage process of the hybrid electrode.50 And specific contributions of the two can be quantitatively divided by using the Equation: i(V) = k1v + k2v1/2.50-52 As demonstrated in Figure 7c, the pseudocapacitive contribution (the blue region) represents ~35.7% of the whole capacity at a high scanning rate of 1.6 mV s-1 for the NCO@NCS. Furthermore, Figure 4d collects the capacitive contributions at various sweep rates. Obviously, with the sweep rate increasing, the pseudocapacitive proportion gradually increases and even can reach ~50.8% in the case of 2.5 mV s-1. It highlights the remarkable capability of the unique NCO@NCS electrode to render convenient Li+



insertion/extraction and diffusion channels, and rapid electronic transport towards highrate capacities/cyclic stability of the hybrid NCO@NCS. To detailedly study the potential commercial prospect of the NCO@NCS, we assembled the NCO@NCS-based full device with the commercial NCM as the positive electrode. Due to the non-ignorable initial capacity loss issues for the NCO@NCS anode, the unique pre-lithiation technique is applied to effectively compensate for the first loss of lithium. Figure 8a displays the discharge-charge plots of the NCM/Li and NCO@NCS/Li half cells at 100 mA g-1, and the differential capacity vs. voltage plots for the NCO@NCS and NCM are profiled in Figure 8b. In relation to the charge-discharge profile (Figure 8a), the mass of the cathode material was 3.8 times that of the anode. Evidently, the average redox potential of the Ni2+/Ni3+/Ni4+ in the NCM is approximately 4.2 V, while the average reversible voltage for the Co2+/Ni2+ is around 1.0 V, which renders the NCO@NCS//NCM full device an average operating potential of 3.2 V. Figure 8c illustrates the discharge-charge plots of the full device at 100 mA g-1 (based on the positive electrode) within the electrochemical window from 2.5 to 4.2 V. Notably, the discharging capacities of the cell are always retained at ~100 ‒ ~124.3 mAh g-1 throughout the various rate cycles. The rate behaviors (Figure 8d) of the device are conducted within the current density range from 20 to 200 mA g-1 after 10 activation charge-discharge cycles at 10 mA g-1. The average discharge capacities of ~149.8, ~139.4, ~128.1, ~120.8, ~113.8 and ~102.7 mAh g-1 can be estimated based on the cathode as 20, 50, 80, 100, 150 and 200 mA g-1, respectively. The superb capacity consevation (~68.6%) of the device with a 10-time increase in current rates highlights its promising power applications. More impressively, the capacity still can return to ~125.7 mAh g-1 with the


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current rate going back to 20 mA g-1 again. Figure 8e presents the long-cycle test of the full battery at 100 mA g-1 after several low-current activation cycles. The charge/discharge capacities decay slowly with cycling for 200 times, along with an average charge capacity degradation of ~0.12 mAh g-1 per cycle, suggesting its good electrochemical stability of the full cell. More significantly, the CE values of the battery are always close to 100% throughout the whole cyclic process. With an average capacity of ~113 mAh g-1 throughout the cycles and average operating potential of 3.2 V in mind, an impressive specific energy density of ~217.0 Wh kg-1, which is larger than current commercial LIBs (~170 Wh kg-1),50 is obtained by the NCO@NCS//NCM device, considering a rational penalty factor (40%) to account for the whole weight of the involved electrolytes and other auxiliary components. CONCLUSION In conclusion, we first developed a three-step synthesis methodology, including the bottom-up synthesis of Ni-Co-MOF, phase transformation in KOH solution, and calcination in the N2 atmosphere, to fabricate the NCO@NCS, where the nanodimensional NCO encapsulated well in porous NCS framework, towards advanced LIBs. Thanks to its remarkable compositional and structural features, the resultant NCO@NCS was rendered with more electroactive sites, convenient electron/ion transportation, extra space/physical buffer for accommodating huge volume expansion, and flexible matrix towards

preventing

agglomeration

of

the

NCO

NPs

during

continuous

lithiation/delithiation cycles. And meanwhile, fast electrolyte immersion into the hybrid NCO@NCS was guaranteed for efficient Li-ion storage. More strikingly, the resultant NCO@NCS composite anode was endowed with appealing electrochemical Li-storage



properties with a large capacity of ~403.0 mAhg-1 and long-span electrochemical stability with ~371.4 mAh g-1 after 1000 cycles both at 1.0 A g-1. Furthermore, the NCO@NCS//NCM full device presented a competitive specific energy density of ~217.0 Wh kg-1 and cyclic stability with an average capacity decay of ~0.12 mAh g-1 per cycle, highlighting the extraordinary electrochemical potential of the NCO@NCS in LIBs. More significantly, we strongly believe that our work would promote the rational design in new-generation hybrid electrode materials with exceptional electrochemical performance for energy-related applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXXXX. XPS, EDX and TG-DSC data, N2 sorption isotherms, pore diameter distribution, particle size diagram, electrochemical comparisons with other anodes, and EIS profiles of the controlled experiments (PDF).

AUTHOR INFORMATION

Corresponding authors *E-mail: [email protected] (Prof. L. R. Hou) [email protected]; [email protected] (Prof. C. Z. Yuan) ORCID Linrui Hou: 0000-0002-3163-3391


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Changzhou Yuan: 0000-0002-6484-8970

Notes The authors declare no competing interests.

ACKNOLEDGEMENTS We greatfully appreciate the financial support from National Natural Science Foundation of China (Grant Nos. 51772127 and 51772131), Major Program of Shandong Province Natural Science Foundation (Grant No. ZR2018ZB0317), Taishan Scholars (Grant No. ts201712050) and Collaborative Innovation Center of Technology and Equipment for Biological Diagnosis and Therapy in Universities of Shandong.



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Figures and Captions

Figure 1 (a) Schematic illustration for the fabrication procedure of the NCO@NCS; (b) Wide-angle XRD patterns of the Ni-Co-MOF, Ni-Co-hydroxide and NCO@NCS, and corresponding digital photos of the three products; (c) Crystallographic structure of the NCO; (d) Raman spectrum of the NCO@NCS; and High resolution elemental (e) Ni, (f) Co and (g) N XPS spectra of the NCO@NCS.


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Figure 2 FESEM images of (a ‒ c) Ni-Co-MOF precursor, (d ‒ f) Ni-Co-hydroxide, and (g ‒ i) NCO@NCS products.



Figure 3 TEM images of (a, b) Ni-Co-MOF, (c ‒ e) Ni-Co-hydroxide, (f, g) NCO@NCS; (h) HRTEM image of the NCO@NCS, (i) SAED pattern, (j) STEM and corresponding EDX elemental Ni, Co, O, C and N mapping images of the NCO@NCS; (k ‒ m) TEM images of the NCO@NCS after partially etched by HCl.


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Figure 4 Electrochemical evaluation of the NCO@NCS anode: (a) CV curves (0.1 mV s1);

(b) Capacity-voltage profiles for the selected cycles (200 mA g-1); (c) Rate behaviors

at different current densities; (d) Cyclic performance at 200 and 500 mA g-1, and (e) longterm cycling stability at a current rate of 1.0 A g-1.



Figure 5 (a, b) FESEM, (c, d) TEM, (e) HRTEM, and (f) STEM and corresponding EDX elemental Ni, Co, O, C and N mapping images for the cycled NCO@NCS anode after 1000 cycles.


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Figure 6 (a) CV curves at various scanning rates as indicated, and (b) the relationship of the peak current (ip) and square root of scan rate (ν1/2) for the Peaka and Peakc of the NCO@NCS anode.



Figure 7 (a) CV curves, (b) corresponding relationship between anodic peak current and scan rate, (c) specific capacitive (blue) and battery-type (red) contributions to the total charge storage at a sweep rate of 1.6 mV s-1, and (d) normalized contribution proportions of the capacitive (blue) and battery-type (red) at various scanning rates with kinetics and quantitative analysis for the NCO@NCS electrode.


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Figure 8 (a) Charge-discharge profiles (100 mA g-1) and (b) differential capacity versus voltage plots of the NCM and NCO@NCS electrodes; (c) Charge-discharge plots (100 mA g-1) within the potential window from 2.5 to 4.0 V, (d) rate behaviors and (e) cycling stability (100 mA g-1) along with the CE data of the NCO@NCS//NCM full battery.



Table of Content


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226x125mm (150 x 150 DPI)

ACS Paragon Plus Environment

Formation of Nano-Dimensional NiCoO2 Encapsulated in Porous

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