NiS0.66 Heterostructures and Their

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Article 2

0.66

The Assembly of Multifunctional NiP/NiS Heterostructures and Their Superstructure for High Lithium and Sodium Anodic Performance Tian Wu, Sanpei Zhang, Qiming He, Xiaoheng Hong, Fan Wang, Xiangwei Wu, Jianhua Yang, and Zhaoyin Wen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07939 • Publication Date (Web): 01 Aug 2017 Downloaded from http://pubs.acs.org on August 1, 2017

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ACS Applied Materials & Interfaces

The

Assembly

of

Multifunctional

Ni2P/NiS0.66

Heterostructures and Their Superstructure for High Lithium and Sodium Anodic Performance Tian Wu a, b, Sanpei Zhang a, b, Qiming He a, b, Xiaoheng Hong a, b, Fan Wang a, b, Xiangwei Wu a, Jianhua Yang a and Zhaoyin Wen a, * a

CAS key laboratory of Materials for Energy Conversion, Shanghai Institute of

Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China b

University of Chinese Academy of Sciences, Beijing, 100039, P.R. China

*E-mail: [email protected]

Key

words:

Li-ion

battery,

Na-ion

battery,

sulfur

dopant,

nanocrystals,

heterostructures, superstructure.

Abstract The combination of the structure designs at the microscopic and macroscopic level can efficiently enable the electrode materials with greatly enhanced lithium and sodium storage. In this manuscript, the construction of Ni2P/NiS0.66 heterostructures and their assembly into a superstructure at the nanoscale were successfully achieved by a facile and effective strategy. In the obtained superstructure, the Ni2P/NiS0.66 heterostructures are homogeneously coated with ultrathin carbon layers (HT-NPS@C) and, at the same time, assembled into a yolk-shell nanosphere. When evaluated as the anode materials for Li-ion batteries, the HT-NPS@C delivers a high reversible

ACS Paragon Plus Environment


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capacity of 430 mA h g−1 after 200 cycles at 200 mA g−1 and ultrastable cyclability with negligible capacity loss over 500 cycles. Furthermore, the coin-type full cell with LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode and HT-NPS@C anode deliver a high specific capacity of 323.5 mA h g-1 after 50 cycles at 0.3 A g-1. Apart from excellent performance as promising anode materials for LIBs, the Na-ion batteries with HT-NPS@C

sphere

electrodes

also

manifest

remarkable

electrochemical

performance.

Introduction With rechargeable batteries technology poised to move into larger-scale applications, such as electric vehicles and portable devices, intensive research has targeted the optimization of Li-ion (LIBs) and Na-ion batteries (NIBs).1-3 Graphite, the most commonly used commercial anode material for LIBs, suffers from limited theoretical specific capacity (372 mA h g−1) and rate capability.4-5 Meanwhile, the graphite anode is still challenging to achieve the ideal practical capacity for the NIBs (only 31 mA h g−1).6-9 To develop high-performance LIBs and NIBs that satisfy the requirements for future energy storage systems, various electrode materials (Si,10 Sn,11 and Ge12) based on alloy/dealloy mechanism have been exploited and studied.13-15 However, these high-capacity anode materials are still faced many problems, such as extreme large-volume variation (~ 400%) during cycling and large irreversible capacity. Apart from the Li-alloying materials, transformation-type materials, such as NiO,16 MnF2,17 MoS2,18 and so forth, with higher theoretical capacity have been


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recently investigated as the alternative anode materials for LIBs. Recently, transition metal phosphides with high conductivity and activity have been attracted great attention as high-performance anode materials for LIBs and NIBs. For instance, Feng et al.19 synthesized sandwiched Ni2P nanoparticles between graphene sheets using a template-assisted strategy. The electrode with sandwiched Ni2P nanoparticles delivered a high reversible capacity of 625 mA h g-1 at 0.2 C and remarkable high-rate cyclability. Qian and his co-workers20 fabricated the core–shell Sn4P3/C nanocomposites by a high-energy mechanical milling method. The electrode based on Sn4P3/C nanocomposites exhibited high reversible capacity of around 500 mA h g-1 at 0.1 A g-1 for 150 cycles. Besides, various structure designs for metal phosphides including nanotubes,21 nanosheets,22 hollow spheres,23 and nanorods23-24 have been developed to acquire encouraging lithium and sodium storage performance. The design and synthesis of transition metal phosphides nanostructures have endowed them with a large quantity of active sites and high surface-to-volume ratio for Li and Na storage. Nonetheless, owing to their high surface energy, the designed nanocrystals with large numbers of active surface are inevitable to aggregate during the long cycles. Moreover, the transport kinetics of charge carriers is greatly hindered across the over-accumulated nanocrystals. Precisely constructing the heterostructures and their assembly into a three-dimensional (3D) superstructure can not only fast the transport kinetics in the crystal structures but also preserve the nanocrystals structures during the discharge and charge process. However, the construction of Ni2P-based



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heterostructures and their assembly into a superstructure for LIBs and SIBs has rarely been reported. Herein, we demonstrate a simple and effective method to construct Ni2P/NiS0.66 heterostructures and control their assembly into a superstructure (denoted as HT-NPS@C) at the nanoscale. In the obtained superstructure, the Ni2P/NiS0.66 heterostructures are homogeneously coated with ultrathin carbon layers and, at the same time, assembled into a yolk-shell nanosphere. When evaluated as the anode materials for Li-ion batteries, the HT-NPS@C delivers a high reversible capacity of 430 mA h g−1 after 200 cycles at 200 mA g−1 and ultrastable cyclability with negligible capacity loss over 500 cycles. Furthermore, the coin-type full cell with LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode and HT-NPS@C anode deliver a high specific capacity of 323.5 mA h g-1 after 50 cycles at 0.3 A g-1. Apart from excellent performance as promising anode materials for LIBs, the Na-ion batteries with HT-NPS@C

sphere

electrodes

also

manifest

remarkable

electrochemical

performance. We believe the present strategy for HT-NPS@C can be further applied for other nanomaterials to get enhanced electrochemical response. Results and discussion


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Figure 1. Schematic representation of the formation process via low-temperature heating processes. Step 1: Formation of the Ni-precursor solid spheres at 150 oC for 10 hours. Step 2: Preparation of the carbon-coated nanocrystals-assembly nickel phosphides spheres with yolk-shell structure (NP@C) after phosphidation in N2 at 450 °C for 2 hours. Step 3: Fabrication of Ni2P/carbon yolk-shell spheres with abundant Ni2P/NiS0.66 heterointerfaces and S/C defects (HT-NPS@C) after subsequent sulfidation in N2 at 450 °C for 2 hours. The overall strategy for the preparation of HT-NPS@C is schematically depicted in Figure 1. The detail preparation procedures are described in the Supporting Information. In the first step, Ni-precursors are prepared from a modified hydrothermal method.25-26 Then the obtained nanospheres are thermally treated at 450



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°C in PH3 vapor (Step 2), during which inner Ni-precursors of the nanospheres are phosphorized to Ni2P. Meanwhile, the outer organic layers convert into ultrathin carbon layers (denoted as NP@C). Subsequently, the fast sulfidation process (Step 3) is carried out to construct the Ni2P/NiS0.66 heterostructures and the bifunctional sulfur-doped coating layers (HT-NPS@C).

Figure 2. (a-b) SEM images of HT-NPS@C. (c) XRD pattern of HT-NPS@C. (d-e) TEM images of HT-NPS@C. (f) Electron diffraction of HT-NPS@C. (g-i) HRTEM


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images of HT-NPS@C, (j) the selected TEM image, and the corresponding elemental EDS mapping images of (k) nickel, (l) phosphorus, (m) carbon, and (n) sulfur.

The scanning electron microscope (SEM) image (Figure S1a) and transmission electron microscopy (TEM) image (Figure S1b) show that the as-synthesized Ni-precursor consists of microspheres with an average diameter of about 1.3 um. The broadness and weakness of all the peaks for Ni-precursors reveal the poor crystallinity and small size of the crystallite (Figure S2). After being annealed in PH3 vapor, the Ni-precursor nanospheres are transformed into Ni2P@C nanospheres, as confirmed by energy dispersive X-ray spectroscopy (EDS) images (Figure S3a-3d) and powder X-ray diffraction (XRD) measurements (Figure S4). Compared with the Ni-precursors, no obvious changes in structure and size can be observed for the NP@C nanospheres. After the subsequent sulfidation with thiourea at the same temperature, the final particles (HT-NPS@C) are obtained. The SEM images (Figure 2a-b) of the HT-NPS@C show that these particles exhibit a morphology of well-maintained spheres. XRD analysis further confirms the phase-pure properties of the final particles. As demonstrated in Figure 2c, the HT-NPS@C sample exhibits a clear hexagonal structure of Ni2P (JCPDS 74-1385). Furthermore, a small and weak peak of NiS0.66 (JCPDS 44-1418) at 2θ = 33.15°can also be found in the pattern. It might be the result of the structural change from Ni2P into NiS0.66 during the sulfidation process. The detail structure and morphology of the HT-NPS@C samples are studied by TEM and high-resolution TEM (HRTEM) analysis. As shown in Figure 2d, Ni2P spheres possess a yolk-shell architecture with average diameters of



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the yolk and shell about 600 and 1000 nm, respectively. The higher magnification TEM image in Figure 2e and corresponding selected-area electron diffraction (SAED) patterns (Figure 2f) show that the shell and yolk are dotted with ultrafine crystallized Ni2P nanoparticles. Each nanocrystal has a diameter of approximately 25 nm. HRTEM images (Figure 2g-f) further reveal that onion-like carbon shells with a thickness of 2~5 nm cover the surface of Ni2P nanoparticles. The carbon content in the HT-NPS@C sample estimated from the TGA/DSC analysis (Figure S5) is 3.3 wt%. Notably, the additional lattice spacing of 0.41 nm (Figure 2i) can also be observed, which is assigned to the (011) facet of hexagonal NiS0.66. Interestingly, the heterostructure constructed by the (011) facet of NiS0.66 and the neighboring (111) facets of Ni2P can be clearly observed from the HRTEM image. More importantly, the Ni2P/ NiS0.66 heterostructure is also homogeneously coated by the ultrathin carbon layers at the nanoscale. Although the transition metals phosphides with well-defined spheres nanostructure have been reported previously23,

27-28

, the construction of

Ni2P/NiS0.66 heterostructures and their assembly into a superstructure are successfully achieved for the first time. X-ray elemental mappings recorded from an individual yolk-shell sphere (Figure 2j-n) reveal that different elements of Ni (green), P (purple), C (red) and S (yellow) are homogeneously distributed throughout the whole yolk-shell sphere. The TEM-EDS analysis was used to determine the S content in the HT-NPS@C sample. As shown in Figure S6, the S content of HT-NPS@C is about 2.8 at%, which is consistent with the X-ray photoelectron spectroscopy (XPS) result (ca. 2.5 at%) (Table. S1).


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Figure 3. High-resolution XPS spectra of HT-NPS@C and NP@C in the Ni 2p (a) and P 2p (b) regions. (c) High-resolution S 2p XPS spectra of HT-NPS@C at the surface and an etched domain of 3 nm depth. (d) Nitrogen adsorption−desorption isotherm of the HT-NPS@C. The inset shows the pore size distribution.

The detailed surface chemistry of the HT-NPS@C and NP@C samples was characterized by XPS measurements. Figure 3a-c present the Ni 2p, P 2p and S 2p spectra of NP@C and HT-NPS@C samples. The survey scan spectra from XPS analysis of the HT-NPS@C and NP@C are shown in Figure S7. The S 2p peak of the HT-NPS@C sample located at ~160 eV provides the evidence of the introduction of S species. As can be seen in Figure 3a, the high-resolution XPS spectrum of Ni element of the NP@C indicates two main peaks located at 854.2 and 871.6 eV, corresponding



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to the Ni 2p3/2 and Ni 2p1/2 of the Ni-P bonding,29-30 respectively. After sulfidation, the Ni 2p3/2 and Ni 2p1/2 peaks of the HT-NPS@C display obvious shift to 853.7 and 871.1 eV, respectively. The shifts of Ni 2p peak and P 2p peak (Figure 3b) suggest the substitution of P by highly electronegative S.31 In order to understand the chemical bonding of S atoms with carbon, the high-resolution C 1s spectra of HT-NPS@C and NP@C were fitted and are presented in Figure S8a-b. It is noted that the peak located at 283.9 eV, which is assigned to C–S–C, demonstrates the effective atomic hybridization of sulfur into the carbon lattice. Furthermore, in-depth XPS tests were conducted to verify the sulfur dopant. Figure 3c presents the S 2p XPS at the surface and an etched domain of 3 nm depth, where S species both are clearly identified. The S 2p peak can be deconvoluted into four peaks located at 164.9 eV, 163.7 eV, 162.4 eV and 161.3 eV, which are assigned to S 2p3/2, S 2p1/2 of –C–Sx–C–, sulfide, and thiolate, respectively.31-33 These results further confirm the successful introduction of the S dopant in the Ni2P/carbon nanocrystals. Full nitrogen sorption isotherms (Figure 3d) of HT-NPS@C were carried out to obtain information on the specific surface

area

and

corresponding

pore

size

distribution.

The

nitrogen

adsorption-desorption isotherms of the as-prepared HT-NPS@C indicate typical IV curves of mesoporous materials. The Barrett−Joyner−Halenda (BJH) pore size distribution curve (inset of Figure 3d) shows a broad peak ranging from 2 to 15 nm and a narrow peak at 5.0 nm, confirming the nanoporous nature of HT-NPS@C. The Brunauer-Emmett-Teller (BET) specific surface area of HT-NPS@C is calculated to be 88.4 m2 g-1. Such a high specific area of yolk-shell structure with mesoporous


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nanocrystals is desirable for Li-ion and Na-ion batteries, which can provide sufficient interface between the electroactive materials and the electrolyte.34

Figure 4.

Electrochemical performance for lithium storage. (a) Cyclic

voltammogram curves of HT-NPS@C electrodes scanned between 0.01 and 3.0 V at a rate of 0.1 mV s−1 (b) Galvanostatic curves during the initial five cycles. (c) Rate cycling behavior of the HT-NPS@C and NP@C electrodes. (d) & (e) Cycling



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performance of the HT-NPS@C and NP@C electrodes at 0.2 A g−1 for 200 cycles and 0.4 A g−1 for 500 continuous cycles.

Hoping these interesting HT-NPS@C spheres with abundant Ni2P/NiS0.66 heterointerfaces, S/C defects, and high surface area as promising anode materials, we have prepared the Li-ion batteries with the HT-NPS@C based cathode and lithium metal foil as the counter electrodes. Figure 4a shows the CV curves of the HT-NPS@C electrode for the initial five scans at a scan rate of 1 mV s-1 from 0.01 to 3 V vs. Li/Li+. By assigning the cathodic and anodic peaks to the redox reactions, the electrochemical mechanism for Ni2P is considered to be a conversion-type process.35-36. Figure 4b displays the Galvanostatic charge/discharge profiles of the HT-NPS@C electrode for the initial five cycles at 0.1 A g-1. The obscure plateau centered at ~ 1.9 V is ascribed to the oxidation of Li2S to Li ions and NiS0.66,37-38 which is in accordance with the CV results. In addition, the HT-NPS@C electrode exhibits a high initial discharge and charge capacity of 992.8 mA h g-1 and 538.5 mA h g−1, respectively. The irreversible capacity is attributed to the formation of a SEI layer in the first cycle.39 Notably, the contribution of NiS0.66 to the capacity should be negligible (less than 30 mA h g−1) as the S content is considerably low (2.58 at % in the average). NP@C contains the same content of Ni2P crystals, but the NP@C-based battery can only deliver a reversible capacity of 400 mA h g−1, lower than that of HT-NPS@C (Figure S9). The enhanced reversible capacity suggests that the Ni2P/NiS0.66 and S/C heterointerfaces in HT-NPS@C nanocrystals can produce more Li-ion insertion sites and faster reaction kinetics for reoxidation of Ni to Ni2P.


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Electrochemical impedance spectroscopy (EIS) measurements were carried out on the HT-NPS@C and NP@C electrodes. Figure S10 shows that the HT-NPS@C electrode exhibits the reduced resistance, further confirming its fast Li reaction kinetics. The superiority of the HT-NPS@C electrode is also demonstrated via their prevailing rate capability in Figure 4c. Moreover, the HT-NPS@C electrode shows a high capacity retention of 78% over 200 cycles (Figure 4d). In contrast, the capacity of the NP@C electrode is 174.2 mA h g−1 after 200 cycles with a much lower capacity retention of 45%. Long-term cycling stability is also a vital parameter for the practicability of high-power batteries. As shown in Figure 4e, the HT-NPS@C electrode was measured at a higher current density of 0.4 A g−1 for 500 cycles. High reversible capacity and sustainable capacity retention could be achieved for the HT-NPS@C electrode, which remains a stable capacity of 423.2 mA h g−1 without significant capacity decay. The enhancement of cycling performance of HT-NPS@C is remarkable compared to the previous reports on Ni2P nanomaterials (Table S2). Meanwhile, as shown in Figure 5a-b, the HT-NPS@C superstructure is still detectable even after 500 cycles under the current density of 400 mA g−1. The structural evolution of the carbon-coated Ni2P/NiS0.66 nanoparticles during the initial lithiation and delithiation processes are schematically depicted in Figure 5c. During the initial lithiation and delithiation processes, the protective layer is elastic and capable of tolerating large volume expansion of Ni2P/NiS0.66 nanoparticles without particles pulverization and structural rupture. Significantly, with the effects of the elastic coating, the electrode is expected to retain the original structure, and a stable



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SEI film is formed on the nanocrystal surface during cycling. Therefore, the carbon-coated Ni2P/NiS0.66 yolk-shell spheres are likely to be preserved over prolonged cycling, thereby leading to high capacity retention and excellent cycling stability.

Figure 5. SEM (a) and TEM (b) images of the HT-NPS@C electrode after 500 cycles at 0.2 A g-1, (c) Structural evolution of the carbon-coated Ni2P/NiS0.66 nanoparticle electrode during cycling.

According to our knowledge, metal phosphides have rarely been reported in a full coin cell. To demonstration of newly fabricated HT-NPS@C as a potential anode in the coin-type full cells, the LiNi1/3Co1/3Mn1/3O2 (LNCMO) cathode was performed


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within the voltage range of 1.5-4.0 V. As the capacity delivered by the commercial LNCMO cathodes is around 150 mA h g−1 (Figure S11), the mass of the HT-NPS@C electrode was limited to about 1.3 mg cm-2 for matching of the electrodes. The specific capacity of the SPNC-YS//LNCMO is calculated based on the mass of the anode. Figure 6a shows the HT-NPS@C anode has the initial specific charge and discharge capacity of 513.3 and 476.7 mA h g-1, respectively. In the inset of Figure 6a, the HT-NPS@C//LNCMO full cell shows a reversible capacity of 349.3 mA h g−1 at the current density of 0.3 A g-1 over 50 cycles, which confirms that the as-prepared anode material is a suitable anode material for lithium ion batteries. Furthermore, the rate capability of the HT-NPS@C//LNCMO full cell was tested at incremental current densities from 0.2 to 4 A g-1 for each five cycles. The HT-NPS@C electrode exhibits reversible capacity 443.5, 326.1, 231.7, 203.6, 140.3, and 83.4 mA h g−1 at 0.2, 0.4, 0.8, 1, 2 and 4 A g−1, respectively (Figure 6b). The rate cycling behavior is similar to the result of half-cells in Figure 4c. When the current density returns to the initial rate after high-rate cycling, the capacity can return 349.8 mA h g−1. These results confirm that the HT-NPS@C holds promise in the anode application for energy storage devices.



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Figure 6. Full cell lithium ion battery performance. (a) Charge–discharge profile of the HT-NPS@C//LNCMO cells, the inset shows cyclic performance profile of the HT-NPS@C//LNCMO cells (b) Rate performance of the HT-NPS@C//LNCMO cells.

Apart from the excellent lithium storage performance, the sodium storage behavior of the HT-NPS@C electrodes was also investigated and shown in Figure 7. Figure 7a shows the CV curves of the HT-NPS@C electrode. In the first cycle, the peak at 0.6 V could be attributed to the formation of SEI films and the reduction of Ni2P to Ni and Na3P. The discharge/charge profiles (Figure 7b) display that the initial discharge and charge capacities of the HT-NPS@C electrodes are 519.1 and 320.8 mA h g−1, respectively. Due to the larger size of Na ion compared to Li ion, the reversible capacity of the HT-NPS@C electrode for sodium storage is lower than that of lithium storage. Rate performance of the HT-NPS@C and NP@C electrodes in NIBs was also studied. As shown in Figure 7c, the reversible capacities of the HT-NPS@C electrode are 320.8, 222.7, 180.6 147.1, 133.2 and 111.5 mA h g−1 at current densities of 0.1, 0.2, 0.4, 0.8, 1, 2 and 2 A g−1, respectively. More importantly, when the current density returns to the initial rate after high-rate cycling, the capacity can recover to


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189.7 mA h g−1. As expected, the rate performance is superior to those of NP@C electrode. Figure 7d shows that the HT-NPS@C electrodes exhibit superior cycle stability with a decaying rate of 0.94% capacity per cycle. It is identified that the HT-NPS@C electrode also has high stability upon Na+ intercalation/deintercalation.

Figure 7. Electrochemical performance of the HT-NPS@C electrodes for NIBs: (a) Cyclic voltammogram curves of the HT-NPS@C electrode collected at a scan rate of 0.2 mV s-1 within the voltage range of 0.01-3 V. (b) Galvanotactic curves during initial cycles. (c) Rate cycling behavior (d) Cycle performance of the HT-NPS@C electrodes and NP@C electrodes at 0.2 A g−1.

The above evidence clearly demonstrates that our HT-NPS@C anode possesses high reversible Li+/Na+ capacities, excellent Coulombic efficiency and super long-life



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cycling capability at high rates, which should be tightly related to its unique superstructure that offers the following advantages. Firstly, the prepared HT-NPS@C with large surface area can offer enough interface area between the HT-NPS@C electrode and electrolyte. Moreover, the nanocrystal-assembled-spheres provide large amounts of active sites, effectively giving rise to the rate performance of the batteries. Secondly, the Ni2P/NiS0.66 heterostructures of the nanocrystal in the spheres facilitate the fast transport of Li+ and electron. More importantly, the highly flexible ultrathin carbon nanosheets that homogeneously anchor on the surface of the Ni2P/NiS0.66 heterostructures can effectively alleviate the volume change of HT-NPS@C during cycling, which is beneficial to the ultralong cycling performance.

Conclusions In summary, uniform nanocrystals-assembly HT-NPS@C were successfully synthesized by a facile hydrothermal method and subsequent heating processes. The Ni2P/NiS0.66 heterointerfaces and S/C defects constructed in HT-NPS@C improves the reaction kinetics and ions-transport speeds, which can accommodate more Li+/Na+ and also facilitate the insertion and extraction of Li+/Na+. In addition, the nanocrystal-assembly yolk-shell structure with a high surface area, appropriate pore size, and large pore volume can serve as high-capacity reservoirs for Li+/Na+, and also provide a large electrode/electrolyte contact area and short diffusion distance. Owing to these unique advantages, the synthesized HT-NPS@C exhibited high and stable performance when used as anode materials in LIBs and NIBs. We believe that our


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work could provide a promising strategy for designing other advanced nanocomposite materials.

Supporting Information Experimental details, SEM, XRD, TEM, TG/DSC, XPS, EDS, EIS, and electrochemical performance of HT-NPS@C and NP@C sample.

Acknowledgements

The authors are grateful for funding from the National Science Foundation of China (NFSC) project No. 51672300, No. 51402330 and No. 51372262; opening projects of CAS Key Laboratory of Materials for Energy Conversion.

We thank Prof. B. V. R. Chowdari (School of Materials Science and Engineering, Nanyang Technological University) for helpful discussion.

Author information

Corresponding Author Tel: +86-21-52411704, Fax: +86-21-52413903

*E-mail: [email protected]

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NiS0.66 Heterostructures and Their

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