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Phase Boundary Derived Pseudocapacitance Enhanced Nickel-Based Composites for Electrochemical Energy Storage Devices Bei Long, Muhammad-Sadeeq Balogun,* Lei Luo, Weitao Qiu, Yang Luo, Shuqin Song,* and Yexiang Tong* trode materials for the replacement of the conventional graphite.[1–6] However, they suffer from a variety of problems such as poor conductivity for the oxides/ hydroxyls/sulfides and bad structural stability for the nitrides.[7–11] Some of the common methods used to solve the above problems include nanostructuring,[12] carbon coating,[13,14] mixing with carbonbased materials,[15,16] and the construction of TMC composites.[17,18] The preparation of TMC composites usually combines the advantages of each material, leading to synergistic effect.[19–21] According to the study by J. Maier, the phase interface present in a composite often leads to lattice mismatch and therefore creates more active sites for energy storage, enabling the electrodes to exceed their theoretical capacity.[22,23] Moreover, the lattice mismatch is also beneficial for the transformation of lithium ions and electrons, which could also result in the improved rate performance. On the other hand, the predominant pseudocapacitive processes in lithium-ion batteries make active materials show better rate capability.[24] Many electrode materials such as MoO3,[25] SnO,[26] and SnS[27] have been found with excellent pseudocapacitive characteristics both in lithium and sodiumion batteries because these materials store more energy at the surface or near surface of the material by Faradaic charge transfer.[1] Pseudocapacitance materials can either be intrinsic (i.e., storage properties not based on particle sizes and morphologies such as RuO2 and MnO2) or extrinsic (i.e., storage properties based on the preparation of porous or nanometersized active materials such as V2O5 and MoO2).[27] Moreover, pseudocapacitive contributions have been obtained in some insertion, conversion, and alloying reaction of individual or single electrode materials, while their application in composites has been less studied.[28] In view of advances of various TMC composite electrodes, it is highly challenging to study their relationship to pseudocapacitance contributions. Among the commonly reported electrode materials with pseudocapacitive properties, nickel-based materials (NiO, Ni3S2, and Ni3N) have attracted intense attention due to their low cost and theoretical capacity higher than that of conventional graphite.[29]

Several strategies have been employed to improve the performance of energy storage devices through the development of new electrode materials. The construction of transition metal compound composite electrodes plays an important role in promoting the performance of energy storage devices. However, understandings of and insight into how to enhance the composites properties are rarely reported. Taking nickel-based compounds as an example, Ni3N@ Ni3S2 hybrid nanosheets are reported as a high-performance anode material for lithium-ion batteries that delivers higher lithium storage properties than the pristine Ni3N and Ni3S2 electrodes. This demonstrates that the phase boundaries between the Ni3N and Ni3S2 may contribute additional lithium storage, which leads to a synergistic effect via the high pseudocapacitance contribution from the outstanding conductivity of Ni3N and enhanced diffusion-controlled capacity of Ni3S2. The use of composites prepared through sulfuration of hydrothermally annealed nickel hydroxide-based precursor provides an enhancement of the energy storage properties. These results provide an important approach for increasing the electrochemical activity of composites by the combined effect of interfacial mismatch and pseudocapacitance, as well as understandings of the mechanism of the enhancement of the composite electrode properties.

1. Introduction Owing to their high theoretical capacities, transition metal compounds (TMCs) have been recognized as promising elecB. Long, Prof. S. Song, Prof. Y. Tong The Key Lab of Low-Carbon Chemistry and Energy Conservation of Guangdong Province School of Materials Science and Engineering Sun Yat-Sen University Guangzhou 510275, P. R. China E-mail: [email protected]; [email protected] Dr. M.-S. Balogun, L. Luo, W. Qiu, Y. Luo, Prof. Y. Tong MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry KLGHEI of Environment and Energy Chemistry School of Chemistry Sun Yat-Sen University Guangzhou 510275, P. R. China E-mail: [email protected] Prof. Y. Tong Department of Chemistry Shantou University Shantou 515063, China

DOI: 10.1002/aenm.201701681

Adv. Energy Mater. 2017, 1701681

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Nickel nitride (Ni3N) displays excellent conductivity that contributes to its outstanding performance in various field, such as lithium-ion batteries,[30] supercapacitors,[31] and electrocatalysis.[32] It also has the strongest pseudocapacitive characteristic compared to nickel oxide and sulfide because good conductivity materials usually exhibit excellent kinetics and higher pseudocapacitance[29] but deliver low storage capacity (i.e., diffusioncontrolled capacity) for lithium-ion batteries, especially at high current densities. Various composites of NiO and Ni3S2 with other TMCs have been reported with improved electrochemical properties,[33–36] while Ni3N-based composites with their corresponding oxide or sulfide are rarely found. Therefore, owing to the high pseudocapacitance of Ni3N, ideally Ni3N-based composites should be developed in order to improve lithium storage capacity (diffusion-controlled capacity) at higher rate capabilities. Meanwhile, compared to Ni3N, Ni3S2 exhibites a higher theoretical capacity and exhibits a parallel discharge plateau at a particular voltage, indicating its higher diffusion-controlled capacity.[37] Its poor conductivity could definitely lead to low capacity at high rates and poor pseudocapacitance properties. Nevertheless, the poor conductivity of Ni3S2 can be improved to enable its use in high-performance lithium-ion batteries. Hence, as mentioned above, the phase interface present between the lattice mismatched components of composite materials could create more active sites for lithium storage, and an electrode material with good conductivity shows high pseudocapacitance. We propose that the synergistic effect arising from the composite interface boundaries and pseudocapacitance contributions could display excellent lithium storage performance even at higher rate. In this work, we prepared Ni3N@ Ni3S2 nanosheets composite by calcinating Ni-based precursor in ammonia followed by sulfuration. Ni3S2 is preferred over NiO due to the better conductivity of the sulfides than oxides. The storage properties of Ni3N, Ni3N@Ni3S2, and Ni3S2 were compared under the same test conditions. Ni3N@Ni3S2 showed the largest capacity, high pseudocapacitance, and best cyclic stability as the anode material for lithium-ion batteries. These findings may provide a new approach for resolving issues in the energy storage field.

2. Result and Discussion 2.1. Synthesis and Composition Characterization Ni3N@Ni3S2 was prepared by calcinating the Ni-based precursor at 380 °C under NH3 atmosphere followed by hydrothermal sulfuration at 180 °C for 8 h, as depicted in Scheme 1. The X-ray diffractometry (XRD) spectra of the samples are shown in Figure 1a. The Ni3N (PDF#10-2080) was obtained with the generation of a by-product (Ni) (Figure 1a(i)). It should be noted that Ni shows a smaller contribution for energy storage

but plays an important role in the reversible formation/decomposition of some constituents in solid electrolyte interface films, therefore decreasing the capacity loss.[38,39] The sulfuration of Ni3N can be controlled by tuning the thiourea concentration. At the desired thiourea concentration of 7 mmol L−1 and sulfuration time of 8 h, Ni3N@Ni3S2 (denoted as Ni3N@Ni3S2-7) was achieved. XRD spectra confirmed the presence of both Ni3N and Ni3S2 phases confirming the formation of the composite (Figure 1a(ii) and Figure S1a, Supporting Information). By increasing the thiourea concentration to 21 mmol L−1, the Ni3N@Ni3S2 composite (denoted as Ni3N@Ni3S2-21) could still be obtained and showed increased intensity of the Ni3S2 phase (Figure S1b, Supporting Information). As the thiourea concentration was increased to 42 mmol L−1, XRD spectra confirmed that there were no traces of Ni3N and the sample was purely Ni3S2 (PDF#44-1418) (Figure 1a(iii) and Figure S1c, Supporting Information) verifying the formation of Ni3S2. To study the effect of sulfuration, the rate performance and electrochemical impedance spectroscopy (EIS) analyses were carried out for the Ni3N@Ni3S2-7 and Ni3N@Ni3S2-21 electrodes. Figure S2 (Supporting Information) shows that the Ni3N@Ni3S2-7 electrode displayed the conductivity and lithium storage performance superior to those of the Ni3N@Ni3S2-21 electrode. Hence, the Ni3N@Ni3S2-7 electrode (denoted as Ni3N@Ni3S2) was selected for further characterization and electrochemical analysis. The possible reactions related to the preparation process of Ni3N, Ni3N@Ni3S2, and Ni3S2 are expressed by the following equation 12Ni (OH)2 + 8NH3 → 3Ni 3N + 3Ni + 24H2O + 5/2N2 (1) 3Ni 3N + 2xCN2H4S + 4 xH2O → 3 − xNi 3N + xNi 3S2 + 4 xNH4+ + 2xCO2 + 1/2xN 2

(2)

where (x < 3) Ni 3N + 2CN 2H4S + 4H2O → Ni 3S2 + 4NH4 + + 2CO2 + 1/2N 2 (3) X-ray photoelectron spectroscopy (XPS) provides detailed information about the composition of the composite. The XPS survey spectra (Figure S3, Supporting Information) show that the three samples exhibited similar elemental compositions but that of Ni3N@Ni3S2 consisted of S 2p, while that of Ni3N showed the N 1s peak. High-resolution S 2p spectra presented in Figure 1b showed that the Ni3N was S-free and that the S peak intensities increased with the addition of thiourea for the Ni3N@Ni3S2 and Ni3S2. This is consistent with the generation of Ni3S2 and the results obtained from the XRD analysis. By contrast, the signals of N 1s (Figure 1c) weakened gradually and its characteristic peak was barely detected in the as-prepared Ni3S2, suggesting that Ni3N was fully transformed into Ni3S2. Therefore, the XRD and XPS results confirmed the successful

Scheme 1. Schematic illustration procedure of Ni3N@Ni3S2.


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Figure 1. a) XRD patterns, b) high-resolution S 2p, and c) N 1s XPS spectra of Ni3N, Ni3N@Ni3S2, and Ni3S2.

preparation of Ni3N@Ni3S2 composite due to the presence of S and N peak signals.

2.2. Morphological Characterization The variation of the morphology in the preparation process was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figures S4a and S2a (Supporting Information), the Ni precursor and Ni3N consisted of uniform nanosheets, demonstrating that the nitridation process had no effect on the morphology of the Ni precursor. After partial sulfuration of Ni3N, the Ni3N@Ni3S2 still exhibited the nanosheets’ morphology, but many nanosheets were assembled in a packing structure during the sulfuration process (Figure 2b). The as-prepared Ni3N@Ni3S2 (with a high Ni3S2 content, i.e., Ni3N@Ni3S2-21) and Ni3S2 also showed similar changes (Figure S4b,c, Supporting Information), suggesting morphological modification of the Ni3N. According to the TEM image of 7 mmol L−1-Ni3N@Ni3S2 in Figure 2c, the assembly of nanoparticles leads to the generation of nanosheets with many holes, which is a very good structure for the relief of volume expansion during cyclic tests.[40,41] The high-resolution TEM (HRTEM) image shown in Figure 2d consists of different lattice spaces, suggesting the presence of different compounds. Lattice spaces observed from different points (square dazes in Figure 2d) were further magnified as depicted in Figure 2e,f and showed distinct interfaces. In Figure 2e, the lattice spacings of 0.203 and 0.238 nm were ascribed to the (111) and (003) planes of Ni3N and Ni3S2, respectively. Moreover, the probable interface marked by a red curve with a 55° angular mismatch between the (111) plane of Ni3N and the (003) plane of Ni3S2 was surveyed. According to Figure 2f, the (003) crystal plane of the Ni3S2 and the (111) and (110) planes of the Ni3N were detected at the same time with a 39° mismatch angle between the (111) plane of Ni3N and the (003) plane of Ni3S2. It was observed that the phase boundaries between the Ni3N and Ni3S2 consisted of the same plane at a random mismatch angle, suggesting uniform formation of the Ni3N and Ni3S2 interfaces within the nanosheets. These lattice boundaries and mismatch angles confirmed the presence of both Ni3N and Ni3S2 and the phase interface relationship that could lead to the formation of active sites toward higher lithium storage.[23] Additionally, the selected area electron diffraction (SAED) pattern (inset of Figure 2d)


corresponding to Figure 2f showed two visible diffraction rings that were assigned to the Ni3S2 (003) plane and the Ni3N (111) plane, further confirming the hybrid heterojunction. Elemental mapping from the composite nanosheets demonstrated the homogenous distribution of the Ni, N, and S elements (Figure 2g–k), further revealing the successful fabrication of the composite electrode. It is expected that the as-prepared Ni3N@ Ni3S2 sample can be a suitable anode material for lithium-ion batteries. Meanwhile, the microstructure and particles size of Ni3N, Ni3N@Ni3S2, and Ni3S2 were compared in order to analyze the effect of microstructural variation on electrochemical properties, as displayed in Figure S5a–c (Supporting Information). The three samples exhibit a similar structure. The HRTEM images (Figure S5d,e, Supporting Information) and magnified TEM images (inset of Figure S5d,e, Supporting Information) with clear lattice fringe prove the successful preparation of pure Ni3N and Ni3S2. However, the Ni3N@Ni3S2 and Ni3S2 nanoparticles show obvious agglomeration after the sulfuration. To prove this, particle-sized statistics were obtained in order to construct the particle size distribution (Figure S5f–h, Supporting Information). Compared to Ni3N, the Ni3N@Ni3S2 nanoparticles show larger size in ≈30–40 nm range and larger particles with the sizes of 60–70 nm also exist. For Ni3S2, the small 10–20 nm particles disappear and the large 40–50 nm particles distinctly increases. This phenomenon led to decrease of the specific surface area and therefore the reduction of the contact area between the active material and the electrolyte. As presented in Figure S6a (Supporting Information), the Brunauer-Emmett-Teller (BET) specific surface area gradually decreases with the generation of Ni3S2. The BET specific surface areas of Ni3N, Ni3N@Ni3S2, and Ni3S2 are 28.9, 20.1, and 17.5 cm3 g−1, respectively. Meanwhile, the pore volume (Figure S6b, Supporting Information) also gradually decreases with the nanoparticle agglomeration. This is also one of the reasons for the rapid decline of the electrochemical performance of Ni3S2. 2.3. Lithium Storage Performance The lithium storage properties of the Ni3N, Ni3N@Ni3S2, and Ni3S2 electrodes were tested in a coin-cell-type configuration. First, cyclic voltammetry (CV) curves are studied to analyze their electrochemical properties. The CV curves of Ni3N (Figure 3a) show four pairs of redox peaks. The reduction peaks

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Figure 2. SEM images of a) Ni3N and b) Ni3N@Ni3S2. c) Low-resolution TEM image and d) high-resolution TEM (HRTEM) image of Ni3N@Ni3S2 with the SAED pattern (inset) corresponding to green dashed boxes. e,f) Magnified HRTEM from the yellow and green dashed boxes of (d). g–j) EDS elemental mapping of the Ni3N@Ni3S2 sample and k) overlay of Ni, S, and N maps.

at 0.56, 0.69, and 1.53 V are ascribed to the gradual lithiation of Ni3N[30,42] Ni 3N + x( x ≤ 3)Li → xNi + Li x Ni 3−x N (4) The peak at 1.26 V may correspond to the lithium insertion of Ni(OH)2 originating from slight incomplete transformation of Ni-based precursor.[43] During the second and third cycles, the peaks at 0.56 and 1.26 V could disappear or decrease, indicating irreversible reaction and capacity decline due to polarization. Meanwhile, the peaks at 0.69 and 1.53 V shift to 0.82 and 1.51 V, respectively, indicating that the oxidation peaks show similar changes. For the CV curves of Ni3S2 (Figure 3b), the cathodic peaks at 0.75, 1.23, and 1.58 V are attributed to the Li insertion of Ni3S2[37,44] Ni 3S2 + 4Li → 3Ni + 2Li 2S (5) The anodic peaks of 1.40 and 1.96 are the corresponding delithiation. It is obvious that the CV curves of Ni3N@Ni3S2 (Figure 3c) are different from those of Ni3N and Ni3S2. The main reduction peak at ≈1.37 V during the first cycle shifts to 0.98 V in the second and third cycles, which is slightly similar to the redox potential of Ni3N and Ni3S2. Moreover, a comparison of the redox peaks of the three electrodes shows that the Ni3N@Ni3S2 electrode exhibited the biggest CV curve area suggesting its high electroactivity over the bare Ni3N and Ni3S2 (Figure 3d).[7]


The lithium storage mechanism and the redox reaction obtained from the CV curves can also be characterized using the charge/discharge profiles of the electrodes. The first charge/discharge curves of the three samples were carried out within the 0.01–3.0 V voltage window at the current density of 0.1 A g−1 as shown in Figure 4a. It is noted that the discharge curves of the Ni3N@Ni3S2 electrode display no obvious plateau as a whole during the first and second discharge cycles (i.e., a quasilinear discharge curve) compared to those of Ni3N and Ni3S2 electrodes (Figure 4a,b, respectively), which is in accordance with the CV profiles. Nevertheless, it shows broad peaks ≈0.75–1.25 V, indicating the lithium intercalation process. The Ni3N@Ni3S2 electrode delivered a high specific lithiation capacity (1012 mA h g−1) that was higher than those of Ni3N (487 mA h g−1) and Ni3S2 (864 mA h g−1). We suggest that the higher capacity of the composite electrode can be attributed to the synergistic activity created by the phase interfaces between the individual Ni3N and Ni3S2 electrodes. According to Wu et al., storage taking place at the interface of two materials usually occurs at the lower voltage region of the discharge curves, which has little effect on the bulk insertion reactions.[23] Therefore, the percentage of capacity at low voltage (