Atomically dispersed Ni induced by ultrahigh N-doped carbon enables stable sodium storage

•Ultrahigh N-doping in Ni3S4@HNC is controllable by the intermediate’s O content•High-loading atomically dispersed Ni is obtained during discharging process•Atomically dispersed Ni enhances solid-phase contact and the conversion depth•High-capacity maintenance of Ni3S4@HNC for SIBs is achieved The phase interface builds an effective bridge between multi-phase reactions, which is significantly important for chemical reaction kinetics and depth in various research fields, for example, high-capacity conversion-type electrodes in rechargeable batteries. However, their lack of enough solid-phase contact degree causes sluggish electrodynamics and continuous loss of reversible capacities during long cycling. Herein, we have explored an ultrahigh phase contact with atomically dispersed Ni via discharging the active material nickel sulfide, leading to high-capacity maintenance. Meanwhile, the loading of Ni atoms reached 54.9 wt % owing to the synergic stabilizing of ultrahigh N-doping and the discharging product Na2S, providing new possibilities for its potential application. This work provides insights into the design of high-capacity conversion-type electrodes and the improvement of solid-state chemical reactions. Building phase interface with enough solid-phase contact is of great importance for improving chemical reaction kinetics and depth. High dispersion of electrode materials, especially at the atomic-level, are known for high interface contact, yet their potential application in batteries is restricted due to low loading. Herein, the atomically dispersed metal Ni (Ni in Ni–N–C is 54.9 wt %) with high loading was achieved by ultrahigh N-doping carbon (N/N–C:29.5 wt %) during the discharging process of nickel sulfide, leading to good reversibility and high-capacity maintenance owing to ultrahigh phase contact during long cycling for sodium-ion batteries. It delivers a stable cycling life (0.061% capacity decay per cycle) compared with the poor cyclability (0.418%) for the Ni agglomeration electrode with lower N-doping. The assembled pouch cells achieve robust stability (92.1% after 50 cycles). DFT calculations reveal that ultrahigh N-doping and electrochemically formed Na2S can provide thermally stable Na2S/Ni/NC structures, inhibiting Ni agglomeration during cycling. Building phase interface with enough solid-phase contact is of great importance for improving chemical reaction kinetics and depth. High dispersion of electrode materials, especially at the atomic-level, are known for high interface contact, yet their potential application in batteries is restricted due to low loading. Herein, the atomically dispersed metal Ni (Ni in Ni–N–C is 54.9 wt %) with high loading was achieved by ultrahigh N-doping carbon (N/N–C:29.5 wt %) during the discharging process of nickel sulfide, leading to good reversibility and high-capacity maintenance owing to ultrahigh phase contact during long cycling for sodium-ion batteries. It delivers a stable cycling life (0.061% capacity decay per cycle) compared with the poor cyclability (0.418%) for the Ni agglomeration electrode with lower N-doping. The assembled pouch cells achieve robust stability (92.1% after 50 cycles). DFT calculations reveal that ultrahigh N-doping and electrochemically formed Na2S can provide thermally stable Na2S/Ni/NC structures, inhibiting Ni agglomeration during cycling. 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Sci. 2017; 10: 1757-1763Crossref Google Scholar controllable ultrahigh N-doped carbon-based materials are likely to present opportunities for exploring more possibilities for performance optimization. Herein, we present the ultrahigh N-doping (N/N–C=29.5 wt %) into carbon matrix for supporting abundant active sites, thereby achieving high-loading atomically dispersed metal Ni (the ratio of Ni/Ni–N–C is as high as 54.9 wt %) during discharging, resulting in highly persistent conversion reaction reversibility. Consequently, stable cycling of the Ni3S4 electrode is easily attained compared with that of the low-N-doping electrodes. The mechanism of adjustable high N-doping in carbon matrix and the stability of high-loading atomic-level dispersed Ni were investigated via density functional theory (DFT) calculations. The practical application was further verified in a pouch cell. Ultrahigh N-doped carbon-based material was fabricated through a two-step strategy (Figure 1A). Initially, Ni-MOF (1) was prepared with nickel nitrate and isophthalic acid by a typical hydrothermal reaction (Figure S1). Then, the calcination process in 673 K was employed in situ to form Ni in a carbon framework with O-containing (22.2 wt %) functional groups ([email protected](O-3)). Finally, the intermediate product was further calcinated with thiourea at 573 K to achieve the ultrahigh N-doped (N/N–C=29.5 wt %) carbon-based nickel sulfide material (Ni3S4@HNC) (Figures 1B and S2). The molar ratio of N/Ni in Ni3S4@HNC is calculated to be 1:0.96 from elemental analysis. Such high N-doping could provide abundant active sites in the carbon material for immobilizing high-loading metal. In addition, via adjusting the initial calcination temperature, the oxygen contents of intermediate products ([email protected](O-2) and [email protected](O-1)) decrease, obtaining decreased N-doped composites under similar synthesis conditions (Figure S3, denoted as Ni3S4@MNC, and NiSx@LNC, in which M and L stand for middle and low, respectively). Moreover, to quantitatively analyze the relationship between N-doping content (yN) in the final materials and O content in the intermediate products (yO), a linear fitting was completed with the yN = 16.93yO+0.64 (Figure 1C) to guide controllable N-doping synthesis. The chemical states were carried out via Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra. Obviously, the C–N intensity gradually decreases from Ni3S4@HNC to Ni3S4@MNC and NiSx@LNC (Figure S4). Moreover, high-resolution N 1s spectra (Figure 1F) can be deconvoluted into three peaks: pyridinic N, pyrrolic N, and graphitic N. Among them, pyridinic N (67.4 at %) is the dominant N-containing functional group in Ni3S4@HNC, and its ratio decreases to 39.7 at % and 26.4 at % for Ni3S4@MNC and NiSx@LNC. Interestingly, the C=O ratio exhibits a similar downtrend with pyridinic N (Figure S5). The above results suggest that the C=O bond in intermediate products might play an important role in improving N-doped content in final materials via promoting the formation of pyridinic N. Also, the organic ligands with a higher O ratio are used to prepare carbon precursor, thereby obtaining improved N-doped content in synthesized composites (Figure S6), which firmly supports the positive effect of the oxygen-containing precursor in the N-doped carbon framework. The relationship between the O atom in its carbon host and its N-doping is revealed via DFT calculations (Figures 1D and S7),39Jung E. Shin H. Lee B.H. Efremov V. Lee S. Lee H.S. Kim J. Hooch Antink W. Park S. 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Mater. 2019; 31: 1903955Crossref PubMed Scopus (295) Google Scholar Meanwhile, pyridinic N is maintained in the obtained N configuration after structural optimization due to its easier formation compared with other N configurations. The as-synthesized Ni3S4@HNC, Ni3S4@MNC, and NiSx@LNC display a uniform hierarchical sphere structure with a diameter of 1–3 μm (Figures S8 and S9), which consists of nanoparticles encapsulated in an ultrathin N-doped carbon layer with the thickness of 1.5 nm via electron energy loss spectroscopy (EELS), indicating their similar microstructures and surface properties. N2 adsorption-desorption analysis confirms a Brunauer-Emmett-Teller (BET) surface area of 7.9 m2 g−1 of Ni3S4@HNC with the mesopore mainly around 3.8 nm (Figure S10A). The Ni3S4 crystal structures along [311] crystallographic directions are demonstrated in Figure S9 with the clear N-doped carbon layer and uniformly distributed Ni, S, N, and C elements. Higher N-doping causes a lower graphitic degree of carbon and introduces plenty of defects in it, confirmed by Raman spectrum (Figure S10C). In addition, pure highly N-doped carbon (HNC) obtained via removing Ni3S4 from Ni3S4@HNC inherits the microsphere-like morphology assembled from nanoparticles (Figure S11). The elemental analysis reveals the Ni3S4 content of 67.8 wt % in Ni3S4@HNC (Table S2), which is close to 69.6 wt % determined by the thermal gravimetric analysis (TGA) (Figure S10B). The interaction between HNC and Ni3S4 during the initial discharge-charge process was investigated by ex situ Raman, XPS, and FTIR spectra, and cell architecture is shown in Scheme S1. A reversible shift process occurred for the C–N bond (Figure 2B): blue shift upon discharging and red shift upon charging. Considering that Ni possesses unpaired electrons in its 3d orbitals due to its outer electron distribution of 3d84s2, a large number of spin-polarized electrons from its 3d orbitals can be formed during discharge process. This led to highly localized d orbitals with high electron density and benefited the bond between Ni and N atoms. Ex situ N 1s spectra further display the continuous downshift in the discharging and upshift in the charging (Figure 2C), corresponding to the formation of a Ni-N bond between the N-doped carbon framework and Ni discharged product. The integrated ratio of the D1 and G bands (ID1/IG)45Ding J. Wang H. Li Z. Kohandehghan A. Cui K. Xu Z. Zahiri B. Tan X. Lotfabad E.M. Olsen B.C. Mitlin D. 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Moreover, the high-angle annular dark field scanning transmission electron microscope (HAADF-STEM) images display numerous bright spots (≈2–3 Å) as Ni atoms with uniform distribution of C, N, Ni, and S elements in the electrode (Figures 2G and S12). High-resolution transmission electron microscope (HRTEM) further indicates the smallest crystal regions around 1–2 nm with short lattice fringes in sodiated Ni3S4@HNC electrode (Figure S13). In contrast, Ni3S4@MNC, and NiSx@LNC electrodes show larger crystals with the size of 5–10 and 10–20 nm, respectively, indicating that a higher N content in bulk materials can effectively inhibit crystal agglomeration and fabricate the formation of atomic-level materials at discharged state via the Ni–N and Ni–C bonds. In addition, a similar phenomenon occurs in the fully desodiated Ni3S4@HNC electrode (Figure S14), which is that small Ni3S4 nanograins spread over the electrode. In comparison, severe phase agglomeration leads to the irreversible conversion reaction, leaving Ni and Na2S crystals in the fully desodiated Ni3S4@MNC and NiSx@LNC electrodes, which demonstrates the irreversible conversion caused by larger-size discharged products. The stability of Na2S/Ni/NC structures for Ni3S4@HNC, Ni3S4@MNC, and NiSx@LNC with different C/N ratio at fully discharged state is calculated by comparing the formation energies. The C/N atomic ratios of HNC, MNC, and LNC are determined to 2.07, 2.85, and 4.1, respectively (Figure 2H). The formation energies of Na2S/Ni/NC compounds gradually decrease from positive to negative with increased N content (Figure 2I), indicating more thermally stable Ni atoms and an energetically favorable process, which suppresses the agglomeration of Ni crystals during the cycling process.48Cui X. Xiao J. Wu Y. Du P. Si R. Yang H. et al.A graphene composite material with single cobalt active sites: a highly efficient counter electrode for dye-sensitized solar cells.Angew. Chem. Int. Ed. 2016; 55: 6708-6712Crossref PubMed Scopus (200) Google Scholar Importantly, this zero-point lies between the C/N value of 2 and 3, demonstrating the key role of HNC in realizing high dispersion and high loading of Ni atoms. Additionally, the calculated mass proportion of Ni in Ni–N–C is about 54.9 wt % (see supplemental experimental procedures). High loading of atomic-level Ni in the composite is mainly attributed to the ultrahigh N-doping in the carbon matrix that provides enough active sites to fix Ni atoms (Scheme 1). The electrochemical behaviors of Ni3S4@HNC, Ni3S4@MNC, and NiSx@LNC are evaluated in a carbonated electrolyte. The cyclic voltammetry (Figure 3A ) of Ni3S4@HNC displays obvious cathodic and anodic peaks at around 0.95 and 1.68 V, belonging to a conversion reaction between Ni3S4 and Ni/Na2S. Additionally, the redox potential difference (ΔE) of Ni3S4@HNC is 0.73 V, which is lower than that of Ni3S4@MNC (0.78 V) and NiSx@LNC (0.85 V) owing to higher interfacial reaction kinetics of atomic-level dispersed Ni. Charge/discharge profiles of Ni3S4@HNC (Figure 3B) indicated an initial discharge and charge capacities of 713.1 and 603.4 mAh g−1 (based on the mass of entire composite) with an initial coulombic efficiency (ICE) of 84.6%, higher than the that of Ni3S4@MNC and NiSx@LNC with 76.4% and 73.3%, respectively. In situ X-ray diffraction (XRD) further examines the conversion reversibility of Ni3S4@HNC during cycling process (Figure 3C). Upon discharging, the (311) plane of Ni3S4 gradually disappears accompanied by the appearance of (220) pattern of Na2S. The redox reaction corresponds to: Ni3S4 + 8Na+ + 8e– → 3Ni + 4Na2S, which involves an eight-electron transfer reaction. Upon charging, the reverse process occurs: original cubic Ni3S4 phase regenerates with low crystalline, suggesting complete conversion of Ni and Na2S into Ni3S4. Ex situ XRD further show the Ni3S4 diffraction peaks in fully desodiated Ni3S4@HNC (Figure S15), in agreement with the in situ XRD results. It is calculated that Ni3S4 of Ni3S4@HNC performs a high reversible capacity (780.7 mAh g−1 based on the mass of Ni3S4) via removing the capacity contribution of HNC (Figure S16), and extra capacity beyond its theoretical capacity of Ni3S4 (702 mAh g−1) mainly ascribed to surface capacitance from spin-polarized electron of Ni product at discharged states.49Li Q. Li H. Xia Q. Hu Z. Zhu Y. Yan S. Ge C. Zhang Q. Wang X. Shang X. et al.Extra storage capacity in transition metal oxide lithium-ion batteries revealed by in situ magnetometry.Nat. Mater. 2021; 20: 76-83Crossref PubMed Scopus (228) Google ScholarScheme 1Schematic illustration of Ni3S4@HNC and atomically dispersed Ni in the highly N-doped carbon in the discharge stateView Large Image Figure ViewerDownload Hi-res image Download (PPT) The Ni3S4@HNC displays a high reversible capacity of 443.4 ± 4.8 mAh g−1 with an average capacity retention of 87.9% ± 0.7% over 200 cycles and an average coulombic efficiency of 99.4% ± 0.5% after 5th cycle at 0.72 A g−1 for 3 cells (Figures 3D and S17). In comparison, Ni3S4@MNC and NiSx@LNC retain only 16.4% ± 0.3% and 4.7% ± 1.7% of the initial capacities, respectively. The average charge and discharge voltages of Ni3S4@HNC are about 1.66 and 0.90 V, respectively. The specific peaks at 1.3 and 1.6