WP Nanocrystals on N,P Dual-Doped Carbon Nanosheets with Deeply Analyzed Catalytic Mechanisms for Lithium–Sulfur Batteries

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLES14 Nov 2022WP Nanocrystals on N,P Dual-Doped Carbon Nanosheets with Deeply Analyzed Catalytic Mechanisms for Lithium–Sulfur Batteries Peng Wang, Zhengchunyu Zhang, Ning Song, Xuguang An, Jie Liu, Jinkui Feng, Baojuan Xi and Shenglin Xiong Peng Wang School of Chemistry and Chemical Engineering, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 , Zhengchunyu Zhang School of Chemistry and Chemical Engineering, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 , Ning Song School of Chemistry and Chemical Engineering, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 , Xuguang An School of Mechanical Engineering, Chengdu University, Chengdu 610106 , Jie Liu The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050 , Jinkui Feng School of Materials Science and Engineering, Shandong University, Jinan 250061 , Baojuan Xi *Correspondence authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 and Shenglin Xiong *Correspondence authors: E-mail Address: [email protected] E-mail Address: [email protected] School of Chemistry and Chemical Engineering, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100 https://doi.org/10.31635/ccschem.022.202202163 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The use of transition-metal phosphides (TMPs) as catalytic materials to accelerate kinetics of lithium polysulfide (LiPS) conversion has unique advantages. Nevertheless, simple and low-cost preparation strategies are still required for the synthesis of novel TMPs with satisfactory performance. Importantly, the in-depth understanding of the effect of intrinsic interaction between catalytic materials and LiPSs on the promoted kinetics remains limited. Herein, a novel structure of tungsten phosphide (WP) nanocrystals decorated on N,P codoped carbon sheets (WP/NPC) with uniform dispersion is designed by a structure-oriented strategy to promote LiPS redox kinetics. The electrochemical kinetics measurements coupled with density functional theory computations and in situ/ex situ characterizations demonstrate that the strong interaction through W–S bonding and the favorable interfacial charge state of WP-LiPSs promote the nucleation and dissociation of Li2S. Benefiting from this superiority, the WP/NPG-based lithium–sulfur batteries indicate significantly improved electrochemical performance with good cycling life and excellent rate capability. This work provides a methodology for the design of TMP-involved electrode materials and a fundamental understanding of the intrinsic mechanism of catalysis. Download figure Download PowerPoint Introduction With the rapid growth in the production of portable electronics and electric vehicles, the traditional lithium–ion battery system shows a series of deficiencies in terms of energy density, safety, and cost, which has motivated the development and application of emerging energy storage devices.1–3 Among a variety of candidates, lithium–sulfur batteries (LSBs) integrate a multielectron electrochemical reaction and high average voltage of about 2.15 V and are endowed with high theoretical energy density.4,5 Combined with the features of richness of S in the earth, low cost, and environmental benignity, LSBs are regarded as the next-generation battery system with the most potential for application.5,6 However, affected by sluggish redox kinetics and the insulating nature of S, LSBs are faced with incomplete conversion of the sulfur and the "shuttle effect" from the dissolved intermediate polysulfides (LiPSs).7–9 In past research, the shuttle effect was alleviated to some extent by designing porous carbon materials as S hosts to physically limit the dissolution of LiPSs.10–12 Considering the continuous generation of LiPSs and the weak affinity toward polar LiPSs, the space confinement and physical adsorption by carbon materials cannot enable the long cycle of batteries. Inspired by the flood management strategy, the shuttle effect can be fundamentally suppressed by accelerating the conversion between LiPSs and Li2S instead of roughly blocking the LiPSs dissolution.13–15 In this respect, carbon-composited transition metal compounds with high catalytic activity can act as an accelerator of S to achieve high-performance LSBs.16 Further, regulating the type of transition metals and the exposure of active sites can successfully engineer the catalytic behavior.17–19 As a nascent catalytic material, transition-metal phosphides (TMPs), characterized by metallic electrical conductivity and moderate adsorption toward LiPSs, can furnish smooth channels for ions/electrons, which have aroused tremendous research interest.20,21 In recent years, a wide range of TMPs (CoP, Ni2P, FeP, and MoP) have been applied to prompt the LiPSs redox kinetics, and the underlying mechanisms of regulating LiPS transformation have been also explored.22–25 Despite these remarkable achievements, research into tungsten phosphide (WP) in the Li–S system has rarely been reported. With regard to synthesis, the phosphating process usually requires multiple complex steps and a large amount of phosphorus sources (such as NaH2PO2 or a high-boiling organic solvent).26 In addition, it is of great urgency to fully expose the active site of TMPs and achieve structural stability in the catalytic process through structural design. In this respect, it is extremely valuable for the development of convenient and environmentally friendly strategies toward synthesizing TMPs with excellent catalytic activity. Furthermore, theoretical studies have been effectively used to investigate the corresponding catalytic mechanism. For example, the catalytic materials can achieve accelerated Li2S decomposition in the charging process by reducing the diffusion energy barrier of lithium ions and promote Li2S nucleation in the discharging process by decreasing the Gibbs-free energy changes required for LiPSs conversion.27,28 However, the effect of interaction between catalytic materials and LiPSs in the acceleration of electron/ion transport remains to be explored. To address the above issues, a "structure-oriented template" combined with an "in situ self-phosphating" strategy was designed to prepare ultrafine tungsten phosphide (WP) nanocrystals on N,P codoped carbon sheets (WP/NPC) as the catalytic S host. This ingenious synthesis process harvested the WP/NPC through a one-step reaction without the addition of an additional phosphorus source. Systematic electrochemical analyses and structural characterizations showed us that the WP/NPC was rich in catalytic active sites and contributed to the strong affinity with and accelerated bidirectional conversion toward LiPSs. Correspondingly, the as-assembled LSBs delivered stable cycling stability of 786 mAh g−1 after 600 cycles at 0.5 C (corresponding to a capacity decay rate of 0.05% per cycle), superior rate performance of 740 mAh g−1 at 6 C, and high S-loading performance. With a combination of between in situ characterizations and density functional theory (DFT) computations, the electrocatalytic mechanisms were further investigated. On the one hand, this favorable interfacial charge state effectively reduces the electron transfer resistance, which promotes the reduction of Li2S4, that is, the Li2S/Li2S2 nucleation. On the other hand, the strong interaction between WP and Li2S through W–S bonding demonstrates that the WP specializes in "tearing" Li–S bonding to facilitate the dissociation of Li2S. We shed some light on a deeper understanding of LiPS electrodeposition and decomposition reactivity, which provides the necessary theoretical and experimental basis for the large-scale application of LSBs. Experimental Section Synthesis of NPC and WP/NPC The WP/NPC was synthesized through a one-step metal chelate-assisted template self-sacrifice method. In a typical synthesis, 0.3 g of ammonium metatungstate (H28N6O41W12) and 1 mL of phytic acid (PA) were mixed into 50 mL of distilled water with violent stirring to form a uniform solution. Then 5 g of melamine (MA) was added into the above solution followed by stirring it at room temperature to guarantee the full reaction. The precursor MA–PA–W12O39 hybrid was obtained after drying the above mixture in an oven at 80 °C. The as-synthesized precursor MA–PA–W12O39 was heated at 850 °C for 2 h under an Ar atmosphere with a heating rate of 5 °C min−1 to obtain the WP/NPG. For comparison, the NPG matrix was synthesized with similar procedures without the introduction of H28N6O41W12. Preparation of [email protected] and [email protected]/NPC-based cathodes The [email protected] and [email protected]/NPC cathodes were prepared through a traditional melting-infiltrating method. First, the sublimed sulfur power and WP/NPC (NPC) were mixed thoroughly (7:3 by mass) and heated at 155 °C for 12 h and then 185 °C for 2 h in an Ar-filled glass bottle to attain [email protected] and [email protected]/NPC. Then the prepared [email protected] or [email protected]/NPC, acetylene black, and polyvinylidene fluoride (PVDF) were mixed in N-methylpyrrolidone with the mass ratio of 8:1:1 and blading onto the carbon-coated Al foil. The obtained electrodes were dried in a vacuum oven at 80 °C for 12 h and cut into discs with a diameter of 12 mm. The sulfur loading in the [email protected] and [email protected]/NPC cathodes was about 1.1–10.5 mg cm−2. Materials characterization The phase composition and surface chemistry of the samples were characterized by X-ray diffraction (XRD, SmartLab, using Cu Kα radiation, Rigaku, Japan) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi spectrometer, Thermo Fisher, Waltham, Massachusetts, USA). The structure and morphology were characterized by high-resolution field-emission transmission electron microscopy (ARM-200F, JEOL, Shojima City, Tokyo, Japan), field-emission scanning electron microscopy (Gemini300, Zeiss, Oberkochen, Bartenburg, Germany). The Brunauer–Emmett–Teller (BET)-specific surface areas and pore size distribution were obtained by N2 sorption measurement (ASAP 2460 system, Micromeritics, Norcross, Georgia, USA). Thermogravimetric analysis (TGA, STA 449 F3, NETZSCH, Selb, Germany) was measured under the air with a heating rate of 10 °C min−1 from 30 to 800 °C to measure the content of WP. It should be mentioned that the WP can be fully oxidized into WO3 after calcining it to 800 °C at air atmosphere. In this contribution, the results were calculated based on the following equation: 2 WP + 11 / 2 O 2 → 2 WO 3 + P 2 O 5 WP was transformed into WO3, and the P2O5 was vaporized at high temperature, accompanied by a mass decrease of 8%. We assume that the total mass of WP/NPG is 1, and the mass percentage of NPG in the WP/NPG is x, so the mass percentage of WP is (1 − x). Then, x − 8% (1 − x) = 8%. So, we get x = 14.8%, and the content of WP in WP/NPG is 85.2%. Electrochemical measurements The electrodes for the measurement of symmetric cells, Li2S nucleation, and decomposition were prepared by the WP/NPC (or NPC) and PVDF with the mass ratio of 9:1. The electrolyte was 0.15 M Li2S6 electrolyte (in 1,3-dioxolane/ dimethoxyethane (DOL/DME) with 2.0 wt % LiNO3). The symmetric cells were assembled with two identical electrodes as working and counter electrodes. And the cyclic voltammetry (CV) measurements were carried out in the voltage range of −0.8 to 0.8 V at the scan rate of 0.2 mV s−1. The electrochemical impedance spectrum (EIS) tests were performed on the CHI–760E electrochemical workstation with the applied frequency range from 0.1 Hz to 100 kHz. The cells for Li2S nucleation and decomposition were assembled by the above active electrodes as working electrode and lithium foil as counter electrode, in which 20 μL of Li2S6 electrolyte and 15 μL of blank electrolyte (without Li2S6) were employed as catholyte and anolyte, respectively. During the Li2S nucleation measurement, the assembled cells were discharged galvanostatically to 2.06 V and afterwards kept at a potentiostatical voltage of 2.05 V until the current decreased to 10−5 A. For the following Li2S dissolution measurement, the cells were potentiostatically charged at 2.35 V until the current was below 10−5 A. The Tafel plots were conducted on the same cell systems with Li2S nucleation, which were tested at a CHI–760E electrochemical workstation at a scan rate of 2 mV s−1 in the voltage range from −150 to +150 mV. And the exchange current density of the oxidation/reduction reactions were calculated through manually fitting them according to the Bulter–Volmer equation. The LSBs were assembled in CR2016 coin-type cells with the as-prepared [email protected] and [email protected] WP/NPC cathodes, Celgard 2325 separator, and conventional Li–S electrolyte (1.0 M LiTFSI in DOL/DME (v/v = 1:1) with 1 wt % LiNO3) in an Ar-filled glove box. Lithium ion diffusion coefficients were calculated by a series of CVs at different scan rates, and the peak current data were analyzed with the Randles–Sevcik equation I P = ( 2.65 × 10 5 ) n 1 .5 S D Li + 0.5 C Li + v 0 .5 where IP represents the peak current, n is the number of transferred electrons (for Li–S batteries, n = 2), S is the electrode area, CLi+ represents the lithium ion concentration in the electrolyte, and v indicates the scanning rate. The CV and charging/discharging test were conducted in a CHI–760E electrochemical workstation and LAND CT-2001A battery test station over the range of 1.7–2.8 V versus Li+/Li. DFT calculations All the DFT calculations were conducted using the Vienna Ab-initio Simulation Package following the projector-augmented wave method. The ground state electronic calculations were calculated by the generalized gradient approximation and the Perdew–Burke–Ernzerhof functions. The supercell of WP and NPG containing 4 × 4 unit cells was used. During the process of geometry optimization, a kinetic energy cutoff of 400 eV and 2 × 2 × 1 Monkhorst-Pack k-mesh for the Brillouin zone were used. The convergence tolerance was reached when the energy change was smaller than 10−6 eV, and 0.01 eV Å−1 for maximum residual force. The spin polarization was considered in all calculations. The optB86b-vdW function was used to describe physical van der Waals interaction, which explicitly accounted for the binding energy and optimization simulations. The binding energies of Li2Sx on different surfaces were obtained as follows: − B E = E ( surface ) + E ( Li 2 S x ) − E ( total ) The transition state of Li2S decomposition and Li+ transfer on the surface of WP and NPC was located by the nudged elastic band (NEB) method, where the initial and finial states were discretized. Results and Discussion Material synthesis and characterization The delicately designed "structure-oriented template" coupled with "in situ self-phosphating" strategy was developed to prepare highly dispersed tungsten phosphide (WP) ultrafine nanocrystals decorated on the N,P co-doped carbon sheets (WP/NPC). The synthetic process of WP/NPC is schematically illustrated in Figure 1a, and the detailed descriptions are presented in the Experimental Section. PA with strong chelating ability coordinated with both metal ions and proteins through its six reactive phosphate groups.29 During the synthesis of the precursor (denoted as MA–PA–W12O39), PA served as a bidirectional functional agent to link the tungsten source (W12O39) with the carbon precursor (MA) under mild conditions.26 The expected WP/NPC was harvested in the subsequent annealing process, during which MA–PA–W12O39 worked as self-sacrificing template for the self-assembly of WP nanocrystals and N/P co-doped carbon sheets. It should be mentioned that the growth of WP crystals was strongly directed by the coordination between PA and W12O39. Benefiting from this, the formation of highly dispersed WP ultrafine nanocrystals was achieved. Figure 1 | (a) Synthesis schematic illustration, (b) TEM image, (c and d) HAADF-STEM images, and (e) corresponding EDS mapping of WP/NPC. Download figure Download PowerPoint As can be seen from Supporting Information Figure S1, the scanning electron microscope (SEM) image of precursor MA–PA–W12O39 shows obvious lamellar structure, and the corresponding energy dispersive spectroscopy (EDS) demonstrates the coexistence of C, N, P and W elements. To further figure out the structure and formation of MA–PA–W12O39, Fourier transform infrared spectra were obtained for the MA, PA, and MA–PA–W12O39 ( Supporting Information Figure S2). The characteristic peak located at 814 cm−1 corresponds to the triazine ring vibration of MA and shifts to 780 cm−1 in MA–PA–W12O39, which originates from the deformation of the aromatic ring and protonation of the triazine rings. The peak at 957 cm−1 of PA assigned to the –PO4 groups shifts to 980 cm−1 in MA–PA–W12O39. The blueshift phenomenon mentioned above demonstrates the formation of hydrogen bonding between MA and PA.26 The structural feature of the as-formed WP/NPC was characterized by transmission electron microscopy (TEM). As shown in Figure 1b, the highly distributed WP ultrafine nanocrystals embed in the NPC, where the WP nanograins have the average crystal size of about 5 nm. Such structure with nanocrystals of small size that are well dispersed on thin carbon sheets, tends to expose abundant active sites for the construction of catalyst/electrolyte/reactant triple-phase boundaries. It can exhibit great potential to provide high catalytic activity for the Li–S electrochemistry. The high-angle annular dark field-scanning TEM (HAADF-STEM) images in Figure 1c,d also show the tightly fixed WP nanocrystals on the NPC and the obvious spacing of 0.288 nm for the (011) plane of WP phase. The corresponding elemental mappings of WP/NPC (Figure 1e) indicate that the C, N, P, and W are uniformly distributed throughout the total scanning region, which further confirms the uniformity of WP NPs and successful N,P of carbon sheets. The phase composition of WP/NPC is further by the As shown in Figure all the diffraction can be to the WP without was to the WP content the WP/NPC As can be seen from Figure the WP loading content is calculated to be about wt % detailed is presented in the Experimental The surface of WP/NPC is to be g−1 by the And it abundant (Figure This favorable surface is to abundant active areas for the adsorption and transformation of LiPSs. In to further investigate the surface state of analysis was used and shown in Figure and Supporting Information Figure at and eV in W spectrum to the W and W In P the at and eV are assigned to the P and P of In addition, the obvious are with and further into the carbon which has been to the adsorption toward the LiPSs. Figure 2 | (a) (b) sorption and high-resolution spectrum of W for WP/NPC. in is the corresponding pore size distribution Download figure Download PowerPoint The and TEM images also show that lamellar structure features NPC with size and the of ( Supporting Information Figure The corresponding elemental mappings the of and P carbon sheets ( Supporting Information Figure In addition, the two at and assigned to the carbon ( Supporting Information Figure The surface of NPC provides space for the dispersion of WP NPs ( Supporting Information Figure Catalytic performance It has been that a highly catalytic process in Li–S chemistry strong smooth and accelerated transformation of LiPSs. In this a adsorption test was to the interaction between WP and LiPSs As shown in Supporting Information S1, the Li2S6 solution was into the containing a amount of NPC, and it was expected to after In the Li2S6 solution added into the WP/NPC solution with an surface to NPC ( Supporting Information S2). for 30 the solution the solution kept its ( Supporting Information Figure The surface between WP and LiPSs can be further by the where high-resolution spectra of WP/NPC and after Li2S6 adsorption were with the of the peak As can be seen from Supporting Information Figure the of W W P and P after Li2S6 adsorption the interaction with Li2S6 on the the W and W by 0.2 eV after with the electrons transfer from Li2S6 to the of W ions to form bonding based on the are that the WP/NPC strong toward LiPSs through which is to the of the shuttle effect and the diffusion and conversion of LiPSs. As the of the catalytic process, the rate of LiPSs the conversion kinetics, which was by the lithium ion diffusion Considering the are by the Randles–Sevcik equation according to the CV of [email protected]/NPC and [email protected] As in Figure the [email protected]/NPC electrode for 1, and 3 × × and × than its [email protected] × × and × It is obvious that with WP/NPC as the sulfur the LiPSs can than in the of the NPC Figure 3 | CV of the LSBs assembled with (a) [email protected] and (b) [email protected]/NPC. The of The fitting results of the peak current as a function of scan rate for different LSBs at peak 1, (e) peak and peak Download figure Download PowerPoint the redox reaction of LSBs by LiPSs and the WP/NPC for LiPSs redox kinetics were investigated. As shown in Figure the electrocatalytic activity of WP/NPC was by the density and the were calculated from the Tafel The WP/NPC for the reaction and for reduction than NPC for both and the reduction Furthermore, the CV of cells were calculated to the In Figure at a scan rate of 0.2 mV the cell two of redox at and V with high current which are to the transformation between Li2S6 and Li2S. In the of redox and current was in the It should be mentioned that the redox were still the scan rate is to 20 mV which indicates rapid electron/ion exchange and superior electrochemical by the of WP/NPC ( Supporting Information Figure from the (Figure the cell has a decreased charge transfer impedance of with the NPC the and the current of the cells that the WP/NPC a in accelerating transformation between S and Li2S. Figure 4 | (a) The Tafel and (b) CV of symmetric cells for the NPC and WP/NPC electrodes. of and symmetric The shows the corresponding at a voltage of 2.05 V for the WP/NPC and (e) NPC electrodes. charge at 2.35 V for the WP/NPC and NPC electrodes. Download figure Download PowerPoint conversion the discharging process, so the of WP/NPC toward the conversion from LiPS to Li2S to be through the Li2S In Figure the WP/NPC based electrode the peak with an peak also Li2S nucleation capacity mAh than the electrode and The significantly capacity is to be by the morphology of the Li2S. To further into the above the was further and the images of electrodes were As shown in Supporting Information and a traditional growth in the NPC with the morphology of This morphology in electrolyte the electronic transfer from to the insulating for the and subsequent decomposition of Li2S. With the introduction of the Li2S a unique and is characterized with tightly on the surface of the WP/NPC electrode ( Supporting Information Figure The formation of favorable Li2S morphology to the high catalytic effect and abundant active sites of which accelerate the nucleation and growth of Li2S. For a of the improved kinetics toward the conversion process, the of Li2S was with a potential of 2.35 V for the fully discharged electrodes. As shown in Figure the cell the significantly current density of g−1 and peak of than the cell g−1 and which an for the Li2S conversion by the introduction of WP DFT calculations This remarkable catalytic performance us to investigate the LiPS conversion by WP nanocrystals in through theoretical electron transport is to the of LiPS redox and improved electrochemical performance. In this respect, the electronic structure of WP was analyzed through the total density of states In Figure the of and the states the the nature of for catalytic activity. To get into the LiPS adsorption of the binding energy between WP and LiPS was further In Figure and Supporting Information and

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