Efficient Activation of Li2S by Transition Metal Phosphides

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Efficient Activation of Li2S by Transition Metal Phosphides Nanoparticles for Highly-stable Lithium-sulfur Batteries Huadong Yuan, Xianlang Chen, Guangmin Zhou, Wenkui Zhang, Jianmin Luo, Hui Huang, Yongping Gan, Chu Liang, Yang Xia, Jun Zhang, Jianguo Wang, and Xinyong Tao ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00465 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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ACS Energy Letters

Efficient Activation of Li2S by Transition Metal Phosphides Nanoparticles for Highly-Stable Lithium-Sulfur Batteries Huadong Yuan,†,┴ Xianlang Chen,‡ Guangmin Zhou,§ Wenkui Zhang,†,┴ Jianmin Luo,† Hui Huang,† Yongping Gan,† Chu Liang,† Yang Xia,† Jun Zhang,† Jianguo Wang,‡ and Xinyong Tao*,† †

College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou

310014, China ‡

College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China

§

Department of Materials Science and Engineering, Stanford University, Stanford, California

94305, USA

Abstract: Considerable research efforts have been devoted to the lithium-sulfur battery due to its advantages such as high theoretical capacity, high energy density and low cost. However, the shuttle effect and the irreversible deposition of Li2S result in severe capacity decay and low Coulombic efficiency. Herein, we discovered that the transition metal phosphides can not only trap the soluble polysulfides, but also effectively catalyze the decomposing of Li2S to improve the utilization of active materials. Compared with the cathodes without transition metal phosphides, the cathodes based on Ni2P, Co2P, and Fe2P all exhibit higher reversible capacity and improved cycling performance. The Ni2P added electrode delivers capacities of 1,165, 1,024, 912, 870, and 812 mAh g-1 at 0.1, 0.2, 0.5, 1.0, and 2.0 C, respectively, and high capacity retention of 96 % after 300 cycles at 0.2 C. Even with a high sulfur mass loading of 3.4 mg cm-2, the capacity retention keeps 90.3 % after 400 cycles at 0.5 C. Both density functional theory calculations and electrochemical tests reveal that the transition metal phosphides show higher adsorption energies and lower dissociation energies of Li2S than those of carbon materials.

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Transition metal phosphides are synthesized and used as sulfur cathode for Li-S batteries for the first time. According to density functional theory calculations, adsorption and activated tests, the transition metal phosphides can not only trap the soluble polysulfides, but also effectively catalyze the decomposing of Li2S to improve the utilization of active materials. Excellent cycling stability is achieved. The advent of ever-increasing demand for power sources eagerly urges us to search for new energy storage systems beyond present lithium-ion batteries (LIBs). Undoubtedly, lithium-sulfur (Li-S) batteries hold the promise for global scale energy storage due to their advantages such as the high theoretical capacity (1672 mAh g-1), high energy density (2500 Wh/kg or 2800 Wh/L), low cost, natural abundance, and environmental

benignity

of

sulfur.1-7

However,

their

application

toward

commercialization has been halted due to five deadly shortcomings: (1) the poor intrinsic electronic conductivity of sulfur and final discharge products (Li2S/Li2S2), (2) the high dissolution of intermediate product lithium polysulphides (LiPSs), (3) large


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volumetric expansion (76%) of sulfur cathode during the lithiation, (4) the irreversible phase transformation and uncontrolled deposition of Li2S, and (5) the gas evolution with cycling in operating Li-S cells.8 Fortunately, intense research efforts have been devoted to overcoming these unsolved issues, including developing new electrolytes, applying functional separators, and constructing nanostructured sulfur cathodes.9-17 Among these efforts, the most effective strategy is constructing carbon-sulfur nanocomposite cathodes, since their inherent good conductivity and diversity in nanostructures.18-22 In addition, it has been proved that polar groups such as oxides,23-28 sulfides,29-31 nitrides,32-34 carbides,35-37 conductive polymer,38-41 and doped N, B, S, O, and P heteroatoms42-47 are necessary to decorate the nonpolar carbonaceous material, which can provide strong adsorption to the polar LiPSs/Li2S and improve the cycling performance of Li-S battery.48 Our recent work pointed out that polar groups such as Ti4O7,24 B-doped CNTS,49 La2O3,50 and MgO50 can increase the interaction between LiPSs/Li2S and electrode and promote the electrochemical reaction kinetics. However, the application of transition metal phosphides (TMPs) for improving Li-S batteries has not been realized up to now. Compared with others polar compounds, TMPs have the following advantages. Firstly, in contrast to poor conductivity of most oxides and sulfides, TMPs exhibit a metallic character or even superconductivity,51 which are beneficial for the electrochemical reaction in Li-S battery. Secondly, compared with the conductive metal nitrides and metal carbides, the synthesis process of TMPs is facile and gentle.52 Actually, TMPs such as Ni2P, CoP, Fe2P, and MoP have attracted extensive research interest as efficient electrocatalysts for oxygen evolution reaction (OER) and other electrocatalytic reactions recently.53-57 It has been demonstrated that these TMP nanoparticles can provide enough active sites to capture and diffuse the proton, and facilitate proton transfer in the OER process. Therefore, TMPs can promote oxygen dissociation to produce O2 at lower current density. Considering that the sulfur belongs to the same main group elements with oxygen, these advantages inspired us to hypothesize that TMPs may have an outstanding performance in Li-S batteries.


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Herein, we developed a one-pot chelated method for the fabrication of ultrafine TMPs nanoparticles decorated N and P-doped carbon nanoflakes (TMPs@NPC). The sulfur composite cathodes based on N, P-doped carbon nanoflakes decorated with nickel phosphides, cobalt phosphides and iron phosphides (Ni2P@NPC, Co2P@NPC, and Fe2P@NPC) display higher capacity and improved cycling stability compared to the ones without metal phosphide additives. Density functional theory (DFT) calculations combined with electrochemical performance, adsorption test, and microstructure analysis revealed the remarkable performance of TMPs for anchoring LiPSs/Li2S and catalyzing the Li2S decomposing. For the host material of Li-S batteries, the LiPSs capture capability is very important for suppressing the shuttle effect of Li-S batteries.58, 59 Our early work has revealed the similar adsorption behavior of LiPSs with the final product Li2S.50 Therefore, in order to understand the function of TMPs in trapping LiPSs/Li2S, we systematically calculate the adsorption energy and the corresponding adsorption sites of TMPs for Li2S. Figure 1a-c show the optimized geometries of the most stable Li2S on Ni2P (111), Co2P (121), and Fe2P (111) surfaces. On Ni2P (111) surface, the most favorable binding sites of Li and S are Ni atoms, on atop and bridge site, respectively. (Figure 1a). Figure 1b is the optimized structure of Li2S on Co2P (121) surface, in which the bridge site of one Co atom and one P atom are the favorable sites for Li, and the connecting type of sulfur atom is same with that on Ni2P (111). Figure 1c shows the optimized adsorption conformations of the steadiest Li2S on the surface of Fe2P (111). It can be clearly seen that two Li ions of Li2S connect with one Fe atom, and the sulfur atom of Li2S is the hollow site of three Fe atoms. The adsorption energy of Ni2P, Co2P, and Fe2P is -3.70, -4.18, and -5.34 eV, respectively (Figure 1a-c). Compared with the adsorption energy of Li2S on pure carbon (-0.37 eV) and N-doped cabon (-0.94 eV), Ni2P, Co2P, and Fe2P show much higher adsorption energy. It is noted that Fe2P has the best ability for trapping Li2S among these TMPs. In addition, these TMPs’ adsorption energies are close to the reported TiS2 (-3.20 eV),60 Ti4O7 (-3.62 eV),24 and TiO (-4.77 eV),61 which are good matrix for confining sulfides.


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Figure 1. The adsorption and decomposition mechanism on the surface of various TMPs. Optimized structures of the most stable Li2S on (a) Ni2P (111), (b) Co2P (121), and (c) Fe2P (111), respectively. The process of Li2S decomposing on the surface of (d) Ni2P (111), (e) Co2P (121), and (f) Fe2P (111), respectively.

Beside the adsorption for Li2S, the dissociated energy of Li2S is another critical characteristic to evaluate the host materials for Li-S batteries. It has been confirmed that the irreversible phase transformation of the end-product Li2S is regarded as one predominant reason for capacity fading of Li-S batteries. However, there are relatively few studies devoted to analyzing the influence of Li2S decomposing energy in matrix materials using DFT calculation. The specific decomposing process is one Li ion cut from the intact Li2S (Li2S → LiS + Li+ +e–). As shown in Figure 1d-f, the dissociation energies of Li2S on the surface of Ni2P, Co2P, and Fe2P are 0.74, 0.69, and 0.65 eV, respectively. It is noted that there is no obvious difference among these TMPs’ dissociation energies. However, compared with the Li2S decomposing on the surface of pristine carbon (1.86 eV) and N-doped carbon (1.52 eV), Fe2P, Co2P, and Ni2P all deliver better catalyzing ability for the Li2S decomposing (Figure S1). Therefore, the TMPs may be efficient polar groups for the carbon host to improve the electrochemical properties of sulfur cathodes. In order to fabricate the TMPs@NPC, a facile and cost-effective chelated method was developed. Figure S2 shows the fabrication strategy and the formation mechanism of TMPs@NPC. Firstly, ethylene diamine tetraacetic acid (EDTA), KOH,


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Na3PO4·12H2O,

melamine,

and

Ni(NO3)2·6H2O

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(or

Fe(NO3)3·9H2O,

or

Co(NO3)·6H2O) were dissolved into deionized water, respectively. In this solution, metal ions will combine with EDTA via chelating, concurrently with the electrostatically attracted phosphate groups on melamine. After drying, the EDTA and melamine form a uniform complex containing metal ions and phosphide groups. During the carbonization, the pyrolysis of the above complex may release abundant NH3, H2, CO, and NO etc, which act as reductive agents to promote the formation of TMPs at high temperature. Finally, the TMPs@NPC was obtained after grinding, purifying, and drying. Compared with conventional synthetic methods for phosphides, our newly developed one-pot strategy is facile and gentle. In addition, the control sample of pristine NPC was prepared by removing the TMPs from TMPs@NPC using concentrated hydrochloric acid. The mass ratio of TMPs in each TMPs@NPC composites is around 12 wt% (Table S1).


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Figure 2. Microstructures of TMPs@NPC. (a) and (b) SEM images of Ni2P@NPC; (c) and (d) SEM images of Co2P@NPC; (e) and (f) SEM images of Fe2P@NPC; (g) and (h) TEM images of Fe2P@NPC. (i) Mapping area and corresponding elemental distributions of C, P, and Co. (j) Mapping area and corresponding elemental distributions of C, P, and Ni.

As shown in the scanning electron microscopy (SEM) images in Figure 2a, c, and e, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC display the similar nanoflakes morphology. The SEM images from the magnified edges of nanoflakes (Figure 2b, d, f) clearly show the sandwich-like structure, which may be beneficial for enlarging the electrode/electrolyte interfaces, and increasing the adsorbed sites for LiPSs. Transmission electron microscope (TEM) was performed to investigate the microstructures of TMPs@NPC (Figure 2g-j and Figure S3-4). As shown in Figure 2g and Figure S3, Fe2P nanoparticles with the diameter around 10 nm are well-dispersed in carbon nanoflakes. Two clear lattice fringes corresponding to inter-planar spacing of 0.22 and 0.20 nm can be readily assigned to the Fe2P (111) and (201) planes,62 respectively (Figure 2h). However, the HRTEM images of Co2P and Ni2P were difficult to be obtained because of that the Co2P and Ni2P will be translated to amorphous at the bombardment of high-energy electron beam in TEM. As displayed in the STEM images, we can clearly see the Co2P and Ni2P nanoparticles uniformly distributed in Co2P@NPC and Ni2P@NPC (Figure S4a, c). Energy dispersive X-ray spectroscopy (EDX) also confirms the presence of Co, Ni, and P elements in Co2P@NPC and Ni2P@NPC (Figure S4b, d). In addition, STEM images and the corresponding element maps of Co2P@NPC and Ni2P@NPC further verified the uniform distribution of C, P, Ni, and Co elements (Figure 2i, j).


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Figure 3. The analysis of phase, composition and pore structures. XRD patterns of (a) Ni2P@NPC, (b) Co2P@NPC, and (c) Fe2P@NPC. (d) Ni 2p XPS spectrum of the Ni2P@NPC. (e) Co 2p XPS spectrum of the Co2P@NPC. (f) Fe 2p XPS spectrum of the Fe2P@NPC. N2 adsorption and desorption isotherms of (g) Ni2P@NPC, (h) Co2P@NPC, and (i) Fe2P@NPC, respectively.

To reveal the phase structure of these samples, the X-ray diffraction (XRD) was performed. As shown in Figure 3a-c and Figure S5, all samples exhibit a similar broad peak at 24 º in the XRD patterns, corresponding to an inter-planar d002-spacing of 0.34 nm. The diffraction peaks at 40.6 º, 44.5 º, 47.3 º, 54.1 º, and 54.9 º are indexed to the (111), (201), (210), (300), and (211) planes of Ni2P (data from PDF#65-3544), respectively (Figure 3a).53 In addition, the peaks in Figure 3b, c are also perfectly assigned to the Co2P (PDF#32-0306)52 and Fe2P (PDF#27-1171),62 respectively. No other peaks can be observed in XRD patterns, indicating the high purity of these samples. Raman spectrum was performed to confirm the graphitization of these samples (Figure S6). As shown in Figure S6, the difference of the graphitization can


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be ignored in these samples. X-ray photoelectron spectroscopy (XPS) was carried out to identify the elemental compositions and valence states of TMPs@NPC. The survey spectra (Figure S7 and Table S2) clearly prove the presence of C, N, O, P and the corresponding metallic elements in these samples, which is consistent with the result of TEM (Figure 2g-j) and EDX (Figure S4). As depicted in Figure 3d, the high-resolution Ni 2p spectra is divided into four peaks, which are located at 856.4 (Ni 2p3/2), 874.4 (Ni 2p1/2), 862.0, and 881.3 eV,63, 64 respectively. The 856.4 eV peak is the binding energy of Ni and P.64, 65 According to Figure 3e, the high-resolution Co 2p spectra yielded four peaks at 781.5 eV (Co 2p3/2) and 797.6 eV (Co 2p1/2) followed two satellites at 786.8 and 803.6 eV,52, 65 respectively. It should be mentioned that the binding energy of Co-P is located at 781.5 eV.52, 66 The high-resolution scan of Fe 2p shows two main peaks with binding energy values of 708.3 and 721.0 eV (Figure 3f), corresponding to the Fe 2p3/2 and Fe 2p1/2,67-69 respectively. The peaks at 713.9 and 727.0 eV are two satellites of Fe 2p3/2 and Fe 2p1/2, respectively. The peak at 708.3 eV is ascribed to the binding energy of Fe and P in Fe2P.68 In order to examine the specific

surface

area

and

pore

features

of

TMPs@NPC,

the

nitrogen

adsorption-desorption isothermal measurement was performed. The surface areas are calculated to be 1,613, 1,147, 1,205, and 1,247 m2 g-1 corresponding to NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC, respectively (Figure 3g-i and Figure S8). The high surface areas of TMPs@NPC can not only provide enough volume to store abundant sulfur, but also provide a broad electrode/electrolyte interface for immobilizing LiPSs.70 In addition, the nitrogen adsorption-desorption isotherms of NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC exhibit characteristics of type IV according to the classification of the International Union of Pure and Applied Chemistry (Figure 3g-i and Figure S8). It is noted that the hysteresis loop is between 0.4 and 1.0 relative pressure range, which indicates that mesopores are dominating in these samples. The pore size of three samples is all around 25 nm (Figure S9). To the best of our knowledge, this kind of mesoporous structure is beneficial for confining sulfur.71, 72 In addition, the pore volume calculated from N2 adsorption-desorption isotherms are 1.1, 0.8, 1.0, and 0.9 cm3 g-1 for NPC, Ni2P@NPC, Co2P@NPC, and


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Fe2P@NPC, respectively (Figure 3g-i and Figure S8). The specific surface area and pore volume of NPC are both increasing due to the removal of TMPs from TMPs@NPC (Figure S8). The first role of polar groups in Li-S batteries is immobilizing the LiPSs to suppress the shuttle effect. Therefore, the LiPSs capture ability of TMPs@NPC was verified by adsorption test combined with inductively coupled plasma-mass spectroscopy (ICP-MS). Considering that the popularly used solvent are dimethoxyethane (DME) and 1,3-dioxolane (DOL) in the normal Li-S battery, 0.005 M Li2S8 in DME/DOL (1:1, v/v) was prepared for adsorption test of LiPSs.18,

50

Figure 4a shows the digital image of the LiPSs capture test using different mass of TMPs@NPC with same total surface area. After 6 h adsorption, the color of the solution containing NPC has no observable change, indicating its low capture ability for LiPSs. It is worth noting that all TMPs@NPC samples have strong capture ability for LiPSs due to their obvious color change after adsorption, especially sample Fe2P@NPC. ICP-MS results (Figure 4b and Table S3) also confirm that Fe2P@NPC has the strongest adsorption ability for LiPSs, and the capturing ability of NPC is the lowest. Both Ni2P@NPC and Co2P@NPC exhibit similar ability for capturing LiPSs. The Li2S8 adsorption quantity of these samples is 2.83, 6.24, 6.32, and 7.10 µmol m-2 for NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC, respectively, which is close to the our previously reported monolayer adsorption of Li2S8 on metal oxides.50 The ability for trapping LiPSs is consistent with above DFT calculation results (Figure 1).


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Figure 4. The experimental verification for DFT calculation. (a) Digital images of Li2S8 adsorbed by TMPs@NPC in DME/DOL. (b) The relative adsorption amounts of NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC. (c) Activation test of Li2S electrodes based on AC, NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC. (d) The magnified area of the red rectangle area in Figure 4c.

To verify the calculation result of the Li2S dissociation on TMPs@NPC, the cathodes containing commercial Li2S particles were assembled to observe the characteristic activating peak of micro-size Li2S at the first charge process.73 The origin of the first activation barrier is phase nucleation, but the height of this barrier is mainly attributed to the poor charge transfer on the surface of Li2S and the limited diffusivity of lithium ions inside Li2S.73 No soluble LiPSs existing before initial activation of Li2S indicates that the Li2S will not be transformed via disproportionation reaction. Therefore, the result of this method is perfect to compare the activation performance of various TMPs@NPC. The cathodes consist of different host materials (NPC, Ni2P@NPC, Co2P@NPC, and Fe2P@NPC), the commercial Li2S, Super-P, and polyvinylidene difluoride (PVDF). In addition, the activation carbon (AC) was introduced as a criterion for comparing the host material. The detailed fabrication process is described in the experimental section. As shown in


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Figure 4c, the test voltage window of the first intact charge curve is from open-circuit voltage to 4.0 V. Figure 4d shows the magnification of the activation peak, revealing that the activation potentials are 3.50, 2.80, 2.59, 2.51, and 2.44 V for AC, NPC, Fe2P@NPC, Co2P@NPC, and Ni2P@NPC based cathodes, respectively. The cathodes based on AC and NPC display a higher activation barrier at 3.50 and 2.80 V, respectively, implying a disappointed kinetic reaction in the initial charging cycle. In contrast, three electrodes on basis of TMPs@NPC exhibit lower activation barriers and longer voltage plateau, indicating the lower charge-transfer resistance due to the high activity of TMPs in decomposing Li2S. The electrochemical performance of Li-S batteries based on TMPs@NPC were assessed by assembling 2025 type coin cells using lithium metal as the anode. The sulfur content is around 76.0 wt% in active materials (Figure S10). It should be mentioned that the TMPs is still remained stable after sulfur loading and cycling (Figure S11). Figure 5a shows the first charge-discharge curves of TMPs@NPC/S cathodes at 0.1 C (1 C = 1, 672 mA g-1). The sulfur mass loading of the electrode ranges from 1.1 to 1.3 mg cm-2. Two typical discharge plateaus can be identified in all discharge curves, indicating the reduction reaction from sulfur to long-chain LiPSs (Li2Sn (4≤n≤8)) and the formation of short-chain LiPS (Li2Sn (1≤n≤4)).6, 74, 75 Figure S12 show the CV curves of Ni2P@NPC sulfur cathode. Among all samples, NPC/S cathode shows the highest polarization, suggesting that TMPs promises much better reaction kinetics and efficient utilization of the active materials. The initial discharge capacities at a current density of 0.1 C corresponding to NPC/S, Fe2P@NPC/S, Co2P@NPC/S, and Ni2P@NPC/S cathodes are 1,090, 1,146, 1,204, and 1,253 mAh g-1, respectively. Next, the rate performance of these cathodes were evaluated at different current densities from 0.1 to 2.0 C (Figure 5b). When the current density is increased from 0.1 to 0.2, 0.5, 1.0, and 2.0 C, the sulfur cathode based on Ni2P@NPC delivers high capacities of 1,165, 1,024, 912, 870, and 812 mAh g-1, respectively. When the current density is returned back to 0.1 C, the discharge capacity of Ni2P@NPC/S cathode is recovered to 1,272 mAh g-1, indicating the excellent stability of this composite cathode (Figure 5c). In addition, compared with


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NPC/S cathode, both Co2P@NPC/S and Fe2P@NPC/S cathodes show better cycling stability at different current densities, implying that the TMPs can effectively block LiPSs and catalyze Li2S to improve the cycling stability. As shown in Figure S13, the calculated potential difference between the charge-discharge voltage plateaus at various rates confirmed that the cathodes containing TMPs display much less polarization and higher utilization of active materials. Electrochemical impedance spectroscopy (EIS) analysis indicates the superior conductivity of Fe2P@NPC/S, Co2P@NPC/S, and Ni2P@NPC/S (Figure S14).

Figure 5. The electrochemical performance of Li-S batteries. (a) The first charge-discharge profiles of the cathode based on different TMPs@NPC at 0.1C. (b) Cycling performance of the composite sulfur cathodes at different current densities. (c) The charge-discharge curves of Ni2P@NPC/S cathode at various current densities from 0.1 to 2 C. (d) Cycling performance and the corresponding Coulombic efficiency of TMPs@NPC/S cathodes at 0.2 C. (e) The Cycling performance of Ni2P@NPC/S cathode with 3.4 mg cm-2 sulfur mass loading at 0.5 C.

Beside the charge-discharge curves, cyclic performance of four samples are


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compared under a current density of 0.2 C (Figure 5d). The electrodes of Fe2P@NPC/S, Co2P@NPC/S, and Ni2P@NPC/S deliver higher initial specific capacities of 1,203, 1,351, and 1226 mAh g-1, respectively, while NPC/S cathode only gives 939 mAh g-1. After 300 cycles, the capacity decay per cycle is 0.14, 0.05, 0.02, and 0.01 % for NPC/S, Fe2P@NPC/S, Co2P@NPC/S, and Ni2P@NPC/S cathodes. Additionally, the capacity retention of NPC/S, Fe2P@NPC/S, Co2P@NPC/S, and Ni2P@NPC/S is 55, 82, 89, and 96 %, which is calculated from the second cycle. Interestingly, although Fe2P@NPC owns higher adsorption energy for LiPSs, the cycling performance of this cathode is not better than that of the cathodes containing Co2P@NPC and Ni2P@NPC. This may be because of that the extremely high ability for capturing LiPSs of Fe2P results in Li2Sx accumulation on the surface of cathodes and hinder the subsequent Li2Sx diffusion and conversion.50 Our previous work proved that although Al2O3 has the best capture capacity for lithium polysulfides, the electrochemical performance of the electrodes based on Al2O3 is really worse than that of electrodes based on other oxides.50 It has been proved that the adsorption of lithium polysulfide is monolayered chemisorption. Therefore, the good diffusion of lithium polysulfide and lithium sulfide on the surface of oxides was very important for the electrochemical performance of Li-S batteries. In current work, cyclic voltammetry (CV) measurements in Figure S15 indicate that the diffusion properties of Ni2P@NPC and Co2P@NPC are better than that of Fe2P@NPC, which is in agreement with the cycling performance. This can explain the electrochemical property difference of the electrodes based on various TMPs. Since high sulfur mass loading is essential for the practical application of Li-S batteries, the cycling stability of Ni2P@NPC/S electrode with different sulfur mass loading is further evaluated (Figure S16).76 When the sulfur mass loading increases from 1.14, 2.27 to 3.40 mg cm-2, the average capacities are 1,100, 950, and 840 mAh g-1 with capacity retention of 97.4, 94.7, and 94.5 % after 60 cycles, respectively. Figure 5e shows the long cyclic performance of Ni2P@NPC/S cathode at 0.5 C, whose sulfur mass loading is 3.4 mg cm-2. After 400 cycles, its capacity retention is as high as 90.3 %. When the sulfur mass loading of Ni2P@NPC/S cathode increases to


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6.50 mg cm-2, the cathode still delivers stable cycling performance (Figure S17). It is notable that the capacities of Ni2P@NPC/S cathodes decrease with the increasing of sulfur mass loading. However, the capacity decay per cycle has no obvious change, implying the stability of Ni2P@NPC/S cathode at high sulfur mass loading. These electrochemical results combined with the microstructure analysis and DFT calculation indicate that TMPs make a great contribution to the high capacity and stability of Li-S battery owning to their following advantages: (1) the good intrinsic electronic conductivity, (2) the uniform distribution in N, P-doped nanoflakes, (3) the high ability for trapping LiPSs, and (4) the remarkable catalyzing capability for decomposing Li2S. In summary, we successfully synthesized Fe2P@NPC, Co2P@NPC, and Ni2P@NPC via a one-pot chelated method. Compared with NPC/S cathode, the cathodes on the basis of Fe2P@NPC, Co2P@NPC and Ni2P@NPC exhibit higher specific capacity and significantly improved cycling stability. In addition, DFT calculation combined with microstructure analysis and electrochemical performance demonstrate that the TMPs can not only effectively capture the soluble LiPSs to lock the shuttle effect, but also promote Li2S conversion reactions to improve the utilization of active materials. We believe that these findings can be generalized to other TMPs and some other host materials with relative high adsorption energy and low decomposing energy for Li2S, which will guide us to design ideal cathode for Li-S batteries.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Experimental section, SEM and TEM images, additional electrochemical data, XRD, XPS, Raman, BET, and TG data, and relative DFT results.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Phone: +86-13857122035

Author Contributions X.T. and H.Y conceived the idea. H.Y. designed the experiments, synthesized the materials and performed electrochemical test. W.Z., H.H., Y.G., Y.X., C.L., J.Z. and X.T. contributed new reagents/analytic tools. X.L. J.L. and J.W. helped conduct materials characterization. H.Y., G.Z. and X.Y. wrote the paper. All authors discussed the electrochemical results and the whole paper.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This research was support by the National Natural Science Foundation of China (Grant no. 51002138 and 51572240), the Natural Science Foundation of Zhejiang Province

(Grant

no.

LQ14E020005,

LR13E020002,

LY13E020010

and

LY15B030003), Scientific Research Foundation of Zhejiang Provincial Education Department (Grant no.Y201432424) and Ford Motor Company.

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Efficient Activation of Li2S by Transition Metal Phosphides

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