Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithium–sulfur battery

Electrochimica Acta xxx (xxxx) xxx

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Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery Feng Ma a, e, 1, Xiaoming Wang b, c, 1, Jiayang Wang a, Yuan Tian a, Jiashun Liang a, Yining Fan a, Liang Wang a, Tanyuan Wang a, e, Ruiguo Cao d, Shuhong Jiao d, Jiantao Han a, Yunhui Huang a, Qing Li a, e, * a State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China b College of Materials Science and Engineering, Hunan Provincial Key Laboratory of Flexible Electronic Materials Genome Engineering, Changsha University of Science & Technology, Changsha, 410114, China c Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of Guangdong Province, Shantou University, Shantou, 515063, China d CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, 230026, China e Shenzhen Institute of Huazhong University of Science and Technology, Shenzhen, 518000, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 May 2019 Received in revised form 18 October 2019 Accepted 14 November 2019 Available online xxx

Lithium-sulfur (LieS) battery has aroused intensive attention due to its intrinsicly high capacity and energy density. However, the sluggish kinetics of lithium polysulfide (LiPS) redox conversion and shuttle effect severely damage the sulfur utilization, rate performance, and cycling stability of LieS batteries. In this work, we report that Ru-doping induced phase transformation from MoP to Mo4P3 (RueMo4P3) could significantly facilitate the electrocatalytic conversion of LiPS. When RueMo4P3 nanoparticles (NPs) are decorated on hollow carbon spheres (HCS), the S/HCS-Ru-Mo4P3 electrode delivers high reversible capacities of 1178 mAh g
1 and 660 mAh g
1 at 0.5C and 4C in LieS battery, respectively. The rate performance of the developed S/HCS-Ru-Mo4P3 is among the best of the reported transition metal phosphide cathodes for LieS batteries. When the S loading is as high as 6.6 mg cm
2, S/HCS-Ru-Mo4P3 retains a reversible areal capacity of 5.6 mAh cm
2 after 50 cycles, higher than that of the commercial Liion battery (4 mAh cm
2). The excellent LieS battery performance can be attributed to the intrinsically active RueMo4P3 phase combining with hollow carbon structure, which significantly facilitates the electrocatalytic conversion and entrapment of LiPS. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Lithium-sulfur battery Lithium polysulfide conversion Mo4P3 Electrocatalysis Phase transformation

1. Introduction Lithium-sulfur (LieS) battery with high theoretical capacity (1675 mAh g
1) and energy density (2600 Wh Kg
1) [1,2] has drawn tremendous attention as an important candidate of the next generation energy storage devices due to the low cost, nontoxicity, and easy access of S sources [3]. However, the performance of LieS battery currently cannot fulfill the rapid growing demand for clean

* Corresponding author. State Key Laboratory of Material Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, 430074, China. E-mail address: [email protected] (Q. Li). 1 These authors contributed equally to this work.

energy because of several obstacles. During the discharge/charge processes of LieS battery, the sluggish kinetics of lithium polysulfide (LiPS) intermediates (denoted as Li2Sx, x ¼ 2e8) transformation and Li2S deposition/decomposition result in limited capacity, large polarization, and low utilization of S [4,5]. Meanwhile, the migration of the high-soluble LiPS generally leads to the loss of active S species and the subsequent shuttle effect which dramatically damages the cycling stability of LieS battery [6,7]. So far, various efforts have been devoted to enhance the kinetic process of LiPS transformation and alleviate the shuttle effect. Electron-conductive porous carbons with high surface area and pore volume are often used as S hosts to improve the electronconductivity of S electrode[8e10], whereas the nonpolar carbon itself has very limited affinity with polar LiPS [11]. In order to provide sufficient anchoring sites for the adsorption and

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Please cite this article as: F. Ma et al., Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135310

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confinement of LiPS, transition metal oxides [12e14], sulfides [15], nitrides [16,17], and phosphides [18e20] have been composited with carbon materials to enhance the interactions with LiPS. Metal phosphides, due to their high electron-conductivity[21] and strong interactions with LiPS, can potentially provide faster redox kinetics of LiPS compared to semi-conductive metal oxides (e.g., MnO2 and V2O5) and sulfides (e.g., MnS, NiS, and MoS2). For instance, molybdenum phosphide (MoP) nanoparticles (NPs) loaded on carbon nanotubes (CNTs) have been reported as LieS battery cathode and deliver a relatively high cycling stability, while the capacity and rate performance are still unsatisfied due to the limited LiPS transformation activity of MoP [18]. Previous studies indicate efficient phase and atomic arrangement controls of metal nanocrystals could induce significant electrocatalytic performance enhancement [22e24], while other molybdenum phosphides such as Mo4P3 have never been studied as electrode materials likely due to the harsh conditions of synthesis (e.g., 4 GPa and 1700  C) [25]. On the other hand, ruthenium (Ru) and its oxides/alloys have demonstrated excellent activity in the oxidation of small organic molecules (e.g., methanol[26,27], dimethyl ether [28]), oxygen evolution reaction (OER) [29,30], and LieS battery [31,32]. Partially replacing Mo with more intrinsically active metal such as Ru holds promise in developing high-performance LieS battery cathode, which has yet to be explored to date. In this study, Ru-doped Mo4P3 NPs were synthesized for the first time via a facile and environmental benign method and employed as high-performance LieS battery cathodes. Interestingly, phase transformation from MoP to Mo4P3 is observed in the presence of Ru, which reveals higher intrinsic activity towards electrocatalytic conversion of LiPS than MoP and RuP2 electrodes. With hollow carbon spheres (HCSs) as supports, the developed S/HCS-Ru-Mo4P3 electrode delivers excellent rate capability with a capacity of 660 mAh g
1 at 4C and a low capacity decay of 0.07% per cycle at 3C with a high S content of 77%. S/HCS-Ru-Mo4P3 also exhibits a high areal capacity of 5.6 mAh cm
2 after 50 cycles which is superior to that of the commercial Li-ion battery. The improved performance of HCS-Ru-Mo4P3 in LieS battery compared to that of the HCS-MoP and HCS-RuP2 counterparts can be attributed to the high activity of RueMo4P3 sites with a possible synergistic effect between Ru and Mo4P3, which significantly facilitates the catalytic conversion of LiPS.

The preparation of pure HCS was similar to that of HCS-RuMo4P3 except that no metal salts were added. To prepare HCS-MoP or HCS-RuP2, all the procedures are the same except no RuCl3 or AHM was added, respectively. 2.1.2. The preparations of S/HCS-Ru-Mo4P3, S/HCS-MoP, S/HCS-RuP2 and S/HCS Each S host (e.g. HCS-Ru-Mo4P3) was well grounded with sublimed S with a weight ratio of 1:5 in a mortar. Then the mixture was transferred to a sealed container which was heated at 155  C for 15 h. After that the S-infusion sample was evaporated at 200  C under flowing N2 to remove the excessive S. 2.1.3. The preparations of Li2S4 solution and Li2S8 catholyte To obtain 0.01 M Li2S4, designed amount of Li2S and S with a molar ratio of 1:3 was added to a 1,2-dimethoxyethane (DME)/1,3dioxolane (DOL) mixed solution and kept stirring at 80  C to afford a Li2S4 solution. The 0.1 M Li2S8 catholyte was obtained by heating the Li2S and S with molar ratio of 1:7 in the electrolyte composed of 1 M lithium bis(trifluoromethylsulphonyl)imide (LiTFSI) dissolved in DME/DOL with 4% LiNO3 as the additive. 2.2. Material characterizations The crystal phase of the samples was observed by X-ray diffraction (XRD) with a PANalytical B.V. machine with Cu Ka radiation (l ¼ 1.5406 Å). The morphologies of the samples were characterized by Nova NanoSEM 450 Scanning Electron Micrograph (SEM) operating at 30 kV and Talos F200X transmission electron microscope (TEM) operating at 200 kV. The thermogravity analysis (TGA) was performed by the TGA8000 analyzer. The surface area and pore volume of the samples were obtained by the nitrogen adsorption/desorption isotherms with ASAP2000 machine. The Xray photoelectron spectroscopy (XPS) was performed by AXISULTRA DLD-600 W analyzer. 2.2.1. LiPS adsorption and XPS tests The samples were soaked with 0.01 M Li2S4 in DME/DOL for 5 h. Then the samples were centrifuged out to form a clear solution which was subject for UVeVis test. As for the XPS tests, the solid obtained by the centrifugation was washed with DME/DOL and then dried in vacuum at 60  C overnight before test.

2. Experimental 2.3. Battery assembly and electrochemical tests 2.1. Material synthesis 2.1.1. The preparations of HCS-Ru-Mo4P3, HCS-MoP, HCS-RuP2, and pure HCS To synthesize HCS-Ru-Mo4P3, 60 mmol of tetraethyl orthosilicate (TEOS) was first dissolved in the mixture of 350 mL ethanol and 50 mL deionized water followed by introducing 11 mL of concentrated ammonia solution. After stirring for 10 min, 2 g of resorcinol and 2.8 mL of formaldehyde solution were subsequently added onto the white suspension. The suspension was kept stirring for 24 h and was subject to centrifugation and washed with deionized water for 3 times. After that the residue was redispersed in 100 mL deionized water and 2/21 mmol ammonium heptamolybdate (AHM), 1/3 mmol RuCl3 and 10 mmol NH4H2PO4 were added in turn into that suspension. After stirring for 5 h, the suspension was slowly evaporated until all the water was removed. After drying in air, the obtained solid was transferred to a tube furnace and calcined under 900  C for 3 h under the forming gas (H2/Ar, 5/95%). The HCS-Ru-Mo4P3 was obtained by an etching treatment in 2.0 M NaOH aqueous solution at 92  C followed by a washing and freeze-dry process.

2.3.1. The preparations of electrodes for LieS battery Each sample (e.g. S/HCS-Ru-Mo4P3) was well grounded with super P and sodium alginate with a mass ratio of 7:2:1 for 10 min in a mortar. After that, a designed amount of deionized water was added into the mixture to form a uniform slurry. The slurry was further casted onto the surface of carbon paper (Toray Industries, Inc) followed by drying at 60  C for 12 h. The LieS battery was assembled with the S/host electrode (e.g. S/HCS-Ru-Mo4P3) as cathode, lithium foil as anode and Celgard polypropylene (PP) membrane as the separator. The electrolyte was 1 M LiTFSI dissolved in DME/DOL with 4% LiNO3 as the additive. The cell was cycled under a certain current density. For the cycling stability test, the voltage range of the coin cell was controlled to be 1.7e2.8 V. The capacities of the Li-S batteries were evaluated on LANHE and Neware battery testers. The preparations of electrodes for LiPS redox conversion, Li2S deposition, and Li2S decomposition tests: The sample without loading S (e.g. HCS-Ru-Mo4P3) was dispersed in the mixed solution of water and alcohol (7:3 in volume) followed by adding 10 mL Nafion solution (5%). The suspension was sonicated for 1 h to form a

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uniform dispersion and dropped onto a carbon paper. The active material loaded carbon paper was used as the electrodes for the LiPS transformation test. These tests were conducted on an Autolab 302 potentiostation (Metrohm). 3. Results and discussion The preparation procedure of the HCS-Ru-Mo4P3 catalyst is illustrated in Fig. 1. Silica sub-microspheres were prepared by the hydrolysis of tetraethyl orthosilicate (TEOS) and further coated with resorcinol formaldehyde resin (RF) through the in-situ polymerization of resorcinol and formaldehyde. The silica spheres could act as the hard template to form the hollow structure and the RF serves as the carbon source of HCS. The Mo and Ru cations were adsorbed onto the surface of RF via the electrostatic interaction, which further reacted with ammonium biphosphate to form the RueMo4P3 NPs under forming gas (H2/Ar 5/95) annealing at 900  C for 3 h. In this calcination process, high-valent Mo (VI) and Ru (IV) compounds were carbothermally reduced to phosphides which may help to form the pores of the HCS due to the consumption of carbon. Finally, the RueMo4P3 NPs decorated on HCS (denoted as HCS-Ru-Mo4P3) sample were obtained by etching the silica cores with 2.0 M NaOH solution at 92  C. For comparison, MoP and RuP2 NPs loaded on HCSs were also prepared and denoted as HCS-MoP and HCS-RuP2, respectively. The crystal phases of HCS-Ru-Mo4P3, HCS-MoP, and HCS-RuP2 samples are measured by XRD (Fig. 2a), and their patterns match with that of the orthorhombic Mo4P3 (JCPDS 18e0846), hexagonal MoP (JCPDS 24e0771), and orthorhombic RuP2 (JCPDS 34e0333), respectively. According to the XPS measurements, the Ru and Mo contents in HCS-Ru-Mo4P3 are 0.26 at% and 2.4 at%, respectively (molar ratio of Mo to Ru is 9.24), suggesting that the Ru atom is doped into the Mo4P3 lattice. The increased Mo/Ru molar ratio compared to that in the precursors (Mo/Ru ¼ 2/1) may result from the different degrees of metal loss in the alkaline leaching process. Importantly, MoP phase is transformed to Mo4P3 after Ru doping

Fig. 1. Schematic illustration of the preparation of the HCS-Ru-Mo4P3 NPs.

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and similar phase transition (e.g. rhombohedral R3c phase BiFeO3 transfers to orthorhombic Pbam phase upon La-doping[33]) induced by heteroatom substitution has also been observed in other works [33,34]. The Mo4P3 phase with a higher metal/P ratio compared to MoP may enable improved electrocatalytic activity according to previous studies [35]. On the contrary to the sharp diffraction peaks in the XRD pattern of RuP2, the relatively weak peaks of HCS-Ru-Mo4P3 suggest that the RueMo4P3 NPs have smaller NP sizes. The content of RueMo4P3 in the HCS-Ru-Mo4P3 composite is measured by thermogravimetric analysis (TGA) to be 20% (Fig. S1). The weight loss in the range of 200e400  C and 400e550  C refers to the oxidation of carbon with different degrees of graphitization. The similar result has also been observed in other carbon materials reported before [36]. As revealed in scanning electron microscope (SEM) (Fig. 2b) and transmission electron microscope (TEM) (Fig. 2c) images, RueMo4P3 NPs are well-dispersed on HCSs with an average size of 5 nm. Thanks to the protection of carbon shell derived from RF, the aggregation of RueMo4P3 NPs can be greatly suppressed. From the HR-TEM image (Figs. 2d and S2), it can be observed that the interplanar spacing is measured to be 0.29 nm which is very close to the interplanar distance of (206) facet of Mo4P3 phase, consistent with the XRD result. The hollow sphere structures can be also observed in HCS (Fig. S3), HCS-MoP (Fig. S4), and HCS-RuP2 (Fig. S5) samples. The uniform distribution of RueMo4P3 NPs can be further illustrated in the elemental mapping images (Fig. 2e), where the signal of Ru is well overlapped with that of Mo and P. The surface area and pore volume of HCS-Ru-Mo4P3 are measured by N2 adsorption/desorption tests to be 964 m2 g
1 and 1.62 cm3 g
1, respectively (Fig. S6). Similarly, the HCS-MoP sample exhibits a high surface area of 1053 m2 g
1 and pore volume of 1.86 cm3 g
1. While the pure HCS only delivers a surface area of 683 m2 g
1 and pore volume of 1.05 cm3 g
1. The enhanced pore volume of HCS-Ru-Mo4P3 and HCS-MoP may result from the introduction of Mo and Ru that act as the pore-forming agents, which has been reported in the cases of Fe and Zn [37,38]. The relatively low surface area and pore volume of HCS-RuP2 (691.7 m2 g
1, 1.14 cm3 g
1) may result from the severe aggregation of RuP2 NPs that prevents the pore-forming process. After infiltrated with S, the obtained S/HCS-Ru-Mo4P3 still remains hollow sphere morphology (Figs. S7eS8). The S content in the S/HCS-Ru-Mo4P3 is calculated by the TGA results with ca. 77% (Figs. S9eS10). There is no obvious change in RueMo4P3 particle size after S loading, pointing to a high stability upon the S interaction (Figs. S11eS13). Importantly, no S particles can be observed, suggesting that almost all of the S is infiltrated into the HCS-RuMo4P3 sphere and S is uniformly distributed in the HCS-Ru-Mo4P3 host (Fig. S8). Considering the high surface area and pore volume of HCS-Ru-Mo4P3, it is concluded that all of the S is encapsulated in the pore of carbon shell. The encapsulated S in the shell of carbon sphere has a close contact with conductive substance which renders a better interfacial electron transfer. Meanwhile, the RueMo4P3 NPs loaded on the HCSs can serve as a guard and chemically suppress the random diffusion of LiPS formed in lithiation process. Besides the morphology and pore volume, the affinity of HCSRu-Mo4P3 with LiPS was also studied. By immersing HCS-Ru-Mo4P3 into Li2S4 solution, the yellow-colored solution gradually turns to colorless after standing for 10 h (Fig. S14), suggesting the high adsorption ability of HCS-Ru-Mo4P3 towards LiPS. For comparison, the color of Li2S4 solution immersed with HCS and HCS-RuP2 cannot fully fade during the 10 h while the HCS-MoP shows a similar adsorption ability with HCS-Ru-Mo4P3. The adsorption ability of those materials can be also measured by the UVeVis (Fig. 3a). As is shown, the HCS-Ru-Mo4P3 exhibits the lowest

Please cite this article as: F. Ma et al., Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135310

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Fig. 2. (a) The XRD patterns of HCS-MoP, HCS-Ru-Mo4P3, and HCS-RuP2, the symbols marked in the figure indicate the peaks of the corresponding patterns. The symbols identify the peak positions of HCS-MoP and HCS-Ru-Mo4P3; (b) and (c) The SEM and TEM images of HCS-Ru-Mo4P3; (d) High resolution TEM image of HCS-Ru-Mo4P3; (e) The HAADF-STEM image and C, Mo, Ru, P elemental mapping images of HCS-Ru-Mo4P3, the scale bars are 200 nm.

absorbance followed by HCS-MoP, HCS-RuP2, and HCS. Considering the higher surface area of HCS-MoP than HCS-Ru-Mo4P3, the introduction of Ru in HCS-Ru-Mo4P3 seems to provide superior intrinsic LiPS adsorption capability to HCS-MoP. The interfacial chemical interaction of LiPS with HCS-Ru-Mo4P3 was studied by XPS measurements. The Ru 3p spectrum of pristine

HCS-Ru-Mo4P3 (Fig. 3b) shows a peak at 484.4 eV, representing the Runþ species [39e41]. After immersion with Li2S4, a new peak observed at 482.6 eV can be assigned to the RueS bond, revealing a strong interaction of Ru with LiPS (Fig. 3c) [39,41]. At the same time, the Mo 3 d spectra also reveal some changes after soaking with Li2S4, with Mo 3d5/2 and 3d3/2 peaks shifting to the lower binding

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Fig. 3. (a) The UVeVis spectra of pristine Li2S4 solution and the Li2S4 solution after immersion with HCS, HCS-MoP, HCS-Ru-Mo4P3, HCS-RuP2. (b)e(f): The XPS study of the interaction between HCS-Ru-Mo4P3 and Li2S4. The XPS spectra of Ru 3p (b and c), Mo 3 d (d and e) before and after Li2S4 immersion, the S 2p spectrum (f) of HCS-Ru-Mo4P3 adsorbing Li2S4.

energy of 232.1 and 228.0 eV (Fig. 3d and e). The downshift of the peaks may reflect the interaction of Mo with LiPS. This hypothesis is confirmed by the S 2p spectrum of the Li2S4-adsorbed HCS-RuMo4P3 which consists of terminal (ST) and bridged (SB) S peaks at 163.1 and 164.3 eV followed by the thiosulfate peak at 167.8 eV (Fig. 3f) [42]. The thiosulfate originates from the interfacial reaction of Li2S4 with the negative oxygen at the MoeRueO surface of RueMo4P3 particle. As a result, there form MoeS and RueS bond while the remaining S turns to the LieOeS form [43]. Those results represent the strong surface interaction between HCS-Ru-Mo4P3 and Li2S4, demonstrating the excellent capability of the developed HCS-Ru-Mo4P3 NPs towards LiPS entrapment. The transformation mechanism of LiPS during charge/discharge processes involves the reduction of long chain Li2Sm (4 < m < 8) to short chain Li2Sn (2 < n < 4), Li2S deposition (discharge process) and Li2S decomposition (charge process) [15]. Previous studies suggest that the conductive polar materials, such as CoS2 and FeP[15,44], can accelerate the redox reaction of long-chain/short-chain conversion and alleviate the diffusion induced loss of LiPS, which could improve the rate capability as well as cycling stability. To better understand the role of HCS-Ru-Mo4P3, in this study, the electrocatalytic conversion of long-chain/short-chain LiPS redox was demonstrated by the cyclic voltammetry (CV) of symmetric cells employing HCS-Ru-Mo4P3, HCS-MoP, HCS-RuP2, and HCS electrodes in 0.1 M Li2S8 catholyte (Fig. 4a). Li2S8 is electrochemically reduced and re-oxidized on the electrodes in the cathodic and anodic processes of CV, respectively. It can be observed in Fig. 4a, the HCS-Ru-Mo4P3 electrode represents the highest onset potential of LiPS electrocatalytic reduction at 0.37 V and a lowest onset potential of Li2S decomposition at
0.32 V compared to other samples, which represents the high intrinsic activity of RueMo4P3 NPs for LiPS transformation. In contrast, HCS-MoP, HCS-RuP2, and HCS show the onset potentials of LiPS reduction and Li2S decomposition at 0.31/-0.25 V, 0.32/-0.246 V, 0.17/-0.13 V, respectively. The HCSRu-Mo4P3 electrode exhibits four distinct peaks at 0.03,
0.181,
0.007, 0.172 V. The former two reductive peaks refer

to the conversion of Li2S4 to Li2S[45,46], while the latter two oxidative peaks represent the oxidation process of Li2S to S8. Compared to previous results on LiPS catalytic transformation [45,46], the gap between the cathodic and anodic peaks of HCS-RuMo4P3 is relatively low (ca. 0.18 V), revealing a low polarization and an accelerated LiPS conversion process. For comparison, the pure HCS electrode delivers only one pairs of redox peak at
0.19/0.175 V with a large polarization gap of 0.365 V, indicating a sluggish LiPS conversion kinetics. The HCS-MoP and HCS-RuP2 electrodes exhibit similar four redox peaks with HCS-Ru-Mo4P3 but significantly larger polarization gaps. The enhanced electrocatalytic activity of HCS-Ru-Mo4P3 towards LiPS conversion compared to that of HCSMoP and HCS-RuP2 may stem from the high intrinsic activity of Ru-doped Mo4P3 sites. Previous studies regarding electrocatalytic hydrogen evolution reaction (HER) on Ru-doped metal compounds [47,48] indicate that the Ru doping can induce the synergistic effect between Ru and metal sites through the electronic interactions and thus optimize the adsorption/desorption of reaction intermediates (e.g., H*) during the electrocatalytic process. In this study, the possible synergistic effect between Ru and Mo in RueMo4P3 electrode may provide optimized adsorption/desorption energy and faster electrocatalytic conversion rate of LiPS. On the other hand, even though the electrochemical performance of Mo4P3 and other P-deficient molybdenum phosphide phase is hardly reported before, the electrochemical performance of ruthenium phosphides (i.e., Ru2P, RuP, and RuP2) for HER has been investigated [35]. Their result reveals that the higher metal/P ratio can lead to a higher electrochemical activity due to the optimized adsorption/desorption of H* intermediate and the more exposed Ru active sites. In our system, it is speculated that the higher metal/P ratio in Mo4P3 than MoP can deliver enhanced activity towards LiPS conversion due to the more exposed Mo sites. As for the Li2S deposition process generated from the lithiation of short-chain LiPS (mainly Li2S4), the nucleation and growth of Li2S can be engineered by the charge access ability and the affinity of substrate and LiPS as it highly depends on the chemical

Please cite this article as: F. Ma et al., Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135310

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Fig. 4. (a) CV plots of HCS, HCS-MoP, HCS-RuP2, and HCS-Ru-Mo4P3 asymmetric cells with 0.1 M Li2S8 catholyte. (b) The potentialstatic discharge profile of 0.1 M Li2S8 catholyte at 2.05 V on HCS, HCS-MoP, HCS-RuP2, and HCS-Ru-Mo4P3 electrodes. (c) The potentialstatic charge profile of 0.1 M Li2S8 catholyte at 2.25 V on HCS, HCS-MoP, HCS-RuP2, and HCS-RuMo4P3 electrodes.

environment according to previous studies [4,49e52]. In Li2S deposition tests (Fig. 4b), the I-t curve of HCS-Ru-Mo4P3 shows a peak at 544 s, obviously earlier than the pristine HCS electrode (749 s), HCS-MoP electrode (631 s) and HCS-RuP2 (690 s). The HCSRu-Mo4P3 with high adsorption ability of RueMo4P3 NPs offers plenty of LiPS-adsorptive interfaces for the Li2S nucleation and growth. Meanwhile, the unique hollow morphology of HCS-RuMo4P3 can provide highways for electron and charge transportation. On the other aspect, the enhanced charge accessibility and LiPSaffinitive interfaces in turn guarantee the rapid Li2S decomposition process (Fig. 4c). The pre-deposited HCS-Ru-Mo4P3 electrode was further subject to a Li2S decomposition test. The peak of HCS-RuMo4P3 arises at 485 s, dramatically earlier than that of pristine HCS (1647 s), HCS-MoP (1142 s), and HCS-RuP2 (1410 s), demonstrating a lower Li2S decomposition barrier. This result matches well with the lowest onset potential of HCS-Ru-Mo4P3 electrode for Li2S decomposition shown in the CV plot of Li2S8 asymmetric cell (Fig. 4a). This improvement can be mainly attributed to the intrinsically higher activity of the novel RueMo4P3 NPs in lowering the Li2S decomposition barrier compared to the undoped MoP ones. Meanwhile, HCS substrate can enhance ion diffusion and charge transfer in the Li2S/substrate interface as is reported earlier [5,53,54]. The S/HCS-Ru-Mo4P3 and other samples are further incorporated in LieS batteries and their rate performance are displayed in Fig. 5a. The 5th cycle discharge capacities of S/HCS-Ru-Mo4P3 under 0.5C, 1C, 2C, 3C, and 4C are 972, 955, 839, 712, and 660 mAh g
1, respectively. When the current density turns back to 0.5C, the capacity comes to 910 mAh g
1, showing a high reversibility. The excellent rate performance of S/HCS-Ru-Mo4P3 is superior to that of the previously reported MoP or other transition metal phosphides for LieS batteries (Table 1).

For comparison, the capacities of S/HCS are 757, 609, 536, 484, and 455 mAh g
1 under the same current densities. The voltagecapacity curves of these electrodes are displayed in Fig. S15. As is shown, the discharge profile of S/HCS-Ru-Mo4P3 at 0.5C delivers a short plateau 2.3 V followed by a long plateau at 2.08 V. The short plateau represents the reduction of Li2S8 to Li2S4 and the Li2S4 is further conversed to Li2S2 or Li2S which is the Li2S deposition process. The decomposition of Li2S in the charge process is reflected in the long plateau at 2.3 V of the charge profile of S/HCS-Ru-Mo4P3. From the difference of discharge/charge profiles of S/HCS-RuMo4P3 and S/HCS, it can be clearly observed that the S/HCS-RuMo4P3 has a much lower polarization gap (270 mV) in contrast to S/ HCS (320 mV) under a higher current density of 1C rate (Fig. S16) in addition to the capacity difference. Meanwhile, the S/HCS-MoP and S/HCS-RuP2 show polarization gap of 280 and 275 mV, both of which are lower than the S/HCS. The high rate capability of S/HCSRu-Mo4P3 demonstrates the fast kinetics of LiPS conversion. The electrochemical impedance spectroscopy (EIS) measurement (Fig. 6a and Fig. S17) is employed to study the distinct kinetics of the electrode. As is shown, the EIS plots after rate test (Fig. 6a) are composed of a semicircle at high and middle frequencies followed by a diagonal line at low frequency. The semicircle refers to the charge transfer resistance (Rct) in the electrochemical process. Obviously, the S/HCS-Ru-Mo4P3 has the lowest Rct of 21.3 U compared to other samples, which is in accordance with its outstanding rate capability. The lowest Rct may result from the intrinsic high conductivity of Mo4P3 phase and the enhanced redox kinetics of LiPS on the surface of RueMo4P3 NPs. Moreover, compared to the EIS plots of the electrodes (Fig. S17) before the cycling, it is clear that the S/HCS-Ru-Mo4P3 shows the lowest Rct change (13 Ue21.3 U). As the cathodic passivation resulting from the accumulation of LiPS and subsequent random deposition of S species strongly affects the Rct, this result reveals the effective

Please cite this article as: F. Ma et al., Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135310

F. Ma et al. / Electrochimica Acta xxx (xxxx) xxx

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Fig. 5. (a) Rate performance and (b) cycling stability at 1C rate of S/HCS, S/HCS-MoP, S/HCS-RuP2, and S/HCS-Ru-Mo4P3. (c) Cycling performance of S/HCS-Ru-Mo4P3 at 3C. The S loading of each electrode is 2 mg cm
2.

Table 1 Summary of the performance of MoP and other transition metal phosphides in LieS Batteries. Materials CNT-MoP/GO MoP/rGOa MoP-CNT Ni2P/N-doped carbon Ni2P/CNT/P-doped CNTb CoeFeeP cubes Ni2P@N,P-doped carbon Ni2P yolk-shell spheres HCS-Ru-Mo4P3

Sulfur Content
2

73 wt%1 mg cm [18] 85.6 wt% 3.88 mg cm
2 72 wt% 4 mg cm
2 61.92 wt% 78.8 wt% 1.2 mg cm
2 ~70 wt% 1 mg cm
2 77 wt% 2 mg cm
2 65.1 wt% 77 wt% 2 mg cm
2

Rate Capability (mAh g
1)

Capacity Retention

Ref.

942 (0.5C) 656 (2C) 816 (0.5C) 570 (2C) 1127 (50 mA g
1) 829 (350 mA g
1) 915.7 (1C) 468.9 (5C) 854 (0.5C) 655 (2C) 1012 (0.5C) 741 (2C) 912 (0.5C) 812 (2C) 1243 (0.5C initial) 499 (5C) 1119 (0.5C initial) 839 (2C) 660 (4C)

80% (1156 cycles at 1C) 86.4% (300 cycles at 0.5C) ~84% (80 cycles at 200 mA g
1) 82% (1000 cycles at 1C) 61% (500 cycles at 1C) 78% (500 cycles at 1C) 96% (300 cycles at 0.2C) 61% (1000 cycles at 5C) 61% (350 cycles at 1C)

[18] [42] [55] [56] [57] [58] [5] [20] This work

* and **: Metal phosphides are used as functional separators. a The cathode material is S/CNT/acetylene black. b The cathode material is S/ketjen black.

Fig. 6. (a) EIS plots of S/HCS, S/HCS-MoP, S/HCS-RuP2, and S/HCS-Ru-Mo4P3 electrodes after rate performance tests. (b) Cycling performance of S/HCS-Ru-Mo4P3 with a S loading of 6.6 mg cm
2 at 0.1C. (c) Discharge/charge profiles of the S/HCS-Ru-Mo4P3 electrode in Fig. 6b at various cycles.

Please cite this article as: F. Ma et al., Phase-transformed Mo4P3 nanoparticles as efficient catalysts towards lithium polysulfide conversion for lithiumesulfur battery, Electrochimica Acta, https://doi.org/10.1016/j.electacta.2019.135310

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F. Ma et al. / Electrochimica Acta xxx (xxxx) xxx

chemical suppression of LiPS migration and alleviated cathodic passivation in the repeating cycles. The S/HCS-Ru-Mo4P3 electrode was also subject to a long-term cycling test. The S/HCS-Ru-Mo4P3 has an initial capacity of 943 mAh g
1 under 1C rate (Fig. 5b). After 350 cycles, it still delivers a capacity of 560 mAh g
1 corresponding to a capacity decay of 0.11% per cycle. In the contrast, S/HCS reveals an initial capacity of 928 mAh g
1 but retains only 148 mAh g
1 after 350 cycles. S/HCSRuP2 delivers similar cycling stability to S/HCS-Ru-Mo4P3 with a capacity decay of 0.13% per cycle while it fails in the lower capacity. S/HCS-MoP exhibits a similar initial cycling stability with S/HCSRu-Mo4P3 while the capacity sharply decreases after 270 cycles and at last retains only 161 mAh g
1 after 350 cycles. In order to evaluate the capability of RueMo4P3 in suppressing the migration of LiPS, the S/HCS-Ru-Mo4P3 and S/HCS (as the reference) electrodes with different cycle numbers were disassembled and the optical photographs were compared in Fig. S18. The S/HCS-Ru-Mo4P3 electrode exhibits negligible color change while the S/HCS electrode turns to yellow since the 5th cycle. This visible difference demonstrates the effective chemical suppression of RueMo4P3 for LiPS. For a prolonged cycling test, the S/HCS-Ru-Mo4P3 electrode is subject to discharge and charge under a constant rate of 3C for 700 cycles, revealing a slight capacity decay of 0.07% per cycle (Fig. 5c). After cycling, the S/HCS-Ru-Mo4P3 cell was disassembled and subject to the TEM measurements. As is shown in Fig. S19, there is no obvious change of the morphology and elemental distributions after long-term cycling, indicating a high stability of HCS-Ru-Mo4P3 in LieS battery testing. For the purpose of the practical use of LieS battery, a high S loading is required to obtain a considerable areal capacity (>4 mAh cm
2). The S/HCS-Ru-Mo4P3 electrode with a high S loading of 6.6 mg cm
2 is employed in a LieS battery. The battery delivers an initial capacity of 1078 mAh g
1 at 0.1C (Fig. 6b and c), corresponding to an areal capacity of 7.7 mAh cm
2. The short plateau at 1.9 V refers to the transition of Li2S2 to Li2S [59e61]. After 50 cycles, it still retains a capacity of 840 mAh g
1 (corresponding to 5.6 mAh cm
2) with a capacity retention of 78%, demonstrating the strong anchoring capability of LiPS and excellent catalytic activity for LiPS transformation of RueMo4P3 NPs even under a high S loading condition.

4. Conclusion In this work, Ru-doped Mo4P3 NPs decorated on HCSs were developed for the first time as high-performance cathode materials in LieS battery. Doping Ru into MoP would induce phase transformation to Mo4P3 phase, which exhibits inherently high activity towards LiPS conversion. As a result, the HCS-Ru-Mo4P3 composite has a large pore volume to accommodate S and exhibits high affinity and adsorption capability in LiPS further guarantees the rapid redox conversion of LiPS. When incorporated in LieS battery, the S/ HCS-Ru-Mo4P3 electrode delivers a higher reversible capacity, better rate capability and cycling stability than the S/HCS-MoP, S/ HCS-RuP2, and S/HCS electrodes. Importantly, the rate capability achieved on S/HCS-Ru-Mo4P3 outperforms other transition metal phosphides for LieS batteries ever reported. The improved LieS battery performance can be primarily attributed to the high activity of Mo4P3 phase and the possible synergistic effect between Ru and Mo4P3, which guarantees faster kinetics of LiPS transformation and stronger binding with LiPS. The concept and preparation method can be also extended for other transition metal phosphides for use in advanced electrochemical energy storage and conversion.

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