Article pubs.acs.org/cm
Molybdenum Phosphide: A Conversion-type Anode for UltralongLife Sodium-Ion Batteries Zhaodong Huang,† Hongshuai Hou,† Chao Wang,§ Simin Li,† Yun Zhang,‡ and Xiaobo Ji*,† †
College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, Hunan, China College of Chemistry and Chemical Engineering, University of Electronic Science and Technology of China, Chengdu 610000, Sichuan, China ‡ Department of Advanced Energy Materials, College of Materials Science and Engineering, Sichuan University, Chengdu 610064, Sichuan, China §
S Supporting Information *
ABSTRACT: High specific capacity and long cycling life of anode materials still remain major challenges in the development of sodium-ion batteries (SIBs). Nowadays, transition metal phosphides have been reckoned as promising anodes in view of their high theoretical specific capacities, low potential for sodium storage, and superior conductivity. Herein, molybdenum phosphide (MoP) nanorods wrapped with thin carbon layer have been prepared and applied as an anode for SIBs. By utilizing in situ X-ray diffraction technology, a conversion-type mechanism of MoP anode has been disclosed. This kind of MoP electrode exhibits extraordinary electrochemical properties, including excellent cycling performance with a discharge capacity of 398.4 mA h g−1 at a current density of 100 mA g−1 after 800 cycles, and remarkable rate capabilities, which remain 104.5 mA g−1 at 1600 mA h g−1 even after 10 000 loops. The distinguished performances stem from synergetic merits of the morphology, structure, and mechanism of MoP. It is expected that MoP will be a promising anode material for SIBs and this work would provide theoretical basis for the scalable research and applications of MoP-based anode materials for SIBs.
1. INTRODUCTION Sodium-ion batteries (SIBs), which are promising substitutes of lithium-ion batteries (LIBs) thanks to having similar electrochemical principles and more extensive reserves of sodium sources, have aroused explosive enthusiasms of researchers lately. Hitherto, many transitional metal oxides1−4 and hard carbon materials5−7 have been applied as anode materials for SIBs, but their specific capacity and cycling life cannot fulfill the demanding requirements of electronic devices and large-scale energy storage.8 Hence, it is imperative to explore more suitable anode candidates with exceptional electrochemical traits to further optimize SIBs technology. Transition metal phosphides, possessing high theoretical specific capacities, low potential for sodium storage, and superior conductivity,9 have been considered as potential electrode materials for SIBs, which were first introduced into anode materials for SIBs by Yang et al.10 They studied the sodium storage mechanism of Sn4P3 and proposed a alloyreaction mechanism accompanying with gigantic volume expansion. Therefore, many measures such as carbon coating and morphological control were adopted to improve the cycling stability of Sn4P3;11−14 additionally, the influences of electrolyte additive and the Sn/P ratio on the electrochemical properties of tin phosphide have also been investigated in detail.15,16 Very © 2017 American Chemical Society
recently, the alloy-type mechanism of Se4P4 has been explored by Chen et al.17 So far, Sn4P3 and Se4P4 are the only two phosphides that undergo an alloy-type electrochemical mechanism, though they delivered attractive specific capacity, the enormous volume variation during the charge/discharge process severely restricted their cycling lives. Compared with the alloy-type phosphide materials, of which bulky volume expansion could result in poor cycling stability, and insertiontype phosphide, with intrinsic deficiency of second-rate specific capacity, conversion-type phosphides seem to be more attractive in view of their superior specific capacity than that of insertion-type materials and smaller volume expansion than alloy-type ones. Hence, a series of phosphides of which sodium storage via conversion reaction have earned widespread research enthusiasm. Nanosized FeP and CoP particles were first reported for SIBs by Chou et al.,18,19 followed by the extension to many other iron-based,20,21 cobalt-based,22 nickelbased,9 and copper-based23 phosphides. Since then, the electrochemical properties of phosphides have been greatly improved, which benefited from the joint efforts of numerous Received: May 29, 2017 Revised: August 14, 2017 Published: August 14, 2017 7313
DOI: 10.1021/acs.chemmater.7b02193 Chem. Mater. 2017, 29, 7313−7322
Article
Chemistry of Materials
Not surprisingly, the MoP nanorods composites displayed extremely stable cycling performance that delivered 398.4 mAh g−1 at a current of 100 mA g−1 with no attenuation observed even after 800 cycles. Benefiting from the morphology of nanorod and the structure of cross-linked MoP connected by amorphous carbon film, the conversion-type MoP composites showed superior rate capability and ultralong cycle life that remained 104.5 mA h g−1 at 1600 mA g−1 even after 10 000 cycles.
researchers. Especially, CoP@FeP core−shell structure prepared by Yin et al. delivered 456.2 mAh g−1 after 200 cycles;21 even so, the cycling life is still less than satisfactory. Particularly, the introduction of transition metal might greatly enhance the electrochemical conductivity12,17,20 and mitigate the volume expansion of phosphorus,18,19,23 while the combination with phosphorus can significantly improve the theoretical specific capacity of composites. Therefore, it is of great significance to explore a new conversion-type phosphide anode. Molybdenum-based materials, including MoS 2 , 24−26 MoSe2,27 MoO3,28,29 which belong to conversion-reaction materials, have shown preferable cycle stability and rate performances serving as anode for SIBs. Specifically, MoS2/ SWNT showed capacity retention of 95% after 100 cycles at 200 mA g−1;24 MoSe2/SWCNT delivered 459 mA g−1 after 90 cycles without obvious decline;27 and MoO3 nanobelts also demonstrated outstanding rate capability,29 indicating the benefits of molybdenum to the electronic conductivity and structure stability of molybdenum-based materials. Nowadays, molybdenum phosphide (MoP) has extremely been a research focus as catalysis for hydrogen evolution reaction for their high electronic conductivity, excellent stability, and high catalytic activity.30−33 Moreover, given that they have high specific capacity for SIBs (633 mA h g−1), it will be a promising anode material that could fulfill the expectation. The synchrotron in situ high energy XRD is an authoritative technique for monitoring reacting system since the high energy X-ray can penetrate the anodes and generate premium data in limitless time during (de)sodiation, which makes measurement feasible.34,35 Hence, it is usually adopted to explore the electrochemical storage mechanism of different electrode materials in various systems, covering anode and cathode in SIBs36−39 and Li-oxygen batteries.40 Typically, Chen et al. explored an insertion-type mechanism of Co-doped FeS2 nanospheres through in situ XRD.37 A sodium storage mechanism that combined Na+ intercalation, conversion reaction, and alloy reaction of Sb2Se3 nanorods was revealed by Huang et al.41 Thus, it can be employed to study the energy storage mechanism of MoP anode for SIBs. Aside from energy storage mechanism, morphology and structure of anode materials also have greatly important influences on their electrochemical performances.42 Special morphology such as nanorod (nanotube)43−45 and hollow sphere46,47 was usually adopted to shorten the diffusion pathway of sodium ion and ease the giant volume expansion. For instance, one-dimensional Sb2S3 materials48 and NiSb intermetallic hollow spheres49 displayed outstanding cycle ability and rate capabilities. The building of stable structure was another efficient measure to accommodate volume expansion. Double wall Sb@TiO2‑x nanotubes took advantage of high specific capacity of Sb and brilliant electrochemical stability of TiO2−x together.50 Graphene-rich wrapped rutile TiO2 prepared by Zhang et al. demonstrated the significant enhancement of electronic conductivity via carbon coating.51 Therefore, it is more likely to search for a new promising anode for SIBs providing excellent cycling performance and outstanding rate capability when taking morphology, structure, and mechanism all into consideration. Hence, in this work, MoP nanorods wrapped by amorphous carbon have been first applied as anode materials for SIBs, and their sodium storage mechanism has been investigated utilizing in situ XRD technology. It turned out that the MoP composites storage sodium via conversion reaction between Na+ and MoP.
2. EXPERIMENTAL SECTION Materials and Chemicals. All chemical reagents in our experiments, including ammonium molybdate ((NH4)6Mo7O24·4H2O, 99%), diammonium hydrogen phosphate ((NH4)2HPO4, 99%), and L-(+)-tartaric acid (C4H6O6, 99.5%), were purchased from Sinopharm and used without any further treatment. Sodium metal foil (Na, 99%), super P (C, 35−45 nm in particle sizes), polyethylene film (Celgard 2500), and carboxymethyl cellulose (CMC, 99.5%) were purchased from Sigma-Aldrich. Sodium perchlorate (NaClO4, 99%), propylene carbonate (PC, C4H6O3, 99.7%), and fluoroethylene carbonate (FEC, C3H3FO3, 99%) additive were purchased from Alfa Aesar. Preparation of MoP Nanorods Composites. The MoP nanorods composites were synthesized through a high temperature solid phase reaction, 2.65 g of ammonium molybdate tetrahydrate ((NH4)6Mo7O24·4H2O), 2 g of ammonium monoacid phosphate ((NH4)2HPO4), and 0.643 g of tartaric acid (C4H6O6, TA) were dissolved in 30 mL of deionized water with the molar ratio Mo/P/TA = 1:1:2. Then the solution was kept aging in a water bath at 80 °C for 24 h, following by dried at 110 °C to obtain the precursor. Subsequently, the dried samples after grinding were calcined under a flow of 5% H2/Ar mixture gas for 5 h at 950 °C in a tube furnace with the heating rate of 6 °C min−1. After nature cooling, the obtained samples were washed with deionized water and alcohol several times, respectively, followed by drying in the vacuum drying oven at 80 °C. To explore the formation process of MoP nanorods, a series of calcinations durations (1 h, 3 h, 4 h) have also been tested with other conditions stay the same as before. Morphology and Structural Characterization. The composition and morphology of MoP were characterized by X-ray diffraction (XRD, Rigaku D/max 2550 VB+ 18 kW, Cu Kα radiation, Japan) (2θ: 10−80°; receiving slit, 0.1542 nm; scintillation counter, 40 mA; 40 kV), thermogravimetric analysis (TGA, NETZSCH STA449F3), field emission scanning electron microscope (FESEM, FEI Quanta 200, Japan), and transmission electron microscopy (TEM, FEI TecnaiG2F20, American). The elemental distribution characteristics were attained via energy-dispersive X-ray spectroscopy (EDX, JEOL JEM-2100F, Japan). The chemical composition of the sample’s surfaces was detected by X-ray photoelectron spectroscopy (XPS, using KAlpha 1063). Electrochemical Measurements. The electrochemical properties of MoP were studied by using half-cells assembled in an MBraun glovebox (mBraun, Germany) full with Ar atmosphere (O2 and H2O levels
