Enhanced Silicon Diphosphide-Carbon Composite Anode for Long

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Enhanced Silicon Diphosphide-Carbon Composite Anode for Long-Cycle, High-Efficient Sodium Ion Batteries Jaffer Saddique, Xu Zhang, Tianhao Wu, Xin Wang, Xiaopeng Chen, Heng Su, Shiqi Liu, Liqiang Zhang, Guangyin Li, Yuefei Zhang, and Haijun Yu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02242 • Publication Date (Web): 28 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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Enhanced Silicon Diphosphide-Carbon Composite Anode for LongCycle, High-Efficient Sodium Ion Batteries Jaffer Saddique,†,‡,# Xu Zhang,†,# Tianhao Wu,† Xin Wang,§ Xiaopeng Chen,‡ Heng Su,† Shiqi Liu,† Liqiang Zhang,*,§ Guangyin Li,† Yuefei Zhang,*,‡ Haijun Yu,*,† †College

of Materials Science and Engineering, Key Laboratory of Advanced

Functional Materials of Education Ministry of China, Beijing University of Technology, Beijing 100124, P. R. China §State

Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Failure,

Corrosion, and Protection of Oil/Gas Facilities, China University of Petroleum, Beijing 102249, P. R. China ‡Institute

of Microstructure and Property of Advanced Materials, Beijing University of

Technology, Beijing 100124, P. R. China *Corresponding authors: E-mail address: [email protected] [email protected] [email protected]

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KEYWOR: silicon diphosphide (SiP2), composite anode material, sodium ion battery (NIBs), cycle performance, mechanism ABSTRACT Advanced anode materials possessing high performance, excellent stability and low cost are quite insufficient for sodium ion batteries (NIBs), and call for huge progress to meet the prerequisites toward practical applications. Herein, a silicon diphosphide (SiP2)/carbon composite anode was synthesized by a facile ball milling method to improve the NIBs performance.

The well-constructed composite

comprising of uniformly distributed SiP2 nanocrystallites within a conductive carbon framework greatly enhanced the electrode conductivity and structural compatibility to repeated sodiation/desodiation conversions, and thus generated an excellent cyclability of >80% capacity retention for 500 cycles, a high Coulombic efficiency of 99% and a promising fast-rate capability. In-situ/ex-situ X-ray diffraction and in-situ transmission electron microscopy investigations on the reversible electrochemical conversion of SiP2 showed the formation of Na3P and NaSi6 as the main sodiation products.

This study sheds lights on the

realization of large-scale, high-performance NIBs through superior phosphorusbased anode materials. Rechargeable batteries, particularly lithium ion batteries (LIBs), have been extensively developed in the last few decades to fulfill the huge and pressed demand in highefficient, long-life, low-cost and eco-friendly energy storage systems. However, the poor abundance (20 ppm) and uneven distribution of lithium in Earth’s crust greatly lift the cost of LIBs and limit their large-scale applications.1-3 Sodium (Na) is therefore considered as a promising low-cost alternative to lithium in rechargeable batteries, owing to its high earth abundance (2.4%), low price and broad geographical 2 ACS Paragon Plus Environment

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distribution.4-8 Considering the material feasibility and chemical properties of Na, sodium ion batteries (NIBs) are anticipated to be an ideal alternative to LIBs, and attract a rapidly growing attention. However, NIBs still face severe deficiencies in many aspects such as energy density, cycle stability, Columbic efficiency and safety, which call for high-performance and low-cost electrode materials to overcome these challenges.9-15 Among many attempted anode materials in NIBs, group 14 and 15 elementary substances such as phosphorus (P), tin (Sn), antimony (Sb) and germanium (Ge) have drawn special attention due to their high theoretical specific capacities when reacting with Na to form various alloys, including Na3P (2596 mAh g−1), Na15Sn4 (847 mAh g−1), Na3Sb (660 mAh g−1) and NaGe (369 mAh g−1).16-20 In particular, P allotropes including black P and red P are fascinating anode candicates because of their extremely high theoretical capacity.16,21-27

However, while black P is synthesized at high

pressure/temperature and thus not cost-effective, red P suffers from a poor electronic conductivity (1 × 10−14 S cm−1) and a volume expansion of ~430% in the formation of Na3P, leading to severe material pulverization, electrical isolation, sluggish kinetics and rapid capacity decay.21,26

P-based materials are thus proposed to solve these

problems.28-33 In particular, Si can not only form compounds with P,34,35 but also possess a theoretical capacity of 954 mAh g−1 (based on NaSi) in NIBs.36-38 Silicon diphosphide (SiP2) is thus expected to be a promising anode material, which may combine the advantages of its two precursors and generate new superiorities because of its specific crystal structure.39,40 However, to the best of our knowledge, there is only one primary study on SiP2-based NIBs, which showed a severe capacity decay within 15 cycles and a poor Columbic efficiency.30 The optimization of SiP2 anode by morphology control, conductivity enhancement and composite fabrication, which are 3 ACS Paragon Plus Environment

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anticipated to greatly improve the battery performance especially in terms of cycle stability and efficiency, is thus urgently required. Herein, a SiP2/super P carbon (SiP2/C) composite anode, prepared by facile ball milling procedures, is reported to be able to greatly improve the performance of NIBs especially in the cyclability and Columbic efficiency.

Structural characterizations on the

composite indicate the maintenance of SiP2 nanocrystallites and their uniform distribution in the carbon conductive framework, ensuring the SiP2/C-based NIBs possessing promising cycle stability with above 80% capacity retention for 500 cycles as well as other enhanced performance.

The reaction mechanism of the SiP2/C

electrode was disclosed by in-situ/ex-situ X-ray diffraction (XRD) and in-situ transmission electron microscopy (TEM), which showed that SiP2 is able to react with Na to form Na3P and NaSi6 during the discharge procedure.

Figure 1. Schemic illustration of the synthesis process of the SiP2/C composite.

The SiP2/C composite was facilely synthesized by a two-step high energy mechanical milling as illustrated in Figure 1. First, SiP2 was stoichiometrically synthesized from commercial silicon nanoparticles and red phosphorus powders at 600 rpm for 20 h under argon, a condition providing a pressure of ~6 GPa and a temperature of ~200 ºC. Such a reaction environment is able to convert nano Si and red P into a high-pressure

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phase of SiP2. The as-prepared SiP2 was further milled with super P carbon at 600 rpm for 6 h under argon to yield the final SiP2/C composite.

Figure 2. (a) XRD pattern and refinement result of the SiP2/C composite. (b) Structural model of SiP2 crystal. (c) TEM image of the SiP2/C composite. (d) HRTEM image of SiP2 in the composite. The inset is the Fourier transform image. (e-h) HAADF image (e) and the corresponding Si (f), P (g) and C (h) elemental distributions.

The crystalline structures of the SiP2 and SiP2/C composite, as revealed by XRD, are displayed in Figure S1 (supporting information, SI) and Figure 1a, respectively. The XRD patterns of both materials agree well with the theoretical pattern of SiP2, suggesting the formation of pure SiP2 and the maintainence of its crystallites in the final composite. The broad peak at around 25º in Figure 1a can be assigned to the XRD peak of graphitic structures from super P carbon, which is absent in the XRD pattern of pure SiP2 (Figure S1). There is no other crystal phase in either SiP2 or SiP2/C composite, although Si and P can also form other high-pressure phases such as Si6P2.54.39 Based on the XRD refienement results, a pyrite-type cubic structure , which belongs to space group Pa3 with lattice parameters a = b = c = 5.7070 Å (5.7045 Å, COD No. 434431) can well explain the experimental results.

In such a structure, each Si atom is

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coordinated with six P atoms to form Si-P bonds with a distance of 3.9065 Å in an octahedron unit, and each P atom is shared by three octahedrons (Figure 2b). The morphology of the SiP2/C composite was examined by scanning electron microscopy (SEM) (Figure S2, SI) and TEM (Figure 2c). It can be seen in Figure S2 that micron-scale clusters were formed by the agglomeration of nanoparticles of ~2050 nm. The fact that SiP2 crystallites and C particles can hardly be discerned by SEM suggests the uniform mixing of the two precursors with no noticeable separation. Such a composite structure ensures a good conductivity and short Na-diffusion pathways. Figure 2c shows that SiP2 nanoparticles are well embedded into a carbon framework, leading to a well-defined composite configuration.

The SiP2 crystallites in the

composite were further confirmed by high-resolution TEM (HRTEM) image in Figure 2d. The crystalline structure of SiP2 with its (210) and (220) planes is clearly identified in the HRTEM image and the corresponding Fourier transform pattern (inset in Figure 2d). The lattice parameters obtained from HRTEM are in good agreement with the XRD results. Figure 2e shows the high-angle annular dark field (HAADF) image of the SiP2/C composite, and the corresponding element distributions of Si, P, and C as revealed by energy dispersive spectroscopy (EDS) maps are presented in Figure 2f-h, respectively. The elemental distributions of Si and P coincide well in intensity with each other due to the single-phase nature of SiP2. Carbon is observed overall the composite, indicating a fine distribution of carbon nanoparticle framework for hosting SiP2 nanoparticles.

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Figure 3. Electrochemical performance of SiP2/C composite electrode in NIBs. (a) Initial five CV curves scanned from 0.01 to 2 V at a rate of 0.01 mV s−1. (b) Cycle performance of SiP2/C electrode acquired at a current density of 50 mA g−1. (c) The corresponding discharge/charge profiles from selected cycles in (b). (d) Rate performance of SiP2/C at various current densities from 50 to 1000 mA g−1. (e) The typical discharge/charge profiles at various current densities. (f) EIS of SiP2/C and the corresponding equivalent circuit. (g) Long cycle performance of SiP2/C composite recorded at 50 mA g−1 (initial 10 cycles) and then at 500 mA g−1 (remaining 490 cycles).

To evaluate the performane of SiP2/C, a series of electrochemical characteriations were conducted on batteries composing the composite as the working electrode and Na foils as the counter and reference electrodes. In the 1st cycle, similar to that of pure SiP2 (Figure S4a, SI), the cyclic voltammetry (CV) curve of SiP2/C (Figure 3a) shows a broad cathodic peak ranging from 0 to 0.7 V, which is most probably owing to the formation of a SEI layer. The CV curves became almost constant from the 2nd cycles, suggesting the good reversibility of the electrochemical conversions of SiP2/C after its initial stabilization.

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The cycle performance and representative galvanostatic discharge/charge profiles of the SiP2/C electrode acquired at 50 mA g−1 are presented in Figure 3b and 3c, respectively. It can be seen that the initial Coulombic efficiency was only 76%. However, from the 2nd cycle, the SiP2/C electrode reached a high Coulombic efficiency of 99%, and maintained a capacity above 410 mAh g−1 for 100 cycles.

Benefiting

from the the conductive carbon framework, the efficiency and cycle stability of the composite are obviously superior to those of SiP2 (Figure S4b, c, SI). The well repeatable charge/discharge profiles in Figure 3c further demonstrate the excellent repeatable electrochemical behavior/performance of the composite electrode.

To

evalucate the influence of super P carbon on the capacity, the electrochemical performance of only super P carbon in NIBs was tested as displayed in Figure S3. It can be seen that the capacity of super P carbon is only 100 mAh/g at a current density of 50 mA/g. The small discharge plateau at around 0.1 V should originate from the sodiation of super P carbon additive,41 contributing a capacity below 50 mAh g−1. Compared with SiP2/C, P/C (Figure S5) and Si/C (Figure S6) composites displayed much worse performance, implying that SiP2 is more compatible to super P carbon in the formation of composite structure. The rate performance of SiP2/C has also been tested at a series of current densities ranging from 50 to 1000 mA g−1 (Figure 3d and e). When the current densities of 50, 100, 200, 300, 500 mA g−1, and 1000 mA g−1were applied, the SiP2/C electrode exhibits average discharge capacities of ~501, 414, 341, 301, 259, and 198 mAh g−1 mAh g−1, respectively. After these tests, the NIB was operated at 50 mA g−1 again, and the average capacity recovery reached a high value of 99.0% (496 mAh g−1), suggesting a good rate performance of the composite.

Significantly, during all these battery

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rate performance demonstrates that SiP2/C favors the reversible sodiation/desodiation transformation at high current densities with no significant capacity fade. Electrochemical impedance spectra (EIS) of the attempted electrodes were measured to attain their electrochemical properties for understanding the origins of the superior electrochemical performance of SiP2/C. The Nyquist plot and the corresponding fitting line of SiP2/C are displayed in Figure 3f, showing a semicircle at high frequencies and a straight line at low frequencies. Based on the fitted results, the SiP2/C electrode exhibit a low impedance (R2 = 356.2 Ω). The spectral radius of SiP2/C is much smaller than SiP2 (Figure S4d), P/C (Figure S5d) and Si/C (Figure S6d), indicating a much lower electrochemical resistance of the former. The EIS results of SiP2/C, P/C and Si/C show that SiP2/C has the lowest resistance, indicating SiP2 and C have a good structure compatibility during the ball milling process to form a compact composite with tight interfaces. Such a structure is advantage to the charge transfer during electrochemical reactions. Also, compared with P, the reduced volume expansion of SiP2 during sodiation/desodiation would stabalize the SiP2/C composite. As a result, super P carbon as the SiP2 loading host greatly increases the electrode conductivity, leading to the better performance of SiP2/C than the other tested materials. The long cycle performance of the SiP2/C electrode is shown in Figure 3g, in which the current density was set at 50 mA g−1 for the initial 10 cycles for activation and then at 500 mA g−1 for the remaining 490 cycles. The SiP2/C composite exhibited a highly stable electrochemical performance with no more than 20% capacity decay and a remarkable Columbic efficiency of 99%, suggesting the SiP2/C composite is a promising anode material for rechargable NIBs during the repeated charge/discharge process.

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Figure 4. In-situ XRD analysis of the SiP2/C composite in the initial two discharge/charge cycles. (a, b) 2D Contour plots (a) and the corresponding discharge/charge profiles (b). (c) Enlarged contour plots in selected 2θ regions.

The real-time monitoring on the electrochemical conversion of SiP2/C electrode was performed by in-situ XRD for initial two cycles. Figure 4a and b shows the 2D Contour plot of in-situ XRD patterns and the coordinated discharge/charge profile, respectively. The characteristic peaks of SiP2 at 31.25°, 35.05°, 38.55° and 44.88° are observed to undergo partially reversible decrease/increase during discharge/charge, which is clearer shown in Figure 4c. Selected in-situ XRD patterns at a series of electrode potentials are presented in Figure S7. In the discharge process, the SiP2 peaks became much weaker, suggesting its sodiation to form other materials. In the charge process, the SiP2 peaks became stronger again, probably owing to the recovery of crystalline SiP2 in a reverse reaction. However, these peaks could not recover their initial intensities, indicating the partial reversibility and the amorphization of SiP2 in repeated electrochemical conversions. 10 ACS Paragon Plus Environment

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Figure 5. (a) Ex-situ XRD of the SiP2/C electrode at different discharge/charge status in the first cycle. (b-d) In-situ TEM investigation of a SiP2-Na nano battery. TEM images of electrodes and the corresponding SAED patterns of the SiP2 electrode in selected regions acquired before battery working (b), when discharged at −3V (c), and when further charged at +3V (d).

To obtain more structure information of materials during the electrochemical reaction, ex-situ XRD analysis was carried out on SiP2/C electrodes of NIBs disassembled at selected potentials in the first cycle (Figure 5a). The pattern 1 recorded before reaction displays typical SiP2 peaks as expected (marked by circles). The discharge to 0.5 V (vs. Na+/Na) led to the appereance of new peaks attributed to Na3P (marked by stars in pattern 2). In addition, a set of other peaks can also be found (as marked by triangles), which exhibit slightly upshifts from the standard pattern of NaSi6 probably due to unknown lattice distortions. These peaks did not alter when further discharged to 0.01 V (pattern 3), indicating Na3P and NaSi6 are final discharge products. However, upon these discharges, SiP2 peaks did not vanish, suggesting a portion of SiP2 did not react

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with Na. When the battery was charged to 1 V (pattern 4), the peaks of Na3P and NaSi6 could not be observed, which means these discharge products converted to other materials such as SiP2 or elemental substances of P and Si. The formation of SiP2 is implied by the enhanced peak intensities in the in-situ XRD plots (Figure 4). The formation of P and Si is also possible due to the phase separation of binary phosphides during battery operation.42 Further charge to 2 V did not change the pattern greatly (pattern 5). To further elucidate the reaction mechanism of SiP2 in NIBs, in-situ TEM investigation of a nano battery consisting of SiP2 working electrode and Na counter electrode was carried out. As shown in Figure 5b, prior to the contact with Na, the pristine SiP2 electrode displayed a pure crystalline structure with its (211) and (220) planes detected in the selected area electron diffraction (SAED) pattern. When the Na electrode was driven by a piezoceramic manipulator to contact SiP2, a constant voltage (−3V) was immediately applied on the battery to launch the sodiation process. After the discharge for 258 s, the TEM image of SiP2 displayed a noticeable change, and the SAED pattern of SiP2 became more complicated, suggesting the generation of some new phases (Figure 5c). Notablely, the (200) and (102) planes of Na3P crystal can be clearly discerned in the SAED pattern as very bright spots, suggesting the dominant formation of Na3P as a sodiation product of SiP2. The (321) plane of NaSi6 can also be detected. The formation of Na3P and NaSi6 as revealed by TEM agrees well with the conclusion obtained from ex-situ XRD. In addition, the (200) and (220) planes of SiP2 were distinguished, further demonstrating the incomplete conversion of SiP2. The nano battery was then charged at +3V to test electrochemial reactions in the reverse operation. The SAED pattern in Figure 5d shows that SiP2 phase was recovered, since the SiP2 (220) and (200) planes could be discerned. However, in contrast to the excellent 12 ACS Paragon Plus Environment

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crystalline structure before cycling, the SiP2 electrode showed a tendancy of amorphorization after the 1st charge. After that, the nano battery was runned for another two cycles, and the results were presented in Figures S8. More severe amorphorization of SiP2 was observed, and a complete amorphorization was observed in the 3rd charge process.

An in-situ TEM movie showing the real-time morphology change of

electrodes in the first cycle is provided in SI.

The in-situ TEM investigation

demonstrates that SiP2 reacts with Na to form Na3P as well as NaSi6 in the discharge process, which may partially recover to (SiP2)crystalline/amorphous in the charge process. From the above in-situ/ex-situ XRD and in-situ TEM studies, it can be concluded that most Na turns to Na3P when reacting with SiP2, which means almost all capacity of SiP2 stems from the formation of Na3P. The possible formation of P due to the phase separation of SiP2 in the battery operation may imply the electrochemical reactions between P and Na in following cycles. It demonstrates the key role of phosphorus in the SiP2 anode material. We thus proposed the following reaction procedures for the key conversion of SiP2/C composites in the initial operation of NIBs. In the discharge process: SiP2→Na3P + NaSi6 In the charge process: Na3P+ NaSi6→(SiP2)crystalline/amorphous + P In summary, a SiP2/carbon composite was prepared by a facile ball milling procedure as a high-performance anode material for sodium ion batteries. The composite structure was examined by a series of techniques, which indicate the maintenance of SiP2 nanocrystallites and their uniform distribution in the conducting carbon framework. 13 ACS Paragon Plus Environment

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Electrochemical measurements of the composite electrode demonstrate a high capacity above 410 mAh g−1 at 50 mA g−1 after 100 cycles, a greatly improved cyclability with a >80% capacity retention for 500 cycles, and a high Columbic efficiency of 99%. The electrochemical stability of the composite greatly surpassed that of pure SiP2, red phosphorus/carbon and silicon/carbon composites, suggesting that both the use of active SiP2 and the efficient loading by the conductive carbon framework are crucial for the improved performance.

In-situ/ex-situ X-ray diffraction and in-situ

transmission electron microscopy studies were employed to elucidate the reaction mechanism, revealing an electrochemical conversion comprising the sodiation of SiP2 into Na3P as well as NaSi6. This study provides important insights into the development of phosphorus-based anode materials for practical sodium ion batteries. ASSOCIATED CONTENT Supporting Information Supporting Information is available free of charge on the ACS Publications website. It contains the experimental details, additional XRD, SEM, electrochemical performance, in-situ XRD and in-situ TEM results. AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected] Author Contributions #J.

S. and X. Z. contributed equally to this work.

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Haijun Yu: 0000-0003-0204-9943 Xu Zhang: 0000-0001-7320-4360 Shiqi Liu: 0000-0003-3230-9099 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This project is supported financially by the Beijing Natural Science Foundation (B) (KZ201610005003), National Natural Science Foundation of China (Grants 21503009, 51622202, U1507107 and 21603009), the National Key R&D Program of China (Grant No. 2018YFB0104302) and Guangdong Provincial Science and Technology Program (2016B010114001). REFERENCE (1) Armand, M.; Tarascon, J. M. Building better batteries. Nature 2008, 451, 652-657. (2) Goodenough, J. B.; Kim, Y. Challenges for Rechargeable Li Batteries. Chem. Mater. 2010, 22, 587-603. (3) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the development of advanced Li-ion batteries: a review. Energ. Environ. Sci. 2011, 4, 3243-3262. (4) Hwang, J. Y.; Myung, S. T.; Sun, Y. K. Sodium-ion batteries: present and future. Chem. Soc. Rev. 2017, 46, 3529-3614. (5) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636-11682. (6) Palomares, V.; Serras, P.; Villaluenga, I.; Hueso, K. B.; Carretero-Gonzalez, J.; Rojo, T. Na-ion batteries, recent advances and present challenges to become 15 ACS Paragon Plus Environment

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(17) Zhu, H. L.; Jia, Z.; Chen, Y. C.; Weadock, N.; Wan, J. Y.; Vaaland, O.; Han, X. G.; Li, T.; Hu, L. B. Tin Anode for Sodium-Ion Batteries Using Natural Wood Fiber as a Mechanical Buffer and Electrolyte Reservoir. Nano Lett. 2013, 13, 3093-3100. (18) Darwiche, A.; Marino, C.; Sougrati, M. T.; Fraisse, B.; Stievano, L.; Monconduit, L. Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism. J. Am. Chem. Soc. 2012, 134, 20805-20811. (19) Baggetto, L.; Keum, J. K.; Browning, J. F.; Veith, G. M. Germanium as negative electrode material for sodium-ion batteries. Electrochem. Commun. 2013, 34, 41-44. (20) Luo, W.; Gaumet, J. J.; Mai, L. Q. Antimony-based intermetallic compounds for lithium-ion and sodium-ion batteries: synthesis, construction and application. Rare Metals 2017, 36, 321-338. (21) Kim, Y.; Park, Y.; Choi, A.; Choi, N. S.; Kim, J.; Lee, J.; Ryu, J. H.; Oh, S. M.; Lee, K. T. An Amorphous Red Phosphorus/Carbon Composite as a Promising Anode Material for Sodium Ion Batteries. Adv. Mater. 2013, 25, 3045-3049. (22) Li, W. J.; Chou, S. L.; Wang, J. Z.; Liu, H. K.; Dou, S. X. Simply Mixed Commercial Red Phosphorus and Carbon Nanotube Composite with Exceptionally Reversible Sodium-Ion Storage. Nano Lett. 2013, 13, 5480-5484. (23) Yabuuchi, N.; Matsuura, Y.; Ishikawa, T.; Kuze, S.; Son, J. Y.; Cui, Y. T.; Oji, H.; Komaba, S. Phosphorus Electrodes in Sodium Cells: Small Volume Expansion by Sodiation and the Surface-Stabilization Mechanism in Aprotic Solvent. Chemelectrochem 2014, 1, 580-589. (24) Sun, J.; Lee, H. W.; Pasta, M.; Yuan, H. T.; Zheng, G. Y.; Sun, Y. M.; Li, Y. Z.;

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Cui, Y. A phosphorene-graphene hybrid material as a high-capacity anode for sodium-ion batteries. Nat. Nanotechnol. 2015, 10, 980-U184. (25) Xu, G. L.; Chen, Z. H.; Zhong, G. M.; Liu, Y. Z.; Yang, Y.; Ma, T. Y.; Ren, Y.; Zuo, X. B.; Wu, X. H.; Zhang, X. Y.; Amine, K. Nanostructured Black Phosphorus/Ketjenblack Multiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries. Nano Lett. 2016, 16, 3955-3965. (26) Xia, Q. B.; Li, W. J.; Miao, Z. C.; Chou, S. L.; Liu, H. K. Phosphorus and phosphide nanomaterials for sodium-ion batteries. Nano Res. 2017, 10, 40554081. (27) Chen, S. Q.; Wu, F. X.; Shen, L. F.; Huang, Y. Y.; Sinha, S. K.; Srot, V.; van Aken, P. A.; Maier, J.; Yu, Y. Cross-Linking Hollow Carbon Sheet Encapsulated CuP2 Nanocomposites for High Energy Density Sodium-Ion Batteries. ACS Nano 2018, 12, 7018-7027. (28) Li, W. J.; Chou, S. L.; Wang, J. Z.; Liu, H. K.; Dou, S. X. A new, cheap, and productive FeP anode material for sodium-ion batteries. Chem. Commun. 2015, 51, 3682-3685. (29) Qian, J. F.; Xiong, Y.; Cao, Y. L.; Ai, X. P.; Yang, H. X. Synergistic Na-Storage Reactions in Sn4P3 as a High-Capacity, Cycle-stable Anode of Na-Ion Batteries. Nano Lett. 2014, 14, 1865-1869. (30) Duveau, D.; Israel, S. S.; Fullenwarth, J.; Cunin, F.; Monconduit, L. Pioneer study of SiP2 as negative electrode for Li- and Na-ion batteries. J. Mater. Chem. A 2016, 4, 3228-3232. (31) Lan, D. N.; Wang, W. H.; Li, Q. Cu4SnP10 as a promising anode material for sodium ion batteries. Nano Energy 2017, 39, 506-512.

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(32) Usui, H.; Domi, Y.; Fujiwara, K.; Shimizu, M.; Yamamoto, T.; Nohira, T.; Hagiwara, R.; Sakaguchi, H. Charge-Discharge Properties of a Sn4P3 Negative Electrode in Ionic Liquid Electrolyte for Na-Ion Batteries. ACS Energy Lett. 2017, 2, 1139-1143. (33) Usui, H.; Domi, Y.; Yamagami, R.; Fujiwara, K.; Nishida, H.; Sakaguchi, H. Sodiation–Desodiation Reactions of Various Binary Phosphides as Novel Anode Materials of Na-Ion Battery. ACS Appl. Energy Mater. 2018, 1, 306-311. (34) Chattopadhyay, T. K.; Vonschnering, H. G. Pyrite-Type Silicon Diphosphide PSip2 - Structural Parameters and Valence Electron-Density Distribution Chemistry and Structural Chemistry of Phosphides and Polyphosphides. Z Kristallogr 1984, 167, 1-12. (35) Carlsson, J. R. A.; Madsen, L. D.; Johansson, M. P.; Hultman, L.; Li, X. H.; Hentzell, H. T. G.; Wallenberg, L. R. A new silicon phosphide, Si12P5: Formation conditions, structure, and properties. J. Vac. Sci. Technol. A 1997, 15, 394-401. (36) Zhao, Q. J.; Huang, Y. H.; Hu, X. L. A Si/C nanocomposite anode by ball milling for highly reversible sodium storage. Electrochem. Commun. 2016, 70, 8-12. (37) Xu, Y. L.; Swaans, E.; Basak, S.; Zandbergen, H. W.; Borsa, D. M.; Mulder, F. M. Reversible Na-Ion Uptake in Si Nanoparticles. Adv. Energy Mater. 2016, 6. (38) Jung, S. C.; Jung, D. S.; Choi, J. W.; Han, Y. K. Atom-Level Understanding of the Sodiation Process in Silicon Anode Material. J. Phys. Chem. Lett. 2014, 5, 1283-1288. (39) Kwon, H. T.; Lee, C. K.; Jeon, K. J.; Park, C. M. Silicon Diphosphide: A SiBased Three-Dimensional Crystalline Framework as a High Performance LiIon Battery Anode. ACS Nano 2016, 10, 5701-5709.

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(40) Reinhold, R.; Stoeck, U.; Grafe, H. J.; Mikhailova, D.; Jaumann, T.; Oswald, S.; Kaskel, S.; Giebeler, L. Surface and Electrochemical Studies on Silicon Diphosphide as Easy-to-Handle Anode Material for Lithium-Based Batteriesthe Phosphorus Path. ACS Appl. Mater. Inter. 2018, 10, 7096-7106. (41) Yu, H. J.; Ren, Y.; Xiao, D. D.; Guo, S. H.; Zhu, Y. B.; Qian, Y. M.; Gu, L.; Zhou, H. S. An Ultrastable Anode for Long-Life Room-Temperature SodiumIon Batteries. Angew. Chem. Int. Edit. 2014, 53, 8963-8969. (42) Usui, H.; Domi, Y.; Yamagami, R.; Fujiwara, K.; Nishida, H.; Sakaguchi, H. Sodiation-Desodiation Reactions of Various Binary Phosphides as Novel Anode Materials of Na-Ion Battery. ACS Appl. Energy Mater. 2018, 1, 306-311.

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