Graphene-Encapsulated CuP2: A Promising Anode Material with High

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Graphene-Encapsulated CuP2: A Promising Anode Material with High Reversible Capacity and Superior Rate-Performance for Sodium-Ion Batteries Yuanjun Zhang, Guanyao Wang, Liang Wang, Liang Tang, Ming Zhu, Chao Wu, Shi Xue Dou, and Minghong Wu Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b00342 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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Graphene-Encapsulated CuP2: A Promising Anode Material with High Reversible Capacity and Superior Rate-Performance for Sodium-Ion Batteries Yuanjun Zhang,†,# Guanyao Wang,†,# Liang Wang,† Liang Tang,‡ Ming Zhu,§ Chao Wu,*,†,§ Shi-Xue Dou,§ and Minghong Wu*,‡ †School

of Environmental and Chemical Engineering and ‡Shanghai Applied Radiation Institute, Shanghai University, Shanghai 200444, China §Institute

for Superconducting & Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong, NSW 2522, Australia. #Yuanjun

Zhang and Guanyao Wang contributed equally to this work.

ABSTRACT: Metal polyphosphides are regarded as the ideal anode candidates for sodium storage because of their high theoretical capacity, reasonable potential, and abundant resource alternative. However, most of them suffer from irreversibility problems, as reflected by their low reversible capacity, inferior Coulombic efficiency (CE), low rate capability, and poor cycling stability. In this work, we systematically compare the electrochemical behaviour of a variety of polyphosphides bulks, discovering that the CuP2 bulks have higher initial reversible capacity (416 mAh g-1 at 0.1 A g-1) and CE (74%) compared to the FeP2, CoP3, and NiP2 bulks, which is related to the unique crystal structure of CuP2. The CuP2 electrode is optimized by the rational design of encapsulating CuP2 nanoparticles into 3D graphene networks (CuP2@GNs), leading to excellent electrochemical performance. In the carbonate electrolyte, the CuP2@GNs electrode can deliver the reversible capacities of 804, 736, 685, 621, and 508 mAh g-1 at 0.1, 0.5, 1, 2 and 5 A g-1, respectively, along with a first CE of 66%. The reversible capacity can be up to 746 mAh g-1 at 0.1 A g-1 with a first CE of 83% in the ether electrolyte. These excellent performance demonstrates that CuP2@GNs could be a promising anode material for sodium-ion batteries. KEYWORDS: Polyphosphides, CuP2, Graphene, Anodes, and Sodium-ion Batteries.

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Over the past decade, sodium-ion batteries (SIBs) have attracted increasing attention owing to low cost and earth-abundant sodium source. Both of the merits render SIBs huge potential for application in large-scale energy storage devises as a replacement of lithium-ion batteries (LIBs).1-3 Just like LIBs, SIBs need appropriate electrode materials to realize excellent electrochemical performance. The state-of-the-art LIBs are employing graphite as anodes, which can deliver more than 300 mAh g-1 capacity and low redox potential. However, the sodium-storage capacity of graphite is less than 100 mAh g-1 at common current rate using carbonate-based electrolyte. Even though ether-based electrolyte with higher cost is used, the capacity of sodium storage is only up to about 150 mAh g-1, much less than that of lithium-storage.4 Therefore, the development of SIB anodes with high capacity and reasonable potential is highly desired. Beyond the intercalation mechanism of sodium storage with graphite as anode, conversion-based transitional metal binary compounds (MaXb) have high theoretical capacity, and provide abundant species for picking up anodes with reasonable redox potential.5 Much effort has been devoted to developing binary metal oxides6,7 and chalcogenides.8-10 Although they deliver high reversible capacity, the oxidation potential corresponding to most of capacity contribution is very high (>1.5 V vs Na+/Na), resulting in low voltage output in the full batteries and limiting their applications. Compared to oxides and chalcogenides, metal polyphosphides (MPn, n≥2) possess high theoretical capacity (more than 1000 mAh g-1) as well as suitable reduction potential (between 0.1 and 0.9 V), which make them ideal candidates as anodes for sodium storage. However, recently reported metal polyphosphides, such as FeP2,11 CuP2,12-14 and CoP315, deliver a reversible capacity (700 oC) to get the desired samples (FeP2, CoP3, NiP2, and CuP2). The transitional metals of Fe, Co, Ni and Cu are selected as representatives because these elements locate the same period and adjacent groups. Initially, we attempted to synthesize the CoP2 bulks for comparison. Unfortunately, CoP2 is not available because of its phase instability at high temperature. Interestingly, even if the molar ratio of P to Co is tuned to 2, the phase of CoP3 is detected rather than that of CoP2. Therefore, the pure CoP3 bulks were synthesized for comparison under the P/Co ratio of 3:1. Figure 1a-d shows the XRD patterns of polyphosphides (FeP2, CoP3, NiP2, and CuP2). For each type of polyphosphides, no extra diffraction peaks are observed, indicating their pure phases. The morphologies of polyphosphide bulks, investigated by scanning electron microscopy (SEM), reveals that they are all composed of aggregation of rod-like particles with an average diameter of more than 0.5 m (Figure 3


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1e-l). Although these polyphosphide bulks have similar morphology, they show distinctive electrochemical behaviours for sodium storage (Figure 1m-p). The CoP3 bulks deliver a discharge and charge capacity of 164 and 62 mAh g-1 at 0.1 A g-1, respectively, along with a CE of 38%. In contrast, the metal polyphosphides (M= Fe, Ni, and Cu) can deliver a discharge capacity of more than 400 mAh g-1. Nonetheless, the reversible capacities of FeP2 and NiP2 are only 202 mAh g-1 and 212 mAh g-1, along with the corresponding CE of 38% and 50%, respectively. Similar electrochemical behaviour is also observed for red phosphorus.18 It can deliver a discharge capacity of 2306 mAh g-1, but the reversible capacity is only 79.6 mAh g-1, with a CE of 3.45 % (Figure S1, Supporting Information). Remarkably, the CuP2 bulks can deliver a reversible capacity of up to 416 mAh g-1, along with a high CE of 74%. Moreover, the profiles of CuP2 bulks show well-defined and flat plateaus on cycling, indicting its better kinetics.

Figure 1. XRD patterns of the (a) FeP2, (b) CoP3, (c) NiP2, and (d) CuP2 bulks; SEM and magnified SEM images of (e, i) FeP2, (f, j) CoP3, (g, k) NiP2, and (h, l) CuP2 bulks, the black and white scale 4


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bars are 1 μm and 200 nm, respectively; The initial charging/discharging profiles of (m) FeP2, (n) CoP3, (o) NiP2, and (p) CuP2 bulks at 0.1 A g-1. The electrochemical performance testing of polyphosphide bulks is conducted at the same conditions. Aforementioned characterization has demonstrated that the polyphosphide bulks have similar morphology and size for crystals. Considering the above facts, the big difference of their electrochemical behaviours should be attributed to the crystal structures. Figure 2 shows the crystal structures of polyphosphides, revealing that CuP2 has a layer-like structure that the puckered ten-membered phosphorus rings give rise to the corrugated layers parallel with the bc-plane (inset of Figure 2d). Pairs of cooper atoms are homogenously distributed between the phosphorus layers. Notably, each pair of cooper atoms is just situated in the octahedral phosphorus holes along crystallographic a axis (Figure S2, Supporting Information). This layer-like structure allows for the Cu atomic layers between the adjacent phosphorus layers to form the percolation conductive pathways for facilitating the electron transfer after discharging. For the other polyphosphides, the metal atoms heterogeneously distributed in the crystals are surrounded by the P atoms, and thus easily form the isolated “islands” after discharging, leading to worse kinetics for the following charge process. Electrochemical impedance testing provides a hard evidence for the better kinetics of CuP2 stemming from its crystal structure (insets of Figure 2a-c). Figure 2 exhibits Nyquist plots for polyphosphides, which are collected at the beginning of first charging. All these plots show a depressed semicircle at high/medium frequency and an inclined line at low frequency, which can be ascribed to the charge-transfer resistance (Rct) and the sodium-diffusion process within the crystals, respectively. Apparently, the semicircle diameter of the CuP2 bulks is the smallest among them and much less than that of others, which demonstrates better transfer of electrons/ions, implying the forming of percolation conductive pathways for CuP2 after discharging.

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Figure 2. Electrochemical impedance spectra of the (a) FeP2, (b) CoP3, (c) NiP2, and (d) CuP2 bulks, measured after the first discharging to 0.005 V. Their corresponding crystal structures are given in the insets. Although the CuP2 bulk has better kinetics compared other three phosphides, its reversible capacity is still much less than theoretical capacity. Previous literatures have demonstrated that nanostructured CuP2 composites show superior electrochemical performance to the bulk samples.12-14 In order to boost the kinetics of CuP2, CuP2@GNs are elaborately designed and synthesized, as shown in Figure 3a. First, Cu2O nanocubes were synthesized by hydrothermal method. Because of surface positive charges, Cu2O nanocubes can form a homogenous colloid aqueous solution without sediments for a certain time, and assemble with graphene oxide with negative charges via the electrostatic interaction.19 Once the above colloid solutions were mixed, the coagulation effect happened immediately. Second, the coagulation consisting of graphene oxide and Cu2O nanocubes was collected and annealed in Ar/H2 at 500 oC to achieve the precursor of Cu nanocrystals encapsulated by graphene networks (denoted as Cu@GNs). Finally, Cu@GNs was reacted with red phosphorus at high temperature to produce the desired samples (CuP2@GNs). The whole synthesis process for CuP2@GNs is quite facile and scalable. 6


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Figure 3. (a) Schematic of synthesis process of CuP2@GNs; SEM images of (b) Cu2O nanocubes, (c) Cu@GNs, and (d) CuP2@GNs; (e) TEM image of CuP2@GNs and HRTEM image shown in the inset; (f) XRD patterns of Cu2O, Cu@GNs, and CuP2@GNs; (g) Raman spectra of graphene oxide (GO) and CuP2@GNs. The white and black scale bars are 200 nm and 2 μm, respectively. The morphologies and crystal structures of CuP2@GNs and its precursors are investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Figure 3b shows the typical image of Cu2O precursor. Large amount of cube-like independent nanoparticles are observed with an edge length of about 100 nm. After assembly with graphene oxide, Cu2O is wrapped by grapheme oxide networks. In the subsequent annealing process, graphene oxide and Cu2O nanocubes transform into graphene and Cu nanoparticles, which is confirmed by the XRD pattern of Cu@GNs. All the diffraction peaks are 7


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indexed into the phase of Cu, and the peak associated to graphene oxide around 2θ=10° disappears in the pattern. Figure 3c exhibits the SEM image of Cu@GNs, which reveals that all the Cu nanoparticles are uniformly embedded into graphene networks. After the phosphorization reaction, the Cu nanoparticles turn into CuP2 nanoparticles, reflected by the XRD pattern of CuP2@GNs (Figure 3f). Furthermore, the morphology of Cu@GNs is inherited to CuP2@GNs, as shown in Figure 3d. The CuP2 nanoparticles are almost encapsulated by graphene networks (Figure 3d,e). High resolution TEM (HRTEM) image of CuP2@GNs reveals the characteristic lattice fringes of CuP2 nanoparticles, indicating a high degree of crystallinity (inset of Figure 3e). For comparison, we had also synthesized the CuP2 nanoparticles without adding graphene oxides. As shown in Figure S3 (Supporting Information), the big CuP2 aggregates, composed of nanoparticles, are noticed after reaction. This indicates that graphene networks can prevent the nanoparticles from agglomerating. The CuP2@GNs samples are further characterized by Raman technique and nitrogen adsorption measurement. Raman spectrum of CuP2@GNs reveals two typical D and G bands around 1333 and 1576 cm-1, which can be attributed to sp3-type disordered carbon and sp2-type graphitized carbon, respectively. Compared to graphene oxide, the G band of CuP2@GNs shifts to low wavelength number (Figure 3g), which further confirms the reduction of graphene oxide (oxygen-containing groups are removed after annealing).

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Figure 4. (a) The initial cyclic voltammetry curves of CuP2@GNs at a scan rate of 0.1 mV s-1; (b) The initial discharging/charging profiles of CuP2@GNs at 0.1 A g-1; (c) Rate performance of CuP2@GNs and CuP2 nanoaggregates; (d) The comparison of rate capability for CuP2@GNs and previously reported polyphosphide electrodes;12-15 (e) The discharging/charging profiles of CuP2@GNs at various current density; (f) Cycling performance of CuP2@GNs. To study the electrochemical behaviour of CuP2@GNs, it serves as the working electrode coupled with the fresh sodium foil as the counter electrode in the coin cell, using 1 M NaClO4 in propylene carbonate containing 5% fluoroethylene carbonate as electrolyte. Figure 4a shows the 9


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cyclic voltammetry (CV) curves for the initial 5 cycles at a scan rate of 0.1 mV s-1. The first cycle exhibits that both of reduction peaks are located at 1 V and 0.1 V, respectively. However, the former peak disappears and a new reduction peak around 0.4 V occurs for the following cycles, implying a change of sodium storage mechanism after the first cycle (discussed below). All the CV curves show two oxidation peaks around 0.67 V and 0.88 V, and the intensities of peaks gradually increase, indicating the presence of active process. Corresponding to the CV curves, the profiles of CuP2@GNs show two discharge plateaus and two charge plateaus at the first cycle, giving rise to a polarization voltage of about 0.7 V (Figure 4b). The average polarization voltage decreases to 0.47 V after 2 cycles, to 0.46 V after 4 cycles, and to 0.45 V after 9 cycles. These values are much less than that of the CuP2 bulks and other conversion-based electrodes (oxides and sulphides),6,20-21 indicating excellent kinetics of CuP2@GNs. The plateau around 1 V in the first cycle should be attributed to the decomposition of electrolyte and the plateau at 0.1 V is ascribed to the sodiation reaction of CuP2, because only the latter plateau is found for CuP2@GNs using the ether-based electrolyte (1M NaFP6 in dimethoxy ethane/1,3-dioxolane by 1:1 volume ratio). Remarkably, the CuP2@GNs electrode delivers a first reversible capacity of 623 mAh g-1, much higher than that of the CuP2 bulks, along with a CE of 59%. This inferior initial CE compared to that of the CuP2/carbon hybrid by mechanical milling12 might be attributed to the reduced size of CuP2 nanoparticles that causes large irreversible capacity loss related to the formation of SEI layer during the first discharge process. The following increase of sodium insertion and extraction capacities indicates that the sodiation of CuP2 requires an initial activation process for the breakage of Cu-P interaction. The reversible capacity rises to 720 mAh g-1 after 2 cycles, to 740 mAh g-1 after 4 cycles, and to 768 mAh g-1 after 9 cycles. Notably, the above specific capacity is calculated based on the mass of both CuP2 and graphene networks. Generally, pure graphene has a very low reversible capacity for sodium storage. If the capacity of graphene is neglected, the capacity contribution from CuP2 is up to 1024 mAh g-1 (based on the mass ratio of CuP2 79.6%), which is close to the theoretical capacity of CuP2 (1281 mAh g-1).5 The initial irreversible capacity origins from two contributions: one is the decomposition of electrolyte decomposition, and the other is incomplete sodiation reaction of CuP2 caused by a big polarization. Interestingly, the CuP2@GNs electrode can deliver a reversible capacity as high as 738 mAh g-1 using the above ether-based electrolyte, with a CE of 83%, which is 10


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attributed to less decomposition of electrolyte and smaller discharging polarization voltage (Figure S4, Supporting Information). The CuP2@GNs electrode shows an excellent rate performance as well, given in Figure 4c. It can deliver the reversible capacities of 751, 707, 666, 621, 595, 520, and 377 mAh g-1 at the current densities of 0.1, 0.3, 0.5, 0.8, 1, 2, and 5 A g-1 respectively. For comparison, the CuP2 aggregates were synthesized without graphene by the same process as CuP2@GNs. The reversible capacities of CuP2 aggregates are 497, 51, and 41 mAh g-1 at current densities of 0.1, 0.3, and 0.5A g-1, respectively. After high rate cycling, the capacity of CuP2@GNs can go back to 766 mAh g-1 and keep stable while the capacity of CuP2 aggregates only reaches 78 mAh g-1. Such superior rate performance is better than that of the previously reported CuP2/carbon composites at room temperature,12-15,22 as shown in Figure 4d. Figure 4e displays the discharging/charging profiles at various current densities. The discharging potential of CuP2@GNs for the majority of capacity contribution ranges from 0.5 V to 0.1 V and the corresponding charging potential is within the scope of 0.5-0.9 V, avoiding the precipitation of sodium metal at high rate and enabling a high voltage output when coupled with a common cathode material. The average polarization voltages are about 0.45 and 0.46 V at 0.1 and 0.3 A g-1, respectively. At high current rates of 2 and 5 A g-1, the polarization voltages are only 0.57 and 0.69 V, respectively. Cycling stability is an important parameter for practical applications. Figure 4f shows the cycling performance of CuP2@GNs. Before cycling at 0.5 A g-1, the CuP2@GNs electrode is activated for 5 cycles at 0.1 A g-1, delivering a reversible capacity of 623 mAh g-1 at 0.5 A g-1 without capacity decay after 50 cycles. Such excellent electrochemical performance including reversible capacity, rate capability, and cycling stability can be attributed to the rational electrode design. Graphene network not only facilitates the electron transfer to greatly promote kinetics, but also stops the agglomeration of CuP2 nanoparticles to keep the electrode structure integration.

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Figure 5. (a) Electrochemical impedance spectra of symmetric electrodes for sodium foils and Sn-protected sodium foils; (b) The first discharging/charging profile of CuP2@GNs coupled with Sn-protected Na at 0.1 A g-1; (c) The comparison of the second discharging/charging profiles of CuP2 bulks, CuP2 nanoaggregates, CuP2@GNs, and CuP2@GNs coupled with Sn-protected Na at 0.1 A g-1; (d) Rate performance of CuP2@GNs coupled with Sn-protected Na. The counter electrode composed of pristine sodium foil has a strong chemical reactivity with carbonate electrolytes, which can give rise to a thick solid electrode interface (SEI) via the side reaction, leading to a high interfacial resistance. Figure 5a displays the impedance spectrum measured at room temperature for the pristine symmetric sodium cell, revealing that the interfacial resistance reaches 400 Ω. In order to reduce the interfacial resistance, the pristine sodium foil was treated with SnCl2 solution to form a Na-Sn alloy film on its surface. The impedance spectrum shows that the interfacial resistance of tin-protected sodium (Na-Sn) foil is less than 100 Ω after the treatment, much less than that of the pristine sodium foil. The pristine sodium foil was replaced by the tin-protected sodium foil to reduce the first discharging polarization of CuP2@GNs. As shown in Figure 5b, the discharging plateau, corresponding to the sodiation reaction of CuP2, rises to 0.15 V. The CuP2@GNs electrode delivers a reversible capacity of 699 mAh g-1 with a CE of 66%, coupled 12


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with the Na-Sn foil. Besides, it is found that the activation process is also shortened. After 2 cycles, the reversible capacity approaches the steady state. The CuP2@GNs electrode coupled with the Na-Sn foil shows the lowest discharging/charging polarization and highest reversible capacity, as shown in Figure 5c. The average polarization voltage is only 0.42 V, less than that of the reported conversion-based electrode materials including phosphides, oxides and sulphides.23-25 Figure 5d demonstrates the rate performance of CuP2@GNs using the Na-Sn foil as the counter electrode. The reversible capacities are up to 804, 768, 736, 685, 621, and 508 mAh g-1 at the current densities of 0.1, 0.3, 0.5, 1, 2, and 5 A g-1, respectively. These results indicate the excellent electrochemical performance of CuP2@GNs, which hold huge potential as anodes to couple with appropriate cathode to achieve high-performance SIBs.

Figure 6. (a) The first discharging/charging profile of CuP2@GNs and (b) the corresponding in-situ XRD patterns; (c) The second discharging/charging profile of CuP2@GNs and (d) the corresponding in-situ XRD patterns. Currently, the sodium-storage mechanism of CuP2 is still not clear. In order to elucidate the storage mechanism of CuP2, in-situ XRD technique is carried out to detect the phase change of CuP2 on cycling. As shown in Figure 6, the strongest diffraction peak located at 2θ=31° gradually become weak with the ongoing discharging. Simultaneously, no new diffraction peaks including those of Na3P appear, possibly because the product of Na3P is amorphous. Notably, the peak at 2θ=31° doesn’t become strong but keeps stable during the first charging. This peak continues to become 13


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weak during the second discharging and keep stable during the following charging. These results indicate the irreversibility of sodiation reaction of CuP2. Second, the first discharging/charging profiles of CuP2@GNs electrode are different from those of the following cycles, which are very similar. Third, the profiles after the first cycle is similar to that of previously reported black phosphorus.26,27 Building on the above facts, we propose the following mechanism for CuP2 storage: CuP2 + 6Na → Cu + 2Na3P at the first discharging

(1)

Na3P → P + 3Na at the first charging

(2)

P + 3Na ↔ Na3P at the following cycles

(3)

Namely, the metallic Cu and Na3P are the initial discharge products. During the following cycles, the active materials transform from CuP2 into amorphous P, and the generated metallic Cu might keep its current form and provide conducting pathways for fast electron transport across the electrodes. In summary, a variety of metal polyphosphides, including FeP2, CoP3, NiP2, and CuP2 bulks, were synthesized and their electrochemical behaviours were investigated, showing that the CuP2 bulks can deliver a reversible capacity of 416 mAh g-1 and CE of 74%, which are much higher than those of other three polyphosphides. This result can be attributed to the unique layer-like crystal structure of CuP2, which is beneficial for the formation of percolation conductive pathways to facilitate electron transfer. To further modify the kinetics of CuP2, we design and fabricate CuP2@GNs, which shows excellent electrochemical performance in terms of high reversible capacity (up to 804 mAh g-1 at 0.1 A g-1), fast rate capability (reaching 508 mAh g-1 at 5 A g-1), and superior cycling stability (without capacity decay after 50 cycles). The first CE of CuP2@GNs is 59% using carbonate electrolyte, and the CE is increased to 83% using ether-based electrolyte. If the interfacial resistance between sodium metal and carbonate electrolyte is reduced by replacing pristine sodium foil with tin-protected sodium foil, the CuP2@GNs electrode delivers a first reversible capacity as high as 699 mAh g-1, along with a CE of 66%, demonstrating its excellent kinetics. The in-situ XRD studies reveal the storage mechanism of CuP2. After first cycle, the active component transforms from CuP2 into amorphous P. The fundamental understanding and the above

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demonstrations of excellent electrochemical performance open a viable route to development of polyphosphides as anodes for advanced SIBs. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional details on the experimental methods. Additional discharging/charging figures; crystal structure illustration; SEM images. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] ORCID Guanyao Wang: 0000-0002-7151-7782 Liang Wang: 0000-0002-3771-4627 Chao Wu: 0000-0002-2825-6337 Minghong Wu: 0000-0002-9776-671X Author Contributions #Y.

Zhang and G. Wang contributed equally to this work.

Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT This work was ﬁnancially supported by the National Natural Science Foundation of China (Nos. 21805180, 21671131, 11875185, and 41430644), Program for Changjiang Scholars and Innovative Research Team in University (No. IRT_17R71), Australia Research Council (DP160102627 and DE170101426), and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. We are grateful to the Analysis and Testing Center of Shanghai University for TEM, XRD, and Raman testing. REFERENCES (1) Larcher, D.; Tarascon, J. M. Nat. Chem. 2014, 7, 19-29. 15


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Graphene-Encapsulated CuP2: A Promising Anode Material with High

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