Cross-Linking Hollow Carbon Sheet Encapsulated CuP2

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Cross-Linking Hollow Carbon Sheet Encapsulated CuP2 Nanocomposites for High Energy Density Sodium Ion Batteries Shuangqiang Chen, Feixiang Wu, Laifa Shen, Yuanye Huang, Shyam Kanta Sinha, Vesna Srot, Peter A. van Aken, Joachim Maier, and Yan Yu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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Cross-Linking Hollow Carbon Sheet Encapsulated CuP2 Nanocomposites for High Energy Density Sodium Ion Batteries Shuangqiang Chen,† Feixiang Wu,† Laifa Shen,† Yuanye Huang,† Shyam Kanta Sinha,† Vesna Srot,† Peter A. van Aken,† Joachim Maier,† and Yan Yu*,†, ‡,§ ‡

Department of Materials Science and Engineering, University of Science and Technology of China, CAS Key Laboratory of Materials for Energy Conversion, Hefei, 230026, Anhui, P. R. China. † Max Planck Institute for Solid State Research, Heisenbergstrasse 1, 70569, Stuttgart, Germany. § State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui, 230026, China.

ABSTRACT: Sodium ion batteries (SIB) are regarded as the most promising competitors to lithium ion batteries in spite of expected electrochemical disadvantages. Here a “cross-linking” strategy is proposed to mitigate the typical SIB problems. We present a SIB full battery that exhibits a working potential of 3.3 V and an energy density of 180 Wh kg-1 with good cycle life. The anode is composed of cross-linking hollow carbon sheet encapsulated CuP2 nanoparticles (CHCS-CuP2) and a cathode of carbon coated Na3V2(PO4)2F3 (C-NVPF). For the preparation of the CHCS-CuP2 nanocomposites, we develop an in-situ phosphorization approach, which is superior to mechanical mixing. Such CHCS-CuP2 nanocomposites deliveres a high reversible capacity of 451 mAh g-1 at 80 mA g-1, showing an excellent capacity retention ratio of 91 % in 200 cycles together with good rate capability and stable cycling performance. Post-mortem analysis reveals that the cross-linking hollow carbon sheet structure as well as the initially formed SEI layers are well preserved. Moreover, the inner electrochemical resistances do not significantly change. We believe that the presented battery system provides a significant progress regarding practical application of SIB. KEYWORDS: sodium-ion battery, cross-linking strategy, porous carbon shell, CuP2 composites, full cell It is a matter of fact that lithium ion batteries (LIB), first commercialized in 1991 by Sony company, have entered our daily life in various aspects and will continue to do so in terms of devices, smart homes, robots and electric vehicles. Current lithium-ion batteries are facing many challenges such as high cost, toxicity, safety issues, poor low-temperature performance and low abundance that are ACS Paragon Plus Environment

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particularly critical as far as grid energy storage is concerned.1-8 Here, sodium ion batteries (SIB) exhibit great potential in view of the aforementioned issues, and the SIB field may also benefit from the relatively mature LIB technology.9-19 Unfortunately, many well-developed LIB materials, such as graphite and Si, do not intercalate sodium ions to an appreciable extent and novel anode materials should be developed for sodium storage. The recently developed SIB anode materials can be classified as follows: (i) Carbon-based materials (hard carbon, amorphous carbon);20-25 (ii) Metal and metal alloys (Na, Sn, Ge, Sb, Sn4P3 and Sn-Ge, etc.);26-30 (iii) Non-metallic, non-carbon coating materials (TiO2, SnO2, MoS2, Co3O4, CoS, SnS2, WS2, Fe3P, Cu3P, CuP2, Ni3P, P and S).15, 31-42

Typical anode materials

are shown in Figure 1a at the bottom of the voltage vs. capacity diagram. Among them, CuP2 is regarded as the most promising one due to its high theoretical capacity (1282 mAh g-1), along with low cost, low charge voltage and environmental benignity. Typical cathode materials are shown at the top of the

voltage

vs.

capacity

diagram.

Na3V2(PO4)3,

Na3V2(PO4)2F3,

NaMnO2

and

Na0.95Li0.15(Ni0.15Mn0.55Co0.1)O2 are outstanding among the various reported cathode materials in terms of the voltage vs. capacity map.13, 43-48 However, when cycling performance is considered, only the Na3V2(PO4)3 (NVP) material and the Na3V2(PO4)2F3 (NVPF) material remain as appropriate candidates. Compared to NVP (a high theoretical capacity of 118 mAh g-1 with a discharge platform of ~3.4 V), NVPF has obvious advantages not only in terms of high theoretical capacity (128 mAh g-1) and high discharge voltage (two discharge voltage plateaus at 4.25 V and 3.6 V),47, 49-51, but also by providing a higher energy density and a high thermal safety because of superior chemical and structural stability. Therefore, we chose NVPF as a cathode for CuP2 to demonstrate its practical advantages in a full SIB. In the literature, CuP2 is described to be normally prepared by either ball-milling or from organophosphorus compounds,35, 36, 52-54 which results in random morphologies and particle sizes. Such typical large CuP2 particles are easily cracked when reacting with sodium ions, leading to a large volume expansion and a loss of effective contacts with current collectors. Simultaneously, the solid electrolyte interface (SEI), formed on the surface layer of bare CuP2 electrodes during first discharge

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process, shows cracks in the absence of additional protection during cycling, and it will be partly regenerated in the subsequent sodiation processes. To address such problems, coating with conductive materials or doping hetero-atom is employed and effectively enhances the electronic conductivity and controls the growth of the SEI layer. Inner voids are helpful in mitigating the volume changes of CuP2, and particle connections facilitate electron conductivity, improving the mechanical properties and stabilizing the material’s structure. The electrolyte is of significant importance for transferring ions, infiltrating electrodes and forming SEI layers.55-57 We choose sodium perchlorate (NaClO4) dissolved in the solvent of ethylene carbonate (EC) and propylene carbonate (PC) by a 1:1 vol. percentage together with the additive of fluoroethylene carbonate (FEC; 5 vol.%) because of its high ionic conductivity, good thermal stability and low cost according to a previous comprehensive electrolyte report.58 The fluoroethylene carbonate additive helps forming stable solid electrolyte interfaces (SEI) on both the cathode and the anode, but slightly slows the electrode kinetics.59 In this communication, we describe the preparation of cross-linking hollow carbon sheet encapsulated CuP2

(CHCS-CuP2)

nanocomposites

via

surfactant-assisted

wet-chemical

precipitation,

surface-polymerization and in-situ phosphorization approaches. The as-prepared CHCS-CuP2 nanocomposites have several advantages. The preserved inner voids mitigate large volume changes during cycles, carbon shells encapsulate individual CuP2 nanoparticles, and their connectivity is helpful for the overall transport, the structural stability and the inner stress resistance. When it is applied as anode materials of SIB, the CHCS-CuP2 nanocomposite delivered a high rechargeable capacity of 451 mAh g-1 with a current density of 80 mA g-1 (0.1 C), and maintained at 410 mAh g-1 after 200 cycles, showing a high retention ratio of 91%. It also exhibited excellent rate capabilities and good cycle stabilities at high rates. Moreover, a full cell consisting of CHCS-CuP2//C-Na3V2(PO4)2F3 nanocomposite provided a capacity of 111 mAh g-1 at a current density of 0.1 C. It showed a high


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discharge voltage of 3.3 V and an energy density value of 180 Wh kg-1 with a high capacity retention of 94.2% during 100 cycles. RESULTS AND DISCUSSION The preparation of CHCS-CuP2 nanocomposites is briefly described in Figure 1b. Cross-linking and flocculent Cu3(PO4)2‧3H2O nanosheets were first precipitated in an ethanol aqueous solution assisted by hexadecyl trimethyl ammonium bromide surfactant with its beneficial hydrophilic/hydrophobic properties, as shown in scanning electron microscopy (SEM) images in Figure S1a-d and transmission electron microscopy (TEM) images in Figure S2a-d.60,

61

Dopamine-hydrochloride was stepwise

polymerized on the surface of the cross-linking sheets (Without it Cu3P and CuP2 bulk will form, in Figure S3a-b and S4a-d) to enhance the mechanical properties of the precursor. It was then carbonized at 600 ℃ in H2/Ar and the cross-linking morphology was well maintained. Simultaneously, Cu3(PO4)2‧3H2O was reduced to Cu3P by hydrogen, while the corresponding volume largely shrunk during the microstructural transformation from nanosheets to nanospheres, leaving sufficient inner caves for volume expansion during sodiation. The as-obtained Cu3P was continuously in-situ-phosphorized by red P at 410 ℃ to finally yield CuP2, and the cross-linking hollow carbon shells were fully encapsulating the CuP2 nanoparticles. Cross-linking hollow carbon shells play an important role on size-control and structural stabilization. Without the carbon shell protection, CuP2 nanoparticles easily form aggregates of several hundred of micrometers, leading to a random morphology (cf. Figure S5a-d for details). The main chemical reactions involved in the preparation of CHCS-CuP2 are as follows: 3Cu(CH3COO)2‧3H2O + 2NH4H2PO4 → Cu3(PO4)2‧3H2O↓+ 4CH3COOH + 2CH3COONH4 ∆

2Cu3(PO4)2‧3H2O + 19H2→2Cu3P + 2PH3↑ + 22H2O↑ ∆

Cu3P + 5P →

3CuP2

(2)

(3)


(1)

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The phase changes according to the as-mentioned reactions are evidenced by X-ray diffraction (XRD) patterns in Figure 2a. Cu3(PO4)2‧3H2O was indexed to a standard JCPDS card 01-0054 and the hexagonal intermediate (Cu3P) was indexed to JCPDS: 71-2261. Both bulk CuP2 and the CHCS-CuP2 nanocomposite are confirmed having a monoclinic structure (JCPDS: 76-1190) and no impurities were detected, implying complete chemical conversion. The weight percentage of carbon in the CHCS-CuP2 nanocomposite was confirmed to be around 36% by combustion analysis. Furthermore, the cross-linking morphology of the CHCS-CuP2 nanocomposite was evidenced by low and high magnification SEM images in Figure 2b-c and Figure S6a-d, exhibiting half-transparent carbon shells with a cross-linked morphology and evenly distributed CuP2 nanoparticles with an average diameter of 80 nm. Those CuP2 nanoparticles are fully encapsulated by carbon shells. The large internal space is a consequence of the huge volume shrinkage and the vapor escape during the thermal treatment and reduction, as shown in the bright-field (BF)-TEM image in Figure 2d and Figure S7a-d. The monoclinic structure is also verified by the high-resolution TEM (HRTEM) image and the corresponding Fourier transform given in Figure 2e, showing the (110) planes of CuP2. The thin carbon layer on the surface of the CuP2 nanoparticles is only around 2 nm. Details of the hollow carbon shell are shown in Figure 2f, indicating a hollow interior part (see white dashed line. The specific surface area of CHCS-CuP2 is around 85 m2 g-1 in Figure S8a-b by nitrogen adsorption/desorption analysis). The outer part exhibits a larger inter-layer distance (0.39 nm) than natural graphite, which buffers volume changes during cycling. Elemental maps determined by scanning TEM (STEM)-energy-dispersive X-ray spectroscopy (EDX) of the CHCS-CuP2 nanocomposite (Figure 2g) further verify the full encapsulation of CuP2 nanoparticles by cross-linking carbon shells. The corresponding EDX spectrum shown in Figure 2h confirms the purity of the as-prepared CHCS-CuP2 nanocomposite. The electrochemical performances of the CHCS-CuP2 nanocomposite and bulk CuP2 are evaluated in terms of half-cells paired with Na metal in 2032 type coin cells, using glass fiber as a separator and a 1M solution of NaClO4 in EC/PC (vol. ratio of 1:1) with 5 vol.% FEC as the electrolyte. Cyclic


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voltammetry (CV) curves in Figure 3a display the potential positons of redox reactions. Specifically, the small peak between 1 V and 0.5 V at the first cathodic sweep is indicative of the formation of solid electrolyte interphase (SEI) and irreversible electrolyte decomposition. This peak is smaller than reported in literature,36,

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implying less consumption of Na+ ions and thinner SEI layers on the

cross-linking carbon shells. The increasing reduction current value from 0.5 V (Figure 3a) implies pronounced sodiation of CuP2 in the low potential range, demonstrating potential advantage as anode for SIB.28 In the initial anodic sweep, there are two apparent peaks around 0.62 V and 0.86 V that are related to multi-step de-sodiation processes. The other sweeps are almost overlapped, indicating good reversible capability and high stability of CHCS-CuP2 nanocomposite. The cycling performances of the CHCS-CuP2 nanocomposite and the bulk CuP2 electrode (Figure 3b) delivered a high rechargeable capacity of 451 mAh g-1 at 0.1 C and a value as high as at 410 mAh g-1, even at the 200th cycle with a capacity retention ratio of 90.9% and high Coulombic efficiency. On the contrary, the bare CuP2 electrode only showed a rechargeable capacity of 189 mAh g-1, which quickly decreased to 51 mAh g-1 within 200 cycles. This is likely related to the large particle size (associated with long transferring paths of Na+ ions and poor electrical contacts after electrode pulverization) and low electronic conductivity of the bulk materials. Typical profiles of discharge/charge behaviors of the CHCS-CuP2 nanocomposite at the 1st, 10th and 200th cycles are displayed in Figure 3c, exhibiting a noticeable discharge plateau at ~0.55 V corresponding to the sodiation reaction of CuP2 and a short oblique line, relating to sodium storage of the carbon layer. However, the electrochemical profiles of bulk CuP2 are slightly different from the CHCS-CuP2 nanocomposite in terms of the potential of SEI formation, Coulombic efficiency and over-potential (as shown in Figure S9). Moreover, the CHCS-CuP2 nanocomposite electrodes also delivered a superior rate capability as shown in Figure 3d with rechargeable capacities of 451, 345, 270, 211, 167 and 137 mAh g-1 at current rates of 0.1 C, 0.2 C, 0.4 C, 0.8 C, 1.6 C and 3.2 C, respectively. The corresponding charge/discharge profiles are shown in Figure 3e, which is in agreement with a two-step sodiation. The conversion reaction is characterized by


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a discharge plateau around 0.5 V (see below). The CHCS-CuP2 nanocomposite electrodes exhibit good capacity retention ratios at different current rates in Figure 3f, and the corresponding Coulombic efficiencies are beyond 98% for 200 cycles. The superior electrochemical performances of the CHCS-CuP2 nanocomposite are ascribed to the cross-linking carbon structure, buffering volume changes during (de)sodiation processes, and the nm-size of the particles in shortening sodium transfer paths and enhancing rate capability. Without cross-linking hollow carbon shells, bulk CuP2 delivered low rechargeable capacities and fast capacity fading. This indicates that such cross-linking hollow carbon shells play a decisive role in controlling the large volume changes during (de)sodiation and in maintaining good electronic contacts among nanoparticles and providing the superior cycling stability. To further understand the reasons for the excellent electrochemical performance, electrochemical impedance spectra of the CHCS-CuP2 nanocomposite and bulk CuP2 were measured, yielding information on the internal electrochemical resistances and interfacial properties (measured for both fresh and cycled cells). The representative Nyquist plot depicted in Figure 4a exhibits a depressed semi-circle at high frequencies and a straight line in the low frequency region. As a fresh cell, the radius of CHCS-CuP2 nanocomposite is much smaller than CuP2, indicating a much lower electrochemical resistance of CHCS-CuP2 nanocomposite with the cross-linked carbon shells. When cycled for 200 cycles, the impedance of CHCS-CuP2 nanocomposite is only slightly increased because of the good electronic conductivity and mechanical stability of electrode with the cross-linked structure and the stable SEI layer. However, the radius of bare CuP2 electrode from fresh cell to cycled cell is markedly increased because of the increase of inner resistance, relating to the gradual growth of SEI and particle pulverization. To examine the sodiation/desodiation reaction mechanism of CHCS-CuP2 nanocomposite, ex-situ XRD analysis was applied to track phase changes during discharge/charge processes as shown in Figure 4b. There is no indication of a phase change from 2.5 V (①) to 0.75 V (②) in this first stage except a weak phase peak corresponding to the formation of SEI. Then, after an apparent discharge plateau, formation ACS Paragon Plus Environment

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of Na3P was detected at 0.25 V (③). The fact that XRD peaks of CuP2 became smaller but did not vanish can be ascribed to the presence of unreacted CuP2 and a wide sodiation reaction window. No intermediate phase formation of Cu3P was detected. When charged, the newly formed Na3P phase almost fully disappeared at 1.5 V (④). In agreement with this, the redox processes can be formulated as: CuP2 + 6Na ⇌ 2Na3P + Cu

(4)

The typical cross-linking hierarchical structure of the CHCS-CuP2 nanocomposite was well maintained after 200 cycles (Figure 4c) and no electrode pulverization or cracks are observed. The detailed inner structure is shown in Figure 4d, displaying that many CuP2 nanoparticles are encapsulated by carbon shells. The fact, that no cracks are observed, points toward high mechanical stability due to a sufficient amount of inner voids of cross-linking carbon shells. The STEM-EDX elemental maps shown in Figure 4e-h give the elemental distributions of C, Cu, P and Na, where C overlaps with the domains of Cu and P. After full de-sodiation of the electrode, the presence of Na may come from both not fully decomposed sodium compound and the thin SEI layer formed on the electrode. That sodium compound is mainly referred to the not fully decomposed Na3P. And the SEI layer on the surface of electrode, confirmed by Raman spectra by Kaghazchi et al.,63-65 may also contain other sodium composites, including Na2O, Na2CO3 or NaF (FEC) etc., which are those irreversible sodium compounds. These structural phase changes are crucial for the superior electrochemical performances, the low electrochemical resistance as well as the impressive cycling stability. To demonstrate the applicability in SIBs, the CHCS-CuP2 nanocomposite was combined with a cathode of Na3V2(PO4)F3 (NVPF) nanoparticles in a standard electrolyte (1M NaClO4 in EC/PC solution with 5 vol.% FEC). NVPF (space group: P42/mnm, in Figure S10a) is regarded as a promising cathode material for SIBs, because of two high discharge plateaus at 4.25 V and 3.6 V (associating to the multi-stage redox reactions of V3+/V4+) and a theoretical capacity of 128 mAh g-1. Carbon coated NVPF (C-NVPF) nanoparticles, prepared by a two-step reaction method,72 exhibit homogenous particles (~100 nm, as shown in Figure S10a-b), coated by a carbon layer of 5 nm thickness (Figure S10c). Such C-NVPF ACS Paragon Plus Environment

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delivered a high rechargeable capacity of 127 mAh g-1 at 0.2 C and superior rate capabilities at 0.5 C, 1 C and 2 C (Figure S10f-g), respectively. The stable cycling performances of the C-NVPF electrode at high current rates for 200 cycles (Figure S11) obviously contribute to the cycling stability of the C-NVPF//CHCS-CuP2 full cell. More detailed measurements of the C-NVPF electrode, including CV, structural analysis and elemental distribution, are shown in Figure S12 and Figure S13a-h. The good electrochemical performances of the C-NVPF electrode are ascribed to the fast Na+ ion transport and charge transfer as well as to the pronounced structural stability. The mass ratio of cathode to anode material is set to 3.5 in accordance with the capacity differences (Figure 5a). The average discharge potential of the full cell is around 3.3 V based on the voltage difference between the cathode and anode, and slightly lower than that of commercial LIBs (3.7 V). The full cell was first charged to 4.5 V to activate it, whereby it delivered a rechargeable capacity of 111 mAh g-1 at 0.1 C (based on the mass of two electrodes) with an initial Coulombic efficiency of 87% (Figure 5b). The irreversible capacity is associated with the SEI formation on the cathode side in the initial cycle as well as the capacity fading of both cathode and anode. The overall cell reaction is described by: 3C-Na3V2(PO4)F3 + CHCS-CuP2 ⇌ 3C-NaV2(PO4)F3 + CHCS-Cu + 2Na3P

(5)

Moreover, the full cell displayed good rate capability and high Coulombic efficiency (Figure 5c), delivering rechargeable capacities of 93 mAh g-1, 78 mAh g-1 and 70 mAh g-1 at 0.2 C, 0.5 C and 1 C, respectively. The full cell features an average working potential of 3.3 V resulting in an effective energy density of 180 Wh kg-1 (based on the mass of two electrodes). Such performance is comparable with typical commercialized LIBs used in electric vehicles. Regarding cycling stability, one recognizes a stable cycling performance at 0.1 C and 1 C rates connected with capacity retention ratios of 81.3% and 94.2% within 100 cycles (Figure 5d). The electrochemical reactions of CHCS-CuP2 electrode with C-NVPF electrode are experienced in two-steps at 0.1C in Fig. 5b. As the current density is increased to 1C, the reversible electrochemical reaction is gradually reduced to one step, which is might associated ACS Paragon Plus Environment

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to the joint contribution from the slow conversion-reaction kinetics from CHCS-CuP2 electrode (only surface CuP2 Participates conversion reaction) and one Na extracted/inserted in C-NVPF electrode with the V3+/V4+ valence change. Therefore, partial CuP2 materials and C-NVPF participate the reversible reactions, which jointly contribute the good cycle stability of the full cell at 1C with the sacrifice of partial cell capacity. In Figure 5e, the electrochemical performance of the full cell is compared to examples described in the literature using NVPF as the cathode (detailed information is listed in the Supporting Information Table S1), revealing a larger energy density and a higher working voltage of our phosphide electrode. As the process of SEI formation will irreversibly consume sodium cations, extra sodium supply is necessary. This may be achieved by a higher Na content in the NVPF or by admixing a sodium-rich second phase.59, 68 Similarly, a pre-sodiated electrode as anode is also beneficial for cycling performance of the full cell. Electrochemical impedance spectra were measured to reveal inner electrochemical resistance changes among the fresh state, the 50th and 100th cycle (Figure 5f). The depressed high frequency semi-circle of the fresh cell only slightly increases in the following cycles due to its good electrical and interfacial properties. The overlapped impedance plots of cycled electrodes are associated to the good mechanical stabilities and highly reversible electrochemical activities of electrodes. The stable SEI layers are also partially contributed to this phenomenon. The cycled electrodes of C-NVPF and CHCS-CuP2 are all examined by ex-situ SEM images (Figure S14 and S15), exhibiting thin SEI layers and superior structural stabilities. Overall, the proposed strategy, viz. using cross-linking hollow carbon shells and a void-preserving strategy, has effectively addressed the challenges of poor cycling performance and low rate capability in terms of practical applications. This procedure is expected to provide reliable solutions also for other rechargeable battery materials. CONCLUSION In conclusion, we proposed a “cross-linking” strategy for obtaining materials with high electrical conductivity and marked structural stability. The “cross-linking” strategy is characterized by ACS Paragon Plus Environment

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interconnected hollow carbon shells which can effectively encapsulate CuP2 nanoparticles, transport electrons and absorb large volume changes during de/sodiation processes. The as-prepared CHCS-CuP2 nanocomposites as the SIB anode delivered a high reversible specific capacity of 451 mAh g-1 at 0.1 C, and maintained a capacity retention ratio of 91 % within 200 cycles. It also exhibited good rate capability and cycling performances at high rates. This can be ascribed to the preserved inner voids in the cross-linked carbon matrix and the connected carbon shells, resulting in a better structural stability control and an increased stress resistance. The redox reactions for the CuP2 electrode were examined by ex-situ XRD technique and it was verified that Na3P and Cu were the main discharge products. A full cell composed of the C-NVPF//CHCS-CuP2 nanocomposite displayed a high rechargeable capacity of 111 mAh g-1 at 0.1 C, delivered an energy density value of 180 Wh kg-1 with a high voltage of 3.3 V and good cycling stability. The “cross-linking” strategy that proved to be helpful for the SIB may be advantageous for advancing electrode materials in other battery systems as well.

EXPERIMENTAL METHODS Preparation of cross-linked hollow carbon sheet coated CuP2 (CHCS-CuP2) nanocomposites. All chemicals were used as received without further processing or purification. In a typical synthesis procedure, Cu(CH3COO)2·3H2O (0.0225 mol/L) was dissolved in a mixture of 80 mL deionized H2O and 20 mL ethanol under strong stirring for 30 min. Ammonium dihydrogen phosphate (0.015 mol/L) and hexadecyl trimethyl ammonium bromide (CTAB; 0.3 g) were dissolved in 80 mL H2O and 20 mL ethanol under strong stirring for 30 min, and then drop-wise added in a cobalt acetate solution, showing a light blue suspension. The mixture was stirred for 12 h at room temperature. Dopamine hydrochloride (2 mmol) was added in the mixture and stirred for 36 h, showing a black suspension. After that, the suspension was vacuum filtered and washed with copious de-ionized water. The resulting fine black powder was collected after overnight drying, and was annealed at 650 ℃ for 2h and reduced by H2/Ar for 4h in a tube furnace. The obtained CHCS-Cu3P composite was mixed and ground with red ACS Paragon Plus Environment

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phosphorus with a weight ratio of 1:5. Then, the CHCS-CuP2 composite was obtained by heat treatment at 410 ℃ for 8h in a tube furnace under Argon atmosphere. The bare CuP2 composite was obtained using the same procedure, but without the addition of dopamine hydrochloride. Electrochemical measurement The CHCS-CuP2 working electrodes were made from 80% of active materials, 10% of carbon black, and 10% of the binder (Polyvinylidene fluoride (PVDF) dissolved in N-methyl-2-pyrrolidone (NMP)). The slurry was pasted on copper current collectors using a medical blade. The work electrodes were dried in a vacuum oven at 80 ℃ overnight. The mass loading of anode materials including bare CuP2, CHCS-CuP2 nanocomposites is around 1 mg on the copper current collector. CR2032 coin cells were assembled in an argon-filled glove box (Mbraun, Unilab, Germany), in which both the moisture and oxygen contents were controlled to be less than 0.1 ppm. Small pieces of a Sodium cube were applied as the counter and reference electrodes. A glass microfiber (Whatman) was used as a separator. The electrolyte solution was composed of 1 M NaClO4 dissolved in the mixture of ethylene carbonate (EC) and propylene carbonate (PC) with a volume ratio of 1:1, and 5 vol.% fluoroethylene carbonate (FEC) as an electrode additive. The cells were galvanostatically discharged and charged in a voltage range of 0.01‒2.5 V at a current density of 80 mA·g-1 (0.1 C). Higher current rates (0.2 C, 0.4 C, 0.8 C, 1.6 C and 3.2 C) were also applied to evaluate the rate capabilities. The CV was measured on a CHI 660E electrochemical workstation at a scan rate of 0.1 mV·s-1. The electrochemical impedance spectra (EIS) were examined using a voltage amplitude 5 mV and a frequency range from 106 to 0.1 Hz. Full sodium-ion batteries, using CHCS-CuP2 nanocomposites as anode and C-NVPF nanoparticles as cathode, were assembled. A similar strategy was applied to prepare the cathode electrode: C-NVPF nanoparticles synthesized according to the previous report was mixed with carbon black and PVDF in a weight ratio of 80:10:10. The mass loading of the cathode materials is around 3.5 mg on the aluminium current collector. The electrolyte and separator used were the aforementioned ones. The galvanostatic charge/discharge method was applied to measure the electrochemical performance between 2 V and 4.5 ACS Paragon Plus Environment

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V at current densities of 16 mA·g-1 (0.1 C), 32 mA·g-1 (0.2 C), 80 mA·g-1 (0.5 C) and 160 mA·g-1 (1 C), respectively. Structural and phase characterization The materials were characterized by X-ray diffraction (Rigaku D/max-2550 V with Cu Kα radiation) operated at 40 kV and 30 mA. The morphologies and crystal structure of the materials were analyzed by a field emission scanning electron microscope (Zeiss Merlin) and a high-resolution TEM (HRTEM) and energy-dispersive X-ray (EDX) spectroscopy experiments. HRTEM was performed at 200 and 80 kV with an advanced TEM (JEOL ARM200F, JEOL Co. Ltd.), equipped with a cold field-emission gun and a CETCOR image corrector (CEOS Co. Ltd.). To perform the ex-situ SEM, TEM and HRTEM measurements, the batteries were fully desodiated after 200 cycles and carefully disassembled in an argon-filled glove box. Then, the electrode was soaked in an EC and PC solvent for two days to dissolve the electrolyte. For ex-situ SEM, the cycled electrode was dried and directly pasted on conductive carbon tape. For ex-situ TEM and HRTEM measurements, after the ultrasonic treatment, the well-dispersed electrode materials were dropped on TEM grid in the glove box. After the evaporation of the solvent, the TEM grid was also completely sealed by three Ar-filled plastic bags and blown with high-purity Ar during loading in the TEM vacuum chamber. In this way the side reactions of the discharge product with oxygen and moisture in air can be effectively prevented.

Supporting information


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The morphologies of precursors and CuP2 bulk are presented in supporting information, and all other detailed information for comparison are all presented in supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Address correspondence to [email protected] Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was financially supported by National Key R&D Pro-gram of China (No. 2018YFB0905400, No. 2016YFB0100305), the National Natural Science Foundation of China (No. 51622210), the Fundamental Research Funds for the Central Universities (WK3430000004), the Sofja Kovalevskaja award and postdoctoral scholarship of the Alexander von Humboldt Foundation. The authors appreciate support from Dr. Helga Hoier (XRD measurement). REFERENCES AND NOTS 1.

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ACS Nano

Figure 1. (a) A prototype full cell of NaVPF//CuP2 with a voltage of 3.3 V based on the materials diagram, showing working voltage vs. capacity.11 (b) Schematic image of the preparation of the CHCS-CuP2 composite and reversible reactions with Na+ ions, showing cross-linking and full encapsulation of discharge products. (c) Schematic image of the preparation of bare CuP2 particles and particle pulverization after cycling.


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Figure 2. (a) X-ray diffraction patterns of precursors of CuP2 and CuP2-based materials. (b and c) SEM images of CHCS-CuP2 composites. (d) BF-TEM image of CHCS-CuP2 nanocomposites, showing the well-distributed CuP2 nanoparticles in cross-linking hollow carbon sheets, indicated by pink arrows. (e) High-resolution TEM (HRTEM) image of the CHCS-CuP2 composite and its corresponding Fourier transformation in the inset, showing a thin carbon layer (~2 nm) and the (110) planes of CuP2. (f) HRTEM image of a hollow carbon sheet with a wide inter-layer distance (~0.39 nm), showing an apparent inner chamber allowing for volume changes of CuP2 during cycling. (g) High-angle annular dark-field (HAADF)-STEM image with corresponding C, Cu and P elemental maps. (h) Corresponding EDX spectrum of a CHCS-CuP2 composite.


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Figure 3. Electrochemical performance of bare CuP2 and CHCS-CuP2 composites. (a) CV curves of CHCS-CuP2 composites in three cycles. (b) Cycling performance of bare CuP2 and CHCS-CuP2 composites at 0.1 C. (c) Electrochemical profiles of CHCS-CuP2 composites at the 1st, 10th, and 200th cycle. (d) Rate capability and Coulombic efficiency of CHCS-CuP2 composites at different current rates. (e) Charge/discharge profiles of CHCS-CuP2 composites at different current densities. (f) Cycling performance of CHCS-CuP2 composites at 0.2 C, 0.4 C, 0.8 C and 1.6 C.


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Figure 4. (a) Electrochemical impedance spectra of bare CuP2 and CHCS-CuP2 composites and the corresponding equivalent circuit. (b) Ex-situ XRD patterns at different charge/discharge states. (c) SEM image of CHCS-CuP2 composites after cycling. (d) HAADF-TEM image of CHCS-CuP2 composites after cycling. (e-h) Corresponding C, P, Cu and Na STEM-EDX elemental maps of CHCS-CuP2 composites after cycling.


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ACS Nano

Figure 5. (a) Schematic illustration and charge/discharge voltage profiles of the CHCS-CuP2 composite and C-NVPF nanoparticles at 0.1 C, respectively. (b) Electrochemical behaviors of the full cell (C-NVPF//CHCS-CuP2) at different current rates. (c) Rate capability and Coulombic efficiencies of the full cell. (d) Cycling performance of the full cell at current rates of 0.1 C and 1 C. (e) Electrochemical performance comparison of full cells with values reported in the literature (1, NVPF//SnS/G+C;66 2, NVPF//Na2Ti6O13;67 3, NVPF//C;68 4, NVPF//hard C;59 5, NVPF/C//Zn;69 6, NVPF//NTP;47 7, NVPF//HC;70 8, NVPF@C@rGO// NVPF@C@rGO;71). (f) The impedance spectra of the full cell at the fresh state, after 50th cycle and 100th cycle, respectively. Table of Content ACS Paragon Plus Environment

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Cross-Linking Hollow Carbon Sheet Encapsulated CuP2

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