Ketjenblack–Multiwalled Carbon

Publication Date (Web): May 25, 2016 ... Abstract Image ... Stable Sodium Storage of Red Phosphorus Anode Enabled by a Dual-Protection ..... preparati...
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Nanostructured Black Phosphorus/Ketjenblack−Multiwalled Carbon Nanotubes Composite as High Performance Anode Material for Sodium-Ion Batteries Gui-Liang Xu,† Zonghai Chen,*,† Gui-Ming Zhong,⊥ Yuzi Liu,‡ Yong Yang,⊥ Tianyuan Ma,†,¶ Yang Ren,§ Xiaobing Zuo,§ Xue-Hang Wu,⊥ Xiaoyi Zhang,§ and Khalil Amine*,† †

Chemical Sciences and Engineering Division, ‡Nanoscience and Technology Division, and §X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ⊥ Collaborative Innovation Center of Chemistry for Energy Materials, State Key Laboratory Physical Chemistry Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen, Fujian 361005, China ¶ Materials Science Program, University of Rochester, Rochester, New York 14627, United States S Supporting Information *

ABSTRACT: Sodium-ion batteries are promising alternatives to lithiumion batteries for large-scale applications. However, the low capacity and poor rate capability of existing anodes for sodium-ion batteries are bottlenecks for future developments. Here, we report a high performance nanostructured anode material for sodium-ion batteries that is fabricated by high energy ball milling to form black phosphorus/Ketjenblack− multiwalled carbon nanotubes (BPC) composite. With this strategy, the BPC composite with a high phosphorus content (70 wt %) could deliver a very high initial Coulombic efficiency (>90%) and high specific capacity with excellent cyclability at high rate of charge/discharge (∼1700 mAh g−1 after 100 cycles at 1.3 A g−1 based on the mass of P). In situ electrochemical impedance spectroscopy, synchrotron high energy X-ray diffraction, ex situ small/wide-angle X-ray scattering, high resolution transmission electronic microscopy, and nuclear magnetic resonance were further used to unravel its superior sodium storage performance. The scientific findings gained in this work are expected to serve as a guide for future design on high performance anode material for sodium-ion batteries. KEYWORDS: Sodium-ion batteries, anode material, nanostructured, black phosphorus, ball milling, sodiation/desodiation

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sodium.5,6 As a consequence, more efforts have been focused on the development of novel anode materials for SIBs including nongraphitized carbon, transition metal oxide/sulfide, and intermetallic metal.7 Among the anode materials for SIBs reported in the open literature, they generally have either a low capacity (e.g., hard carbon) or a relatively high working potential (e.g., metal oxides and metal sulfides).8 A high energydensity anode for SIBs requires the right combination of a high capacity and a low working potential. Phosphorus has recently been found to be a very promising anode material for SIBs owing to its high theoretical capacity of about 2600 mAh g−1 and reasonable discharge and charge potential (0.3 and 0.65 V vs Na/Na+, respectively).9 Solid P exists as three main allotropes: white, red, and black, which exhibit strikingly different properties. White P is chemically unstable and ignites spontaneously in air at about 50 °C, so it is not suitable as electrode material when battery safety is considered. Despite its low electronic conductivity and large

ithium-ion batteries (LIBs) have been widely applied in portable electrical devices since their commercialization in 1991.1 In recent times, sodium-ion batteries (SIBs) have increasingly been considered as an attractive alternative to LIBs owing to the greater abundance and lower cost of sodium compared to lithium.2 As lithium and sodium both belong to group I of the alkali metals, they have many similarities in their physical and chemical properties. Therefore, extensive efforts have been devoted to replacing lithium with sodium based on the already well-developed electrode materials of LIBs. Compared with corresponding LIBs cathodes, layered NaxMO2 (M = Co, Mn, Fe, Ni, etc.), olivine NaFePO4 and Na3V2(PO4)3, NaFeSO4F, and their analogues have been reported to present similar electrochemical behavior in terms of specific capacity, cycle life, and rate capability, except for a lower working potential.3 However, in the area of anodes, Kang et al. found that the commercially used graphite anode for LIBs can only store a small amount of sodium due to the insufficient interlayer distance and stretched C−C bonds induced by sodiation, which makes the Na−graphite intercalation compound thermodynamically unstable.4 Moreover, while silicon has been widely considered as a high-capacity anode (>1000 mAh g−1) for LIBs, it has little capability to alloy with © XXXX American Chemical Society

Received: May 2, 2016 Revised: May 19, 2016

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Figure 1. (a) Scheme for the structures of BPC composite; (b) HEXRD patterns and (c) Raman spectra of BPC composite and bulk BP.

composite, which exhibits an ultrastable cyclic performance and excellent rage performance (809 mAh g−1 at 1.5 A g−1).16 Very recently, Yu et al. reported an amorphous red phosphorus/ highly ordered mesoporous carbon composite, which could deliver a reversible capacity of 1020 mAh g−1 based on the mass of red P at 5 °C but with only 31.54 wt % of phosphorus loading in the composite.17 In addition to red P, orthorhombic black phosphorus (BP) has also drawn much attention as BP is thermodynamically the most stable allotrope of the three, insoluble in most solvents, and chemically the least reactive.18 Moreover, with its layered structure and good electrical conductivity, BP has a similar structure to graphite and can be mechanically exfoliated just like graphene from graphite.19 Therefore, BP has attracted lots of attention in many fields including field-effect transistors,20 photodetectors,21 and lithium-ion batteries.22−24 Ramireddy et al. prepared a phosphorus/graphite composite by using red phosphorus as a starting material and claimed that they could transform the red P to black P through ball milling.25 Although the resulting P/C composites could deliver a high initial reversible capacity of about 1300 mAh g−1 based on the whole composite, the reversible capacity rapidly decayed to about 200 mAh g−1 after only 30 cycles. Komaba et al. systematically investigated the effect of the electrolyte additives on the cycle performance of BP/carbon composite.26 However, the cycle life is limited to less than 25 cycles. Cui and co-workers recently reported a phosphorene/graphene hybrid with an optimized phosphorene loading of 48.3 wt % as SIB anode,27 which has high reversible capacity with good cycle stability and high rate capability. Although significant improvement on the specific capacity and cycle life of phosphorus-based anode for SIBs has been achieved by the pioneer works through fabrication of different phosphorus/carbon composites, a combination of high initial Coulombic efficiency, high phosphorus loading, and high capacity with good stability at high rate of charge/ discharge (>1 A g−1) has been less reported (see Table S1).

volume expansion during cycling, red P has been intensively investigated as an SIB anode material owing to its abundance, low cost, and high chemical stability. The key challenge related with phosphorus anode is the rapid structural degradation caused by the large volume change of up to around 500% during charge/discharge. For the electrode materials that experience large volume changes during charge/discharge such as Si anodes, phosphorus anodes, transition metal oxide/sulfide anodes, and sulfur cathodes, how to mitigate the negative impact of the volumetric changes on the electrochemical performance is the key to enable this class of alloying or conversion reaction electrode materials. Kim et al. reported an amorphous red P/carbon composite, which can deliver a stable capacity about 1890 mAh g−1 in 30 cycles at a current density of 0.143 A g−1.10 Li and co-workers found that by simply mixing commercial microsized red P with multiwalled carbon nanotubes (MWCNTs) through hand grinding, the red P/carbon composite could deliver a reversible capacity of 1675 mA h g−1 with capacity retention of 76.6% after 10 cycles.11 Song and co-workers reported a red P/graphene hybrid prepared by facile ball milling, which can maintain 1700 mAh g−1 for 60 cycles at 0.26 A g−1, benefiting from the formation of a P−O−C bond in the P/graphene hybrid.12 The performance was further improved by formation of chemical bonding between phosphorus, carbon nanotube, and crosslinked polymer binder, which could maintain 1586.2 mAh g−1 after 100 cycles at 0.52 A g−1.13 Wang et al. have developed a vaporization−condensation method to synthesize red P/ SWCNTs composite with a P content of 40 wt %, which can attain an overall capacity of 700 mAh g−1 based on the mass of P/SWCNTs at 0.05 A g−1.14 Sun et al. reported a carbothermic reduction synthesis of red phosphorus-filled 3D carbon material with 36 wt % of phosphorus loading, which could deliver an overall capacity of 1027 mAh g−1 at 0.21 A g−1 and high capacity retention of 88% after 160 cycles.15 Zhang et al. reported an amorphous phosphorus/N-doped graphene paper B

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A Hahn-echo pulse sequence with a recycle delay of 10 s and a 90° pulse length and pulse width of 1.6 μs was applied to acquire 31P NMR spectra. The chemical shift of 31P was calibrated using 85% H3PO4 aqueous solution (0 ppm). Electrochemical Characterization. The phosphorus electrodes were prepared by spreading a mixture of 70 wt % active material, 15 wt % super-P, and 15 wt % carboxymethyl cellulose sodium salt (2 wt %) onto a copper foil current collector. The as-prepared electrodes were then dried at 100 °C in a vacuum oven for 12 h. The loading density was controlled at about 1.5 mg cm−2. The electrochemical performance of the BPC composite was characterized by assembling it as an anode in coin cells (type CR2032) in an argon-filled glovebox under conditions where the contents of moisture and oxygen were both below 0.5 ppm. The electrode was separated from the sodium counter electrode by a separator (glass fiber, grade GF/ F Glass Microfiber Filter Binder Free, circle, 125 mm). The electrolyte used in the cell was 1 M NaPF6 in propylene carbonate (PC) with 2 vol % fluoroethylene carbonate (FEC) as additive. The cells were charged and discharged using a MACCOR cycler. Cyclic voltammograms of the BPC electrode were recorded on a Solartron Analytical 1470 System between 0.01 and 2.0 V (vs Na/Na+) at 0.1 mV s−1. EIS of the BPC electrode at different charge/discharge states and after different cycles was recorded on a Solartron Analytical 1470 System in a frequency range of 100 kHz to 0.1 Hz. The electrochemical reaction kinetic current of the BPC electrode was measured in a home-build high-precision source meter (Keithley 2401, see Figure S1). Full Cell Investigation. To investigate the potential of BPC in a Na metal-free battery, we carried out a full cell test by coupling a BPC anode and Na0.66Ni0.26Zn0.07Mn0.67O2 cathode. The Na0.66Ni0.26Zn0.07Mn0.67O2 cathode was prepared according to our previous reported work.28 The Na0.66Ni0.26Zn0.07Mn0.67O2 electrodes were prepared by spreading a mixture of 80 wt % active material, 10 wt % super-P, and 10 wt % PVDF (8 wt %) onto a alumnium foil current collector. The as-prepared electrodes were then dried at 100 °C in a vacuum oven for 12 h. To ensure the reversibility of the cathode material, the potential range of the Na0.66Ni0.26Zn0.07Mn0.67O2 cathode test was limited to 2−4.25 V versus Na/Na+. For the full cell, because of the difference in specific capacities of the anode and the cathode, the masses of the electrodes have to be matched (N/P ratio is around 1.2:1). In a typical cell, the active mass of the Na0.66Ni0.26Zn0.07Mn0.67O2 cathode is 7.2 mg, and that of the BPC anode is set at 0.77 mg. The full cell was charged and discharged between a voltage of 1 and 4.15 V. The specific capacities and current densities for the BPC/NNZMO cell were calculated based on the active mass of the NNZMO cathode. Results and Discussions. The BPC composite with a high phosphorus content of 70 wt % was prepared by HEBM. Although similar ball milling process has been used to fabricate P/C composites, the properties of carbon matrix such as specific surface area, pore volume, and the electrical conductivity, and the status of phosphorus in the P/C composites are deemed very important, which significantly determine their electrochemical performances. The Ketjenblack (EC-600JD) and MWCNTs used in this work are both commercially available, and their morphologies are shown in Figure S2. The high surface area of Ketjenblack (1400 m2 g−1) could enable a uniform distribution of BP in the BPC

This may be achievable by controlling the existing state and distribution of phosphorus in the hybrid structures. Herein, to achieve high reversible capacity with stable cyclability at high rate of charge/discharge, in the present study, we fabricated a nanostructured BP/Ketjenblack− MWCNTs (BPC) composite with a high phosphorus loading of 70 wt % by high energy ball milling (HEBM). The representative structure of BPC composite is illustrated in Figure 1, panel a. During the HEBM process, phosphorus nanoparticles are formed and assembled with the highly conductive Ketjenblack matrix to form microsized secondary particles. MWCNTs are also introduced to form a dual conductive network and increase the structural stability of the composite. Thanks to its unique nanostructure, the as-prepared BPC composite as anode of SIBs attains a very high initial Coulombic efficiency (>90%) and high specific capacity with excellent cycle performance at high rate of charge/discharge (∼1700 mAh g−1 after 100 cycles at 1.3 A g−1 based on the mass of BP). Moreover, in situ electrochemical impedance spectroscopy (EIS), synchrotron high energy X-ray diffraction (HEXRD), ex situ small/wide-angle X-ray scattering (S/ WAXS), transmission electronic microscopy (TEM), and nuclear magnetic resonance (NMR) were used to understand its superior sodium storage performance. BP nanocrystalline was transformed to crystalline Na3P through an amorphous NaP intermediate during the sodiation process and then converted back to amorphous phosphorus with a small amount of amorphous NaP remaining during the desodiation process. We expect the results gained in this work could serve as a guide for future design on high performance anode material for sodium ion batteries. Experimental Section. Synthesis of BP/Ketjenblack− MWCNTs (BPC) Composite. The bulk BP (99.998%) used in this work was purchased from Smart Elements. Ketjenblack (EC-600JD) with a high surface area of 1400 m2 g−1 was purchased from AkzoNobel. MWCNTs (SMW 200X) were provided by SouthWest NanoTechnologies. The BPC composite was prepared through a HEBM process. BP, Ketjenblack, and MWCNTs were mixed with a mass ratio of 7/2.5/0.5, and then the mixture was transferred to the HEBM machine (8000D Mixer/Mill, SPEX Sample Prep.) and was ball-milled for 700 min in a stainless steel jar under an argon atmosphere. The weight ratio of the balls to the mixture was maintained in the ratio of 10:1, typically 1 g of the mixtures of BP, Ketjenblack, and MWCNTs versus 10 g of ball. Structure Characterization. Ex situ and in situ HEXRD experiments were carried out at the Beamline 11-ID-C and 11ID-D of the Advanced Photon Source (APS) at Argonne National Laboratory using X-rays with wavelengths of 0.11725 and 0.799898 Å, respectively. The morphologies and structures of the materials were characterized by field emission scanning electron microscopy (HITACHI S-4700-II) and TEM (JEOL2100F). Raman experiments were performed using a Renishaw inVia microscope spectrometer. S/WAXS data were collected at Beamline 12-ID-B of APS. The 2D S/WAXS images were radially averaged to 1D profiles and presented as X-ray scattering intensities versus momentum transfer, q (q = 4π sin θ/λ, where θ is one-half of the scattering angle, and λ = 0.886 Å is the wavelength of the X-ray at 12-ID-B), measured in the range of 0.01−2.3 Å−1. The NMR experiments were performed in a 9.4T magnetic field with a Bruker Avance III spectrometer. The 31P NMR spectra were recorded using 1.3 mm probe heads at a MAS speed of 60 kHz with a single pulse. C

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Figure 2. SEM images of (a) bulk BP and (b) the BPC composite; (c) low magnification SEM image and the corresponding elemental mapping of (d) carbon and (e) phosphorus in the BPC composite.

Figure 3. (a) Low and (b) high magnification TEM images of the BPC composite; (c, d) different magnification TEM images of BPC composite. D

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Figure 4. (a) Charge/discharge profiles, (b) Coulombic efficiency versus cycle number, and (c) cycle performance of BPC anode at 0.416 A g−1; (d) overall capacities of Ketjenblack−MWCNTs and BPC composite versus cycle number; (e) cycle performance at 1.3 A g−1 and (f) rate performance of BPC anode.

attributed to the D and G bands of the carbon matrix (Ketjenblack and MWCNTs) in the composite, respectively.29 The microstructures of the BP and BPC composite were characterized by scanning electron microscopy (SEM). The particle size of bulk BP is about hundreds of micrometers, as shown in Figure 2, panel a, which is too large to be used as electrode material for batteries. After HEBM, these large BP particles cannot be observed. They are homogeneously assembled with Ketjenblack and MWCNTs and form a unique nanostructure. Secondary particles with a particle size of about hundreds of nm to 1 μm can be observed in Figure 2, panels b and c. In addition, SEM elemental mapping (Figure 2d,e) reveals that carbon and phosphorus can be found throughout the whole area in Figure 2, panel c, demonstrating uniform distribution of BP nanocrystals in the BPC composite. The lowmagnification TEM image (Figure 3a) of the BPC composite further confirms that the BPC particles have a particle size of about 1 μm. The high-magnification TEM image of the BPC composite in Figure 3, panel b clearly shows that BP nanoparticles with crystal lattices (marked by dashed circles)

composite, while the high conductivity of Ketjenblack and MWCNTs yields a dual conductive network that facilitates electron transport within the BPC composite. The phase structures of the bulk BP and BPC composite were characterized by HEXRD. As shown in Figure 1, panel b, the bulk BP presents very strong diffraction peaks, which may be due to its large crystal size. However, the diffraction peaks of phosphorus become very broad in the BPC composite, indicating the formation of nanocrystalline BP after ball milling. The structures of BP and BPC composite were further characterized by Raman spectra (Figure 1c). The Raman spectra of bulk BP show sharp peaks at 362.5 cm−1, 439.8 cm−1, and 467.1 cm−1, which are assigned to the vibrations of the crystalline lattice and match the Raman shifts attributed to the A1g, B2g, and A2g phonon modes.19 These sharp peaks, however, cannot be seen in the Raman spectra of the BPC composite. A weak and broad peak appears instead, which suggests that the particle size of BP may be very small and homogeneously distributed in the BPC composite. The Raman spectrum also shows large broad peaks at 1323 and 1604 cm−1, which are E

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Figure 5. (a) Representative charge/discharge curves, (b) cycle performance at 12 mA g−1, and (c, d) rate performance of a Na0.66Ni0.26Zn0.07Mn0.67O2/P cell with electrolyte of 1 M NaPF6/PC with 2 vol % fluorinated ethylene carbonate additive.

The charge/discharge profiles of the BPC anode for cycle number of 1, 2, and 50 at 0.416 A g−1 are shown in Figure 4, panel a. In the discharge profiles, the first sodiation curve is different from the others due to the solid-electrolyte interphase (SEI) formation. The general sodiation curves consist of a sloping region from 1.0−0.5 V followed by an inclined plateau from 0.5−0.10 V and another sloping region from 0.10−0.01 V. Such minor plateau at around 0.07−0.11 V was also observed in the phosphorus−carbon composite reported by Ramireddy and co-workers25 and red phosphorus/carbon nanotube composite reported by Dou and co-workers,11 which may be ascribed to the formation of fully discharged Na3P. In the charge profiles, the desodiation curves comprise a sloping region from 0.01−0.4 V and an inclined plateau from 0.4−0.8 V followed by a sloping region up to 2.0 V. The first discharge capacity of the BPC anode was measured to be 2206.7 mAh g−1 based on the mass of BP, corresponding to 85% phosphorus utilization compared to the theoretical capacity of phosphorus (2600 mAh g−1). Unless specified otherwise, the capacities of the BPC composite in the present study were calculated based on the phosphorus only. The first reversible capacity was 2011.1 mAh g−1, corresponding to an initial Coulombic efficiency as high as 91.1%. Such a high Coulombic efficiency is very important when coupling an anode with a cathode material in a full cell system. To the best of our knowledge, this is the highest initial Coulombic efficiency among all the phosphorus-based anode material for SIBs.9−12,16,17,25,26 Upon continuous charging/ discharging, as shown in Figure 4, panels b and c, the BPC anode maintained very stable capacities with Coulombic efficiency around 100% for 50 cycles, indicating that the sodiation/desodiation process is highly reversible. The reversible capacity after 50 cycles is 1826.9 mAh g−1 at 0.416 A g−1, corresponding to capacity retention as high as 90.8%.

uniformly distributed in the amorphous carbon matrix, confirming the nanocrystalline property of BP in the BPC composite. The interval between lattices was measured to be 0.334 nm from the high resolution TEM image (Figure 3c), coinciding with the d-spacing for the (021) plane of BP (PDF no. 76−1962). The high magnification TEM image of the BPC composite in Figure 3, panel d shows that MWCNTs are distributed along the edge of the composite to form a dual conductive network. Such a unique structure could greatly enhance the electronic conductivity and could also increase the mechanical strength of the composite, and hence improve its rate performance and prolong its cycle life. The electrochemical performance of the BPC composite anode was evaluated by assembling them into coin cells with sodium as reference and counter electrode. The electrolyte was 1 M NaPF6 in propylene carbonate (PC) with 2 vol % fluorinated ethylene carbonate (FEC) (see Figure S3 for a comparison of cycling performance using different electrolytes). The effect of FEC additive has been well revealed by Komaba and co-workers using hard X-ray photoelectron spectroscopy analysis and time-of-flight secondary ion mass spectroscopy.26 They found that the FEC additive could form a homogeneous and relatively thinner surface layered on the electrode compared to additive-free electrolytes, which could protect the electrode against electrolyte decomposition and also provide lower resistance on the electrode surface probably enhances the efficiency and conductivity and thus leads the black P material to have relatively good rate performance and cycle stability. However, NaPF6 has been found to have a higher ionic conductivity than NaClO4 by Adelhelm and coworkers30 and better stability than NaClO4 by Komaba and coworkers,31 leading to the better electrochemical performance of hard carbon and Na0.7CoO2 in the NaPF6-based electrolytes. F

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Figure 6. (a) Cyclic voltammogram at 0.1 mV s−1 and (b) in situ HEXRD in the first cycle at 0.1 C of BPC anode; (c) ex situ HEXRD and (d) 31P NMR spectra of BPC anode at different charge/discharge states.

reversible capacity of 2119 mAh g−1 in five cycles. With successively increasing the current density up to 2 A g−1, the average reversible capacities are still over 1750 mAh g−1. Even at a high current density of 3 A g−1, the composite still delivers a high reversible capacity of 928 mAh g−1. When the rate is decreased back to 0.2 A g−1, the specific capacity in the BPC anode could be recovered to 1953 mAh g−1, which is 92.2% of the initial average capacity at 0.2 A g−1, demonstrating superior rate capability. To further demonstrate the feasibility of the BPC composite as an SIB anode, a BPC/Na0.66Ni0.26Zn0.07Mn0.67O2 (NNZMO) full cell was fabricated. The NNZMO cathode was prepared according to our previous reported work,28 and its half-cell performance is shown in Figure S5. The NNZMO cathode has multiple sodiation/desodiation plateaus, ranging from 3.0−4.2 V as depicted in its charge/discharge profiles (Figure S5a) and differential capacity curves (Figure S5b). As shown in Figure S5c, a reversible capacity of about 100 mAh g−1 after 100 cycles with the NMZMO cathode was obtained with an electrolyte of 1 M NaClO4/PC and 2 vol % FEC additive. A BPC/NNZMO cell was then assembled with a controlled negative/positive electrode (N/P) capacity ratio of about 1.2:1. To maximize the performance of the BPC anode, the electrolyte used in the full cell test was 1 M NaPF6/PC with 2 vol % FEC additive. The cell was then charged/discharged within a voltage range of 1− 4.15 V. Figure 5, panel a illustrates the charge/discharge curves of the BPC/NNZMO cell in the first, second, and 100th cycle at a constant current density of 12 mA g−1. The initial charge and discharge capacities were 122.9 mAh g−1 and 92.1 mAh g−1, respectively, leading to a reasonable initial Coulombic efficiency of about 75%. The current density and specific capacities were both calculated based on the active material mass of the NNZMO cathode. After the formation cycle, the BPC/

Red phosphorus/carbon composite (RPC) with 70 wt % of phosphorus loading was also prepared by the same ball milling process. Its cycle performance tested at the same condition is given in the Figure S4. As shown, the RPC anode exhibits lower initial Coulombic efficiency (87.8%), lower reversible capacities, and higher capacity fading rate in 50 cycles, which may be due to its lower electronic conductivity than BPC. However, it should be noted that the sodium storage performance of RPC anode is still comparable to most of P/C composites, convincing the advantages of Ketjenblack−MWCNTs network used in this work.9−12,17,25,26 The capacity contributed by Ketjenblack−MWCNTs in the BPC composite is negligible, as indicated by their significantly lower capacities (∼66 mAh g−1) as an SIB anode (Figure 4d). Owing to the high phosphorus content (70 wt %) in the BPC composite, the BPC anode could deliver a high overall capacity, about 1300 mAh g composite−1 (calculating the specific capacity based on the total mass of BP and carbon matrix) in 50 cycles. Given its excellent cycle stability at low rate of charge/ discharge, the cycle stability of BPC anode at high rate of charge/discharge was further evaluated. After an initial formation cycle at 0.26 A g−1, the cell was continuously cycled at a high current density of 1.3 A g−1. As shown in Figure 4, panel e, the BPC anode illustrates excellent cycle stability, which could deliver a very high reversible capacity of about 1700 mAh g−1 after 100 cycles based on the mass of BP. Such a high reversible capacity with excellent cycle stability at high rate of charge/discharge (>1 A g−1) together with phosphorus loading as high as 70 wt % has been less reported in the previous P/C composites. Figure 4, panel f further illustrates the rate performance of the BPC composite with charge/ discharge current density from 0.2−3 A g−1. At a low current density of 0.2 A g−1, the BPC anode could deliver an average G

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Nano Letters NNZMO cell maintained a reversible capacity of ∼100 mAh g−1 after 100 cycles (Figure 5b), with Coulombic efficiency of around 100%. The rate performance test (Figure 5c) of the BPC/NNZMO cell showed that the reversible capacity undergoes very low capacity fading when the charge/discharge rate is increased from 12 mA g−1 to 120 mA g−1 (10×). The capacity retention at 120 mA g−1 is about 90% of that at 12 mA g−1, indicating good rate capability. When the rate was returned to 12 mA g−1, the initial capacity was almost 100% recovered. The cell was further cycled at 60 mA g−1 after the rate performance test (Figure 5d) and still delivered about 80 mAh g−1 in the subsequent 100 cycles, illustrating good cycle stability at high rate of charge/discharge. The specific capacity of the BPC/NNZMO cell is currently limited by the NNZMO cathode. When the BPC composite anode is coupled with other sodium cathode materials such as Na2/3Ni1/3Mn2/3O232 or Na[Li0.05(Ni0.25Fe0.25Mn0.5)0.95]O2,33 the specific capacity and energy density of the corresponding full cell could be further increased. Thus, the nanostructured BPC composite proposed in this work has the potential to be a promising anode material for the room-temperature SIB. To understand the superior sodium storage performance of BPC anode, a series of techniques were used to characterize its sodiation/desodiation process. Figure S6a shows the in situ Nyquist plots of the BPC anode at different charge/discharge states in the first cycle. The Nyquist plot comprises one semicircle at the high and medium frequency region and a straight line at the low frequency region. The semicircle corresponds to the interfacial resistance including sodium ions passing through the SEI film and the electrochemical charge transfer resistance between the active material and the electrolyte. The low-frequency straight line is attributed to sodium ion diffusion inside the active material. As shown in Figure S6a, the cell had a large interfacial resistance of about 750 Ω at open circuit voltage. The interfacial resistance gradually decreased with the discharge/charge process and reached a low value of ∼15 Ω when the cell was charged back to 2.0 V. Figure S6b further reveals that the interfacial resistance did not significantly increase and was still kept at a very small value after tens of cycles, indicating good interfacial stability of the BPC anode. To further understand the sodiation/desodiation mechanism of the BPC anode, cyclic voltammetry (CV) was performed at a scan rate of 0.1 mV s−1 within a voltage range of 0.01−2.0 V. Figure 6, panel a illustrates CV data for the first five cycles of the BPC anode. A weak and broad peak occurs at around 1.0 V in the first cathodic scan and is attributed to the reduction of electrolytes to form the SEI layer. In the subsequent scans, a new peak centered at around 0.95 V appeared, which should be attributed to the initial sodiation of BP.12 When the potential was further scanned from 0.8−0.01 V, a major cathodic peak appeared at about 0.45 V, which corresponds to sodium ion insertion and formation of NaxP (1 < x ≤ 3). This peak shifted to a lower potential and stabilized after four cycles. The potential shift may be ascribed to an activation process. Peaks centered at 0.59, 0.90, and 1.43 V appeared during the first anodic scan and were also detected in subsequent scans. These peaks possibly correspond to stepwise sodium-ion extraction from the fully charged Na3P phase. In situ synchrotron HEXRD is a powerful technique for monitoring reacting systems since it can produce high-quality data in a very short time (e.g., 1 min or even less depending on the material properties). Specially designed coin cells were used

for in situ HEXRD studies. The schematic structure of in situ coin cells for in situ HEXRD can be seen in our previous reported work.34 High energy X-rays, which can penetrate the sodium chips anode, make the measurements possible. As shown in Figure 6, panel b, no crystalline phase is present at the beginning of discharge, indicating the reaction intermediate at this stage is in an amorphous state. When the cell was discharged to around 0.3 V, crystalline Na3P phase first appeared, and its intensity gradually increased with sodiation. During the desodiation process, the Na3P intensity gradually decreased and completely disappeared at about 1.4 V. The HEXRD patterns at the beginning of discharge and at the end of charge are quite similar, indicating a highly reversible sodiation/desodiation process of the BPC anode. To track the formation of intermediate phases during sodiation/desodiation, a home-built system for high-precision leakage current measurement was utilized to track the rate of parasitic reaction between the sodiated anode and electrolyte. The method for the leakage current measurement is described in Figure S7 and described more specifically in our recent work.35 Figure S7c plots the leakage current for the BPC anode versus potential. As shown, the leakage current gradually increased and stabilized at about 0.8 V, and then it rapidly increased up to about 0.2 V. Clearly, the dependence of the leakage current on the potential severely deviated from the simple Tafel equation, which is an exponential decay function predicting a monotonically decreasing leakage current with increasing potential. The stepwise current−potential relationship in Figure S7c implies that the parasitic reaction between the sodiated BPC and the electrolyte is chemical, where the reaction rate slightly depends on the species and concentration of the sodiated BPC. The plateau at around 0.8 V and the downward peak at 0.2 V in Figure S7c also imply that two different species may be involved during the sodiation of BPC. To clearly identify the reaction intermediates and disclose the sodiation/desodiation mechanism of the BPC anode, we further performed HEXRD, S/WAXS, NMR, and highresolution TEM on the cycled electrodes at different charge/ discharge states. The cells were all precycled for two cycles at 0.2 C and then discharged to 0.8, 0.2, and 0.01 V, and finally charged back to 2.0 V, respectively. All the potentials were held for 10 h to stabilize the reaction intermediates. The cycled cells were disassembled in a glovebox and then washed with dimethyl carbonate for several minutes. The powder samples were collected and then loaded into the corresponding sample holders to prevent the contamination of oxygen and moisture. Figure 6, panel c shows the HEXRD patterns of the cycled electrodes at 0.8 V (discharge), 0.2 V (discharge), 0.01 V (discharge), and 2.0 V (charge), respectively. As shown in Figure 1, panel b, the pristine BPC composite presents very broad diffraction peaks, indicating the existence of nanocrystalline BP. When the cell was discharged to 0.8 V, however, no diffraction peaks were observed, demonstrating that the reaction intermediates at 0.8 V have a perfect amorphous structure. When the cell was further discharged to 0.2 V, diffraction peaks belonging to Na3P (PDF no. 04−0764) could be clearly indexed, confirming the alloying reaction between one P atom and three Na atoms. The diffraction peaks of Na3P slightly increased when the cell was further discharged to 0.01 V, which may be due to formation of more Na3P during the continuous sodiation. However, when the cell was charged back to 2.0 V, it was found that the desodiation product for BPC anode converted to an amorphous structure. S/WAXS was also H

DOI: 10.1021/acs.nanolett.6b01777 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 7. (a, e, i) Low magnification TEM image, (b, f, j) high resolution TEM image, (c, g, k) SAED pattern, and (d, h, l) elemental mapping of BPC anode at (a−d) 0.8 V (discharge), (e−h) 0.2 V (discharge), and (i−l) 2.0 V (charge). The red, blue, and green colors represent Na, C, and P, respectively.

spectrum showed a very strong signal for Na3P when the cell was discharged to 0.01 V, indicating that most of the BP had been converted to Na3P. We also noticed that a weak peak attributed to phosphorus could be still observed, which may be gradually alloyed with sodium to form Na3P, resulting in the discharge minor discharge plateau at 0.07−0.11 V in the subsequent cycles. When the cell was charged back to 2.0 V, the strong signal of pristine BP cannot be completely recovered, and a weak signal of NaP was also seen, indicating that the phosphorus may exist in an amorphous state, and a small amount of NaP remained in the electrode during the desodiation process. The sodiation/desodiation mechanism of the BPC composite was further investigated by high-resolution TEM analysis to understand its morphological and crystalline structure evolution. Figure 7, panels a, e, and i show the corresponding TEM images of the electrodes at 0.8 V (discharge), 0.2 V (discharge), and 2.0 V (charge). These images clearly show that the morphology of the BPC composite barely changes during charge and discharge. The particle size remained at about 1 μm, and no obvious aggregation can be seen, indicating very good structural stability. The high resolution TEM image in Figure 7, panel b shows that the reaction intermediate at 0.8 V is a perfect amorphous structure, no crystal lattice is evident, which is further confirmed by the corresponding selected area electron diffraction (SAED) pattern in Figure 7, panel c. A clear crystal lattice can be found in the high resolution TEM image of the electrode at 0.2 V (see Figure 7f and Figure S8). It was measured to be about 0.231 nm, corresponding to the d-spacing for the (103) plane of Na3P. The diffraction dots in the SAED pattern in Figure 7, panel g confirm the polycrystalline structure

used to reveal the structure changes during discharging and charging (see Figure S8). We do not observe significant structural changes in the SAXS region (0.01 < q < 0.1 Å−1), which is related to the particle size variation during cycling. However, the WAXS curves clearly indicate a crystalline structure of the pristine electrode and the intermediates at 0.02 and 0.1 V, and they also show an amorphous structure of the intermediates at 0.8 and 2.0 V. To further reveal the structures of sodiation/desodiation intermediates or products, especially those in amorphous states, we conducted solid-state 31P magic angle spinning (MAS) NMR spectroscopy. Figure 6, panel d shows the massnormalized 31P MAS NMR spectra of the BPC anode at different charge/discharge states. The pristine BPC anode shows one strong peak at δ = 10 ppm and one weak shoulder peak at around 70 ppm, which is similar to that reported for BPC composite25 and indicates an amorphous structure with chemical shift and structure closer to BP. When the cell was discharged to 0.8 V, the signal for BP was obviously decreased, and two weak and broad peaks (δ = −111 and −154 ppm) appeared at the same time, confirming that the amorphous intermediate at 0.8 V is NaP, which has two P sites. A new peak at δ = −212 ppm appeared when the cell was further discharged to 0.2 V and was assigned to Na3P. The peak at δ = −317 ppm currently cannot be firmly assigned but may be NaxP intermediates with 1 < x < 3, as this peak disappeared after further sodiation, and the sodiation of phosphorus is a multistep process. The chemical shift of 31P for BP, NaP, NaxP (1 < x < 3), and Na3P highly coincided with the calculated 31P NMR chemical shifts by Ab Initio calculation that was recently reported by Mayo and co-workers.36 The 31P I

DOI: 10.1021/acs.nanolett.6b01777 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters of the Na3P. The crystal lattice and the diffraction dots were not observed when the cell was charged back to 2.0 V, confirming that the desodiation product is amorphous. The elemental mapping images in Figure 7, panel d with energy dispersive X-ray spectroscopy (EDS) reveal that sodium and phosphorus are uniformly distributed in the electrode at 0.8 V, and the atomic ratio for Na/P is very close to 1 (see Figure S10 for the EDS analysis on the cycled electrode at different charge/ discharge states), suggesting that the reaction intermediate at 0.8 V may be NaP phase, which coincides with the NMR result. As shown in Figure 7, panel h, sodium and phosphorus were still uniformly distributed in the electrode at 0.2 V, but sodium was the dominant element, indicating an increased content of sodium upon continuous sodiation. The EDS spectrum at 0.2 V indicates that the atomic ratio for Na/P is about 2.43:1, which is in accordance with the results from NMR that the intermediate phase at 0.2 V is a compound of NaP and Na3P. When the cell was charged back to 2.0 V, the sodium signal largely decreased, as shown in Figure 7, panel i, and the atomic ratio for Na/P was calculated to be only 0.27:1 (see Figure S10c), indicating partially remaining NaP. On the basis of the HEXRD, S/WAXS, NMR, and TEM study, we concluded that nanocrystalline BP is transformed to crystalline Na3P through an amorphous NaP intermediate during the sodiation process and then converted back to amorphous phosphorus with a small amount of amorphous NaP remaining during the desodiation process. Conclusion. In summary, we have demonstrated a high performance nanostructured anode material for room-temperature SIBs. This material is a BP/Ketjenblack−MWCNTs composite with a high phosphorus content (70 wt %) fabricated by HEBM. Thanks to its unique nanostructure, the as-prepared BPC anode material delivers very high initial Coulombic efficiency (91.1%) and high specific capacity with excellent cyclability (∼1700 mAh g−1 after 100 cycles at 1.3 A g−1 based on the mass of P) as well as high rate capability (∼1000 mAh g−1 at 3 A g−1 based on the mass of P). Most importantly, nanosized phosphorus particles and nanocarbon matrix were successfully assembled into micron-sized secondary particles to maximize the loading efficiency of active materials in the electrode laminate while maintaining excellent capacity retention for long-term cycling as demonstrated in this work. Considering that the volumetric expansion of forming Na3P is up to around 500%, the findings of this work also imply that an effective integration of nanostructured composite can be a practical solution to develop high capacity and high energy density metallic anode, such as Si with up to 400% volumetric change for LIBs and Sn with up to 420% for Na/Li-ion batteries.

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discharge states and after different cycles; leakage current measurement for the BPC anode; S/WAXS of BPC anode at different charge/discharge states; HRTEM image of BPC electrode when the cell was discharged to 0.2 V; EDX analysis of the BPC anode at different charge/discharge states (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research at the Argonne National Laboratory was funded by U.S. Department of Energy (DOE), Vehicle Technologies Office. Support from Tien Duong of the U.S. DOE’s Office of Vehicle Technologies Program is gratefully acknowledged. Use of the Advanced Photon Source, Center of Nanoscale Materials Office of Science User Facilities operated for the U.S. DOE Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357. Research at State Key Laboratory of Xiamen University was funded by National Natural Science Foundation of China (Grant Nos. 21233004, 21428303, and 21473148).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b01777. Digital photo of home-build high precision source meter; SEM images of Ketjenblack and MWCNTs; cycling performance of BPC anode at 0.416 A g−1 using different electrolytes; cycling performance of BPC and RPC anode at 0.416 A g−1; half-cell performance of Na0.66Ni0.26Zn0.07Mn0.67O2 cathode; electrochemical impedance spectra of BPC anode at different charge/ J

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