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Antimony/Graphitic Carbon Composite Anode for High-Performance Sodium-Ion Batteries Xin Zhao, Sean A. Vail, Yuhao Lu, Jie Song, Wei Pan, David R. Evans, and Jong-Jan Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01761 • Publication Date (Web): 12 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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ACS Applied Materials & Interfaces

Antimony/Graphitic Carbon Composite Anode for HighPerformance Sodium-Ion Batteries Xin Zhao, Sean A. Vail, Yuhao Lu*, Jie Song, Wei Pan, David R. Evans, Jong-Jan Lee Sharp Laboratories of America, Camas, WA 98607, USA

Abstract Although room-temperature rechargeable sodium-ion battery has emerged as an attractive alternative energy storage solution for large-scale deployment, major challenges towards practical sodium-ion battery technology remain including identification and engineering of anode materials that are both technologically feasible and economical. Herein, an antimony-based anode is developed by incorporating antimony into graphitic carbon matrices using low-cost materials and scalable processes. The composite anode exhibits excellent overall performance in terms of packing density, fast charge/discharge capability and cyclability, which is enabled by the conductive and compact graphitic network. A full cell design featuring this composite anode with a hexacyanometallate cathode achieves superior power output and low polarization, which offers the potential for realizing high-performance, cost-effective sodium-ion battery. Keywords Na-ion batteries, antimony, graphite, anode, energy storage

Introduction The demand for more economical and efficient energy storage systems continues to emerge as intermittent renewable energy generating technologies become more prevalent and widely integrated into the electric grid.1-3 Ambient-temperature rechargeable batteries offer distinct advantages for energy storage applications, amongst which lithium-ion batteries are particularly versatile in offering flexible physical expandability, ease of maintenance, high energy storage efficiency and power output, as compared to other energy storage technologies.2,3 However, the widespread implementation of Li-ion battery in energy storage systems is impeded by the

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spiraling cost of lithium raw materials, which arises as a consequence of both increasing global demand for Li commodity chemicals and geographically-constrained terrestrial Li reserves.4-7 In contrast, the rich abundance, low cost and appropriate electrochemical equivalent and standard potential of sodium have made Na-ion battery technology an attractive alternative, yet Na-ion batteries are significantly less developed and incapable of meeting the power requirements for frequency regulation and load balancing in grid storage. In the pursuit of high-power Na-containing cathode, we recently demonstrated several transitionmetal hexacyanoferrates with unprecedented charge-storage and rate capability in non-aqueous Na-ion cells.8-10 Despite the constant progress being achieved for cathode materials, the negative electrode remains as a major challenge for Na-ion batteries. Metallic Na appears to form dendrites and poses similar safety concerns to metallic Li.11,12 Conventional carbonaceous materials, e.g. graphite and amorphous carbon, suffer from either electrochemical inactivities or low sodiation rates along with corresponding small densities (< 1.0 g/cm3).13-21 Porous carbonaceous structures as well as heteroatom-doped carbon have shown improved rate capability,22-27 while the initial coulombic efficiencies were not sufficiently high (typically 2565%), which would necessarily require excess cathode to compensate for the Na-ion consumption in full cells.28,29 Furthermore, their safety is called into question by both the low sodiation voltage that overlaps the Na metal plating regime and the highly reactive sodiated species formed during cycling.30,31 Elemental antimony (Sb) can accommodate Na ions via reversible alloying reactions and has demonstrated the potential to dramatically increase the energy and power density of Na-ion batteries.32,33 However, the structural instability of Sb arising from drastic volumetric variation during the alloying process leads to reduction of electrode integrity and considerable performance degradation during successive charge/discharge cycles. The sodiation/desodiation rate and cycling life is further hindered by the surface passivation of Sb and loss of electrical contact within the electrodes.34,35 To mitigate the adverse effects associated with mechanical fracture and failure, research is progressing towards reducing the Sb domain size and/or creating nanostructured Sb embedded in carbon matrices.36-39 Nonetheless, elaborate processing procedures are inevitably required, while the high surface area of the electrode intensifies reductive decomposition of electrolyte, leaving the irreversible Na-ion consumption, inferior rate


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capability and prolonged cyclability issues unresolved. Until now, there has been no demonstration of feasible Na-ion cell design utilizing Sb-based anode that is compatible with existing battery configurations and manufacturing protocols. In order to achieve satisfactory rate and cycling performance, anode designs that can facilitate the formation of a stable solid electrolyte interphase (SEI) while not disrupting ionic and electronic conduction would be critical. Herein, we describe a Sb-based composite anode constructed from Sb particles and graphitic carbon matrix featuring design capabilities to match the rate performance of a sodium manganese hexacyanoferrate cathode. The Sb/graphite composites were prepared by a facile, high-throughput ball-milling method, which benefits from direct use of commercially available Sb powders or beads, and offers a cost-effective and scalable anode formulation and processing approach. Attributed to the compact and robust structure of the interconnecting graphitic layers, the composite anode achieves a packing density that is twice that of hard carbon, a markedly reduced polarization and high capacity exceeding 200 mAh/g at 20C-rate (equivalent to full charge/discharge in 150 seconds), which collectively afford superior rate performance in full cells. The performance influencing factors including choice of carbon matrix, substrate and sodiation rates were comprehensively studied in a systematic manner.

Experimental Section Materials Sb powders (325 mesh, Strem Chemicals) were ground with N-methyl-2-pyrrolidone (NMP, Sigma-Aldrich) in a weight ratio of 1:2 for 48 hours using a 2 L planetary ball mill (Across International, PQ-N2). The milling was conducted under ambient atmosphere using a 500 mL ceramic milling jar at a powder-to-ball weight ratio of 1:4. The milled powders were separated and washed with acetone by centrifugation, followed by vacuum drying at 80 °C. The dried Sb particles were further pulverized with expanded graphite (Asbury) or high-purity laminate graphite (Nihon-Kokuen J-SP-α) flakes by the planetary ball mill at a powder-to-ball weight ratio of 1:1. Briefly, 75 g refined Sb powders and 50 g graphitic carbon along with 125 g balls were introduced into a 500 mL milling jar. Ball milling was then performed at 250 rpm for 12



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hours. The anode film was fabricated by pasting an aqueous slurry containing the milled Sb/graphite powders, carbon black and sodium alginate (MP Biomedicals) with a weight ratio of 85:5:10 onto either a copper or carbon coated aluminum foil (MTI). In a controlled experiment, the graphitic carbon was substituted by carbon black (Timcal Super C45), and the Sb/carbon black mixture was processed in the same way. The anode films contained milled Sb/carbon black composite, pristine carbon black and sodium alginate with a weight ratio of 85:5:10. The typical mass loading was on the order of 1.5-3 mg/cm2. The film was calendered by a roll press and subsequently dried at 120 °C under vacuum for >12 h. Characterization and Electrochemical Measurement Sample morphology was investigated using JEOL 6500F scanning electron microscopy. X-ray diffraction patterns were collected by a Rigaku Ultima IV X-ray diffractometer with CuKα radiation (λ=1.5418 Å) at 40 kV, a step size of 0.02° and a step time of 0.5 s. Surface area was determined from N2 adsorption/desorption isotherm at 77K, and the data was collected using a Multi-Station AnyGas Sorption Analyzer NOVA 2200 (Quantchrome Instruments) and analyzed using Brunauer-Emmett-Teller (BET) method. Electrochemical measurements were performed using standard CR2032 coin cells which were assembled in an argon-filled glove box. A metallic sodium disc was used as the counter electrode for half-cell study, and a microporous borosilicate glass-fiber membrane (Whatman) was used as the separator. The electrolyte comprised 1 M sodium hexafluorophosphate (NaPF6) in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 1:1 plus 5 vol% fluoroethylene carbonate (FEC) additive. Galvanostatic measurements were carried out with a BT2000 Potentiostat/Galvanostat system (Arbin Instruments) at various current densities using a constant-current (CC) mode. Galvanostatic intermittent titration technique (GITT) was applied on fully charged and discharged working electrodes using a series of current pulses of 1C for 6 min, each followed by an interval of five hours to obtain the quasi-open-circuit voltage. For full cell assembly, a cathode film was fabricated by pasting a mixture of sodium manganese hexacyanoferrate (NMHFC), carbon black, polyvinylidene fluoride (Kynar HSV 900) and NMP onto an Al foil (MTI Corporation). The NMHFC active material was synthesized by a coprecipitation approach reported elsewhere.9 The cathode film with a composition of 85 wt% NMHFC, 8 wt% carbon black and 7 wt% PVDF was calendered and vacuum dried for >12


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hours. In the full cell, the cathode and anode films were positioned facing each other and isolated by a layer of Celgard trilayer membrane. Results and Discussion The Sb/graphitic carbon composites were fabricated using a facile, dual-step grinding approach, as illustrated in Figure 1. 50 µm-sized bulk Sb powders were firstly subjected to a wet-milling process in a planetary ball mill, using NMP as a grinding medium. The pre-treated powders were collected by centrifugation and subsequently ball milled with graphite flakes in a mass ratio of 3:2. By varying the ball milling duration, the primary particle size of Sb powders was reduced to below 1 µm. At laboratory scale, greater than 500 g of composite product can be obtained in a single batch, which corresponds to a yield of nearly 100%. Based on the cost of raw materials and processing i.e. electricity consumption and associated equipment usage incurred in our experiment, the total cost of the composite powders has been estimated at $17-20 per kg for a daily capacity production of 1 kg (Table S1). It is reasonable to assume that these costs could be reduced substantially if the precursor materials are purchased in bulk and the ball milling route is adapted to the manufacturing environment. In order to construct a compact and conductive matrix capable of accommodating Nainsertion/extraction stresses, expanded graphite (EG) with partially exfoliated graphite layers40 was initially selected. This material is an analogue to graphene flakes with a high width-tothickness aspect ratio, whereas the inter-sheet connection and lack of functional groups in expanded graphite would substantially promote electron conduction in the cross-plane direction (along the c axis), as well as alleviate the irreversible capacity typically induced by the use of graphene, which is characterized by extreme surface area and structural defects.41,42 To validate the influence of carbon matrix on the anode performance, a high-purity thin graphite (J-SP-α) was investigated in parallel (Figure S1). The average diameters (D50) and Brunauer-EmmettTeller (BET) surface areas were 8.0 µm and 23.7 m2/g for EG and 6.4 µm and 9.5 m2/g for graphite J-SP-α, respectively. Figure 2 shows the scanning electron microscopy (SEM) images taken on the surface of coated anode films. Fragmentation and curling of graphite flakes under mechanical crushing led to formation of micron-sized granules that comprised Sb particles and crumpled EG or J-SP-α graphite layers. In both cases, the Sb particles dispersed both in between and underneath the



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graphene stacks were visible. The milled Sb powders had a broad particle size distribution of ~200 nm to 1 µm, consisting of many polydispersed agglomerates comprising submicron-sized primary particles (Figure S2). X-ray diffraction confirmed the good crystallinity of Sb and graphite species before and after ball milling, as shown in Figure 3. The d002 spacings were nearly equal for EG and J-SP-a (3.4 Å) without shifting after mechanical pulverization. From the line width of the characteristic (012) diffraction peak, the grain size of Sb was estimated to be 21 nm prior to milling and 16-19 nm for the milled Sb composites. Despite some Sb particles in direct contact, they were uniformly distributed within the graphite scaffold and attached to the conductive graphene sheets. Consolidated by the compact graphitic domains, the structure would be favorable for achieving a high packing density, sustained integrity and continuous electrical contact. The electrochemical performance of the composite anode was evaluated using deep galvanostatic charge/discharge cycling in the voltage range of 2-0.02 V (vs. Na/Na+) in a coin cell, with Na foil as the counter electrode and NaPF6 in EC/DEC solution containing 5% FEC as the electrolyte.17,43 As a comparison, hard carbon electrode with a mass loading of 1.5 mg/cm2 was prepared and evaluated at the same time. Figure 4a presents the capacity and coulombic efficiency (CE) of Sb/graphite composite anode deposited on copper foil at sequential charge/discharge rates of 0.1C to 20C. At a low rate of 0.1C (30 mA/g), the Sb/EG composite delivered a reversible charge capacity of 275 mAh/g along with an initial CE of 68%. The EG itself does not allow Na ions to be inserted notably even after moderate grinding, and the reversible capacity was negligible (~5.3 mAh/g), as shown in Figure S3. The initial capacity loss was ascribed to the formation of SEI layers and parasitic Na-ion reactions with the residual surface groups, e.g. oxygenated groups on graphite and Sb particles.31-33,44 At an ultrahigh rate of 20C (6 A/g), the reversible capacity stabilized at 167 mAh/g, which corresponds to approximately 61% retention of initial capacity. After returning to the initial rate, the original capacity was fully recovered, which implies excellent structural tolerance of the composite anode for rapid Na-ion insertion/extraction. The Sb/J-SP-α composite demonstrated an initial CE of 72% and similar capacity retention at high rates, while the reversible capacity was consistently greater by ca. 35 mAh/g compared to Sb/EG at all rates. The lower initial CE and Na ion uptake for Sb/EG should be associated with


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the higher proportion of porous graphite domains and larger surface area of EG which passivated more intensively and therefore compromised the ionic/electronic conduction, as indicated by Figure S3. As the content of carbon black C45 was extremely low in the anode, its contribution to the reversible capacity is negligible.31 Thus, considering the fact that the mass of Sb is 60% in the composite, the contribution from Sb to the capacity of Sb/J-SP-α composite anode would be approximately 516 mAh/g, which implies relatively high utilization of Sb. In sharp contrast, the rate capability of the Sb/carbon black composite anode was limited to 1C-sodiation/desodiation rate and for which the CE rapidly declined. This was further hindered by the irreparable capacity degradation during cycling, as illustrated in Figure S4. Likewise, an optimized hard carbon anode was severely deactivated upon increasing the rate. At charge/discharge rates beyond 10C (2.5 A/g), the capacity decreased by >95% (Figure S5), which is in agreement with the highly sluggish kinetics of Na-ion insertion into the turbostratic hard carbon structure.29,45,46 Indeed, the rate capability of the Sb/graphite composite anode design outperformed the previously reported bulk Sb,33 nanostructured Sb,47,48 Sb alloys35,49 and Sb/amorphous carbon anode.35-37 A comparison of the performance of the composite anode with previously reported Sb-based anodes is presented in Table S2. Although considerably higher capacity values of ca. 600 mAh/g have been shown by Darwiche et al.32 and He et al.33 using microcrystalline Sb, the active Sb mass loading was smaller in their reports, while at the same time, the significant amount of amorphous carbon additives employed were not considered when calculating the capacity. Attributed to a high compression density of 2.5 g/cm3 for the present material system, the volumetric capacity of the entire Sb composite anode reached 658 mAh/cm3, which is three times the value for conventional hard carbon (213 mAh/cm3). In addition to the characteristics of carbon matrices, the rate performance proved to be a complex function of the electrode substrate. Since aluminum does not alloy with Na in the voltage window for anode testing, it enables the application of Al as anode substrate.17,50 A systematic comparison of Cu and carbon coated Al current collectors was performed, as shown in Figure 4a and S6. Interestingly, the Sb/graphite composites deposited onto carbon coated Al foils with identical compositions attained higher reversible capacity (350-400 mAh/g) at 0.1 to 0.5C-rate, but were accompanied by more significant capacity decay at high rates (10-20C). Such a capacity variation was similarly observed in the hard carbon system. The carbon coating is a 1 µm thick conformal deposition of nanographite particles, which is electrochemically inactive.



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The most plausible explanation for the higher capacity at relatively low rates may be the larger surface roughness of carbon coated Al foil which reinforced its affinity towards the anode slurry and modified the adhesion and orientation of active components within the anode. At higher rates, the lower electrical resistance of Cu (1.71×10-6 Ω⋅cm) compared to carbon coated Al (3.39×10-6 Ω⋅cm) could prove more beneficial in terms of maintaining high electrode utilization. To further assess the rate capability and reveal the charge-transfer kinetics, a modified test protocol was applied to the composite anode, as shown in Figure 4b. Following five cycles at 0.1C (activation), the charge current was increased from 1C to 20C while maintaining the discharge current at 1C. The hard carbon anode displayed a capacity retention of 16% at 20C versus 0.1C charge rate. In comparison, the capacity losses for Sb/J-SP-α and Sb/EG composite anodes were as low as 13% when the charge rate was increased 200 times from 0.1C to 20C. Additional hints as to the charge-transfer kinetics were provided by means of GITT, as shown in Figure 5. The Na-ion diffusivity calculated as a function of specific capacity shows that the ion transport inside the Sb/graphite composites was kinetically more favorable than the Sb/carbon black composite, which indicates that the graphite matrices remained both more intact and conducting than carbon black. The variation of diffusivity associated with state of charge and discharge was a consequence of a combination of factors including electronic conductivity and volume change. The evolution of electrical conductivity stemmed from a series of phase transformations of Sb species; the sodiation/desodiation-induced changes of particle morphology and arrangement modified the ion transfer behaviors, which resembles conventional data for intermetallics such as Li-Si and Li-Sn alloys.51,52 Moreover, the Na-ion diffusivity in Sb composites was nearly two orders of magnitude higher than in Na containing phases of hard carbon,20 which explains the superiority of Sb-based anode for high-rate applications. The galvanostatic charge/discharge profiles of the Sb/J-SP-α composite anode at various charge/discharge rates using the two different test protocols are presented in Figures 6a and 6b, respectively. The first Na-ion insertion occurs along a single extended plateau at ca. 0.50 V vs Na/Na+, while the additional voltage shoulder at ca. 0.90 V corresponds to parasitic electrolyte decomposition. In the subsequent discharge cycles, the three distinct characteristic plateaus at 0.71-0.35 V remained distinguishable even at very high rates, and which correspond to the phase transformation from amorphous Sb into an amorphous intermediate (NaxSb) and ultimately


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crystalline NaSb and Na3Sb.32 In addition to the expected increase in polarization between charge and discharge at higher rates, the capacity decay was markedly suppressed by fixing the discharge rate to 1C (Figure 6b). Since a metallic Na counter electrode undergoes stronger corrosion than its Li counterpart,11,53-55 a more accurate examination of anode voltage was performed by subtracting the voltage of Na foil from the overall cell voltage. The polarization voltage (ViR) of metallic Na was obtained by cycling a symmetric Na/Na cell under equivalent current densities per unit area of electrode surface, as shown in Figure 6c. Such measurements confirmed that the coin cell polarization was predominantly Na plating and stripping on the counter electrode rather than from the Sb composite anode. In other words, the Sb composite anode can be desodiated completely within 2-2.5 min in a single charge cycle in the absence of ohmic polarization. To examine the cycle life of Sb composite anode, charge/discharge cycling was performed galvanostatically at 1C-rate as shown in Figure 7a. After 160 cycles, reversible capacities of 280 and 241 mAh/g were obtained for Sb/J-SP-α and Sb/EG composites, respectively, which corresponds to a small average capacity loss of ca. 0.04-0.05% per cycle. The CE of the composite anode increased rapidly and reached greater than 99% after 5 cycles and 99.5% after 15 cycles. At a relatively fast rate of 4C, the Sb/J-SP-α and Sb/EG composites retained ca. 86% and 75% of their original capacities following 100 cycles (Figure S11). Composites prepared by direct ball milling of pristine Sb powders and J-SP-α demonstrated only 63% and 96% CE during the 1st and 15th cycle at 0.1C, respectively, revealing unstable SEI formation associated with Sb deformation as compared to the pre-milled Sb/J-SP-α composite (Figure 7b). On the contrary,

the

graphitic

carbon

promoted

initial

electrolyte

decomposition

at

the

electrode/electrolyte interface, forming comparatively stable SEI layers that were more permeable to alkaline ions.34,56,57 It is known that graphite flakes exhibit an inherently high Liion diffusivity along the direction parallel to the graphene plane.56 This feature could be imparted to Na-ion systems, which can be extended to rational design of carbonaceous and composite anode for Na-ion batteries with high rate capability. To validate the performance and applicability of the Sb composite anode more comprehensively, full cells were assembled by coupling the Sb composite anode with a rhombohedral sodium manganese hexacyanoferrate (NMHFC) cathode. The NMHFC with a nominal formula



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Na2MnIIFeII(CN)6 is a Prussian-blue analogue with a theoretical capacity of 171 mAh/g. The material used herein was synthesized in house and had a molar Na: Fe: Mn ratio of 1.77: 1.00: 0.72 (ICP). Previous investigations have revealed its high rate capability in non-aqueous Na-ion half-cells.9 Figure 8 depicts the galvanostatic rate and cycling performance of NMHFC – Sb/graphite full cells. The designed cell capacity per unit area was 0.5 mAh/cm2, along with a pre-determined balancing anode/cathode capacity ratio of 1.05:1. At 0.1C-rate, the cells reversibly charged and discharged with capacity close to the designed value according to the following electrochemical process: Sb/C + Na1.77Mn0.72Fe(CN)6 ↔ NaxSb/C + Na1.77-xMn0.72Fe(CN)6 When normalized to the active mass of NMHFC, the cathode achieved an initial charge capacity of 155 mAh/g and discharge capacity of 115 mAh/g, which corresponds to an initial CE of 74%. Interestingly, this value is consistent with the half-cell measurements of anode. Upon increasing the charge/discharge current, the cells showed stable operation including well-preserved voltage plateaus. The voltage drop was a derivative of cathode ohmic polarization, as evidenced by the half-cell measurement of NMHFC.9 When the cells were charged at 3C and discharged at 20C, the voltage hysteresis increased by merely ca. 600 mV compared to 0.1C charge/discharge. Meanwhile, approximately 50% of the initial capacity (measured at 0.1C) was retained. Postcycling at 3C-rate led to over 80% capacity retention after 40 cycles, suggesting good structural stability for both cathode and anode. On the other hand, increasing the areal loadings further led to more pronounced polarization and capacity degradation at high rates, presumably due to higher cell resistance as revealed from half-cell measurements of thicker cathode and anode films. We postulate that this issue can be further resolved via i) appropriate selection of conductive additives and binders for cathode and anode, and ii) minimizing the physical distance and displacement of electrode alignment and other cell components during cell assembly. The cycled Sb/graphite composite electrodes maintained their integrity while Na metal deposition was not visually detected on the surface. In contrast, plated Na metal was observed on the surface of the hard carbon anode under the same cycling conditions, as shown in Figure S12. The minimal polarization and high energy efficiencies qualify the Sb/graphite material system as an extremely attractive candidate for high power applications.


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Conclusions In summary, Sb/graphite composites prepared by facile mechanical grinding demonstrated impressive charge storage capability, rate capability and cycling stability for sodium-ion batteries. The ductile and compact graphitic matrices provided highly conductive pathways and effective protection to the active material, which ameliorated the polarization behavior and retained the integrity of Sb anode as compared to amorphous carbon. Optimization on the characteristics of graphite led to further enhanced performance by increasing the cycling reversibility. Full cells constructed from these anodes in combination with a hexacyanometallate cathode, in the absence of additional Na source, rendered exceptionally high rate capability and small voltage polarization that rivals conventional high-power Li-ion batteries. In the current drive for sustainable and cost-effective energy storage systems, this cathode/anode material combination, along with the preparation methods employed herein, may provide practical energy-storage solutions by offering competitive performance and cost advantages, as well as excellent compatibility with existing high-throughput manufacturing protocols and cell formats for current Li-ion state-of-the-art.

Associated Contents Supporting Information Further details on the cost estimation (Table S1), comparison with literatures (Table S2), additional SEM images (Figure S1-S2), galvanostatic charge/discharge profiles, and cycling/rate performance measurements (Figure S3-S12). This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgement



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This work was supported by the Advanced Research Projects Agency-Energy, U.S. Department of Energy, under contract DE-AR0000297. The work was also supported by CRDD in Sharp Corporation, Japan. The authors thank Mr. Motoaki Nishijima at KRI Inc., Japan for his valuable input in the research, and Dr. David Ji’s group at Oregon State University for the XRD measurements.

Figure 1. (a) Schematic illustration of the mechanical fabrication of Sb/graphitic carbon composites via planetary ball milling; (b) digital images showing the processing of Sb/graphite composite powders into anode films.


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Figure 2. SEM images taken on the surface of Sb/graphite composite anode films showing the uniform distribution of Sb particles in the graphitic matrix: (a)(b) Sb/EG and (c)(d) Sb/J-SP-α composite films.



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Figure 3. Power XRD patterns of EG, J-SP-α, Sb before and after milling, and processed Sb/graphite composites.


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Figure 4. Electrochemical performance of Sb composite anodes in half cell configuration: (a) specific desodiation capacities and coulombic efficiencies of Sb/EG, Sb/J-SP-α and Sb/carbon black anodes evaluated between 0.02-2.0 V at charge/discharge current densities of 0.1C-20C (1C based on a theoretical capacity of 300 mAh/g for Sb composites); (b) specific desodiation capacities and coulombic efficiencies of Sb/EG, Sb/J-SP-α and Sb/carbon black anodes evaluated between 0.02-2.0 V at constant discharge current densities of 0.1C/1C and charge current densities of 0.1C-20C (charge current densities are labeled).

Figure 5. Na-ion diffusivity of Sb/EG, Sb/J-SP-α and Sb/carbon black anodes obtained from GITT during (a) discharge and (b) charge.



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Figure 6. Galvanostatic charge/discharge profiles of (a) Sb/J-SP-α anode tested at charge/discharge current densities of 0.1C-20C, (b) Sb/J-SP-α anode tested at constant discharge current densities of 0.1C/1C and charge current densities of 0.1C-20C, and (c) a symmetric Na/Na cell at equivalent current densities. The polarization voltages of charge cycles at various rates are highlighted. The voltage is read when ½ total capacity is attained.


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Figure 7. (a) Cycle life evaluation of Sb composite anodes at a constant current density of 1C; (b) cycle life evaluation of Sb/J-SP-α anodes with and without pre-milled Sb at a constant current density of 0.1C.

Figure 8. Electrochemical performance of full cells featuring NMHFC cathode and Sb/J-SP-α composite anode: (a) specific desodiation capacity and coulombic efficiency of full cells with designed areal capacity of 0.5 mAh/cm2 evaluated between 0.4-3.4 V at charge/discharge current densities of 0.1C-20C; (b) galvanostatic charge/discharge profiles of full cells. The nominal voltages at 1C, 10C and 20C-discharge rates are indicated in the figure.



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