Stable Sodium Storage of Red Phosphorus Anode Enabled by a Dual

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Stable Sodium Storage of Red Phosphorus Anode Enabled by a Dual-Protection Strategy Quan Xu, Jian-Kun Sun, Feng-Shu Yue, Jin-Yi Li, Ge Li, Sen Xin, Ya-Xia Yin, and Yu-Guo Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12571 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Applied Materials & Interfaces

Stable Sodium Storage of Red Phosphorus Anode Enabled by a Dual-Protection Strategy Quan Xu,a,d,† Jian-Kun Sun,a,d,† Feng-Shu Yue,b Jin-Yi Li,a,d Ge Li,a,d Sen Xin,c,* Ya-Xia Yin,a,d,* and Yu-Guo Guoa,d,* a

CAS

Key

Laboratory

of

Molecular

Nanostructure

and

Nanotechnology,

CAS

Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100190, P.R. China. b

Shenzhen EUBO New Material Technology Co., Ltd., Shenzhen 518100, P.R. China

c

Materials Science and Engineering Program & Texas Materials Institute, The University of Texas

at Austin, 1 University Station, C2201, Austin, Texas 78712, USA d

University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

†

These authors contributed equally to this work.

KEYWORDS: Na-ion batteries, phosphorus/carbon, nano/microspheres, scalable synthesis, electrochemistry *

To whom correspondence should be addressed. E-mail: [email protected] (X.S.); [email protected] (Y.X.Y.); [email protected] (Y.G.G.)

ACS Paragon Plus Environment

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ABSTRACT

Red phosphorus is appealing for anode use in sodium-ion batteries. However, the synthesis of electrochemically-stable red P anodes remains challenging due to a notable volume variation upon (de)sodiation, and limited synthetic methods arising from the low ignition and sublimation temperatures. To address the above problems, we herein successfully develop an industrially adaptable process for scalable synthesis of affordable phosphorus/carbon (APC) anode materials with an excellent electrochemical performance and at a significantly reduced cost. The key to our success is a delicately-designed, self-organized, strongly-interactive porous P/C structure filled with sodium alginate binder, which maintains the structural integrity of anode and enhances the electrical contact of red P upon its volume variation via a dual protection from porous structure and strong surface interactions. The APC anodes hence present ultrahigh initial Coulombic efficiency (86.2%), excellent cycling stability, and superior rate capability. The industrially adaptable process and excellent electrochemical performance endow the novel APC nano/microspheres with promising applications in high-performance Na-ion batteries.


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1. INTRODUCTION Though lithium-ion batteries (LIBs) currently dominate the energy storage markets for portable electronic devices and electric vehicles, their predictably increasing cost in the near future, as resulted from a limited reserve and an inhomogeneous distribution of Li resource on a global scale, is challenging their sustainability.1-4 Recently, sodium-ion batteries (SIBs) have aroused an unparalleled interest as promising alternatives to LIBs because of the rich abundance of Na across the world, bringing a significant cost advantage of Na over Li.5-11 Meanwhile, the exploration of electrode materials for SIBs is traceable from the LIBs because of the chemical similarities between Na and Li.12,13 Though the cathode materials of SIBs present a comparable electrochemical performance to their counterparts in LIBs,14-17 major challenges still exist in developing high-performance anode materials that accommodate reversible (de)intercalation of large Na-ions.18,19 Since the ionic radius of Na+ (102 pm) is 26 pm larger than that of Li+, most of the conventional anode materials for LIBs (such as graphite) are not suitable for SIBs unless solvent molecules are consorted or expanded graphite is used.20-22 Hard carbon materials show Na+ host ability in use as SIB anodes, yet their low reversible capacity seriously hinders the battery application.23-28 To further improve the electrochemical performance and specific energy of SIBs, anode materials with high Columbic efficiencies and specific capacities, and appropriate redox potentials are highly desired. Among all emerging materials proposed for SIB anode,29-34 red phosphorus is identified as one of the most promising candidates because of its ultrahigh theoretical capacity (2595 mA h g−1) from the three-electron reaction with Na+, which is twice as much as that of the Na metal (1166 mA h g−1), as well as its significant cost advantage and non-toxic nature.35-39 Also, red P is chemically stable and widely used as flame retardant.40 Hence, the battery use of red P offers


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improved safety over that of white P which is easily initiated by shock or friction. However, the practical applications of red P as SIB anodes are impeded by two intrinsic disadvantages which are similar with the alloy anodes (such as Si, Sn) in LIBs.41-43 One is the poor electric conductivity (1 × 10−14 S cm−1) and notable volume variation (~300%) upon (de)sodiation, which result in poor electrochemical activity and stability, and lead to unsatisfactory anode performance. The other is the relatively low ignition (~260 °C) and sublimation (416 °C) temperatures of red P, which raises challenges on material synthesis and anode fabrication.44-48 With the former experience obtained on alloy anodes for LIBs, one can expect to address the first problem by integrating nanosized P particles into a surface-coated, three-dimensional porous conductive networks to improve the electric conductivity and electrochemical stability of the P anode.47-50 However, to date there are only limited methods to prepare red P-based composites, in which simple ball milling approach and vaporization-condensation approach are the mostly employed. For example, an early work by Wang’s Group reported loading of red P into a single-walled carbon nanotube (SWCNT) network through a modified vaporization-condensation approach, and demonstrated an extended cycle life of P anode.47 Another work employed ball milling to form chemical bonds between P and graphene nanosheets, yielding a conductive P/graphene composite anode material for SIBs.48 The above P anodes, with single-carbon-component (e.g., SWCNT or graphene) conductive networks to improve their Na storage performance, are insufficient for maintaining their structural integrity when confronted with repeated and drastic volume changes during cycling. The structural instability inevitably leads to pulverization of anode and accounts for an unstable electrochemistry. Meanwhile, the use of expensive carbon materials (e.g., SWCNT or graphene) offsets the cost advantage of red P, deteriorating the engineering practicality and manufacture scalability of the P/C composite materials. Furthermore, both the simple ball milling method and the vaporization-


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condensation method have their inherent disadvantages. For example, the composite material prepared via the simple ball milling process usually have a high specific surface area (SSA), low tap density and large interparticle resistance, which are unsuitable for use in battery. The vaporization-condensation method has severe safety problems due to the unavoidable generation of white P. Referring to the former experience on dealing with the similar problems in Si-C anodes,51-53 here we show affordable P/carbon (APC) nano/microspheres with a porous bottom-up design and strong P-C interactions, and their large-scale synthesis via an effective combination of ball milling and closed-spray drying. The conductive network co-constructed by three nanocarbon components, i.e., multi-walled carbon nanotubes (MWCNTs), Super P carbon black, and flake graphite improves the electrical conductivity of APC nano/microspheres. Nanosized red P particles are embedded in the nanopores pre-formed by solvent evaporation in the conductive network, which provide sufficient void space to accommodate P expansion, so that the structural integrity of APC anode can be maintained upon cycling. Additionally, sodium alginate (SA), a biomass binder rich in carboxylic groups,54 is utilized to bind red P and carbon components via chemical bonds so that the composites’ electrochemical stability can be further improved. As a consequence, the APC anodes present a high reversible capacity of 1408 mA h g−1 P and superior rate performance. Furthermore, a scalable synthesis (>500 g per batch, >95% yield) is successfully achieved for the mass production the APC composite nano/microspheres, rendering its engineering practicality to build better SIBs.

2. EXPERMENTAL SECTION 2.1 Materials Synthesis


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Unless otherwise specified, all materials used in the experiment were of analytical grade, and were used without any pretreatment. The APC nano/microspheres were synthesized through a combined process of ball-milling and closed spray drying. Red P, flake graphite, CNTs, Super P carbon and SA were used as the starting materials at a mass ratio of 5:3.5:0.5:0.5:0.5. In a typical synthesis, red P, CNTs, Super P carbon and SA (0.5 wt. % water solution) were sealed in a stainless-steel jar preloaded with yttria-stabilized zirconia milling balls (diameter: 1 mm), then a ball-milling process was performed at a rotational speed of 800 rpm and lasted for 24 h. After that, flake graphite and ethanol were added into the above jar, and another ball-milling process was performed at a reduced rotational speed of 400 rpm for 12 h (milling balls with a diameter of 5 mm were used). During the whole ball milling process, the jar was filled with nitrogen and its temperature was maintained at 20 °C using a circulating cooling water system. The obtained slurry was utilized to fabricate APC nano/microspheres in a closed spray drying system under N2 atmosphere. The detailed parameters were displayed as follows: inlet temperature (180 °C), outlet temperate (100 °C), feeding speed (3 L h-1), rotate speed of nebulizer (25000 r min-1) and solid content (20%). The product was finally dried in a vacuum oven at 80 °C for 12 h. To synthesize the control material (P/C composites), red P, Super P carbon, CNTs and flake graphite at a mass ratio of 5:0.5:0.5:3.5 were sealed in the stainless-steel jar preloaded with the above 5-mm milling balls (without addition of water or ethanol), and then a ball-milling was performed at 400 rpm for 36 h to yield the control product. 2.2 Morphological, Structural and Elemental Characterizations The morphologies of all materials and the EDX elemental mappings were investigated by a field-emission SEM (JEOL 6701F). XRD patterns were collected by a Rigaku D/max 2500 diffractometer using Cu Kα radiation. Raman spectra was obtained using a Digilab FTS3500


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system. PSD was measured using a laser particle size analyzer (Malvern, Mastersizer 3000). XPS was conducted on an ESCALab 250Xi (Thermo Scientific) using a 200W monochromatized Al Kα radiation. FT-IR was performed on a Nicolet Nexus 670. 2.3 Electrochemical Measurement The APC anodes were prepared through coating a homogeneous slurry consisting of APC nano/microspheres, Super P carbon, sodium carboxymethyl cellulose and styrene butadiene rubber at a mass ratio of 85:5:5:5 on the C-coated copper foil, and then dried in a vacuum oven at 60 °C for 12 h. The mass loading of active material was ~2 mg cm-2. The electrochemical measurements of the APC anodes were performed using CR2032-type coin cells composed of Na foil as counter electrode, glass fibers as separator (Whatman, GE healthcare UK limited, CAT No. 1823-090), the APC working electrode and electrolyte (1M NaClO4 in a mixture of ethylene carbonate and propylene carbonate (v:v = 1:1) containing 10% fluoroethylene carbonate). The cells were assembled in an Ar-filled glove box. The galvanostatic discharge/charge of cells were performed between 0.01 and 2.0 V vs. Na+/Na under a constant current density of 100 mA g-1. The EIS measurements were carried out using an Autolab workstation in a frequency range from 100 kHz to 0.1 Hz.

3. RESULTS AND DISCUSSIONS Scheme 1. Schematic illustration showing the synthetic process of APC nano/microspheres.


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Scheme 1 shows the typical preparation process of APC, in which micron-sized red P particles (Figure 1a) are thoroughly mixed with various nanocarbon components (Figure 1b-d) via ball milling, forming a uniform slurry of embedded red P nanoparticles in carbon matrix (Figure 1f). The obtained slurry is used to fabricate the APC through the closed-spray drying process. Eventually, optimized design and scale-up synthesis of APC nano/microspheres are successfully implemented through the facile and industrially adaptable process. To demonstrate the superiority of the above synthetic strategy, a control material was prepared by simply mixing red P and the above carbon materials via ball milling, and was denoted as the P/C composites. As shown in Figure 1e, the morphology of P/C composites is disordered and unsystematic. However, the APC composites present nano/microspherical structure after spray drying (Figure 1g), in which APC microspheres actually consist of numerous nanosized building blocks. Firstly, the phosphorus and carbon are homogeneously dispersed in the slurry after ball milling process (Figure S1). Then uniform distributions of P are found throughout the APC, as confirmed by the energy dispersive X-ray (EDX) elemental mappings (Figure 1h and 1i). This result suggests that, compared with the simple solid-state ball milling, the ball milling with water and ethanol is more conducive to achieve a uniform dispersion of red P and carbon, and the high viscosity of the SA solution can effectively prevent P/C nanoparticles from aggregating during the whole fabrication process.


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Figure 1. The SEM images of (a) micron-P, (b) flake graphite, (c) CNTs, (d) Super P, (e) P/C materials, (f) the slurry after ball milling process, (g) APC nano/microspheres, (h) Typical morphology of APC nano/microspheres, and (i) the corresponding EDX mapping of P. The particle size distribution (PSD) of the components for APC was investigated using a laser particle size analyzer. As shown in Figure 2a, the average diameter (D50) of red P is reduced to 95 nm after ball milling, whereas the D50 of the original micron-P is 4.2 μm (Figure S2). The P particles with shrunk size are beneficial to prevent the anode pulverization during the repeated Na+ (de)intercalation and to facilitate the Na+ transport,18,41 yet also brings a high specific surface area (SSA), which may cause severe side reactions and result in low initial Coulombic efficiency. This is also demonstrated by the N2 adsorption/desorption isotherms of the P/C composite (Figure S3),


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which show a large SSA of 93.8 m2 g-1 calculated by the Brunauer-Emmett-Teller (BET) method. The PSD curve of slurry (D50 = 0.07 μm, Figure S4a) presents two peaks separately associated with nano-sized components (e.g., nano-P and carbon black, which agrees with Figure 2a) and micron-sized flake graphite (which agrees with Figure S2). After the spray drying process, the peak denoting nanocomponents disappears, while the other peak in the micrometer region shifts to reach a higher average diameter (D50 = 11.5 μm). Such a change in PSD curves indicates the formation of a secondary micron-sized structure by self-assembly of primary building blocks, and is consistent with the hierarchically nano-/micron-architecture of the APC. Meanwhile, a scale-up synthesis (>500 g per batch, >95% yield) is successfully reached for the mass production of the APC nano/microspheres (Figure 2b) via the closed-spray drying process. The pore structure of the APC and its components are also investigated by N2 adsorption/desorption isotherms (Figure 2c and d), which shows a low SSA of 17.6 m2 g-1, and a hierarchically porous structure mainly consisting of mesopores and macropores. The low SSA could be ascribed to the following two aspects: one is the large-sized flake graphite (D50 = 5.2 μm, SSA = 11.8 m2 g-1 according to Figure S3a), and the other is the SA additive, which facilitates the space-efficient occupation inside APC nano/microspheres by establishing an intimate contact between phosphorus and carbon. The characteristics of porous structures are consistent with the scanning electron microscopic (SEM) finding in Figure 1g.


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Figure 2. (a) PSD curves of APC nano/microspheres and red P after ball milling. (b) APC nano/microspheres produced at a 500-gram scale. (c) N2 adsorption-desorption isotherms and (d) Pore size distribution of APC nano/microspheres. Figure 3a and 3b show the X-ray diffraction (XRD) patterns and Raman spectra of APC nano/microspheres, carbon and red phosphorus. The characteristic peaks of red phosphorus are dramatically weakened, indicating that nano-P particles are uniformly dispersed in conductive carbon networks and compactly wrapped by carbon substrate.45 In contrast, the characteristic Raman peaks of red phosphorus in 300-500 cm-1 is easily observed in the P/C composite (Figure S4b), which indicates a weak P-C interaction. No new phase exists in APC nano/microspheres because of the effective fabrication process. Compared with the Raman spectra of red P, the Raman spectra of APC nano/microspheres mainly present two peaks at 1332 and 1576 cm−1 for D-band and G-band, respectively. The lower ratio (ID/IG) of APC nano/microspheres implies a high degree


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of graphitization,55 which can improve the poor electrical conductivity of P-based materials. Ball milling has been previously utilized to enhance surface interactions by chemical bonding,46,48 which could maintain the continuous contact of phosphorus and carbon during the huge volume variation. Furthermore, SA is an excellent binder for anode materials due to their strong interfacial interactions.56 Therefore, the intimate contact in APC nano/microspheres is induced by combining ball milling and SA additive. The surface interactions among phosphorus, carbon and SA are also demonstrated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). Figure 3c shows the C1s and P2p XPS spectra of APC composites. An obvious shift is observed in XPS spectra, indicating surface interactions between red P and carbon. FT-IR spectral measurements suggest further evidence for the interactions in APC nano/microspheres. As shown in Figure 3d, the SA shows four characteristic peaks at 1023, 1302, 1403 and 1592 cm−1, which correspond to C-O-C (asymmetric), pyranose ring, O-C-O (symmetric), and O-C-O (asymmetric) vibrations, respectively.44-48,54 Compared with the FT-IR spectra of pure red P and SA, some characteristic peaks are weakened or disappear in the FT-IR spectra of APC nano/microspheres, suggesting the interfacial interactions among red P, carbon and SA.45,48 The strong interactions are conducive to facilitate the formation of stable solid electrolyte interphase (SEI) and ensure the superior electrical contact between phosphorus and carbon, thus resulting in the excellent electrochemical performance of APC anodes.44,54


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Figure 3. (a) XRD patterns, (b) Raman spectra, (c) C1s and P2p XPS spectra of the APC nano/microspheres, carbon and red P, respectively. (d) FT-IR spectra of red P, carbon, SA and APC nano/microspheres. The electrochemical properties of APC nano/microspheres were investigated to reflect the electrochemical advantage of the optimized P anode structure. A initial Coulombic efficiency of 86.2% is achieved in a voltage range of 0.01–2.0 V vs Na+/Na under a constant current density of 100 mA g−1 (Figure 4a). Such a high Coulombic efficiency can be attributed to the following aspects: i) the low SSA of the APC nano/microspheres (17.6 m2 g−1) decreases the electrode– electrolyte contact area; ii) the conductive carbon substrates present a low irreversible capacity of 100 mA g−1. Flake graphite has been widely used in LIBs due to its favorable electrochemical performance and low cost (Figure S5). Inexpensive flake graphite possesses superior electrical conductivity and exceptional process ability because of its two-dimensional layered structure.


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Compared with the irreversible capacity of CNT and graphene (>200 mA h g−1),39 the initial irreversible capacity of flake graphite is only ~30 mA h g−1 and the reversible capacity contribution of graphite and CNT is less than 25 mA h g-1 in subsequent cycles (Figure S6). Furthermore, the conductive carbon substrates of the APC could exhibit a high reversible capacity of ~115 mA h g1

and a stable cycling performance in an appropriate electrolyte (Figure S7). These advantages of

flake graphite make it a promising conductive substrate for SIBs.

Figure 4. (a) Discharge-charge profiles of the APC anodes during the initial two cycles. (b) Cycling performance of anodes under a constant current density of 100 mA g-1, (c) the corresponding Coulombic efficiencies and (d) rate performance of the P/C (black) and APC (red) anodes. All of the discharge capacity values are calculated based on the mass of red P. Figure 4b shows the cycling performance of APC anodes in SIBs under a constant current density of 100 mA g-1. Due to the porous structure and strong interfacial interactions of the APC composite, a significantly improved cycling stability is seen on the anode. As a result, the APC


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−1 anodes delivers a high reversible capacity of 1408 mA h g−1 P (704 mA h gcomposites) and an impressive

capacity retention of 82.6% for over 300 cycles. This means a high reversible capacity of 704 mA h g−1 based on the mass of APC composites (Figure S8). In contrast, the P/C composite anode, can merely deliver a capacity of 231 mA h g−1 P after 300 cycles. It is generally hard to achieve such an impressive cyclability of red P because of its serious volume variation. As shown in Table S1, compared with some previous reports, the APC anodes present excellent electrochemical performance in terms of ICE, capacity retention and cycle life. The huge volume variation can usually cause concurrent fracture of active materials and SEI layer and poor electrical contact between red P and conductive substrates, thereby resulting in rapid capacity fading.57 In this study, the dual protection strategy is adopted to overcome the disadvantages in addition to decreasing the particles size to nanoscale. The benefits are obvious: i) the porous structure accommodates the volume expansion of red P without causing the pulverization of APC nano/microspheres; ii) the strong interfacial interactions not only maintain the electrical contact between P and carbon but also contribute to forming a stable SEI layer during a large volume variation.31,41 As shown in Figure 4c, the Coulombic efficiency of APC anodes is over 99% in the following cycles. Electrochemical impedance spectroscopy (EIS) is conducted to understand the advantages of APC nano/microspheres (Figure S9). The low resistance of charge transfer and Na+ conduction indicates a stable SEI layer and a well-maintained electrical contact in APC anodes. To better highlight the advantages of porous structure and strong surface interactions, the SEM images of P/C anodes and APC anodes before cycles, after the initial sodiation to 0.01 V vs Na+/Na, after the first desodiation to 2 V vs Na+/Na and after 150 cycles are investigated, respectively. As shown in Figure 5, the APC anodes still maintain the original structure after the first (de)sodiation and 150 cycles. The excellent structure integrity of APC nano/microspheres could ensure the superior electrical contact


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and prevent electrode pulverization during cycling. In contrast, the morphologies of P/C anodes undergo obvious variation during (de)sodiation process (Figure 5a-d), resulting in the capacity fading. Figure 4d presents the rate capability of APC anodes when the current density is varied from 50 mA g−1 to 2000 mA g−1. The APC anodes still deliver high reversible capacity of 986 mA −1 −1 −1 h g−1 P (493 mA h gcomposites) at 2000 mA g . As the current density is restored to 100 mA g , the −1 capacity recovers to 1520 mA h g−1 P (760 mA h gcomposites), indicating superior reversibility at a high

current density.

Figure 5. SEM images of (a-d) P/C anodes and (e-h) APC anodes before cycles, after the initial sodiation to 0.01 V vs Na+/Na, after the first desodiation to 2 V vs Na+/Na and after 150 cycles, respectively.

4. CONCLUSIONS In summary, novel APC nano/microspheres with an optimized structural design are synthesized to address the electrochemical disadvantages of red P anode in SIBs. A facile and industrially adaptable process is developed and performed, for the first time, to produce APC nano/microspheres in large-scale through the effective combination of ball milling and spray drying. Low cost carbon components, including flake graphite and Super P carbon black are


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utilized to establish a highly-efficient conductive network because of their superior electrical conductivity and exceptional processability. The dual protection strategy, namely, porous structure and strong surface interactions, maintains the structural integrity and enhances the electrical contact during volume variation. APC anodes hence present ultrahigh initial Coulombic efficiency, excellent cycling stability, and superior rate capability. The industrially adaptable process and excellent electrochemical performance endow the novel APC nano/microspheres with promising applications in high-performance SIBs. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. SEM/EDX result of the slurry, PSD curves of the micron-P, flake graphite and slurry, N2 adsorption-desorption isotherms of the flake graphite and P/C composites, Raman spectrum of the P/C composites, cycling performance of the flake graphite in Li/Na half cells, cycling performance of the bare C matrix, the APC anode and the P/C anode in a Na half cells, and Nyquist plots of the Na half cells built with the APC anodes. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (X.S.) E-mail: [email protected] (Y.-X.Y.) E-mail: [email protected] (Y.-G. G.) Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the Basic Science Center Project of National Natural Science Foundation of China (NSFC) under grant No. 51788104, the NSFC (Grant Nos. 21773264 and 51772301), the National Key R&D Program of China (Grant No. 2016YFB0100100), and the "Transformational Technologies for Clean Energy and Demonstration", Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDA 21070300). REFERENCES (1)

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Stable Sodium Storage of Red Phosphorus Anode Enabled by a Dual

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