Solid-State Sintering Strategy for Simultaneous Nanosizing and

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A Solid-state Sintering Strategy for Simultaneous Nanosizing and Surface Coating of Iron Oxides as High-capacity Anodes for Long-life Li-ion Batteries Xiaolei Qu, Zhuanghe Ren, Yaxiong Yang, Yongjun Wu, Mingxia Gao, Yongfeng Liu, and Hongge Pan ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01308 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Applied Energy Materials

A Solid-state Sintering Strategy for Simultaneous Nanosizing and Surface Coating of Iron Oxides as High-capacity

Anodes

for

Long-life

Li-ion

Batteries Xiaolei Qu,† Zhuanghe Ren,† Yaxiong Yang,† Yongjun Wu,† Mingxia Gao,† Yongfeng Liu,*,†,‡ and Hongge Pan†

†

State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and

Applications for Batteries of Zhejiang Province and School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China. ‡

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai

University, Tianjin 300071, China.

Corresponding Author *E-mail: [email protected]

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ABSTRACT: Rapid capacity degradation and poor rate capability are still critical challenges for the utilization of iron oxides as high-capacity anodes of advanced Li-ion batteries (LIBs). To alleviate these problems, nanoengineering and surface coating are commonly adopted. However, the preparation of surface-coated nanostructures is usually complicated and consequently difficult and expensive to scale up. Herein, we demonstrate the successful simultaneous achievement of nanosizing and surface coating of iron oxides with a facile and scalable solid-state sintering process, which is of particularly practical importance for using these materials as anodes in LIBs. Heating mixtures of micron-sized Fe2O3 and NaBH4 to 350 ºC gives rise to the formation of a NaBO2-coated Fe3O4 nanocomposite, thanks to the high reducing ability of NaBH4. The particle size of Fe3O4 ranges from 30 to 60 nm, and the thickness of the NaBO2 coating layer is 3~4 nm. While used the NaBO2-coated Fe3O4 nanocomposite as anode materials for LIBs, the prepared sample from Fe2O3-0.2NaBH4 delivers a stable discharge capacity as high as 1228 mAh g-1 after 400 charge/discharge cycles at 100 mA g-1, exhibiting a significantly improved cycling duration. Moreover, the specific capacity also reaches 733 mAh g-1 even cycling at 2 A g-1. These excellent electrochemical performances mainly originate from the nanosized particles of the Fe3O4 matrix and the high viscosity and good ionic conductivity of the NaBO2 coating layer. KEYWORDS: lithium ion battery; anode materials; iron oxides; nanocomposites; surface coating; solid-state sintering


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1. INTRODUCTION Developing new electrode materials with high energy densities is a critical challenge to meet the ever-growing demand for high-performance LIBs.1-3 Recently, transition metal oxides have attracted intense interest as promising anode candidates for replacing graphite due to their high electrochemical lithium storage capacity, environmental friendliness, natural abundance and low cost.4-6 Iron oxides, such as Fe2O3 and Fe3O4, have been extensively studied and evaluated because they are cheap, abundant, non-toxic and easily processed with a high lithium storage capacity.7 For example, Fe2O3 reacts with 6 Li+ ions per formula unit based on a conversion reaction process, which corresponds to a theoretical capacity of 1007 mAh g-1.8 Moreover, it also offers around 6 times higher volumetric capacity than graphite, thanks to a high inherent density (5.26 g cm-3). Unfortunately, the utilization of iron oxides as anode materials often suffers from poor kinetics and rapid capacity fading, which are caused by their low electrical conductivity and large hysteresis between charging and discharging voltage, and a large volume change upon cycling, respectively.9 Studies by many research groups have demonstrated that nanostructuring is an effective approach to alleviate the volume expansion of iron oxides during the lithiation and delithiation process because nanostructured materials not only accommodate the mechanical strain generated from repeated volume expansion and contraction but also shorten the lithium ion diffusion pathway.10 A variety of nanostructures, including nanoparticles, nanotubes, nanorods, nanoflakes, nanospheres, nanospindles, nanoflowers, porous materials, hollow structures and hierarchical structures, have been designed and fabricated for iron oxides.11-18 As early as 2003, Larcher et al. observed that nanosized Fe2O3 particles exhibited significantly improved cycling stability and rate capacity compared to their micro-sized counterpart.11 Porous Fe2O3 microspheres composed 3 ACS Paragon Plus Environment


of well-crystalline nanoparticles exhibited the feature of ordered microstructure, hierarchical porosity, good electron pathways and easy penetration of the electrolyte, consequently offering a stable and reversible capacity of 705 mA h g-1 after 430 cycles.12 Kang et al. successfully produced Fe2O3 nanotubes with an excellent electrochemical performance of a highly reversible discharge capacity of 929 mAh g-1 after 30 cycles.13 In addition, Wang et al. used a high-pressure hydrogen surface treatment and controlled etching to compound Fe2O3 nanoflakes that showed remarkably high rates of performance and stability by maintaining 599 mA h g-1 at 5 A g-1 and retained 96.5% capacity after 500 cycles.14 Alternatively, α-Fe2O3 hollow spheres assembled by sheet-like subunits showed 710 mAh g−1 of reversible capacity after 100 cycles while operated at 200 mA g−1 and 0.05 - 3 V.15 Hollow microspheres composed of Fe3O4 nanoplates exhibited high cycling stability with a reversible capacity of 580 mAh g-1 after 100 cycles.16 Nanoporous Fe2O3 synthesized by one-pot chemical etching delivered a significantly improving electrochemical characterization of a high reversible capacity of ~700 mAh g-1 even after 200 cycles at a current density of 100 mA g-1.17 Moreover, nanospindle Fe2O3 exhibited quite stable cycling performance of a larger-than-theoretical reversible capacity of 1176 mAh g−1 after cycling 200 times at 100 mA g−1 and good high-rate performance.18 However, it was noted that the increased electrode/electrolyte interfaces of nanostructured iron oxides increases the risk of undesirable side reactions. Moreover, the active nanoparticles also tended to aggregate into larger, inactive clusters during long-time cycling. A frequently used strategy to protect active materials from serious side reactions is surface coating because the coating layers can physically insulate the direct contact of the electrolyte and active materials. Previously, a series of carbon-coated iron oxide composites were designed, synthesized and examined as anode materials for LIBs, such as carbon-coated Fe3O4 nanospindles, nanowires, nanorods, nanospheres, and so on.19-23 For example, carbon-decorated 4 ACS Paragon Plus Environment

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Fe3O4 nanowires delivered a high charge capacity of 600 mA h g-1 at a high rate of 5 C.21 Moreover, Li2CO3 and LiF can work as preformed artificial SEI layers to effectively retard the capacity loss and enhance the Coulombic efficiency upon cycling, since the SEI layers are electronically insulating but ionically conductive, which can prevent the direct contact between the electrolyte and active materials.24,25 Furthermore, some polymers, such as polypyrrole (PPy), have also been used to coat low-conductivity electrode materials because of their good conductivity, remarkable electrical charge reserving ability, and high stability in air or water. A reversible capacity as high as 785 mAh g-1 was retained over 100 cycles for PPy-coated Fe2O3.26 Here, the conductive carbon layers not only improve the electronic conductivity of active materials and prevent the aggregations of nanoparticles but also overcome the undesirable side reactions between the electrolyte and active materials. Alternatively, attempts have been conducted to coat iron oxides with other functional layers, including Li4Ti5O12.27 However, it should be noted that the surface-coated nanosized iron oxides were usually prepared by a multistep chemical reaction process, typically including primarily nanosizing iron oxides and subsequently coating carbon and polymer or loading nanoparticles on the surface of a preprepared carbon matrix. Such a process is quite complicated and consequently difficult and expensive to scale up. It is therefore highly desirable to develop a novel, easy accessible strategy for achieving simultaneous nanosizing and surface coating of iron oxides. In this work, we propose a facile, scalable one-step solid-state sintering approach to achieve simultaneous nanosizing and surface coating of Fe3O4. By heating a mixture of Fe2O3 and NaBH4, a 3~4 nm-thick NaBO2-coated Fe3O4 nanocomposite ranging from 30 nm to 60 nm was successfully prepared. Due to the high viscosity and good ionic conductivity of NaBO2 and the fine particle sizes, the prepared Fe3O4 nanocomposites exhibited remarkably improved electrochemical properties, especially a good long-term cycling stability, as anode materials for 5 ACS Paragon Plus Environment


LIBs. The prepared sample from Fe2O3-0.2NaBH4 delivers a specific capacity as high as 1228 mAh g-1 even after 400 cycles while cycling at 100 mA g-1, and the specific capacity remains at 733 mAh g-1 at 2 A g-1. These performances are remarkably superior to pristine Fe2O3 and Fe3O4. 2. EXPERIMENTAL SECTION 2.1 Preparation of NaBO2-coated Iron Oxide Nanocomposites: Commercial raw chemicals, micron-sized Fe2O3 (purity 98%) and NaBH4 (purity > 98%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and Thermo Fisher Scientific (Acros Organics, USA), respectively, and used as received. Fe2O3 was first mixed with NaBH4 at a 1:x molar ratio, x = 0, 0.1, 0.2, 0.3, 0.4, in a QM-3SP4 planetary ball mill (Nanjing, China) rotating at 300 rpm. To achieve uniform mixing in the given period of time (2 h), the ratio of the weight of the balls to the powder was set to be approximately 40:1. The prepared mixtures were then loaded into a stainless-steel tube reactor, which was heated from room temperature to 350 °C at a ramping rate of 2 °C/min and maintained for 2 h to give rise to the formation of the products. 2.2 Structure and Morphology Characterization: X-ray diffraction (XRD) measurements were carried out on a MiniFlex 600 XRD unit (Rigaku, Japan) equipped with Cu Kα radiation operating at 40 kV and 15 mA. A Bruker Tensor 27 unit (Germany) was employed to acquire the Fourier infrared (FTIR) spectra. A pellet was prepared by cold pressing the mixture of the sample powder and potassium bromide (KBr) with a weight ratio of 1:100, and transmission mode was used. An ESCALAB 250Xi system (Thermo Scientific, USA) equipped with an Al Kα (1486.6 eV) X-ray source was used to characterize X-ray photoelectron spectroscopy (XPS). The XPS data were fitted by XPSPEAK software and the obtained results were calibrated with the adventitious C 1s signal (284.8 eV) as a reference. The sample morphology observation was conducted on a Hitachi SU70field emission scanning electron microscopy (SEM, Japan) and a 6 ACS Paragon Plus Environment

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FEI Tecnai G2 F20 transmission electron microscopy (TEM, USA, operating at 200 kV). A Nano Indenter G200 unit was used to carry out the nanoindentation experiments with a continuous stiffness measurement technique. A 200-μm-thick pellet was prepared by cold-pressing the powder sample at a pressure of 20 MPa, which was then attached to a stainless-steel substrate. 2.3 Electrochemical Measurements: The prepared samples were assembled into CR 2025 coin cells as active anode materials to evaluate their electrochemical properties. Approximately 70 wt% active materials were first mixed with 10 wt% sodium alginate (the binder) and20 wt% acetylene black (the conductive agent). Here, sodium alginate was used as the binder instead of polyacrylic acid or polyvinylidene fluoride because it is more favourable for long-term cyclability. The resultant slurry was then coated on 13-mm-diameter copper foils, which were dried in a vacuum oven at 120 °C for 12 h. For each working electrode, the mass loading of the active material was estimated to be ~1.5 mg cm−2. The cell assembly was carried out in an argonfilled glove box (MBRAUN, Germany), in which the oxygen and water content was kept less than 0.1 ppm, using a pure lithium foil as the counter/reference electrode, a Celgard 2400 membrane as the separator, and a 1 M LiPF6 solution with ethylene carbonate/dimethyl carbonate/diethyl carbonate (EC/DMC/DEC, 1:1:1 by volume) and 1 vol% fluoroethylene carbonate (FEC) as the electrolyte. Moreover, a full cell composed of LiNi1/3Co1/3Mn1/3O2 (NCM111) cathode and NaBO2-coated iron oxide anode was also constructed with the same separator and electrolyte. To match the capacities, the mass loading density of cathode was around 11 mg cm-2 and NaBO2-coated iron oxide anode 1.5 mg cm-2. The galvanostatic chargedischarge measurements were performed at 26 ± 1 °C on a Neware battery testing system (Shenzhen, China). A constant current density of 100 mA g−1 was used and the working potential range was set at 0.01–3 V (vs Li/Li+). Cyclic voltammetry (CV) data were collected on an Arbin BT-2000 potentiostat (USA) with a scan rate of 0.1 mV s−1. A Vertex electrochemical 7 ACS Paragon Plus Environment


workstation (Ivium, The Netherlands) was employed to record electrochemical impedance spectrometry (EIS) curves. The measurement frequency range was from 100 kHz to 10 mHz, and a 5-mV amplitude was used to obtain a good signal-noise ratio. 3. RESULTS AND DISCUSSION The Fe2O3-xNaBH4 (x = 0, 0.1, 0.2, 0.3, 0.4) mixtures were heated to 350 °C from room temperature at a ramping rate of 2 °C min-1. The resultant products were detailedly characterized by means of XRD, FTIR, XPS, SEM and TEM. As shown in Figure 1a, the characteristic reflections of α-Fe2O3 dominate the XRD profile of the prepared sample from Fe2O3-0.1NaBH4. However, careful comparison revealed a relative change in the intensities of the two strongest peaks at 33°and 35.5°because the peak at 33°weakened and that at 35.5°intensified. For the prepared sample from Fe2O3-0.2NaBH4, the reflections belonging to α-Fe2O3 were nearly undetectable and Fe3O4 was identified as the major phase. Meanwhile, a minor amount of metallic Fe was also observed at 2θ = 44.6°and 82.3°with weak intensities. After reacting with 0.3 and 0.4 mol NaBH4, the signals of Fe2O3 completely disappeared in the XRD profiles, and only two phases, Fe3O4 as the primary phase and metallic Fe as the minor phase, were detected. The XRD results presented here clearly indicated that Fe2O3 was reduced to Fe3O4 and even metallic Fe while heating with NaBH4. However, no Na or B-containing species were distinguished by means of XRD, possibly due to the relatively low content and/or their amorphous forms. To understand the existing state of Na and B, the quantity of NaBH4 in the Fe2O3-xNaBH4 mixture was increased to 1 mole, and the corresponding XRD result of the solidstate product after heating at 350 ºC revealed the presence of NaBO2 (Figure S1, Supporting Information). Furthermore, FTIR examination confirmed the formation of NaBO2 because three obvious absorbances assignable to B-O bonding emerged at 1445, 1240 and 710 cm-1 (Figure 8 ACS Paragon Plus Environment

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1b).28 The high-resolution XPS spectra of B 1s, Na 1s and Fe 2p were present in Figure 1c-e. In the B 1s and Na 1s XPS spectra (Figure 1c and d), only one single peak was observed in each at 191.8 eV and 1072.0 eV, respectively. They are in good agreement with the signals of NaBO2.29 This further evidences the presence of NaBO2 after the thermal chemical reaction between Fe2O3 and NaBH4. For the Fe 2p XPS peaks, a gradual redshift was observed with increasing content of NaBH4, representing the conversion from Fe2O3 to Fe3O4,30 which agrees well with the XRD results described above. In addition, it is noteworthy that the intensities of B 1s and Na 1s XPS peaks gradually intensified with increasing content of NaBH4. In contrast, those of Fe 2p peaks weakened. This makes us believe that the newly formed NaBO2 mainly located on the surface of iron oxide nanoparticles because the XPS results represent the chemical states of elements on the surface. Figure 2 displays SEM and high resolution TEM (HR-TEM) images of the samples prepared from Fe2O3-xNaBH4. For comparison purposes, the morphology of pristine Fe2O3 was also observed with SEM. As shown in Figure 2a, the pristine Fe2O3 displayed very irregular particles in shape and size, and the size distribution range was from 100 nm to 1.0 μm. After reacting with 0.1 mol NaBH4 with heating, a large number of nanoparticles measured at 30~60 nm were separated out on the surface of the large Fe2O3 particles (Figure 2b). As x in Fe2O3-xNaBH4 was increased to 0.2-0.4, the larger particles were invisible in the SEM images and only nanosized particles were observed (Figure 2c-e). For the samples prepared from x = 0.3 and 0.4, however, the nanosized particles appeared to be sticking together. Further HRTEM observation presented a core-shell-like morphology for these nanosized particles. The inner core was identified to mainly consist of Fe3O4 and the outer shell was NaBO2 because two sets of fringes with interplanar spacings of 0.253 and 0.261 nm, which correspond to the separations between (311) planes of Fe3O4 and (131) planes of NaBO2, respectively, were detected (Figure 2f). In other words, the 9 ACS Paragon Plus Environment


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Fe3O4 nanoparticle (30~60 nm) was coated with a thin NaBO2 layer (3~4 nm). This is consistent with the XRD, FTIR and XPS results. Further nanoindentation measurement demonstrated a remarkably increased elastic moduli value for the prepared NaBO2-coated sample, representing an enhanced mechanical property possibly due to the high viscosity of NaBO2 (Figure S2, Supporting Information). According to the discussion above, we can conclude that by reacting with NaBH4 with heating, Fe2O3 was reduced to Fe3O4 (x ≤ 0.2) and even metallic Fe (x > 0.2), accompanied with a decrease in the particle size, and NaBH4 converted to NaBO2 along with the release of H2, as described as follows. 6 Fe2O3  NaBH 4  4 Fe3O4  NaBO2  2 H 2

(1)

Fe3O4  2 NaBH 4  3Fe  2 NaBO2  4 H 2

(2)

More importantly, NaBO2 coated on the surface of Fe3O4 nanoparticles as a thin layer. Thus, we successfully prepare a novel NaBO2-coated Fe3O4 nanocomposite by heating the mixture of Fe2O3 and NaBH4 in the present study, successfully achieving simultaneous nanosizing and surface coating of Fe3O4. Such a solid-state sintering technique is simple, facile and scalable, which is of particularly practical importance for using iron oxides as anodes in LIBs. The electrochemical lithium storage performances of the samples prepared from Fe2O3xNaBH4 as anode materials of LIBs were evaluated by assembling into CR2025 coin-type half cells. The results are shown in Figures 3 and 4. As expected, the samples prepared from Fe2O3xNaBH4 exhibited significantly improved electrochemical lithium storage performance. From Figure 3a, the initial discharge/charge capacities were determined to 1238/940, 1350/1015, 1172/897 and 1086/808 mAh g-1 for the prepared samples from x = 0.1, 0.2, 0.3, and 0.4, respectively, as the charge/discharge process operated at 100 mA g-1. These values are still 3-4 times higher than that of commercial graphite anode (370 mAh g-1).31 In addition, the initial 10 ACS Paragon Plus Environment

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coulombic efficiencies were calculated to be approximately 75.9%, 75.2%, 76.5% and 74.3% for the prepared samples from x = 0.1, 0.2, 0.3, and 0.4, respectively, very close to that of pristine Fe2O3 (76.8%). This suggests that the NaBO2 coating layer does not efficiently suppress the side reaction and the formation of SEI film during the initial cycling process. Comparison on the first CV curves (Figure 3b) revealed that the lithiation/delithiation behaviour of the prepared sample from Fe2O3-0.2NaBH4 is much similar to that of Fe3O4 as three peaks were observed, including one big peak at 0.48 V for the cathodic scan and two adjacent peaks at 1.65 and 1.86 V for the anodic scan. This is explained well with the XRD results because Fe2O3 was gradually reduced to Fe3O4 after reacting with NaBH4. The fact that nearly no electrochemical activity for lithiation/delithiation was observed for pristine NaBO2 (~500 nm in size) at 0.01 - 3 V (Figure S3, Supporting Information) further confirmed that the electrochemical lithium storage capacity mainly originated from Fe3O4 in the present study. In addition, NaBO2 is quite stable at the test potential range because it was still discernable after 1 deep charging/discharging cycle (Figure S4, Supporting Information). More importantly, the prepared samples from Fe2O3-xNaBH4 exhibited a significantly retarded capacity fading, and even a slight but gradual increase in the specific capacity was observed at the middle cycling stage (Figure 3c). For example, the specific capacity of the prepared sample from Fe2O3–0.2NaBH4 was reduced from 1038 to 937 mAh g−1 within the first 10 cycles, and subsequently increased gradually at 10-100 cycles and stabilized at approximately 1167 mAh g−1 after 200 cycles. However, the pristine Fe2O3 exhibited a rapid decrease in the specific capacity from 981 to 610 mAh g−1 within the first 60 cycles and then a sluggish decrease to 525 mAh g−1 in the following cycles. Thus, the value of available capacity of the prepared sample from Fe2O3–0.2NaBH4 after 200 cycles is twice that of pristine Fe2O3. This cycling performance is also greatly superior to pristine Fe3O4. Here, to shed light on the capacity fading and rising upon cycling for the prepared sample from Fe2O3–0.2NaBH4, the 11 ACS Paragon Plus Environment


discharge/charge curves after different cycles were collected and compared. As shown in Figure 3d, a gradual shortening in the discharge voltage plateau was observed within the initial 10 cycles, implying a degradation in the intrinsic capacity due to the consumption of active materials. This result can be attributed to the occurrence of the chemical reaction between active materials and the organic electrolyte at the exposed fresh surface of particles caused by the pulverization in the initial cycling stage.32 After 100 additional cycles, the discharge voltage plateau remained nearly constant, but the sloping part of discharge curve at lower voltage (< 0.85 V) was gradually extended, representing an increase in the non-intrinsic capacity. A similar phenomenon was also observed in other transition metal oxides and their composites, including Fe2O3, NiO, Co3O4, SnO2/Fe2O3, SnO2/CoO, SnO2/NiO, Sn-Fe3O4@graphite, and so on.32-38 The additional increased capacity was recently attributed to the reversible formation and dissolution of SEI due to the high electrocatalysis of sufficient nanosized Fe particles.38 We believe that such a mechanism also reasonably explains the capacity rising at 10-100 cycles in the present study because a minor amount of metallic Fe was produced after reacting Fe2O3 with NaBH4, as identified in the XRD profiles. After that, the specific capacity maintained nearly unchanged (~1228 mAh g-1), even after cycling 400 times (Figure 3e). This long-term cyclability is largely superior to pristine and other materials-coated Fe3O4 reported previously (Table S1, Supporting Information). Moreover, an attractive electrochemical energy storage performance was also attained by assembling the prepared sample as anode into a full cell with LiNi1/3Co1/3Mn1/3O2 (NCM111) as cathode. As shown in Figure S5 (Supporting Information), the assembled full cell delivered 930 mAh g-1 of initial discharge capacity at 0.1 C based on anode mass loading and two full cells powered a 22yellow-LED-bulb array. The energy density was calculated to be approximately 179 Wh kg-1, slightly higher than that of NCM111-graphite full cell (174 Wh kg-1) reported previously.39 Further evaluation showed 733 mAh g−1 of the specific capacity for the prepared sample from 12 ACS Paragon Plus Environment

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Fe2O3-0.2NaBH4 while cycling at 2 A g-1 (Figure S6, Supporting Information), exhibiting good rate capability. This should correlate closely to the charge transfer resistance of the surface and the Li+ diffusivity in the bulk. Figure 4a presents the EIS data of the pristine Fe2O3 and samples prepared from Fe2O3xNaBH4. The Nyquist plots of the studied samples share a common feature of two semicircles in the high- and middle-frequency regions followed by a straight line in the low-frequency region. According to the proposed equivalent circuit,40 the high-frequency semicircle corresponds to the impedance of the SEI film (Rsei), the middle-frequency semicircle is connected with the charge transfer impedance (Rct), and the linear tailing in the low-frequency region is related to Warburg impedance (Wo), which represents lithium diffusion in the electrode. By fitting the EIS curves with Zview software, the Rsei and Rct values were obtained as listed in Table 1. The results show a lower Rct value for the samples prepared from Fe2O3-xNaBH4 compared to pristine Fe2O3, which correlates closely to their reduced particle sizes and the high ionic conductivity of NaBO2 coating layer. In general, the reduced particle sizes can effectively increase the specific surface area of electrode materials, which provides much more active reaction sites. Moreover, the high ionic conductivity also facilitates the transmission of Li ions on the particle surface. All these factors contributed a lower surface reaction resistance for the prepared samples from Fe2O3xNaBH4. The Li ion diffusion behaviour was further studied and compared by fitting the slope of the straight line at the low frequency region and calculating the values of Warburg impedance coefficient (σω) and the Li+ diffusion coefficient (DLi) according to the following equations.41,42 Z Re  R et  Rsei  Rct    1/2 1  RT  DLi    2  AF 2C  

(3)

2

(4) 13

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in which R, T, F, A and C are the gas constant, the absolute temperature, Faraday’s constant, the area of the electrode surface and the molar concentration of Li+, respectively. As shown in Table 1, the DLi values were calculated to range from 1.7 × 10-12 to 4.4 × 10-12 cm2 s-1, exhibiting an order of magnitude higher than that of pristine Fe2O3 (8.8 × 10-13 cm2 s-1). This result is possibly due to the shortened diffusion distance of Li+ and the abundant grain boundaries caused by the decreased particle size and the creation of a multiphase structure. As observed in Figure 4b, the surface reaction activity was improved and the Li+ diffusivity was enhanced after reacting with NaBH4, which are reasonably responsible for the good rate capability. To understand the reasons for the remarkably improved cyclability of the prepared sample from Fe2O3-xNaBH4, Nyquist plots of pristine Fe2O3 and the x = 0.2 sample were collected after different cycles and analysed. The results are shown in Figure 4c-d and Table 2. A progressive increase in the Rsei and Rct values was observed for both pristine Fe2O3 and the x = 0.2 sample due to the deteriorated surface states. However, it should be noted that the Rsei and Rct values of the x = 0.2 sample exhibited a smaller increase with respect to pristine Fe2O3. For example, the Rct value was increased from 18.3 Ω to 254.6 Ω for pristine Fe2O3 after 200 cycles, but it was only increased from 12.6 Ω to 84.6 Ω for the x = 0.2 sample. This result represents a relatively stable surface state for the x = 0.2 sample. Such conjecture was further confirmed by XPS analyses. Figure 5 compares the high-resolution F 1s XPS spectra of pristine Fe2O3 and the prepared sample from Fe2O3-0.2NaBH4 as a function of cycle. It is generally accepted that LiF is one of important SEI components in the LiPF6-based electrolyte, and its content represents the quality of the SEI film formed on the surface of electrode materials.43 As observed in Figure 5a, the F 1s XPS peak exhibited a change tendency of first increase and then decrease for pristine Fe2O3 with cycling. This peak possibly originates from the repeated formation, damage and even exfoliation 14 ACS Paragon Plus Environment

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of the SEI film because of the pulverization and fracture of Fe2O3 particles caused by the large volume change during lithiation/delithiation, as reported extensively.44 In contrast, the F 1s XPS peak remained nearly unchanged for the prepared sample from Fe2O3-0.2NaBH4 after five cycles, representing a stable SEI film. This can be attributed to the reduced particle sizes of Fe3O4 and the high mechanical strength of NaBO2 as characterized by the elastic moduli, which works together to accommodate the volume change of Fe3O4 particles upon lithiation/delithiation, consequently preventing the pulverization of active particles. The suppressed pulverization of active particles was further evidenced by morphology observation on the cycled wafer electrodes. As shown in Figure 6, top-view SEM observations revealed that unlike pristine Fe2O3 sample (Figure 6a, b), in which there are a plenty of cracks and interstices on the electrode surface after 200 cycles, the prepared sample from Fe2O30.2NaBH4 displayed a quite smooth surface for the cycled electrode without obvious cracks (Figure 6e, f). Cross-sectional observations further confirmed a largely retarded increase in the thickness of the active material layer for the prepared sample from Fe2O3-0.2NaBH4 compared with pristine Fe2O3 after cycling (Figure 6c, d, g, h). Moreover, the exposed copper current collector was observed by optical microscope on the surface of pristine Fe2O3 electrode after only cycling 5 times and further cycling induced the loss of much more active materials (Figure 6i-l). In contrast, the wafer electrode of the prepared sample from Fe2O3-0.2NaBH4 still maintained good integrity and nearly no loss of active materials was observed even after 200 cycles (Figure 6m-p). All these results sufficiently indicate that the particle pulverization and fracture of Fe3O4 was effectively retarded with the reduced particle sizes and the presence of the NaBO2 coating layer. The reduced particle size offers better accommodation of the strain of lithiation/delithiation, and the high viscosity and stretchability of NaBO2 maintains good contact between the broken particles, consequently contributing significantly to improved cycle stability. 15 ACS Paragon Plus Environment


4. CONCLUSION In summary, a NaBO2-coated Fe3O4 nanocomposite was successfully synthesized by heating mixtures of micron-sized Fe2O3 and NaBH4 to 350 ºC. The prepared Fe3O4 particles ranged from 30 to 60 nm in particle size with a 3~4-nm-thick NaBO2 coating layer. When used as active anode materials, the NaBO2-coated Fe3O4 nanocomposite delivered 1350/1015 mA h g−1 of initial discharge/charge capacities at 100 mA g−1, and an excellent long-term cyclability because the specific capacity remained at 1228 mAh g−1 after 400 cycles. The significantly improved performance was mainly ascribed to the positive effects of the high viscosity and good ionic conductivity of NaBO2 and the decreased particle sizes of the prepared Fe3O4 nanocomposites. The proposed synthetic strategy in this work could be extended to other active oxides, consequently providing new opportunities for designing and fabricating a wide range of highperformance LIB electrode materials. ASSOCIATED CONTENT Supporting Information: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ Experimental details, Figure S1-S6 and Table S1 (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] NOTES The authors declare no competing financial interest. ACKNOWLEDGMENTS


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We gratefully acknowledge the financial support from the National Natural Science Foundation of China (51471152, 51571178), the National Materials Genome Project (2016YFB0700600), and the National Program for Support of Top-notch Young Professionals. REFERENCES 1.

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Artificial Solid Electrolyte Interphase-Protected LixSi Nanoparticles: an Efficient and Stable Prelithiation Reagent for Lithium-ion Batteries. J. Am. Chem. Soc. 2015, 137, 8372-8375. 26. Liu, J.; Zhou, W.; Lai, L.; Yang, H.; Lim, S. H.; Zhen, Y.; Yu, T.; Shen, Z.; Lin, J. Three Dimensionals α-Fe2O3/Polypyrrole (Ppy) Nanoarray as Anode for Micro Lithium Ion Batteries. Nano Energy 2013, 2, 726-732. 27. Chen, M.; Li, W.; Shen X.; Diao, J. Fabrication of Core–Shell α-Fe2O3@Li4Ti5O12 Composite and Its Application in the Lithium Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 4514-4523. 28. Nine, M. J.; Tran, D. N. H.; ElMekawy, A.; Losic. D. Interlayer Growth of Borates for Highly Adhesive Graphene Coatings with Enhanced Abrasion Resistance, Fire-retardant and Antibacterial Ability. Carbon 2017, 117, 252-262. 29. Xing, M.; Fang, W.; Nasir, M.; Ma, Y.; Zhang, J.; Anpo, M. Self-Doped Ti3+-Enhanced TiO2 Nanoparticles with a High-Performance Photocatalysis. J. Catal. 2013, 297, 236-243. 30. Fan, L.; Li, B.; Rooney, D. W.; Zhang, N.; Sun, K. In situ Preparation of 3D Graphene Aerogels@Hierarchical Fe3O4 Nanoclusters as High Rate and Long Cycle Anode Materials for Lithium Ion Batteries. Chem. Commun. 2015, 51, 1597-600. 31. Zhao, L. Y.; Bennett, J. C.; Obrovac, M. N. Hexagonal Platelet Graphite and Its Application in Li-Ion Batteries. Carbon 2018, 134, 507-518. 32. Wu, Z. S.; Ren, W.; Wen, L.; Gao, L.; Zhao, J.; Chen, Z.; Zhou, G.; Li, F.; Cheng, H. M. Graphene Anchored with Co3O4 Nanoparticles as Anode of Lithium Ion Batteries with Enhanced Reversible Capacity and Cyclic Performance. ACS Nano 2010, 4, 3187-3194. 33. Zhu, J.; Yin, Z.; Yang, D.; Sun, T.; Yu, H.; Hoster, H. E.; Hng, H. H.; Zhang, H.; Yan, Q. Hierarchical Hollow Spheres Composed of Ultrathin Fe2O3 Nanosheets for Lithium Storage and Photocatalytic Water Oxidation. Energy Environ. Sci. 2013, 6, 987-993. 20 ACS Paragon Plus Environment

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Intensity (a.u.)

x = 0.4

b

x = 0.3 x = 0.2 x = 0.1 Pristine Fe2O3 01-1262 Fe 99-0073 Fe3O4

Fe-O Pristine Fe3O4

Intensity (a.u.)

a

B-O

B-O

Pristine NaBO2 x = 0.4 x = 0.3 x = 0.2 x = 0.1 Pristine Fe2O3

79-1741 Fe2O3

20

30

40

50

60

2 Theta ()

c

187.3 eV

B 1s

NaBH4

x= 0.4 x= 0.3 x= 0.2 x= 0.1 Pristine Fe2O3

196

194

192

190

188

Binding Energy (eV)

186

80

d Na 1s Intensity (a.u.)

191.8 eV

70

90 2000

1600

1200

800

400

-1

Wavenumber (cm ) 1072.0 eV

Pristine NaBO2 x = 0.4 x = 0.3 x = 0.2 x = 0.1 Pristine Fe2O3

e

Intensity (a.u.)

10

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

1078 1076 1074 1072 1070 1068 1066

Binding energy (eV)

Fe 2p

x = 0.4 x = 0.3 x = 0.2 x = 0.1 Pristine Fe2O3

730

725

720

715

710

Binding Energy (eV)

Figure 1 XRD patterns (a), FTIR spectra (b), and high-resolution XPS spectra of B 1s (c), Na 1s (d), Fe 2p (e) of the NaBO2-coated iron oxide nanoparticles.


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a

b

c

500 nm

500 nm

f

e

d

500 nm

d(311)=0.253 nm Fe3O4

500 nm

500 nm

d(131)=0.261 nm NaBO2

5 nm

Figure 2 SEM images of pristine Fe2O3 (a) and NaBO2-coated iron oxide nanoparticles samples for x = 0.1 (b), x = 0.2 (c), x = 0.3 (d), and x = 0.4 (e). High-resolution TEM image of the NaBO2coated iron oxide nanoparticles (x = 0.2) sample (f).



a

b

Potential (V vs Li/Li

-1

+

2.5 2.0 Pristine Fe2O3

1.5

x =0.1 x =0.3

1.0

x =0.2 x =0.4

0.5 0.0 0

400

800

Current density (A g

)

)

3.0

0.0 -0.8 -1.6

0.48 V

-0.8

Fe3O4

-1.6 0.0 -0.8 -1.6

1200

0.46 V 1.55 V

-1

0.5

d )

800 600 400

Fe2O3

Fe3O4

x = 0.1 x = 0.3

x = 0.2 x = 0.4

0

Specific Capacity (mAh g )

50

100

2.0

2.5

+

3.0

)

2.5 2.0 5th 50th

1.5

10th 100th

1.0 0.5 0.0

NaBO2

200

e

1.5

3.0

+

1000

1.0

Potential (V vs. Li/Li

Potential (V vs Li/Li

Specific Capacity (mAh g

-1

)

1165 mAh g

Fe2O3

0.56 V

0.0

-1

1200

1.86 V

0.0

Capacity (mAh g )

c

-1

x=0.2 1.65 V

Cycle number

150

200

0

200

400

600

Capacity (mAh g

800 -1

1000

)

100

1600 1200

90

800 Discharging Charging Coulombic efficiency

400 0

0

50

100

150

200

250

300

80 350

400

Coulombic Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

Cycle number Figure 3 First charge/discharge curves of NaBO2-coated iron oxide nanocomposites at 100 mA g−1 (a). First CV curves of pristine Fe2O3, Fe3O4, and NaBO2-coated iron oxide (x = 0.2) (b). Cycling performance curves of pristine Fe2O3, Fe3O4 and NaBO2-coated iron oxide nanocomposites (c). The galvanostatic discharge/charge voltage profiles of NaBO2-coated iron oxide (x = 0.2) at selected cycles (d). Specific capacity and coulombic efficiency of the NaBO2coated iron oxide (x = 0.2) after 400 cycles at 100 mAh g−1 (e).


Page 25 of 29

a200

b -11.2

Pristine Fe2O3 x=0.1

2

-1

x=0.2 x=0.3

-11.6

x=0.4

100

-11.4

Log[DLi+] (cm s )

-Z'' ()

150

-11.8

50

-12.0

0 0

100 Z' ()

150

0.2

0.3

200

0.5

1st 5th 50th 200th

200

0

0.4

400

200

0

0.1

value of x

d 600

1st 5th 50th 200th

400

-12.2 -0.1 0.0

200

-Z'' ()

c

50

600

-Z'' ()

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Z' ()

400

600

0 0

200

Z' ()

400

600

Figure 4 Nyquist plots (a) and diffusion coefficient values (b) for Li+ ions (DLi) of pristine Fe2O3 and NaBO2-coated iron oxide nanocomposites after 1 charge/discharge cycle. Nyquist plots of pristine Fe2O3 (c) and NaBO2-coated Fe3O4 nanoparticle (x = 0.2) (d) after different cycles.



a F 1s LixPFy

Pristine Fe2O3 LiF

690

688

b

LiF

686

684

x=0.2

LixPFy

200th

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

200th

50th

50th

5th

5th

1st

1st 690

682

688

686

684

682

Binding energy (eV) Figure 5 High-resolution XPS spectra of F 1s for pristine Fe2O3 (a) and NaBO2-coated Fe3O4 nanocomposites (x = 0.2) electrodes (b) at selected cycles.


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e

a

50 μm b

i

m

j

n

k

o

50 μm

f

50 μm

50 μm g

c 25.1 μm

25.5 μm

Cu

Cu

50 μm

50 μm d

l

(d) h 87.5 μm

Cu

p

41.2 μm

50 μm

Cu

50 μm

Figure 6 Top-view and cross-sectional SEM images of pristine Fe2O3 electrodes (a-d) and NaBO2-coated Fe3O4 nanocomposite electrodes (x = 0.2) before (a, e, c, g) and after (b, f, d, h) cycling, and photographs of cycled wafer electrodes of pristine Fe2O3 (i–l) and NaBO2-coated Fe3O4 (x = 0.2) (m–p): after 1 cycle (i, m), after 5 cycles (j, n), after 50 cycles (k, o), and after 200 cycles (l, p).



Page 28 of 29

Table 1. Impedance parameters of the pristine Fe2O3 and NaBO2-coated iron oxide (x = 0.2) samples after first charge/discharge cycle. sample Rsei (Ω) Rct (Ω) σω (Ω cm2 s-1/2) DLi (cm2 s-1) Fe2O3 17.7 18.3 190.6 8.8×10-13 x=0.1 15.7 16.4 137.1 1.7×10-12 x=0.2 11.7 12.6 122.5 2.1×10-12 x=0.3 9.6 10.4 99.3 3.2×10-12 x=0.4 5.6 9.2 85.6 4.4×10-12

Table 2. Impedance parameters of the pristine Fe2O3 and the NaBO2-coated iron oxide (x = 0.2) samples at different cycles. σω (Ω Rsei Rct σω (Ω cm2 s- DLi (cm2 sRsei Rct DLi Fe2O3 x=0.2 cm2 s1/2 1 2 -1 (Ω) (Ω) ) ) (Ω) (Ω) (cm s ) 1/2 ) 3.3×10-13 1st 17.7 18.3 190.6 8.8×10 1st 11.7 12.6 97.3 12 10th

30.8

52.4

193.9

8.5×10-13

10th

13.2

15.4

49.2

50th

42.4

106.1

267.1

4.4×10-13

50th

17.5

28.7

39.5

100th

67.3

134.6

288.8

3.8×10-13

100th 22.8

51.3

33.2

200th

142.6

254.6

320.8

3.1×10-13

200th 45.2

84.6

28.5


1.3×1011

2.1×1011

2.9×1011

3.9×1011

Page 29 of 29

Table of Contents Image

NaBH4

heating

mixing

Fe2O3 Milling balls

Fe3O4

-1

Specific Capacity (mAh g )

NaBO2

100

1600 1200

90

800

Discharging Charging Coulombic efficiency

400 0

0

50

100

150

200

250

300

Cycle number


350

80 400

Coulombic Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Solid-State Sintering Strategy for Simultaneous Nanosizing and

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