Etching Preparation of (010)-Defective LiFePO4 Platelets to Visualize

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Etching Preparation of (010)-Defective LiFePO Platelets to Visualize the One-Dimensional Migration of Li Ions +

Miaomiao Zhang, Rui Liu, Fan Feng, ShaoJie Liu, and Qiang Shen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b02270 • Publication Date (Web): 11 May 2015 Downloaded from http://pubs.acs.org on May 15, 2015

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The Journal of Physical Chemistry

Etching Preparation of (010)-Defective LiFePO4 Platelets to Visualize the One-Dimensional Migration of Li+ Ions Miaomiao Zhang, Rui Liu, Fan Feng, Shaojie Liu1 and Qiang Shen* Key Laboratory for Colloid and Interface Chemistry of Education Ministry, School of Chemistry & Chemical Engineering, Shandong University, Jinan 250100, China.

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ABSTRACT To visualize the one-dimensional (1D) migration of Li+ ions along direction, two kinds of (010)-defective LiFePO4 platelets with different hollow interiors are prepared using the polyethylene glycol (PEG)-assisted hydrothermal reaction of LiOH, FeSO4 and H3PO4 at the molar ratio of 3:1:1. Owing to the acidic and viscous reaction circumstance in the presence of polymeric PEG, the relatively high concentration of PEG induces the formation of (010)defective LiFePO4 platelets with a small average length and a small hollow interior. As a Li-ion battery cathode, (010)-defective crystallites with a small hollow interior exhibit the higher reversible capacity at each charge-discharge cycle, the smaller concentration polarization and charge transfer resistance and the bigger Coulombic efficiency and Li-ion diffusion coefficient than (010)-defective LiFePO4 platelets with a large hollow interior. Furthermore, (010)-manifest LiFePO4 micro-rhombohedra and their surface-etched derivatives could be treated as a comparative couple to prove the probable formation mechanism of (010)-defective LiFePO4 platelets and to visualize their sluggish charge transfers along direction.


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1. INTRODUCTION In crystallography, lithium iron phosphate (LiFePO4) possesses olivine structure and belongs to orthorhombic system with a space group Pnma,1,2 wherein oxygen atoms localize in a slightly distorted, hexagonal-close-packed arrangement, phosphorus atoms take up tetrahedral sites, iron and lithium atoms occupy octahedral 4a and c sites, respectively. That is, crystalline LiFePO4 is spatial framework composed of FeO6 octahedra, PO4 tetrahedra and LiO6 octahedra, each FeO6 octahedron is linked with four FeO6 octahedra through common corners in bc plane, LiO6 octahedra form edge-sharing chains along the b-axis. Each FeO6 octahedron shares common edges with two LiO6 octahedra and each PO4 tetrahedron shares one edge with a FeO6 octahedron and two edges with LiO6 octahedra. It has been demonstrated that Li+-ion migration within the orthorhombic lattices is one-dimensional (1D) and preferentially proceeds along the baxis via a nonlinear, curved trajectory between Li sites,3-8 however, it is still difficult to visualize the 1D channels so far. Since the pioneering work of Goodenough et al.,1,2 LiFePO4 has been widely applied as one of the most promising cathode materials for rechargeable lithium-ion batteries, possessing a high theoretical capacity (170 mAh g-1) and flat operating voltage (3.4 V vs. Li+/Li). Based on crystallization habit, polygonal crystallites of LiFePO4, produced under hydrothermal or solvothermal conditions, generally possess a shape of (010)-manifest platelet with the side crystal faces of (100), (101), etc.9,10 Regardless of particle aggregation, these grown-up (010)manifest platelets may satisfy the well-known theory of surface energy minimization for the hydrothermal or solvothermal reaction systems.9,10 Also, these shape- and/or size-controlled preparation routes exhibit incomparable advantages comparing with solid-state reactions.11-23 For example, the addition of reducing agent could effectively prevent the undesirable oxidation of



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Fe(II) species and simultaneously avoid the doping of unnecessary impurities, while the modification of influential factors (e.g., pH or solvent) may prohibit crystal growth along the direction, resulting in (010)-manifest LiFePO4 platelets.7,24-27 As for the hydrothermal formation of (010)-manifest LiFePO4 platelets, the previously added citric acid (CA) or tetra(ethylene glycol) (TEG) helps to dissolve/etch metallic cations preferentially from (010) planes and then induces the final production of hollow structures.28,29 Taking the polyol-assisted hydrothermal synthesis of (010)-manifest LiFePO4 platelets into consideration, a monomeric additive of ethylene glycol (EG) has been demonstrated to serve not only as co-solvent but also as chelating agent of Fe2+ ions.25,27 When trimeric ethylene glycol (TEG) was added into the water-based reaction system of FeSO4, LiH2PO4 and LiOH, (010)manifest LiFePO4 hollow structures with a single-crystalline shell were uniformly obtained, emphasizing a specific adsorption of the additives to prohibit crystal growth along the b axes.29 Therein, the etching effectiveness of Lewis acid should be in charge of the formation of (010)manifest LiFePO4 platelets with a hollow interior.28,29 Aroused by an ever-observed rodlike intermediates during the EG-assisted formation of (010)-manifest LiFePO4 platelets at a reaction interval of 0.5 h,25 herein multimeric ethylene glycol (PEG) was tentatively used as a polymeric additive to subjectively produce crystallites with a high aspect ratio. In this study, a facile hydrothermal route was adopted to synthesize the solid and hollow platelets of (010)-manifest LiFePO4 in the absence and presence of polyethylene glycol (PEG), respectively. The variation of PEG concentration could modify the volume of hollow interior of (010)-defective LiFePO4 platelets, whereas the subsequent treatment of (010)-manifest LiFePO4 rhombohedra with a solid interior facilitates the surface etching of acetic acid. When applied as Li-ion battery cathodes, these samples were comparatively investigated and the obtained


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electrochemical properties unexpectedly support the 1D migration of Li+ ions along direction.

2. EXPERIMENTAL SECTION 2.1. Synthesis of LiFePO4 Platelets All the chemicals are of analytical grade and were used without further purification and ultrapure water (18.2 MΩ·cm) was used throughout solution preparation. (010)-manifest LiFePO4 hollow platelets were synthesized by the modified hydrothermal method using polyethylene glycol (PEG, molecular weight~4000) as a crystal modifier. In a typical procedure, solid-state LiOH·H2O (24.0 mmol) was added into a beaker containing 6.0 mL of ultrapure water, then H3PO4 aqueous solution (2.0 M, 4.0 mL) was transferred into the beaker under vigorous stirring and N2 bubbling, followed by the addition of powdered PEG (10.0 or 40.0 g). After the complete dissolution of PEG, the freshly prepared FeSO4 aqueous solution (0.8 M, 10.0 mL) was dropwise added and then transferred into a 100-mL Teflon-lined stainless steel autoclave. The sealed autoclave was allowed to stand still in a thermostatic chamber at 170oC for 12 h. After cooling to room temperature, the precipitates were collected by centrifugation, washed with ultrapure water and then ethanol for 3 times, dried at 80oC under vacuum condition, and then finally resulting in two kinds of LiFePO4 platelets with different hollow features. In contrast with the above mentioned synthesis procedure, a facile hydrothermal route was adopted to prepare (010)-manifest LiFePO4 solid platelets in the absence of PEG.25 The asobtained solid platelets (1.0 mmol) were dispersed into ultrapure water (50.0 mL) under magnetic stirring, followed by the dropwise adding of liquid-state CH3COOH (2.0 mL), and then the wet etching was performed in 30 minutes.30 Finally, the surface-etched platelets were



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collected by centrifugation, washed with ultrapure water and then ethanol for 3 times, dried at 80oC under vacuum condition. It should be pointed out that each LiFePO4 sample was mixed with its 20 percent of powdered sucrose in weight, sufficiently grinded using an agate mortar, followed by the N2atmosphere calcination at 700oC for 6 h with a heating rate of 10oC min-1, resulting in the corresponding LiFePO4/C composite for electrochemical measurements. 2.2. Structural Characterization X-ray diffraction (XRD) measurements were performed on a Rigaku D/max-2400 power Xray diffractometer with Cu-Kα radiation (40 kV, 120 mA) and 0.08 steps/(25 s) in the 2θ range of 10° to 70°. Scanning electron microscopy (SEM) observation was conducted on a JEOL JSM6700F, fitted with a field emission source and operating at an accelerating voltage of 15 kV. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements were carried out on a JEM 2100 microscope (200 kV). N2 adsorption–desorption isotherms (i.e., Brunauer–Emmett–Teller or BET isotherms) of each LiFePO4 sample was measured on a Micromeritics ASAP 2020 sorptometer, and the corresponding pore size distribution was evaluated using Barrett-Joyner-Halenda (BJH) method. To measure the tap density of each LiFePO4 sample, about 0.2 g of powder was placed in a small plastic vial and tapped on a laboratory bench 1000 times by hand. 2.3. Electrochemical Characterization Typically, a composite sample of LiFePO4/C was mixed and grounded with acetylene black and poly(vinylidene fluoride) at a weight ratio of 8:1:1, followed by the addition of Nmethylpyrrolidone to form a uniform slurry. The resulting slurry was casted onto an aluminum


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foil and dried at 80oC for 12 h in a vacuum desiccator. Subsequently, the LiFePO4-loaded foil was cut into discs (12 mm in diameter) and used as lithium ion battery cathodes with a loading density of 2.1 ± 0.4 mg cm-2. Finally, CR2032-type coin cells were assembled in an argon-filled glove box, using metallic lithium metal, nickel foam, Celgard 2300 microporous membrane and commercial LBC 305-01 LiPF6 solution as counter electrode, current collector, separator and electrolyte, respectively. All the electrochemical experiments were performed at 30oC. Galvanostatic cycling tests were conducted on a LAND CT2001A system within a potential range of 2.5-4.3 V (vs. Li+/Li and hereafter). Electrochemical impedance spectroscopy (EIS) tests were measured in a frequency range from 100 kHz to 0.1 Hz with AC voltage amplitude of 5 mV at open-circuit voltage.

3. RESULTS AND DISCUSSION 3.1. The Etching Preparations of (010)-Defective LiFePO4 Platelets It should be mentioned at first that the aqueous solution of PEG is acidic, possessing a pH value of 5.60 and 4.61 at the concentrations of 0.8 and 0.4 g mL-1, respectively. To clarify the possible acidic etching, 0.8 or 0.4 g mL-1 PEG was previously added into the hydrothermal reaction system of LiOH, FeSO4 and H3PO4 at the molar ratio of 3:1:1. After the 12-h hydrothermal crystallization, residual supernatant acquired a pH value of 4.91 or 4.12 and the collected precipitates were referred to as LiFePO4-1 or LiFePO4-2 correspondingly. (FIG. 1)



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Figure 1 shows that the observed diffraction peaks of LiFePO4-1 and LiFePO4-2 can be indexed using the standard XRD data of orthorhombic LiFePO4 (JCPDS No. 83-2092). The absence of crystalline impurities may be used to estimate the specific adsorption of additive PEG onto the exposed (010) crystal faces and the possible crystal growth along direction simultaneously, defined as the preferential orientation of LiFePO4 crystallites deduced from the peak intensity ratio of (020)- to (200)-reflections (i.e., I(020)/I(200)).10,19 According to the standard XRD data of randomly oriented LiFePO4 crystals (JCPDS No. 83-2092), the theoretical value of I(020)/I(200) is 2.06, whereas from the XRD patterns shown in Figure 1, the calculated I(020)/I(200) values of powdered LiFePO4-1 and LiFePO4-2 are 3.33 and 2.07, respectively. If both of them adopt the same hydrothermal crystallization mechanism, at the higher concentration of PEG (i.e., 0.8 g mL-1) the resulting LiFePO4-1 crystallites might easily lay down with their {020} crystal planes parallel to the XRD supporting substrate. (FIG. 2) SEM images of LiFePO4-1 and LiFePO4-2 present a shape of the (010)-manifest platelet with irregular sizes and outside concaves/holes (Figure 2). By statistical analysis, the average particle sizes of LiFePO4-1 and LiFePO4-2 are 2.8 ± 0.3 and 2.9 ± 0.2 µm in length, respectively. Visually, the particles’ shape and its outside concaves/holes are similar to those obtained in the presence of TEG, indicating the demonstrated formation mechanism of selective etching and reversed recrystallization therein.27-29 Insets in Figure 2a and b are the schematic drawing and TEM image of a representative LiFePO4-1 platelet obtained at the high concentration of 0.8 g mL-1 PEG, exhibiting the major platelets of LiFePO4 with a small hollow interior and the clear concaves at side surfaces. Perhaps, the relative high viscous medium with a high pH value benefits the specific adsorption of polymeric PEG onto the {010} faces of nuclei and then slows


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down the interior dissolution and recrystallization of (010)-manifest LiFePO4-1 platelets (supernatant pH~4.91). Similarly, the relatively low concentration of PEG (0.4 g mL-1) and low pH value of residual supernatant (pH~4.12) account for the precipitation of (010)-defective LiFePO4-2 platelets with a large hollow interior, also shown as the insets of Figure 2c and d. The edges of each LiFePO4-1 platelet are irregular and its hollow interior is small, whereas the crystalline platelets of LiFePO4-2 are the comparatively smooth hexagons with a large hollow interior. Based on the degree of the selective etching and recrystallization, the (010)manifest platelets of LiFePO4-1 might be defined as “intermediates” of (010)-defective LiFePO42 crystallites. The morphological differences between LiFePO4-1 and LiFePO4-2 samples are in agreement with the corresponding XRD results that the estimated I(020)/I(200) ratio of LiFePO4-1 is bigger than that of LiFePO4-2. (FIG. 3) Considering the fact that additive citric acid exerts a great influence on the selective etching of the interior parts of (010)-manifest LiFePO4 particles, the previously observed (010)-manifest LiFePO4 micro-rhombohedra with a solid interior were obtained and defined as the target sample of LiFePO4-3 herein.25,28 And then, LiFePO4-3 sample was treated using a dilute solution of acetic acid (CH3COOH), resulting in the comparative sample LiFePO4-4. No matter the acidic etching was applied or not, XRD patterns of LiFePO4-3 and LiFePO4-4 coincide well with the standard data of orthorhombic LiFePO4 (JCPDS No. 83-2092) as shown in Figure 3. By comparison, the XRD peak intensities of LiFePO4-3 crystallites are stronger than those of their etched LiFePO4-4 ones, and the I(020)/I(200) ratio of the former (3.72) is bigger than that of the latter (2.55). Therefore, the decreasing I(020)/I(200) ratio could assure an involved selective etching



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of Lewis acids for the formation of (010)-defective LiFePO4 platelets in the presence of polymeric PEG. (FIG. 4) Figure 4a and b are the gradually magnified SEM images of LiFePO4-3 micro-rhombohedra with a solid interior obtained in the absence of any additives (3.9 ± 0.6 µm in length), and insets are the corresponding schematic drawing and TEM image of a representative (010)-manifest crystallite.25 After a 30-min treatment using the dilute aqueous solution of CH3COOH, irregular ravines, concaves and crater-like holes appear on the ac-planes of (010)-manifest LiFePO4-4 solid rhombohedra (3.8 ± 0.8 µm in length) as shown in Figure 4c, d and the insets. TEM image and corresponding SAED patterns of an acid-etched crystallite (Figure 4e, f) clearly interpret the single-crystalline nature of (010)-manifest LiFePO4 solid rhombohedra shown in Figure 4a-d, giving the marked diffraction spots of (200), (101) and (301) faces in [020] zone axis. These indicate that small-molecular organic acids (e.g., acetic and citric acids) may help to selectively dissolve the Li+ and/or Fe2+ ions of crystalline LiFePO4 along the direction, resulting in the so-called (010)-defective platelets.28 Especially, when the exposed {010} faces of LiFePO4 crystallites are “stabilized” by pendent PEG molecules, the selective etching and recrystallization interprets the morphological evolution from (010)-manifest LiFePO4-1 platelets with a rough surface and a small hollow interior to (010)-defective LiFePO4-2 platelets with a smooth skeleton and a large hollow interior shown in Figure 2. 3.2. The Sluggish Charge Transfer of (010)-Defective LiFePO4 Platelets After mixing with sucrose in ethanol, the above described LiFePO4 samples were separately heat-treated under an inert circumstance and the resulting LiFePO4/C composites were applied as


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the active substances to estimate the specific capacity of each working electrode. At a current density of 0.2 C, initial charge-discharge profiles of LiFePO4-1/C and LiFePO4-2/C electrodes were comparatively recorded, and their typical voltage plateaus are indicative of orthorhombic LiFePO4 crystals (Figure 5a).1 (FIG. 5) Generally, the voltage gap between the discharging and charging voltage plateaus can be defined as the parameter of electrode polarization to denote electrochemical irreversibility, and the bigger value of the voltage gap and the higher degree of the electrode polarization.31,32 Despite of the possibly positive effects of conductive acetylene black (10 wt%) and sucrosederived amorphous carbon (4.8 ± 0.2 wt%), the electrochemical irreversible degree of LiFePO4-1 (e.g., 65.4 mV) is lower than that of LiFePO4-2 (e.g., 104.6 mV), shown in the inset of Figure 5a. As shown in panels (a) and (b) of Figure S1, both of the irreversible degrees (i.e., LiFePO4-1, 283.5 mV; LiFePO4-2, 530.7 mV) increase when cycling at 0.4 C, comparatively indicating a concentration polarization originated from the insufficient diffusion of Li+ ions between electrode and electrolyte.33,34 Therefore, considering a large hollow interior to block Li-ion migration across, the concentration polarization of (010)-defective LiFePO4-2 should become more serious than that of (010)-defective LiFePO4-1 with small hollow interior (Fig. 5a). Figure 5b presents the cycling stabilities and corresponding Coulombic efficiencies (i.e., CEs) of LiFePO4-1/C and LiFePO4-2/C electrodes, and both of the (010)-defective platelets experience a slight increase of discharge capacity during initial 4 cycles and then suffer from a gradual decay of the reversible value thereafter. After the possibly initial activation the reversible capacity of LiFePO4-1/C is 165.4 mAh g-1 in the 5th cycle, while that of LiFePO4-2/C is 162.3



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mAh g-1. Over 100 charge-discharge cycles the retention ratios of reversible capacity are 96.2% (LiFePO4-1/C) and 94.9% (LiFePO4-2/C), respectively. In Figure 5b, the plots of discharge capacity against cycle number are visually “parallel”, and also, the comparative constant CEs of LiFePO4-1/C (99.2% in 2nd cycle and thereafter) and LiFePO4-2/C electrodes (96.4% in 30th cycle and thereafter) suggest that the volume of hollow interior determines the sluggish charge transfer for the reversible intercalation-deintercalation of Li+ ions. Rate performances of LiFePO4-1/C and LiFePO4-2/C electrodes are comparatively shown in Figure 5c, giving an initial discharge capacity of 153.8 and 144.7 mAh g-1 at 0.2 C, respectively. Normally, the observed reversible capacity of LiFePO4-1/C gradually decreases with the increasing current rate and then rapidly recovers when the current rate returns from 2.0 to 0.2 C. As for (010)-defective LiFePO4-2 platelets with a large hollow interior, the corresponding electrode of LiFePO4-2/C unexpectedly exhibits a sharp capacity fading at 0.4 C and even reaches zero at a higher current density of 0.8, 1.2, 1.6 or 2.0 C (Figure 5c). It should be mentioned that, as shown in panel (c) of Figure S1, this abnormal phenomenon is reproducible and can be reasonably assigned to the serious electrode polarization of LiFePO4-2/C. Both the macroscopic drawing of assembled electrode (left) and the atomic-scale drawing of active substance LiFePO4 (right) are shown in Figure 5d. This schematically indicates that, without the penetration of electrolyte, the hollow interior of a crystallite could effectively block the involved diffusion paths of Li+ ions along direction. Therefore, the relatively poor electrochemical properties of (010)-defective LiFePO4-2 (Figure 5a-c) may be explained using the 1D migration models of Li+ ions (Figure 5d) which can be confined by the relatively large hollow interior (Figure 2c, d). Actually, for these (010)-defective LiFePO4 samples, their relationships between structural (e.g., tap density or BET surface area and pore diameter shown


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in Figure S2) and electrochemical parameter (e.g., electrode polarization shown in Figure S1 or Li+-ion diffusion coefficient) could further confirm the 1D models, as summarized in Table S1. (FIG. 6) Nyquist plots of LiFePO4-1/C and LiFePO4-2/C electrodes operated at open circuit voltage are shown in Figure 6a, which can be described using a Randles equivalent circuit inserted therein. Both of the EIS spectra exhibit a semicircle in high and/or semi-high frequency region and a slop line in low frequency region. According to the theoretical model of equivalent circuit, parameter Re represents the resistance of electrolyte; Rf stands for the resistance of the surface film formed on electrodes; Rct designates the charge-transfer impedance; ZW is associated with Warburg impedance corresponding to the diffusion of Li+ ions into bulk materials; Qdl1 and Qdl2 corresponds to the constant phase element of SEI film and electrode/electrolyte interface, respectively.35 By analysis, the charge transfer resistances (i.e., the Rct values) of LiFePO4-1/C and LiFePO4-2/C are 573.1 and 799.2 Ω, respectively. This indicates that the charge transfer ability of (010)-defective LiFePO4-1 platelets is better than that of (010)-defective LiFePO4-2 rhombohedra for the reversible insertion/extraction of Li+ ions, coinciding well with their comparative electrochemical properties shown in Figure 5a-c. Furthermore, the diffusion coefficient of Li+ ions (DLi) can be derived according to the following equations:36,37 DLi = R2T2 / (2n4A2F4C2σ2)

(1)

Zre = Re + Rct + σω-0.5

(2)

Where R is the gas constant (8.314 J mol-1 K-1), T is the temperature (298.5 K), n is electron number for each molecule during Li-ion insertion, A is the area of the cathode/electrolyte



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interface (~1.130 cm2), F is the Faraday’s constant (96500 C mol-1) and C is the molar concentration of lithium ions (5.702 × 10-3 mol cm-3), σ is the Warburg impedance coefficient which has the relationship with Zre, and ω is the angular frequency in the low frequency range. The interdependence between Zre and the root square angular frequency ω-0.5 in the low frequency region is illustrated in Figure 6b. Therein, the slopes of the two fitted lines (i.e., the two σ values) are 353.40 and 479.25 Ω cm2 s-0.5, giving the DLi values of 6.83 × 10-15 and 3.71 × 10-15 cm2 s-1 for LiFePO4-1 and LiFePO4-2, respectively. According to literature reports,4,23,36,38 the theoretical 1D diffusion constant (i.e., the DLi value) is of the order of magnitude of 10-8 cm2 s-1, the measured DLi value of the naked LiFePO4 platelets with a solid interior is ca. 3.08 × 10-14 cm2 s-1, while that of LiFePO4/C composites is as high as 1.86 × 10-12 cm2 s-1. By contrast, the relatively low DLi values of (010)-defective platelets indicate a sluggish transport of Li+ ions within the olivine structures of LiFePO4-1 and LiFePO4-2. Generally, hollow or porous structure can improve the electrochemical performance of an active substance by increasing its close contact with electrolyte and by shorting the diffusion path of Li+ ions, facilitating Li+-ion diffusion within the electrode material.14,39-41 This is contrary to our experimental results that the electrochemical properties of LiFePO4-1/C electrode are better than those of LiFePO4-2/C electrode as shown in Figure 5 and 6, assigned to the more defective feature of LiFePO4-2 crystallites with a large hollow interior. Probably, this is one of the reasons why the tap density of an olivine-structured LiFePO4 electrode should be high for practical purposes. (FIG. 7)


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As an experimental control, as Li-ion battery cathodes the electrochemical properties of LiFePO4-3/C and LiFePO4-4/C electrodes are comparatively shown in Figure 7 and 8. At 0.2 C LiFePO4-3/C (i.e., (010)-manifest LiFePO4 micro-rhombohedra with a solid interior) electrode delivers the low initial discharge capacity of 98.7 mAh g-1 with a low CE of 76.9%, which needs a continuous “activation” to attain the specific value of 112.2 mAh g-1 at the 100th cycle (Figure 7a). Through the etching treatment using CH3COOH, at 0.2 C the corresponding LiFePO4-4/C (i.e., surface-etched LiFePO4 micro-rhombohedra) electrode initially exhibits the extremely low discharge capacity of 47.1 mAh g-1 with a relatively high CE of 95.4% and then reaches a reversible value of 65.1 mAh g-1 even after the 100-time “activation” (Figure 7a). Aside from the surface-etched deficiency, the generally decreased tap density (Table S1) and the “reversely” increased BET surface area or porous diameter (Figure S2) could be used to explain the relatively low discharge capacity (or CE) of LiFePO4-4/C in each cycle (Figure 7a). Rate performances of LiFePO4-3/C and LiFePO4-4/C electrodes are comparatively shown in Figure 7b, giving an initial discharge capacity of 110.1 and 50.3 mAh g-1 at 0.2 C, respectively. If the initial capacity of LiFePO4-3/C (i.e., 110.1 mAh g-1) is due to the relatively big average size of (010)-manifest LiFePO4-3 solid micro-rhombohedra, the lower value of LiFePO4-4/C (i.e., 50.3 mAh g-1) should relate to the surface-etched ac-planes of (010)-manifest LiFePO4 solid micro-rhombohedra (i.e., (010)-defective LiFePO4-4 crystallites). Furthermore, this is always the same for the comparison between LiFePO4-3/C and LiFePO4-4/C in every charge-discharge cycle at each current rate (Figure 7b). (FIG. 8)



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EIS results show that the charge transfer resistance Rct of LiFePO4-3/C is 260.2 Ω, smaller than that (i.e., 580.9 Ω) of LiFePO4-4/C (Figure 8a and the inset). By further analyses, the estimated DLi values are 5.90 × 10-15 and 3.05 × 10-15 cm2 s-1, corresponding to the σ values of 380.08 and 529.12 Ω cm2 s-0.5 for (010)-manifest LiFePO4-3 and (010)-defective LiFePO4-4, respectively (Figure 8b). Considering the selective (010)-face etching and the increasing BET surface area and pore diameter (Figure S2), the comparative results shown in Figure 7 and 8 surely demonstrate that the hollow interior of (010)-manifest LiFePO4 platelets could block the 1D migration of Li+ ions within the orthorhombic lattices as presented in Figure 5 and 6.

4. CONCLUSIONS In summary, a polymeric PEG-assisted hydrothermal method has been successfully used to prepare (010)-defective LiFePO4 platelets with a hollow interior and the variation of PEG concentration can modify the volume of the hollow interior through a possible formation mechanism of selective etching and recrystallization. (010)-defective LiFePO4 crystallites with a small hollow interior deliver the higher reversible capacity at each charge-discharge cycle than (010)-defective LiFePO4 crystallites with a large hollow interior. Simultaneously, the former possesses the smaller values of electrode polarization, charge transfer resistance and lithium-ion diffusion coefficient than the latter. These indicate that the hollow interior of (010)-manifest LiFePO4 platelets could block the 1D migration of lithium ions along direction. Also, these suggest that the preparation of (010)-manifest LiFePO4 crystallites without lattice defects may satisfy the high-rate demand for practical purposes.

ASSOCIATED CONTENT Supporting Information


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The structural and electrochemical parameters of various (010)-defective LiFePO4 samples; the initial charge-discharge profiles and cycling performances of LiFePO4-1/C and LiFePO4-2/C electrodes recorded at 0.2 and 0.4 C; and the N2 adsorption–desorption isotherms of various (010)-defective LiFePO4 samples. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGEMENTS Authors thank the financial support from the National Basic Research Program of China (2011CB935900).

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FIGURE CAPTIONS Figure 1. XRD patterns of (a) LiFePO4-1 and (b) LiFePO4-2 obtained in the presence of 0.8 and 0.4 g mL-1 PEG, respectively. The nether lines are the standard XRD data of orthorhombic LiFePO4 (JCPDS 83-2092). Figure 2. SEM images of (a, b) as-obtained LiFePO4-1 and (c, d) LiFePO4-2. In panel (a) or (b) inset is schematic drawing or TEM image of the representative (010)-defective LiFePO4 platelet with a small hollow interior, while in panel (c) or (d) inset is schematic drawing or TEM image of the representative (010)-defective LiFePO4 platelet with a large hollow interior. Figure 3. XRD patterns of (a) LiFePO4-3 and (b) LiFePO4-4 obtained before and after the acetic acid-induced surface etching, respectively. Figure 4. SEM images of (a, b) as-obtained LiFePO4-3 and (c, d) acid-etched LiFePO4-4 crystallites, and insets are the corresponding schematic drawings or TEM images of a representative (010)-manifest LiFePO4 crystallite before and after the acetic acid-induced surface etching, respectively. (e) TEM image and (f) corresponding SAED patterns of a representative LiFePO4-4 crystallite, indicating a single-crystalline nature for both the as-obtained and acidetched crystallites. Figure 5. (a) Initial charge-discharge profiles, (b) cycling and (c) rate performances of the LiFePO4-1/C and LiFePO4-2/C electrodes operated at various current rates. (d) One-dimensional migration models of Li+ ions.



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Figure 6. (a) EIS results and (b) the low-frequency relationships between Zre and w-0.5 of LiFePO4-1/C and LiFePO4-2/C electrodes operated at open circuit voltage within the frequency range from 100 kHz to 0.1 Hz. An inset in panel (a) is the Randles equivalent circuit to analyze the two EIS spectra. Figure 7. (a) Cycling and (b) rate performances of the LiFePO4-3/C and LiFePO4-4/C electrodes operated at various current rates. Figure 8. (a) EIS results and (b) the low-frequency relationships between Zre and w-0.5 of LiFePO4-3/C and LiFePO4-4/C electrodes operated at open circuit voltage within the frequency range from 100 kHz to 0.1 Hz. An inset in panel (a) is the Randles equivalent circuit to analyze the two EIS spectra.


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FIG. 1



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FIG. 2


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FIG. 3



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FIG. 4


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FIG. 5



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FIG. 6


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FIG. 7



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FIG. 8


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TABLE OF CONTENTS


Etching Preparation of (010)-Defective LiFePO4 Platelets to Visualize

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