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Controllable synthesis and morphology evolution of hierarchical LiFePO4 cathode materials for Li-ion batteries
Materials Characterization 157 (2019) 109927
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Controllable synthesis and morphology evolution of hierarchical LiFePO4 cathode materials for Li-ion batteries
T
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Liang Baoa,b, , Gang Xub, Meiyan Wangc a
College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hang Zhou 310018, China School of Materials Science and Engineering, Zhejiang University, Hang Zhou 310027, China c School of Landscape and Architecture, Zhejiang A&F University, Lin'an 311300, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li-ion batteries Nanocomposites Structural evolution
Hierarchical LiFePO4 crystals self-assembled with subunits were successfully synthesized via a simple hydrothermal method. The X-ray diffraction (XRD) curve indicates the highly crystalline nature of the prepared samples. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images reveal that with increased KOH concentrations, the LiFePO4 products evoluted from microspheres selfassembled with microwires to spindle-like structures self-assembled with micro-needles and eventually to flower-like hierarchical nanostructures self-assembled with nanorods. The crystal growth mechanism could be explained by the synergetic effect of pH value and the surface-adsorbed K+ ions. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. Moreover, charge–discharge curves and CV measurements demonstrated good reversible capacities and stable cycle performances of the LFP/C samples.
1. Introduction Olivine structured phosphate compounds, LiMPO4 (M = Fe, Mn, Co, etc.), have been identified as candidates of the most promising cathode substitutes for Li-ion batteries (LIBs) to replace the conventional LiCoO2 and LiMnO3 materials due to their outstanding thermal stability by strong PO4 covalent bonding and low toxicity. Among them, the LiFePO4 olivine could provide smooth voltage plateau (~3.4 V versus Li+/Li) and high theoretical capacity (170 mAh∙g−1), becoming a promising cathode material for Li-ion batteries [1,2]. However, the olivine LiFePO4 suffers from very low electronic conductivity and poor lithium ion diffusion kinetics, which could strongly block its further application. Over the past years, many attempts have been carried out to overcome the obstacles of the olivine LiFePO4, including coating with an conductive phase (carbon) [3–8], particle size narrowing [9–11], morphology control [10,12–14] and bivalent cation doping [15–17]. Among these practicable strategies, carbon coating is the most basic and important part, because it could instantaneously facilitate the electron mobility between LiFePO4 particles and the electrolyte. However, the effect of carbon coating is marginal and restricted, for it only functions when carbon phase sufficiently contacts active particles.
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Moreover, the ionic conductivity (ca. 10−11–10−10 S∙cm−1) of olivine LiFePO4 is much smaller than the electronic conductivity (> 10−9 S∙cm−1). Therefore, more emphasis should be placed on particle size narrowing [18]. Early in 2006, Delacourt and his co-workers [19] study on the size effects on LiFePO4 Powders, demonstrating that if the particles size were as small as 140 nm, high electrochemical energy density could also be achieved even without carbon. Afterwards, a similar particle size effect on LiMnPO4 was also demonstrated by D. Rangappa et al [20]. As the particle size decreases from 80 to 60, 20 nm, the discharge capacity enhances to the highest (ca. 156 mAh·g−1 at 0.1C). Moreover, considering that the Li ion diffusion takes one-dimensional channel of b-axis in the crystal structure of orthorhombic LiFePO4 framework, suppressing the crystal growth along [010] could further improve the rate performance [10,21–24]. In our previous work, monodispersed LiFePO4 [001] nanorods were synthesized via a simple hydrothermal method [25]. Since the size in [010] falls in nanoscale, the LiFePO4@C core-shell [001] nanorods express excellent reversible capacity and rate performance. Besides particle size narrowing, ensuring the monodispersity of the primary LiFePO4 particles seems to be effective for improving the electrochemical activity via adequately contacting with electrolyte. However, the high specific surface areas of the monodispersed
Corresponding author at: College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hang Zhou 310018, China. E-mail address: [email protected] (L. Bao).
https://doi.org/10.1016/j.matchar.2019.109927 Received 8 April 2019; Received in revised form 18 July 2019; Accepted 10 September 2019 Available online 11 September 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
Materials Characterization 157 (2019) 109927
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The electrochemical performances of the LiFePO4 samples were evaluated by assembling CR2025 coin-type cells. Before assembling, the LFP/C samples were vacuum dried at 110 °C for 12 h. The samples were then mixed with polyvinylidene fluoride (PVDF) and acetylene black in a weight ratio of 80: 10:10 by blending in N-methylpyrrolidone before brushed on an aluminum foil discs with a loading of ca. 2 mg/cm2. The dried Lithium foil was used as counter electrode, and LB 301 LiPF6 solution (Guotail huarong Zhangjiagang, China) was adopted as the electrolyte. The charge and discharge capacities were measured using a constant-current–constant-voltage (CC–CV) protocol at various rates with potentiostatic steps at the cutoff potential in a voltage range of 2.5–4.2 V (vs. Li+/Li) with a Newware BTS 5V1A system at room temperature. Cyclic voltammetry (CV) curves were depicted in a voltage range of 2.5–4.5 V with scanning rate of 0.1 mV·s−1, using a Chenhua CHI604D Electrochemical analyzer (Chenhua, Shanghai China). Electrochemical impedance spectroscopy (EIS) measurements were performed in an alternating current frequency range from 1 mHz to 1 MHz using the same electrochemical workstation.
nanoparticles would lead to low volumetric density, which would definitely hinder the future applications. Accordingly, fabricating hierarchical architectures are attractive for LIB cathodes [26–31]. The unique geometrical structures could express superior rate capability and reversible capacity upon the samples with basic structures. However, developing an effective route to control both the hierarchical architectures and the crystal growth direction (suppressing [010]) of LiFePO4 is still a challenge for conventional synthesis methods. Hydrothermal method has been taken as a flexible wet chemical route to obtain diverse inorganic functional nanocrystals for many years. It is facilely manipulated for synthesizing such hierarchical nanostructures via modulating test temperature, precursors concentration, addition, solvent, etc. Generally, agglomeration is caused by the interactions between colloidal particles and to subside the agglomeration rely on the repulsive potential. In our recently researching works, by adjusting KOH concentration, flower-like and stamen-like LiMnPO4 (with space group of Pmnb) nanostructures self-assembled with (010) nanosheets and [001] nanorods have been synthesized via a facile hydrothermal route [32,33]. However, the driving force for agglomeration or dispersion of LiFePO4 is unequal to LiMnPO4, the dispersant effect of KOH on the crystal formulation is quite different. Herein, based on our previously reported method in ref. 21, a facile one-step hydrothermal route to synthesize hierarchical LiFePO4 powders was proposed. The size and shape of the LiFePO4 crystals were modified by adopting different dosage of the additive KOH. The synergy of the absorption fuction of K+ cations attached to the crystal surfaces and the protected nucleation by the OH– ions make the as-prepared particles grow to a series of hierarchical self-assembly LiFePO4 micro/ nanostructures. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. The fundamental differences and relationship were discussed in detail. Moreover, the electrochemical performances of the synthesized samples with carbon coating were also investigated.
3. Results and discussion Fig. 1 presents the XRD patterns of the hydrothermally synthesized LiFePO4 samples at 200 °C for 16 h. In all the samples, all the diffraction peaks could be indexed to the pure orthorhombic LiFePO4 phase of Pmna with lattice parameters of a = 10.334 Å, b = 6.01 Å and c = 4.693 Å, agreeing well with the reported data of JCPDS No.83–2092. The sharp and strong diffraction peaks indicate the well crystallinity of the obtained LiFePO4 samples. Otherwise, the diffraction peaks become boarder and lower from LFP-a to LFP-c, indicating that the sub-grain sizes of the synthesized LiFePO4 particles decrease on the basis of Scherrer equation. Fig. 2 shows the SEM images of the as-prepared LiFePO4 products. All the samples present hierarchical self-assembly architectures, and the morphology of the crystals are determined by the concentration of additive potassium hydroxide during the hydrothermal treatment. The low-magnification SEM image in Fig. 2a clearly show that the LFP-a samples synthesized with low amount of KOH (0.336 g) exhibited microspheres with diameters of ca. 10 μm. Fig. 2b reveals that the microspheres were self-assembled with microwires. Fig. 2c shows that when the dosage of KOH increased to 0.392 g, the LFP-b sample exhibited spindle-like morphologies. Moreover, the spindle-like structure was constructed by needle-like subunits with diameters of ~100 nm, as depicted in Fig. 2d. When the amount of KOH was further increased to 0.448 g, the as-prepared LFP-c sample changed to flower-like architectures self-assembled with nanorod subunits, which were shown in Fig. 2e and f. Moreover, the nanorod subunits of LFP-c sample are of ~50 nm in diameter, which were thinner than the microwires of LFP-a
2. Experimental procedures All chemicals including lithium sulfate (Li2SO4·H2O), ferrous sulfate (FeSO4·7H2O), potassium hydroxide (KOH), potassium dihydrogen phosphate (KH2PO4), ethylene glycol (EG), ethanol and sucrose, used in this work were of analytical grade. The detailed procedures are as follows: 0.96 g Li2SO4·H2O, 0.68 g KH2PO4, a certain amount of KOH, and 1.39 g FeSO4·7H2O were dissolved in 40 mL EG/H2O (1:1 by volume) mixture solvent by magnetic stirring, forming a suspension as feedstock for the hydrothermal synthesis. The KOH was added by a series of 0.336 g, 0.392 g, and 0.448 g in prepare different LiFePO4 samples which were denoted as LFP-a, LFP-b, and LFP-c, respectively. After further stirring over 30 min, the suspension was transferred to a 50 mL Teflon-lined autoclave made by stainless steel. The molar ratio of PO43+, Li+, and Fe2+ was set as 1: 3: 1. Then, the sealed autoclave was heated to 200 °C for 16 h in an oven for hydrothermal reaction. The cooled LiFePO4 products were washed by distilled water and ethanol. The washed and dried LiFePO4 powders were then modified by carbon by mixed with sucrose at 600 °C for 4 h under N2 atmosphere. The final LiFePO4 samples were denoted as LFP/ C-a, LFP/C-b and LFP/C-c, respectively. X-ray diffraction (XRD) of Rigaku D/max-RA with Cu Kα (λ = 0.15418 nm) radiation with a step size of 0.02° was used to identify the phase composition of the samples. Field-emission scanning electron microscopy (FESEM) of Hitachi S-400 was adopted to observe the morphology. Thermogravimetric analysis (TG) was operated with a NETZSCH 209 thermal analyzer from room temperature to 700 °C at a heating rate of 10 °C min−1 in air. Raman spectra were acquired by a Renishaw 109907hk Microscopic Raman spectrometer with a laser wavelength of 532 μm.
Fig. 1. XRD patterns of hydrothermally synthesized (a) LFP-a, (b) LFP-b and (c) LFP-c. 2
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Fig. 2. SEM images of hydrothermally synthesized (a, b) LFP-a, (c, d) LFP-b and (e, f) LFP-c samples.
Fig. 3. (a)TEM image of LFP-c sample, and (b) HRTEM image of an individual nanorod of LFP-c.
transmission electron microscopy (TEM) images of LFP-C were taken, as shown in Fig. 3. It can be found that in Fig. 3a the nanorods have diameters of 30–50 nm and lengths with inhomogeneous distribution. Moreover, the nanorods overlap and intersect with each other, resulting in some kind of flower-like hierarchical nanostructures self-assembled with nanorod subunits. The HRTEM image shown in Fig. 3b was taken
and the needle-like subunits of LFP-b. These results indicate that the concentration of KOH in the hydrothermal system played an essential role in the LiFePO4 morphology forming process. However, due to the aggregation of the particles, specific characteristics of the LiFePO4 nanorods, including lengths were difficult to be determined in SEM image. Therefore, in order to confirm the morphology of the LFP-c, 3
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Fig. 4. Schematic illustration of the formation mechanism of the as-prepared LFP samples and the LMP samples in ref. [32].
multiplicated on account of the enhanced KOH concentration. Therefore, the mean size of the as-prepared particles decreased from LFP-a, LFP-b to LFP-c, and the accompanying subunits grow from sub-micron order to nanoscale, as Fig. 2 demonstrates. Moreover, as a kind of organic solvent, Ethylene glycol (EG) can be adsorbed along the crystal surfaces by hydrogen bonding with hydroxyls. Such adsorption yields a kinetic function of growth rate controlling and size dropping of the LiFePO4 crystals. At the same time, the crystal growth along [010] direction is suppressed due to the binding K+ ions, rendering LiFePO4 primary blocks to form rod-like architectures. Therefore, when more KOH was added into the hydrothermal system, more K+ ions are adhered onto the surfaces of LiFePO4. The crystal growth, as the oriented attachment polymerization (OAP) mechanism reveals, is restrained. This effect together with the enhanced nucleation by the increased OH– ions make the as-prepared particles and the accompanying subunits decreased in size from LFP-a, LFP-b to LFP-c. Moreover, when the dosage of KOH further increased to 0.5 g in the suspension feedstock, all the surfaces of the obtained LiFePO4 [001] nanorods are covered by positive K+ ions. Since the electrostatic repulsion, the synthesized LiFePO4 [001] nanorods display good monodispersity, as our previous work depicted [25]. Therefore, the major forming factor for the hierarchical morphology is the pH value working on the ever increasing nucleus under low KOH concentration. Then, the driving force become the additive KOH as dispersants to synthesize monodispersed nanorods with medium KOH dosage. Finally, the pH value dominates again via generating massive crystal nucleus, resulting in agglomerated LiFePO4 particles. To summarize the discussion above, the formation mechanism was simply depicted in Fig. 4. Whereas nanocrystallization can reduce the diffusion distance for both lithium ions and electrons within the bulk of the LiFePO4 cathode, it is prospective that the LFP/C-c sample would deliver more excellent electrochemical performances than LFP/C-a and LFP/C-b. Before electrochemical measurements, the as-prepared LiFePO4 samples were modified by a carbon-coating method, as the experimental procedures section depicted. Fig. 5 depicted the status of the LFP/C samples. The TG curves of the LFP/C samples confirm the absorption of carbon on the surfaces of the primary LiFePO4 samples, as Fig. 5a shows. The weighting process of 4.2% observed in the range of 300–500 °C in raw LFP should be indexed to the Fe2+ ions oxidization. Therefore, the percentage of coated carbon in LFP/C samples can be calculated to 5.5%, 6.5% and 8.5% for LFP/C-a, LFP/C-b and LFP/C-c, respectively. Since two peaks located at 1598 and 1345 cm−1 respectively reflect the
from an individual LiFePO4 nanorod presented in Fig. 3a. All the lattice fringes with equivalent interval run over the entire observed area and cross through the boundary, demonstrating that the primary LiFePO4 nanorod subunits are single crystals. The 0.30 and 0.23 nm intervals of the fringes agree well with the spacings of (020) and (002) planes, respectively. Since these two planes are both perpendicular and parallel to the axial orientation, the growth direction of the primary LiFePO4 nanorods can be inferred to be parallel to the c-axis, as the white arrow shows in Fig. 3b. Under hydrothermal environment, on the basis of formation mechanism [34], the crystal morphology is determined by lattice structure and outside conditions, such as additive, temperature, and even stir. In the hydrothermal process, when LiFePO4 species accumulate to a moderate concentration and grow larger than the critical dimension, nuclei turn up and gradually grow to LiFePO4 crystals by incorporating with additional species. In general, pH value is thought to be the most important factor, because agglomeration is usually inhibited by the electrostatic surface potential and strengthened by van der Waals interactions. In 2013, Christoph Neef et al. work on the morphology and agglomeration control of LiMnPO4 micro- and nanocrystals [35]. With the pH value raised from neutral to alkalescent, LiMnPO4 products change form dispersion to agglomeration, and to dispersion eventually. In our recently researching works with KH2PO4 as phosphorus source, the LiMnPO4 architecture vary from mono-dispersed nanobelts to mono-dispersed nanorods with enhanced KOH dosage [36]. Then, owing to the synergistic effect of the bound K+ ions on (010) planes and the preferential combination of SO42− ions on (100) planes, monodispersed LiFePO4 [001]-oriented nanorods were prepared [25]. Therefore, to manipulate the dispersity of olivine LiFePO4, addition of dispersants is also critical. It should be noticed that the surface redox potential for (010) facets of LiFePO4 (Pnma lattice group) is lower than that of LiMnPO4 [37], prohibiting the SO42− ions absorption on (010). Therefore, under the same KOH concentration, the LiFePO4 grows to flower-like nanostructures self-assembled with nanorod subunits while LiMnPO4 crystallizes to mono-dispersed single-crystalline nanobelts. Compared to LiFePO4, the pH value isoelectric point of LiMnPO4 is a little higher [38], making the dispersity behavior of LiFePO4 share tiny differences with LiMnPO4. Therefore, with relatively lower KOH dosage, monodispersed LiMnPO4 nanorods could be synthesized rather than LiFePO4. Since OH– and K+ concentrations both affect the coagulation process, with the promoting function of OH– radicals on oxide species formation [39], the quantities of LiFePO4 nuclei were 4
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Fig. 5. (a) TG and (b) raman profiles of LFP/C samples, and (C) HRTEM image of LFP/C-c sample.
Sevcik equation. Fig. 7 exhibits the electrochemical performances of the LFP/C samples under electrochemical window of 2.5–4.2 V. In Fig.7a, from the long flat plateaus observed in all the samples we can know that the charge-discharge processes are dominated by the two-phase reaction, LiFePO4 < − > (1-x) LiFePO4 + x FePO4 + x Li+ + x e−. The reversible capacity of LFP/C-c sample (150.7 mAh·g−1) is much larger than that of LFP/C-a (115.6 mAh·g−1) and LFP/C-b (93.9 mAh·g−1). Nonetheless, the LFP/C-c sample seems to possess the smallest separation between the charge and discharge plateau, indicating the weakest polarization. This result agrees well with the CV data. The superior electrochemical performance of LFP/C-c sample upon LFP/C-a and LFP/C-b sample is also demonstrated by the rate performance shown in Fig. 7b. When the rate was increased to 5C, discharge capacity of 73.8, 50.6 and 43.5 mAh·g−1 was detected for LFP/C-c, LFP/C-b and LFP/C-a, respectively. When the rate was back to 0.1C, the reversible capacities climbed to 148.5, 108.5 and 89.6 mAh·g−1, corresponding to 98.5, 93.9 and 95.4% of the initial performances for LFP/C-c, LFP/C-b and LFP/C-a, respectively, indicating that all the LFP/C samples maintain stable cycle performances. Fig.8c shows the long-time cycle performances of the LFP/C samples at 0.1C. After 100 cycles, the reversible capacities of LFP/C-c, LFP/C-b and LFP/C-a samples could still achieves 145.7, 113.1 and 92.6 mAh·g−1, corresponding to 96.7, 97.8 and 98.6% retention of the initial capacities of 150.7, 115.6 and 93.9 mAh·g−1, respectively. Fig. 8d expresses the corresponding Coulomb efficiency files. Except for the first cycle, the Coulombic efficiency of all samples corresponds to nearly 100%, further indicating the high electrochemical stability. The good discharge capacities and stable cycle performances of LFP/C-c can be attributed to the well-organized carbon coating, the nano-sized particle dimension and the consequent larger area of active materials with the electrolyte. Moreover, as the HRTEM image shown in Fig. 3b, the nanorods subunits of the nanostructures of LFP-c are of [001] direction, which means the Li-ion diffusion pathway of [010] is effectively suppressed and accordingly facilitated the Li-ion diffusion in the charge-discharge process. This inference was confirmed by the electrochemical impedance spectroscopy (EIS) spectra in Fig. 8. The radii of the quasi-semicircle located at the high and medium frequency of the LFP/C-c is much smaller than that of LFP/C-a and LFP/C-b. Whereas the charge-transfer resistance (Rct) was depicted by the hinder
G-band (ordered graphitic carbon) and D-band (disordered carbon) in the Raman spectrum (Fig. 5b), it can be understood that conductive carbon has been coated on LiFePO4 samples. Generally, the ratio of IG to ID represents the extent of graphitization of the carbon materials. The differentiation in the morphologies leads to the incongruous graphitization extent. For LFP/C-c sample, more carbon could be coated on the crystal surface due to the relatively bigger specific surface area. Therefore, LFP/C-c sample possesses a higher graphitizable extent than that of LFP/C-a and LFP/C-b. The morphology of LFP/C-c was characterized by HRTEM. As Fig. 5c shows, the thickness of the carbon film is ca. 2 nm. Fig. 6 presents the typical cyclic voltammetry curves of the LiFePO4@C samples measured at a scanning rate of 0.1 mV·s−1. Redox peaks at 3.62 and 3.26 V, 3.59 and 3.30 V, 3.54 and 3.34 V are observed for LFP/C-a, LFP/C-b and LFP/C-c, respectively, corresponding to the extraction and insertion of Li+ ions. The separation between the redox peaks of the LFP/C-c is slightly narrower than that of LFP/C-a and LFP/ C-b, indicating that the former suffered from weaker polarization. Moreover, the redox peaks of the LFP/C-c samples are higher and of larger area than that of the others, it can be inferred that the Li-ion diffusion in LFP/C-c is faster due to the smaller particle size and larger area of active materials with the electrolyte, according to the Randles-
Fig. 6. Typical cyclic voltammogram curves of LFP/C samples. 5
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Fig. 7. (a) charge-discharge curves and (b) rate performances of LFP/C samples, and (c) cycle performances of LFP/C-c sample.
assembled with nanorods. The synergy of the absorption fuction of K+ cations attached to the crystal surfaces and the protected nucleation by the OH– ions make the as-prepared particles grow to a series of hierarchical self-assembly LiFePO4 micro/nanostructures. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. Because the particle size falls into nanoscale and the nanorods subunits are of [001] direction, after carbon coating the flower-like hierarchical nanostructures express good reversible capacity and stable cycle performances. Considering the advantage of the conveniently manipulated hierarchical structures, our work provides a simple and efficient controllable synthesis route for micro−/nanostructure designing and a selection for the cathode material of lithium ion batteries.
part of the quasi-semicircle, it is reputed that the Li ions diffuse more easily in LFP/C-c sample. Fig. 8b presents the plot of Zre vs. the reciprocal root square of the lower angular frequencies (ω-1/2) for LFP/C samples. The low-frequency region of the EIS test indicates the surfacereaction-controlled electrochemical behavior and the mid-frequency region usually represents a Warburg resistance with semi-infinite diffusion controlled electrochemical behavior. Accordingly, the Li+ ion diffusion coefficient for LFP/C samples were calculated to be 1.7 × 10−14, 1.3 × 10−14 and 8.5 × 10−15 cm2·S−1 for LFP/C-a, LFP/ C-b and LFP/C-c, respectively, corresponding well with the morphology results. 4. Conclusion In summary, a facile one-step hydrothermal method was proposed for controllable synthesis of hierarchical LiFePO4 crystals self-assembled with subunits. With KOH concentration increasing, the LiFePO4 products changed from microspheres self-assembled with microwires to spindle-like structures self-assembled with micro-needles and eventually to flower-like hierarchical nanostructures self-
Acknowledgements This work is supported by the National Natural Science Foundation of China under grant no. 61274004, 51232006 and 51201037, and the
Fig. 8. (a) EIS spectra and (b) The plot of Zre vs. the reciprocal root square of the lower angular frequencies (ω-1/2) for LFP/C samples. 6
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Research Foundation of Hangzhou Dianzi University (KYS205618009).
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Controllable synthesis and morphology evolution of hierarchical LiFePO4 cathode materials for Li-ion batteries
T
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Liang Baoa,b, , Gang Xub, Meiyan Wangc a
College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hang Zhou 310018, China School of Materials Science and Engineering, Zhejiang University, Hang Zhou 310027, China c School of Landscape and Architecture, Zhejiang A&F University, Lin'an 311300, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Li-ion batteries Nanocomposites Structural evolution
Hierarchical LiFePO4 crystals self-assembled with subunits were successfully synthesized via a simple hydrothermal method. The X-ray diffraction (XRD) curve indicates the highly crystalline nature of the prepared samples. The field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images reveal that with increased KOH concentrations, the LiFePO4 products evoluted from microspheres selfassembled with microwires to spindle-like structures self-assembled with micro-needles and eventually to flower-like hierarchical nanostructures self-assembled with nanorods. The crystal growth mechanism could be explained by the synergetic effect of pH value and the surface-adsorbed K+ ions. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. Moreover, charge–discharge curves and CV measurements demonstrated good reversible capacities and stable cycle performances of the LFP/C samples.
1. Introduction Olivine structured phosphate compounds, LiMPO4 (M = Fe, Mn, Co, etc.), have been identified as candidates of the most promising cathode substitutes for Li-ion batteries (LIBs) to replace the conventional LiCoO2 and LiMnO3 materials due to their outstanding thermal stability by strong PO4 covalent bonding and low toxicity. Among them, the LiFePO4 olivine could provide smooth voltage plateau (~3.4 V versus Li+/Li) and high theoretical capacity (170 mAh∙g−1), becoming a promising cathode material for Li-ion batteries [1,2]. However, the olivine LiFePO4 suffers from very low electronic conductivity and poor lithium ion diffusion kinetics, which could strongly block its further application. Over the past years, many attempts have been carried out to overcome the obstacles of the olivine LiFePO4, including coating with an conductive phase (carbon) [3–8], particle size narrowing [9–11], morphology control [10,12–14] and bivalent cation doping [15–17]. Among these practicable strategies, carbon coating is the most basic and important part, because it could instantaneously facilitate the electron mobility between LiFePO4 particles and the electrolyte. However, the effect of carbon coating is marginal and restricted, for it only functions when carbon phase sufficiently contacts active particles.
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Moreover, the ionic conductivity (ca. 10−11–10−10 S∙cm−1) of olivine LiFePO4 is much smaller than the electronic conductivity (> 10−9 S∙cm−1). Therefore, more emphasis should be placed on particle size narrowing [18]. Early in 2006, Delacourt and his co-workers [19] study on the size effects on LiFePO4 Powders, demonstrating that if the particles size were as small as 140 nm, high electrochemical energy density could also be achieved even without carbon. Afterwards, a similar particle size effect on LiMnPO4 was also demonstrated by D. Rangappa et al [20]. As the particle size decreases from 80 to 60, 20 nm, the discharge capacity enhances to the highest (ca. 156 mAh·g−1 at 0.1C). Moreover, considering that the Li ion diffusion takes one-dimensional channel of b-axis in the crystal structure of orthorhombic LiFePO4 framework, suppressing the crystal growth along [010] could further improve the rate performance [10,21–24]. In our previous work, monodispersed LiFePO4 [001] nanorods were synthesized via a simple hydrothermal method [25]. Since the size in [010] falls in nanoscale, the LiFePO4@C core-shell [001] nanorods express excellent reversible capacity and rate performance. Besides particle size narrowing, ensuring the monodispersity of the primary LiFePO4 particles seems to be effective for improving the electrochemical activity via adequately contacting with electrolyte. However, the high specific surface areas of the monodispersed
Corresponding author at: College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hang Zhou 310018, China. E-mail address: [email protected] (L. Bao).
https://doi.org/10.1016/j.matchar.2019.109927 Received 8 April 2019; Received in revised form 18 July 2019; Accepted 10 September 2019 Available online 11 September 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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The electrochemical performances of the LiFePO4 samples were evaluated by assembling CR2025 coin-type cells. Before assembling, the LFP/C samples were vacuum dried at 110 °C for 12 h. The samples were then mixed with polyvinylidene fluoride (PVDF) and acetylene black in a weight ratio of 80: 10:10 by blending in N-methylpyrrolidone before brushed on an aluminum foil discs with a loading of ca. 2 mg/cm2. The dried Lithium foil was used as counter electrode, and LB 301 LiPF6 solution (Guotail huarong Zhangjiagang, China) was adopted as the electrolyte. The charge and discharge capacities were measured using a constant-current–constant-voltage (CC–CV) protocol at various rates with potentiostatic steps at the cutoff potential in a voltage range of 2.5–4.2 V (vs. Li+/Li) with a Newware BTS 5V1A system at room temperature. Cyclic voltammetry (CV) curves were depicted in a voltage range of 2.5–4.5 V with scanning rate of 0.1 mV·s−1, using a Chenhua CHI604D Electrochemical analyzer (Chenhua, Shanghai China). Electrochemical impedance spectroscopy (EIS) measurements were performed in an alternating current frequency range from 1 mHz to 1 MHz using the same electrochemical workstation.
nanoparticles would lead to low volumetric density, which would definitely hinder the future applications. Accordingly, fabricating hierarchical architectures are attractive for LIB cathodes [26–31]. The unique geometrical structures could express superior rate capability and reversible capacity upon the samples with basic structures. However, developing an effective route to control both the hierarchical architectures and the crystal growth direction (suppressing [010]) of LiFePO4 is still a challenge for conventional synthesis methods. Hydrothermal method has been taken as a flexible wet chemical route to obtain diverse inorganic functional nanocrystals for many years. It is facilely manipulated for synthesizing such hierarchical nanostructures via modulating test temperature, precursors concentration, addition, solvent, etc. Generally, agglomeration is caused by the interactions between colloidal particles and to subside the agglomeration rely on the repulsive potential. In our recently researching works, by adjusting KOH concentration, flower-like and stamen-like LiMnPO4 (with space group of Pmnb) nanostructures self-assembled with (010) nanosheets and [001] nanorods have been synthesized via a facile hydrothermal route [32,33]. However, the driving force for agglomeration or dispersion of LiFePO4 is unequal to LiMnPO4, the dispersant effect of KOH on the crystal formulation is quite different. Herein, based on our previously reported method in ref. 21, a facile one-step hydrothermal route to synthesize hierarchical LiFePO4 powders was proposed. The size and shape of the LiFePO4 crystals were modified by adopting different dosage of the additive KOH. The synergy of the absorption fuction of K+ cations attached to the crystal surfaces and the protected nucleation by the OH– ions make the as-prepared particles grow to a series of hierarchical self-assembly LiFePO4 micro/ nanostructures. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. The fundamental differences and relationship were discussed in detail. Moreover, the electrochemical performances of the synthesized samples with carbon coating were also investigated.
3. Results and discussion Fig. 1 presents the XRD patterns of the hydrothermally synthesized LiFePO4 samples at 200 °C for 16 h. In all the samples, all the diffraction peaks could be indexed to the pure orthorhombic LiFePO4 phase of Pmna with lattice parameters of a = 10.334 Å, b = 6.01 Å and c = 4.693 Å, agreeing well with the reported data of JCPDS No.83–2092. The sharp and strong diffraction peaks indicate the well crystallinity of the obtained LiFePO4 samples. Otherwise, the diffraction peaks become boarder and lower from LFP-a to LFP-c, indicating that the sub-grain sizes of the synthesized LiFePO4 particles decrease on the basis of Scherrer equation. Fig. 2 shows the SEM images of the as-prepared LiFePO4 products. All the samples present hierarchical self-assembly architectures, and the morphology of the crystals are determined by the concentration of additive potassium hydroxide during the hydrothermal treatment. The low-magnification SEM image in Fig. 2a clearly show that the LFP-a samples synthesized with low amount of KOH (0.336 g) exhibited microspheres with diameters of ca. 10 μm. Fig. 2b reveals that the microspheres were self-assembled with microwires. Fig. 2c shows that when the dosage of KOH increased to 0.392 g, the LFP-b sample exhibited spindle-like morphologies. Moreover, the spindle-like structure was constructed by needle-like subunits with diameters of ~100 nm, as depicted in Fig. 2d. When the amount of KOH was further increased to 0.448 g, the as-prepared LFP-c sample changed to flower-like architectures self-assembled with nanorod subunits, which were shown in Fig. 2e and f. Moreover, the nanorod subunits of LFP-c sample are of ~50 nm in diameter, which were thinner than the microwires of LFP-a
2. Experimental procedures All chemicals including lithium sulfate (Li2SO4·H2O), ferrous sulfate (FeSO4·7H2O), potassium hydroxide (KOH), potassium dihydrogen phosphate (KH2PO4), ethylene glycol (EG), ethanol and sucrose, used in this work were of analytical grade. The detailed procedures are as follows: 0.96 g Li2SO4·H2O, 0.68 g KH2PO4, a certain amount of KOH, and 1.39 g FeSO4·7H2O were dissolved in 40 mL EG/H2O (1:1 by volume) mixture solvent by magnetic stirring, forming a suspension as feedstock for the hydrothermal synthesis. The KOH was added by a series of 0.336 g, 0.392 g, and 0.448 g in prepare different LiFePO4 samples which were denoted as LFP-a, LFP-b, and LFP-c, respectively. After further stirring over 30 min, the suspension was transferred to a 50 mL Teflon-lined autoclave made by stainless steel. The molar ratio of PO43+, Li+, and Fe2+ was set as 1: 3: 1. Then, the sealed autoclave was heated to 200 °C for 16 h in an oven for hydrothermal reaction. The cooled LiFePO4 products were washed by distilled water and ethanol. The washed and dried LiFePO4 powders were then modified by carbon by mixed with sucrose at 600 °C for 4 h under N2 atmosphere. The final LiFePO4 samples were denoted as LFP/ C-a, LFP/C-b and LFP/C-c, respectively. X-ray diffraction (XRD) of Rigaku D/max-RA with Cu Kα (λ = 0.15418 nm) radiation with a step size of 0.02° was used to identify the phase composition of the samples. Field-emission scanning electron microscopy (FESEM) of Hitachi S-400 was adopted to observe the morphology. Thermogravimetric analysis (TG) was operated with a NETZSCH 209 thermal analyzer from room temperature to 700 °C at a heating rate of 10 °C min−1 in air. Raman spectra were acquired by a Renishaw 109907hk Microscopic Raman spectrometer with a laser wavelength of 532 μm.
Fig. 1. XRD patterns of hydrothermally synthesized (a) LFP-a, (b) LFP-b and (c) LFP-c. 2
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Fig. 2. SEM images of hydrothermally synthesized (a, b) LFP-a, (c, d) LFP-b and (e, f) LFP-c samples.
Fig. 3. (a)TEM image of LFP-c sample, and (b) HRTEM image of an individual nanorod of LFP-c.
transmission electron microscopy (TEM) images of LFP-C were taken, as shown in Fig. 3. It can be found that in Fig. 3a the nanorods have diameters of 30–50 nm and lengths with inhomogeneous distribution. Moreover, the nanorods overlap and intersect with each other, resulting in some kind of flower-like hierarchical nanostructures self-assembled with nanorod subunits. The HRTEM image shown in Fig. 3b was taken
and the needle-like subunits of LFP-b. These results indicate that the concentration of KOH in the hydrothermal system played an essential role in the LiFePO4 morphology forming process. However, due to the aggregation of the particles, specific characteristics of the LiFePO4 nanorods, including lengths were difficult to be determined in SEM image. Therefore, in order to confirm the morphology of the LFP-c, 3
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Fig. 4. Schematic illustration of the formation mechanism of the as-prepared LFP samples and the LMP samples in ref. [32].
multiplicated on account of the enhanced KOH concentration. Therefore, the mean size of the as-prepared particles decreased from LFP-a, LFP-b to LFP-c, and the accompanying subunits grow from sub-micron order to nanoscale, as Fig. 2 demonstrates. Moreover, as a kind of organic solvent, Ethylene glycol (EG) can be adsorbed along the crystal surfaces by hydrogen bonding with hydroxyls. Such adsorption yields a kinetic function of growth rate controlling and size dropping of the LiFePO4 crystals. At the same time, the crystal growth along [010] direction is suppressed due to the binding K+ ions, rendering LiFePO4 primary blocks to form rod-like architectures. Therefore, when more KOH was added into the hydrothermal system, more K+ ions are adhered onto the surfaces of LiFePO4. The crystal growth, as the oriented attachment polymerization (OAP) mechanism reveals, is restrained. This effect together with the enhanced nucleation by the increased OH– ions make the as-prepared particles and the accompanying subunits decreased in size from LFP-a, LFP-b to LFP-c. Moreover, when the dosage of KOH further increased to 0.5 g in the suspension feedstock, all the surfaces of the obtained LiFePO4 [001] nanorods are covered by positive K+ ions. Since the electrostatic repulsion, the synthesized LiFePO4 [001] nanorods display good monodispersity, as our previous work depicted [25]. Therefore, the major forming factor for the hierarchical morphology is the pH value working on the ever increasing nucleus under low KOH concentration. Then, the driving force become the additive KOH as dispersants to synthesize monodispersed nanorods with medium KOH dosage. Finally, the pH value dominates again via generating massive crystal nucleus, resulting in agglomerated LiFePO4 particles. To summarize the discussion above, the formation mechanism was simply depicted in Fig. 4. Whereas nanocrystallization can reduce the diffusion distance for both lithium ions and electrons within the bulk of the LiFePO4 cathode, it is prospective that the LFP/C-c sample would deliver more excellent electrochemical performances than LFP/C-a and LFP/C-b. Before electrochemical measurements, the as-prepared LiFePO4 samples were modified by a carbon-coating method, as the experimental procedures section depicted. Fig. 5 depicted the status of the LFP/C samples. The TG curves of the LFP/C samples confirm the absorption of carbon on the surfaces of the primary LiFePO4 samples, as Fig. 5a shows. The weighting process of 4.2% observed in the range of 300–500 °C in raw LFP should be indexed to the Fe2+ ions oxidization. Therefore, the percentage of coated carbon in LFP/C samples can be calculated to 5.5%, 6.5% and 8.5% for LFP/C-a, LFP/C-b and LFP/C-c, respectively. Since two peaks located at 1598 and 1345 cm−1 respectively reflect the
from an individual LiFePO4 nanorod presented in Fig. 3a. All the lattice fringes with equivalent interval run over the entire observed area and cross through the boundary, demonstrating that the primary LiFePO4 nanorod subunits are single crystals. The 0.30 and 0.23 nm intervals of the fringes agree well with the spacings of (020) and (002) planes, respectively. Since these two planes are both perpendicular and parallel to the axial orientation, the growth direction of the primary LiFePO4 nanorods can be inferred to be parallel to the c-axis, as the white arrow shows in Fig. 3b. Under hydrothermal environment, on the basis of formation mechanism [34], the crystal morphology is determined by lattice structure and outside conditions, such as additive, temperature, and even stir. In the hydrothermal process, when LiFePO4 species accumulate to a moderate concentration and grow larger than the critical dimension, nuclei turn up and gradually grow to LiFePO4 crystals by incorporating with additional species. In general, pH value is thought to be the most important factor, because agglomeration is usually inhibited by the electrostatic surface potential and strengthened by van der Waals interactions. In 2013, Christoph Neef et al. work on the morphology and agglomeration control of LiMnPO4 micro- and nanocrystals [35]. With the pH value raised from neutral to alkalescent, LiMnPO4 products change form dispersion to agglomeration, and to dispersion eventually. In our recently researching works with KH2PO4 as phosphorus source, the LiMnPO4 architecture vary from mono-dispersed nanobelts to mono-dispersed nanorods with enhanced KOH dosage [36]. Then, owing to the synergistic effect of the bound K+ ions on (010) planes and the preferential combination of SO42− ions on (100) planes, monodispersed LiFePO4 [001]-oriented nanorods were prepared [25]. Therefore, to manipulate the dispersity of olivine LiFePO4, addition of dispersants is also critical. It should be noticed that the surface redox potential for (010) facets of LiFePO4 (Pnma lattice group) is lower than that of LiMnPO4 [37], prohibiting the SO42− ions absorption on (010). Therefore, under the same KOH concentration, the LiFePO4 grows to flower-like nanostructures self-assembled with nanorod subunits while LiMnPO4 crystallizes to mono-dispersed single-crystalline nanobelts. Compared to LiFePO4, the pH value isoelectric point of LiMnPO4 is a little higher [38], making the dispersity behavior of LiFePO4 share tiny differences with LiMnPO4. Therefore, with relatively lower KOH dosage, monodispersed LiMnPO4 nanorods could be synthesized rather than LiFePO4. Since OH– and K+ concentrations both affect the coagulation process, with the promoting function of OH– radicals on oxide species formation [39], the quantities of LiFePO4 nuclei were 4
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Fig. 5. (a) TG and (b) raman profiles of LFP/C samples, and (C) HRTEM image of LFP/C-c sample.
Sevcik equation. Fig. 7 exhibits the electrochemical performances of the LFP/C samples under electrochemical window of 2.5–4.2 V. In Fig.7a, from the long flat plateaus observed in all the samples we can know that the charge-discharge processes are dominated by the two-phase reaction, LiFePO4 < − > (1-x) LiFePO4 + x FePO4 + x Li+ + x e−. The reversible capacity of LFP/C-c sample (150.7 mAh·g−1) is much larger than that of LFP/C-a (115.6 mAh·g−1) and LFP/C-b (93.9 mAh·g−1). Nonetheless, the LFP/C-c sample seems to possess the smallest separation between the charge and discharge plateau, indicating the weakest polarization. This result agrees well with the CV data. The superior electrochemical performance of LFP/C-c sample upon LFP/C-a and LFP/C-b sample is also demonstrated by the rate performance shown in Fig. 7b. When the rate was increased to 5C, discharge capacity of 73.8, 50.6 and 43.5 mAh·g−1 was detected for LFP/C-c, LFP/C-b and LFP/C-a, respectively. When the rate was back to 0.1C, the reversible capacities climbed to 148.5, 108.5 and 89.6 mAh·g−1, corresponding to 98.5, 93.9 and 95.4% of the initial performances for LFP/C-c, LFP/C-b and LFP/C-a, respectively, indicating that all the LFP/C samples maintain stable cycle performances. Fig.8c shows the long-time cycle performances of the LFP/C samples at 0.1C. After 100 cycles, the reversible capacities of LFP/C-c, LFP/C-b and LFP/C-a samples could still achieves 145.7, 113.1 and 92.6 mAh·g−1, corresponding to 96.7, 97.8 and 98.6% retention of the initial capacities of 150.7, 115.6 and 93.9 mAh·g−1, respectively. Fig. 8d expresses the corresponding Coulomb efficiency files. Except for the first cycle, the Coulombic efficiency of all samples corresponds to nearly 100%, further indicating the high electrochemical stability. The good discharge capacities and stable cycle performances of LFP/C-c can be attributed to the well-organized carbon coating, the nano-sized particle dimension and the consequent larger area of active materials with the electrolyte. Moreover, as the HRTEM image shown in Fig. 3b, the nanorods subunits of the nanostructures of LFP-c are of [001] direction, which means the Li-ion diffusion pathway of [010] is effectively suppressed and accordingly facilitated the Li-ion diffusion in the charge-discharge process. This inference was confirmed by the electrochemical impedance spectroscopy (EIS) spectra in Fig. 8. The radii of the quasi-semicircle located at the high and medium frequency of the LFP/C-c is much smaller than that of LFP/C-a and LFP/C-b. Whereas the charge-transfer resistance (Rct) was depicted by the hinder
G-band (ordered graphitic carbon) and D-band (disordered carbon) in the Raman spectrum (Fig. 5b), it can be understood that conductive carbon has been coated on LiFePO4 samples. Generally, the ratio of IG to ID represents the extent of graphitization of the carbon materials. The differentiation in the morphologies leads to the incongruous graphitization extent. For LFP/C-c sample, more carbon could be coated on the crystal surface due to the relatively bigger specific surface area. Therefore, LFP/C-c sample possesses a higher graphitizable extent than that of LFP/C-a and LFP/C-b. The morphology of LFP/C-c was characterized by HRTEM. As Fig. 5c shows, the thickness of the carbon film is ca. 2 nm. Fig. 6 presents the typical cyclic voltammetry curves of the LiFePO4@C samples measured at a scanning rate of 0.1 mV·s−1. Redox peaks at 3.62 and 3.26 V, 3.59 and 3.30 V, 3.54 and 3.34 V are observed for LFP/C-a, LFP/C-b and LFP/C-c, respectively, corresponding to the extraction and insertion of Li+ ions. The separation between the redox peaks of the LFP/C-c is slightly narrower than that of LFP/C-a and LFP/ C-b, indicating that the former suffered from weaker polarization. Moreover, the redox peaks of the LFP/C-c samples are higher and of larger area than that of the others, it can be inferred that the Li-ion diffusion in LFP/C-c is faster due to the smaller particle size and larger area of active materials with the electrolyte, according to the Randles-
Fig. 6. Typical cyclic voltammogram curves of LFP/C samples. 5
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Fig. 7. (a) charge-discharge curves and (b) rate performances of LFP/C samples, and (c) cycle performances of LFP/C-c sample.
assembled with nanorods. The synergy of the absorption fuction of K+ cations attached to the crystal surfaces and the protected nucleation by the OH– ions make the as-prepared particles grow to a series of hierarchical self-assembly LiFePO4 micro/nanostructures. The OH– anions represent pH value dominate the hierarchical morphology under both low and high KOH concentration, while K+ ions play the major role in synthesizing monodispersed LiFePO4 nanorods under medium KOH concentration. Because the particle size falls into nanoscale and the nanorods subunits are of [001] direction, after carbon coating the flower-like hierarchical nanostructures express good reversible capacity and stable cycle performances. Considering the advantage of the conveniently manipulated hierarchical structures, our work provides a simple and efficient controllable synthesis route for micro−/nanostructure designing and a selection for the cathode material of lithium ion batteries.
part of the quasi-semicircle, it is reputed that the Li ions diffuse more easily in LFP/C-c sample. Fig. 8b presents the plot of Zre vs. the reciprocal root square of the lower angular frequencies (ω-1/2) for LFP/C samples. The low-frequency region of the EIS test indicates the surfacereaction-controlled electrochemical behavior and the mid-frequency region usually represents a Warburg resistance with semi-infinite diffusion controlled electrochemical behavior. Accordingly, the Li+ ion diffusion coefficient for LFP/C samples were calculated to be 1.7 × 10−14, 1.3 × 10−14 and 8.5 × 10−15 cm2·S−1 for LFP/C-a, LFP/ C-b and LFP/C-c, respectively, corresponding well with the morphology results. 4. Conclusion In summary, a facile one-step hydrothermal method was proposed for controllable synthesis of hierarchical LiFePO4 crystals self-assembled with subunits. With KOH concentration increasing, the LiFePO4 products changed from microspheres self-assembled with microwires to spindle-like structures self-assembled with micro-needles and eventually to flower-like hierarchical nanostructures self-
Acknowledgements This work is supported by the National Natural Science Foundation of China under grant no. 61274004, 51232006 and 51201037, and the
Fig. 8. (a) EIS spectra and (b) The plot of Zre vs. the reciprocal root square of the lower angular frequencies (ω-1/2) for LFP/C samples. 6
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Research Foundation of Hangzhou Dianzi University (KYS205618009).
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