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Design of Li2FeSiO4 cathode material for enhanced lithium-ion storage performance
Chemical Engineering Journal 379 (2020) 122329
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Design of Li2FeSiO4 cathode material for enhanced lithium-ion storage performance ⁎
Hailong Qiua, Di Jina, Chunzhong Wanga, Gang Chena, Lei Wangb, Huijuan Yuec, , Dong Zhanga,
T ⁎
a
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China Key Laboratory of Eco-chemical Engineering (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
H I GH L IG H T S
/C/Cu/Li PO composites were synthesized by a sol-gel method. • LiLi FeSiO FeSiO /C is modified by electronic conductor and ionic conductor. • LFS-4 exhibits enhanced rate capability and superior cycle performance. • 2
4
2
4
3
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium ion battery Li2FeSiO4 Multi-functional engineering Enhanced electrochemical performance
The practical use of Li2FeSiO4 (LFS) is largely restricted by its inherent low electronic conductivity and slow lithium ions diffusion rate. Carbon (C) coating has been widely used to improve the electrochemical performance of LFS, but it can reduce the tap density and block lithium ion transmission. Herein, we design and synthesize the LFS/C/Cu/Li3PO4 composites with elaborate structure that the elemental Cu is encapsulated by LFS particles and Li3PO4 distributes both inside LFS particles and the surface carbon layer. The as-prepared samples with the unique structure depict enhanced electronic/ionic conductivities and kinetic properties due to the synergistic effect of Li3PO4 and Cu. LFS-4, which contains 2 wt% Cu and 2.42 wt% Li3PO4, exhibits the best rate performance with an average discharge specific capacity of 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C rate (1 C = 165.6 mA g−1). Over 800 cycles, a capacity of 116.2 mAh g−1 is maintained under the high current density of 10 C with the capacity retention of 88.9%. The study indicates that the rational design of hybrid material with multi-functional engineering possesses important significance in cathode materials for lithium-ion batteries.
1. Introduction Li2FeSiO4 (LFS) is believed to be one of the most promising cathode for the next-generation of high energy–density rechargeable batteries due to the high theoretical specific capacity of 332 mAh g−1 and many other advantages, such as low cost, environment friendly and high thermal stability [1–5]. However, low electronic conductivity and slow lithium ions diffusion rate of LFS severely limit its actual electrochemical performance [6–9]. Extensive efforts have been devoted to addressing these problems by constructing LFS and carbon-based materials to hybrid composites [10–18]. Zhang synthesized high performance LFS/C composite with reduced graphene oxide and glucose as carbon sources, achieving an initial discharge capacity of 178 and
⁎
119 mAh g−1 at 0.1 and 2 C [19]. Wang et al. designed a carbon nanotube-directed three-dimensional porous Li2FeSiO4/C composite, affording enhanced rate performance with capacities of 214, 158, 129, 103 and 79 mAh g−1 at rates of 0.2, 0.5, 1, 2 and 5 C, respectively [20]. Although these LFS/C hybrids have shown improved electrochemical performance during cycles, the simplistic modification will pose negative effects and limitations: 1) The electron conductivity of amorphous carbon (1.25–2 × 103 S cm−1) is relatively low. 2) The lithium ions diffusion is limited to some extent because carbon is not a good ionic conductor [21]. 3) Because the carbon-based materials usually modify the outer surface of the LFS particles rather than the reconstruction of internal practical, the reaction will terminate and will not continue to consume the active material inside the particles when the diffusion rate
Corresponding authors. E-mail addresses: [email protected] (H. Yue), [email protected] (D. Zhang).
https://doi.org/10.1016/j.cej.2019.122329 Received 29 April 2019; Received in revised form 19 June 2019; Accepted 23 July 2019 Available online 24 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
Chemical Engineering Journal 379 (2020) 122329
H. Qiu, et al.
2. Experimental section
binder (10 wt%) to obtain a homogeneous slurry. Then, the slurry was coated onto Al foil and dried at 120 °C in a vacuum for 12 h. The cathode was cut into 0.8 × 0.8 cm2 pieces wafers with average active masses of about 1–2 mg. Electrochemical tests were carried out by 2032 coin cells on LAND 2100 battery test system in 1 M LiPF6 electrolyte solution dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC/DEC = 1:1, v/v) with lithium foil as counter electrode and Celgard 2320 membrane as a separator. The cells were assembled in an argon-filled glove box with water and oxygen content lower than 1 ppm. Galvanostatic charge-discharge tests were conducted in the voltage window of 1.5–4.5 V versus Li/Li+ at room temperature. The charge/discharge specific capacities were calculated on the mass of the active materials only removing the carbon content. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were measured to study the electrochemical kinetics performance of LFS/C/Cu/Li3PO4 composites by using a VSP and MPG multichannel potentiostatic-galvanostatic system (Biologic France). The EIS data was recorded in the frequency range between1 MHz and 1 mHz.
2.1. Synthesis of Cu3(PO4)2
3. Results and discussion
First, Cu(NO3)2·3H2O (3 mmol) and (NH4)2HPO4 (2 mmol) were dissolved in 15 mL deionized water. Then, (NH4)2HPO4 solution was added dropwise to Cu(NO3)2·3H2O solution and stirred for another 0.5 h. The solvent was evaporated and dried at 100 °C. The dried powder was calcined in air at 900 °C (5 °C min−1) for 10 h and naturally cooled to obtain Cu3(PO4)2.
XRD was used to determine the phase composition and crystal structure of the prepared samples. As shown in Fig. S1, it can be seen that Cu3(PO4)2 is pure phase. SEM further reveals that it consists of large particles with a particle size between 1.5 and 4 μm (inset of Fig. S1a). The XRD patterns of LFS-x (x = 0, 2, 4, 6 and 20) are shown in Fig. 1. All the diffraction peaks of LFS-0 belong to monoclinic structure LFS with a P21/n space group [23,24]. For LFS-x (x = 2, 4, 6 and 20) obtained by adding different amount of Cu3(PO4)2, although the diffraction peaks corresponding to LFS main phase do not change, several new diffraction peaks corresponding to Cu0 are observed. Furthermore, with the increased amount of Cu3(PO4)2, the diffraction peaks intensity of Cu gradually enhances, indicating that the relative content of Cu increases. In addition, when the amount of Cu3(PO4)2 increases to 20 wt %, Li3PO4 is also found in the obtained sample (LFS-20) (Fig. 1b). It should be pointed out that the diffraction peaks corresponding to Li3PO4 are not obviously observed in the LFS-x (x = 2, 4 and 6), which probably is due to the relatively low content and degree of crystallization in these materials. Moreover, the diffraction peaks of LFS-x (x = 2, 4, 6, and 20) do not shift from the original LFS-0, which concludes that the addition of Cu3(PO4)2 does not significantly affect the crystal structure of LFS. In order to study the formation mechanism of Li3PO4 and Cu, firstly, the thermal stability of Cu3(PO4)2 under N2 atmosphere was tested. Fig. S2 shows that Cu3(PO4)2 does not decompose and is quite stable under N2 condition even heated up to 800 °C. Then, the reaction of Cu3(PO4)2 with P123 and LiAc·2H2O heated at 650 °C for 10 h under N2 was studied. As shown in Fig. S3, the final product consists of Li3PO4 and Cu, suggesting that Cu is derived from the reduction of Cu2+ in the heat treatment process, and Li3PO4 is the reactant between PO43− and Li+ in the raw materials. Thus, the chemical reaction equation for the formation of the LFS/C/Cu/Li3PO4 composites is as follows:
of lithium ions cannot maintain the electrode reaction, which will reduce the utilization of the active material and affect the cycle performance [22]. To summarize, LFS-based cathode should not only be highly conductive to improve electrical conductivity and the lithium ion diffusion rate, but also enable high active material utilization by the rearrangement of inner surface of the particles. Therefore, the welldesigned structure of LFS matrix with strong electronic/ionic conductivity is crucially important. In this work, we report LFS/C/Cu/Li3PO4 composites using a sol-gel method. The hybrid material combines several advantages: 1) carbon layer on the surface of LFS particles improves electronic conductivity. 2) Cu, which is encapsulated inside the active material LFS particles, enhances the utilization of active material and further increases the electronic conductivity. 3) Li3PO4 located in the surface and the interior of the LFS particles plays an essential role in enhancing the ionic conductivity. As respected, the hybrid material exhibits excellent electrochemical properties.
2.2. Synthesis of LFS/C/Cu/Li3PO4 composites The LFS/C/Cu/Li3PO4 composites are synthesized by a sol-gel method, details of which are as follows. First, a certain amount of polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) was dispersed into 20 mL ethanol. Then, Ethylsilicate (TEOS) (4 mmol), LiAc·2H2O (4 mmol) and Fe (NO3)3·9H2O (4 mmol) as well as a certain amount of Cu3(PO4)2 (2 wt%: 0.013 g, 4 wt%: 0.026 g, 6 wt%: 0.039 g, 20 wt%: 0.13 g) were added into the above solution under magnetic stirring in sequence. The amount of Cu3(PO4)2 is the weight ratio to the theoretical yield of LFS. After stirred for 2 h, the obtained solution was evaporated at 70 °C and dried at 100 °C. The dried precursor was sintered under N2 atmosphere at 650 °C (5 °C min−1) for 10 h to obtain the final product LFS/C/Cu/Li3PO4. In this work, the final products obtained by adding 0%, 2 wt%, 4 wt%, 6 wt% and 20 wt% of Cu3(PO4)2 are denoted as LFS-0, LFS-2, LFS-4, LFS-6, and LFS-20, respectively. 2.3. Materials characterization The morphology and elemental mapping of the samples were studied by field-emission scanning electron microscope (FESEM; HITACHI SU8020) and transmission electron microscope (TEM; FEI Tecnai G2). X-ray diffraction (XRD; Bruker AXS D8 powder diffractometer) was used to determine the crystal structure of the products operated at a voltage of 40 kV and a current of 100 mA using Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) investigation was carried out on a VG scientific ESCALAB 250 spectrometer. Raman scattering spectra were conducted on a Labram HR Evolution Horiba using laser excitation at 532 nm. The carbon content was determined by CHNSO elemental analyzer (Elementar Varon EL cube type). TGA (SDTQ600) was used to examine the stability of Cu3(PO4)2 in N2 atmosphere with a temperature ramp of 10 °C min−1.
xCu3 (PO4 )2 + Fe3+ + 2Li+ + TEOS + P123 → 3xCu + Li2−6x FeSiO4 + 2xLi3 PO4 + C+ H2 O↑ + CO2 ↑ From the above equation, it can be found that the generated LFS is non-stoichiometric with lithium vacancies, which is favorable for Li+ transmission [25]. Additionally, the amounts of Cu and Li3PO4 are relative to the LFS mass ratio, for instance, 2 wt% Cu3(PO4)2 can produce about 1 wt% Cu and 1.21 wt% Li3PO4. The presence of carbon can be confirmed by Raman spectroscopy and elemental analysis. Fig. 1c shows the Raman spectra of LFS-x (x = 0, 2, 4, and 6). The two diffraction peaks of the Raman spectra locate at 1355 and 1596 cm−1, corresponding to the D and G band
2.4. Electrochemical test The work electrode was prepared by mixing the LFS/C/Cu/Li3PO4 composites (70 wt%), Super P (20 wt%) and poly(vinylidene fluoride) 2
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Fig. 1. (a, b) XRD patterns of LFS-x (x = 0, 2, 4, 6 and 20). (c) Raman spectrum of LFS-x (x = 0, 2, 4 and 6). SEM images of (d) LFS-0, (e) LFS-2, (f) LFS-4 and (g) LFS6. (h, i) TEM and HRTEM images of LFS-4. The inset in (h) shows the SAED pattern of LFS-4 and the inset in (i) shows the enlarged view of the rectangular.
Fig. 2. XPS spectra of (a) Cu and (b) P element in LFS-4.
better conductivity. The carbon contents of LFS-0, LFS-2, LFS-4 and LFS-6 samples are 17.53 wt%, 17.31 wt%, 17.14 wt% and 16.85 wt% by CHNSO elemental analyzer. SEM was used to observe the effect of Cu3(PO4)2 on the microstructure of the composites. As shown in Fig. 1d-g, LFS-0 consists of nanoparticles. After adding different amounts of Cu3(PO4)2, the microstructure of the obtained composites is remained, suggesting that Cu3(PO4)2 does not affect the microstructure of the final products. EDS mapping shows the distribution of Fe, Si, O, Cu and P elements in LFS-4 (Fig. S4), indicating that Cu and Li3PO4 are not uniformly distributed in
characteristic peaks of the carbon, respectively. Among them, the D peak represents the disordered vibration of the sp3 type hybrid carbon atom (disordered carbon), and the G peak represents the stretching vibration in-plane of the sp2 type hybrid carbon atom (graphitized carbon). ID/IG represents the degree of graphitization of the carbon materials, the smaller the ID/IG, the higher the degree of graphitization of carbon [26,27]. For LFS-0, LFS-2, LFS-4 and LFS-6, the values of ID/IG are 0.907, 0.879, 0.853 and 0.876, respectively. Compared with LFS-0, the ID/IG of LFS-2, LFS-4 and LFS-6 are slightly reduced, indicating that they are coated by carbon with higher degree of graphitization and 3
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performance of LFS, four samples of LFS-x (x = 0, 2, 4 and 6) were subjected to galvanostatic charge–discharge test (1 C = 165 mA g−1) in a voltage range from 1.5 to 4.5 V. Fig. S6 shows the charge and discharge curves of the four samples at 0.1 C, the shapes of which are similar, indicating that the electrochemical reactions are the same during the Li ion insertion and deinsertion process. The same phenomenon is observed at 1 C (Fig. 3b). Among all samples, LFS-4 shows the highest discharge specific capacity of 180.7 mAh g−1, whereas, the discharge capacities of LFS-0, LFS-2 and LFS-6 are 156.5, 169.4 and 157.2 mAh g−1. Furthermore, LFS-4 exhibits the best rate capability from 1 to 40 C (Fig. 3c), remaining an average discharge specific capacity of 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C rate, respectively, which are higher than those reported LFS cathode materials (Table S1, Supporting Information). On the contrary, the average discharge specific capacity of LFS-0 at different rates is 147.5, 130.1, 104.4, 72.7, 46.5 and 33.6 mAh g−1. Obviously, it can be found that Cu and Li3PO4 significantly enhance the electrochemical performance of LFS at the high rates. As is well known, Cu and Li3PO4 (like carbon) are electrochemical inactive, overdosage will reduce the utilization of active material. Therefore, the electrochemical performance of as-prepared samples did not increase linearly with the increase of Cu and Li3PO4. The cycle performance of LFS-0 and LFS-4 is presented in Fig. 3d-f. As can be seen, although LFS-0 and LFS-4 have similar capacity retention rate at 1 and 5 C rates, LFS-4 shows a higher specific capacity than LFS-0, still delivering 136.5 and 116.2 mAh g−1 after long cycles at 1 and 5 C rates (128.4 and 89.0 mAh g−1 for LFS-0). At higher rate of 10 C, the discharge specific capacity of LFS-4 is 116.2 mAh g−1 with the capacity retention of 88.9%, however, the discharge specific capacity of LFS-0 is only 69.3 mAh g−1 after 800 cycles. Fig. S7 further give the effects of Cu and Li3PO4 on the capacity and cycle performance of LFS electrode materials at 20 C high rates, the discharge capacity of LFS-4 is 47.8 mAh g−1, which is twice that of LFS-0 after 3000 cycles at 20 C. In terms of capacity retention, LFS-4 is 60.1%, while LFS-0 is only 38.5%. It should be pointed out that we chose the discharge specific capacity of the 100th cycle as the initial capacity to calculate the capacity retention.
the whole samples. The detailed microstructure of LFS-4 was determined by TEM. As can be seen from Fig. 1h, LFS-4 processes a porous structure stacked by nanoparticles with a diameter of 10–25 nm. The electron diffraction pattern of LFS-4 proves that the composition is polycrystalline. In the HRTEM pattern (Fig. 1i), the interplanar spacings of 0.20, 0.28, 0.30, and 0.38 nm correspond to (1 1 1) of Cu, (1 1 2) of LFS, (2 0 0) and (1 0 1) crystal plane of Li3PO4, respectively. In order to further determine the chemical state of Cu and P element in LFS/C/Cu/LP composites and the distribution of Cu and Li3PO4, LFS4 was subjected to XPS test. The binding energies were corrected by using the C 1 s peak at 284.6 eV. As shown in Fig. 2a, at first, Cu element is not observed on the surface of LFS-4. However, the Fe and Si element can be observed (Fig. S5). After etching for 300 and 600 s, Cu element is finally detected for XPS testing. There are two peaks of Cu 2p1/2 and Cu 2p3/2 locating at 932.8 and 952.1 eV, respectively, indicating that Cu is present as Cu0 in LFS-4 [28]. In contrast, the P element is founded both on the surface and interior of sample with the peak at 133.98 eV that is a typical PO4 tetrahedral structure [29] (Fig. 2b). XPS results again confirm that Cu and P are present as Cu0 and Li3PO4. Furthermore, Cu0 is encapsulated by LFS particles, meanwhile, Li3PO4 is not only present inside of the LFS particles but also mixed with carbon on the surface of LFS particles. Analyzing the above structure character, it can hypothesize that the special structure (Fig. 3a) and appropriate amount of Cu and Li3PO4 can improve the rate and cycle performance of LFS. The surface carbon layer can prevent the reaction between the active material and the electrolyte. The combination of Cu inside the LFS particles with carbon on the surface can effectively improve the electronic conductivity of the composite electrode. At the same time, the internal material of LFS has been partially replaced by Cu, which can increase the active material’s utilization, improving the cycle performance. In addition, Li3PO4 is not only distributed in the carbon layer, but also inside the LFS particles, which will significantly enhance the lithium ion transfer rate. As a result, the optimized LFS/C electrode materials by the appropriate amount of Cu and Li3PO4 can achieve the best rate performance and cycle stability. We studied the effect of Cu and Li3PO4 on the electrochemical
Fig. 3. (a) The structure schematic of LFS/C/Cu/Li3PO4 composites. (b) Charge-dicharge profiles at 1 C rate and (c) rate dependent cycling performance of the LFS-x (x = 0, 2, 4 and 6) samples. Cycling performance of LFS-0 and LFS-4 at (d) 1 C, (e) 5 C and (f) 10 C. 4
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Fig. 4. (a) Nyquist plots of the LFS-0, LFS-2, LFS-4 and LFS-6 samples. Inset: equivalent circuit model of Nyquist plots. (b) Linear fitting of the Z′ versus ω−1/2 relationship. (c) The GITT curves of LFS and LFS-4, and (d) the calculated DLi+ as a function of cell voltage processes.
In order to study the effect of dispersion of Cu and Li3PO4 on the electrochemical performance, the LFS-Cu-LP sample is prepared by simply mixing LFS/C, Cu and Li3PO4, in which the amount of Cu and Li3PO4 in LFS-Cu-LP is the same as that in LFS-4. Depicted in Fig. S8, LFS-Cu-LP shows lower performance regardless of the rate performance or cycle performance than LFS-4. Additionally, even compared with LFS-0, the electrochemical performance of LFS-Cu-LP has no advantage. From the above results, we confirm that the in situ formation of Cu and Li3PO4 is better than simple mixing, and the dispersion mode of Cu and Li3PO4 significantly affects the electrochemical performance of the product. EIS was further used to evaluate the effects of Cu and Li3PO4 on electrochemical dynamic properties of electrode materials and tested at 0.2 C for the first charge to 4.5 V. The Nyquist curves for both samples are shown in Fig. 4a. The Nyquist curves are composed of three parts. In the high frequency region, the intercept Rs at the Z’ axis represents the ohmic resistance. In the middle frequency region, the semicircle Rct corresponds to the charge transfer resistance. The straight line located in the low frequency region is related to lithium ions diffusion in the electrode and is Warburg impedance [30,31]. According to the above description, the Nyquist curves are fitted using the equivalent circuit model in the inset of Fig. 4a. The specific values are listed in Table 1. We can see that as the Cu and Li3PO4 content increases, the Rct
gradually decreases, which can be inferred that the Cu improves the electronic conductivity of LFS. The Warburg portion of the low frequency region can be used to determine the diffusion coefficient of lithium ions, DLi+. The calculation is based on the formula:
D+Li = R2T 2/2A2n4F 4C2σ 2
In the formula, R is the gas constant, T is the absolute temperature, A is the surface area of electrode, n is the number of electrons transferred by each molecule in the oxidation-reduction process, F is the Faraday constant, C is the concentration of lithium ions. σ is the Warburg factor that obeys the relationship between the Z' and the reciprocal of the square root of the frequency in the low frequency region (ω−1/2) (Fig. 4b) and can be calculated by the formula (2)
Z′ = RD + RL + σω−1/2
Rs (Ω)
Rct (Ω)
σ (Ω S−1/2)
DLi+ (cm2 s−1)
LFS-0 LFS-2 LFS-4 LFS-6
3.493 6.845 2.704 1.371
324.1 226.1 173.5 95.13
25.46 21.89 16.50 11.41
3.397 × 10−13 4.681 × 10−13 8.274 × 10−13 1.702 × 10−12
(2)
+
The DLi was calculated using the formula (1), and the specific values are shown in Table 1. LFS-x (x = 2, 4 and 6) shows higher DLi+ than LFS, verifying that Cu and Li3PO4 in LFS/C/Cu/Li3PO4 composites can promote lithium ion transport. GITT is a reliable method for calculating the diffusion kinetics of lithium ions and can be used to evaluate the change of lithium ion diffusion coefficient with lithium content or voltage in solid solution systems [32–34]. If the ohmic voltage drop, charge transfer kinetics, double electric layer charge and phase transition are not taken into account, the ion diffusion coefficient in the solid solution system follows Fick's law and can be calculated using the following formula:
Table 1 Impedance parameters of the LFS-0, LFS-2, LFS-4 and LFS-6 samples. Samples
(1)
2
DLi + =
4 ⎛ mB VM ⎞ ΔEs 2 ⎛ ⎞ πτ ⎝ MB S ⎠ ⎝ ΔEτ ⎠ ⎜
⎟
(3)
In the formula, mB and MB are the mass and molecular weight of the compound, respectively. VM is the molar volume of the compound, ΔEs 5
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is the change of the equilibrium voltage, ΔEτ is the change of the voltage within τ time after applying the constant current Ip (ignoring the IR drop). Fig. 4c shows the GITT curves (voltage as a function of time) of the LFS-0 and LFS-4 for the first discharge with a voltage range of 1.5–4.5 V. A constant current flux of 16.5 mA g−1 is applied to charge for an interval of 0.5 h, then stood for 4 h in the open-circuit to allow the cell potential to quasi-balanced state (Es). The procedure is repeated for the full voltage range of operation. Fig. 4d shows the change of DLi+ with voltage for LFS-0 and LFS-4. It can be seen that the change of DLi+ depends on the discharge process. At the beginning of the discharge, the DLi+ of the LFS-0 is 1.59 × 10−10 cm2 s−1, subsequently, gradually reduces to minimum of 1.15 × 10−12 cm2 s−1 at the discharge platform. Then, DLi+ increases to 7.23 × 10−10 cm2 s−1, and again reduces to 1.17 × 10−10 cm2 s−1. Obviously, lithium diffusion coefficients of the LFS-4 is overall larger than those of LFS-0, indicating that the presence of Cu and Li3PO4 can facilitate the lithium ion transport, which well explains its improved rate performance.
[8]
[9]
[10]
[11]
[12]
[13]
[14]
4. Conclusion [15]
In conclusion, we successfully synthesize the LFS/C/Cu/Li3PO4 composites by adding different amounts of Cu3(PO4)2 in the raw materials. The physical and electrochemical properties of all prepared materials are characterized. The elemental Cu is encapsulated by LFS particles, and Li3PO4 distributes inside LFS particles and the surface carbon layer. The synergistic effect of Cu and Li3PO4 in LFS/C/Cu/ Li3PO4 composites could further improve the electron conductivity, lithium ion transport rate and the utilization of active materials, leading to high rate performance and long cycle stability. Among all materials, LFS-4 shows the best rate performance, and the average discharge specific capacity is 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C, respectively. It is believed that such a modified method can be applied to other materials.
[16]
[17]
[18]
[19]
[20]
[21]
Acknowledgements This work was supported by funding from “973” project (No. 2015CB251103), National Natural Science Foundation of China (No. 21771086), S&T Development Program of Jilin Province (Nos. 20160101320JC, 20180101293JC), Jilin Provincial Department of Education “13th Five-Year” scientific research project (No JJKH20180116KJ).
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[23]
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Appendix A. Supplementary data
[25]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122329.
[26]
[27]
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Li, Carbon nanotube directed three-dimensional porous Li2FeSiO4 composite for lithium batteries, Nano Res. 10 (2017) 229–237. M. Holzapfel, A. Martinent, F. Alloin, B. Le Gorrec, R. Yazami, C. Montella, First lithiation and charge/discharge cycles of graphite materials, investigated by electrochemical impedance spectroscopy, J. Electroanal. Chem. 546 (2003) 41–50. Z. Yang, Y. Dai, S. Wang, J. Yu, How to make lithium iron phosphate better: a review exploring classical modification approaches in-depth and proposing future optimization methods, J. Mater. Chem. A 4 (2016) 18210–18222. Z. Zheng, Y. Wang, A. Zhang, T. Zhang, F. Cheng, Z. Tao, J. Chen, Porous Li2FeSiO4/ C nanocomposite as the cathode material of lithium-ion batteries, J. Power Sources 198 (2012) 229–235. J. Yang, X. Kang, L. Hu, X. Gong, D. He, T. Peng, S. Mu, Synthesis and electrochemical performance of Li2FeSiO4/C/carbon nanosphere composite cathode materials for lithium ion batteries, J. Alloy. Compd. 572 (2013) 158–162. 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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Design of Li2FeSiO4 cathode material for enhanced lithium-ion storage performance ⁎
Hailong Qiua, Di Jina, Chunzhong Wanga, Gang Chena, Lei Wangb, Huijuan Yuec, , Dong Zhanga,
T ⁎
a
Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, China Key Laboratory of Eco-chemical Engineering (Ministry of Education), College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China c State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China b
H I GH L IG H T S
/C/Cu/Li PO composites were synthesized by a sol-gel method. • LiLi FeSiO FeSiO /C is modified by electronic conductor and ionic conductor. • LFS-4 exhibits enhanced rate capability and superior cycle performance. • 2
4
2
4
3
4
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium ion battery Li2FeSiO4 Multi-functional engineering Enhanced electrochemical performance
The practical use of Li2FeSiO4 (LFS) is largely restricted by its inherent low electronic conductivity and slow lithium ions diffusion rate. Carbon (C) coating has been widely used to improve the electrochemical performance of LFS, but it can reduce the tap density and block lithium ion transmission. Herein, we design and synthesize the LFS/C/Cu/Li3PO4 composites with elaborate structure that the elemental Cu is encapsulated by LFS particles and Li3PO4 distributes both inside LFS particles and the surface carbon layer. The as-prepared samples with the unique structure depict enhanced electronic/ionic conductivities and kinetic properties due to the synergistic effect of Li3PO4 and Cu. LFS-4, which contains 2 wt% Cu and 2.42 wt% Li3PO4, exhibits the best rate performance with an average discharge specific capacity of 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C rate (1 C = 165.6 mA g−1). Over 800 cycles, a capacity of 116.2 mAh g−1 is maintained under the high current density of 10 C with the capacity retention of 88.9%. The study indicates that the rational design of hybrid material with multi-functional engineering possesses important significance in cathode materials for lithium-ion batteries.
1. Introduction Li2FeSiO4 (LFS) is believed to be one of the most promising cathode for the next-generation of high energy–density rechargeable batteries due to the high theoretical specific capacity of 332 mAh g−1 and many other advantages, such as low cost, environment friendly and high thermal stability [1–5]. However, low electronic conductivity and slow lithium ions diffusion rate of LFS severely limit its actual electrochemical performance [6–9]. Extensive efforts have been devoted to addressing these problems by constructing LFS and carbon-based materials to hybrid composites [10–18]. Zhang synthesized high performance LFS/C composite with reduced graphene oxide and glucose as carbon sources, achieving an initial discharge capacity of 178 and
⁎
119 mAh g−1 at 0.1 and 2 C [19]. Wang et al. designed a carbon nanotube-directed three-dimensional porous Li2FeSiO4/C composite, affording enhanced rate performance with capacities of 214, 158, 129, 103 and 79 mAh g−1 at rates of 0.2, 0.5, 1, 2 and 5 C, respectively [20]. Although these LFS/C hybrids have shown improved electrochemical performance during cycles, the simplistic modification will pose negative effects and limitations: 1) The electron conductivity of amorphous carbon (1.25–2 × 103 S cm−1) is relatively low. 2) The lithium ions diffusion is limited to some extent because carbon is not a good ionic conductor [21]. 3) Because the carbon-based materials usually modify the outer surface of the LFS particles rather than the reconstruction of internal practical, the reaction will terminate and will not continue to consume the active material inside the particles when the diffusion rate
Corresponding authors. E-mail addresses: [email protected] (H. Yue), [email protected] (D. Zhang).
https://doi.org/10.1016/j.cej.2019.122329 Received 29 April 2019; Received in revised form 19 June 2019; Accepted 23 July 2019 Available online 24 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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2. Experimental section
binder (10 wt%) to obtain a homogeneous slurry. Then, the slurry was coated onto Al foil and dried at 120 °C in a vacuum for 12 h. The cathode was cut into 0.8 × 0.8 cm2 pieces wafers with average active masses of about 1–2 mg. Electrochemical tests were carried out by 2032 coin cells on LAND 2100 battery test system in 1 M LiPF6 electrolyte solution dissolved in ethylene carbonate (EC) and diethyl carbonate (DEC) (EC/DEC = 1:1, v/v) with lithium foil as counter electrode and Celgard 2320 membrane as a separator. The cells were assembled in an argon-filled glove box with water and oxygen content lower than 1 ppm. Galvanostatic charge-discharge tests were conducted in the voltage window of 1.5–4.5 V versus Li/Li+ at room temperature. The charge/discharge specific capacities were calculated on the mass of the active materials only removing the carbon content. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) were measured to study the electrochemical kinetics performance of LFS/C/Cu/Li3PO4 composites by using a VSP and MPG multichannel potentiostatic-galvanostatic system (Biologic France). The EIS data was recorded in the frequency range between1 MHz and 1 mHz.
2.1. Synthesis of Cu3(PO4)2
3. Results and discussion
First, Cu(NO3)2·3H2O (3 mmol) and (NH4)2HPO4 (2 mmol) were dissolved in 15 mL deionized water. Then, (NH4)2HPO4 solution was added dropwise to Cu(NO3)2·3H2O solution and stirred for another 0.5 h. The solvent was evaporated and dried at 100 °C. The dried powder was calcined in air at 900 °C (5 °C min−1) for 10 h and naturally cooled to obtain Cu3(PO4)2.
XRD was used to determine the phase composition and crystal structure of the prepared samples. As shown in Fig. S1, it can be seen that Cu3(PO4)2 is pure phase. SEM further reveals that it consists of large particles with a particle size between 1.5 and 4 μm (inset of Fig. S1a). The XRD patterns of LFS-x (x = 0, 2, 4, 6 and 20) are shown in Fig. 1. All the diffraction peaks of LFS-0 belong to monoclinic structure LFS with a P21/n space group [23,24]. For LFS-x (x = 2, 4, 6 and 20) obtained by adding different amount of Cu3(PO4)2, although the diffraction peaks corresponding to LFS main phase do not change, several new diffraction peaks corresponding to Cu0 are observed. Furthermore, with the increased amount of Cu3(PO4)2, the diffraction peaks intensity of Cu gradually enhances, indicating that the relative content of Cu increases. In addition, when the amount of Cu3(PO4)2 increases to 20 wt %, Li3PO4 is also found in the obtained sample (LFS-20) (Fig. 1b). It should be pointed out that the diffraction peaks corresponding to Li3PO4 are not obviously observed in the LFS-x (x = 2, 4 and 6), which probably is due to the relatively low content and degree of crystallization in these materials. Moreover, the diffraction peaks of LFS-x (x = 2, 4, 6, and 20) do not shift from the original LFS-0, which concludes that the addition of Cu3(PO4)2 does not significantly affect the crystal structure of LFS. In order to study the formation mechanism of Li3PO4 and Cu, firstly, the thermal stability of Cu3(PO4)2 under N2 atmosphere was tested. Fig. S2 shows that Cu3(PO4)2 does not decompose and is quite stable under N2 condition even heated up to 800 °C. Then, the reaction of Cu3(PO4)2 with P123 and LiAc·2H2O heated at 650 °C for 10 h under N2 was studied. As shown in Fig. S3, the final product consists of Li3PO4 and Cu, suggesting that Cu is derived from the reduction of Cu2+ in the heat treatment process, and Li3PO4 is the reactant between PO43− and Li+ in the raw materials. Thus, the chemical reaction equation for the formation of the LFS/C/Cu/Li3PO4 composites is as follows:
of lithium ions cannot maintain the electrode reaction, which will reduce the utilization of the active material and affect the cycle performance [22]. To summarize, LFS-based cathode should not only be highly conductive to improve electrical conductivity and the lithium ion diffusion rate, but also enable high active material utilization by the rearrangement of inner surface of the particles. Therefore, the welldesigned structure of LFS matrix with strong electronic/ionic conductivity is crucially important. In this work, we report LFS/C/Cu/Li3PO4 composites using a sol-gel method. The hybrid material combines several advantages: 1) carbon layer on the surface of LFS particles improves electronic conductivity. 2) Cu, which is encapsulated inside the active material LFS particles, enhances the utilization of active material and further increases the electronic conductivity. 3) Li3PO4 located in the surface and the interior of the LFS particles plays an essential role in enhancing the ionic conductivity. As respected, the hybrid material exhibits excellent electrochemical properties.
2.2. Synthesis of LFS/C/Cu/Li3PO4 composites The LFS/C/Cu/Li3PO4 composites are synthesized by a sol-gel method, details of which are as follows. First, a certain amount of polyethylene oxide-polypropylene oxide-polyethylene oxide (P123) was dispersed into 20 mL ethanol. Then, Ethylsilicate (TEOS) (4 mmol), LiAc·2H2O (4 mmol) and Fe (NO3)3·9H2O (4 mmol) as well as a certain amount of Cu3(PO4)2 (2 wt%: 0.013 g, 4 wt%: 0.026 g, 6 wt%: 0.039 g, 20 wt%: 0.13 g) were added into the above solution under magnetic stirring in sequence. The amount of Cu3(PO4)2 is the weight ratio to the theoretical yield of LFS. After stirred for 2 h, the obtained solution was evaporated at 70 °C and dried at 100 °C. The dried precursor was sintered under N2 atmosphere at 650 °C (5 °C min−1) for 10 h to obtain the final product LFS/C/Cu/Li3PO4. In this work, the final products obtained by adding 0%, 2 wt%, 4 wt%, 6 wt% and 20 wt% of Cu3(PO4)2 are denoted as LFS-0, LFS-2, LFS-4, LFS-6, and LFS-20, respectively. 2.3. Materials characterization The morphology and elemental mapping of the samples were studied by field-emission scanning electron microscope (FESEM; HITACHI SU8020) and transmission electron microscope (TEM; FEI Tecnai G2). X-ray diffraction (XRD; Bruker AXS D8 powder diffractometer) was used to determine the crystal structure of the products operated at a voltage of 40 kV and a current of 100 mA using Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) investigation was carried out on a VG scientific ESCALAB 250 spectrometer. Raman scattering spectra were conducted on a Labram HR Evolution Horiba using laser excitation at 532 nm. The carbon content was determined by CHNSO elemental analyzer (Elementar Varon EL cube type). TGA (SDTQ600) was used to examine the stability of Cu3(PO4)2 in N2 atmosphere with a temperature ramp of 10 °C min−1.
xCu3 (PO4 )2 + Fe3+ + 2Li+ + TEOS + P123 → 3xCu + Li2−6x FeSiO4 + 2xLi3 PO4 + C+ H2 O↑ + CO2 ↑ From the above equation, it can be found that the generated LFS is non-stoichiometric with lithium vacancies, which is favorable for Li+ transmission [25]. Additionally, the amounts of Cu and Li3PO4 are relative to the LFS mass ratio, for instance, 2 wt% Cu3(PO4)2 can produce about 1 wt% Cu and 1.21 wt% Li3PO4. The presence of carbon can be confirmed by Raman spectroscopy and elemental analysis. Fig. 1c shows the Raman spectra of LFS-x (x = 0, 2, 4, and 6). The two diffraction peaks of the Raman spectra locate at 1355 and 1596 cm−1, corresponding to the D and G band
2.4. Electrochemical test The work electrode was prepared by mixing the LFS/C/Cu/Li3PO4 composites (70 wt%), Super P (20 wt%) and poly(vinylidene fluoride) 2
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Fig. 1. (a, b) XRD patterns of LFS-x (x = 0, 2, 4, 6 and 20). (c) Raman spectrum of LFS-x (x = 0, 2, 4 and 6). SEM images of (d) LFS-0, (e) LFS-2, (f) LFS-4 and (g) LFS6. (h, i) TEM and HRTEM images of LFS-4. The inset in (h) shows the SAED pattern of LFS-4 and the inset in (i) shows the enlarged view of the rectangular.
Fig. 2. XPS spectra of (a) Cu and (b) P element in LFS-4.
better conductivity. The carbon contents of LFS-0, LFS-2, LFS-4 and LFS-6 samples are 17.53 wt%, 17.31 wt%, 17.14 wt% and 16.85 wt% by CHNSO elemental analyzer. SEM was used to observe the effect of Cu3(PO4)2 on the microstructure of the composites. As shown in Fig. 1d-g, LFS-0 consists of nanoparticles. After adding different amounts of Cu3(PO4)2, the microstructure of the obtained composites is remained, suggesting that Cu3(PO4)2 does not affect the microstructure of the final products. EDS mapping shows the distribution of Fe, Si, O, Cu and P elements in LFS-4 (Fig. S4), indicating that Cu and Li3PO4 are not uniformly distributed in
characteristic peaks of the carbon, respectively. Among them, the D peak represents the disordered vibration of the sp3 type hybrid carbon atom (disordered carbon), and the G peak represents the stretching vibration in-plane of the sp2 type hybrid carbon atom (graphitized carbon). ID/IG represents the degree of graphitization of the carbon materials, the smaller the ID/IG, the higher the degree of graphitization of carbon [26,27]. For LFS-0, LFS-2, LFS-4 and LFS-6, the values of ID/IG are 0.907, 0.879, 0.853 and 0.876, respectively. Compared with LFS-0, the ID/IG of LFS-2, LFS-4 and LFS-6 are slightly reduced, indicating that they are coated by carbon with higher degree of graphitization and 3
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performance of LFS, four samples of LFS-x (x = 0, 2, 4 and 6) were subjected to galvanostatic charge–discharge test (1 C = 165 mA g−1) in a voltage range from 1.5 to 4.5 V. Fig. S6 shows the charge and discharge curves of the four samples at 0.1 C, the shapes of which are similar, indicating that the electrochemical reactions are the same during the Li ion insertion and deinsertion process. The same phenomenon is observed at 1 C (Fig. 3b). Among all samples, LFS-4 shows the highest discharge specific capacity of 180.7 mAh g−1, whereas, the discharge capacities of LFS-0, LFS-2 and LFS-6 are 156.5, 169.4 and 157.2 mAh g−1. Furthermore, LFS-4 exhibits the best rate capability from 1 to 40 C (Fig. 3c), remaining an average discharge specific capacity of 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C rate, respectively, which are higher than those reported LFS cathode materials (Table S1, Supporting Information). On the contrary, the average discharge specific capacity of LFS-0 at different rates is 147.5, 130.1, 104.4, 72.7, 46.5 and 33.6 mAh g−1. Obviously, it can be found that Cu and Li3PO4 significantly enhance the electrochemical performance of LFS at the high rates. As is well known, Cu and Li3PO4 (like carbon) are electrochemical inactive, overdosage will reduce the utilization of active material. Therefore, the electrochemical performance of as-prepared samples did not increase linearly with the increase of Cu and Li3PO4. The cycle performance of LFS-0 and LFS-4 is presented in Fig. 3d-f. As can be seen, although LFS-0 and LFS-4 have similar capacity retention rate at 1 and 5 C rates, LFS-4 shows a higher specific capacity than LFS-0, still delivering 136.5 and 116.2 mAh g−1 after long cycles at 1 and 5 C rates (128.4 and 89.0 mAh g−1 for LFS-0). At higher rate of 10 C, the discharge specific capacity of LFS-4 is 116.2 mAh g−1 with the capacity retention of 88.9%, however, the discharge specific capacity of LFS-0 is only 69.3 mAh g−1 after 800 cycles. Fig. S7 further give the effects of Cu and Li3PO4 on the capacity and cycle performance of LFS electrode materials at 20 C high rates, the discharge capacity of LFS-4 is 47.8 mAh g−1, which is twice that of LFS-0 after 3000 cycles at 20 C. In terms of capacity retention, LFS-4 is 60.1%, while LFS-0 is only 38.5%. It should be pointed out that we chose the discharge specific capacity of the 100th cycle as the initial capacity to calculate the capacity retention.
the whole samples. The detailed microstructure of LFS-4 was determined by TEM. As can be seen from Fig. 1h, LFS-4 processes a porous structure stacked by nanoparticles with a diameter of 10–25 nm. The electron diffraction pattern of LFS-4 proves that the composition is polycrystalline. In the HRTEM pattern (Fig. 1i), the interplanar spacings of 0.20, 0.28, 0.30, and 0.38 nm correspond to (1 1 1) of Cu, (1 1 2) of LFS, (2 0 0) and (1 0 1) crystal plane of Li3PO4, respectively. In order to further determine the chemical state of Cu and P element in LFS/C/Cu/LP composites and the distribution of Cu and Li3PO4, LFS4 was subjected to XPS test. The binding energies were corrected by using the C 1 s peak at 284.6 eV. As shown in Fig. 2a, at first, Cu element is not observed on the surface of LFS-4. However, the Fe and Si element can be observed (Fig. S5). After etching for 300 and 600 s, Cu element is finally detected for XPS testing. There are two peaks of Cu 2p1/2 and Cu 2p3/2 locating at 932.8 and 952.1 eV, respectively, indicating that Cu is present as Cu0 in LFS-4 [28]. In contrast, the P element is founded both on the surface and interior of sample with the peak at 133.98 eV that is a typical PO4 tetrahedral structure [29] (Fig. 2b). XPS results again confirm that Cu and P are present as Cu0 and Li3PO4. Furthermore, Cu0 is encapsulated by LFS particles, meanwhile, Li3PO4 is not only present inside of the LFS particles but also mixed with carbon on the surface of LFS particles. Analyzing the above structure character, it can hypothesize that the special structure (Fig. 3a) and appropriate amount of Cu and Li3PO4 can improve the rate and cycle performance of LFS. The surface carbon layer can prevent the reaction between the active material and the electrolyte. The combination of Cu inside the LFS particles with carbon on the surface can effectively improve the electronic conductivity of the composite electrode. At the same time, the internal material of LFS has been partially replaced by Cu, which can increase the active material’s utilization, improving the cycle performance. In addition, Li3PO4 is not only distributed in the carbon layer, but also inside the LFS particles, which will significantly enhance the lithium ion transfer rate. As a result, the optimized LFS/C electrode materials by the appropriate amount of Cu and Li3PO4 can achieve the best rate performance and cycle stability. We studied the effect of Cu and Li3PO4 on the electrochemical
Fig. 3. (a) The structure schematic of LFS/C/Cu/Li3PO4 composites. (b) Charge-dicharge profiles at 1 C rate and (c) rate dependent cycling performance of the LFS-x (x = 0, 2, 4 and 6) samples. Cycling performance of LFS-0 and LFS-4 at (d) 1 C, (e) 5 C and (f) 10 C. 4
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Fig. 4. (a) Nyquist plots of the LFS-0, LFS-2, LFS-4 and LFS-6 samples. Inset: equivalent circuit model of Nyquist plots. (b) Linear fitting of the Z′ versus ω−1/2 relationship. (c) The GITT curves of LFS and LFS-4, and (d) the calculated DLi+ as a function of cell voltage processes.
In order to study the effect of dispersion of Cu and Li3PO4 on the electrochemical performance, the LFS-Cu-LP sample is prepared by simply mixing LFS/C, Cu and Li3PO4, in which the amount of Cu and Li3PO4 in LFS-Cu-LP is the same as that in LFS-4. Depicted in Fig. S8, LFS-Cu-LP shows lower performance regardless of the rate performance or cycle performance than LFS-4. Additionally, even compared with LFS-0, the electrochemical performance of LFS-Cu-LP has no advantage. From the above results, we confirm that the in situ formation of Cu and Li3PO4 is better than simple mixing, and the dispersion mode of Cu and Li3PO4 significantly affects the electrochemical performance of the product. EIS was further used to evaluate the effects of Cu and Li3PO4 on electrochemical dynamic properties of electrode materials and tested at 0.2 C for the first charge to 4.5 V. The Nyquist curves for both samples are shown in Fig. 4a. The Nyquist curves are composed of three parts. In the high frequency region, the intercept Rs at the Z’ axis represents the ohmic resistance. In the middle frequency region, the semicircle Rct corresponds to the charge transfer resistance. The straight line located in the low frequency region is related to lithium ions diffusion in the electrode and is Warburg impedance [30,31]. According to the above description, the Nyquist curves are fitted using the equivalent circuit model in the inset of Fig. 4a. The specific values are listed in Table 1. We can see that as the Cu and Li3PO4 content increases, the Rct
gradually decreases, which can be inferred that the Cu improves the electronic conductivity of LFS. The Warburg portion of the low frequency region can be used to determine the diffusion coefficient of lithium ions, DLi+. The calculation is based on the formula:
D+Li = R2T 2/2A2n4F 4C2σ 2
In the formula, R is the gas constant, T is the absolute temperature, A is the surface area of electrode, n is the number of electrons transferred by each molecule in the oxidation-reduction process, F is the Faraday constant, C is the concentration of lithium ions. σ is the Warburg factor that obeys the relationship between the Z' and the reciprocal of the square root of the frequency in the low frequency region (ω−1/2) (Fig. 4b) and can be calculated by the formula (2)
Z′ = RD + RL + σω−1/2
Rs (Ω)
Rct (Ω)
σ (Ω S−1/2)
DLi+ (cm2 s−1)
LFS-0 LFS-2 LFS-4 LFS-6
3.493 6.845 2.704 1.371
324.1 226.1 173.5 95.13
25.46 21.89 16.50 11.41
3.397 × 10−13 4.681 × 10−13 8.274 × 10−13 1.702 × 10−12
(2)
+
The DLi was calculated using the formula (1), and the specific values are shown in Table 1. LFS-x (x = 2, 4 and 6) shows higher DLi+ than LFS, verifying that Cu and Li3PO4 in LFS/C/Cu/Li3PO4 composites can promote lithium ion transport. GITT is a reliable method for calculating the diffusion kinetics of lithium ions and can be used to evaluate the change of lithium ion diffusion coefficient with lithium content or voltage in solid solution systems [32–34]. If the ohmic voltage drop, charge transfer kinetics, double electric layer charge and phase transition are not taken into account, the ion diffusion coefficient in the solid solution system follows Fick's law and can be calculated using the following formula:
Table 1 Impedance parameters of the LFS-0, LFS-2, LFS-4 and LFS-6 samples. Samples
(1)
2
DLi + =
4 ⎛ mB VM ⎞ ΔEs 2 ⎛ ⎞ πτ ⎝ MB S ⎠ ⎝ ΔEτ ⎠ ⎜
⎟
(3)
In the formula, mB and MB are the mass and molecular weight of the compound, respectively. VM is the molar volume of the compound, ΔEs 5
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is the change of the equilibrium voltage, ΔEτ is the change of the voltage within τ time after applying the constant current Ip (ignoring the IR drop). Fig. 4c shows the GITT curves (voltage as a function of time) of the LFS-0 and LFS-4 for the first discharge with a voltage range of 1.5–4.5 V. A constant current flux of 16.5 mA g−1 is applied to charge for an interval of 0.5 h, then stood for 4 h in the open-circuit to allow the cell potential to quasi-balanced state (Es). The procedure is repeated for the full voltage range of operation. Fig. 4d shows the change of DLi+ with voltage for LFS-0 and LFS-4. It can be seen that the change of DLi+ depends on the discharge process. At the beginning of the discharge, the DLi+ of the LFS-0 is 1.59 × 10−10 cm2 s−1, subsequently, gradually reduces to minimum of 1.15 × 10−12 cm2 s−1 at the discharge platform. Then, DLi+ increases to 7.23 × 10−10 cm2 s−1, and again reduces to 1.17 × 10−10 cm2 s−1. Obviously, lithium diffusion coefficients of the LFS-4 is overall larger than those of LFS-0, indicating that the presence of Cu and Li3PO4 can facilitate the lithium ion transport, which well explains its improved rate performance.
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4. Conclusion [15]
In conclusion, we successfully synthesize the LFS/C/Cu/Li3PO4 composites by adding different amounts of Cu3(PO4)2 in the raw materials. The physical and electrochemical properties of all prepared materials are characterized. The elemental Cu is encapsulated by LFS particles, and Li3PO4 distributes inside LFS particles and the surface carbon layer. The synergistic effect of Cu and Li3PO4 in LFS/C/Cu/ Li3PO4 composites could further improve the electron conductivity, lithium ion transport rate and the utilization of active materials, leading to high rate performance and long cycle stability. Among all materials, LFS-4 shows the best rate performance, and the average discharge specific capacity is 165.8, 142.9, 119.2, 102.1, 83.3 and 60.4 mAh g−1 at 1, 2, 5, 10, 20 and 40 C, respectively. It is believed that such a modified method can be applied to other materials.
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Acknowledgements This work was supported by funding from “973” project (No. 2015CB251103), National Natural Science Foundation of China (No. 21771086), S&T Development Program of Jilin Province (Nos. 20160101320JC, 20180101293JC), Jilin Provincial Department of Education “13th Five-Year” scientific research project (No JJKH20180116KJ).
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Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122329.
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