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C cathode material of lithium ion battery with enhanced electrochemical performance
Journal of Alloys and Compounds 782 (2019) 413e420
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Synthesis of LiFe0.4Mn0.4Co0.2PO4/C cathode material of lithium ion battery with enhanced electrochemical performance Zhi-Gen Huang a, Jun-Tao Li b, *, Kai Wang b, Wen-Feng Ren a, Yan-Qiu Lu b, Li Deng b, Ling Huang a, Shi-Gang Sun a, ** a b
State Key Lab of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China College of Energy, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2018 Received in revised form 22 November 2018 Accepted 11 December 2018 Available online 13 December 2018
An olivine-structured solid solution, LiFe0.4Mn0.4Co0.2PO4/C nanocomposite, was synthesized as cathode materials for Li-ion batteries by the coprecipitation-and-milling method using stearic acid as the carbon source. Thus-formed LiFe0.4Mn0.4Co0.2PO4/C solid solution was well confirmed and characterized by various techniques such as SEM, TEM and XRD. The LiFe0.4Mn0.4Co0.2PO4/C-based cathode delivered an initial discharge capacity of 163.3 mAh g1 at 0.1 C, together with a capacity retention rate of 86.6% after 50 cycles in the potential range of 2.5e5 V. Even at 1 C, a discharge capacity of 104.7 mAh g1 was maintained after 100 cycles. The good electrochemical performance of the LiFe0.4Mn0.4Co0.2PO4/C could be attributed to the homogeneous mixing of Fe, Mn and Co elements in relevant precursor solid solution Fe0.4Mn0.4Co0.2C2O4$2H2O. Its characteristic CV profiles reveal three pairs of anodic/cathode peaks which could be assigned to the redox reactions of Co3þ/Co2þ, Mn3þ/Mn2þ and Fe3þ/Fe2þ with potential plateaus at about 4.7 V, 4.1 V and 3.5 V in the charge/discharge curves, respectively. Our study demonstrates that the LiFe0.4Mn0.4Co0.2PO4/C is a promising cathode material of Li-ion battery and the coprecipitation-andmilling method represents a workable strategy towards the synthesis of high-performance phosphatetype composite cathode. © 2018 Elsevier B.V. All rights reserved.
Keywords: LiFe0.4Mn0.4Co0.2PO4/C Solid solution Coprecipitation-and-milling method Cathode Li-ion batteries
1. Introduction Echoing to the ever-increasing demand for energy, the yearly decreasing reserves of fossil fuels and the growing global concern on the environmental issues, the efficient, safe and low-pollution Li-ion batteries have attracted more and more attention, especially the high specific energy batteries for applications in vehicles [1e3]. Among various cathode materials, the olivine-structured LiFePO4 cathode materials have been successfully commercialized thanks to its safety, low cost, environment friendliness, excellent electrochemical performance and remarkable thermal stability [4,5]. However, LiFePO4 cathodes generally operate at a low voltage (3.4 V versus Li/Liþ), which results in a low specific energy density of the battery, and they also suffer from inherently low electronic conductivity. In a closely related issue, olivine type cathode materials of LiMnPO4 and LiCoPO4, though which own the same
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J.-T. Li). https://doi.org/10.1016/j.jallcom.2018.12.169 0925-8388/© 2018 Elsevier B.V. All rights reserved.
theoretical capacity (about 170 mAh g1) as LiFePO4, have received great attention due to their higher operating voltage (4.1 and 4.8 V versus Li/Liþ, respectively) [6e10]. However, the LiMnPO4 cathode suffers from severe capacity fading, mainly due to the Jahn-Teller effect of Mn3þ ions, and its extremely low intrinsic electronic conductivity [11]. For the LiCoPO4, it suffers from poor cycling stability due to the decomposition of common carbonate-based electrolytes at high voltage [12,13]. To overcome these challenges and improve the electrochemical properties of cathode materials, various olivine-type solid solutions such as LiFe1-xMnxPO4 [14e20], LiFe1-xCoxPO4 [21,22], and LiFexMnyCozPO4 [2,23e29] (x þ y þ z ¼ 1) were explored, especially the LiFe1-xMnxPO4 which has been successfully commercialized. Moreover, LiFexMnyCozPO4 with a variety of chemical compositions such as LiFe1/3Mn1/3Co1/3PO4 [2,23e26], LiFe0.5Mn0.3Co0.2PO4 [27], LiFe0.25Mn0.5Co0.25PO4 [28], and LiMn0.35Co0.2Fe0.45PO4 [29] were also successfully synthesized, which generally demonstrated good performances. The existing researches indicated that, after doping with Fe and Co, the activity of Mn in the LiFexMnyCozPO4 cathode is obviously improved due to enhanced conductivity and suppressed
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Jahn-Teller deformation. In the current study, LiFe0.4Mn0.4Co0.2PO4/C nanocomposite was synthesized through a coprecipitation process followed by a wetmilling process. The effects of preparation conditions on the morphologies and electrochemical properties of the resulting LiFe0.4Mn0.4Co0.2PO4/C composite were systematically evaluated. The assynthesized LiFe0.4Mn0.4Co0.2PO4/C cathode can deliver an initial discharge capacity of 163.3 mAh g1 at 0.1 C and also a capacity retention rate of 86.6% after 50 cycles in the voltage range of 2.5e5 V. Even at of 1 C, a discharge capacity of 104.7 mAh g1 after 100 cycles was maintained. We attributed such remarkable electrochemical performance to the homogeneous mixing of Fe, Mn and Co elements in the solid solution. 2. Experimental 2.1. Materials and chemicals All chemical reagents in this study were of analytical grade and used directly without further purification. Among them, FeC2O4$2H2O, FeSO4$7H2O, MnSO4$H2O, MnCO3, CoCO3, LiH2PO4, and stearic acid were provided by Xiqiao Chemical Co., Ltd. (NH4)2C2O.4H2O and CoSO4$7H2O were provided by Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of Fe0.4Mn0.4Co0.2C2O4·2H2O and LiFe0.4Mn0.4Co0.2PO4/C The Fe0.4Mn0.4Co0.2C2O4$2H2O precursor was synthesized by the coprecipitation method. FeSO4$7H2O, MnSO4$H2O and CoSO4$H2O with a stoichiometric ratio were added and stirred in a beaker, followed by dropwise addition of a sufficient of 0.3 M (NH4)2C2O4$H2O solution. During this process, the solution became turbid yellow. The resulting yellow particles which were obtained by filtration and were washed with deionized water for 5 times were dried in oven at 80 C for 8 h to yield the Fe0.4Mn0.4Co0.2C2O4$2H2O product. Subsequently, the as-prepared Fe0.4Mn0.4Co0.2C2O4$2H2O, LiH2PO4, and stearic acid (the molar ratio between Fe0.4Mn0.4Co0.2C2O4$2H2O and stearic acid is about 2:1.1) with a stoichiometric ratio was mixed with a portion of ethanol in a milling tank. The mixture was ground for 12 h through a planetary ball mill at 400 rpm with a ball diameter as about 8 mm and the weight ratio between balls and powder as about 10:1. Afterward, after being dried at 45 C in a vacuum oven overnight, the mixture was pre-calcined at 400 C for 3 h and then at 700 C for 16 h in argon atmosphere with a ramping rate of 5 C min1 to produce the LiFe0.4Mn0.4Co0.2PO4/C sample (denoted as C-LFMCP). A control sample was also prepared by a one-step milling method (denoted as O-LFMCP). The starting materials, namely LiH2PO4, FeC2O.42H2O, MnCO3 and CoCO3 with a stoichiometric ratio, and stearic acid (the molar ratio between FeC2O.42H2O and stearic acid is about 0.8:1.1) were mixed with a portion of ethanol in the milling tank. The mixture then was subjected to the same thermal treatment used for the synthesis of C-LFMCP to give the OLFMCP. 2.3. Characterizations The crystalline phases of the samples were characterized by Xray diffraction (XRD) using a Rigaku D/Max III diffractometer with Cu Ka radiation. The lattice parameters were obtained by using PDXL software (Rigaku Co., Ltd., PDXL 2.1) [30]. The particles size, micro-morphologies and structure information of the samples were observed by scanning electron microscopy (SEM, Hitachi S4800 N) equipped with an energy dispersive X-ray spectroscopy
(EDS) instrument and TEM (JEOLJEM-2100 microscopy). The material impedance was measured using a PARSTAT 2263 electrochemical tester (Princeton applied researcher, USA) and fitting through the ZView software. As shown in inset of EIS (electrochemical impedance spectroscopy), Re, Rct and W1 represent the electrolyte impedance, charge impedance and the Warburg impedance, respectively, and CPE is the constant phase angle element. The lithium ion diffusion coefficients (Dþ Li) of the coin cells for the two samples were calculated using the following two equations (1) and (2) [31].
. 2A2 n4 F 4 C 2 s2 DLi þ ¼ R2 T 2
(1)
Zre ¼ Re þ Rct þ su1=2
(2)
where R and F represent gas and Faraday constant, and T, A, n and C denote the absolute temperature, the surface area of the electrode sheet, the number of electrons per mole and the molar concentration of lithium ions in LiFe0.4Mn0.4Co0.2PO4/C, respectively. Then s (Warburg factor) is calculated according to the relationship between Zre (which is the Liþ diffusion resistance in the LiFe0.4Mn0.4Co0.2PO4/C cathode) and u (the angular frequency) in equation (2). 2.4. Electrochemical studies Electrochemical tests were conducted in the CR2025 coin cells, in which a lithium foil acted as a counter electrode and LiFe0.4Mn0.4Co0.2PO4/C served as the cathodes. For the preparation of the LiFe0.4Mn0.4Co0.2PO4/C cathode, LiFe0.4Mn0.4Co0.2PO4/C, acetylene black, and PVDF (polyvinylidene fluoride) binder in a weight ratio of 8: 1: 1 in N-methylpyrrolidone (NMP) were mixed to form a homogeneous slurry through ball-milling. The slurry was then uniformly casted on Al foils and heated at 100 C in vacuum overnight. The mass loadings of the cathodes were about 2.5e3.0 mg cm2. Celgard 2400 was used as the separator. 1 M LiPF6 and some additives dissolved in 1 L of ethylene carbonate and dimethyl carbonate (1: 1 in volume, from Fosai New Materials Co. Ltd., Jiangsu, China) was used as the electrolyte. All the cells were subjected to galvanostatic charge and discharge tests under operating voltages ranging from 2.5 to 5 V at 30 C. 3. Results and discussion Fig. 1a displays the XRD pattern of the Fe0.4Mn0.4Co0.2C2O4$2H2O. The major peaks at 18.3, 18.7, 22.6 and 29.3 can be indexed to FeC2O.42H2O, CoC2O4$2H2O and MnC2O4$2H2O [32], indicating that the as-prepared material is a composite of FeC2O.42H2O, CoC2O4$2H2O and MnC2O4$2H2O. As shown in Fig. 1b, for the resulting C-LFMCP, most of the peaks in its XRD pattern could be referred to the orthorhombic structure with a Pnma space group. Note that their diffraction peaks locate at positions similar to those of the LiFePO4 (JCPDS, 40-1499), but with slight shift, which proves the sample as a good solid solution instead of a simple physical mixture of LiFePO4, LiMnPO4, and LiCoPO4. This is further confirmed by the resulting lattice parameters (Table 1) obtained from Rietveld refinement. No significant peak of carbon was found in the XRD pattern, signifying very low carbon content or the amorphous nature of the carbon in the sample. Fig. 1c shows the thermogravimetric curve of the C-LFMCP material, which indicates that the carbon mass fraction is about 2.9% wt in the sample. The morphology and size of the C-LFMCP and O-LFMCP were characterized by SEM. As shown in Fig. 2a and b, the particles in the C-LFMCP generally assume an irregular nano-morphology, and
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Fig. 1. XRD patterns of the as-prepared Fe0.4Mn0.4Co0.2C2O4.2H2O precursor (a) and the C-LFMCP sample (b), the thermogravimetric curve of the C-LFMCP sample (c).
Table 1 Lattice parameters obtained by XRD Rietveld refinement of C-LFMCP and other LiMPO4 (M ¼ Fe, Mn, Co). Compound
a(Å)
b(Å)
c(Å)
V (Å3)
LiFe0.4Mn0.4Co0.2PO4 (this work) LiFe0.25Mn0.5Co0.25PO4 [28] LiFe0.5Mn0.3Co0.2PO4 [17] LiCo1/3Mn1/3Fe1/3PO4 [23] LiFe1/3Mn1/3Co1/3PO4 [26] LiFePO4(JCPDS:40-1499) LiMnPO4(JCPDS:33-0804) LiCoPO4(JCPDS:32-0552)
10.338 10.377 10.321 10.327 10.326 10.347 10.454 10.206
6.018 6.041 6.010 6.012 6.011 6.019 6.106 5.922
4.708 4.735 4.705 4.717 4.713 4.704 4.749 4.701
292.908 296.82 291.847 292.95 292.611 292.959 303.139 284.128
their size ranges from 50 to 400 nm. In Fig. 2e and f, the particles in the O-LFMCP show an irregular nano-morphology, and the size is also between 50 and 400 nm. Compared to the other materials synthesized by ball milling, the particles in our C-LFMCP and OLFMCP are smaller in size, mainly attributing to the presence of stearic acid which not only serves as the carbon source but also can limit the growth of particles [17,33]. The C-LFMCP is composed of aggregated nanoparticles closely interconnected by carbon, as demonstrated in Fig. 2b, indicating their good conductivity. Their relevant TEM images are displayed in Fig. 2cee and Fig. 2hej. The particle sizes of the samples shown in Fig. 2c and h are in good agreement with that obtained from the SEM images. The presence of carbon which links the neighboring particles and forms an effective conductive carbon network in C-LFMCP sample
was further verified. Fig. 2d illustrates a typical C-LFMCP nanoparticle, from which one can observe an amorphous carbon layer that is uniformly coated on the entire nanoparticle. From the highresolution TEM images in Fig. 2e, the thickness of the carbon shell is about 3 nm, further implying remarkable conductive contact between the C-LFMCP particles. In addition, the legible lattice fringes in the C-LFMCP nanoparticle were also observed in the highresolution TEM image in Fig. 2e with a d-spacing of about 0.416 nm, corresponding to the (101) planes of C-LFMCP, which agrees well with the above Rietveld refinement of XRD data. Such results indicate that the C-LFMCP sample is a single-phase solid solution with high crystallinity, which could be attributed to the homogeneous mixing of Fe, Mn and Co elements in the sample by the coprecipitation method. In particular, it shall also be mentioned that the large spacing gap of the C-LFMCP sample can allow relatively high Li-ion transport capability [34]. In comparison, as shown in the TEM images (Fig. 2 h-j) though the surface of the O-LFMCP was also coated with a carbon layer with a thickness of 3 nm. EDS element analysis (Table 2) demonstrates that the molar ratio of Mn, Fe and Co in the C-LFMCP composite is close to 2:2:1, which is consistent with the molar ratio of the starting materials. Elemental mappings of the C-LFMCP sample (Fig. 3a) illustrated the homogeneous distribution of Fe, Mn, Co, and O in the sample together with an even distribution of carbon. Meanwhile, EDS mapping of Fe0.4Mn0.4Co0.2C2O.42H2O (Fig. 3b) further verified homogeneous distribution of Fe, Mn and Co element in the sample. To further evaluate the electrochemical properties of C-LFMCP
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Fig. 2. SEM (a, b) and TEM (c, d and e) images of the C-LFMCP sample, SEM (e, f) and TEM (h, i and j) images of the O-LFMCP sample.
Table 2 Parameters of Fe, Mn and Co content of C-LFMCP from the EDS. samples
Element
Atomic ratio
C-LFMCP
Fe Mn Co
0.42 0.38 0.21
cathode, galvanostatic charge-discharge, cycling and rate capability tests were carried out. Fig. 4a displays the discharge curves of selected cycles of the C-LFMCP cathode at a low rate of 0.1 C (1 C ¼ 170 mAh g1) between 2.5 and 5 V. Three characteristic potential plateaus appear clearly at about 4.7 V, 4.1 V and 3.5 V, corresponding to the redox processes of Co3þ/Co2þ, Mn3þ/Mn2þ and
Fe3þ/Fe2þ, respectively. The redox potentials of Fe or Co in the LiFe0.4Mn0.4Co0.2PO4/C were different from those in the pure LiFePO4 or LiCoPO4 [35,36], while the redox potential of Mn is the same as those in the pure LiMnPO4. Such differences may originate from the strong interaction between the Fe-O-Mn (Co) ions [35]. In addition, compared with Fe which generally have stable activity, the activities of Mn and Co decay rapidly in the discharge process, which may be attributed to the Jahn-Teller effect of Mn3þ ions [11] and the decomposition of electrolytes due to the excessively high redox potential of Co [12,13]. The cycle performance of C-LFMCP sample prepared by coprecipitation-and-milling at 0.1 C is shown in Fig. 4b. At the first cycle, the discharge specific capacity of C-LFMCP cathode is 163.3 mAh g1, which is close to its theoretical capacity (170 mAh
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Fig. 3. (a) SEM image and EDS mapping of the corresponding elemental distributions of C, O, Fe, Mn and Co element of C-LFMCP, (b) SEM image and EDS mapping of Fe, Mn and Co element of Fe0.4Mn0.4Co0.2C2O.42H2O.
g1). A capacity retention of 86.6% was obtained after 50 cycles. The sample displayed an initial coulombic efficiency of 86%, which increased gradually along with the increasing cycles and eventually maintained at about 96% after 50 cycles. Fig. 4c and d further depict the stable cycling behavior of CLFMCP sample at 0.5 and 1 C, respectively. Compared to O-LFMCP sample, C-LFMCP sample exhibits higher initial discharge capacities at 0.5 and 1 C. The discharge capacities of C-LFMCP sample were 110 and 104.7 mAh g1 after 100 cycles at 0.5 and 1 C, respectively, which are much higher than that of O-LFMCP sample (92.4 and 83.4 mAh g1 after 100 cycles at 0.5 and 1 C, respectively). During the overall cycle process, both samples yielded columbic efficiencies close to 98% at 0.5 C and 99% at 1 C. However, the initial coulombic efficiencies of the C-LFMCP sample were 86.6% and 80.9% at 0.5 C and 1 C, respectively, in comparison to the coulombic efficiencies of 75.1% and 68.3% at the corresponding rates yielded by the O-LFMCP sample. Such comparative results proved the excellent chemical performance of our C-LFMCP sample prepared through the coprecipitation-and-milling method. To further evaluate their electrochemical performance, the rate
performance test was also performed. The rate capability is shown in Fig. 5, which was carried out at various discharge rates ranging from 0.1 to 2 C, and all the cells for testing were charged at the corresponding rates. One could see that the C-LFMCP electrode exhibited better rate performance than the O-LFMCP. As shown in Fig. 5c, the C-LFMCP sample presents discharge specific capacities of 160.5, 145.6, 126.8, 109.1 and 84.7 mAh g1 at rates of 0.1, 0.2, 0.5, 1 and 2 C, respectively. When the current was recovered to 0.2 C, a discharge specific capacity of 138.8 mAh g1 could be restored, suggesting that the composite electrode delivers good structural stability and rate performance. As a comparison, the O-LFMCP sample prepared by the one-step milling method delivers discharge specific capacities of 143.8, 129.5, 109.3, 88.6 and 45.7 mAh g1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively. Such comparative result indicates the C-LFMCP possesses better Liþ diffusion dynamics than the control sample, proving the advantages of the stepwise coprecipitation-and-milling method. Park et al. prepared the Fe1/3Mn1/3Co1/3C2O4$2H2O by coprecipitation and further synthesized LiFe1/3Mn1/3Co1/3PO4 [26]. The material exhibits a capacity of 81 mAh g1 at rate of 6 C, which further supports the above
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Fig. 4. Electrochemical properties analysis of two samples, (a) Discharge plateau profiles at 0.1 C of C-LFMCP sample, (b) Cycling performance of C-LFMCP sample at 0.1 C for 50 cycles, (c, d) Cycling performance for the two samples at 0.5 and 1 C for 100 cycles, respectively.
Fig. 5. (a, b) Discharge curves and (c) rate properties of O-LFMCP sample and C-LFMCP sample between 2.5 and 5 V.
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Fig. 6. (a) CV profiles with scanning rate of 0.2 mV s1 of two samples, and (b) CVs of the first three cycles of C-LFMCP sample.
conclusions. Fig. 5a and b correspond to the discharge voltage curves of O-LFMCP and C-LFMCP sample at different rates, respectively. It can be seen that the reversible capacities contributed by the Mn3þ/Mn2þ and Co3þ/Co2þ redox couples are obviously attenuated with increasing discharge current. It is obvious that the capacity of the C-LFMCP sample decay slower than that of the OLFMCP sample. The better rate capability and higher discharge specific capacity from the C-LFMCP nanocomposite cell are most likely supported by the homogenous mixing Fe, Mn and Co element and the enhanced conductivity from the extensive presence of carbon. In this regard, CV (cyclic voltammetry) and EIS measurements were performed to further understand their merits. Fig. 6a shows the initial CV curves of the two samples in the voltage range of 2.5e5.0 V at a scanning speed of 0.2 mV s1. Three pairs of redox peaks were observed, which could be assigned to the redox reactions of Co3þ/Co2þ, Mn3þ/ Mn2þ and Fe3þ/Fe2þ couples respectively. Moreover, as listed in Table 3, significant difference in the potential difference between the oxidation peak (EO) and the reduction peak (ER) could be identified between these two samples. EO1-R1, EO2-R2 and EO3-R3 in the C-LFMCP sample electrode are 0.222 V, 0.397 V and 0.186 V, compared to those in the O-LFMCP sample which are 0.28 0 V, 0.533 V and 0.226 V, respectively. This indicates that C-LFMCP cathode material has a lower potential polarization and better reaction kinetics compared to O-LFMCP sample during the charge and discharge processes. Fig. 6b shows CVs of the first three cycles of the C-LFMCP sample, which display peaks at the same position and well overlap with each other after the second cycle, proving its excellent reversibility and cycling performance. The EIS tests were conducted for electrodes under open circuit conditions. The impedance spectra of the two LiFe0.4Mn0.4Co0.2PO4/ C electrodes are shown as Nyquist plots in Fig. 7a. The impedance diagrams consist of both the semicircle and the slanted line, corresponding to the high-frequency and low-frequency regions. The values of Rct for the O-LFMCP sample and C-LFMCP sample are 393.5 and 236.7 U by the given equivalent circuit (inset of Fig. 7a),
respectively. Fig. 7b shows the relationship between Zre and u1/2 at low frequency, from which the Dþ Li of the O-LFMCP sample and the C-LFMCP sample were calculated to be 1.863 1014 and 2.835 1013 cm2 s1, respectively. The higher Dþ Li and lower Rct of the C-LFMCP sample are favorable for efficient kinetic behavior during the electrode reaction process [37], which is consistent with
Table 3 Potential difference between the oxidation peak and the reduction peak of the two LiFe0.4Mn0.4Co0.2PO4/C samples. samples
EO1-R1(V)
EO2-R2(V)
EO3-R3(V)
C-LFMCP O-LFMCP
0.222 0.28
0.397 0.533
0.186 0.226
Fig. 7. (a) EIS profiles of two samples under open-circuit condition, inset: equivalent circuit. (b) The relationship between Zre and u1/2 at low frequency.
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their enhanced electrochemical properties. On this basis, our comparative results proved the crucial role of the synthetic strategy on the resulting cathode materials and thus the diffusion kinetics of lithium ions. 4. Conclusion In conclusion, the C-LFMCP nanocomposite with particles size between 50 and 400 nm is successfully synthesized through a stepwise coprecipitation-and-milling method. The particles in such C-LFMCP cathode material are an olivine-structured solid solution, and on each particle there exists a continuous 3-nm-thick carbon layer which well connects the particles forming a network and enables a better electronic conductivity. The C-LFMCP sample exhibited super electrochemical properties including high specific capacity and impressive rate performance. A first discharge capacity of 163.3 mAh g1 at 0.1 C was obtained, which could still maintain 110 and 104.7 mAh g1 after 100 cycles at 0.5 and 1 C, respectively. The enhanced electrochemical performance of the CLFMCP is attributed to the homogeneous mixing of the chemical elements, the uniform distribution of carbon and the small particle size of the LiFe0.4Mn0.4Co0.2PO4/C composite cathode material. Such a stepwise coprecipitation-and-milling method may offer an effective strategy for preparing phosphate type composite cathode material with excellent electrochemical performance. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21621091, 21273184), and the National Key Research and Development Program of China (No. 2016YFB0100202). References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2018) 2930e2946. [2] H. Li, Y. Chen, L. Chen, H. Jiang, Y. Wang, H. Wang, G. Li, Y. Li, Y. Yuan, Improved cycling and high rate performance of core-shell LiFe1/3Mn1/3Co1/ 3PO4/carbon nanocomposites for lithium-ion batteries: effect of the carbon source, Electrochem. Acta 143 (2014) 407e414. [3] W. Yang, Y. Bi, Y. Qin, Y. Liu, X. Zhang, B. Yang, Q. Wu, D. Wang, S. Shi, LiMn0.8Fe0.2PO4/C cathode material synthesized via co-precipitation method with superior high-rate and low-temperature performances for lithium-ion batteries, J. Power Sources 275 (2015) 785e791. [4] H. Liu, F.C. Strobridge, O.J. Borkiewicz, K.M. Wiaderek, K.W. Chapman, P.J. Chupas, C.P. Grey, Capturing metastable structures during high-rate cycling of LiFePO4 nanoparticle electrodes, Science 344 (2014), 1252817. [5] L. Wang, X. He, W. Sun, J. Wang, Y. Li, S. Fan, Crystal Orientation Tuning of LiFePO4 nanoplates for high rate lithium battery cathode materials, Nano Lett. 12 (2012) 5632e5636. [6] M.K. Devaraju, I. Honma, Hydrothermal and solvothermal process towards development of LiMPO4 (M ¼ Fe, Mn) nanomaterials for lithium-ion batteries, Adv. Energy Mater. 2 (2012) 284e297. [7] D. Choi, D. Wang, I.T. Bae, J. Xiao, Z. Nie, W. Wang, V.V. Viswanathan, Y.J. Lee, J.G. Zhang, G.L. Graff, Z. Yang, J. Liu, LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for Li-ion battery cathode, Nano Lett. 10 (2010) 2799e2805. [8] C. Wang, S. Li, Y. Han, Z. Lu, Assembly of LiMnPO4 nanoplates into microclusters as a high-performance cathode in lithium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 27618e27624. [9] F. Wang, J. Yang, Y. NuLi, J. Wang, Novel hedgehog-like 5V LiCoPO4 positive electrode material for rechargeable lithium battery, J. Power Sources 196 (2011) 4806e4810. [10] B. Wu, H. Xu, D. Mu, L. Shi, B. Jiang, L. Gai, L. Wang, Q. Liu, L. Ben, F. Wu, Controlled solvothermal synthesis and electrochemical performance of LiCoPO4 submicron single crystals as a cathode material for lithium ion batteries, J. Power Sources 304 (2016) 181e188. [11] M. Yonemura, A. Yamada, Y. Takei, N. Sonoyama, R. Kanno, Comparative kinetic study of olivine LixMPO4 (M¼Fe, Mn), J. Electrochem. Soc. 151 (2004) A1352eA1356. [12] S.M. Oh, S.T. Myung, Y.K. Sun, Olivine LiCoPO4ecarbon composite showing
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Chen, Continuous solid solutions LiFe1xCoxPO4 and its electrochemical performance, J. Power Sources 146 (2005) 580e583. [22] K. Wu, G. Hu, K. du, Z. Peng, Y. Cao, Synthesis and electrochemical characterization of LiCo1/3Fe2/3PO4/C composite using nano CoFe2O4 as precursor, J. Alloys Compd. 650 (2015) 718e723. [23] S. Akimoto, I. Taniguchi, Synthesis and characterization of LiCo1/3Mn1/3Fe1/ 3PO4/C nanocomposite cathode of lithium batteries with high rate performance, J. Power Sources 242 (2013) 627e630. [24] Y. Wang, Y. Wang, X. Liu, B. Zhu, F. Wang, Solvothermal synthesis of LiFe1/ 3Mn1/3Co1/3PO4 solid solution as lithium storage cathode materials, RSC Adv. 7 (2017) 14354e14359. [25] Y. Zhang, C.S. Sun, Z. Zhou, Solegel preparation and electrochemical performances of LiFe1/3Mn1/3Co1/3PO4/C composites with coreeshell nanostructure, Electrochem. Commun. 11 (2009) 1183e1186. [26] Y.U. Park, J. Kim, H. Gwon, D.H. Seo, S.W. Kim, K. Kang, Synthesis of multicomponent olivine by a novel mixed transition metal oxalate coprecipitation method and electrochemical characterization, Chem. Mater. 22 (2010) 2573e2581. [27] Y. Zhong, Z. Wu, J. Li, W. Xiang, X. Guo, B. Zhong, S. Sun, Synthesis of core-shell structured LiFe0.5Mn0.3Co0.2PO4@C with remarkable electrochemical performance as the cathode of a lithium-ion battery, ChemElectroChem 2 (2015) 896e902. [28] D.D. Lecce, R. Brescia, A. Scarpellini, M. Prato, J. Hassoun, A high voltage olivine cathode for application in lithium-ion batteries, ChemSusChem 9 (2016) 223e230. [29] H.T. Kuo, T.S.C, N.C. Bagkar, G.Q. Liu, R.S. Liu, C.H. Shen, D.S. Shy, X.K. Xing, J.M. Chen, Effect of Co2P on electrochemical performance of Li(Mn0.35Co0.2Fe0.45)PO4/C, J. Phys. Chem. B 112 (2008) 8017e8023. [30] K. Wang, Z.G. Wu, T. Zhang, Y.P. Deng, J.T. Li, X.D. Guo, B.B. Xu, B.H. 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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Synthesis of LiFe0.4Mn0.4Co0.2PO4/C cathode material of lithium ion battery with enhanced electrochemical performance Zhi-Gen Huang a, Jun-Tao Li b, *, Kai Wang b, Wen-Feng Ren a, Yan-Qiu Lu b, Li Deng b, Ling Huang a, Shi-Gang Sun a, ** a b
State Key Lab of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China College of Energy, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 February 2018 Received in revised form 22 November 2018 Accepted 11 December 2018 Available online 13 December 2018
An olivine-structured solid solution, LiFe0.4Mn0.4Co0.2PO4/C nanocomposite, was synthesized as cathode materials for Li-ion batteries by the coprecipitation-and-milling method using stearic acid as the carbon source. Thus-formed LiFe0.4Mn0.4Co0.2PO4/C solid solution was well confirmed and characterized by various techniques such as SEM, TEM and XRD. The LiFe0.4Mn0.4Co0.2PO4/C-based cathode delivered an initial discharge capacity of 163.3 mAh g1 at 0.1 C, together with a capacity retention rate of 86.6% after 50 cycles in the potential range of 2.5e5 V. Even at 1 C, a discharge capacity of 104.7 mAh g1 was maintained after 100 cycles. The good electrochemical performance of the LiFe0.4Mn0.4Co0.2PO4/C could be attributed to the homogeneous mixing of Fe, Mn and Co elements in relevant precursor solid solution Fe0.4Mn0.4Co0.2C2O4$2H2O. Its characteristic CV profiles reveal three pairs of anodic/cathode peaks which could be assigned to the redox reactions of Co3þ/Co2þ, Mn3þ/Mn2þ and Fe3þ/Fe2þ with potential plateaus at about 4.7 V, 4.1 V and 3.5 V in the charge/discharge curves, respectively. Our study demonstrates that the LiFe0.4Mn0.4Co0.2PO4/C is a promising cathode material of Li-ion battery and the coprecipitation-andmilling method represents a workable strategy towards the synthesis of high-performance phosphatetype composite cathode. © 2018 Elsevier B.V. All rights reserved.
Keywords: LiFe0.4Mn0.4Co0.2PO4/C Solid solution Coprecipitation-and-milling method Cathode Li-ion batteries
1. Introduction Echoing to the ever-increasing demand for energy, the yearly decreasing reserves of fossil fuels and the growing global concern on the environmental issues, the efficient, safe and low-pollution Li-ion batteries have attracted more and more attention, especially the high specific energy batteries for applications in vehicles [1e3]. Among various cathode materials, the olivine-structured LiFePO4 cathode materials have been successfully commercialized thanks to its safety, low cost, environment friendliness, excellent electrochemical performance and remarkable thermal stability [4,5]. However, LiFePO4 cathodes generally operate at a low voltage (3.4 V versus Li/Liþ), which results in a low specific energy density of the battery, and they also suffer from inherently low electronic conductivity. In a closely related issue, olivine type cathode materials of LiMnPO4 and LiCoPO4, though which own the same
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J.-T. Li). https://doi.org/10.1016/j.jallcom.2018.12.169 0925-8388/© 2018 Elsevier B.V. All rights reserved.
theoretical capacity (about 170 mAh g1) as LiFePO4, have received great attention due to their higher operating voltage (4.1 and 4.8 V versus Li/Liþ, respectively) [6e10]. However, the LiMnPO4 cathode suffers from severe capacity fading, mainly due to the Jahn-Teller effect of Mn3þ ions, and its extremely low intrinsic electronic conductivity [11]. For the LiCoPO4, it suffers from poor cycling stability due to the decomposition of common carbonate-based electrolytes at high voltage [12,13]. To overcome these challenges and improve the electrochemical properties of cathode materials, various olivine-type solid solutions such as LiFe1-xMnxPO4 [14e20], LiFe1-xCoxPO4 [21,22], and LiFexMnyCozPO4 [2,23e29] (x þ y þ z ¼ 1) were explored, especially the LiFe1-xMnxPO4 which has been successfully commercialized. Moreover, LiFexMnyCozPO4 with a variety of chemical compositions such as LiFe1/3Mn1/3Co1/3PO4 [2,23e26], LiFe0.5Mn0.3Co0.2PO4 [27], LiFe0.25Mn0.5Co0.25PO4 [28], and LiMn0.35Co0.2Fe0.45PO4 [29] were also successfully synthesized, which generally demonstrated good performances. The existing researches indicated that, after doping with Fe and Co, the activity of Mn in the LiFexMnyCozPO4 cathode is obviously improved due to enhanced conductivity and suppressed
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Jahn-Teller deformation. In the current study, LiFe0.4Mn0.4Co0.2PO4/C nanocomposite was synthesized through a coprecipitation process followed by a wetmilling process. The effects of preparation conditions on the morphologies and electrochemical properties of the resulting LiFe0.4Mn0.4Co0.2PO4/C composite were systematically evaluated. The assynthesized LiFe0.4Mn0.4Co0.2PO4/C cathode can deliver an initial discharge capacity of 163.3 mAh g1 at 0.1 C and also a capacity retention rate of 86.6% after 50 cycles in the voltage range of 2.5e5 V. Even at of 1 C, a discharge capacity of 104.7 mAh g1 after 100 cycles was maintained. We attributed such remarkable electrochemical performance to the homogeneous mixing of Fe, Mn and Co elements in the solid solution. 2. Experimental 2.1. Materials and chemicals All chemical reagents in this study were of analytical grade and used directly without further purification. Among them, FeC2O4$2H2O, FeSO4$7H2O, MnSO4$H2O, MnCO3, CoCO3, LiH2PO4, and stearic acid were provided by Xiqiao Chemical Co., Ltd. (NH4)2C2O.4H2O and CoSO4$7H2O were provided by Sinopharm Chemical Reagent Co., Ltd. 2.2. Preparation of Fe0.4Mn0.4Co0.2C2O4·2H2O and LiFe0.4Mn0.4Co0.2PO4/C The Fe0.4Mn0.4Co0.2C2O4$2H2O precursor was synthesized by the coprecipitation method. FeSO4$7H2O, MnSO4$H2O and CoSO4$H2O with a stoichiometric ratio were added and stirred in a beaker, followed by dropwise addition of a sufficient of 0.3 M (NH4)2C2O4$H2O solution. During this process, the solution became turbid yellow. The resulting yellow particles which were obtained by filtration and were washed with deionized water for 5 times were dried in oven at 80 C for 8 h to yield the Fe0.4Mn0.4Co0.2C2O4$2H2O product. Subsequently, the as-prepared Fe0.4Mn0.4Co0.2C2O4$2H2O, LiH2PO4, and stearic acid (the molar ratio between Fe0.4Mn0.4Co0.2C2O4$2H2O and stearic acid is about 2:1.1) with a stoichiometric ratio was mixed with a portion of ethanol in a milling tank. The mixture was ground for 12 h through a planetary ball mill at 400 rpm with a ball diameter as about 8 mm and the weight ratio between balls and powder as about 10:1. Afterward, after being dried at 45 C in a vacuum oven overnight, the mixture was pre-calcined at 400 C for 3 h and then at 700 C for 16 h in argon atmosphere with a ramping rate of 5 C min1 to produce the LiFe0.4Mn0.4Co0.2PO4/C sample (denoted as C-LFMCP). A control sample was also prepared by a one-step milling method (denoted as O-LFMCP). The starting materials, namely LiH2PO4, FeC2O.42H2O, MnCO3 and CoCO3 with a stoichiometric ratio, and stearic acid (the molar ratio between FeC2O.42H2O and stearic acid is about 0.8:1.1) were mixed with a portion of ethanol in the milling tank. The mixture then was subjected to the same thermal treatment used for the synthesis of C-LFMCP to give the OLFMCP. 2.3. Characterizations The crystalline phases of the samples were characterized by Xray diffraction (XRD) using a Rigaku D/Max III diffractometer with Cu Ka radiation. The lattice parameters were obtained by using PDXL software (Rigaku Co., Ltd., PDXL 2.1) [30]. The particles size, micro-morphologies and structure information of the samples were observed by scanning electron microscopy (SEM, Hitachi S4800 N) equipped with an energy dispersive X-ray spectroscopy
(EDS) instrument and TEM (JEOLJEM-2100 microscopy). The material impedance was measured using a PARSTAT 2263 electrochemical tester (Princeton applied researcher, USA) and fitting through the ZView software. As shown in inset of EIS (electrochemical impedance spectroscopy), Re, Rct and W1 represent the electrolyte impedance, charge impedance and the Warburg impedance, respectively, and CPE is the constant phase angle element. The lithium ion diffusion coefficients (Dþ Li) of the coin cells for the two samples were calculated using the following two equations (1) and (2) [31].
. 2A2 n4 F 4 C 2 s2 DLi þ ¼ R2 T 2
(1)
Zre ¼ Re þ Rct þ su1=2
(2)
where R and F represent gas and Faraday constant, and T, A, n and C denote the absolute temperature, the surface area of the electrode sheet, the number of electrons per mole and the molar concentration of lithium ions in LiFe0.4Mn0.4Co0.2PO4/C, respectively. Then s (Warburg factor) is calculated according to the relationship between Zre (which is the Liþ diffusion resistance in the LiFe0.4Mn0.4Co0.2PO4/C cathode) and u (the angular frequency) in equation (2). 2.4. Electrochemical studies Electrochemical tests were conducted in the CR2025 coin cells, in which a lithium foil acted as a counter electrode and LiFe0.4Mn0.4Co0.2PO4/C served as the cathodes. For the preparation of the LiFe0.4Mn0.4Co0.2PO4/C cathode, LiFe0.4Mn0.4Co0.2PO4/C, acetylene black, and PVDF (polyvinylidene fluoride) binder in a weight ratio of 8: 1: 1 in N-methylpyrrolidone (NMP) were mixed to form a homogeneous slurry through ball-milling. The slurry was then uniformly casted on Al foils and heated at 100 C in vacuum overnight. The mass loadings of the cathodes were about 2.5e3.0 mg cm2. Celgard 2400 was used as the separator. 1 M LiPF6 and some additives dissolved in 1 L of ethylene carbonate and dimethyl carbonate (1: 1 in volume, from Fosai New Materials Co. Ltd., Jiangsu, China) was used as the electrolyte. All the cells were subjected to galvanostatic charge and discharge tests under operating voltages ranging from 2.5 to 5 V at 30 C. 3. Results and discussion Fig. 1a displays the XRD pattern of the Fe0.4Mn0.4Co0.2C2O4$2H2O. The major peaks at 18.3, 18.7, 22.6 and 29.3 can be indexed to FeC2O.42H2O, CoC2O4$2H2O and MnC2O4$2H2O [32], indicating that the as-prepared material is a composite of FeC2O.42H2O, CoC2O4$2H2O and MnC2O4$2H2O. As shown in Fig. 1b, for the resulting C-LFMCP, most of the peaks in its XRD pattern could be referred to the orthorhombic structure with a Pnma space group. Note that their diffraction peaks locate at positions similar to those of the LiFePO4 (JCPDS, 40-1499), but with slight shift, which proves the sample as a good solid solution instead of a simple physical mixture of LiFePO4, LiMnPO4, and LiCoPO4. This is further confirmed by the resulting lattice parameters (Table 1) obtained from Rietveld refinement. No significant peak of carbon was found in the XRD pattern, signifying very low carbon content or the amorphous nature of the carbon in the sample. Fig. 1c shows the thermogravimetric curve of the C-LFMCP material, which indicates that the carbon mass fraction is about 2.9% wt in the sample. The morphology and size of the C-LFMCP and O-LFMCP were characterized by SEM. As shown in Fig. 2a and b, the particles in the C-LFMCP generally assume an irregular nano-morphology, and
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Fig. 1. XRD patterns of the as-prepared Fe0.4Mn0.4Co0.2C2O4.2H2O precursor (a) and the C-LFMCP sample (b), the thermogravimetric curve of the C-LFMCP sample (c).
Table 1 Lattice parameters obtained by XRD Rietveld refinement of C-LFMCP and other LiMPO4 (M ¼ Fe, Mn, Co). Compound
a(Å)
b(Å)
c(Å)
V (Å3)
LiFe0.4Mn0.4Co0.2PO4 (this work) LiFe0.25Mn0.5Co0.25PO4 [28] LiFe0.5Mn0.3Co0.2PO4 [17] LiCo1/3Mn1/3Fe1/3PO4 [23] LiFe1/3Mn1/3Co1/3PO4 [26] LiFePO4(JCPDS:40-1499) LiMnPO4(JCPDS:33-0804) LiCoPO4(JCPDS:32-0552)
10.338 10.377 10.321 10.327 10.326 10.347 10.454 10.206
6.018 6.041 6.010 6.012 6.011 6.019 6.106 5.922
4.708 4.735 4.705 4.717 4.713 4.704 4.749 4.701
292.908 296.82 291.847 292.95 292.611 292.959 303.139 284.128
their size ranges from 50 to 400 nm. In Fig. 2e and f, the particles in the O-LFMCP show an irregular nano-morphology, and the size is also between 50 and 400 nm. Compared to the other materials synthesized by ball milling, the particles in our C-LFMCP and OLFMCP are smaller in size, mainly attributing to the presence of stearic acid which not only serves as the carbon source but also can limit the growth of particles [17,33]. The C-LFMCP is composed of aggregated nanoparticles closely interconnected by carbon, as demonstrated in Fig. 2b, indicating their good conductivity. Their relevant TEM images are displayed in Fig. 2cee and Fig. 2hej. The particle sizes of the samples shown in Fig. 2c and h are in good agreement with that obtained from the SEM images. The presence of carbon which links the neighboring particles and forms an effective conductive carbon network in C-LFMCP sample
was further verified. Fig. 2d illustrates a typical C-LFMCP nanoparticle, from which one can observe an amorphous carbon layer that is uniformly coated on the entire nanoparticle. From the highresolution TEM images in Fig. 2e, the thickness of the carbon shell is about 3 nm, further implying remarkable conductive contact between the C-LFMCP particles. In addition, the legible lattice fringes in the C-LFMCP nanoparticle were also observed in the highresolution TEM image in Fig. 2e with a d-spacing of about 0.416 nm, corresponding to the (101) planes of C-LFMCP, which agrees well with the above Rietveld refinement of XRD data. Such results indicate that the C-LFMCP sample is a single-phase solid solution with high crystallinity, which could be attributed to the homogeneous mixing of Fe, Mn and Co elements in the sample by the coprecipitation method. In particular, it shall also be mentioned that the large spacing gap of the C-LFMCP sample can allow relatively high Li-ion transport capability [34]. In comparison, as shown in the TEM images (Fig. 2 h-j) though the surface of the O-LFMCP was also coated with a carbon layer with a thickness of 3 nm. EDS element analysis (Table 2) demonstrates that the molar ratio of Mn, Fe and Co in the C-LFMCP composite is close to 2:2:1, which is consistent with the molar ratio of the starting materials. Elemental mappings of the C-LFMCP sample (Fig. 3a) illustrated the homogeneous distribution of Fe, Mn, Co, and O in the sample together with an even distribution of carbon. Meanwhile, EDS mapping of Fe0.4Mn0.4Co0.2C2O.42H2O (Fig. 3b) further verified homogeneous distribution of Fe, Mn and Co element in the sample. To further evaluate the electrochemical properties of C-LFMCP
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Fig. 2. SEM (a, b) and TEM (c, d and e) images of the C-LFMCP sample, SEM (e, f) and TEM (h, i and j) images of the O-LFMCP sample.
Table 2 Parameters of Fe, Mn and Co content of C-LFMCP from the EDS. samples
Element
Atomic ratio
C-LFMCP
Fe Mn Co
0.42 0.38 0.21
cathode, galvanostatic charge-discharge, cycling and rate capability tests were carried out. Fig. 4a displays the discharge curves of selected cycles of the C-LFMCP cathode at a low rate of 0.1 C (1 C ¼ 170 mAh g1) between 2.5 and 5 V. Three characteristic potential plateaus appear clearly at about 4.7 V, 4.1 V and 3.5 V, corresponding to the redox processes of Co3þ/Co2þ, Mn3þ/Mn2þ and
Fe3þ/Fe2þ, respectively. The redox potentials of Fe or Co in the LiFe0.4Mn0.4Co0.2PO4/C were different from those in the pure LiFePO4 or LiCoPO4 [35,36], while the redox potential of Mn is the same as those in the pure LiMnPO4. Such differences may originate from the strong interaction between the Fe-O-Mn (Co) ions [35]. In addition, compared with Fe which generally have stable activity, the activities of Mn and Co decay rapidly in the discharge process, which may be attributed to the Jahn-Teller effect of Mn3þ ions [11] and the decomposition of electrolytes due to the excessively high redox potential of Co [12,13]. The cycle performance of C-LFMCP sample prepared by coprecipitation-and-milling at 0.1 C is shown in Fig. 4b. At the first cycle, the discharge specific capacity of C-LFMCP cathode is 163.3 mAh g1, which is close to its theoretical capacity (170 mAh
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Fig. 3. (a) SEM image and EDS mapping of the corresponding elemental distributions of C, O, Fe, Mn and Co element of C-LFMCP, (b) SEM image and EDS mapping of Fe, Mn and Co element of Fe0.4Mn0.4Co0.2C2O.42H2O.
g1). A capacity retention of 86.6% was obtained after 50 cycles. The sample displayed an initial coulombic efficiency of 86%, which increased gradually along with the increasing cycles and eventually maintained at about 96% after 50 cycles. Fig. 4c and d further depict the stable cycling behavior of CLFMCP sample at 0.5 and 1 C, respectively. Compared to O-LFMCP sample, C-LFMCP sample exhibits higher initial discharge capacities at 0.5 and 1 C. The discharge capacities of C-LFMCP sample were 110 and 104.7 mAh g1 after 100 cycles at 0.5 and 1 C, respectively, which are much higher than that of O-LFMCP sample (92.4 and 83.4 mAh g1 after 100 cycles at 0.5 and 1 C, respectively). During the overall cycle process, both samples yielded columbic efficiencies close to 98% at 0.5 C and 99% at 1 C. However, the initial coulombic efficiencies of the C-LFMCP sample were 86.6% and 80.9% at 0.5 C and 1 C, respectively, in comparison to the coulombic efficiencies of 75.1% and 68.3% at the corresponding rates yielded by the O-LFMCP sample. Such comparative results proved the excellent chemical performance of our C-LFMCP sample prepared through the coprecipitation-and-milling method. To further evaluate their electrochemical performance, the rate
performance test was also performed. The rate capability is shown in Fig. 5, which was carried out at various discharge rates ranging from 0.1 to 2 C, and all the cells for testing were charged at the corresponding rates. One could see that the C-LFMCP electrode exhibited better rate performance than the O-LFMCP. As shown in Fig. 5c, the C-LFMCP sample presents discharge specific capacities of 160.5, 145.6, 126.8, 109.1 and 84.7 mAh g1 at rates of 0.1, 0.2, 0.5, 1 and 2 C, respectively. When the current was recovered to 0.2 C, a discharge specific capacity of 138.8 mAh g1 could be restored, suggesting that the composite electrode delivers good structural stability and rate performance. As a comparison, the O-LFMCP sample prepared by the one-step milling method delivers discharge specific capacities of 143.8, 129.5, 109.3, 88.6 and 45.7 mAh g1 at 0.1, 0.2, 0.5, 1 and 2 C, respectively. Such comparative result indicates the C-LFMCP possesses better Liþ diffusion dynamics than the control sample, proving the advantages of the stepwise coprecipitation-and-milling method. Park et al. prepared the Fe1/3Mn1/3Co1/3C2O4$2H2O by coprecipitation and further synthesized LiFe1/3Mn1/3Co1/3PO4 [26]. The material exhibits a capacity of 81 mAh g1 at rate of 6 C, which further supports the above
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Fig. 4. Electrochemical properties analysis of two samples, (a) Discharge plateau profiles at 0.1 C of C-LFMCP sample, (b) Cycling performance of C-LFMCP sample at 0.1 C for 50 cycles, (c, d) Cycling performance for the two samples at 0.5 and 1 C for 100 cycles, respectively.
Fig. 5. (a, b) Discharge curves and (c) rate properties of O-LFMCP sample and C-LFMCP sample between 2.5 and 5 V.
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Fig. 6. (a) CV profiles with scanning rate of 0.2 mV s1 of two samples, and (b) CVs of the first three cycles of C-LFMCP sample.
conclusions. Fig. 5a and b correspond to the discharge voltage curves of O-LFMCP and C-LFMCP sample at different rates, respectively. It can be seen that the reversible capacities contributed by the Mn3þ/Mn2þ and Co3þ/Co2þ redox couples are obviously attenuated with increasing discharge current. It is obvious that the capacity of the C-LFMCP sample decay slower than that of the OLFMCP sample. The better rate capability and higher discharge specific capacity from the C-LFMCP nanocomposite cell are most likely supported by the homogenous mixing Fe, Mn and Co element and the enhanced conductivity from the extensive presence of carbon. In this regard, CV (cyclic voltammetry) and EIS measurements were performed to further understand their merits. Fig. 6a shows the initial CV curves of the two samples in the voltage range of 2.5e5.0 V at a scanning speed of 0.2 mV s1. Three pairs of redox peaks were observed, which could be assigned to the redox reactions of Co3þ/Co2þ, Mn3þ/ Mn2þ and Fe3þ/Fe2þ couples respectively. Moreover, as listed in Table 3, significant difference in the potential difference between the oxidation peak (EO) and the reduction peak (ER) could be identified between these two samples. EO1-R1, EO2-R2 and EO3-R3 in the C-LFMCP sample electrode are 0.222 V, 0.397 V and 0.186 V, compared to those in the O-LFMCP sample which are 0.28 0 V, 0.533 V and 0.226 V, respectively. This indicates that C-LFMCP cathode material has a lower potential polarization and better reaction kinetics compared to O-LFMCP sample during the charge and discharge processes. Fig. 6b shows CVs of the first three cycles of the C-LFMCP sample, which display peaks at the same position and well overlap with each other after the second cycle, proving its excellent reversibility and cycling performance. The EIS tests were conducted for electrodes under open circuit conditions. The impedance spectra of the two LiFe0.4Mn0.4Co0.2PO4/ C electrodes are shown as Nyquist plots in Fig. 7a. The impedance diagrams consist of both the semicircle and the slanted line, corresponding to the high-frequency and low-frequency regions. The values of Rct for the O-LFMCP sample and C-LFMCP sample are 393.5 and 236.7 U by the given equivalent circuit (inset of Fig. 7a),
respectively. Fig. 7b shows the relationship between Zre and u1/2 at low frequency, from which the Dþ Li of the O-LFMCP sample and the C-LFMCP sample were calculated to be 1.863 1014 and 2.835 1013 cm2 s1, respectively. The higher Dþ Li and lower Rct of the C-LFMCP sample are favorable for efficient kinetic behavior during the electrode reaction process [37], which is consistent with
Table 3 Potential difference between the oxidation peak and the reduction peak of the two LiFe0.4Mn0.4Co0.2PO4/C samples. samples
EO1-R1(V)
EO2-R2(V)
EO3-R3(V)
C-LFMCP O-LFMCP
0.222 0.28
0.397 0.533
0.186 0.226
Fig. 7. (a) EIS profiles of two samples under open-circuit condition, inset: equivalent circuit. (b) The relationship between Zre and u1/2 at low frequency.
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their enhanced electrochemical properties. On this basis, our comparative results proved the crucial role of the synthetic strategy on the resulting cathode materials and thus the diffusion kinetics of lithium ions. 4. Conclusion In conclusion, the C-LFMCP nanocomposite with particles size between 50 and 400 nm is successfully synthesized through a stepwise coprecipitation-and-milling method. The particles in such C-LFMCP cathode material are an olivine-structured solid solution, and on each particle there exists a continuous 3-nm-thick carbon layer which well connects the particles forming a network and enables a better electronic conductivity. The C-LFMCP sample exhibited super electrochemical properties including high specific capacity and impressive rate performance. A first discharge capacity of 163.3 mAh g1 at 0.1 C was obtained, which could still maintain 110 and 104.7 mAh g1 after 100 cycles at 0.5 and 1 C, respectively. The enhanced electrochemical performance of the CLFMCP is attributed to the homogeneous mixing of the chemical elements, the uniform distribution of carbon and the small particle size of the LiFe0.4Mn0.4Co0.2PO4/C composite cathode material. Such a stepwise coprecipitation-and-milling method may offer an effective strategy for preparing phosphate type composite cathode material with excellent electrochemical performance. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (21621091, 21273184), and the National Key Research and Development Program of China (No. 2016YFB0100202). References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2018) 2930e2946. [2] H. Li, Y. Chen, L. Chen, H. Jiang, Y. Wang, H. Wang, G. Li, Y. Li, Y. Yuan, Improved cycling and high rate performance of core-shell LiFe1/3Mn1/3Co1/ 3PO4/carbon nanocomposites for lithium-ion batteries: effect of the carbon source, Electrochem. Acta 143 (2014) 407e414. [3] W. Yang, Y. Bi, Y. Qin, Y. Liu, X. Zhang, B. Yang, Q. Wu, D. Wang, S. Shi, LiMn0.8Fe0.2PO4/C cathode material synthesized via co-precipitation method with superior high-rate and low-temperature performances for lithium-ion batteries, J. Power Sources 275 (2015) 785e791. [4] H. Liu, F.C. Strobridge, O.J. Borkiewicz, K.M. Wiaderek, K.W. Chapman, P.J. Chupas, C.P. Grey, Capturing metastable structures during high-rate cycling of LiFePO4 nanoparticle electrodes, Science 344 (2014), 1252817. [5] L. Wang, X. He, W. Sun, J. Wang, Y. Li, S. Fan, Crystal Orientation Tuning of LiFePO4 nanoplates for high rate lithium battery cathode materials, Nano Lett. 12 (2012) 5632e5636. [6] M.K. Devaraju, I. Honma, Hydrothermal and solvothermal process towards development of LiMPO4 (M ¼ Fe, Mn) nanomaterials for lithium-ion batteries, Adv. Energy Mater. 2 (2012) 284e297. [7] D. Choi, D. Wang, I.T. Bae, J. Xiao, Z. Nie, W. Wang, V.V. Viswanathan, Y.J. Lee, J.G. Zhang, G.L. Graff, Z. Yang, J. Liu, LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for Li-ion battery cathode, Nano Lett. 10 (2010) 2799e2805. [8] C. Wang, S. Li, Y. Han, Z. Lu, Assembly of LiMnPO4 nanoplates into microclusters as a high-performance cathode in lithium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 27618e27624. [9] F. Wang, J. Yang, Y. NuLi, J. Wang, Novel hedgehog-like 5V LiCoPO4 positive electrode material for rechargeable lithium battery, J. Power Sources 196 (2011) 4806e4810. [10] B. Wu, H. Xu, D. Mu, L. Shi, B. Jiang, L. Gai, L. Wang, Q. Liu, L. Ben, F. Wu, Controlled solvothermal synthesis and electrochemical performance of LiCoPO4 submicron single crystals as a cathode material for lithium ion batteries, J. Power Sources 304 (2016) 181e188. [11] M. Yonemura, A. Yamada, Y. Takei, N. Sonoyama, R. Kanno, Comparative kinetic study of olivine LixMPO4 (M¼Fe, Mn), J. Electrochem. Soc. 151 (2004) A1352eA1356. [12] S.M. Oh, S.T. Myung, Y.K. Sun, Olivine LiCoPO4ecarbon composite showing
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