Article pubs.acs.org/EF
Electrochemical Behaviors of LiMn1−xFexPO4/C Cathode Materials in an Aqueous Electrolyte with/without Dissolved Oxygen Mingshu Zhao,*,† Guanliang Huang, Weigang Zhang, Hanyuan Zhang, and Xiaoping Song MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, School of Science, Xi’an Jiaotong University, 710049, Xi’an, China S Supporting Information *
ABSTRACT: Olivine LiMn1−xFexPO4 (x = 0.5, 0.4, 0.3, 0.2) coated with carbon are prepared as the cathode materials for the aqueous rechargeable lithium-ion batteries (ARLBs). Li+ insertion-extraction behaviors of the as-prepared materials in LiNO3 aqueous electrolyte with/without dissolved oxygen are studied by cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS). The Li+ diffusion coefficient of LiMn1−xFexPO4 in LiNO3 aqueous electrolyte is first reported. The results indicate that eliminating the dissolved oxygen in the aqueous electrolyte could decrease the charge transfer-resistance and increase the Li+ diffusion coefficient of (LiMn1−xFexPO4/C)//LiV3O8 ARLBs, which also improves the electrochemical properties of the ARLBs. The initial discharge specific capacity of LiMn0.6Fe0.4PO4/C is 112 mA h g−1 at 0.1 C-rate, which provides a new candidate cathode material for ARLBs.
1. INTRODUCTION
without dissolved oxygen are studied by cyclic voltammogram (CV) and electrochemical impedance spectra (EIS) testing.
The traditional lithium-ion batteries have been widely used in the mobile electric devices, electric vehicles, and energy storage devices; however, they still have safety issues caused by the usage of the organic electrolyte. About twenty years ago, Dahn et al. put forward the aqueous rechargeable lithium-ion battery (ARLB), which has the advantages of greater safety and lower cost due to its inorganic aqueous electrolyte.1 The aqueous electrolyte has higher ionic conductivity than that of the organic electrolyte. Hence, research works about the ARLB arose in recent years, including LiMn2O4,2−4 LiCr0.15Mn1.85O4,5,6 LiCoO2,7,8 LiNiO2,9 LiFePO4,10−12 LiMnPO4,13,14 and LiCoPO4.15,16 It is well-known that LiMnPO4 has higher power density compared to that of LiFePO4. Yet, its electric conductivity and ionic conductivity are lower. Moreover, during the charge−discharge process, the volume change between LiMnPO4 and MnPO4 make Li+ extract from LiMnPO4 incompletely, leading to worse capacity and rate capability. Therefore, it is meaningful to improve the Li+ insertion−extraction ability of LiMnPO4 in the aqueous electrolyte. For the traditional lithium-ion batteries, the electrochemical properties of LiMnPO4 have been improved significantly by carbon coating,17,18 particles size minimization,19,20 and cation doping21−26 such as Zr, Fe, Mg, Cu, and so on. But, for the ARLB system, there are few research works about the electrochemical performances of LiMnPO4 cathode materials. In this paper, LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) are synthesized by a sol−gel method and sintering process. The crystalline structure and morphology of as-prepared materials are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and high-resolution transmission microscopy (HRTEM). These above materials are successfully used as the cathode materials for the ARLBs. And, their Li+ insertion-extraction behaviors in the aqueous electrolyte with/ © 2013 American Chemical Society
2. EXPERIMENTAL SECTION LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) were prepared by the sol− gel method and calcination process. CH3COOLi·2H2O, MnCl2·4H2O, FeCl2·4H2O, P2O5, and critic acid with a molar ratio of 1:(1− x):x:0.5:1 were dissolved in 50 mL ethanol solution. The obtained mixture solution was rigorously stirred for 10 h in nitrogen gas and heated at 353 K to get xerogel. Then, the xerogel was heated at 773 K for 8 h in the flowing purified argon gas to obtain LiMn1−xFexPO4/C. LiV3O8 was synthesized as described in ref 26. The three-electrode cell was constructed with the LiMn1−xFexPO4/ C (x = 0.5, 0.4, 0.3, 0.2) cathode (15 mm in length, 5 mm in width, 0.2 mm in thickness, and the weight of the active material was about 12 mg) as the working electrode (WE), LiV3O8 anode (with identical parameters to the cathode) was used as counter electrode (CE), and the saturated calomel electrode (0.242 vs SHE/V) was used as reference electrode (RE). The WE was made by mixing active materials (LiMn1−xFexPO4/C), acetylene black, and polyvinlidene fluoride (PVDF) in a weight ratio of 80:10:10 using Nmethylpyrrolidone as solvent; the black slurry was uniformly mixed with an ultrasonic process for about 2 min and then, coated on nickel mesh followed by drying at 373 K for 10 h under vacuum. The CE was prepared by the same method as the WE described above. The neutral electrolyte was prepared by dissolving an appropriate amount of LiOH·H2O in the saturated LiNO3 solution. It is noted that the experimental deoxygenated electrolyte was prepared by bubbling highpurity nitrogen for 3 h before electrochemical testing. The as-prepared materials were characterized by XRD analysis using Bruker D8-Adavanced diffractometer. The lattice parameters were refined by Rietveld analysis using the Topas software. The morphologies were observed by JEOL JEM-2100. The galvanostatic charge−discharge tests of the ARLBs were performed using an Arbin BT2000 instrument. For the as-prepared nanocomposites, the CV Received: November 8, 2012 Revised: January 9, 2013 Published: January 9, 2013 1162
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Figure 1. XRD analysis of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2): (a) XRD patterns; (b−e) Rietveld refinements; (f) lattice parameters. curves within −0.4−1.2 V (vs SCE) at 0.2 mV s−1 and the EIS within 0.01−100 Hz were performed by Ametek VMC-4 in the aqueous electrolyte with/without dissolved oxygen.
structure. Figure 1b−e illustrates the refinement results of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) (details in the Supporting Information), including the refinement parameters of Rwp and Rp. Using the Rietveld refinement method, the calculated lattice parameters are presented in Figure 1f. It can be indicated that lattice parameters and the cell volume of LiMn1−xFexPO4/C increased with the increased Mn content. The lattice parameters a, b, c, and V of LiMnPO4 (ICDD no. 33-0804) are 10.454, 6.106, and 4.749 Å and 303.14 Å3, and
3. RESULTS AND DISCUSSION Figure 1a shows the XRD patterns of LiMn1−xFexPO4/C materials (x = 0.5, 0.4, 0.3, 0.2), and they exhibit a highly crystalline olivine structure (ICDD no. 13-0336). Almost all the diffraction peaks can be indexed into an orthorhombic olivine 1163
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Figure 2. TEM images of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2): (a) LiMn0.5Fe0.5PO4/C; (b) LiMn0.6Fe0.4PO4/C; (c) LiMn0.7Fe0.3PO4/C; (d) LiMn0.8Fe0.2PO4/C. Note: Right upper insets are ED patterns of the region marked “Sp1”; right lower insets are HRTEM photos.
cycles, two pairs of redox peaks also existed at about 0.38/0.42 and 0.82/1.05 V (vs SCE), respectively. Compared to the potentials of the hydrogen and the oxygen evolutions in the aqueous electrolyte, the redox potentials of LiMn1−xFexPO4/C are more positive and more negative. It suggests that LiMn1−xFexPO4/C is stable when it is used as candidate cathode materials for the ARLB. It is noted that the additional peaks marked with the dark arrows are observed in the aqueous electrolyte with the dissolved oxygen for the as-prepared materials (including LiMn0.5Fe0.5PO4/C, LiMn0.6Fe0.4PO4/C, LiMn0.7Fe0.3PO4/C, LiMn0.8Fe0.2PO4/C); however, they are not found in the aqueous electrolyte without the dissolved oxygen for the asprepared materials (including LiMn 0 . 5 Fe 0 . 5 PO 4 /C, LiMn0.6Fe0.4PO4/C, LiMn0.7Fe0.3PO4/C, LiMn0.8Fe0.2PO4/C). Since the resting time of (LiMn1−xFexPO4/C)//LiV3O8 ARLBs in the aqueous electrolyte without the dissolved oxygen is longer than that with the dissolved oxygen, we presume the additional peak may be attributed to the resting time and the dissolved oxygen. The shorter resting time results in insufficient interfacial contact between the electrode and electrolyte, which leads to uncompleted reaction. Then in Figure 3, we can observe the additional peak in the CV profiles. Moreover, in the aqueous electrolyte with the dissolved oxygen, it is observed that the capacity fades relatively fast upon cycling, especially for LiMn0.5Fe0.5PO4 and LiMn0.8Fe0.2PO4. If the dissolved oxygen
those of LiFePO4 (ICDD no. 40-1499) are 10.347, 6.0189, and 4.7039 Å and 292.95 Å3. Comparing the lattice parameters of the as-prepared materials (shown in Figure 1f) to that of LiMnPO4 and LiFePO4, we can prove that these values of LiMn1−xFexPO4 satisfy Vegard’s law. Figure 2 presents the TEM images of LiMn1−xFexPO4/C. The particles of the asprepared materials show uniform morphology with the carbon layers. The right upper inset images are the electron diffraction (ED) patterns of the regions marked “Sp1”, which demonstrate that these particles consist of many small crystallites. The right lower inset images are HRTEM photos, indicating that the thickness of the carbon coating is about 3−4 nm. After the calcinations process, LiMn1−xFexPO4 (x = 0.5, 0.4, 0.3, 0.2) are coated with 9.78, 9.82, 9.80, and 9.71 wt % carbon, respectively. The weights of the coated carbon were measured using thermogravimetry measurement. CV curves of the synthesized materials in the neutral LiNO3 aqueous electrolyte with/without dissolved oxygen at the scanning rate of 0.2 mV s−1 are shown in Figure 3. While LiMn1−xFexPO4/C material charged/discharged in the neutral LiNO3 aqueous electrolyte with the dissolved oxygen in the seventh cycles, there are two pairs of redox peaks situated at about 0.4/0.45 and 0.82/1.05 V (vs SCE), respectively, corresponding to the reversible insertion/extraction behaviors of Li+ into/out of LiV3O8 materials.27 When it was in LiNO3 aqueous electrolyte without the dissolved oxygen in the seven 1164
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Figure 3. CV curves of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) in the aqueous electrolyte with/without dissolved oxygen at 0.2 mV s−1.
is eliminated from the aqueous electrolyte, there exists irreversibility in the first cycle for the as-prepared cathode materials, but relatively good overlaps appear in the subsequent six cycles, which suggests that the dissolved oxygen in the aqueous electrolyte decreases Li+ insertion/extraction reversibility of LiMn1−xFexPO4/C. Hence, eliminating the dissolved oxygen in the aqueous electrolyte would improve the capacity of LiMn1−xFexPO4/C. Figure 4 shows the first charge−discharge curves of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) in the neutral LiNO3 aqueous electrolyte with/without the dissolved oxygen. It is observed that two distinct potential plateaus were at about 0.95 and 0.37 V (vs SCE), respectively, corresponding to the extraction/insertion of Li+ from/into LiMn1−xFexPO4/C. This is in good agreement with the two couples of redox peaks observed in the CV curves, which is related to the Mn2+/Mn3+ and Fe2+/Fe3+ redox couples. In the aqueous solution with oxygen, the as-prepared materials deliver a discharge capacity of 106.17, 110.60, 77.88, and 73.21 mA h g−1 at 0.1 C-rate, respectively. However, in the aqueous solution without oxygen, their discharge capacities become 110.22, 112.66, 111.08, and 90.50 mA h g−1 separately. The results indicate that the discharge capacities could be enhanced by eliminating the dissolved oxygen in the aqueous electrolyte, and LiMn0.6Fe0.4PO4/C has the highest discharge capacity among these materials, which can be competitive with the reported results: the initial discharge capacity of Li 1−xFePO4// LiTi2(PO4)3 in 1 M Li2SO4 at 1 C was 55 mA h g−1,10 the
Figure 4. Initial charge−discharge curves of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) in the aqueous electrolyte with/without the dissolved oxygen.
first discharge capacity of LiMn0.05Ni0.05Fe0.9PO4//LiTi2(PO4)3 in saturated Li2SO4 solution at the current density of 0.2 mA cm−2 was 103.9 mA h g−1,11 and the initial discharge capacity of LiFe0.5Mn0.5PO4//LiV3O8 in saturated LiNO3 solution at 0.1 C was 107 mA h g−1.27 Figure 5 show the EIS curves of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) measured in the delithiated state after the first cycle. The EIS profiles exhibit a semicircle in the high frequency region and an inclined line in the low frequency region. Using the Z-view software, EIS data can be fitted by an equivalent 1165
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inverse square root of angular frequency in the aqueous electrolyte with/without dissolved oxygen. According to the slopes of these lines and eqs 1 and 2, we can calculate the DLi+ of LiMn1−xFexPO4/C in the aqueous electrolyte with/without dissolved oxygen. Table 1 shows these kinetic parameters. It is obviously seen that the resistance of the electrolyte (Rs) increases by Table 1. Electrochemical Kinetics Parameters Obtained from the Equivalent Circuit Fitting of the EIS for the Delithiated LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) Electrodes after the Initial Cycle
Figure 5. EIS results of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) after the 1st cycle in the aqueous electrolyte with/without dissolved oxygen.
Figure 6. Equivalent circuit used in analyzing the EIS results.
(1)
Z im = σω−1/2
(2)
Rs (Ω)
Rct (Ω)
LiMn0.5Fe0.5PO4 LiMn0.6Fe0.4PO4 LiMn0.7Fe0.3PO4 LiMn0.8Fe0.2PO4 sample (electrolyte without oxygen)
2.549 3.147 3.128 2.475 Rs (Ω)
2.297 1.496 2.752 3.426 Rct (Ω)
LiMn0.5Fe0.5PO4 LiMn0.6Fe0.4PO4 LiMn0.7Fe0.3PO4 LiMn0.8Fe0.2PO4
2.673 3.743 3.725 3.548
1.272 0.358 0.497 1.661
DLi+ (cm2 s−1) 3.66 × 10−14 5.24 × 10−14 3.31 × 10−14 2.76 × 10−14 DLi+ (cm2 s−1) 1.37 2.06 1.47 1.22
× × × ×
10−13 10−13 10−13 10−13
eliminating the dissolved oxygen in the aqueous solution. Rs value of LiMn0.5Fe0.5PO4 and LiFe0.2Mn0.8PO4 are lower than that of the other materials. Moreover, it is clear that Rct drastically decreases and DLi+ increases by eliminating the dissolved oxygen in the aqueous electrolyte. It is noted that DLi+ order of magnitude for LiMn1−xFexPO4/C in the aqueous electrolyte without dissolved oxygen is larger than that in the aqueous electrolyte with dissolved oxygen. Especially, DLi+ of LiMn0.6Fe0.4PO4/C in the aqueous electrolyte without dissolved oxygen is the highest, 2.06 × 10−13 cm2 s−1, among these asprepared materials, which is corresponding to its highest initial discharge specific capacity. Meanwhile, the Rct value of LiMn0.6Fe0.4PO4/C is lowest among these as-prepared materials, which is 0.358 Ω. Therefore, the discharge capacities of LiMn1−xFexPO4/C in the neutral LiNO3 solution without dissolved oxygen are higher than those in aqueous electrolyte with dissolved oxygen. Nevertheless, how the dissolved oxygen in the aqueous electrolyte does chemically or electrochemically influence the electrochemical performance of the cathode materials is not clear, and research is further underway.
circuit model. Figure 6 gives the equivalent circuit. In Figure 6, Rs represents the ohmic resistance of the aqueous electrolyte, Rct denotes the resistance in the charge-transfer process. The constant-phase element (CPE) is used instead of a pure capacitor, and Zw represents Li+ diffusion resistance through the material. The Li+ diffusion coefficients of LiMn1−xFexPO4/C are calculated according to the following equations:28 D = (R2T 2)/(2A2 n 4F 4C 2σ 2)
sample (electrolyte with oxygen)
+
Where, D is the Li diffusion coefficient, R is the gas constant, T is the absolute temperature, A is the surface of the cathode and its value calculated by BET method, n is the number of electrons for Li+ transmission, F is the Faraday constant, C is the concentration of Li+, Zim is Li+ diffusion resistance in the electrode material, and σ is the Warburg factor related with Zim. Figure 7 illustrates the linear relationship between Zim and the
4. CONCLUSION LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) are synthesized by sol−gel and sintering method. The as-prepared materials show a similar morphology with the homogeneous particle size distribution. The materials are successfully used as the cathode for the ARLBs. The initial discharge capacities of LiMn1−xFexPO4/C (x = 0.5, 0.4, 0.3, 0.2) in the aqueous electrolyte without dissolved oxygen are better than those in the aqueous electrolyte with dissolved oxygen. The Li+ insertion-extraction kinetic parameters of LiMn1−xFexPO4/C in the aqueous electrolyte with/without dissolved oxygen are calculated, which suggest that eliminating the dissolved oxygen in the aqueous electrolyte could decrease the charge transfer resistance and increase Li+ diffusion coefficients. The obtained CV and the EIS results suggest it can also improve Li+ insertion-extraction ability of LiMn1−xFexPO4/C in the aqueous electrolyte. It is crucial for the ARLBs in the application of power device and energy storage.
Figure 7. Relationship between Zim and inverse square root of the angular frequency in the aqueous electrolyte with/without dissolved oxygen. 1166
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ASSOCIATED CONTENT
S Supporting Information *
Analysis reports. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86-029-82663034, +86-029-82667872. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. † International Society of Electrochemistry member.
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ACKNOWLEDGMENTS The authors acknowledge the Province Natural Science Foundation of Shaan Xi (2012JM6005), Xi’an Science and technology project (CXY1124), and the China Fundamental Research Funds for the Central University (xjj2012095, 2012jdh235).
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