C composite cathode materials for lithium ion batteries

C composite cathode materials for lithium ion batteries

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CERAMICS INTERNATIONAL

Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint

Preparation and electrochemical properties of LiMn1  xFexPO4/C composite cathode materials for lithium ion batteries Shanshan Lia, Zhi Sua,n, Arzugul Muslima, Xiaokang Jiangb, Xinyu Wanga a

College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi 830054, Xinjiang, China b College of Physical Science and Technology, Yili Normal University, Yining 835000, Xinjiang, China Received 19 April 2015; received in revised form 11 May 2015; accepted 12 May 2015

Abstract Carbon-coated LiMn1  xFexPO4/C (x ¼0, 0.1, 0.3, 0.5) nanoparticles have been synthesized by a two-step sol–gel method. The materials show good crystallinity and a uniform grain size of about 100 nm. The LiMn1  xFexPO4/C composites deliver the best initial specific discharge capacities of 160 mA h g  1 at 0.05 C for x ¼0.3, LiMn0.7Fe0.3PO4/C retained 157 mA h g  1 (97.8%) after 40 cycles, demonstrating excellent cyclability. Results suggest that the improved cycling stability of the doped composites results from mitigated Mn2 þ dissolution due to Fe2 þ substitution. Impedance measurements reveal the Rct and Ws impedance of the doped composites to be much lower than the non-doped samples. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: LiMn1  xFexPO4/C; Nanoparticles; Sol–gel; Electrochemical properties

1. Introduction Olivine-structured lithium transition metal phosphates (LiMPO4; M¼ Mn, Fe, Co, and Ni) are currently receiving much attention for their potential applications as lithium ion battery cathodes given their high performance, electrochemical stability, low cost, low toxicity, and environmental friendliness [1–3]. Among them, LiMnPO4 shows particular potential given its electrochemical properties; however, the inherently low ionic and electrical conductivity of this entire class of materials seriously limits Li þ insertion and extraction and lowers charge transport rates, restricting further commercialization. Furthermore, major Jahn–Teller lattice distortions induced by Mn3 þ result in poor cycling stability [4–9]. These disadvantages can be overcome by coating particles with a carbon layer; this decreases the interfacial resistance on the particle boundaries and reduces particle size, which not only shortens the Li þ diffusion path but the electron conduction path as well. Doping with small amounts of Fe, Al, and Mg has also been reported to improve electrochemical performance [9–12].

Many works have investigated that doping with Fe2 þ ion can improve the cycling and rate performance of LiMnPO4 material [9,13–15]. Aravindan et al. reported that LiMnxFe1  xPO4/C composites were synthesized by many methods and showed better discharge capacity than LiMnPO4/C [9]. The carbon-coated LiMn0.71Fe0.29PO4 can provide better electronic conductivity and has better electrochemical performance [13]. Morphology-controlled LiMn0.9Fe0.1PO4 nanoplatelets with high performance have been obtained via a simple ascorbic acid-assisted solvothermal process in water–ethanol solvent [14]. LiFe1  xMnxPO4/C (x ¼ 0.85, 0.75, 0.65) composites with good discharge capacity have been synthesized by solid-state reactions [15]. In this work, we report the preparation of a nanostructured, Fe-doped LiMnPO4/C composite through a two-step sol–gel method that employs ethanol as a dispersing agent. The morphology and electrochemical properties of the prepared samples were then investigated. 2. Experimental

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Corresponding author. Tel./fax: +86 9914332683. E-mail address: [email protected] (Z. Su).

All reagents were of analytical grade. First, manganese acetate tetrahydrate and ferrous acetate tetrahydrate were

http://dx.doi.org/10.1016/j.ceramint.2015.05.061 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: S. Li, et al., Preparation and electrochemical properties of LiMn1  xFexPO4/C composite cathode materials for lithium ion batteries, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.061

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dissolved in 50 mL of ethanol. Meanwhile, citric acid, the carbon source for the reaction, and phosphoric acid were dissolved in 20 mL of ethanol. The two solutions were then mixed by stirring to obtain a sol, which was dried at 40 1C overnight to yield the gel. The gel was heated to 700 1C for 1 h under argon (ultra-high purity) and then cooled slowly to room temperature, yielding the precursor. Finally, the precursor was mixed with lithium hydroxide monohydrate for 1 h using a ball mill set to 500 rpm, then heated to 700 1C for 12 h under argon before being cooled slowly back to room temperature, yielding the LiMn1  xFexPO4/C composites. The microstructure of the prepared samples was characterized by X-ray diffraction (XRD, Bruker D2) with a Cu-Kα radiation source (λ ¼ 1.5408 Å) and by transmission electron microscopy (TEM, FEI Tecnai 20) with an accelerating voltage of 200 kV. The exact carbon amount has been verified by the technique using an elemental analyzer (VarioEL III, Elementar, Germany). Electrochemical performance was assessed using coin-type half-cells (LIR 2025) that were assembled in an argon glove box. The cathode materials were made from a mixture of the synthesized LiMn1  xFexPO4/C, a poly(tetrafluoroethylene) binder, and acetylene black in a 80:15:5 weight ratio. Lithium metal was used for the counter and reference electrodes, while a 1 M solution of LiPF6 dissolved in a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate in a weight ratio of 1:1:1 was used as the electrolyte. Aluminum foil was used as current collector. The galvanostatic method (LAND CT2001A) was used to measure the electrochemical capacity and cycle performance of the electrodes at room temperature. The charge and discharge cut-off potentials were set to 2.5 and 4.4 V versus Li þ /Li and measured at variable current densities. Cyclic voltammetry (CV, LK2500) was performed from 3.0 to 4.7 V with a scan rate of 0.1 mV s  1. Electrochemical impedance spectroscopy (EIS, Zahner IM6ex) was completed over a frequency range of 10 kHz to 10 mHz, with a 5 mV a.c. input signal applied between the working and reference electrodes. 3. Results and discussion 3.1. Composition and structure characterization XRD patterns for these samples are shown in Fig. 1. All peaks are highly ordered and can be indexed as resulting from a single-phase orthorhombic structure in space group Pnma (based on a comparison with PDF standard Card 74-0375). Diffraction features shift to a higher angle with increasing Fe2 þ content, likely because Fe2 þ has a smaller radius than Mn2 þ (radii for these ions are 0.076 and 0.080 nm, respectively); this in turn leads to a lower d-value. The lack of carbon diffraction peaks indicates that any carbon in the sample is amorphous [16]. The observed carbon content is 7.1 wt% for LiMnPO4/C, 6.9 wt% for LiMn0.9Fe0.1PO4/C, 6.9 wt% for LiMn0.7Fe0.3PO4/C, 7.0 wt% for LiMn0.5Fe0.5PO4/C. TEM images of the composite samples are shown in Fig. 2. They all show similar morphologies, with a grain diameter of

Fig. 1. XRD patterns of the composite samples.

about 100 nm. Overall, the phosphate particles are clearly dispersed throughout the carbon matrix that had formed during the synthesis of the precursor. Overall, results indicate that this two-step sol–gel method, employing an alcohol as the dispersing agent, can effectively control particle diameter.

3.2. Electrochemical properties Fig. 3 depicts initial discharge curves for the LiMn1  xFexPO4/C samples at a charge–discharge rate of 0.05 C. In comparison with non-doped composites, the doped composites showed two typical potential plateaus at 4.0 V and 3.5 V versus Li/Li þ . The initial discharge capacities of composites with different Fe2 þ contents of 0, 0.1, 0.3, 0.5 are 74, 115, 160 and 158 mA h g  1 at 0.05 C, respectively. Discharge capacity increases rapidly from 74 mA h g  1 for LiMnPO4/C to 160 mA h g  1 for LiMn0.7Fe0.3PO4/C, with only a slight decrease over that observed for LiMn0.5Fe0.5PO4/C. LiMn0.7Fe0.3PO4/C shows the best discharge capacities with 160 mA h g  1 at 0.05 C. Cycle life data for the LiMn1  xFexPO4/C samples at a charge–discharge rate of 0.05 C are presented in Fig. 4. All of them demonstrates excellent cyclic stability, with hardly any decrease after 40 cycles. LiMn0.7Fe0.3PO4/C retained 157 mA h g  1 (97.8%) of the initial specific discharge capacity after 40 cycles, demonstrating excellent cyclability. These improved properties likely result from the presence of Fe2 þ . Note that the discharge capacity for samples with 0.3 and 0.5 Fe2 þ have a slight decrease, and LiMn0.7Fe0.3PO4/C provides the best performance. CV data for both the doped and non-doped composites is displayed in Fig. 5. The non-doped samples show a pair of oxidation–reduction peaks at 4.38 and 3.80 V, respectively, corresponding to Mn3 þ and Mn2 þ . Meanwhile, the LiMn0.7Fe0.3PO4/C sample showed two pairs of peaks, one at 4.29 and 3.84 V and the other at 3.66 and 3.46 V, corresponding to Mn3 þ /Mn2 þ and Fe3 þ /Fe2 þ , respectively. The voltage difference between the LiMn0.7Fe0.3PO4/C peaks is smaller than for LiMnPO4/C. Results suggest that the reversibility of the Li

Please cite this article as: S. Li, et al., Preparation and electrochemical properties of LiMn1  xFexPO4/C composite cathode materials for lithium ion batteries, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.061

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Fig. 2. TEM images of the composite samples: (a) x ¼0, (b) x¼0.1, (c) x¼ 0.3, (d) x ¼0.5.

Fig. 3. Initial charge and discharge curves for the composite samples.

Fig. 5. CV data for the doped and non-doped composite samples.

EIS values for the materials were measured in a charged state after the first cycle to provide additional information on electrochemical performance. The Nyquist plots and corresponding circuits in Fig. 6 all take the shape of a semicircle in the highfrequency region and a line in the low-frequency region. The reaction is controlled by both Warburg impedance in Li þ diffusion, which is inversely proportional to the diffusion coefficient, and the temporary, steady-state surface electrochemical reaction. Table 1 shows the calculated electrochemical parameters for all samples; overall, the doped materials show much lower charge transfer resistance and Warburg impedance, contributing to significantly improved electrical conductivity; conductivity values for LiMnPO4 and LiFePO4 are on the order of 10  14 and 10  9 S/cm, respectively [17,18]. Fig. 4. Cycle life data for the composite samples.

insertion and extraction processes improved with added Fe2 þ content. Furthermore, the intensity of the LiMn0.7Fe0.3PO4/C peaks is greater, suggesting higher electrical conductivity.

4. Conclusion LiMn1  xFexPO4/C nanoparticles were successfully synthesized by a two-step sol–gel method. The materials showed

Please cite this article as: S. Li, et al., Preparation and electrochemical properties of LiMn1  xFexPO4/C composite cathode materials for lithium ion batteries, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.061

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Fig. 6. Nyquist plots for doped and non-doped composite samples integrated into test cells.

Table 1 Calculated electrochemical parameters from ac impedance spectra of the LiMn1  xFexPO4/C samples at the charge voltage 4.2 V (versus Li þ /Li) using ZView 2.8 softwarea. Sample

Rct (Ω)

CPE (F  10  6)

Ws (Ω)

LiMnPO4/C LiMn0.9Fe0.1PO4/C LiMn0.7Fe0.3PO4/C LiMn0.5Fe0.5PO4/C

335.18 203.72 148.27 172.21

1.89 2.46 3.17 3.19

132.71 63.28 48.65 55.69

a Rct represents the charge-transfer resistance, CPE represents the constant phase element, and Ws represents the Warburg impedance.

similar morphologies, an average particle size of 100 nm, and overall excellent cycle stability. LiMn0.7Fe0.3PO4/C delivered the best electrochemical performance, with discharge capacities of 160 mA h g  1 at 0.05 C, and retained 157 mAh g  1 (97.8%) after 40 cycles. Overall, CV and EIS data suggest that doping with the appropriate amount of Fe2 þ can efficiently improve Li insertion and extraction, as well as electrical conductivity. Acknowledgments This study was supported by the National Natural Science Foundation of China (Nos. 21061015 and 2106014), National Natural Science Foundation of China for Youth Scholars (No. 21104063), and the Xinjiang Normal University Students' Academic Technology Innovation Project (No. 20131215). References

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Please cite this article as: S. Li, et al., Preparation and electrochemical properties of LiMn1  xFexPO4/C composite cathode materials for lithium ion batteries, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.05.061