- Nano Express
- Open Access
Preparation of PPy-Coated MnO2 Hybrid Micromaterials and Their Improved Cyclic Performance as Anode for Lithium-Ion Batteries
© The Author(s). 2017
- Received: 14 June 2017
- Accepted: 20 August 2017
- Published: 2 September 2017
MnO2@PPy core-shell micromaterials are prepared by chemical polymerization of pyrrole on the MnO2 surface. The polypyrrole (PPy) is formed as a homogeneous organic shell on the MnO2 surface. The thickness of PPy shell can be adjusted by the usage of pyrrole. The analysis of SEM, FT-IR, X-ray photoelectron spectroscopy (XPS), thermo-gravimetric analysis (TGA), and XRD are used to confirm the formation of PPy shell. Galvanostatic cell cycling and electrochemical impedance spectroscopy (EIS) are used to evaluate the electrochemical performance as anode for lithium-ion batteries. The results show that after formation of MnO2@PPy core-shell micromaterials, the cyclic performance as anode for lithium-ion batteries is improved. Fifty microliters of PPy-coated caddice-clew-like MnO2 has the best cyclic performances as has 620 mAh g−1 discharge specific capacities after 300 cycles. As a comparison, the discharge specific capacity of bare MnO2 materials falls to below 200 mAh g−1 after 10 cycles. The improved lithium-storage cyclic stability of the MnO2@PPy samples attributes to the core-shell hybrid structure which can buffer the structural expansion and contraction of MnO2 caused by the repeated embedding and disengagement of Li ions and can prevent the pulverization of MnO2. This experiment provides an effective way to mitigate the problem of capacity fading of the transition metal oxide materials as anode materials for (lithium-ion batteries) LIBs.
- Manganese dioxide
- Lithium-ion battery
- Anode material
Since 3d transition metal oxides (MO; where M is Fe, Co, Ni, and Cu) were proposed to serve as high theoretic capacity anodes for lithium-ion batteries by Tarascon et al. , many efforts have been made in preparing micro/nano-metal oxides with various morphologies and researching their electrochemical performance as anode for lithium-ion batteries [2–6]. For examples, Zhu’s research group had made monodisperse Fe3O4 and γ-Fe2O3 microspheres via a surfactant-free solvothermal method . They had a high initial discharge capacity of 1307 and 1453 mAh g−1, respectively. After 110 cycles, the discharge capacity remained at 450 mAh g−1 for Fe3O4 and 697 mAh g−1 for γ-Fe2O3. Hongjing Wu et al. had prepared uniform multi-shelled especially quintuple-shelled NiO hollow spheres by a simple shell-by-shell self-assembly hydrothermal treatment. The merit of this research made a significant contribution to the synthetic methodology of multi-shelled hollow structures. But the lithium storage performances of the NiO hollow spheres were not very excellent . MnO2 possess high theoretically gravimetric lithium storage capacity of about 1230 mAh g−1; therefore, many researches are made to the design, synthesis, and applications of MnO2 anodes for lithium-ion battery [7–10]. For instance, Chen’s research group had made γ-MnO2 with hollow microspherical shape and nanocubic shape . After 20 cycles, the discharge capacities of the nanocubes and microspheres were 656.5 and 602.1 mAh g−1. In addition, they had made many researches on MnO2 materials for lithium-ion battery from the year 2000 to now [12, 13]. We also studied the applications of MnO2 anodes for lithium-ion battery, but the discharge specific capacity of bare MnO2 materials felled so fast to below 200 mAh g−1 after 10 cycles .
Although transition metal oxides materials have large theoretical specific capacities, all these materials including MnO2 anodes are generally plagued by rapid capacity fading. The reasons for the poor cycling stability are as follows: (1) the electronic conductivity of transition metal oxides materials are usually low, and the electron or ion have difficulties in the diffusion process, resulting in irreversible electrode reaction and fast capacity decay. (2) After charge/discharge cycles, transition metal oxides suffer from enormous mechanical stress and pulverize, leading to electrical contact loss between active particles and current collector. The transition metal oxide particles without electrical contact can no longer participate in the charge/discharge cycles, resulting in capacity fading [15, 16].
Shell coating is an effective strategy to improve the cycling stability. In this structure, to a certain extent, the shell can buffer the structural expansion and contraction of metal oxide materials caused by the repeated embedding and disengagement of the Li ions. For the moment, carbon coating, organic conducting polymer coating, graphene hybrid, and other inorganic compound coating have been used [17, 18]. For instance, Yin et al. prepared polypyrrole (PPy)-coated CuO nanocomposites. The core-shell sample had a high reversible capacity of 760 mAh g−1 which was much better than those of the bare CuO sample . Li et al. prepared graphene-wrapped MnO2 nanoribbons. The reversible specific discharge capacity reached 890 mAh g−1 at 0.1 A g−1 after 180 cycles. Therefore, it is necessary and urgent to make PPy shell coating on MnO2 materials to improve the cyclic stability as anode for lithium-ion batteries .
In the present work, to improve the cyclic performance of MnO2 materials as anode for lithium-ion batteries, polypyrrole (an organic conducting polymer) coating had been prepared by chemical polymerization. As a result, the cyclic performance was improved after formation of MnO2@PPy core-shell micromaterials. This experiment provides an effective way to mitigate the problem of capacity fading of the transition metal oxides materials as anode materials for (lithium-ion batteries) LIBs.
Preparation of Samples
All reagents were of analytical grade and purchased from the Shanghai Chemical Company. The pyrrole was purified by decompressional distillation prior to use and stored at 0–5 °C and guarded against exposure to light to prevent residual polymerization. Other reagents were used without further purification.
The MnO2 micromaterials were prepared using the similar method described by Yu et al. [14, 21] as some modification. To prepare caddice-clew-like MnO2 micromaterial, 1.70 g MnSO4·H2O was dissolved in 15 mL distilled water with vigorous stirring. When the solution was clear, 20 mL aqueous solution containing 2.72 g K2S2O8 were added to the above solution under continuous stirring. Then, the resulting transparent solution was transferred into a Teflon-lined stainless steel autoclave (50 mL) of 80% capacity of the total volume. The autoclave was sealed and maintained at 110 °C for 6 h. After the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The solid black precipitate was filtered, washed several times with distilled water to remove impurities, and then dried at 80 °C in air for 3 h. The obtained caddice-clew-like MnO2 micromaterial was collected for the fabrication of PPy-coated MnO2 materials. Urchin-like MnO2 micromaterial was prepared by the similar method; after adding 1.70 g MnSO4·H2O and 2.72 g K2S2O8 into 35 mL distilled water, 2 mL H2SO4 was then added.
The MnO2@PPy hybrid micromaterials were prepared by chemical polymerization of pyrrole on the MnO2 surface using sodium benzenesulfonate (BSNa) as surfactant and FeCl3 as oxidant. The molar ratio of monomer pyrrole to BSNa was 3:1. First, 0.2 g MnO2 was dispersed into a beaker containing 50 mL of 0.01 mol L−1 BSNa aqueous solution and stirred for 0.5 h. The mixture was put in an ice/water bath (0–5 °C) under stirring. Then, a certain amount of pyrrole was added to the mixture. After stirring for 0.5 h, a small amount of FeCl3 solution was dropwise added into the aqueous solution to start the polymerization process. The gradual change of color from light black to deep black indicated the formation of PPy. The mixture was kept at 0–5 °C under stirring for 12 h to form MnO2@PPy core-shell micromaterials. The thickness of PPy was controlled by pyrrole usage. Finally, the obtained composite was filtered, washed with water and ethanol, and then dried under vacuum at 60 °C for 4 h.
Characterization of Samples
The morphological investigations of SEM images and energy dispersive spectroscopy (EDS) were taken on a scanning electron microscope (QUANTA-200 America FEI Company). The crystallographic structures of the products were determined with XRD which were recorded on a Rigaku D/max-2200/PC with Cu target at a scanning rate of 7°/min with 2θ ranging from 10° to 70°. Fourier transform infrared (FT-IR) spectra of the MnO2@PPy hybrid micromaterials palletized with KBr were performed on a Nicolet IS10 spectrometer. Thermo-gravimetric analysis (TGA) was also used to determine the weight loss of MnO2@PPy hybrid micromaterials at 10 °C/min from 25 to 800 °C in air (MELER/1600H Thermogravimetric Analyzer). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Ulvac-PHI, PHI5000 Versaprobe-II X-ray photoelectron spectroscope, using Al Kα X-rays as the excitation source. The binding energy obtained in the XPS analysis was calibrated against the C1s peak at 284.8 eV.
Cell Assemply and Electrochemical Studies
Electrochemical lithium-storage properties of the synthesized products were measured by using CR2025 coin-type test cells assembled in a dry argon-filled glove box. To fabricate the working electrode, a slurry consisting of 60 wt.% active materials, 10 wt.% acetylene black, and 30 wt.% poly-vinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone was casted on a copper foil, dried at 80 °C under vacuum for 5 h. A lithium sheet was served as counter and reference electrode, while a Celgard 2320 membrane was employed as a separator. The electrolyte was a solution of 1 M LiPF6 in ethylene carbonate (EC)-1,2-dimethyl carbonate (DMC) (1:1 in volume). Galvanostatical charge-discharge experiments were performed by Land electric test system CT2001A (Wuhan Land Electronics Co., Ltd.) at a current density of 0.2 C between 0.01 and 3.00 V (versus Li/Li+). When calculating the specific capacity of MnO2@PPy core-shell micromaterials, the mass of PPy was included. Electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical workstation (CHI604D, Chenhua, Shanghai), and the frequency ranged from 0.1 Hz to 100 KHz with an applied AC signal amplitude of 5 mV.
Morphological Features of Samples
In this work, caddice-clew-like MnO2 micromaterial is also coated by PPy using the similar method. The SEM morphologies are shown in Additional file 1: Supporting information 1. The caddice-clew-like MnO2 micromaterial is nanowire shaped and aggregates into 2–4 μm diameter spheres which look like a caddice-clew. When the amount of pyrrole is small, the PPy first forms as small particles and adheres on the surface of the MnO2 samples. With the amount of pyrrole increasing, PPy gradually cover the caddice-clew-like MnO2 completely to form a large block structure which looks like rocks.
EDX data for PPy, MnO2, and PPy-coated urchin-like MnO2 sample
MnO2@PPy(10 μL) At%
MnO2@PPy(20 μL) At%
MnO2@PPy(30 μL) At%
MnO2@PPy(50 μL) At%
FT−IR Analysis of Samples
XPS data for urchin-like MnO2@PPy sample
MnO2@PPy(10 μL) At%
XRD Characterization of Samples
To evaluate their lithium-storage cyclic stability, discharge/charge measurements are performed for 300 cycles on MnO2@PPy samples with different pyrroles coated. The thickness of PPy is controlled by the amount of pyrrole. As shown in Fig. 6c, d, when the amount of pyrrole is small (such as 30 uL for caddice-clew-like MnO2 and 10 uL for urchin-like MnO2), the lithium-storage capacity of this hybrid MnO2@PPy sample improves not clearly. This indicates that the PPy film is too thin to prevent MnO2 materials suffering from pulverization. However, when the amount of pyrrole increases, the discharge specific capacities of hybrid MnO2@PPy samples are remarkably enhanced. For caddice-clew-like MnO2, when the amount of pyrrole increases to 50 uL, the hybrid MnO2@PPy sample has the biggest discharge specific capacities as 620 mAh g−1 after 300 cycles. For urchin-like MnO2, the biggest discharge specific capacity appears when 30 uL pyrrole is used. The discharge specific capacity at the 300th cycle is 480 mAh g−1. Furthermore, as can be seen from Fig. 6c, d, all the hybrid MnO2@PPy samples have improved cyclic stabilities. The improved lithium-storage cyclic stabilities of the hybrid MnO2@PPy samples can attribute to the unique structure of the metal oxide/conducting polymer core-shell hybrid products. In this structure, the flexible PPy shell can effectively buffer the structural expansion and contraction of MnO2 caused by the repeated embedding and disengagement of the Li ions. In addition, the PPy shell can prevent the pulverization of MnO2, as well as protect the loss of electrical contact between the MnO2 material and the current collector (copper foil). Whereas, the low capacity and fast capacity fading of bare MnO2 can attribute to the pulverization and loss of inter-particle contact of MnO2 or the contact of MnO2 with copper foil collector due to large volume expansion/contraction during repeated charging-discharging processes. Therefore, this experiment of PPy coating provides an effective way to mitigate the problem of capacity fading of all the transition metal oxide materials as anode materials for LIBs.
As shown in the rate performance, the urchin-like MnO2 micromaterial has relatively higher discharge specific capacity than caddice-clew-like MnO2 micromaterial, which is consistent with previous reports . However, after PPy coating, the caddice-clew-like MnO2@PPy sample has better lithium-storage cyclic stability. Here, the conjugate degree of the PPy may be one reason. The FT-IR analysis indicates that the PPy conjugate degree of the caddice-clew-like MnO2@PPy sample is higher. So, the caddice-clew-like MnO2@PPy sample should have better conductivity and better electrochemical performance. To confirm it, the EIS tests are carried out.
In summary, MnO2@PPy core-shell micromaterials are successfully prepared by chemical polymerization of pyrrole on the MnO2 surface. The thickness of the PPy shell can be adjusted by the usage of pyrrole. After formation of MnO2@PPy core-shell micromaterials, the cyclic performances as an anode for lithium-ion batteries are improved. Fifty microliters of PPy-coated caddice-clew-like MnO2 has the best cyclic performances and has 620 mAh g−1 discharge specific capacities after 300 cycles. As a comparison, the discharge specific capacity of bare MnO2 materials falls below 200 mAh g−1 after 10 cycles. The improved lithium-storage cyclic stability of the MnO2@PPy samples can attribute to the core-shell hybrid structure. In this structure, the flexible PPy shell can effectively buffer the structural expansion and contraction of MnO2 caused by the repeated embedding and disengagement of Li ions and can prevent the pulverization of MnO2. Therefore, this experiment of PPy coating provides us an effective way to mitigate the problem of capacity fading of the transition metal oxide materials as anode materials for LIBs.
This work was financially supported by the Program for National Natural Scientific Fund (Nos. 21463028, 21761035, 21565031, and 21665027), YMU-DEAKIN International Associated Laboratory on Functional Materials, Key Laboratory of Resource Clean Conversion in Ethnic Region, Education Department of Yunnan Province, the general program of the application and basic research foundation of Yunnan province (2013FZ080, 2016FD059); the key project of scientific research foundation of educational bureau of Yunnan province (2015Z118); and the Innovative Training Program of Yunnan Minzu University for Undergraduates and Open Laboratory Project of Yunnan Minzu University for Undergraduates.
The experiments and characterization presented in this work were carried out by LF, ZX, YZ, and RW. The experiments were designed by LF. The experiments were discussed in the results by LF, Y Z, WB, SJ JY, ZZ, and HG. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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