Polypyrrole-coated Mn–Fe bimetallic oxides as high stability anode for lithium-ion batteries

Transition metal oxides as anode materials have received extensive research owing to the high specific capacity. However, the rapid decline of battery capacity caused by volume expansion and low electrical conductivity hinders the practical application of transition metal oxides. This study reported a pseudo-capacitance material polypyrrole-coated Fe2O3/Mn2O3 composites material as a high stability anode for lithium-ion batteries. The polypyrrole coating layer can not only serve as a conductive network to improve electrode conductivity but also can be used as a protective buffer layer to suppress the volume change of Fe2O3/Mn2O3 during the charging and discharging process. At the same time, the porous structure of Fe2O3/Mn2O3 composite can not only provide more active sites for lithium storage but also play a certain buffer effect on the volume change of the material. Polypyrrole-coated Fe2O3/Mn2O3 composite as the anode for lithium-ion batteries shows great electrochemical storage performance, with high specific capacity (627 mAh g−1 at a current density of 1 A g−1), great cycle stability (the capacity not shows obvious signs of attenuation after 500 cycles), and rate performance (432 mAh g−1 at a current density of 2.0 A g−1).


Introduction
Lithium-ion batteries (LIBs) have become the primary electrochemical energy storage devices due to the high energy density and excellent cycling stability. However, with the continuous development of battery technology, its energy density requirements are getting higher and higher. The traditional anode material graphite has been unable to meet the rapid development of LIBs due to its relatively low specific capacity (372 mAh g -1 ) [1][2][3]. Therefore, the development of a new generation of great performance anode materials with high energy density and cycle stability is an inevitable trend for the development of LIBs.
In recent years, transition metal oxides, with high theoretical specific capacity and low cost, have received widespread attention as promising anode candidates for LIBs [4,5]. Additionally, the transition metal oxides as anode materials have a high lithium insertion potential, which can effectively prevent the accumulation of lithium metal forming lithium branch crystals when the battery is overcharged, thereby improving the safety performance of LIBs [6]. Among them, Mn 2 O 3 and Fe 2 O 3 are considered as one of the ideal anode materials by reason of the higher theoretical capacity (1018 and 1007 mAh g -1 , respectively) and abundant resource reserves. Mn 2 O 3 and Fe 2 O 3 bimetal oxides have a large number of heterojunction interfaces, and the existence of heterojunction interfaces and the synergistic effect between the interfaces can also effectively promote the oxidation-reduction reaction [7]. However, owing to the poor electronic conductivity and a large amount of volume expansion during the charge/ discharge cycling, the battery capacity will decrease rapidly, and the rate performance will be poor, which limits its practical application [3]. In view of the above problems, the composite method of nano-porous transition metal oxides, carbon nanotubes, and graphene materials was be designed to alleviate the volume expansion of oxides during the cycling and improve the conductivity [8][9][10][11]. For example, manganese oxide/graphene and manganese oxide/carbon nanotubes have been successfully prepared, and have shown good electrochemical performance. Yuan et al. introduced nitrogen-doped graphene-buffered Mn 2 O 3 nanocomposite as an anode to enhance Mn 2 O 3 volume utilization and conductivity [12]. Lee et al. introduced Mn 3 O 4 nanorods on graphene sheets as electrodes, which significantly improved their power density, and capacitance retention upon cycling [13]. However, in the preparation process of most graphene composite materials, highly toxic hydrazine hydrate is used as a reducing agent, and the prices of graphene and carbon nanotubes are relatively high, which will further increase the production cost of the material. Polypyrrole (PPy), as a pseudo-capacitance material for supercapacitors, has the advantages of high conductivity, good flexibility, simple production, and low price. This pseudo-capacitance material can store charge on the electrode material surface, so compared with batteries, the electrode is less degraded and has better cycle stability. It is widely used in batteries, supercapacitors, and sensors [14][15][16]. Introducing it into the transition metal oxide anode material can effectively improve its electrochemical stability [17,18]. Studies have found that the interface pseudo-capacitance can increase the storage capacity of lithium [19]. Unlike other metal oxides, manganese oxide's redox reaction occurs in a wide potential range and allows the existence of pseudo-capacitance charge and discharge properties [20]. Therefore, the composite of manganese oxide and this flexible polymer conductive material is an important method to improve its electrochemical performance.
In this study, PPy-coated Fe 2 O 3 /Mn 2 O 3 composite was successfully synthesized by the hydrothermal method and the in situ polymerization (SIP) method, and the effect of polypyrrole on the electrochemical performance of Fe 2 O 3 /Mn 2 O 3 was studied. It is found that the existence of the conductive network of polypyrrole can store the charge in the electrode material. The surface can provide a more convenient channel for electrons and Li ? to shuttle back and forth during the charging and discharging process, thereby promoting the charge transfer rate between the active material, conductive carbon, and the binder electrochemical performance of the material. The research results have certain guiding significance for the preparation of high specific capacity, high cycling stability lithium-ion battery anode materials.

Material synthesis and preparation
All of the reagents used in this experiment are analytical reagents. MnSO 4 ÁH 2 O, Fe (NO 3 ) 3 Á9H 2 O, urea, ammonium sulfate, P-toluene sulfonic acid, and pyrrole were bought from Sinopharm Chemical Reagent Co., Ltd. Polyvinylidene fluoride, acetylene black, and N-methyl pyrrolidone was provided by Tianjin Comio Chemical Reagent Co., Ltd.  (15 mmol) and ammonium sulfate (9 mmol) in 50 ml of ultrapure water. Subsequently, the pyrrole solution was then added drop by drop to the suspension and continuously stirred in ice water for 1 h for further polymerization. The reaction product was dried at 60°C for 12 h to obtain MnFeO@PPy composite material. For comparison, MnFeO@PPy samples with 5, 10, and 20 ll of pyrrole solution were prepared according to the above steps, and they were denoted as MnFeO@PPy5, MnFeO@PPy10, and MnFeO@PPy20, respectively.

Material characterizations
The phase structure of the samples was investigated by X-ray diffractometer with Cu-Ka radiation (XRD, D/Max 2200PC, Japan). The Fourier infrared spectrum of the sample was measured on Bruker's infrared spectrometer (Vertex70) and passed FESEM (Hitachi S-4800, Japan) and TEM (Tecnai G2 F20, FEI company) to characterize the micromorphology and structure of the sample.

Electrochemical test
The electrochemical performance of the MnFeO@PPy electrode was characterized by the CR2032 half-cell. The preparation method of the half-cell is as follows: First, the active material, acetylene black, and polyvinylidene fluoride binder were uniformly mixed at a ratio of 5:4:1 (w/w), and an appropriate amount of N-methyl pyrrolidone was added to prepare the electrode slurry. It is then evenly coated on copper foil to prepare the working electrode (the content of electrode material is about 0.4 mg cm -2 substrate). Then the negative electrode shell, shrapnel, gasket, lithium sheet, electrolyte, diaphragm, electrolyte, MnFeO@PPy electrode, and positive electrode shell were placed in the order, and then pressed and sealed with a sealing machine. The electrolyte is 1 M LiPF6 solution (using a volume ratio of 1:1:1 solvent mixture of ethylene carbonate, diethyl carbonate, and methyl vinyl carbonate as electrolyte). Cyclic voltammetry (CV) curves with various scanning rates and electrochemical impedance spectroscopy of the battery were characterized by Koster electrochemical workstation (CS350H). The galvanostatic charge-discharge performance and rate performance of the battery were tested on NEWARE test equipment (CT-4008T).

Results and discussion
The phase structure of MnFeO, MnFeO@PPy5, MnFeO@PPy10, MnFeO@PPy20, and pure phase PPy are shown in Fig. 1a. The diffraction peaks of MnFeO, MnFeO@PPy5, MnFeO@PPy10, and MnFeO@PPy20 correspond to Fe 2 O 3 (JCPDS No. 99-0060) and Mn 2 O 3 (JCPDS No. 89-4836) in the standard card. There are no obvious impurity peaks in the diffraction pattern, which indicates that the prepared sample has a higher purity. The XRD pattern of the pure phase PPy is also shown in Fig. 1a, and there is a clear C peak at about 26°, which is consistent with the literature report [21,22]. To further prove the existence of PPy in the composite material, the sample was tested by FTIR, and the result is shown in Fig. 1b. The peaks at 524 and 570 cm -1 correspond to the characteristic peaks of the Fe-O bond and Mn-O bond, respectively [23,24]. The peaks at 1554 and 1463 cm -1 are corresponding to the characteristic peaks of the pyrrole ring. The bands at 1180, 1105, 1045, and 914 cm -1 are the characteristic peaks of N-H, C-N-C, C-H, and C-C [25][26][27]. TG is used to determine the chemical composition of the MnFeO@PPy10 (Fig. 1c). The results show that the MnFeO@PPy10 composite contains 12.1 wt% PPy, which was measured using TG analysis at air atmosphere based on the complete burning up of the PPy. Figure 1d shows the nitrogen adsorption-desorption curve of MnFeO@PPy10, which shows a typical type-IV isotherm representing the mesoporous structure with a specific surface area of 5.3 m 2 g -1 . The pore size distribution shown in the inset of Fig. 1d suggests that the MnFeO@PPy10 product has an average mesopore size of 22 nm, which is conducive to the release volume effect.
The microscopic morphology of the sample was characterized by FESEM, and the result are shown in Fig. 2. Figure 2a and d shows the MnFeO microscopic morphologies which have a porous structure. This channel structure can establish a channel for the rapid transport of lithium ions, and it can effectively alleviate the volume expansion of the active material during charge and discharge, thereby improving the cycle stability of the material. Figure 2b and e shows the microscopic morphology of MnFeO@PPy10. Unlike MnFeO, the surface of MnFeO@PPy10 has a layer of PPy film. Its high conductivity and good flexibility can further improve its electrical conductivity and stability of chemical properties. The microstructure of MnFeO@PPy10 was further studied by TEM (Fig. 2c, f). It can be observed from Fig. 2c that MnFeO@PPy10 has an obvious porous structure. In the TEM image shown in Fig. 2f, there is a clear PPy coating layer at the boundary of the MnFeO matrix, with a thickness of about 23 nm. The conductive polymer film can further improve the conductivity of MnFeO composite, and the existence of the coating layer can effectively reduce the large volume expansion during the charge and discharge process, and prevent the material structure from being damaged and losing activity.
XPS was used to analyze the chemical composition and valence state of the prepared MnFeO@PPy10 sample surface. The test results are shown in Fig. 3. The high-resolution energy spectrum of Mn2p (Fig. 3a) in the MnFeO@PPy10 sample has two peaks at 641.2 eV and 652.7 eV corresponding to Mn2p3/2 and Mn2p1/2, respectively. Its spin separation energy is 2.0 eV, which corresponds to the reported peak of Mn 3? [28]. The characteristic peak of Fe2p (Fig. 3b) has two peaks at 710.7 eV and 724.3 eV, corresponding to Fe2p3/2 and Fe2p1/2, respectively. In addition, a wider satellite peak was detected at 715.3 eV, which further revealed the presence of Fe 3? [29]. Figure 3c shows the high-resolution energy spectrum of C1s in MnFeO@PPy10. Among the C1s characteristic peaks, three peaks can be observed at 284.6 eV, 285.8 eV, and 288.2 eV, corresponding to C-C, C=C, and C-N, respectively. The characteristic peaks of N1s in MnFeO@PPy10 are shown in Fig. 3d. The peaks at 398.7 eV, 399.7 eV, and 400.5 eV correspond to pyridinic-N, pyrrolic-N, and graphitic-N, respectively. Pyridinic-N and pyrrolic-N as functional groups of electrochemically active sites can enhance the surface-induced capacitive process of lithium ions and can enhance the diffusion capacity of Li ? in the material, thereby increasing the overall lithium storage capacity of the material [25].
To characterize the electrochemical reaction kinetics and reversibility of the composites, CV tests were carried out on the battery with MnFeo@PPy10 and MnFeO as the anode at the scanning rate of 0.1 mV s -1 . The test results are shown in Fig. 4a and b. At the first cathodic scanning process of MnFeO@PPy10, three obvious reduction peaks were observed at 0.35 V, 0.95 V, and 1.1 V, respectively. The sharp peaks at 0.35 V and 0.95 V are corresponding to Mn(III) ? Mn(0) and Fe(III) ? Fe(0), respectively. The peak at 1.1 V results from the process of solid electrolyte film forming by the irreversible reduction reaction of the electrolyte. At the next cathodic scanning test, the reduction peak at 1.1 V disappeared, which further indicates that the solid electrolyte film is an irreversible phase transition. Two obvious oxidation peaks appeared during the anode scanning test. The oxidation peak at 1.31 V was attributed to the reversible oxidation of Fe (0) ? Fe(II) and Mn (0) ? Mn(II). The oxidation at 1.67 V. The formation of the peak is attributed to the reversible oxidation of Fe(II) ? Fe(III) and Mn(II) ? Mn(III) [30][31][32]. These two groups of redox peaks did not change significantly in the subsequent tests, and the peak positions basically overlapped, indicating that the electrochemical reaction has good reversibility. Different from MnFeO, the two reduction peaks of MnFeO@PPy10 show a certain left shift in the subsequent cycle test. This is because the introduction of pseudo-capacitance material polypyrrole can store a large amount of charge on the surface of the material during charging. In this process, these stored charges can further reduce the reaction energy barrier and promote the reduction reaction [33]. Figure 4c and d shows the first three charging/ discharging curves of MnFeO@PPy10 and MnFeO electrodes at a current density of 0.1 A g -1 . It can be observed from Fig. 4c and d that there are three obvious discharge platforms in both MnFeO@PPy10 and MnFeO electrodes during the first discharge (platform 1: 1.1 V; platform 2: 0.9 V; platform 3: 0.3 V). These three platforms correspond to the formation of solid electrolyte membranes, the reduction of Fe(III) ? Fe(0), and the reduction of Mn(III) ? Mn(0). Among them, platform 1 corresponds to irreversible capacity, and platforms 2 and 3 are reversible capacity. The existence of irreversible capacity determines the size of the first coulombic efficiency [34,35]. The first charge-discharge specific capacity of MnFeO@PPy10 composite is 995 and 1551 mAh g -1 , respectively. The first coulombic efficiency is 64%, which is caused by the irreversible electrochemical reaction and the decomposition of the electrolyte. It has been consistent with the CV curve analysis. According to CV curve and charge-discharge curve analysis, the electrochemical reaction mechanism of the electrode is as follows: The cycling performance curves of PPy, MnFeO, MnFeO@PPy5, MnFeO@PPy10, and MnFeO@PPy20 electrode materials were characterized by galvanostatic charge-discharge tests at a current density of 1 A g -1 , and the results are shown in Fig. 5. The initial charge-discharge specific capacity of MnFeO (Fig. 5a) electrode is 661/1100 mAh g -1 , and its capacity loss is mainly caused by the formation of solid electrolyte film by an irreversible electrochemical reaction, which is consistent with CV curve analysis. The battery capacity dropped sharply after 250 cycles of charge and discharge, and after 500 cycles, its capacity dropped to 230 mAh g -1 . The average specific capacity of pure PPy (Fig. 5a) is about 191 mAh g -1 at 1 A g -1 after 500 cycles.
Unlike MnFeO electrodes, MnFeO@PPy5, MnFeO@PPy10, and MnFeO@PPy20 electrode materials did not show obvious signs of capacity degradation after 500 cycles. After 500 cycles, the specific capacity ratio was 473, 628, and 492 mAh g -1 . The average lithium storage capacities of MnFeO@PPy5, MnFeO@PPy10, and MnFeO@PPy20 electrode materials after 500 charge-discharge cycles were 483, 627, and 619 mAh g -1 , respectively, among which MnFeO@PPy10 showed the most stable lithium storage performance. The MnFeO@PPy10 electrode exhibits a certain capacity decay in the first 50 charge and discharge cycles, which is mainly caused by the irreversible structural transformation caused by the partial volume change in the early stage [28,36,37]. After 50 cycles, its capacity tends to be stable. Its specific capacity has been fluctuating at 617 mAh g -1 during the discharge process, showing good cycling stability.
The rate performance of MnFeO and MnFeO@PPy10 were investigated at different current densities (0.1, 0.2, 0.5, 1, 2 A g -1 ), and the test results are shown in Fig. 6. The average reversible discharge capacity of MnFeO@PPy10 electrode at 0.1, 0.2, 0.5, 1, and 2 A g -1 is 946, 775, 643, 506, and 432 mAh g -1 , respectively, and the current density increases from 0.1 to 2 A g -1 ; its capacity loss is 54.3%. After charging and discharging tests under increasing current density, when the current density decreases again (0.5, 0.1 A g -1 ), the specific capacity also sequentially recovers to 657, 881 mAh g -1 . Compared with MnFeO electrode (the specific capacity is 660, 535, 490, 401, and 257 mAh g -1 at the current density of 0.1-2 A g -1 , and the capacity loss is 61%), MnFeO@PPy10 shows more excellent rate performance, which further illustrates the MnFeO@PPy10. The specific capacity has better stability and reversibility.
To further study the capacitance behavior of the MnFeO@PPy10 electrode material, the capacitance effect of the MnFeO@PPy10 composite electrode system was analyzed through the CV curves of different scan rates (Fig. 7a). Generally, two charge storage behaviors (including capacitive contribution caused by the surface charge transfer and diffusion contribution caused by lithiation/desalination reactions) play an important role in the cycling process. The charge storage process can be characterized by analyzing the relationship between the current response (i) and the scan rate (v). Combined with the data shown in Fig. 7a, the capacitance effect of the electrode material can be qualitatively analyzed according to the formulas (3) and (4): Here a and b are constants. When b is close to 0.5, the electrochemical process is controlled by diffusion, and when b is close to 1.0, the reaction is controlled by the contribution of capacitance. For b values between 0.5 and 1.0, there is a mixing mechanism during charge storage [10,38]. Figure 7b shows the fitting curve of log i-log v; calculating from it, the b values of the oxidation peak (peaks 1, 2) and the reduction peak (peaks 3, 4) are 0.88, 0.92, 1.0, and 0.7, respectively, which indicates that the MnFeO@PPy10 electrode has a good capacitance effect. The proportion of capacitance contribution can be quantitatively calculated by the formula (5), where V is the voltage, i is the corresponding current response, k 1 v is the surface capacitance contribution, and k 2 v 1/2 contributes to diffusion: Figure 7c shows the capacitance evaluation of the capacitive behavior contribution when the scan rate is 0.1 mV s -1 , in which the red shaded part is the capacitive contribution. The capacitive control capacity accounts for 75% of the total charge storage at 0.1 mV s -1 . Figure 7d shows the ratio of the amount of charge stored by the diffusion contribution and the capacitance contribution at various scan rates. The results show that as the scan rate increases from 0.1 to 1 mV s -1 , the capacitance contribution of the capacitance behavior increases to 97%. The conductive polymer polypyrrole coating in the MnFeO@PPy10 electrode material can promote the charge transfer of the active material and the capacitance behavior of the MnFeO@PPy10 electrode. This feature is also conducive to the rapid transport of Li, thereby making its cycle life and reversible capacity stable enhanced [14].
To investigate the impact of PPy on the conductivity of MnFeO, electrochemical impedance spectroscopy tests were carried out on half-cells with MnFeO and MnFeO@PPy10 as working electrodes. The test results are shown in Fig. 8a. The semicircular area (high frequency) corresponds to the charge transfer process at the electrode/electrolyte interface. The linear part (low frequency) corresponds to the diffusion process of lithium ions in the electrode. The equivalent circuit diagram is shown in the inset of Fig. 8a, where R s represents the half-cell ohmic impedance, R f and R ct represent the solid electrolyte film resistance and the surface charge transfer resistance, C1 and C2 represent the phasing elements of the corresponding double-layer capacitors, and W1 represents the Warburg impedance caused by the diffusion of lithium ions in the electrode. The impedance value of R s , R f , and R ct is fitted: MnFeO (R s = 6.4 X, R f = 157 X, R ct = 49.3 X); MnFeO@PPy10 (R s = 5.4 X, R f = 72.3 X, R ct = 36.9 X). As shown in Fig. 8b, the resistance of each circuit element corresponding to MnFeO@PPy10 is lower than MnFeO electrode. Among them, the resistance of the solid electrolyte membrane (R f ) and the charge transfer resistance (R ct ) decrease the most. The main reason is that the existence of the pseudo-capacitance material polypyrrole conductive network can store charge in the surface of the electrode material and provides a more convenient channel for electrons and Li ? to shuttle back and forth during the charging and discharging process, thereby promoting the charge transfer between the active material, conductive carbon, and the binder, and improving the conductivity of the material. In addition, the porous structure of the MnFeO@PPy10 electrode material can also reduce the aggregation and contact resistance of the electrode and provide more active sites for the transfer of electrons and ions, thereby promoting the effective progress of the reaction.

Conclusion
In this study, MnFeO@PPy10 composite anode material was successfully synthesized through two steps of hydrothermal method and SIP method. The charging and discharging tests show that the MnFeO@PPy10 composite anode material has a high discharge/charge specific capacity and great stability. At 1 A g -1 , the specific discharge capacity of MnFeO@PPy10 composite is 627 mAh g -1 . After 500 cycles, the specific capacity does not show obvious signs of attenuation, which shows the MnFeO@PPy10 anode has a good cycling stability. At 2.0 A g -1 , the specific capacity of MnFeO@PPy10 is 432 mAh g -1 , which shows good rate performance. Through charge contribution analysis and electrochemical impedance spectroscopy analysis, it is found that the existence of the conductive network of polypyrrole can store the charge in the electrode material. The surface can provide a more convenient channel for electrons and Li ? to shuttle back and forth during the charging and discharging process, thereby promoting the charge transfer rate between the active material, conductive carbon, and the binder electrochemical performance of the material. In addition, the porous structure of Fe 2 O 3 /Mn 2 O 3 and polypyrrole coating layer of pseudo-capacitance material can effectively alleviate the volume effect leading to the phenomenon of material inactivation during charging and discharging, thereby improving material stability.