Trans. Nonferrous Met. Soc. China 30(2020) 2188-2199

Preparation and electrochemical performance of nitrogen-doped carbon-coated Bi₂Mn₄O₁₀ anode materials for lithium-ion batteries

Jing ZHAN^1,2, Chang-fan XU¹, Yi-yu LONG¹, Qi-hou LI¹

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China

2. National Engineering Laboratory for High Efficiency Recovery of Refractory Nonferrous Metals Resources,  Central South University, Changsha 410083, China

Received 1 January 2020; accepted 4 June 2020

Abstract: To inhibit rapid capacity attenuation of Bi₂Mn₄O₁₀ anode material in high-energy lithium-ion batteries, a novel high-purity anode composite material Bi₂Mn₄O₁₀/ECP-N (ECP-N: N-doped Ketjen black) was prepared via an uncomplicated ball milling method. The as-synthesized Bi₂Mn₄O₁₀/ECP-N composite demonstrated a great reversible specific capacity of 576.2 mA·h/g after 100 cycles at 0.2C with a large capacity retention of 75%. However, the capacity retention of individual Bi₂Mn₄O₁₀ was only 27%. Even at 3C, a superior rate capacity of 236.1 mA·h/g was retained. Those remarkable electrochemical performances could give the credit to the introduction of ECP-N, which not only effectively improves the specific surface area to buffer volume expansion and enhances conductivity and wettability of composites but also accelerates the ion transfer and the reversible conversion reaction.

Key words: Bi₂Mn₄O₁₀ nanoparticles; N-doped Ketjen black; rate capability; lithium-ion batteries

1 Introduction

Higher requirements have been put forward for the lithium-ion batteries (LIBs) with higher energy and power density, as well as more excellent cycle performance because of the rapid development of portable electronic equipment and electric vehicles and the limited theoretical specific capacity of commercial graphite anodes (372 mA·h/g) [1,2]. Among a multitude of anode materials, manganese- based oxides, such as MnO [3], Mn₂O₃ [4], MnO₂ [5] have attracted tremendous research attention owing to their stable chemical states, environmental friendliness, low cost, and high specific capacities between 600 and 1000 mA·h/g [6]. Particularly, the metallic Bi can furnish a desirable volumetric capacity of 3760 mA·h/cm³. Bi-based materials, as anode materials for LIBs, have also exhibited distinctive characteristics of excellent volumetric density and specific capacity due to their tailor-made morphologies, microstructures, and multifarious composition [7-9]. For example, Bi₂O₃/rGO composite anode materials showed a specific capacity of 347.3 mA·h/g after 100 electro- chemical cycles at 1C [10]. However, most metal oxides generally display low conductivity, and their commercial application is severely impeded by dreadful capacity degradation caused by the extreme volume expansion, dramatic structure pulverization, and the loss of electronic connection during continuously repeated lithiation/delithiation processes [6,11]. Multicomponent metal oxides, such as ZnMn₂O₄ [12] and ZnCo₂O₄ [13] usually exhibit better electrochemical properties than single-component metal oxides on account of their abundant composition and synergistic effects among various metals [14,15].

Especially, mullite-type compound Bi₂Mn₄O₁₀ with a considerable theoretical specific capacity of 873 mA·h/g has been investigated as an anode material for advanced Li-ion battery systems. SONG et al [16] prepared a Bi₂Mn₄O₁₀ anode material via the sol-gel method, which displayed a high reversible specific capacity of 711.6 mA·h/g at 60 mA/g. Furthermore, our group synthesized Bi₂Mn₄O₁₀ nanoparticles, which maintained a stable reversible specific capacity of 402.3 mA·h/g after 50 cycles at 0.2C [17]. It is essential to improve cycling lifespan and rate performances of Bi₂Mn₄O₁₀. Downsizing the bulk material to nanoscale and integrating with highly conductive and excellent chemical stable carbon were considered as one of the most effective strategies for LIBs to circumvent capacity fading, inferior rate performance, and poor cyclic durability [18,19]. Reducing the size of particles can not only tailor the properties of surface interface and expose numerous active sites, but also ensure the adequate diffusion of electrolyte and ions [20]. As known, the carbon-based materials not only have high conductivity and stability but also can provide a larger reaction space to relieve the volume change during the continuous insertion/extraction of Li⁺, thereby reliably inhibiting the pulverization of electrode materials. In addition, doping elemental nitrogen (N) into the conductive carbon matrix is propitious to create extensive defects and active sites without causing lattice mismatch, which significantly strengthens the electrical conductivity and wettability, thus accelerating the faster transfer of ions and the highly reversible conversion reaction [21]. Hence, the economical and efficient preparation of Bi₂Mn₄O₁₀/C anode composites using carbon materials with excellent electro- chemical properties is of great significance for improving the cycle stability, lithium storage capacity, and rate performance of Bi₂Mn₄O₁₀ in portable electronic equipment and electric vehicles.

In this work, the N-doped Ketjen black (ECP-N) was prepared by a low-cost calcination reduction method. Subsequently, the Bi₂Mn₄O₁₀/ ECP-N nanocomposites were prepared via a simple wet ball milling method. Most significantly, the Bi₂Mn₄O₁₀/ECP-N composites serving as anodes for LIBs demonstrated higher cycling stability than bare Bi₂Mn₄O₁₀. An attractive specific capacity of 576.2 mA·h/g was obtained at 0.2C after 100 cycles with prominent retention of 75%, while bare Bi₂Mn₄O₁₀ only conveyed 169.5 mA·h/g with inferior retention of 27%. The ECP with N dopants not only suppresses the aggregation of Bi₂Mn₄O₁₀ nanoparticles but also offers substantial defect  sites to enhance ions transport and electronic conductivity.

2 Experimental

2.1 Material synthesis

All chemical reagents were used directly to prepare target samples without further purification. Bi₂Mn₄O₁₀/ECP-N material was synthesized via a typical method. Firstly, the preparation of Bi₂Mn₄O₁₀ powders was given in our previous report [17]. In this work, the ball milling time for preparing Bi₂Mn₄O₁₀ powders was 18 h. ECP-N was synthesized via calcining Ketjen black (ECP600JD, Triquo Chemical Technology Co., Ltd.) at 800 °C under NH₃ for 10 h. Then, 20 g Bi₂Mn₄O₁₀ powders and 2 g ECP-N were added into a 250 mL ball mill tank for ball milling for 8 h to obtain the precursor, which was calcined at 300 °C in air for 3 h with a heating ramp of 2 °C/min, marked as Bi₂Mn₄O₁₀/ECP-N.

2.2 Material characterization

The crystallographic structures of Bi₂Mn₄O₁₀ and Bi₂Mn₄O₁₀/ECP-N were investigated by an X-ray powder diffractometer (XRD, Rigaku- TTRIII) allocated with Cu K_α at 8 (°)/min. The morphologies of the as-prepared materials were observed by the scanning electron microscope (SEM, JSM-6360LV) and the transmission electron microscope (TEM, TEM, JEM-2100F). The Brunauer-Emmett-Teller (BET) specific surface areas and Barrett–Joyner–Halenda (BJH) pore  size distribution of as-prepared materials were calculated according to N₂ adsorption isotherms (ASAP2020). X-ray photoelectron spectroscopy analysis (XPS, Kratos Axis Ultra DLD) was used to testify the chemical composition of Bi₂Mn₄O₁₀/ ECP-N.

2.3 Electrochemical test

In this work, the 2025-type coin cells were employed to measure the electrochemical properties of as-prepared materials. The assembling of cells was performed in the glove box filled with high purity argon, using lithium foil as the counter electrode. 1 mol/L LiPF₆ in a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and diethyl carbonate (DEC) with a volume ratio of 1:1:1 was employed as the electrolyte. Moreover, porous polypropylene membranes were applied as the separator. For obtaining the working electrode, the as-prepared electroactive materials were mixed with conductive carbon and polyvinylidene fluoride (PVDF) at a mass ratio of 7:2:1. The mixture was homogeneously painted on the copper foil and dried in a vacuum oven for 12 h at 80 °C, which was further cut into wafers with a diameter of 12.0 mm. The final mass of electroactive materials was approximately 1.2 mg/cm. The galvanostatic charge/discharge cycling characterization was performed on LANHE cell tester (CT2001A, Wuhan, China) between 0.05 and 3.0 V at room temperature. The electrochemical workstation  (CHI 760e) was used to obtain the cyclic voltammogram (CV) between 0.05 and 3.0 V at a scan rate of 0.1 mV/s. Electrochemical impedance spectroscopy (EIS) measurements were also carried out on a CHI 760e system in the frequency range of 0.01-100000 Hz.

3 Result and discussion

3.1 Phase structure

The crystalline nature of the as-prepared Bi₂Mn₄O₁₀ nanoparticles and Bi₂Mn₄O₁₀/ECP-N composites was investigated by XRD and the corresponding results were depicted in Fig. 1. As can be seen from Fig. 1, the ball-milled Bi₂Mn₄O₁₀ nanoparticles and Bi₂Mn₄O₁₀/ECP-N composites exhibit nearly the same XRD patterns, which perfectly match with the pure Bi₂Mn₄O₁₀ phase (JCPDS No. 27-0048). However, there is a diffraction peak of Bi₂O₃ in Fig. 1(b), which might be attributable to the relatively short milling time. The bismuth and manganese cannot be mixed uniformly, resulting in the existence of the Bi₂O₃ phase. After the Bi₂Mn₄O₁₀ powders and ECP-N were mixed by ball milling for 8 h, no peak of Bi₂O₃ was detected, indicating the higher purity of the as-obtained Bi₂Mn₄O₁₀/ECP-N. Moreover, the peak intensities of Bi₂Mn₄O₁₀/ECP-N composite become weaker than those of Bi₂Mn₄O₁₀, which indicates that the crystallinity of different crystal faces declines and the grain size is smaller after ball milling with ECP-N. The interplanar spacing (d) and grains size (D_hkl) can be estimated through the Bragg equation 2dsin θ=nλ (n is the diffraction series, θ is the angle of incidence on a crystal  plane and λ is the wavelength), and Scherrer equation D_hkl=Kλ/(βcos θ) (K=0.89, λ=0.154 nm, and β is the full width at half-maximum (FWHM)). Corresponding results are listed in Table 1. After ball milling with ECP-N, the interplanar spacing and grain size of Bi₂Mn₄O₁₀ nanoparticles decreased obviously, which were of great significance to increase the specific surface area of the sample and release substantial active sites for Li⁺ storage. In addition, no obvious peak of ECP-N was detected on account of the higher peak intensity of Bi₂Mn₄O₁₀, indicating that the low content of ECP-N had no obvious effect on the crystallization behavior of the Bi₂Mn₄O₁₀.

Fig. 1 XRD patterns of as-synthesized Bi₂Mn₄O₁₀/ ECP-N (a) and bare Bi₂Mn₄O₁₀ (b)

Table 1 Structural parameters of Bi₂Mn₄O₁₀ and Bi₂Mn₄O₁₀/ECP-N determined from XRD Rietveld refinement

3.2 SEM and TEM morphologies

The typical SEM and TEM images of as-synthesized materials were revealed in Fig. 2.  As shown in Fig. 2(a), the Bi₂Mn₄O₁₀/ECP-N composites were mainly composed of well- developed nanoparticles with particle sizes  ranging from one hundred to several hundred nanometers. The morphology of Bi₂Mn₄O₁₀/ECP-N nanoparticles demonstrated uniform distribution because the Bi₂Mn₄O₁₀/ECP-N nanocrystals could be well separated and stabilized by the doped carbon matrix. Figure 2(c) displayed that the fine lattice fringes of Bi₂Mn₄O₁₀/ECP-N nanoparticles with an interplanar spacing of approximately  0.265 nm, which corresponded to the (130) crystalline plane of the Bi₂Mn₄O₁₀ phase. This result further demonstrated the Bi₂Mn₄O₁₀/ECP-N nanocrystals with superior crystallization and purity. As can be seen from the SEM and TEM images of ECP-N in Figs. 2(b, d), ECP-N showed a nanoparticle structure with the particle size distribution between tens of nanometers and hundreds of nanometers, which could be conducive to uniform doping during the ball-milling.

To evidence elements distribution in Bi₂Mn₄O₁₀/ ECP-N composites, the element mapping was measured via the energy-dispersive spectroscopy (EDS), as demonstrated in Fig. 3. The elemental mapping of Bi₂Mn₄O₁₀/ECP-N nanoparticles indicated the existence of Bi, Mn, N, C and O elements (Figs. 3(a-f)). The uniform distribution of element C will improve more abundant reaction area and strengthen the electrical conductivity of Bi₂Mn₄O₁₀, and the uniform distribution of element N provides substantial defects and active sites, which will improve the wettability of Bi₂Mn₄O₁₀ in the electrolyte and the electrochemical properties of the Bi₂Mn₄O₁₀ anode material.

Fig. 2 Low and high magnification SEM images of Bi₂Mn₄O₁₀/ECP-N (a), SEM image of ECP-N (b), high-resolution TEM image of Bi₂Mn₄O₁₀/ECP-N (c) and TEM image of ECP-N (d)

Fig. 3 EDS image of Bi₂Mn₄O₁₀/ECP-N (a), corresponding element mapping of Bi (b), Mn (c), N (d), C (e), O (f) and comparison of interface contact angle of Bi₂Mn₄O₁₀ (g), ECP (h) and ECP-N (i)

In particular, the interaction among Li⁺ and Bi₂Mn₄O₁₀, ECP, N-ECP was also measured via the contact angle test. As shown in Figs. 3(g-i), the three electrodes with Bi₂Mn₄O₁₀, ECP, and ECP-N were adopted to characterize the contact angle, respectively, which used the same electrolytes (1 mol/L LiPF₆ in a mixture of EC, DMC and  DEC (1:1:1, volume ratio)). As shown in Fig. 3(i), ECP-N electrode manifested minimum contact angle of 1.36°, which is smaller than that on the Bi₂Mn₄O₁₀ electrode (45.61°, Fig. 3(g)) and ECP electrode (2.34°, Fig. 3(h)), indicating good wettability for electrolyte after being N-doped. The enhanced affinity with electrolyte would promote Li⁺ transmission, thus increasing the lithium-ion diffusion rate on the surface of the electrode [22], which could be conducive to the improvement of the cyclicity and rate performance.

3.3 N₂ adsorption/desorption isotherms

Generally, the specific surface area, total pore volume, and pore diameter have significant impact on the electrochemical performance of the electroactive materials, thus typical N₂ adsorption/ desorption isotherm measurements were implemented to characterize the porous features and textural properties of the as-prepared Bi₂Mn₄O₁₀/ ECP-N and Bi₂Mn₄O₁₀ samples more clearly. As shown in Fig. 4, all N₂ isotherms could be classified as typical type-IV characteristics with a type-H1 hysteresis loop [23,24], revealing mesoporous characteristics. Their BET specific surface area and total pore volume were computed to be 33.72 m²/g and 0.1538 cm³/g for Bi₂Mn₄O₁₀/ECP-N, 11.87 m²/g and 0.0232 cm³/g for Bi₂Mn₄O₁₀, respectively. The relatively large surface area and total pore volume of Bi₂Mn₄O₁₀/ECP-N were beneficial to providing the electrochemical uptake and release of Li⁺ ions, and favoring enough room to accommodate volume expansion/ contraction during continuous cycling. Furthermore, the mesoporous characteristics could also be further testified by the Barrett-Joyner-Halenda (BJH) pore-size distribution (PSD) in the insets of    Figs. 4(a, b), the pore distributions of Bi₂Mn₄O₁₀/ ECP-N and Bi₂Mn₄O₁₀ were mostly in the range of 2-20 nm, and their average pore sizes were approximately 18.24 and 7.83 nm respectively, which strongly demonstrated that the Bi₂Mn₄O₁₀/ ECP-N contained a great number of mesoporous architectures. Indeed, the abundant mesoporous configuration not only promotes the contact between electroactive materials and electrolytes  but also provides sufficient surfaces/interfaces to boost charge transfer and shorten path length of ion diffusion, thus strongly increasing the electrochemical performance of samples [23].

Fig. 4 N₂ adsorption/desorption isotherms of Bi₂Mn₄O₁₀/ ECP-N (a) and Bi₂Mn₄O₁₀ (b) (Insets in (a, b) show corresponding BJH pore size distribution plots)

3.4 XPS spectra

The XPS was employed to characterize the chemical elements and the surface chemical valence states. The as-obtained XPS data and typically fitted curves by using the Gaussian fitting method were collected and shown in Fig. 5. The main peaks of  Bi 4f, C 1s, O 1s, and Mn 2p were delivered in the survey spectrum of Bi₂Mn₄O₁₀/ECP-N in Fig. 5(a), and another weak peak was situated at about 398 eV, which corresponded to the N 1s. As shown in the high-resolution spectrum of Bi 4f (Fig. 5(b)), the binding energies situated at 160.2 and 165.5 eV were contributed to Bi 4f_5/2 and Bi 4f_7/2, respectively [15,25]. As shown in Fig. 5(c), the high-resolution spectrum of Mn 2p exhibited two the characteristic peaks at 641.3 and 652.9 eV, which corresponded to Mn 2p_3/2 and Mn 2p_1/2, respectively. Moreover, the spectrum of Mn 2p could be separated into four peaks, in which 641.2 and 652.9 eV could characterize the existence of Mn³⁺, and the other two peaks at 642.5 and 654.1 eV could be assigned to Mn⁴⁺ species [23]. According to the high-resolution spectra of C 1s in Fig. 5(d), three peaks at 284.4, 285.6 and 288.6 eV were attributed to C—C, C—N, and C=O groups, which indicated that N atoms had been doped into the electroactive materials [26]. The existence of N species was further testified by N 1s peak in Fig. 5(e). The peaks located at 398.8, 399.6 and 401.4 eV could be assigned to pyridinic-N, pyrrolic-N and graphitic-N, respectively [22,27]. The total content of N was approximately 1.9%, which could enhance lithium-storage capability.

3.5 Electrocatalytic behavior

Subsequently, the Bi₂Mn₄O₁₀/ECP-N was configured as an anode in a half cell, in which metallic lithium foil was utilized as the counter electrode, highlighting the application prospects of the material for high-performance LIBs and evaluating its electrochemical lithium-storage performance. The lithium-ion storage mechanism of Bi₂Mn₄O₁₀/ECP-N composites was preliminarily investigated using cyclic voltammetry in the voltage range of 0.05-3.0 V (vs Li⁺/Li) at a scan rate of 0.1 mV/s. As shown in Fig. 6(a), the CV curves displayed multipeak characteristics, indicating that the reaction between Li⁺ and the Bi₂Mn₄O₁₀/ECP-N was multistep. During the first negative scan, two irreversible cathodic peaks at about 1.19 and 0.73 V could be attributed to the decomposition of the electrolyte and the reduction of Mn³⁺, Mn⁴⁺ and Bi³⁺ to Mn⁰ and Bi⁰ with the production of Li₂O, respectively [28,29]. The cathodic peak at around 0.38 V could be associated with the formation of SEI films and alloying reaction between Bi and Li, which could be considered as a multistep alloying reaction. However, the broad peak was divided into two small peaks at ~1.37 and ~1.06 V in subsequent cycles, which corresponded to the reduction of BiO to metallic Bi and MnO to metallic Mn, respectively [28,29]. During the positive scan, a strong anodic peak at around 1.12 V could be related to the dealloying processes of Li₃Bi. Furthermore, two broad anodic peaks at about 1.4 and 1.7 V might have originated from the oxidation of Bi and Mn metal clusters. The aforementioned main reactions were shown in Table 2. From the CV curves, it was obvious that the reduction peaks and oxidation peaks exhibited apparent overlapped characteristics, which verified that the electrode exhibited excellent reversibility and stability during the electrochemical reaction.

Fig. 5 Full XPS spectrum of Bi₂Mn₄O₁₀/ECP-N (a), high-resolution XPS spectrum of Bi 4f (b), Mn 2p (c), C 1s (d) and N 1s (e)

Table 2 Chemical reactions during charge/discharge processes

Fig. 6 CV curves of Bi₂Mn₄O₁₀/ECP-N (a), charge/discharge curves of Bi₂Mn₄O₁₀/ECP-N (b), cycling performance of Bi₂Mn₄O₁₀, Bi₂Mn₄O₁₀/ECP and Bi₂Mn₄O₁₀/ECP-N at 0.2C (1C=800 mA/g) (c), rate performance curve of Bi₂Mn₄O₁₀/ ECP-N (d) and EIS Nyquist curves of Bi₂Mn₄O₁₀ and Bi₂Mn₄O₁₀/ECP-N (e)

As illustrated in Fig. 6(b), the initial discharge specific capacity and charge specific capacity were 1207 and 823 mA·h/g, delivering Coulombic efficiency of 68%, the initial irreversible capacity loss could be attributed to the irreversible redox reaction and the formation of SEI layer on the anode electrode [30]. The formation of SEI layer is indispensable to conversion-type anode materials for large volume expansion [31]. Expectably, the stable redox processes could be obtained after the first cycle, resulting in high capacity retention, which could be clearly demonstrated in Fig. 6(c). As we could see the cycle performance at 0.2C in Fig. 6(c), it was obvious that the Bi₂Mn₄O₁₀/ECP-N displayed a higher discharge specific capacity than the Bi₂Mn₄O₁₀/ECP and Bi₂Mn₄O₁₀. The specific capacity of Bi₂Mn₄O₁₀/ECP-N showed a slight decrease and stabilized at 576.2 mA·h/g after 100 cycles, while the Bi₂Mn₄O₁₀/ECP and Bi₂Mn₄O₁₀ only demonstrated the specific capacity of 467.3 and 169.5 mA·h/g, respectively. It was obvious that the introduction of ECP-N could significantly strengthen the cycle stability, which could be ascribed to the protection of layered ECP-N for structural change during long-term continuous charge and discharge processes, and could be related to the better wettability for electrolyte after being nitrogen-doped. Figure 6(d) indicated that the Bi₂Mn₄O₁₀/ECP-N composite showed superior rate performance and delivered reversible capacities of 723.1, 607.4, 470.5, 314.6 and 236.1 mA·h/g, at the current densities of 0.2C, 0.5C, 1C, 2C and 3C, respectively. These results are more satisfactory than previous work of bare Bi₂Mn₄O₁₀ [17]. The electrochemical performance based on other Bi-M-O compounds anode materials for lithium-ion batteries was summarized in Table 3.

Table 3 Electrochemical performance comparison of Bi₂Mn₄O₁₀/ECP-N with other reported Bi-M-O compound anode materials for lithium-ion batteries

The heightened cyclic persistence and rate performance of Bi₂Mn₄O₁₀/ECP-N could be closely related to their excellent conductivity owing to the existence of ECP-N, which could be evidenced by EIS characterization. The fitted Nyquist plots and relative equivalent circuit model were displayed in Fig. 6(e). The semicircle in high-medium frequency was characterized as the charge transfer resistance (R_ct), which was ascribed to the electrochemical reactions between the interface of electrolyte and electrodes, the oblique line in low-frequency regions reflected Warburg impedance (Z_w) for Li⁺ diffusion [35,36]. In addition, R_e and R_f were ascribed to the resistances of the solvent electrolyte and SEI films, respectively. The resistance value of R_ct+R_e+R_f for the Bi₂Mn₄O₁₀/ECP-N (185.808 Ω) was smaller than that of Bi₂Mn₄O₁₀ (252.971 Ω), suggesting that Bi₂Mn₄O₁₀/ECP-N composite anode material possessed a faster charge transfer than the bare Bi₂Mn₄O₁₀ anode. Additionally, the linear slope of Bi₂Mn₄O₁₀/ECP-N composite in low-frequency region was larger than that of bare Bi₂Mn₄O₁₀, indicating the Li⁺ diffusion rate could be increased by using Bi₂Mn₄O₁₀/ECP-N composite anode. Furthermore, the Li⁺ diffusion coefficient (D_Li⁺) could be estimated from the low-frequency regions by the following formula: D_Li⁺=0.5(RT)²/(An²₁F²Cσ)², where R is the mole gas constant and R=8.314 J/(mol·K), T is the thermodynamic temperature and T=298 K, electrode surface area A=1.13 cm², F is the Faraday constant and F=96500 C/mol, the concentration of Li⁺ C=0.001 mol/cm³, n₁ is the electronic transfer number, and σ is the Warburg coefficient resulting from Z′=R_ct+R_e+R_f+σω^-1/2, ω=2πf (Z', ω and f represented the real impedance, angular frequency and frequency, respectively). The EIS fitting statistic values and Li⁺ diffusion were presented in Table 4. The calculated D_Li⁺ value of Bi₂Mn₄O₁₀/ ECP-N (8.54×10^-13 cm²/s) is much higher than that of pristine Bi₂Mn₄O₁₀ (9.71×10^-14 cm²/s), revealing the high Li⁺ diffusion coefficient and excellent reaction kinetics after the introduction of ECP-N. Bi₂Mn₄O₁₀/ECP-N illuminated smaller charge transfer resistance and Warburg impedance than bare Bi₂Mn₄O₁₀, reflecting that the introduction of ECP-N can not only advance Li⁺ diffusion and enhance the conductivity of the electroactive materials but also reduce the resistance of the charge transfer and enhance the reaction kinetics. Therefore, Bi₂Mn₄O₁₀/ECP-N composite anode exhibited a superior cycle and rate performance.

Table 4 EIS fitting statistic values and Li⁺ diffusion coefficient

4 Conclusions

(1) The high-purity mesoporous architecture Bi₂Mn₄O₁₀/ECP-N composite anode material with uniform distribution of C and N elements was designed via a convenient wet ball-milling method.

(2) Benefiting from the ECP-N layers, the Bi₂Mn₄O₁₀/ECP-N composite anode demonstrates a high capacity of 576.2 mA·h/g after 100 cycles at 0.2C with a capacity retention of 75%, which is much higher than that of bare Bi₂Mn₄O₁₀ (only 27%). Even at 3C, a superior rate capacity of  236.1 mA·h/g is retained.

(3) The N dopants make ECP have good wettability and electronic affinity, which can be beneficial to promoting the electronic conductivity of Bi₂Mn₄O₁₀/ECP-N and enhance kinetics. In addition, using ECP-N as the buffer layer is a pretty effective technique to mitigate volume expansion and alleviate the rapid capacity decline. This work provides a potential electroactive material for lithium-ion batteries and a design strategy for large-scale synthesizing nanocomposite electrode materials.

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氮掺杂碳包覆Bi₂Mn₄O₁₀锂离子电池负极材料的制备及其电化学性能

湛 菁^1,2，徐昌藩¹，龙怡宇¹，李启厚¹

1. 中南大学 冶金与环境学院，长沙 410083；

2. 中南大学 难冶有色金属资源高效利用国家工程实验室，长沙 410083

摘  要：为抑制高能锂离子电池负极材料Bi₂Mn₄O₁₀容量的快速衰减，通过简单球磨法制备新型高纯Bi₂Mn₄O₁₀/ECP-N(ECP-N为氮掺杂科琴黑)负极复合材料。所合成的Bi₂Mn₄O₁₀/ECP-N复合材料在 0.2C倍率下  循环100次后可保持576.2 mA·h/g的比容量，容量保持率为75%，而纯Bi₂Mn₄O₁₀的容量保持率仅为27%。3C倍率下Bi₂Mn₄O₁₀/ECP-N复合材料的放电容量仍保持在236.1 mA·h/g。引入氮掺杂的科琴黑ECP-N不仅可以有效地提高比表面积以缓冲体积膨胀，增强材料的电导率和可湿性，而且还可以促进离子传输和可逆转化反应。

关键词：Bi₂Mn₄O₁₀纳米颗粒；氮掺杂科琴黑；倍率性能；锂离子电池

(Edited by Wei-ping CHEN)

Foundation item: Project (2019zzts502) supported by the Fundamental Research Funds for the Central Universities of Central South University, China; Project (2018GK4001) supported by the Scientific and Technological Breakthrough and Major Achievements Transformation of Strategic Emerging Industries of Hunan Province, China

Corresponding author: Jing ZHAN, Tel: +86-13975147556, E-mail: zhanjing@csu.edu.cn;

Qi-hou LI, E-mail: li_qihou@126.com

DOI: 10.1016/S1003-6326(20)65371-7