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Miao-miao Deng, Da-wei Zhang, Yu Shao, Xiao-dong He, Aqsa Yasmin, Chun-hua Chen. Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 485-490. doi: 10.1063/1674-0068/cjcp1906118
Citation: Miao-miao Deng, Da-wei Zhang, Yu Shao, Xiao-dong He, Aqsa Yasmin, Chun-hua Chen. Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 485-490. doi: 10.1063/1674-0068/cjcp1906118

Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4

doi: 10.1063/1674-0068/cjcp1906118
More Information
  • Corresponding author: Chun-hua Chen. Tel.: +86 551 63606971; fax: +86-551-63601952. E-mail:cchchen@ustc.edu.cn
  • Received Date: 2019-06-12
  • Accepted Date: 2019-08-14
  • Publish Date: 2020-08-27
  • In this work the surface of LiNi$_{0.5}$Mn$_{1.5}$O$_{4}$ (LMN) particles is modified by Mn$_{3}$O$_{4}$ coating through a simple wet grinding method, the electronic conductivity is significantly improved from 1.53$\times$10$^{-7}$ S/cm to 3.15$\times$10$^{-5}$ S/cm after 2.6 wt% Mn$_{3}$O$_{4}$ coating. The electrochemical test results indicate that Mn$_{3}$O$_{4}$ coating dramatically enhances both rate performance and cycling capability (at 55 ℃) of LNM. Among the samples, 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM not only exhibits excellent rate capability (a large capacity of 108 mAh/g at 10 C rate) but also shows 78% capacity retention at 55 ℃ and 1 C rate after 100 cycles.
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Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4

doi: 10.1063/1674-0068/cjcp1906118
    Corresponding author: Chun-hua Chen. Tel.: +86 551 63606971; fax: +86-551-63601952. E-mail:cchchen@ustc.edu.cn

Abstract: In this work the surface of LiNi$_{0.5}$Mn$_{1.5}$O$_{4}$ (LMN) particles is modified by Mn$_{3}$O$_{4}$ coating through a simple wet grinding method, the electronic conductivity is significantly improved from 1.53$\times$10$^{-7}$ S/cm to 3.15$\times$10$^{-5}$ S/cm after 2.6 wt% Mn$_{3}$O$_{4}$ coating. The electrochemical test results indicate that Mn$_{3}$O$_{4}$ coating dramatically enhances both rate performance and cycling capability (at 55 ℃) of LNM. Among the samples, 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM not only exhibits excellent rate capability (a large capacity of 108 mAh/g at 10 C rate) but also shows 78% capacity retention at 55 ℃ and 1 C rate after 100 cycles.

Miao-miao Deng, Da-wei Zhang, Yu Shao, Xiao-dong He, Aqsa Yasmin, Chun-hua Chen. Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 485-490. doi: 10.1063/1674-0068/cjcp1906118
Citation: Miao-miao Deng, Da-wei Zhang, Yu Shao, Xiao-dong He, Aqsa Yasmin, Chun-hua Chen. Improving Interfacial Electrochemistry of LiNi0.5Mn1.5O4 Cathode Coated by Mn3O4[J]. Chinese Journal of Chemical Physics , 2020, 33(4): 485-490. doi: 10.1063/1674-0068/cjcp1906118
  • Spinel-type LiNi$_{0.5}$Mn$_{1.5}$O$_{4}$ (LNM) exhibits a theoretical capacity of 147 mAh/g and a competitive energy density of 640 Wh/kg at a higher working voltage ($\sim$5 V) [1-3]. Nevertheless, the practical use of a LNM cathode is restricted by its poor high-rate capability and dramatic capacity decay at high temperature, which mainly result from the side reaction that occurs on the cathode-electrolyte interphase. An effective strategy of surface coating has been introduced to strengthen the stability of the cathode-electrolyte interphase and eliminate the side reaction. The reported surface coating materials, including Al$_{2}$O$_{3}$ [4], CuO [5], ZnO [6], RuO$_{2}$ [1], TiO$_{2}$ [7], carbon [8, 9], Li$_{3}$PO$_{4}$ [10] and Li$ _{4}$Ti$_{5}$O$_{12}$ [11] have been proven to boost the electrochemical property of LNM. However, the rate performance especially at a large current density (10 C), of LNM modified with the reported coating materials (except carbon materials) is unsatisfactory. With respect to the carbon materials, the redox reaction between carbon and Mn$^{4+}$ cannot be neglected during high temperature treatment. Therefore, discovering for a new coating material LNM is necessary and urgent.

    According to the literature reports [12-14], electrochemically inactive Mn$^{4+}$ existing on the surface of cathode particles can provide excellent structural stability during electrochemical cycling. Additionally, our previous work [15] reported that the right amount of Mn$^{3+}$ on the surface of LNM particles is beneficial to improve the electrical conductivity of LNM. Hence, we deposit a Mn$_{3}$O$_{4}$ coating layer on the surface of LNM particles, which can combine the electrochemical inactivity of Mn$^{4+}$ and active Mn$^{3+}$ ions. Moreover, the Mn$_{3}$O$_{4}$ coating matches well with LNM due to their same spinel-type crystal structure, which is favorable for Li$^{+}$ diffusion at the interface. Undoubtedly, the results of electrochemical measurements demonstrate the significant rate performance and cycling stability (at 55 ℃) after Mn$_{3}$O$_{4}$ coating. Therefore, this work provides a different direction of thinking while selecting the materials for the surface modification of LNM material, which is not only limited to the common inert materials, but will be extended to some electrochemical active materials in the future research.

  • The LNM sample was obtained via a thermopolymerisation method reported in the previous work [15]. Lithium nitrate (LiNO$_{3}$, 5% excess), nickel nitrate (Ni(NO$_{3}$)$_{2}$$\cdot$6H$_{2}$O), and manganese acetate (Mn(CH$_{3}$COO)$_{2}$$\cdot$4H$_{2}$O) were together dissolved in distilled water. Then, acrylic acid (AA) was added to form a mixed solution with volume ratio of 1:2, and then placed in an oven at 150 ℃ for 10 h to obtain a fluffy powder product. The powder was first calcined at 500 ℃ for 5 h and then ground into a fine powder to obtain an intermediate product, which was sintered at 900 ℃ for 15 h and subsequently annealed at 700 ℃ for 48 h. All these heat treatment processes were carried out in air atmosphere.

    For the preparation of Mn$_{3}$O$_{4}$-coated LNM samples, different amounts of manganese acetate (Mn(CH$_{3}$COO)$_{2}$$\cdot$4H$_{2}$O) were dissolved in ethanol in mortar, and then mixed with the as-prepared LNM powder to obtain the intermediate powders. Finally, the calcining process was performed at 400 ℃ in air for 5 h to obtain the final $\textit{X}$ wt% Mn$_{3}$O$_{4}$-coated LNM ($\textit{X}$=0.9, 2.6, 4.4) samples.

    To understand the state of Mn$_{3}$O$_{4}$ coating, we also followed the same procedure to treat manganese acetate alone at 400 ℃ for 5 h. As evident in FIG. S1 in supplementary materials, in comparison with Mn$_{3}$O$_{4}$ (PDF\#24-0734), the product was indeed Mn$_{3}$O$_{4}$.

    A scanning electron microscope (SEM, JSM-6390LA), transmission electron microscope (TEM, JEM-2100F), and X-ray diffraction (XRD) measurement at a scanning rate of 10 $^{\textrm{o}}$/min were introduced to analyse the structure and morphologies of particles. The oxidation state change of the Mn ions was checked by X-ray photoelectron spectroscopy (XPS, ESCALAB 250). The Fourier transformed infrared (FTIR) tests were studied with a Nicolet 8700 infrared spectrometer.

    Electrochemical impedance spectroscopy (EIS) was performed at an electrochemical workstation (CHI 604A) over the frequency range from 0.01 Hz to 10$^{5}$ Hz. The prepared cells for the EIS measurements must be cycled at 0.2 C for 3 times and then charged to half of the capacity. Cyclic voltammetry (CV) was performed using the same electrochemical workstation at a scan rate of 0.1 mV/s.

    The positive electrodes were prepared as follows: the prepared homogenous slurries, fabricated with 8:1:1 mass ratio of active materials ($\textit{i.e.}$ LNM and Mn$_{3}$O$_{4}$@LNM), acetylene black, and PVDF in N-methyl-2-pyrrolidone, were casted onto an aluminum foil and dried at 70 ℃ overnight in a vacuum oven. The loading content of the electrode materials is around 3.3 mg/cm$^{2}$. The electrodes were tested by CR2032 cells assembled in an Ar-glove box. Next, 1 mol/L LiPF$_{6}$ was mixed in ethylene carbonate---dimethyl carbonate (1:1, w/w) to prepare the electrolyte. The cells were measured on a Neware BTS2300 battery cycler within a voltage range of 3.5-4.9 V at room temperature (25 ℃).

    To measure the electrical conductivity of LNM and the Mn$_{3}$O$_{4}$-coated LNM powders, firstly, the powders were respectively pressed into discs (diameter in 14 mm), and then coated with silver paste on both sides of the discs.

  • The XRD patterns for all the samples are presented in FIG. 1(a). Impurities such as Li$_{x}$Ni$_{1-y}$O with diffraction peaks located at 2$\theta$ =37.5$^{\textrm{o}}$, 43.6$^{\textrm{o}}$ and 63.4$^{\textrm{o}}$ can be hardly detected in the XRD patterns [15, 16]. Besides, the sharp and intensive diffraction peaks can be well indexed as cubic spinel-structured LNM. Nevertheless, no diffraction peaks of Mn$_{3}$O$_{4}$ or other manganese oxides are detected in the XRD patterns. In comparison with Mn$_{3}$O$_{4}$ (PDF\#24-0734), we find that the characteristic diffraction peaks of Mn$_{3}$O$_{4}$ overlap with that of LNM.

    Figure 1.  (a) XRD patterns and (b) FTIR spectra of the samples: 0 wt%, 0.9 wt%, 2.6 wt%, 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM.

    The space group of LNM is verified by the FTIR measurement shown in FIG. 1(b). Generally, eight IR adsorption peaks could be detected for LNM with a P4$_{3}$32 structure in the FTIR spectra while only five could be detected for their disordered counterparts (Fd3m) [17, 18]. In FIG. 1(b), the Mn$_{3}$O$_{4}$-coated LNM samples display eight intensive adsorption peaks that are the same as those observed for pristine LNM, which proves that the coating modification process maintains the P4$_{3}$32 space group.

    The SEM images of the pristine and coated samples are shown in FIG. 2. The pristine LNM samples are octahedron-shaped particles with an average size of 2 μm, and their surfaces are smooth and clean (FIG. 2(a)). Meanwhile, the Mn$_{3}$O$_{4}$-coated samples display much rougher surfaces (FIG. 2(b-d)).

    Figure 2.  SEM images of (a) 0 wt%, (b) 0.9 wt%, (c) 2.6 wt%, and (d) 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM.

    As a representative, the coating layer of 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM is observed by HRTEM, the results are shown in FIG. 3 (b) and (c). The surface of the pristine sample is smooth in FIG. 1(a), while the surface of the coated sample is covered by a coating layer of an average thickness of 20 nm. In the HRTEM image (FIG. 3(c)), only a few of lattice fringes can be detected for the Mn$_{3}$O$_{4}$ coating layer, which indicates its poor crystallization degree.

    Figure 3.  TEM images of (a) LNM, and (b) 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM, (c) HRTEM image of 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM.

    In addition, the valence states of Mn in these samples were analysed by XPS. FIG. 4 (a-c) show the XPS spectra of Mn 2p region, in which the peaks at 643.0 and 655.0 eV are assigned to Mn 2p$_{3/2}$ and Mn 2p$_{1/2}$. For the major peaks of Mn 2p$_{3/2}$, the peaks at 642.4 and 643.3 eV are attributed to Mn$^{3+}$ and Mn$^{4+}$ [1, 19, 9, 20], respectively. The content of Mn$^{3+}$ was estimated based on the peak area ratio of Mn$^{3+}$/(Mn$^{3+}$+Mn$^{4+})$ in Mn 2p$_{3/2}$ (FIG. 4(d)). Thus, the Mn$^{3+}$/(Mn$^{3+}$+Mn$^{4+})$ ratio of 0 wt%, 2.6 wt%, and 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM samples were calculated to be 33.45%, 45.54%, and 55.9%, respectively, which indicates that the amount of Mn$^{3+}$ ions on the surface of samples increases obviously after coating with Mn$_{3}$O$_{4}$. This further proves that the coating layer is Mn$_{3}$O$_{4}$.

    Figure 4.  XPS spectra of samples: Mn 2p for (a) 0 wt%, (b) 2.6 wt%, and (c) 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM. (d) Concentration of Mn$^{3+}$ ions calculated by the peak of 2p$_{3/2}$.

    FIG. 5(a) shows the cyclic voltammograms (CV) of 0 wt%, 0.9 wt%, 2.6 wt%, and 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM samples. The small peaks at 4.0 V are ascribed to Mn$^{3+}$/Mn$^{4+}$ redox couples, while the strong peak at approximately 4.7 V to Ni$^{2+}$/Ni$^{4+}$ redox couples; both can be detected for all samples [1, 3, 19]. From the enlarged CV curves in FIG. 5(b), it can be clearly observed that the content of Mn$^{3+}$ ion increases with the coating content of Mn$_{3}$O$_{4}$, which is consistent with the XPS results.

    Figure 5.  CV plots of (a) pristine and Mn$_{3}$O$_{4}$-coated LNM, (b) an enlarged CV curves in the range of 3.8-4.2 V.

  • FIG. 6(a) shows the initial charge-discharge curves of the samples. A long voltage plateau at 4.7 V and a short voltage plateau at 4.0 V are observed for Ni$^{2+}$/Ni$^{4+}$ and Mn$^{3+}$/Mn$^{4+}$ redox couple, respectively [1, 3] in FIG. 6(a). Furthermore, the plateau at 4.0 V can only be detected for the Mn$_{3}$O$_{4}$-coated LNM samples. For the 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM sample, the initial discharge capacity was 128 mAh/g, which is close to that of pristine LNM sample. A slight increase in the discharge capacity (about 2 mAh/g) was observed for 0.9 wt% and 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM samples, which indicates that the lithium ion diffusion between the surface of LNM particles and the electrolyte is not hindered by the Mn$_{3}$O$_{4}$ coating layer. As shown in FIG. 6(b), the cycling stability and coulomb efficiency of LNM are not enhanced by the addition Mn$_{3}$O$_{4}$ coating layer.

    Figure 6.  (a) The initial charge-discharge curves at 0.2 C, (b) cycling performance at 1 C, (c) rate performance, (d) EIS spectra of 0 wt%, 0.9 wt%, 2.6 wt% and 4.4 wt% Mn$_{3}$O$_{4}$-coated LNM at RT and (e) the EIS impedance fitting results of samples.

    Then, we tested the rate performance at different current densities, as shown in FIG. 6(c). The rate performance of LNM samples were significantly improved with a Mn$_{3}$O$_{4}$ coating layer. Further, it should be noted that 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM sample delivers a discharge capacity of 108 mAh/g at 10 C rate, which is considerably higher than that of pristine LNM (80 mAh/g).

    Electrochemical impedance spectroscopy (EIS) was also used to confirm the improved electronic conductivity of Mn$_{3}$O$_{4}$-coated LNM. The Nyquist plots of these samples are shown in FIG. 6(d). The equivalent circuits and fitting results of the EIS are shown in insets of FIG. 6(d), where $\textit{R}_{1}$ is the ohmic resistance and $\textit{R}_{2}$ is the charge-transfer resistance. Clearly, $\textit{R}_{2}$ decreases from 161 $\Omega$ (for LNM) to 36.2 $\Omega$ (for 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM), which suggests the enhanced electronic conductivity of Mn$_{3}$O$_{4}$-coated LNM. We performed EIS measurements on the sintered pellets of the above two samples (FIG. S2), and the electrical conductivity was calculated using the EIS fitting results. The electrical conductivity of 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM (3.15$\times$10$^{-5}$ S/cm) was considerably higher than that of pristine LNM (1.53$\times$10$^{-7}$ S/cm). Therefore, the enhanced rate performance of the 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM is attributed to the high electrical conductivity.

    The lithium ion diffusion coefficient is another key factor that determines the rate performance of the electrode materials. The cyclic voltammetry tests for the pristine and 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM at different scanning rates are shown in FIG. 7. The diffusion coefficient can be calculated by using the Randlnes-Sevick equation [21, 22]:

    Figure 7.  Cyclic voltammograms of (a) pristine and (b) 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM at different scanning rates.

    where $\textit{i}$$_{\textrm{p}}$ is the peak current (mA), $\textit{n}$ is the number of electrons per molecule participating in the reaction, $\textit{D}_{\textrm{Li}}$ is just the diffusion coefficient (cm$^{2}$/s), $\upsilon$ is the scanning rate (V/s), $\textit{A}$ is the electrode area (cm$^{-2}$), and $\textit{C}_{\textrm{Li}}$ is the concentration of the lithium ion in the electrode (mol/cm$^{3}$). For the LNM sample, the $\textit{C}_{\textrm{Li}}$ value was 0.0243 mol/cm$^{3}$ and the electrode area was 1.13 cm$^{-2}$. Finally, the charge and discharge lithium ion diffusion coefficient of the pristine is 4.57$\times$10$^{-10}$ cm$^{2}$/s and 3.33$\times$10$^{-10}$ cm$^{2}$/s, respectively. For the 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM samples, the values were 1.69$\times$10$^{-10}$ cm$^{2}$/s and 2.83$\times$10$^{-10}$ cm$^{2}$/s, respectively. Based on the above results, the lithium ion diffusion coefficient of these two samples are of the same order of magnitudes, and the difference in the value of the previous digits is small. Therefore, we conclude that the 2.6 wt% Mn$_{3}$O$_{4}$ coating layer has no effect on the lithium ion diffusion, and its excellent rate performance is attributed to the high electrical conductivity.

  • FIG. 8 shows the cycling performances and EIS spectra of the pristine and 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM samples at 55 ℃. For the pristine sample, after 100 cycles, the capacity decreased to 84.6 mAh/g with only 67% capacity retention. However, 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM exhibited improved cycling stability. Its capacity retention still remains 78%. Moreover, as shown in FIG. 8(b), the 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM has a lower increase in the impedance compared with the LNM/Li cell; this may be attributed to the Mn$_{3}$O$_{4}$ coating layer. In fact, at the elevated temperature, the Mn$_{3}$O$_{4}$ coating layer can reduce the side reactions to some extent, which can be demonstrated from the improved coulomb efficiency in FIG. 8(a). Hence, Mn$_{3}$O$_{4}$ coating is favorable for improving the cycling stability of LNM at elevated temperatures.

    Figure 8.  (a) Cycling performance of the pristine and 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM at 55 ℃ and 1 C rate. (b) EIS spectra of these two samples after 3rd and 100th cycles at 55 ℃.

    The electrochemical performance of our optimal sample LNM/2.6 wt% Mn$_{3}$O$_{4}$ was also compared with those of previous reports (Table S1 in supplementary materials), it had one of the best of its rate capability and cycling stability.

  • In this study, we demonstrate that the electrochemical performance of LNM can be significantly improved by the Mn$_{3}$O$_{4}$ coating layer. 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM exhibits excellent rate performance with the discharge capacity of 108 mAh/g, which is considerably higher than that of the uncoated LNM (78 mAh/g). The calculation of lithium ion diffusion coefficient in the materials illustrate that lithium diffusion is not hindered by the Mn$_{3}$O$_{4}$ coating layer. Meanwhile, the electrical conductivity of LNM (1.53$\times$10$^{-7}$ S/cm) is improved with 2.6 wt% Mn$_{3}$O$_{4}$ coating layer (3.15$\times$10$^{-5}$ S/cm), which may be because of the substantial content of Mn$^{3+}$ ions on the surface. Moreover, 2.6 wt% Mn$_{3}$O$_{4}$-coated LNM shows good cycling stability with a capacity retention of 78% ($\textit{vs.}$ 67% for LNM) after 100 cycles at 55 ℃.

    Supplementary materials: FIG. S1 shows the XRD patterns of Mn$_3$O$_4$ formed by the manganese acetate alone at 400 $^\circ$C for 5 h and in comparison with Mn$_3$O$_4$-PDF#24-0734. The EIS measurements are also carried on the sintered pellets of the non-coated and 2.6% Mn$_3$O$_4$-coated LNM samples, as shown in FIG. S2. Table S1 is aimed to compare the surface modified LNM samples reported in literatures with this work.

  • This work was supported by the National Key R & D Program of China (No.2018YFB0905400), the Fundamental Research Funds for the Central Universities (No.JZ2019HGBZ0140), the National Natural Science Foundation of China (No.U1630106, No.51577175) and China Postdoctoral Science Foundation (No.172731). We are also grateful to Elementec Ltd. in Suzhou for the technical support.

    Figure S1.  XRD patterns of Mn3O4 formed by the manganese acetate alone at 400 ℃ for 5 h and Mn3O4-PDF#24-0734.

    Figure S2.  EIS spectra for the sintered pellets: non-coated and 2.6% Mn3O4-coated LNM samples.

    Table S1.  Comparison of the electrochemical performance for surface modified LNM samples reported in literatures with this work.

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