F− co-doping on electrochemical performance of LiNi0.5Mn1.5O4 for 5 V lithium-ion batteries

Electrochimica Acta 323 (2019) 134692

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Effect of Mg2þ/F co-doping on electrochemical performance of LiNi0.5Mn1.5O4 for 5 V lithium-ion batteries Aijia Wei a, b, c, Wen Li b, c, *, Qian Chang a, b, Xue Bai b, c, Rui He b, c, Lihui Zhang b, c, Zhenfa Liu a, b, c, **, Yanji Wang a, *** a b c

School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, China Institute of Energy Resources, Hebei Academy of Sciences, Shijiazhuang, Hebei Province, 050081, PR China Hebei Engineering Research Center for Water Saving in Industry, Shijiazhuang, Hebei Province, 050081, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 March 2019 Received in revised form 12 August 2019 Accepted 12 August 2019 Available online 13 August 2019

Mg2þ/F co-doped LiNi0.5Mn1.5O4 cathode material was synthesized by a facile one-step solid-state process. The effect of Mg2þ/F co-doping on grain morphology, phase structure, and electrochemical properties was studied by a series of characterizations. Scanning-electron-microscopy images show that Mg2þ/F co-doped LiNi0.5Mn1.5O4 (denoted LNMO-MF) particles grow larger than pure LiNi0.5Mn1.5O4 particles. X-ray diffraction, Raman spectra, Fourier transformation infrared spectroscopy, X-ray photoelectron spectroscopy, and cyclic-voltammetry tests indicate that all samples mainly display a Fd-3m space group and more Mn3þ ions in the LNMO-MF sample after Mg2þ/F co-doping, which is conducive to increasing the cationic disorder degree and enhancing the electronic conductivity of electrode material. Results show that the LNMO-MF cathode material delivers an excellent rate performance with discharge capacities of 142, 144, 140 136, 132, 124, 115, and 100 mAh g1 at 0.2, 0.5, 1, 2, 3, 5, 7, and 10C (1C ¼ 140 mAh g1), respectively. Remarkably, LNMO-MF also shows cycling stability with a capacity retention of 86.2% at 5C after 400 cycles, which is much higher than that of pure LiNi0.5Mn1.5O4 (67.7%). The improvement of LNMO-MF's electrochemical properties could be ascribed to the Mg2þ/F co-doping, delivering a more stable structure, better crystallinity, the highest Liþ diffusion coefficient, and the lowest charge-transfer resistance. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Lithium-ion battery LiNi0.5Mn1.5O4 Mg2þ/F co-doping Solid-state process

1. Introduction In recent years, with the rapid application of lithium-ion batteries in portable electronic products, electric vehicles, and energystorage systems [1e3], the development of low-cost, high-energydensity, and long-cycle-life lithium-ion batteries is a hot research topic. The choice of cathode materials is one of the key factors that limit the energy density of lithium-ion batteries. In this regard, spinel LiNi0.5Mn1.5O4 (LNMO) is considered a promising cathode material because of its high energy density (~658 Wh kg1), high

* Corresponding author. Institute of Energy Resources, Hebei Academy of Sciences, Shijiazhuang, Hebei Province, 050081, PR China. ** Corresponding author. Institute of Energy Resources, Hebei Academy of Sciences, Shijiazhuang, Hebei Province, 050081, PR China. *** Corresponding author. School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130, China. E-mail addresses: [email protected] (W. Li), [email protected] (Z. Liu), [email protected] (Y. Wang). https://doi.org/10.1016/j.electacta.2019.134692 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

operating voltage (at ~4.7 V versus Liþ/Li), and the use of low-cost and nontoxic manganese source [4,5]. However, there are also some problems in practical application of LNMO-based full cells. One of the main issues is that LNMO will react with organic electrolyte at a 5 V charging voltage, resulting in severe side reactions with electrolytes, leading to capacity fading. In addition, the impurities (such as LixNi1xO or NiO) produced during hightemperature calcination also deteriorate the electrochemical performance of LNMO. In an effort to improve its cycling performance and rate capacity, researchers have mainly been devoted to the modification of LNMO, including surface coatings (comprising many different kinds of materials, such as ZnO [6], TiO2 [7], Al2O3 [8], Fe2O3 [9], RuO2 [10], AlF3 [11], LaF3 [12], Li2SiO3 [13], YPO4 [14], and LeFeO3 [15]) and ion doping for Ni2þ, Mn4þ or O2 (comprising a series of elements, such as Al3þ [16], Mg2þ [17e19], Cu2þ [20], Zr4þ [21], Ti4þ [22], Fe3þ [23], Y3þ [24], Sm3þ [25], Ru4þ [26], Co3þ [27], Cr3þ [28], Zn2þ [29], P3þ [30], F [31,32], and Cl [33]), to suppress electrolyte decomposition and stabilize the morphology

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and structure. In particular, cation or anion doping is an effective way to improve the electrochemical performance of LNMO. Among these doping elements, Mg2þ and F have received considerable attention. Several studies have been conducted on Mg2þ-doped LNMO. The Mg2þ substitution for Ni2þ or Mn4þ can reduce electrode polarization and improve the electronic conductivity of LNMO [17e19]. For example, Liu et al. fabricated spherical Mg2þ gradient-doped LNMO by a co-precipitation method, which delivers a good rate capability with a discharge capacity of 122 mAh g1 at 0.1C and 81 mAh g1 at 4C [19]. In addition, because of the MneF bonds being stronger than the MneO bonds, a small amount of F doping could not only improve the structural stability, but also inhibit the formation of LixNi1xO or NiO. Moreover, F doping can suppress HF attacking the electrode materials and can enhance the electrochemical performance of LNMO [31,32]. For example, Luo et al. synthesized a fluorine gradient-doped LNMO via a ballmilling method using commercial LNMO and NH4F (5:1 M ratio) as raw materials followed by sintering at 300e900  C [31]. The results show that the F-doped LNMO calcined at 400  C delivers the best discharge capacity with 125 mAh g1 at 1C and 104 mAh g1 at 10C, and a capacity retention of 85.8% after 150 cycles at 5C. However, a study of the effect on electrochemical performance by Mg2þ/F co-doping in the LNMO is not found in the literature. There may be a synergistic effect by cation and anion co-doping to improve the rate capacity and cycle stability of LNMO [34,35]. Sun et al. synthesized Mg2þ/F co-doped Li[Ni1/3Co1/3Mn(1/3x)Mgx] O2yFy via a co-precipitation method followed by high-temperature calcination [36]. Mg2þ/F co-doping could reduce the cation mixing and contribute to the improvement of crystallinity and tap density, which enhances the capacity retention and thermal stability, even though the electrodes were cycled between 2.8 and 4.6 V. To investigate Mg2þ/F co-doped LNMO, as reported in this paper, undoped LiNi0.5Mn1.5O4, Mg2þ-doped LiNi0.5Mn1.49Mg0.01O4, Fdoped LiNi0.5Mn1.5O3.97F0.03, and Mg2þ/F co-doped LiNi0.50Mn1.49Mg0.01O3.97F0.03 samples were synthesized via a one-step solid-state ball-milling method. The phase structure, grain morphology, and electrochemical performance of the products were systematically investigated.

with a step mode of 0.02 per 10 s. The Rietveld refinements of XRD data were done with the analysis software (TOPAS 4.2). The particle morphology and size distribution were studied by scanning electron microscopy (SEM, SU 8020, Hitachi, Japan) and transmission electron microscopy (TEM, JEM-2100 plus, JEOL, Japan). The chemical composition analysis was carried out using the EDXmapping equipped in the SEM. X-ray photoelectron spectroscopy (XPS, PHI5600, Physical Electronics, USA) was used to identify the surface chemical state of all of the samples. Fourier-transform infrared spectroscopy (FTIR, VERTEX70, Bruker, Germany) and Raman microscopy (RM2000, Renishaw, England) were utilized to determine the space group of all samples. 2.2. Electrochemical measurements The electrochemical properties of the working electrodes were evaluated on coin cells (CR-2032) with Li foil as the counter electrode. The cathode slurry was prepared by homogeneous mixing of the active material (80 wt%), Super-P (10 wt%), and polyvinylidene fluoride (10 wt%) in N-methyl-2-pyrrolidone solvent. The blended slurry was then cast onto an aluminum foil and dried at 105  C to evaporate the solvent. The electrode laminate was cut into a circular electrodes with a diameter of 10 mm and dried again in a vacuum oven at 120  C for 10 h. The loading density of the active materials was approximately 1.44 mg cm2. The batteries were assembled in an Ar-filled glovebox (both O2 and H2O concentration lower than 1 ppm) with Celgard 2400 (Celgard Inc., USA) as a separator and 1 M LiPF6 in ethylene carbonate, dimethyl carbonate and diethyl carbonate (1:1:1 vol ratio) as an electrolyte (Capchem Technology (Shenzhen) Co., Ltd., China). Charge/discharge tests were performed by using a battery tester (Land 2001A, Wuhan, China) between 3.5 and 5.0 V ranging from 0.2 to 10C at 25  C (1C ¼ 140 mAh g1). Cyclic voltammetry (CV) and electrochemical impedance spectra (EIS) tests were conducted using an electrochemical workstation (Interface 1000, Gamry, USA). The CV tests were carried out with scan rates varying from 0.1 to 0.5 mV s1. The EIS tests were conducted from 105 to 0.01 Hz. 3. Results and discussion

2. Experiments 3.1. Materials characterization 2.1. Synthesis and characterization Fig. 1 shows the XRD patterns of all of the samples, which are Li2CO3 (99.0%, Aladdin), MnO2 (90.0%, Aladdin), NiO (99.9%, Aladdin), MgO (99.9%, Aladdin), and NH4F (99%, Aladdin) were used as raw materials. The Mg2þ/F co-doped LiNi0.50Mn1.49Mg0.01O3.97F0.03 was synthesized by a one-step solid-state reaction. First, stoichiometric amounts of Li2CO3 (3.92 g), MnO2 (14.44 g), NiO (3.77 g), MgO (0.0403 g), and NH4F (0.113 g) were dispersed in ethanol. The mixture was ground in a planetary ball mill at 300 rpm for 16 h and then dried at 105  C for 4 h to evaporate the ethanol solvent. Finally, the precursor powders were preheated at 500  C for 5 h and then calcined at 800  C for 8 h under air atmosphere with a heating rate of 5  C min1. After naturally cooling to room temperature, the Mg2þ/F co-doped LiNi0.50Mn1.49Mg0.01O3.97F0.03 (denoted LNMO-MF) was obtained. For comparison, Mg2þ-doped LiNi0.5Mn1.49Mg0.01O4 and F-doped LiNi0.5Mn1.5O3.97F0.03 were prepared under the same conditions with the addition of MgO and NH4F alone, and the obtained materials were denoted LNMO-M0.01 and LNMO-F0.03. Pure LNMO was synthesized without Mg2þ and F addition. An extra amount of Li (5% molar content) was used to compensate the Li loss during high-temperature calcination. The crystal structure of all of the samples was examined by Xray diffraction (XRD, Ultima IV, Rigaku, Japan) with Cu Ka radiation. The diffraction patterns were collected in the range of 2q ¼ 10e90

Fig. 1. XRD patterns of all samples compared with standard LNMO XRD patterns.

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indexed to the standard spinel structure of LNMO (JPCS Card No. 80e2162). All of the diffraction peaks are narrow and sharp, demonstrating that all of the samples show good crystallinity and that the doping of Mg2þ and F ions did not destroy the formation of spinel LNMO. However, a tiny but observable peak of NiO impurity was detected at 2q ¼ 37.5 and 43.6 for all of the samples, which may result from the raw materials of NiO during hightemperature calcination. The NiO contents are 1.9, 1.5, 1.3, and 1.6 wt% for pure LNMO, LNMO-M0.01, LNMO-F0.03, and LNMO-MF, respectively, which are calculated from the Rietveld refinement as shown in Fig. 2aed. The results show that Mg2þ and F doping could reduce the amount of NiO impurity compared to pure LNMO. Table 1 lists the crystal lattice parameters of all of the samples. The crystal lattice parameters for pure LNMO, LNMO-M0.01, LNMOF0.03, and LNMO-MF are 8.17681, 8.17933, 8.17915, and 8.17946, respectively, suggesting that the lattice parameters become larger after doping with Mg2þ and F ions. For the Mg2þ-doped LNMOM0.01, the crystal lattice parameters increase because the ionic radius of Mn4þ (0.053 nm) is smaller than that of Mg2þ (0.072 nm) [36]. For the F-doped LNMO-F0.03, although the ionic radius of F (0.0133 nm) is slightly smaller than that of O2 (0.0140 nm), the lattice parameter of LNMO-F0.03 increased to 8.17915 compared to pure LNMO. These results may be due to the partial reduction of

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Table 1 Lattice parameters of all samples. Samples

Pure LNMO LNMO-M0.01 LNMO-F0.03 LNMO-MF

Lattice parameters

Interplanar spacing

a/Å

V/Å3

d/Å

8.17681 8.17933 8.17915 8.17946

546.70333 547.20895 547.17282 547.23504

4.72088 4.72234 4.72223 4.72241

Mn4þ (0.053 nm) to Mn3þ (0.064 nm) and to the fact that the increase of Mn3þ could compensate for the small ionic radius of F [31]. Moreover, the MneF bonds are stronger than the MneO bonds, which is beneficial to enhancing the structural stability of LNMO-F0.03 during the repeated de-lithiation/lithiation process [37]. It is expected that the synergistic effect of Mg2þ/F co-doping results in the maximum lattice parameter of LNMO-MF. According to reports, XRD tests cannot clearly distinguish the as-prepared materials belonging to Fd-3m or P4332 space groups [38]. Therefore, FTIR and Raman spectroscopy analyses were conducted and the results are shown in Fig. 3. Fig. 3a shows that there are eight IR absorption peaks located at 655, 622, 591, 556, 505, 478, 468, and 437 cm1. The peaks at 655, 468, and 437 cm1 belong to

Fig. 2. Rietveld refinements of all samples.

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Fig. 3. (a) FTIR and (b) Raman spectra of all samples.

ordered P4332 structure. In addition, the peak intensity of the MneO band at 622 cm1 was higher than that of the NieO band at 591 cm1 for all of the samples, which proves the existence of disordered Fd-3m structure. The intensity ratios of the peaks at 622 and 591 cm1 (I(622)/I(591)) for pure LNMO, LNMO-M0.01, LNMOF0.03, and LNMO-MF are 1.18, 1.18, 1.21, and 1.21, respectively, indicating that the degree of cationic disordering for LNMO-F0.03 and LNMO-MF is greater than that of pure LNMO and LNMOM0.01. These results illustrate that there are more Mn3þ ions in the LNMO-F0.03 and LNMO-MF samples, which is conducive to improving the electronic conductivity of the electrode materials [29]. Based on the above-mentioned analysis, Fd-3m and P4332 space groups coexist in the four samples. Raman spectra of the prepared materials are shown in Fig. 3b, from which it can be seen that the peak located at 580e630 cm1 exhibits no obvious splitting, which is characteristic of the typical disordered Fd-3m structure. Therefore, combining FTIR and Raman spectra, all of the samples are composed of dominant disordered Fd-3m and less P4332 structure. The SEM images of all of the samples are presented in Fig. 4. All of the prepared samples show smooth-surfaced and wellcrystallized octahedral morphology. Pure LNMO displays a large

proportion of small particles in Fig. 4a, while a gradually decreasing ratio of small particles is found with Mg2þ and F doping. Compared with pure LNMO, the particle sizes of LNMO-M0.01, LNMO-F0.03, and LNMO-MF samples become significantly larger. Fig. 5 shows the particle size distribution of each sample, counting for over 250 particles. The mean particle sizes for pure LNMO, LNMO-M0.01, LNMO-F0.03, and LNMO-MF are 0.471 (±0.199), 0.580 (±0.253), 0.639 (±0.282), and 0.695 (±0.268) mm, respectively. This implies that the presence of small amounts of Mg2þ and F doping may be helpful for crystal growth. The large particles of LNMO-MF would decrease the contact area between the electrode materials and electrolyte and improve structural stability [32,36]. A clear distribution of O, Ni, Mn, Mg, and F elements can be seen in the EDX-mapping of LNMO-MF (in Fig. 6), revealing that the Mg2þ and F ions have been doped into the LNMO-MF material. To further investigate the crystallization of LNMO-MF, the TEM images, high-resolution TEM (HRTEM) images, and selected-area electron-diffraction (SAED) patterns of the LNMO-MF sample were obtained. As can be seen in Fig. 7a, LNMO-MF displays octahedral morphology, which is consistent with the results of SEM. Fig. 7b shows the HRTEM image of the lattice fringe; the measured lattice spaces are 0.472 nm, which is in agreement with the lattice space (111) planes of spinel LNMO. Meanwhile, Fig. 7c also clearly shows d111 diffraction spots in the SAED pattern, indicating that the particles of octahedral LNMO-MF are single crystals. These aforementioned results show that LNMO-MF is well crystallized, which is beneficial for better electrochemical properties. XPS analysis was conducted to elucidate the surface chemical state of the prepared materials. Fig. 8aed shows two main peaks centered at ~654 and 642 eV that are assignable to Mn 2p1/2 and Mn 2p3/2. The binding energies of Mn 2p2/3 are located at 642.3 and 643.4 eV, respectively, indicating that both Mn3þ and Mn4þ ions exist in all of the samples. According to Gaussian-Lorentz curve fitting, the rough Mn3þ/Mn4þ ratio can be estimated by peak area. Pure LNMO, LNMO-M0.01, LNMO-F0.03, and LNMO-MF exhibit Mn3þ/Mn4þ ratio of 37.4%:62.6%, 37.7%:62.3%, 45.8%:54.2%, and 45.9%:54.1%, respectively. The increase of Mn3þ content in LNMO-F and LNMO-MF samples is in accordance with the FTIR and Raman results. It can be seen from Fig. 8e and f that the peak observed at 1303.9 eV in the Mg 1s spectrum and the peak observed at 684.9 eV in the F 1s spectrum prove that Mg2þ and F ions exist in the LNMO-MF sample. 3.2. Electrochemical properties To study the kinetic behavior of Liþ migration in the LNMO, CV tests of all of the samples were conducted using the LNMO/Li halfcell in the voltage range 3.5e5.0 V. Fig. 9a shows that all of the samples at 0.1 mV s1 possess two major peaks at approximately 4.6e4.8 V and that the redox peaks split into two peaks, corresponding to the redox couples of Ni2þ/Ni3þ and Ni3þ/Ni4þ [34]. The small peak at ~4.0 V is associated with Mn3þ/Mn4þ redox reaction, and the larger the peak area, the more Mn3þ ions there are. Fig. 9b shows a magnified image of the ~4.0 V peaks of all of the samples at 0.1 mV s1. The rough contents of Mn3þ ions in LNMO-MF and LNMO-F0.03 are higher than those of pure LNMO and LNMO-M0.01, in accordance with the above discussions. With increasing scanning rate, the potential of anodic peaks (de-lithiation) for all samples becomes larger, while the potential of cathodic peaks (lithiation) becomes smaller, as shown in Fig. 9cef. As is well known, the potential differences represent the polarization degree of electrode materials. Table 2 compares the potential differences of all samples. LNMO-MF displays the smallest potential differences, especially at a high scanning rate of 0.5 mV s1, indicating that LNMO-MF has the lowest electrode polarization among all samples. The linear

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Fig. 4. SEM images of pure LNMO (a), LNMO-M0.01 (b), LNMO-F0.03 (c), and LNMO-MF (d) samples.

Fig. 5. Particle size distributions of pure LNMO (a), LNMO-M0.01 (b), LNMO-F0.03 (c), and LNMO-MF (d) samples.

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Fig. 6. EDX-mapping images of Ni, Mn, O, Mg and F elements in the LNMO-MF sample.

Fig. 7. (a) TEM, (b) HRTEM image, and (c) SAED pattern of LNMO-MF sample.

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Fig. 8. XPS spectrum of Mn 2p regions for pure LNMO (a), LNMO-M0.01 (b), LNMO-F0.03 (c), and LNMO-MF (d) samples; XPS spectrum of Mg 1s (e) and F 1s (f) for LNMO-MF sample.

relationship of peak current (ip) versus the square root of scan rates (v1/2) is usually used to calculate the diffusion coefficient of Liþ according to the Randle-Sevcik equation [20]: ip ¼ 2.69  105 A 1/2 n3/2 D1/2 , where A is the electrode area (1.13 cm2), n the Li C v number of electrons per reaction species, DLi (cm2 s1) the diffusion coefficient of Liþ, and C the bulk concentration of Liþ (0.02378 mol cm3). Fig. 9g and h shows the slope of ip versus v1/2 during the extraction/insertion process, and the DLi values of all samples are summarized in Table 3. The DLi values of LNMO-M0.01, LNMO-F0.03, and LNMO-MF are larger than that of pure LNMO, and LNMO-MF exhibits the largest DLi (6.16  1011/ 11 2 1 2þ  7.43  10 cm s ), suggesting that the Mg /F co-doping could improve the Liþ accessibility and reversibility because of the better

electronic conductivity and structural stability of LNMO-MF. The rate capability characteristics of all of the samples were fully investigated at different C-rates ranging from 0.2 to 10C. (Figs. S1 and S2 show that the rate capability of Mg2þ-doped LNMO (LNMO-M0.005, LNMO-M0.01, and LNMO-M0.02) and F-doped LNMO (LNMO-F0.02, LNMO-F0.03, and LNMO-F0.04) samples, and the discharge capacities are listed in Tables S1 and S2. The optimum doping contents of Mg2þ (LNMO-M0.01) and F (LNMO-F0.03) were used to synthesize the Mg2þ/F co-doped LNMO-MF.) Fig. 10a and b shows that the discharge capacities of LNMO-M0.01, LNMOF0.03, and LNMO-MF are higher than that of pure LNMO at each Crate and are listed in Table 4. In particular, at a high C-rate of 10C, LNMO-M0.01, LNMO-F0.03, and LNMO-MF still deliver discharge

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Fig. 9. Cyclic voltammetry curves for all samples (a) at 0.1 mV s1; (b) magnified image of 4.0 V peak at 0.1 mV s1; (cef) at scan rates from 0.1 mV s1 to 0.5 mV s1; (g, h) the plotting of peak current (ip) vs square root of scan rate (v1/2).

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Table 2 Potential differences (DV, V) between anodic(4pa, V) peaks and cathodic (4pc, V) peaks.

y (mVs1)

0.1 0.2 0.3 0.4 0.5

Pure LNMO

LNMO-M0.01

LNMO-F0.03

LNMO-MF

4pa

4pc

DV

4pa

4pc

DV

4pa

4pc

DV

4pa

4pc

DV

4.837 4.879 4.919 4.946 4.971

4.594 4.554 4.513 4.482 4.464

0.243 0.325 0.406 0.464 0.507

4.829 4.864 4.890 4.912 4.924

4.626 4.591 4.566 4.542 4.518

0.203 0.273 0.324 0.370 0.406

4.818 4.846 4.871 4.890 4.907

4.637 4.603 4.581 4.561 4.543

0.181 0.243 0.290 0.329 0.364

4.811 4.842 4.862 4.882 4.896

4.635 4.605 4.581 4.558 4.540

0.176 0.237 0.281 0.324 0.356

Table 3 The diffusion coefficients of Liþ in all samples. Samples

Li-extraction DLi (cm2 s1)

Li-insertion DLi (cm2 s1)

Pure LNMO LNMO-M0.01 LNMO-F0.03 LNMO-MF

2.18  1011 3.35  1011 4.59  1011 6.16  1011

2.30  1011 3.79  1011 5.68  1011 7.43  1011

capacities of 66, 89, and 100 mAh g1, respectively, whereas pure LNMO maintains almost no discharge capacity (only 8 mAh g1), suggesting that LNMO-MF displays the best rate capability. These results may be ascribed to the fact that LNMO-MF possesses the largest Liþ diffusion coefficient. Fig. 10cef shows the galvanostatic charge-discharge curves of all samples. The four materials exhibit three plateaus in the charging process, displaying two long plateaus

Fig. 10. (a) Rate capability of all samples from 0.2C to 10C; (b) discharge capacity as a function of C-rates for all samples and (cef) galvanostatic charge/discharge curves at different C-rates for all samples at 3.5e5.0 V (1C ¼ 140 mAh g1).

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Table 4 Discharge capacity at different C-rates for all samples between 3.5 and 5 V. Samples

Pure LNMO LNMO-M0.01 LNMO-F0.03 LNMO-MF

Discharge capacity (mAh g1) 0.2C

0.5C

1C

2C

3C

5C

7C

10C

127 133 133 142

126 131 133 144

125 127 131 140

118 121 125 136

110 115 119 132

88 105 110 124

54 92 102 115

8 66 89 100

Fig. 11. Cycling performance of all samples at 1C for 200 cycles (a) and 5C for 400 cycles (b) between 3.5 and 5.0 V.

at 4.6e4.8 V and a short slope at ~4.0 V. Moreover, the order of the

potential differences (DV) between the charge and discharge plateaus is LNMO-MF < LNMO-F0.03
Table 5 Discharge capacity retention of all samples at 1C after 200 cycles and 5C after 400 cycles at 25  C. Samples

Discharge capacity at 1C (mAh g1) 1st cycle

200th cycle

Pure LNMO LNMO-M0.01 LNMO-F0.03 LNMO-MF

124 125 133 143

108 110 118 130

Retention at 1C (%)

Discharge capacity at 5C (mAh g1) 1st cycle

400th cycle

87.1 88.0 88.7 90.9

90 108 110 123

61 89 93 106

Retention at 5C (%)

67.7 82.4 84.5 86.2

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Fig. 12. SEM images of pure LNMO electrode before cycling (a) and after 400 cycles (b); SEM images of LNMO-MF electrode before cycling (c) and after 400 cycles (d).

Moreover, there are more Mn3þ ions in LNMO-MF, which is conducive to enhancing the electronic conductivity. As-prepared LNMO-MF exhibits superior electrochemical performance among all of the samples, and displays a remarkable discharge capacity of 100 mAh g1 at 10C and a capacity retention of 86.2% after 400 cycles at 5C. The improved electrochemical performance may be because LNMO-MF delivers a more stable structure, better crystallinity, the highest Liþ diffusion coefficient, and the lowest charge-transfer resistance after Mg2þ/F co-doping. Therefore, the Mg2þ/F co-doped LNMO-MF cathode material is a potential industrial application for 5 V lithium-ion batteries. Acknowledgments

Fig. 13. EIS curves (Nyquist plots) for all samples after 50 cycles at 2C.

This work was financially supported by Hebei Province Applied Basic Research ProgrameKey Basic Research Project (17964407D), Science and Technology Program of Hebei Province (18214404D), Project of Hebei Academy of Science (191410) and Project of Hebei Academy of Science (191409). Appendix A. Supplementary data

Table 6 The values of Rs and Rct of all samples after 50 cycles at 2C. Samples

Rs(U)

Rct(U)

Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2019.134692.

Pure LNMO LNMO-M0.01 LNMO-F0.03 LNMO-MF

1.6 1.5 1.5 1.3

156.9 122.6 119.2 96.1

References

4. Conclusions We prepared Mg2þ/F co-doped LNMO-MF cathode material through a facile one-step solid-state method for the first time. The characterization results show that the lattice parameter increases and the particle size grows larger after Mg2þ/F co-doping.

[1] S.T. Myung, K. Amine, Y.K. Sun, Nanostructured cathode materials for rechargeable lithium batteries, J. Power Sources 283 (2015) 219e236. [2] B. Kang, G. Ceder, Battery materials for ultrafast charging and discharging, Nature 458 (2009) 190e193. [3] L.H. Saw, Y.H. Ye, A.A.O. Tay, Integration issues of lithium-ion battery into electric vehicles battery pack, J. Clean. Prod. 113 (2016) 1032e1045. [4] R. Santhanam, B. Rambabu, Research progress in high voltage spinel LiNi0.5Mn1.5O4 material, J. Power Sources 195 (2010) 5442e5451. [5] T.F. Yi, J. Mei, Y.R. Zhu, Key strategies for enhancing the cycling stability and rate capacity of LiNi0.5Mn1.5O4 as high-voltage cathode materials for high power lithium-ion batteries, J. Power Sources 316 (2016) 85e105. n, J. Morales, Re-examining the effect of [6] J.C. Arrebola, A. Caballero, L. Herna

12

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

A. Wei et al. / Electrochimica Acta 323 (2019) 134692 ZnO on nanosized 5 V LiNi0.5Mn1.5O4 spinel: an effective procedure for enhancing its rate capability at room and high temperatures, J. Power Sources 195 (2010) 4278e4284. S. Tao, F.J. Kong, C.Q. Wu, X.Z. Su, T. Xiang, S.M. Chen, H.H. Hou, L. Zhang, Y. Fang, Z.C. Wang, W.S. Chu, B. Qian, L. Song, Nanoscale TiO2 membrane coating spinel LiNi0.5Mn1.5O4 cathode material for advanced lithium-ion batteries, J. Alloy. Comp. 705 (2017) 413e419. J.W. Kim, D.H. Kim, D.Y. Oh, H. Lee, J.H. Kim, J.H. Lee, Y.S. Jung, Surface chemistry of LiNi0.5Mn1.5O4 particles coated by Al2O3 using atomic layer deposition for lithium-ion batteries, J. Power Sources 274 (2015) 1254e1262. G. Wang, W.C. Wen, S.H. Chen, R.Z. Yu, X.Y. Wang, X.K. Yang, Improving the electrochemical performances of spherical LiNi0.5Mn1.5O4 by Fe2O3 surface coating for lithium-ion batteries, Electrochim. Acta 212 (2016) 791e799. S.H. Jung, D.H. Kim, P. Bruner, H. Lee, H.J. Hah, S.K. Kim, Y.S. Jung, Extremely conductive RuO2-coated LiNi0.5Mn1.5O4 for lithium-ion batteries, Electrochim. Acta 232 (2017) 236e243. Q. Wu, Y.F. Yin, S.W. Sun, X.P. Zhang, N. Wan, Y. Bai, Novel AlF3 surface modified spinel LiNi0.5Mn1.5O4 for lithium-ion batteries: performance characterization and mechanism exploration, Electrochim. Acta 158 (2015) 73e80. Y.P. Li, Q. Zhang, T.H. Xu, D.D. Wang, D. Pan, H.L. Zhao, Y. Bai, LaF3 nanolayer surface modified spinel LiNi0.5Mn1.5O4 cathode material for advanced lithiumion batteries, Ceram.Int. 44 (2018) 4058e4066. Y.L. Deng, L.H. He, J. Ren, Q.J. Zheng, C.G. Xu, D.M. Lin, Reinforcing cycling stability and rate capability of LiNi0.5Mn1.5O4 cathode by dual-modification of coating and doping of a fast-ion conductor, Mater. Res. Bull. 100 (2018) 333e344. T.H. Xu, Y.P. Li, D.D. Wang, M.Y. Wu, D. Pan, H.L. Zhao, Y. Bai, Enhanced electrochemical performance of LiNi0.5Mn1.5O4 cathode material by YPO4 surface modification, ACS Sustain. Chem. Eng. 6 (2018) 5818e5825. J.R. Mou, Y.L. Deng, L.H. He, Q.J. Zheng, N. Jiang, D.M. Lin, Critical roles of semiconductive LaFeO3 coating in enhancing cycling stability and rate capability of 5 V LiNi0.5Mn1.5O4 cathode materials, Electrochim. Acta 260 (2018) 101e111. Y. Luo, T.L. Lu, Y.X. Zhang, L.Q. Yan, S.S. Mao, J.Y. Xie, Surface-segregated, highvoltage spinel lithium-ion battery cathode material LiNi0.5Mn1.5O4 cathodes by aluminium doping with improved high-rate cyclability, J. Alloy. Comp. 703 (2017) 289e297. G.Y. Liu, H.Y. Sun, X. Kong, Y.N. Li, B.S. Wang, Facile synthesis of high performance LiNi0.5Mn1.4Mg0.1O4 and LiNi0.5Mn1.4Al0.1O4 by a low temperature solution combustion synthesis method, Int. J. Electrochem. Sci. 10 (2015) 6651e6662. J.J. Shiu, W.K. Pang, S.H. Wu, Preparation and characterization of spinel LiNi0.5xMgxMn1.5O4 cathode materials via spray pyrolysis method, J. Power Sources 244 (2013) 35e42. M.H. Liu, H.T. Huang, C.M. Liu, J.M. Chen, S.C. Liao, Mg gradient-doped LiNi0.5Mn1.5O4 as the cathode material for Li-ion batteries, Electrochim. Acta 120 (2014) 133e139. H.Y. Sun, X. Kong, B.S. Wang, T.B. Luo, G.Y. Liu, Cu doped LiNi0.5Mn1.5-xCuxO4 (x¼0, 0.03, 0.05, 0.10, 0.15) with significant improved electrochemical performance prepared by a modified low temperature solution combustion synthesis method, Ceram. Int. 44 (2018) 4603e4610. S.P. Feng, X. Kong, H.Y. Sun, B.S. Wang, T.B. Luo, G.Y. Liu, Effect of Zr doping on LiNi0.5Mn1.5O4 with ordered or disordered structures, J. Alloy. Comp. 749 (2018) 1009e1018. A. Howeling, S. Glatthaar, D. Notzel, J.R. Bind, Evidence of loss of active lithium in titanium-doped LiNi0.5Mn1.5O4/graphite cells, J. Power Sources 274 (2015) 1267e1275. N.K. Yavuz, M. Yavuz, S. Indris, N.N. Bramnik, M. Knapp, O. Dolotko, B. Das,

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

H. Ehrenberg, A. Bhaskar, Enhancement of electrochemical performance by simultaneous substitution of Ni and Mn with Fe in Ni-Mn spinel cathodes for Li-ion batteries, J. Power Sources 327 (2016) 507e518. W. Wu, J.L. Guo, X. Qin, C.B. Bi, J.F. Wang, L. Wang, G.C. Liang, Enhanced electrochemical performances of LiNi0.5Mn1.5O4 spinel in half-cell and fullcell via yttrium doping, J. Alloy. Comp. 721 (2017) 712e730. M.Y. Mo, K.S. Hui, X.T. Hong, J.S. Guo, C.C. Ye, A.J. Li, N.Q. Hu, Z.Z. Huang, J.H. Jiang, J.Z. Liang, H.Y. Chen, Improved cycling and rate performance of Smdoped LiNi0.5Mn1.5O4 cathode materials for 5 V lithium ion batteries, Appl. Surf. Sci. 290 (2014) 412e418. J.S. Chae, M.R. Jo, Y.I. Kim, D.W. Han, S.M. Park, Y.M. Kang, K.C. Roh, Kinetic favorability of Ru-doped LiNi0.5Mn1.5O4 for high-power lithium-ion batteries, J. Ind. Eng. Chem. 21 (2015) 731e735. Y. Yang, S. Li, Q. Zhang, Y. Zhang, S.M. Xu, Spherical Agglomeration of octahedral LiNi0.5Co4xMn1.5-3xO4 cathode material prepared by a continuous coprecipitation method for 5 V lithium-ion batteries, Ind. Eng. Chem. Res. 56 (2017) 75e182. J. Mao, K.H. Dai, M.J. Xuan, G.S. Shao, R.M. Qiao, W.L. Yang, V.S. Battaglia, G. Liu, The effect of chromium and niobium doping on the morphology and electrochemical performance of high-voltage spinel LiNi0.5Mn1.5O4 cathode material, ACS Appl. Mater. Interfaces 8 (2016) 9116e9124. H.Y. Sun, X. Kong, B.S. Wang, T.B. Luo, G.Y. Liu, LiNi0.5Mn1.45Zn0.05O4 with excellent electrochemical performance for lithium ion batteries, Int. J. Electrochem. Sc. 12 (2017) 8609e8621. Y.F. Deng, S.X. Zhao, Y.H. Xu, K. Gao, C.W. Nan, Impact of P-Doped in spinel LiNi0.5Mn1.5O4 on degree of disorder, grain morphology, and electrochemical performance, Chem. Mater. 27 (2015) 7734e7742. S.W. Oh, S.H. Park, J.H. Kim, Y.C. Bae, Y.K. Sun, Improvement of electrochemical properties of LiNi0.5Mn1.5O4 spinel material by fluorine substitution, J. Power Sources 157 (2006) 464e470. Y. Luo, H.Y. Li, T.L. Lu, Y.X. Zhang, S.S. Mao, Z. Liu, W. Wen, J.Y. Xie, L.Q. Yan, Fluorine gradient-doped LiNi0.5Mn1.5O4 spinel with improved high voltage stability for Li-ion batteries, Electrochim. Acta 238 (2017) 237e245. W.K. Kim, D.W. Han, W.H. Ryu, S.J. Lim, J.Y. Eom, H.S. Kwon, Effects of Cl doping on the structural and electrochemical properties of high voltage LiNi0.5Mn1.5O4 cathode materials for Li-ion batteries, J. Alloy. Comp. 592 (2014) 48e52. O. Sha, Z.Y. Tang, S.L. Wang, W. Yuan, Z. Qiao, Q. Xu, L. Ma, The multisubstituted LiNi0.475Al0.01Cr0.04Mn1.475O3.95F0.05 cathode material with excellent rate capability and cycle life, Electrochim. Acta 77 (2012) 250e255. Y.P. Zeng, X.L. Wu, P. Mei, L.N. Cong, C. Yao, R.S. Wang, H.M. Xie, L.Q. Sun, Effect of cationic and anionic substitutions on the electrochemical properties of LiNi0.5Mn1.5O4 spinel cathode materials, Electrochim. Acta 138 (2014) 493e500. G.H. Kim, S.T. Myung, H.J. Bang, J. Prakash, Y.K. Sun, Synthesis and electrochemical properties of Li[Ni1/3Co1/3Mn(1/3-x)Mgx]O2-yFy via coprecipitation, Electrochem. Solid St. 17 (2004) A477eA480. N.M. Hagh, G.G. Amatucci, Effect of cation and anion doping on microstructure and electrochemical properties of the LiNi0.5Mn1.5O4-d spinel, J. Power Sources 256 (2014) 457e469. J. Wang, W.Q. Lin, B.H. Wu, J.B. Zhao, Syntheses and electrochemical properties of the Na-doped LiNi0.5Mn1.5O4 cathode materials for lithium-ion batteries, Electrochim. Acta 145 (2014) 245e253. C.J. Yin, H.M. Zhou, Z.H. Yang, J. Li, Synthesis and electrochemical properties of LiNi0.5Mn1.5O4 for Li-ion batteries by the metalorganic framework method, ACS Appl. Mater. Interfaces 10 (2018) 13625e13634.