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Boosting the stable Li storage performance in one-dimensional LiLaxMn2-xO4 nanorods at elevated temperature
Ceramics International 45 (2019) 19351–19359
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Boosting the stable Li storage performance in one-dimensional LiLaxMn2-xO4 nanorods at elevated temperature
T
Chengyi Zhua, Jianxiong Liua, Xiaohua Yua, Yingjie Zhanga,b, Peng Dongb, Xiao Wanga, Yannan Zhangb,∗ a
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China National and Local Joint Engineering Laboratory for Lithium-ion Batteries and Materials Preparation Technology, Key Laboratory of Advanced Battery Materials of Yunnan Province, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion batteries One-dimensional fabrication LiLaxMn2-xO4 nanorod Elevated temperature
Spinel LiMn2O4 is regarded as a promising positive material candidate for contemporary lithium-ion batteries (LIBs). However, its development is hindered by the poor Li+ diffusion during high charge/discharge rates and unstable cyclic performance associated with Mn dissolution under elevated temperature. Herein, a facile hydrothermal method with solid-state calcination is introduced to design one-dimensional (1D) La-doped LiMn2O4 nanorods and ultimately enhance their cathode utilization in high-temperature and long-cycling conditions. Surprisingly, LiLa0.03Mn1.97O4 nanorods exhibit excellent high-temperature cyclic performance for over 1000 cycles, and deliver a promising discharge capacity of 87.2 mAh g−1 at 20C rate. Furthermore, theoretical calculations confirm that compared with the pristine sample, a lower bandgap and dispersed distribution of electrons are also strong evidence of the improved electrochemical performance of LiMn2O4 after La doping. This research offers a new strategy to achieve high-performance LiMn2O4-based positive materials for the development of advanced LIBs.
1. Introduction Much attention has been given to spinel LiMn2O4 as a promising positive material for advanced lithium-ion batteries (LIBs) for wide applications in portable devices, hybrid electric vehicles and smart grids due to its nontoxicity, lower price, and abundant manganese resource [1–4]. However, despite the mentioned advantages, the largescale commercial application of LiMn2O4 has been restricted due to its rapid capacity fading upon the cycling process, especially after the long-term operation at elevated temperature [5–7]. Moreover, the capacity of LiMn2O4 fades severely under high rate serving conditions. The proposed mechanisms can be broadly classified into two aspects: (1) the unremarkable electronic conductivity and low lithium diffusion kinetics and (2) the manganese dissolution in the electrolyte caused by disproportion reaction [8–10]. Recently, to resolve the former issues, design and tailoring to generate specific shapes and sizes of nanomaterials has become a research hotspot [11,12]. Noticeably, one-dimensional (1D) nanostructures, including nanowires, nanorods nanoribbons are widely investigated since they can shorten the pathway of Li+ and provide more active sites. Zhao et al. obtained LiMn2O4 nanorods after calcination at 700 °C using ∗
γ-MnOOH and LiOH·H2O, and the products delivered a promising discharge capacity of 128.7 mAh g−1 at a 0.5C rate [13]. Ding et al. synthesized LiMn2O4 nanotubes by a hydrothermal process which delivered a favourable rate capability performance at a normal temperature [14]. However, although one-dimensional LiMn2O4 can facilitate Li+ transport and effectively improve the capacity density of LiMn2O4, the capacity fading at high ambient temperature has not been addressed. Cation doping is considered to be effective in inhibiting the JahnTeller distortion and dissolution of Mn3+ by introducing the cations (Al3+, Fe3+, Sc3+, Ni2+, Nb3+, Cr3+) to partially substitute the Mn3+ ions [15–20]. As a representative rare-earth cation of the lanthanide series, La3+ can effectively alleviate the lattice deformation attributed to its pillar effect in some electrode materials [21]. Therefore, the La doping for LiMn2O4 nanorods can couple the advantages of the fast Li+ transport and favourable structural stability. Hence, we attempt to fabricate a spinel La-doped LiMn2O4 nanorod via a facile hydrothermal combined with solid-state calcination strategy using β-MnO2 as a sacrificial precursor. The synthetic method, physical characteristics and electrochemical tests of the as-synthesized LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods are
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.06.187 Received 23 April 2019; Received in revised form 16 June 2019; Accepted 18 June 2019 Available online 21 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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investigated in detail. Compared with the pristine LiMn2O4, the asprepared LiLaxMn2-xO4 nanorods deliver superior long-cycling performance and excellent rate ability. Furthermore, the mechanism for improving the electrochemical performance of La-doped LiMn2O4 nanorods is clarified by first-principles calculations. 2. Experimental methods 2.1. Synthesis of β-MnO2 nanorods The β-MnO2 nanorods were successfully synthesized through a hydrothermal method. First, 2 g of KMnO4 (Shanghai Aladdin Reagent, 99.9%) and 10 mL of ethanol were fully dispersed using deionized water (120 mL) under vigorous stirring. Subsequently, the mixed solution was heated in a Teflon autoclave (150 mL) at 180 °C for 12 h. Next, the mixture was washed with distilled water thrice using a laboratory centrifuge (LX-200, Beijing, China) to separate the liquids. The obtained powers were then transferred in a vacuum oven and dried at 100 °C for 6 h. Ultimately, the β-MnO2 nanorods were procured after heating at 350 °C after 80 min under air flow. 2.2. Synthesis of LiLaxMn2-xO4 nanorods LiLaxMn2-xO4 nanorods were prepared after calcination using asprepared β-MnO2 as sacrificial precursors. Typically, stoichiometric amounts of LiOH·H2O (Shanghai Aladdin Reagent, 99.9%), β-MnO2 nanorods, La(OH)3 (Shanghai Aladdin Reagent, 99.9%) (Li:Mn:La = 1.06:2-x:x, x = 0, 0.01, 0.03, 0.05 and 0.10) were fully dispersed by deionized water with strong mechanical stirring for 2 h. Subsequently, the samples were dried in a vacuum freeze-dryer (LGJ18S, Beijing, China). Ultimately, the as-prepared product was calcined under 700 °C for 10 h using a tubular furnace to obtain the LiLaxMn2xO4 nanorods with different amounts of La-doping (denoted as LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10). The flowchart for the preparation of LiLaxMn2-xO4 nanorods is shown in Fig. 1. 2.3. Material characterization The crystalline structure analysis of the obtained LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods was accomplished on an Xray diffraction (XRD, Rint-2000, Japan) using Cu-Kα radiation (1.5412 Å). Raman spectra were acquired in spectral range 100–800 cm−1 from Raman spectrometer (Cora 7 × 00, Xi’an, China) with a laser light source of 508.6 nm. The determination of surface elements of the obtained powers was taken by an X-ray photoelectron spectroscopy (XPS, Quantera ULVAC-PHI, Inc.). The surface morphologies of the obtained powers were confirmed through a field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP). The crystalline microstructures of as-prepared samples were observed by the transmission electron microscopy (TEM, FEI Tecnai F20). The elemental distribution was carried out by energy dispersive spectrometry (EDS, PHI5000 Versa probe-II). The inductively coupled plasma (ICP, THERMO-6000, USA) was carried out to measure the amounts of
manganese dissolution of as-prepared samples. 2.4. Electrochemical measurements The preparation of the electrode materials and the assembly of the coin-cells are based on our previous report [5]. Galvanostatic tests were conducted at 25 °C ( ± 1 °C) and 55 °C ( ± 1 °C) with different rates over a voltage window ranging of 3.0–4.5 V by a battery test system (LAND-CT2001D, Wuhan, China). The electrochemical impedance spectroscopy measurements (EIS) were recorded by an electrochemical workstation (Autolab PGSTAT 302 N). 2.5. Computational details A Vienna Ab initio Simulation Package (VASP) based on Density functional theory (DFT) was considered to explore the electronic structure calculations of as-prepared samples. The Perdew-BurkeErnzerhof (PBE) functional was carried out to implement the exchange and correlation energy. The electronic properties of pure LiMn2O4 and La-doped LiMn2O4 were calculated using 56-atom supercell with a 5 × 5 × 5 evenly Monkhorst Pack of k-point grid using a 400 eV cut-off energy. The supercell containing La atoms is depicted in Fig. 2f. 3. Results and discussion The crystal structures of β-MnO2 and LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods were examined using XRD. As shown in Fig. S1a, the patterns at 28.6°, 37.3°, 41.0°, 42.8°, 46.1°, 56.7°, 59.4°, 64.8°, and 67.3° can be ideally indexed as the tetragonal β-MnO2 phase (JCPDS No.24–0735) [22]. The observed XRD patterns, the calculated XRD patterns and their difference pattern of LiLaxMn2-xO4 nanorods with different masses of La doping are shown in Fig. 2a–e, respectively. All of the samples can be identified as the cubic space of space group Fd3m (JCPDS 35–0782) (Fig. S1b), which adequately illustrates that the La element is favourably integrated into the lattice without altering the spinel structure of LiMn2O4 [23]. Furthermore, with the addition of La of x = 0.10, a characteristic peak belonging to La2O3 appears, which indicates that the excessive La element cannot be integrated into the spinel lattice structure to replace the Mn3+ in the 16d position of the octahedron but exists in the form of La2O3. The Rietveld refinement is performed to assess the distinctions in the lattice parameters of asprepared samples, and the crystal structural parameters are calculated and displayed in Table 1. It can be seen that the lattice constants (Å) and lattice volume (Å3) for LiLaxMn2-xO4 nanorods decrease after La doping. Although the La3+ delivers an ionic radii of 1.15 Å, which is almost twice that of Mn3+ (0.66 Å), the La3+ ions incorporated into the octahedral sites also have a larger binding energy with O (ELa−1 ) than that of Mn3+ ions (EMn-O = 401 kJ mol−1), O = 782 kJ mol which leads to the lattice constriction of LiMn2O4 [24]. Noticeably, the I(311)/I(400) ratio of samples decreases from 0.982 to 0.948 as the quantity of lanthanum doping grows from 0% to 10%, which means that LiLaxMn2-xO4 nanorods can effectively decrease the Li/Mn cation mixing and reduce the resistance of insertion/extraction for Li+ after La doping [25,26].
Fig. 1. Flowchart for the preparation of LiLaxMn2-xO4 nanorods. 19352
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Fig. 2. Refined XRD results show the observed, calculated and difference patterns of LMO-0 (a), LMO-1 (b), LMO-3 (c), LMO-5 (d) and LMO-10 (e), respectively. The schematic illustration of the lattice of LiLaxMn2-xO4 (f). Table 1 Crystal structural parameters and the intensity ratio of the (311)/(400) peaks of the samples. Samples
a (Å)
c (Å3)
I(311)/I(400)
Rp (%)
Rwp (%)
Rexp (%)
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
8.2406 8.2369 8.2306 8.2263 8.2201
559.5984 558.8450 557.5640 556.6903 555.4325
0.982 0.974 0.965 0.948 0.944
7.89 7.69 8.38 8.27 7.90
11.57 10.86 11.69 11.87 10.88
4.33 4.52 5.12 4.66 4.89
Fig. 3 exhibits the Raman spectra of as-prepared LiLaxMn2-xO4 nanorods. As can be seen from Fig. 3a, A1g + Eg + 3F2g are Raman active modes for the spinel structure of LiMn2O4 [27]. All of the Raman scattering spectra of LiLaxMn2-xO4 nanorods are governed by a strong peak at 620 cm−1 (corresponding to the A1g symmetry) with a shoulder-peak at approximately 580 cm−1 (corresponding to the F2g(1) symmetry). A band assigned to the F2g(2) symmetry can be observed at approximately 485 cm−1. Two low wavenumber bands at approximately 387 and 301 cm−1 and can be attributed to the Eg and F2g(3) symmetry, respectively [28]. As seen in Fig. 3b, after La doping, a slight shift in the strong broad-band towards higher wavenumbers is observed, which can be ascribed to the greater atomic weight of the La
atom compared to the Mn atom [29]. Furthermore, it is obvious that the intensities of shoulder peaks at approximately 580 cm−1 are slightly enhanced after La doping, the shoulder peak appearing at 585 cm−1 of the F2g(1) symmetry is mainly attributed to the vibration of MnIV–O bonding, and its intensity reflects the average valence of Mn in the spinel LiMn2O4 [30]. This result demonstrates that the average ionic valence of the Mn ion is increased after La doping. Besides the XRD patterns and Raman scattering spectra, XPS is applied to confirm the elemental distribution of the LiLaxMn2-xO4 nanorods. The full-scan spectra of LMO-0 and LMO-3 samples are exhibited in Fig. 4a. Compared with the LMO-0, besides the Mn 2p, C 1s and O 1s peaks, two characteristic peaks at 835.17 eV and 851.28 eV belonging to La 3d3/2 and La 3d5/2 can be detected in the spectrum of LMO-3, and the binding energy between these two peaks is 16.69 eV, which indicates that the La element doped into the crystal lattice of LiMn2O4 exists in the form of La3+ ions [31]. Furthermore, the Mn 2p spectra of LMO-0 and LMO-3 are illustrated in Fig. 4c and d, respectively. The characteristic peaks appeared at 642.1 and 653.3 eV are assigned to Mn3+, while the characteristic peaks appeared at 643.4 and 654.4 eV are assigned to Mn4+, which illustrates that the average Mn valence in LiLaxMn2-xO4 nanorods is between +3 and + 4 [32]. Moreover, the Mn3+: Mn4+ ratios for LMO-0 and LMO-3 samples have been confirmed by their peak area ratios. LMO-0 exhibits Mn3+: Mn4+
Fig. 3. Raman scattering spectra of LiLaxMn2-xO4 nanorods (x = 0, 0.01, 0.03, 0.05 and 0.10) (a) and their microscopic view between 500 cm−1 and 700 cm−1 (b). 19353
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Fig. 4. XPS full spectra of LMO-0 and LMO-3 samples (a); high-resolution spectra for La 3d of the LMO-3 sample (b); high-resolution spectra for Mn 2p of LMO-0 (c) and LMO-3 samples (d).
ratios of ca. 50.2 : 49.8%. In contrast, LMO-3 displays a lower Mn3+: Mn4+ ratio (ca. 48.4:51.6%), which indicates the increase of the average ionic valence of the Mn ion after La doping. Less Mn3+ ions exist on the surface of LiMn2O4 and can effectively suppress the manganese dissolution as well as stabilize the spinel framework of LiMn2O4 [17]. The SEM image of β-MnO2 sample is exhibited in Fig. S2a. The obtained β-MnO2 displays a rod-shaped structure with an average diameter of 60 nm. The SEM images of LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 samples are shown in Fig. 5a–e, respectively, and all of the samples exhibit a well-defined one-dimensional rod morphology. Compared to the morphology of β-MnO2 nanorods, the obtained LiLaxMn2-xO4 nanorods are generally thicker and shorter, after the calcination process. With the increased doping amount, some granular particles appear on the surfaces of nanorods (Fig. 5d–e), which may be due to that the excessive La element cannot be incorporated into the spinel structure but attaches to the surface of the sample in the form of La2O3. The elemental mapping images of the LMO-3 sample detected by EDS measurement (Fig. 5f) show the homogeneous distribution of Mn, O and La elements. The mass ratio of O, Mn and La elements of the LMO-3 sample is 36.7 wt%:60.2 wt%:2.2 wt%, respectively, which is almost the same as its theoretical mass distribution (O: Mn: La = 36.3 wt%:61.4 wt%:2.4 wt%). The TEM images and SEAD patterns are carried out for confirming the microstructures of the LMO-0 and LMO-3 samples, as shown in Fig. 5h–i. Consistent with SEM results, both samples exhibit rod-like structures with smooth surfaces. It can be clearly seen that both LMO-0 and LMO-3 grow along the (111) crystal plane. The interplanar spacing of LMO-3 is 0.471 nm, which is smaller than that of LMO-0 (0.477 nm). Fig. 6a displays the initial charge-discharge curves of LMO-0, LMO1, LMO-3, LMO-5 and LMO-10 at 0.1C, respectively. It can be found that all of these samples present two potential plateaus at around 4.1 V and 3.9 V, which correspond to the following chemical reactions [33,34]:
MnO2 + 0.5Li+ + 0.5e− = Li 0.5Mn2 O4
(1)
Li 0.5Mn2 O4 + 0.5Li+ + 0.5e− = LiMn2 O4
(2)
In addition, the discharge capacities for LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 nanorods are, respectively, 134, 132, 129, 125 and 121 mAh g−1 at 0.1C. while the discharge capacity of commercial LiMn2O4 (purchased from the Hunan Shanshan New Material Co., Ltd.) is only 118 mAh g−1 (Fig. S3a). The high capacities of as-prepared samples are attributed to their unique one-dimensional morphology, which can shorten the pathway of Li+ and provide more active sites. Noticeably, the initial discharge capacities slowly decrease with the increase of the La-doping amount. It may be because the La3+ doped into the lattice of LiMn2O4 is electrochemically inactive in a voltage window of 3.0–4.5 V. The decrease of the initial capacities of LiLaxMn2xO4 samples after La doping are owing to the reduction of active materials attached to the positive electrodes [21,35]. Furthermore, the rate performance of LiLaxMn2-xO4 nanorods was evaluated, as shown in Fig. 7b and Table 2. As increasing the current density, compared with the pristine sample, all of the samples exhibit significantly improved discharge capacities after doping under high current densities (above 5C). By contrast, the LMO-3 presents best rate performance, and its discharge capacity can remain up to 87.2 mAh g−1 even under 20C rate. The remarkable rate capability of the LMO-3 is probably attributed to its unique one-dimensional morphology and enhanced lattice stability after La doping. Furthermore, Fig. 6c–d displays the initial discharge curves of LMO-0 and LMO-3 samples from 0.1C to 20C, respectively. As can be observed, both of the samples exhibit profiles with nearly the same shapes at 0.1C and 0.5C. As the increasing of the current density, the potential plateau for LMO-0 becomes inconspicuous, and the difference in the voltage plateaus between 0.1C and 20C is 0.52 V, while the potential difference for LMO-3 is shrunk to 0.29 V. Consequently, it can be inferred that the polarization of LiMn2O4 is markedly decreased after La doping, which indicates the good kinetic characteristics and remarkable electrochemical
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Fig. 5. SEM images of LMO-0 (a), LMO-1 (b), LMO-3 (c), LMO-5 (d), and LMO-10 (e); EDS mapping images (f–g) of LMO-3; TEM images of LMO-0 (h) and LMO-3 (i), where the insets in (h) and (i) are the HRTEM images and SEAD patterns of LMO-0 and LMO-3, respectively.
Fig. 6. Initial charge-discharge curves (a) and rate performances (b) of LMO-0, LMO-1, LMO-3, LMO-5 and LMO-10. Discharge curves of LMO-0 (c) and LMO-3 samples (b) at different rates at 25 °C. 19355
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Fig. 7. Cycling performance at 25 °C (a) and 55 °C (b) of LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 nanorods. Concentrations of Mn dissolved from LMO-0 and LMO-3 samples stored in LiPF6/DEC: EC (1:1) electrolyte at 25 °C and 55 °C (c) for 30 days. The schematic illustration of Li+ diffusion and electron transmission in the lattice of LiLaxMn2-xO4 nanorods (d).
Table 2 The discharge capacities (mAh g−1) of as-prepared samples at different rates at 25 °C. sample
0.1C
1C
2C
5C
10C
20C
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
133.0 131.3 129.6 125.6 120.3
123.6 120.2 120.2 116.1 113.5
112.2 114.2 113.0 110.5 106.6
93.6 100.4 106.6 97.1 96.2
72.8 90 105.4 84.6 80.0
47.2 74.6 87.2 68.7 47.3
To demonstrate the Li+ diffusion mechanism, the EIS measurements are performed for the LiLaxMn2-xO4 electrodes after 50 cycles, as shown in Fig. 8a. All of the as-prepared electrodes display typical Nyquist plots, including low-frequency sloping lines and the high-frequency semicircles [40]. According to the equivalent circuit (Fig. 8a inset), the results of solution resistance (Rs) and charge transfer resistance (Rct) values were fitted and listed in Table 3. Furthermore, the Li+ diffusion coefficient change of electrodes can be obtained from equations (3) and (4) [41,42]:
Z′ = Rs + R ct + σω⋅ω−0.5 reversibility for La-doped LiMn2O4 nanorods [36,37]. To evaluate the cycle life of LiMn2O4 nanorods before and after doping, a range of long-term cycling measurements for La-doped LiMn2O4 samples were performed under both normal and high ambient temperature. Fig. 7a presents the cyclic stability test results of the asprepared samples under 25 °C at 2C. After 1000 cycles, the discharge capacity of the LMO-0 decreases from 120 to 43 mAh g−1 with a low retention of 35.8%, whereas the LMO-3 maintains an excellent capacity retention of 86.8% with high coulombic efficiency (almost 100.0%). For comparison, the capacity of commercial LiMn2O4 decays rapidly and delivers an almost ignorable capacity of 32 mAh g−1 after 500 cycles. Fig. 7b shows the cyclic stability of as-prepared LiLaxMn2-xO4 nanorods at 2C at 55 °C. All of the samples display declined capacities at elevated temperature, which may be due to the accelerated Mn dissolution in the electrolyte [38]. After 1000 cycles, the discharge capacity of LMO-0 severely decays to 31.5 mAh g−1, while the LMO-3 only decreases to 93.9 mAh g−1 with a high capacity retention of 78.8%. The superior cyclic stability of LMO-3 is mainly probably attributed to the suppression of manganese dissolution after La doping. To confirm this viewpoint, the amounts of Mn dissolution for LMO-0 and LMO-3 are examined by ICP measurements after 30 days of storage in LiPF6/DEC: EC (1:1) electrolyte at 25 °C and 55 °C, respectively, as shown in Fig. 7c. Compared to LMO-0, LMO-3 exhibits a significant reduction of the amount of Mn dissolution, whether at normal and high ambient temperature. The superior stability of the LMO-3 sample against Mn dissolution can be attributed to the increased manganese valence after La doping [39].
D Li =
R2T 2 2n4A2 F 4σω2 C 2
(3)
(4)
in which DLi, R, T, F, A, C and n represents the diffusion coefficient of Li+, the gas constant (8.314 J mol−1K−1), absolute temperature (298 K), the Faraday constant (9.65 × 104 C mol−1), the area of the electrode (1.74 cm2), the concentration of Li+ (4.35 × 10−3 mol cm−3) and the amount of electrons transferred per LiMn2O4 molecule, respectively [43]. The Warburg impedance coefficient σω can be obtained from the slope of the fitting line, as can be seen from Fig. 8b. The diffusion coefficient (DLi) of the LiLaxMn2-xO4 electrode are calculated from equations (3) and (4), as listed in Table 3. Compared with other samples, LMO-3 delivers the highest DLi of 4.33 × 10−12 cm2s−1, indicating that La doping can effectively enhance the Li+ diffusion transportation. This may be primarily attributed to the stronger La–O bond than the parent Mn–O bond, which can weaken the neighbouring Li–O interaction, thus making it easier for Li+ transportation along the pathway of 8a-16c-8a in the spinel structure of LiLaxMn2-xO4 electrodes [44]. The enhanced Li+ diffusion coefficient after La doping contributes significantly to the improved rate performance and cycle stability of the LiLaxMn2-xO4 nanorods. To further reveal the electronic properties of LiMn2O4 before and after La doping, the electronic structures of LiMn2O4 (LMO) and Ladoped LiMn2O4 are calculated by first-principles calculations [45,46]. Fig. 9a and b shows the band structures and total density of LMO and LLMO obtained by the GGA + U method. As shown in Fig. 9a, the band gap of LLMO is 0.19 eV, which is smaller than that of LMO (0.26 eV).
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Fig. 8. Nyquist plots and equivalent circuit (a) and Z′-ω−1/2 pattern in the low-frequency region (b) of LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) cathode materials.
Table 3 Solution resistance (Rs), charge transfer resistance (Rct), σ values and Li+ diffusion coefficients (DLi) for LiLaxMn2-xO4 cathode materials. Sample
Rs/Ω
Rct/Ω
σ/(Ω/s1/2)
DLi/(cm2s−1)
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
7.86 6.20 4.88 5.61 6.74
186.46 96.87 78.31 104.86 122.39
192.54 96.33 82.90 112.47 140.72
3.96 × 10−13 2.15 × 10−12 4.33 × 10−12 8.11 × 10−12 1.56 × 10−13
The electrons of LLMO are more susceptible to transition due to the smaller band gap, which makes it more conductive [47]. In addition, Fig. 9c and d exhibit the differential charge densities of the O–Mn–O planes of LMO and LLMO, respectively. After La doping, the electrons around the La atom are delocalized, and a significant directional shift occurred in the electron transfer direction of Mn3+/Mn4+, which means that the oriented electron transport channels are more easily formed in the LLMO lattice after La doping [48]. The first-principles calculation results of LMO and LLMO correspond well to the previous results of electrochemical measurements. To investigate the change in the crystal structure and the manganese dissolution of as-prepared samples after cycling, XRD, XPS, and TEM
Fig. 9. Band structures and total density of LiMn2O4 (a) and La-doped LiMn2O4 (b) obtained by the GGA + U method. The differential charge density of the Mn–O–Mn plane of pure LiMn2O4 (c) and La-doped LiMn2O4 (d). 19357
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Fig. 10. XRD patterns (a) and XPS spectra of Mn 2p (b) for the LMO-0 and LMO-3 after 200 cycles at 5C, respectively.
temperature. The promising specific capacity could be related to its unique one-dimensional morphology. The superior long-term cycling stability could be associated with the enhanced lattice stability and the suppression of manganese dissolution after La-doping. Furthermore, the results of DFT calculations illuminate that La-doped LiMn2O4 exhibits a smaller band gap and provides more oriented electron transport channels to facilitate the transport of electrons. The facile fabrication strategy mentioned herein provides a new thinking for the development of spinel-based cathodes with favourable cycling performance at elevated temperature. Fig. 11. HRTEM images of the LMO-0 (a) and LMO-3 (b) after 200 cycles at 5C, respectively.
measurements were conducted on the samples of LMO-0 and LMO-3 after 200 cycles at 5C under 55 °C, respectively. Fig. 10a plots the XRD patterns of the LMO-0 and the LMO-3 samples after cycling. It can be clearly seen that diffraction peaks belonging to λ-MnO2 and Li2MnO3 are detected in the sample of LMO-0 except for the main peaks of spinel LiMn2O4, while the λ-MnO2 and Li2MnO3 are considered as the product of the HF-induced decomposition and manganese dissolution [49]. By comparison, there is no impure peak observed in the LMO-3 sample. In addition, the XPS spectra of Mn 2p for LMO-0 and LMO-3 after cycling is carried out to determine the valence change of manganese, as can be seen in Fig. 10b. After long-term cycling, the amount of Mn4+ for the LMO-3 sample (52.3%) is significantly lower than that of the LMO-0 sample (55.2%), while the Mn4+ stems from the solid product (such as λ-MnO2 and Li2MnO3) [50]. From the results of XRD and XPS measurements, we can deduce that the LMO-3 sample after La-doping can significantly suppress the manganese dissolution. Fig. 11 show the HRTEM images of the LMO-0 and LMO-3 samples after long cycling, respectively. It can be seen from Fig. 11a that there is an obvious impurity phase in the interface of LMO-0 with a lattice distance of 0.204 nm, corresponding to the (202) plane of the Li2MnO3 [51]. By contrast, as shown in Fig. 11b, the LMO-3 can still maintain the basic spinel structure without any other phase after long cycling, which means that the LMO-3 with La-doping can evidently restrain the distortion of the spinel lattice during the charge/discharge process. This discovery is accordant well with the results of XRD and XPS measurements. 4. Conclusion In summary, a facile strategy for fabricating LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods was developed by combining hydrothermal and solid-state calcination methods. The characterization results of La-doped LiLaxMn2-xO4 nanorods exhibit excellent electrochemical performance under both normal and high ambient
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Boosting the stable Li storage performance in one-dimensional LiLaxMn2-xO4 nanorods at elevated temperature
T
Chengyi Zhua, Jianxiong Liua, Xiaohua Yua, Yingjie Zhanga,b, Peng Dongb, Xiao Wanga, Yannan Zhangb,∗ a
Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, 650093, China National and Local Joint Engineering Laboratory for Lithium-ion Batteries and Materials Preparation Technology, Key Laboratory of Advanced Battery Materials of Yunnan Province, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Lithium-ion batteries One-dimensional fabrication LiLaxMn2-xO4 nanorod Elevated temperature
Spinel LiMn2O4 is regarded as a promising positive material candidate for contemporary lithium-ion batteries (LIBs). However, its development is hindered by the poor Li+ diffusion during high charge/discharge rates and unstable cyclic performance associated with Mn dissolution under elevated temperature. Herein, a facile hydrothermal method with solid-state calcination is introduced to design one-dimensional (1D) La-doped LiMn2O4 nanorods and ultimately enhance their cathode utilization in high-temperature and long-cycling conditions. Surprisingly, LiLa0.03Mn1.97O4 nanorods exhibit excellent high-temperature cyclic performance for over 1000 cycles, and deliver a promising discharge capacity of 87.2 mAh g−1 at 20C rate. Furthermore, theoretical calculations confirm that compared with the pristine sample, a lower bandgap and dispersed distribution of electrons are also strong evidence of the improved electrochemical performance of LiMn2O4 after La doping. This research offers a new strategy to achieve high-performance LiMn2O4-based positive materials for the development of advanced LIBs.
1. Introduction Much attention has been given to spinel LiMn2O4 as a promising positive material for advanced lithium-ion batteries (LIBs) for wide applications in portable devices, hybrid electric vehicles and smart grids due to its nontoxicity, lower price, and abundant manganese resource [1–4]. However, despite the mentioned advantages, the largescale commercial application of LiMn2O4 has been restricted due to its rapid capacity fading upon the cycling process, especially after the long-term operation at elevated temperature [5–7]. Moreover, the capacity of LiMn2O4 fades severely under high rate serving conditions. The proposed mechanisms can be broadly classified into two aspects: (1) the unremarkable electronic conductivity and low lithium diffusion kinetics and (2) the manganese dissolution in the electrolyte caused by disproportion reaction [8–10]. Recently, to resolve the former issues, design and tailoring to generate specific shapes and sizes of nanomaterials has become a research hotspot [11,12]. Noticeably, one-dimensional (1D) nanostructures, including nanowires, nanorods nanoribbons are widely investigated since they can shorten the pathway of Li+ and provide more active sites. Zhao et al. obtained LiMn2O4 nanorods after calcination at 700 °C using ∗
γ-MnOOH and LiOH·H2O, and the products delivered a promising discharge capacity of 128.7 mAh g−1 at a 0.5C rate [13]. Ding et al. synthesized LiMn2O4 nanotubes by a hydrothermal process which delivered a favourable rate capability performance at a normal temperature [14]. However, although one-dimensional LiMn2O4 can facilitate Li+ transport and effectively improve the capacity density of LiMn2O4, the capacity fading at high ambient temperature has not been addressed. Cation doping is considered to be effective in inhibiting the JahnTeller distortion and dissolution of Mn3+ by introducing the cations (Al3+, Fe3+, Sc3+, Ni2+, Nb3+, Cr3+) to partially substitute the Mn3+ ions [15–20]. As a representative rare-earth cation of the lanthanide series, La3+ can effectively alleviate the lattice deformation attributed to its pillar effect in some electrode materials [21]. Therefore, the La doping for LiMn2O4 nanorods can couple the advantages of the fast Li+ transport and favourable structural stability. Hence, we attempt to fabricate a spinel La-doped LiMn2O4 nanorod via a facile hydrothermal combined with solid-state calcination strategy using β-MnO2 as a sacrificial precursor. The synthetic method, physical characteristics and electrochemical tests of the as-synthesized LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods are
Corresponding author. E-mail address: [email protected] (Y. Zhang).
https://doi.org/10.1016/j.ceramint.2019.06.187 Received 23 April 2019; Received in revised form 16 June 2019; Accepted 18 June 2019 Available online 21 June 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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investigated in detail. Compared with the pristine LiMn2O4, the asprepared LiLaxMn2-xO4 nanorods deliver superior long-cycling performance and excellent rate ability. Furthermore, the mechanism for improving the electrochemical performance of La-doped LiMn2O4 nanorods is clarified by first-principles calculations. 2. Experimental methods 2.1. Synthesis of β-MnO2 nanorods The β-MnO2 nanorods were successfully synthesized through a hydrothermal method. First, 2 g of KMnO4 (Shanghai Aladdin Reagent, 99.9%) and 10 mL of ethanol were fully dispersed using deionized water (120 mL) under vigorous stirring. Subsequently, the mixed solution was heated in a Teflon autoclave (150 mL) at 180 °C for 12 h. Next, the mixture was washed with distilled water thrice using a laboratory centrifuge (LX-200, Beijing, China) to separate the liquids. The obtained powers were then transferred in a vacuum oven and dried at 100 °C for 6 h. Ultimately, the β-MnO2 nanorods were procured after heating at 350 °C after 80 min under air flow. 2.2. Synthesis of LiLaxMn2-xO4 nanorods LiLaxMn2-xO4 nanorods were prepared after calcination using asprepared β-MnO2 as sacrificial precursors. Typically, stoichiometric amounts of LiOH·H2O (Shanghai Aladdin Reagent, 99.9%), β-MnO2 nanorods, La(OH)3 (Shanghai Aladdin Reagent, 99.9%) (Li:Mn:La = 1.06:2-x:x, x = 0, 0.01, 0.03, 0.05 and 0.10) were fully dispersed by deionized water with strong mechanical stirring for 2 h. Subsequently, the samples were dried in a vacuum freeze-dryer (LGJ18S, Beijing, China). Ultimately, the as-prepared product was calcined under 700 °C for 10 h using a tubular furnace to obtain the LiLaxMn2xO4 nanorods with different amounts of La-doping (denoted as LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10). The flowchart for the preparation of LiLaxMn2-xO4 nanorods is shown in Fig. 1. 2.3. Material characterization The crystalline structure analysis of the obtained LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods was accomplished on an Xray diffraction (XRD, Rint-2000, Japan) using Cu-Kα radiation (1.5412 Å). Raman spectra were acquired in spectral range 100–800 cm−1 from Raman spectrometer (Cora 7 × 00, Xi’an, China) with a laser light source of 508.6 nm. The determination of surface elements of the obtained powers was taken by an X-ray photoelectron spectroscopy (XPS, Quantera ULVAC-PHI, Inc.). The surface morphologies of the obtained powers were confirmed through a field emission scanning electron microscopy (FESEM, Zeiss Supra 55VP). The crystalline microstructures of as-prepared samples were observed by the transmission electron microscopy (TEM, FEI Tecnai F20). The elemental distribution was carried out by energy dispersive spectrometry (EDS, PHI5000 Versa probe-II). The inductively coupled plasma (ICP, THERMO-6000, USA) was carried out to measure the amounts of
manganese dissolution of as-prepared samples. 2.4. Electrochemical measurements The preparation of the electrode materials and the assembly of the coin-cells are based on our previous report [5]. Galvanostatic tests were conducted at 25 °C ( ± 1 °C) and 55 °C ( ± 1 °C) with different rates over a voltage window ranging of 3.0–4.5 V by a battery test system (LAND-CT2001D, Wuhan, China). The electrochemical impedance spectroscopy measurements (EIS) were recorded by an electrochemical workstation (Autolab PGSTAT 302 N). 2.5. Computational details A Vienna Ab initio Simulation Package (VASP) based on Density functional theory (DFT) was considered to explore the electronic structure calculations of as-prepared samples. The Perdew-BurkeErnzerhof (PBE) functional was carried out to implement the exchange and correlation energy. The electronic properties of pure LiMn2O4 and La-doped LiMn2O4 were calculated using 56-atom supercell with a 5 × 5 × 5 evenly Monkhorst Pack of k-point grid using a 400 eV cut-off energy. The supercell containing La atoms is depicted in Fig. 2f. 3. Results and discussion The crystal structures of β-MnO2 and LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods were examined using XRD. As shown in Fig. S1a, the patterns at 28.6°, 37.3°, 41.0°, 42.8°, 46.1°, 56.7°, 59.4°, 64.8°, and 67.3° can be ideally indexed as the tetragonal β-MnO2 phase (JCPDS No.24–0735) [22]. The observed XRD patterns, the calculated XRD patterns and their difference pattern of LiLaxMn2-xO4 nanorods with different masses of La doping are shown in Fig. 2a–e, respectively. All of the samples can be identified as the cubic space of space group Fd3m (JCPDS 35–0782) (Fig. S1b), which adequately illustrates that the La element is favourably integrated into the lattice without altering the spinel structure of LiMn2O4 [23]. Furthermore, with the addition of La of x = 0.10, a characteristic peak belonging to La2O3 appears, which indicates that the excessive La element cannot be integrated into the spinel lattice structure to replace the Mn3+ in the 16d position of the octahedron but exists in the form of La2O3. The Rietveld refinement is performed to assess the distinctions in the lattice parameters of asprepared samples, and the crystal structural parameters are calculated and displayed in Table 1. It can be seen that the lattice constants (Å) and lattice volume (Å3) for LiLaxMn2-xO4 nanorods decrease after La doping. Although the La3+ delivers an ionic radii of 1.15 Å, which is almost twice that of Mn3+ (0.66 Å), the La3+ ions incorporated into the octahedral sites also have a larger binding energy with O (ELa−1 ) than that of Mn3+ ions (EMn-O = 401 kJ mol−1), O = 782 kJ mol which leads to the lattice constriction of LiMn2O4 [24]. Noticeably, the I(311)/I(400) ratio of samples decreases from 0.982 to 0.948 as the quantity of lanthanum doping grows from 0% to 10%, which means that LiLaxMn2-xO4 nanorods can effectively decrease the Li/Mn cation mixing and reduce the resistance of insertion/extraction for Li+ after La doping [25,26].
Fig. 1. Flowchart for the preparation of LiLaxMn2-xO4 nanorods. 19352
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Fig. 2. Refined XRD results show the observed, calculated and difference patterns of LMO-0 (a), LMO-1 (b), LMO-3 (c), LMO-5 (d) and LMO-10 (e), respectively. The schematic illustration of the lattice of LiLaxMn2-xO4 (f). Table 1 Crystal structural parameters and the intensity ratio of the (311)/(400) peaks of the samples. Samples
a (Å)
c (Å3)
I(311)/I(400)
Rp (%)
Rwp (%)
Rexp (%)
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
8.2406 8.2369 8.2306 8.2263 8.2201
559.5984 558.8450 557.5640 556.6903 555.4325
0.982 0.974 0.965 0.948 0.944
7.89 7.69 8.38 8.27 7.90
11.57 10.86 11.69 11.87 10.88
4.33 4.52 5.12 4.66 4.89
Fig. 3 exhibits the Raman spectra of as-prepared LiLaxMn2-xO4 nanorods. As can be seen from Fig. 3a, A1g + Eg + 3F2g are Raman active modes for the spinel structure of LiMn2O4 [27]. All of the Raman scattering spectra of LiLaxMn2-xO4 nanorods are governed by a strong peak at 620 cm−1 (corresponding to the A1g symmetry) with a shoulder-peak at approximately 580 cm−1 (corresponding to the F2g(1) symmetry). A band assigned to the F2g(2) symmetry can be observed at approximately 485 cm−1. Two low wavenumber bands at approximately 387 and 301 cm−1 and can be attributed to the Eg and F2g(3) symmetry, respectively [28]. As seen in Fig. 3b, after La doping, a slight shift in the strong broad-band towards higher wavenumbers is observed, which can be ascribed to the greater atomic weight of the La
atom compared to the Mn atom [29]. Furthermore, it is obvious that the intensities of shoulder peaks at approximately 580 cm−1 are slightly enhanced after La doping, the shoulder peak appearing at 585 cm−1 of the F2g(1) symmetry is mainly attributed to the vibration of MnIV–O bonding, and its intensity reflects the average valence of Mn in the spinel LiMn2O4 [30]. This result demonstrates that the average ionic valence of the Mn ion is increased after La doping. Besides the XRD patterns and Raman scattering spectra, XPS is applied to confirm the elemental distribution of the LiLaxMn2-xO4 nanorods. The full-scan spectra of LMO-0 and LMO-3 samples are exhibited in Fig. 4a. Compared with the LMO-0, besides the Mn 2p, C 1s and O 1s peaks, two characteristic peaks at 835.17 eV and 851.28 eV belonging to La 3d3/2 and La 3d5/2 can be detected in the spectrum of LMO-3, and the binding energy between these two peaks is 16.69 eV, which indicates that the La element doped into the crystal lattice of LiMn2O4 exists in the form of La3+ ions [31]. Furthermore, the Mn 2p spectra of LMO-0 and LMO-3 are illustrated in Fig. 4c and d, respectively. The characteristic peaks appeared at 642.1 and 653.3 eV are assigned to Mn3+, while the characteristic peaks appeared at 643.4 and 654.4 eV are assigned to Mn4+, which illustrates that the average Mn valence in LiLaxMn2-xO4 nanorods is between +3 and + 4 [32]. Moreover, the Mn3+: Mn4+ ratios for LMO-0 and LMO-3 samples have been confirmed by their peak area ratios. LMO-0 exhibits Mn3+: Mn4+
Fig. 3. Raman scattering spectra of LiLaxMn2-xO4 nanorods (x = 0, 0.01, 0.03, 0.05 and 0.10) (a) and their microscopic view between 500 cm−1 and 700 cm−1 (b). 19353
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Fig. 4. XPS full spectra of LMO-0 and LMO-3 samples (a); high-resolution spectra for La 3d of the LMO-3 sample (b); high-resolution spectra for Mn 2p of LMO-0 (c) and LMO-3 samples (d).
ratios of ca. 50.2 : 49.8%. In contrast, LMO-3 displays a lower Mn3+: Mn4+ ratio (ca. 48.4:51.6%), which indicates the increase of the average ionic valence of the Mn ion after La doping. Less Mn3+ ions exist on the surface of LiMn2O4 and can effectively suppress the manganese dissolution as well as stabilize the spinel framework of LiMn2O4 [17]. The SEM image of β-MnO2 sample is exhibited in Fig. S2a. The obtained β-MnO2 displays a rod-shaped structure with an average diameter of 60 nm. The SEM images of LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 samples are shown in Fig. 5a–e, respectively, and all of the samples exhibit a well-defined one-dimensional rod morphology. Compared to the morphology of β-MnO2 nanorods, the obtained LiLaxMn2-xO4 nanorods are generally thicker and shorter, after the calcination process. With the increased doping amount, some granular particles appear on the surfaces of nanorods (Fig. 5d–e), which may be due to that the excessive La element cannot be incorporated into the spinel structure but attaches to the surface of the sample in the form of La2O3. The elemental mapping images of the LMO-3 sample detected by EDS measurement (Fig. 5f) show the homogeneous distribution of Mn, O and La elements. The mass ratio of O, Mn and La elements of the LMO-3 sample is 36.7 wt%:60.2 wt%:2.2 wt%, respectively, which is almost the same as its theoretical mass distribution (O: Mn: La = 36.3 wt%:61.4 wt%:2.4 wt%). The TEM images and SEAD patterns are carried out for confirming the microstructures of the LMO-0 and LMO-3 samples, as shown in Fig. 5h–i. Consistent with SEM results, both samples exhibit rod-like structures with smooth surfaces. It can be clearly seen that both LMO-0 and LMO-3 grow along the (111) crystal plane. The interplanar spacing of LMO-3 is 0.471 nm, which is smaller than that of LMO-0 (0.477 nm). Fig. 6a displays the initial charge-discharge curves of LMO-0, LMO1, LMO-3, LMO-5 and LMO-10 at 0.1C, respectively. It can be found that all of these samples present two potential plateaus at around 4.1 V and 3.9 V, which correspond to the following chemical reactions [33,34]:
MnO2 + 0.5Li+ + 0.5e− = Li 0.5Mn2 O4
(1)
Li 0.5Mn2 O4 + 0.5Li+ + 0.5e− = LiMn2 O4
(2)
In addition, the discharge capacities for LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 nanorods are, respectively, 134, 132, 129, 125 and 121 mAh g−1 at 0.1C. while the discharge capacity of commercial LiMn2O4 (purchased from the Hunan Shanshan New Material Co., Ltd.) is only 118 mAh g−1 (Fig. S3a). The high capacities of as-prepared samples are attributed to their unique one-dimensional morphology, which can shorten the pathway of Li+ and provide more active sites. Noticeably, the initial discharge capacities slowly decrease with the increase of the La-doping amount. It may be because the La3+ doped into the lattice of LiMn2O4 is electrochemically inactive in a voltage window of 3.0–4.5 V. The decrease of the initial capacities of LiLaxMn2xO4 samples after La doping are owing to the reduction of active materials attached to the positive electrodes [21,35]. Furthermore, the rate performance of LiLaxMn2-xO4 nanorods was evaluated, as shown in Fig. 7b and Table 2. As increasing the current density, compared with the pristine sample, all of the samples exhibit significantly improved discharge capacities after doping under high current densities (above 5C). By contrast, the LMO-3 presents best rate performance, and its discharge capacity can remain up to 87.2 mAh g−1 even under 20C rate. The remarkable rate capability of the LMO-3 is probably attributed to its unique one-dimensional morphology and enhanced lattice stability after La doping. Furthermore, Fig. 6c–d displays the initial discharge curves of LMO-0 and LMO-3 samples from 0.1C to 20C, respectively. As can be observed, both of the samples exhibit profiles with nearly the same shapes at 0.1C and 0.5C. As the increasing of the current density, the potential plateau for LMO-0 becomes inconspicuous, and the difference in the voltage plateaus between 0.1C and 20C is 0.52 V, while the potential difference for LMO-3 is shrunk to 0.29 V. Consequently, it can be inferred that the polarization of LiMn2O4 is markedly decreased after La doping, which indicates the good kinetic characteristics and remarkable electrochemical
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Fig. 5. SEM images of LMO-0 (a), LMO-1 (b), LMO-3 (c), LMO-5 (d), and LMO-10 (e); EDS mapping images (f–g) of LMO-3; TEM images of LMO-0 (h) and LMO-3 (i), where the insets in (h) and (i) are the HRTEM images and SEAD patterns of LMO-0 and LMO-3, respectively.
Fig. 6. Initial charge-discharge curves (a) and rate performances (b) of LMO-0, LMO-1, LMO-3, LMO-5 and LMO-10. Discharge curves of LMO-0 (c) and LMO-3 samples (b) at different rates at 25 °C. 19355
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Fig. 7. Cycling performance at 25 °C (a) and 55 °C (b) of LMO-0, LMO-1, LMO-3, LMO-5, and LMO-10 nanorods. Concentrations of Mn dissolved from LMO-0 and LMO-3 samples stored in LiPF6/DEC: EC (1:1) electrolyte at 25 °C and 55 °C (c) for 30 days. The schematic illustration of Li+ diffusion and electron transmission in the lattice of LiLaxMn2-xO4 nanorods (d).
Table 2 The discharge capacities (mAh g−1) of as-prepared samples at different rates at 25 °C. sample
0.1C
1C
2C
5C
10C
20C
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
133.0 131.3 129.6 125.6 120.3
123.6 120.2 120.2 116.1 113.5
112.2 114.2 113.0 110.5 106.6
93.6 100.4 106.6 97.1 96.2
72.8 90 105.4 84.6 80.0
47.2 74.6 87.2 68.7 47.3
To demonstrate the Li+ diffusion mechanism, the EIS measurements are performed for the LiLaxMn2-xO4 electrodes after 50 cycles, as shown in Fig. 8a. All of the as-prepared electrodes display typical Nyquist plots, including low-frequency sloping lines and the high-frequency semicircles [40]. According to the equivalent circuit (Fig. 8a inset), the results of solution resistance (Rs) and charge transfer resistance (Rct) values were fitted and listed in Table 3. Furthermore, the Li+ diffusion coefficient change of electrodes can be obtained from equations (3) and (4) [41,42]:
Z′ = Rs + R ct + σω⋅ω−0.5 reversibility for La-doped LiMn2O4 nanorods [36,37]. To evaluate the cycle life of LiMn2O4 nanorods before and after doping, a range of long-term cycling measurements for La-doped LiMn2O4 samples were performed under both normal and high ambient temperature. Fig. 7a presents the cyclic stability test results of the asprepared samples under 25 °C at 2C. After 1000 cycles, the discharge capacity of the LMO-0 decreases from 120 to 43 mAh g−1 with a low retention of 35.8%, whereas the LMO-3 maintains an excellent capacity retention of 86.8% with high coulombic efficiency (almost 100.0%). For comparison, the capacity of commercial LiMn2O4 decays rapidly and delivers an almost ignorable capacity of 32 mAh g−1 after 500 cycles. Fig. 7b shows the cyclic stability of as-prepared LiLaxMn2-xO4 nanorods at 2C at 55 °C. All of the samples display declined capacities at elevated temperature, which may be due to the accelerated Mn dissolution in the electrolyte [38]. After 1000 cycles, the discharge capacity of LMO-0 severely decays to 31.5 mAh g−1, while the LMO-3 only decreases to 93.9 mAh g−1 with a high capacity retention of 78.8%. The superior cyclic stability of LMO-3 is mainly probably attributed to the suppression of manganese dissolution after La doping. To confirm this viewpoint, the amounts of Mn dissolution for LMO-0 and LMO-3 are examined by ICP measurements after 30 days of storage in LiPF6/DEC: EC (1:1) electrolyte at 25 °C and 55 °C, respectively, as shown in Fig. 7c. Compared to LMO-0, LMO-3 exhibits a significant reduction of the amount of Mn dissolution, whether at normal and high ambient temperature. The superior stability of the LMO-3 sample against Mn dissolution can be attributed to the increased manganese valence after La doping [39].
D Li =
R2T 2 2n4A2 F 4σω2 C 2
(3)
(4)
in which DLi, R, T, F, A, C and n represents the diffusion coefficient of Li+, the gas constant (8.314 J mol−1K−1), absolute temperature (298 K), the Faraday constant (9.65 × 104 C mol−1), the area of the electrode (1.74 cm2), the concentration of Li+ (4.35 × 10−3 mol cm−3) and the amount of electrons transferred per LiMn2O4 molecule, respectively [43]. The Warburg impedance coefficient σω can be obtained from the slope of the fitting line, as can be seen from Fig. 8b. The diffusion coefficient (DLi) of the LiLaxMn2-xO4 electrode are calculated from equations (3) and (4), as listed in Table 3. Compared with other samples, LMO-3 delivers the highest DLi of 4.33 × 10−12 cm2s−1, indicating that La doping can effectively enhance the Li+ diffusion transportation. This may be primarily attributed to the stronger La–O bond than the parent Mn–O bond, which can weaken the neighbouring Li–O interaction, thus making it easier for Li+ transportation along the pathway of 8a-16c-8a in the spinel structure of LiLaxMn2-xO4 electrodes [44]. The enhanced Li+ diffusion coefficient after La doping contributes significantly to the improved rate performance and cycle stability of the LiLaxMn2-xO4 nanorods. To further reveal the electronic properties of LiMn2O4 before and after La doping, the electronic structures of LiMn2O4 (LMO) and Ladoped LiMn2O4 are calculated by first-principles calculations [45,46]. Fig. 9a and b shows the band structures and total density of LMO and LLMO obtained by the GGA + U method. As shown in Fig. 9a, the band gap of LLMO is 0.19 eV, which is smaller than that of LMO (0.26 eV).
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Fig. 8. Nyquist plots and equivalent circuit (a) and Z′-ω−1/2 pattern in the low-frequency region (b) of LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) cathode materials.
Table 3 Solution resistance (Rs), charge transfer resistance (Rct), σ values and Li+ diffusion coefficients (DLi) for LiLaxMn2-xO4 cathode materials. Sample
Rs/Ω
Rct/Ω
σ/(Ω/s1/2)
DLi/(cm2s−1)
LMO-0 LMO-1 LMO-3 LMO-5 LMO-10
7.86 6.20 4.88 5.61 6.74
186.46 96.87 78.31 104.86 122.39
192.54 96.33 82.90 112.47 140.72
3.96 × 10−13 2.15 × 10−12 4.33 × 10−12 8.11 × 10−12 1.56 × 10−13
The electrons of LLMO are more susceptible to transition due to the smaller band gap, which makes it more conductive [47]. In addition, Fig. 9c and d exhibit the differential charge densities of the O–Mn–O planes of LMO and LLMO, respectively. After La doping, the electrons around the La atom are delocalized, and a significant directional shift occurred in the electron transfer direction of Mn3+/Mn4+, which means that the oriented electron transport channels are more easily formed in the LLMO lattice after La doping [48]. The first-principles calculation results of LMO and LLMO correspond well to the previous results of electrochemical measurements. To investigate the change in the crystal structure and the manganese dissolution of as-prepared samples after cycling, XRD, XPS, and TEM
Fig. 9. Band structures and total density of LiMn2O4 (a) and La-doped LiMn2O4 (b) obtained by the GGA + U method. The differential charge density of the Mn–O–Mn plane of pure LiMn2O4 (c) and La-doped LiMn2O4 (d). 19357
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Fig. 10. XRD patterns (a) and XPS spectra of Mn 2p (b) for the LMO-0 and LMO-3 after 200 cycles at 5C, respectively.
temperature. The promising specific capacity could be related to its unique one-dimensional morphology. The superior long-term cycling stability could be associated with the enhanced lattice stability and the suppression of manganese dissolution after La-doping. Furthermore, the results of DFT calculations illuminate that La-doped LiMn2O4 exhibits a smaller band gap and provides more oriented electron transport channels to facilitate the transport of electrons. The facile fabrication strategy mentioned herein provides a new thinking for the development of spinel-based cathodes with favourable cycling performance at elevated temperature. Fig. 11. HRTEM images of the LMO-0 (a) and LMO-3 (b) after 200 cycles at 5C, respectively.
measurements were conducted on the samples of LMO-0 and LMO-3 after 200 cycles at 5C under 55 °C, respectively. Fig. 10a plots the XRD patterns of the LMO-0 and the LMO-3 samples after cycling. It can be clearly seen that diffraction peaks belonging to λ-MnO2 and Li2MnO3 are detected in the sample of LMO-0 except for the main peaks of spinel LiMn2O4, while the λ-MnO2 and Li2MnO3 are considered as the product of the HF-induced decomposition and manganese dissolution [49]. By comparison, there is no impure peak observed in the LMO-3 sample. In addition, the XPS spectra of Mn 2p for LMO-0 and LMO-3 after cycling is carried out to determine the valence change of manganese, as can be seen in Fig. 10b. After long-term cycling, the amount of Mn4+ for the LMO-3 sample (52.3%) is significantly lower than that of the LMO-0 sample (55.2%), while the Mn4+ stems from the solid product (such as λ-MnO2 and Li2MnO3) [50]. From the results of XRD and XPS measurements, we can deduce that the LMO-3 sample after La-doping can significantly suppress the manganese dissolution. Fig. 11 show the HRTEM images of the LMO-0 and LMO-3 samples after long cycling, respectively. It can be seen from Fig. 11a that there is an obvious impurity phase in the interface of LMO-0 with a lattice distance of 0.204 nm, corresponding to the (202) plane of the Li2MnO3 [51]. By contrast, as shown in Fig. 11b, the LMO-3 can still maintain the basic spinel structure without any other phase after long cycling, which means that the LMO-3 with La-doping can evidently restrain the distortion of the spinel lattice during the charge/discharge process. This discovery is accordant well with the results of XRD and XPS measurements. 4. Conclusion In summary, a facile strategy for fabricating LiLaxMn2-xO4 (x = 0, 0.01, 0.03, 0.05 and 0.10) nanorods was developed by combining hydrothermal and solid-state calcination methods. The characterization results of La-doped LiLaxMn2-xO4 nanorods exhibit excellent electrochemical performance under both normal and high ambient
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