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La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries
Author’s Accepted Manuscript La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries Lin Zhou, Mijie Tian, Yunlong Deng, Qiaoji Zheng, Chenggang Xu, Dunmin Lin www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(16)31073-2 http://dx.doi.org/10.1016/j.ceramint.2016.07.016 CERI13241
To appear in: Ceramics International Received date: 27 May 2016 Revised date: 1 July 2016 Accepted date: 3 July 2016 Cite this article as: Lin Zhou, Mijie Tian, Yunlong Deng, Qiaoji Zheng, Chenggang Xu and Dunmin Lin, La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries Lin Zhou, Mijie Tian, Yunlong Deng, Qiaoji Zheng, Chenggang Xu, Dunmin Lin* College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China *
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Abstract Li-rich layered oxides are the most promising cathode candidate for new generation rechargeable
lithium-ion
batteries.
In
this
work,
La2O3-coated
Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials were fabricated via a combined method of sol-gel
and
wet
chemical
processes.
The
structural
and
morphological
characterizations of the materials demonstrate that a thin layer of La2O3 is uniformly covered on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 particles, and the coating of La2O3 has no obvious effect on the crystal structure of Li-rich oxide. The electrochemical performance of La2O3-coated Li-rich cathodes including specific capacity, cycling stability and rate capability has been significantly improved with the coating of La2O3. The Li1.2Mn0.54Ni0.13Co0.13O2 coated with 2.5 wt% La2O3 exhibits the highest discharge capacity, improved cycling stability and reduced charge transfer resistance, delivering a large discharge capacity of 276.9 mAh g-1 in the 1st cycle and a high capacity retention of 71% (201.4 mAh g-1) after 100 cycles. The optimal rate capability of the materials is observed at the coating level of 1.5 wt% La2O3 such that 1
the material exhibits the highest discharge capacity of 90.2 mAh g-1 at 5 C. The surface coating of La2O3 can effectively facilitate Li+ interfacial diffusion, reduce the structural change and secondary reactions between cathode materials and electrolyte during the charge-discharge process, and thus induce the great enhancement in the electrochemical properties of the Li1.2Mn0.54Ni0.13Co0.13O2 materials.
Key words: lithium-ion batteries; Li-rich layered oxide; La2O3 coating layer; electrochemical performance
1. Introduction Lithium-ion batteries (LIBs) have been extensively used in information, transportation, military and other fields due to their high energy density, high operating voltage, and environment friendliness [1,2]. They are also deemed as the most viable energy storage devices for hybrid electric vehicles (HEVs) and electric vehicles (EVs) [3,4]. As known, the energy density of the LIBs relies heavily on the cathode materials [5]. However, most of the extensively investigated cathodes of LIBs including layered LiCoO2 (~140 mAh g-1) [6], olivine LiFeO4 (~170 mAh g-1) [7] and spinel LiMn2O4 (~120 mAh g-1) [8] cannot meet the increasing requirements of the power density and energy density for HEVs and EVs [3,5]. Therefore, there is an urgent demand to explore new cathodes with large capacity and excellent rate capability for new generation rechargeable LIBs. Recently, Li-rich layered oxides xLi2MnO3·(1–x)LiMO2 (0 < x < 1, M = Mn, Ni, Fe, 2
Mn0.5Ni0.5, Mn0.33Ni0.33Co0.33…) have attracted considerable attention due to their high discharge capacity (> 200 mAh g-1), high operating potential (4.6-4.8 V vs. Li/Li+), low cost, hypotoxicity and high safety [9-11]. Among various Li-rich cathodes,
Li1.2Mn0.54Ni0.13Co0.13O2
(LMNC,
equivalently
0.5Li2MnO3·0.5LiMn0.33Ni0.33Co0.33O2) has been widely studied because of its large discharge capacity (~ 250 mAh g-1) and good stability [12,13]. However, Li-rich cathode materials possess a few disadvantages that limit their practical applications: (1) large irreversible capacity loss (ICL) in the 1st cycle, which is attributed to the removal of Li2O from the Li2MnO3 component during the 1st charging process; this process accompanies with the removing of oxygen-ion vacancies, leading to the reduction in Li+ ion insertion sites in the subsequent cycles [9,14,15]; (2) poor rate capability relating to the low electronic conductivity and poor lithium ion diffusion coefficient of Li2MnO3 component [9,16,17]; (3) fast capacity fading arising from the structure evolution from layered to spinel during further cycling [15,18,19]. Many approaches have been taken to solve these disadvantages of Li-rich cathodes, including structure and morphology control [20,21], mild acidic treatment [22-24], cationic substitution [25-28] and the formation of composite cathode with lithium-free insertion host materials [29,30]. Among these approaches, simple surface coating has been demonstrated to be an effective way to enhance cycling stability and rate capability. It has been proved that the coating layer could protect the active materials against the attack of hydrofluoric acid by separating them from electrolyte, suppress the oxygen loss by reducing the activity of oxygen ions, and retain the oxygen-ion 3
vacancies [31,32]. Many compounds have been explored as the coating layer, such as metallic phosphates [34,35], oxides [36-38] and fluorides [39-41]. Oxides of lanthanide series (e.g., Ce2O3, Sm2O3, etc.) have been frequently explored as effective surface modifiers for cathode materials of LIBs because of their high voltage stability and great thermal stability [36,42]. Many researchers have reported that lanthanide oxides can significantly improve electrochemical properties of cathode materials [43-46]. For instance, G.T. K. Fey et al. [47] successfully coated layered LiCoO2 with La2O3 by polymeric method and enhanced the cycling stability and thermal stability of the bare sample. L. Feng et al. [48] found that the cycling stability and rate performance of the spinel LiMn2O4 can be enhanced with the coating of 5 wt% nano-La2O3. In addition, P. Mohan et al. [49] reported that 2 wt% La2O3-coated LiNiO2 electrode has satisfied structural stability (187 mAh g-1 at 0.5 C after 60 cycles with 5% capacity loss), good reversibility and enhanced rate capability (152 mAh g-1 at 5 C) compared with the uncoated sample. It can be clearly noted that La2O3 is an excellent modifier. Therefore, it can be reasonably anticipated that the coating of La2O3 may effectively enhance the specific capacity, rate performance and cycling stability of Li-rich layered oxide LMNC. In this work, La2O3-coated LMNC materials were fabricated via a combined approach of sol-gel and wet chemical processes. The structure, morphology and electrochemical performance of the pristine and La2O3-coated LMNC samples were investigated. Our results show that the La2O3-coated LMNC electrodes exhibit much higher specific capacity, better cyclic performance and rate capability than the bare 4
material.
2. Experimental
2.1. Sample preparation LMNC powders were prepared via a sol-gel method. Stoichiometric amounts of Manganous acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99%), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%) and cobaltous acetate tetrahydrate (Co(CH3COO)2·4H2O, 99.5%) were dissolved into distill water. Afterwards, the mixed solution of citric acid monohydrate (C6H8O7·H2O, 99.5%) and lithium acetate dehydrate (Li(CH3COO)2·2H2O, 99%, 5% excess) with a molar ratio of about 1:1 were slowly dripped into the former solution under continually stirring. The pH value of the resulting mixtures was adjusted to ~ 7.5 by ammonium hydroxide (25-28%). The solution was heated at 80 oC until the purple gel was obtained. The prepared gel was then heat treated at 120 oC for 12 h in the vacuum oven, sequentially preheated in a muffle furnace at 450 oC for 5 h. The resulting powders were pressed into disk samples and sintered in the furnace at 900 oC for 10 h. La2O3-coated LMNC (LMNC-L) was fabricated via a wet chemical process. Stoichiometric amounts of lanthanum nitrate hexahydrate (La(NO3)3·6H2O, 99.9%) was dissolved in distill water. The prepared LMNC powders were dispersed in the solution containing La(NO3)3·6H2O. Ammonium hydroxide was dripped into the mixture. Afterward, the mixture was stirred at 80 oC to evaporate water. The obtained 5
products were sintered at 500 oC for 5 h in air to get LMNC-L powders. The coating amounts of La2O3 were set at 0.5, 1.5, 2.5, 3 and 5 wt% and the corresponding products were referred as LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5, respectively.
2.2. Sample characterizations The crystalline structure of the LMNC and LMNC-L materials were characterized using X-ray diffraction (XRD) analysis with Cu Kα radiation (SmartLab, Rigaku, Tokyo, Japan) at a scan rate of 1o min-1 in the 2 range of 10-80o. The microstructure of LMNC, LMNC-L2.5 and LMNC-L5 powders was examined using transmission electron microscopy (TEM, JEM2100). A field-emission scanning electron microscope (FE-SEM, JSM-7500, Japan) equipped with energy dispersive spectrum X-ray detector (EDS) was used for the observation of the morphologies, element composition and distribution for all the materials.
2.3. Electrochemical measurements The cell cathodes were fabricated by dissolving active material, conductive agents (acetylene black) and PVDF binder (at a mass ratio of 8:1:1) into NMP solvent with thoroughly stirring to get a homogenous slurry. The black and ropy slurry was coated onto Al current collectors and heat treated at 120 oC for 12 h. The CR-2025 coin-type half-cells were assembled and sealed in an Ar-filled glove box (MB-Labstar, Germany). The electrolyte consists of 1 mol L-1 LiPF6 dissolved in ethylene 6
carbonate-dimethyl carbonate (at a volume ratio of 1:1). Metallic lithium plates were used as reference electrodes and Celgard 2400 were the separators. The galvanostatic charge and discharge tests were measured using a cell test system (LANHE CT-2001A, Wuhan, China) between 2.0 and 4.8 V (vs. Li/Li+) at different rates at room temperature. The cyclic voltammograms (CV) for the cells were recorded on an electrochemical station (CHI660E, Shanghai, China) at a scanning rate of 0.1mV s -1 in the voltage range of 2.0-4.8 V. Electrochemical impedance spectroscopy (EIS) measurements of the cells were carried out after 1 cycle and 30 cycles at 0.2 C and monitored at the electrochemical station (CHI660E, Shanghai, China) in the frequency range of 0.01-100000 Hz at a voltage amplitude of 5 mV. Zview2 software was used for the simulation of the EIS results.
3. Results and discussion
3.1. Structure and morphology of LMNC and LMNC-L Fig. 1 shows the XRD patterns of LMNC and LMNC-L. It can be seen that all the materials possess a characteristic XRD pattern of Li-rich layered oxides, indicating that the surface coating of La2O3 has no significant influence on the structure of LMNC materials. For LMNC and LMNC-L materials, the strong diffraction peaks are —
indexed to a hexagonal layered -NaFeO2 structure (space group symmetry: R3m). In addition, the extra diffraction peaks between 20o and 25o are corresponding to the LiMn6 cation arrangement in the transition metal layer of monoclinic Li2MnO3 with 7
C2/m space group. All the materials are proved to be with a well-layered structure by the completely splitting of (006)/(012) and (118)/(110) peaks [25,41]. In general, the I(003)/I(104) intensity ratio (R) is an important symbol to characterize the extent of cation mixing between Ni2+ and Li+ due to their similar ionic radii [38], and it was found that the serious cation mixing comes up when R < 1.2. The value of R increases from 1.17 for the bare LMNC to 1.679, 1.501, 1.734, 1.697 and 1.546 for the LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 samples, respectively, confirming that the LMNC-L samples have lower degree of cation mixing. The much lower cation mixing is favorable to the improvement of cycling stability in the La2O3-coated LMNC materials [36]. The diffraction peaks of La2O3 cannot be observed in Fig.1, which may be due to the low content or low crystalline degree of La2O3 calcined at low temperatures [50]. Similar phenomenon was observed in the CeO2-coated materials [36]. Fig. 2 presents the SEM images of LMNC and LMNC-L powders. From Fig. 2a, the bare LMNC sample is composed of nanoparticles with the sizes of 100~400 nm. After modification with 0.5~2.5 wt% La2O3, the morphology and particle size of LMNC-L0.5, LMNC-L1.5 and LMNC-L2.5 samples (Figs. 2b, c and d) have no obvious change due to the low content of La2O3. These samples with a well-defined morphology have high crystallinity and loose aggregation. When the content of La2O3 increases from 3 wt% to 5 wt%, LMNC-L3 and LMNC-L5 samples (Figs. 2e and f) exhibit inhomogeneous morphology, serious agglomeration and smaller particle size. The EDS spectra of LMNC and LMNC-L samples are given in Fig.3, while Fig. 4 8
exhibits the EDS elemental mapping images of LMNC-L2.5. As shown in Fig. 3, it can be clearly observed that lanthanum is present for all of LMNC-L. From Fig. 3a, the peaks of lanthanum are not found in the pristine LMNC. As the coating level of lanthanum element increases, the peak intensity of lanthanum element increases gradually (Figs. 3b-f). The peaks of Mn, Ni, Co and O are similar for all the samples, indicating that the surface modification of La2O3 has no obvious influence on the elemental composition of the materials. From Fig. 4, Mn, Ni, Co, O and La are homogeneously distributed in the material, which suggests that LMNC particles are successfully coated with La2O3. In order to further reveal the morphology and microstructure of LMNC and LMNC-L, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM) and fast fourier transform pattern (FFT) images have been analyzed and shown in Fig. 5. Compared with the bare LMNC, the morphology of the La2O3-modified LMNC exhibits slight sticky (Figs. 5a1, b1 and c1). From Figs. 5a2, b2 and c2, it can be seen that the LMNC and La2O3-modified LMNC materials consist of sphere-like nanoparticles. The LMNC shows good crystallinity with clear interference fringes that can extend to the edge of the particle as shown in Fig. 5a3. A thin La2O3 layer is clearly observed on the particle edge of LMNC-L2.5 (~ 2 nm) and LMNC-L5 (~ 5 nm) in Figs. 5b3 and c3. Compared with the clear interference fringes in LMNC and LMNC-L2.5 (Fig. 5a3 and b3), the LMNC-L5 sample exhibits much lower crystallinity with blurry interference fringes (Fig. 5c3). This may be ascribed to the diffusion of some La3+ ions into the host structure of LMNC [5]. Similar 9
phenomenon has been observed in ZrF4-coated and LiAlSiO4-coated Li-rich cathode materials [50,51]. From Figs. 5a3 and b3, the lattice spacing of 0.48 nm is indexed to —
the planar spacing of (003) plane of the layered R3m phase for the LMNC and LMNC-L2.5 samples. The lattice spacings of 0.43 and 0.25 nm for LMNC-L5 (Fig. 5c3) correspond to the (020) plane of monoclinic C/2m phase and the (101) plane of —
layered R3m phase, respectively. The inset FFT images can further confirm the above results.
3.2. Electrochemical performance of LMNC and LMNC-L electrodes
Fig. 6 exhibits the initial charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C (1 C = 250 mAh g-1). From Fig. 6, all the initial charge profiles have two voltage plateaus: one is located at 3.7~4.4 V and the other is at about 4.5 V. The first plateau is connected with the extraction of Li+ from LiNi0.33Co0.33Mn0.33O2 phase along with the oxidation of transition metal ions (Ni2+ Ni4+ and Co3+ Co4+). The second plateau is related to the activation of the electrochemical inactive Li2MnO3. At 4.5 V plateau, Li2O is irreversibly removed from the Li2MnO3 lattice, which leads to the formation of the electrochemical active [MnO2] component [11]. It can be seen that the charge profiles of the LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes lie below that of LMNC electrode, while the discharge profiles of LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes are above that of LMNC electrode. The phenomenon implies that the appropriate content of La2O3 can lower the charge potential plateaus and elevate the discharge potential plateaus. The lower 10
charge potential plateaus and higher discharge potential plateaus indicate the lower polarization of 1.5 ~ 3 wt% La2O3-coated LMNC compared to the bare LMNC electrode. This should be ascribed to the improved electronic conductivity and lithium ion diffusion induced by surface coating layers [52]. Compared with the initial discharge capacity of LMNC electrode (242.2 mAh g-1), LMNC-L0.5 electrode presents a close value of 241.7 mAh g-1. However, when compared with LMNC and LMNC-L0.5 electrodes, LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes deliver much higher discharge capacities of 259.2, 276.9 and 250.8 mAh g-1, respectively. Since excess coating of La2O3 significantly degrades the specific capacity of the material, LMNC-L5 electrode exhibits the minimum discharge capacity of 184.7 mAh g-1. All the electrodes have a large ICL, which should be attributed to the reduction of oxygen-ion vacancies and Li+ sites along with the secondary reaction of electrolyte at high working voltages [53]. It can be concluded that the coating of 1.5 ~ 3 wt% La2O3 can effectively enhance the initial discharge capacity of LMNC due to the acceleration of the Li+ interfacial diffusion kinetics, which is verified by the increased discharge plateau [51]. In addition, the low discharge capacity of LMNC-L5 electrode is due to the excessively thick coating layer, which increases the Li+ diffusion path and decreases the electronic tunneling rate [51]. The charge/discharge profiles of LMNC and LMNC-L electrodes in the 1st, 2nd, 3rd, 10th, 50th and 100th cycles at 0.1 C are shown in Fig. 7. From Figs. 7a and b, the discharge capacities of LMNC and LMNC-L0.5 electrodes successively fade with cycle number increasing from 1 to 100. However, the first three discharge capacities 11
of LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes increase gradually, indicating that the complete activation of Li2MnO3 phase takes several cycles after coating with La2O3 [41,54]. In other words, the activation of electrochemical inactive Li2MnO3 is delayed by the La2O3 coating layer. For LMNC-L electrodes, the activation process in the subsequent cycles may be attributed to the strong interface interaction that stabilizes the electrodes in the electrochemical environment [54]. Therefore, the La2O3 coating layer can stabilize the surface structure of LMNC. After the activation process, the discharge capacities of LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes reach the maximum values of 275.9, 282.0, 260.6 and 202.0 mAh g-1 in the 3rd cycle, respectively. For all the samples, the discharge profiles move to a lower voltage plateau with increasing the cycle number, which implies the increment of polarization with cycling. As marked by a black arrow in Fig. 7a, the discharge midpoint potential for the LMNC electrode decays 0.72 V (V) from the 1st to 100th cycles. However, with La2O3 coating, the LMNC-L electrodes present smaller V values in comparison with that of LMNC electrode. It is because the La2O3 layer can stabilize the extraction/reinsertion of Li+ in the interface of electrode/electrolyte and enhance the structural stability of LMNC cathode [50,54]. Fig. 8a shows the cycling performance of LMNC and LMNC-L electrodes during 100 cycles at 0.1 C, while Fig. 8b exhibits the variations of discharge capacity and capacity retention in the 100th cycle with the content of La2O3. From Fig. 8a, it can be clearly seen that 1.5 ~ 3 wt% La2O3-coated LMNCL electrodes exhibit much higher 12
discharge capacity than the bare LMNC electrode during long-term cycling. From Fig. 8b, the discharge capacity of LMNC electrode decreases to 140.5 mAh g-1 after 100 cycles and just remains 58.3% of the initial discharge capacity. However, after coating with La2O3 layer, the discharge capacity and capacity retention of the samples after 100 cycles are significantly enhanced. Compared with the bare LMNC material, all the La2O3-coated materials deliver much higher discharge capacities of 148.1, 188.9, 201.4, 178.2 and 150.9 mAh g-1 with capacity retentions of 61.3%, 72.7%, 71%, 71.1% and 81.7% after 100 cycles for LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes, respectively. The LMNC-L2.5 electrode possesses the highest specific capacity among all samples, which implies that 2.5 wt% La2O3-coating is the optimal content for improving the discharge capacity and cycling performance of LMNC electrode. For the materials coated with La2O3, the La2O3 coating layer can hinder cathodes from directly contacting electrolyte so as to protect the cathodes against the erosion in electrolyte, thus stabilizing the structure of cathode material [55]. Consequently, the cycling stability of the materials is significantly improved. The rate capability of the LMNC and LMNC-L electrodes at various rates and the variations of discharge capacity with discharge current density are shown in Figs. 9a and 9b, respectively. The cells are charged at 0.1 C and then discharged at different rates for 5 cycles. The discharge capacity for all the electrodes gradually reduces with increasing discharge rate due to the increased polarization at high current density [56]. It can be seen that LMNC-L1.5 and LMNC-L2.5 electrodes deliver higher discharge 13
capacities at different discharge rates than other electrodes as shown in Fig. 9, reflecting that the 1.5 ~ 2.5 wt% La2O3-coated LMNCL materials exhibit excellent rate performance. The discharge capacities at 5 C of LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes are 55.2, 90.2, 77.4, 74 and 80.8 mAh g-1, respectively, which are higher than that of LMNC electrode (32.6 mAh g-1). This implies the surface modification with La2O3 can greatly improve the rate performance of LMNC material. The above results reveal that the La2O3 coating layer can accelerate the rate of Li+ transport and improve ion exchange during the charge-discharge process. Fig. 10 displays the cycle voltammograms (CV) of LMNC and LMNC-L electrodes. All the samples exhibit two anodic peaks at ~ 4.1 V and 4.6 V in the 1st cycle, relating to the two voltage plateaus in the initial charge profiles. The 4.1 V anodic peak is related to the oxidation of transition metal ions (Ni2+ Ni4+ and Co3+ Co4+), while the 4.6 V anodic peak is due to the irreversible elimination of Li2O from Li2MnO3 to form MnO2 compound. From Fig. 10a, the anodic peak at 4.6 V in the 1st charging process is strong and narrow for the LMNC electrode, but it becomes weak and broad after surface modification with La2O3 (Figs. 10b-d). The weak and broad anodic peak at 4.6 V for LMNC-L electrodes implies a relaxative oxygen loss process or dissolution of electrolyte at high potential voltages [4,25]. This phenomenon suggests that the La2O3 layer may reduce the structural change and side reactions of electrolyte, and thus improve the cycling stability. In addition, the 4.6 V anodic peak for LMNC electrode vanishes away in subsequent cycles, while it is still distinct for the LMNC-L 14
electrodes in the previous several cycles because of the continuous activation of Li2MnO3 phase. As a result, the increased discharge capacity is exhibited in the initial three cycles (Figs. 7c-f). Meanwhile, for all samples, the two cathodic peaks at 3.7 V and 4.4 V are attributed to the reduction of transition metal ions (Ni4+ Ni2+ and Co4+ Co3+, respectively). Another cathodic peak at 3.2 V appearing in the second cycle for LMNC electrode is related to the reduction of Mn4+ to Mn3+ that balances the charge of oxygen vacancies caused by the oxygen loss in the initial charge process [4]; however, it can be only observed in the 4th and 5th cycles for LMNC-L electrodes after the complete activation of Li2MnO3 phase. This phenomenon also suggests that the oxygen loss can be alleviated. In addition, the CV curves of LMNC-L electrodes in the 4th and 5th cycles are highly overlapped, demonstrating an excellent cyclic reversibility that agrees with the improvement in the cycling stability. EIS measurements are carried out to comprehend the enhanced electrochemical properties and investigate the electrochemical kinetics of the La2O3-modified LMNC electrodes. Figs. 11a and 11b present the Nyquist plots of LMNC and LMNC-L electrodes after 1 cycle and 30 cycles, respectively. The equivalent circuit shown in Fig. 11c is used to further analyze the Nyquist plots. In general, the Nyquist plot consists of three parts: a high-frequency semicircle that is attributed to the lithium ion diffusion through the surface layer (including solid electrolyte interface (SEI) film and coated layer); a mid-frequency semicircle that is assigned to the charge transfer resistance in the interface of electrode/electrolyte, and a low-frequency slope corresponding to the Warburg impedance that is correlated to the lithium ion diffusion 15
process in electrode materials [57]. The simulated electrochemical parameters from EIS spectra are shown in Table 1. In the equivalent circuit (Fig. 11c), Rs, Rsl and Rct represent the internal resistance of the cell, the surface layer resistance and the charge transfer resistance, respectively. From Table 1, all the samples have a quite small Rs, indicating that the three investigated materials have negligible ohmic polarization [58,59]. The value of Rsl for LMNC electrode increases from 228 Ω to 432 Ω after 30 cycles, suggesting the fast growth of SEI film and structural instability on the surface. However, unlike the bare LMNC, as the cycle number increases from 1 to 30, the Rsl value for LMNC-L1.5 electrode decreases from 83 Ω to 52 Ω, while that for LMNC-L2.5 electrode decreases slightly from 41 Ω to 35 Ω. Moreover, the value of Rct for LMNC electrode drastically increases from 5062 Ω to 12789 Ω (Rct = 7727 Ω), while those of LMNC-L1.5 and LMNC-L2.5 electrodes only show slight change from 2973 Ω and 3843 Ω at 1st cycle to 3048 Ω and 3912 Ω at 30th cycle (Rct = 75 Ω and 65 Ω), respectively. The sharp decrement of Rsl, Rct and Rct suggests that the La2O3 coating layer can reduce the secondary reactions between cathode materials and electrolyte by hindering the direct contract between cathode materials and electrolyte, alleviate the growth of SEI film and accelerate the Li+ interfacial diffusion kinetics between electrode and electrolyte. These would result in the significantly enhanced electrochemical performance of the La2O3-coated LMNC materials.
4. Conclusions
16
La2O3-coated LMNC cathodes with significantly enhanced specific capacity, rate capability and cycling performance were fabricated via a combined method of sol-gel and wet chemical processes. A thin La2O3 layer is evenly coated on the surface of LMNC particles. There is no distinct influence on the crystal structure of the materials after modification with La2O3. The surface modification of La2O3 leads to the great enhancement in the specific capacity, cycling stability and rate capability of the Li-rich cathode. The optimal La2O3-coating level is 2.5 wt%. The LMNC coated with 2.5 wt% La2O3 presents the highest discharge capacity, greatly enhanced cycling stability and sharply reduced charge transfer resistance, which delivers a large discharge capacity of 276.9 mAh g-1 in the 1st cycle and high capacity retention of 71% (201.4 mAh g-1) after 100 cycles. The Li-rich oxide with the coating level of 1.5 wt% La2O3 exhibits the optimal rate capability. The improved electrochemical performance of La2O3-modified LMNC can be ascribed to the La2O3 layer that reduces the side reactions between cathodes and electrolyte by hindering the direct contract of cathodes and electrolyte, alleviates the growth of SEI film and accelerates the Li+ interfacial diffusion kinetics between electrode and electrolyte. Our study shows that La2O3 is an efficient modifier to enhance the electrochemical performance of Li-rich cathode materials. The La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 cathodes with enhanced specific capacity, rate capability and cycling stability are the most promising candidate for new generation rechargeable lithium-ion batteries.
Acknowledgement 17
This work was supported by large precision instrument projects of Sichuan Normal University (DJ 2015-48 and DJ 2015-42).
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26
Figure captions Fig. 1 XRD patterns of LMNC and LMNC-L. Fig. 2 SEM images of LMNC and LMNC-L: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 3 EDS spectrums of LMNC and LMNC-L: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 4 EDS elemental mapping images of LMNC-L2.5. Fig. 5 TEM images of LMNC (a1-a2), LMNC-L2.5 (b1-b2), LMNC-L5 (c1-c2); HRTEM images of LMNC (a3), LMNC-L2.5 (b3), LMNC-L5 (c3) (the insert is the corresponding fast fourier transform pattern). Fig. 6 Initial charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C between 2.0 and 4.8 V Fig. 7 Charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C between 2.0 and 4.8 V: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 8 (a) Cycling performances of LMNC and LMNC-L electrodes during 100 cycles at 0.1 C between 2.0 and 4.8 V, (b) variations of discharge capacity and capacity retention in the 100th cycle with content of La2O3. Fig. 9 (a) Rate capability of the LMNC and LMNC-L electrodes at various rates between 2.0 and 4.8 V, (b) variations of discharge capacity with discharge current density. Fig. 10 Cycle voltammograms of LMNC and LMNC-L electrodes between 2.0 and 27
4.8 V, at a scan rate of 0.1 mV s-1: (a) LMNC, (b) LMNC-L1.5, (c) LMNC-L2.5, (d) LMNC-L5. Fig. 11 Nyquist plots of LMNC and LMNC-L electrodes: (a) after 1 cycle, (b) after 30 cycles and (c) the equivalent circuit for the impedance spectra.
Table 1 Simulated data from EIS spectra of the LMNC and LMNC-L electrodes after 1 cycle and 30 cycles. after 1cycle
after 30 cycles
Rs (Ω)
Rsl (Ω)
Rct (Ω)
Rs (Ω)
Rsl (Ω)
Rct (Ω)
Rct (Ω)
LMNC
3
228
5062
9
432
12789
7727
LMNC-L1.5
2
83
2973
11
52
3048
75
LMNC-L2.5
5
41
3843
5
35
3912
65
28
29
30
31
32
PII: DOI: Reference:
S0272-8842(16)31073-2 http://dx.doi.org/10.1016/j.ceramint.2016.07.016 CERI13241
To appear in: Ceramics International Received date: 27 May 2016 Revised date: 1 July 2016 Accepted date: 3 July 2016 Cite this article as: Lin Zhou, Mijie Tian, Yunlong Deng, Qiaoji Zheng, Chenggang Xu and Dunmin Lin, La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.07.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 as cathode materials with enhanced specific capacity and cycling stability for lithium-ion batteries Lin Zhou, Mijie Tian, Yunlong Deng, Qiaoji Zheng, Chenggang Xu, Dunmin Lin* College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610066, China *
Corresponding author: Tel.: +86 [email protected] (Dunmin Lin)
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84760802;
Fax:
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E-mail:
Abstract Li-rich layered oxides are the most promising cathode candidate for new generation rechargeable
lithium-ion
batteries.
In
this
work,
La2O3-coated
Li1.2Mn0.54Ni0.13Co0.13O2 cathode materials were fabricated via a combined method of sol-gel
and
wet
chemical
processes.
The
structural
and
morphological
characterizations of the materials demonstrate that a thin layer of La2O3 is uniformly covered on the surface of Li1.2Mn0.54Ni0.13Co0.13O2 particles, and the coating of La2O3 has no obvious effect on the crystal structure of Li-rich oxide. The electrochemical performance of La2O3-coated Li-rich cathodes including specific capacity, cycling stability and rate capability has been significantly improved with the coating of La2O3. The Li1.2Mn0.54Ni0.13Co0.13O2 coated with 2.5 wt% La2O3 exhibits the highest discharge capacity, improved cycling stability and reduced charge transfer resistance, delivering a large discharge capacity of 276.9 mAh g-1 in the 1st cycle and a high capacity retention of 71% (201.4 mAh g-1) after 100 cycles. The optimal rate capability of the materials is observed at the coating level of 1.5 wt% La2O3 such that 1
the material exhibits the highest discharge capacity of 90.2 mAh g-1 at 5 C. The surface coating of La2O3 can effectively facilitate Li+ interfacial diffusion, reduce the structural change and secondary reactions between cathode materials and electrolyte during the charge-discharge process, and thus induce the great enhancement in the electrochemical properties of the Li1.2Mn0.54Ni0.13Co0.13O2 materials.
Key words: lithium-ion batteries; Li-rich layered oxide; La2O3 coating layer; electrochemical performance
1. Introduction Lithium-ion batteries (LIBs) have been extensively used in information, transportation, military and other fields due to their high energy density, high operating voltage, and environment friendliness [1,2]. They are also deemed as the most viable energy storage devices for hybrid electric vehicles (HEVs) and electric vehicles (EVs) [3,4]. As known, the energy density of the LIBs relies heavily on the cathode materials [5]. However, most of the extensively investigated cathodes of LIBs including layered LiCoO2 (~140 mAh g-1) [6], olivine LiFeO4 (~170 mAh g-1) [7] and spinel LiMn2O4 (~120 mAh g-1) [8] cannot meet the increasing requirements of the power density and energy density for HEVs and EVs [3,5]. Therefore, there is an urgent demand to explore new cathodes with large capacity and excellent rate capability for new generation rechargeable LIBs. Recently, Li-rich layered oxides xLi2MnO3·(1–x)LiMO2 (0 < x < 1, M = Mn, Ni, Fe, 2
Mn0.5Ni0.5, Mn0.33Ni0.33Co0.33…) have attracted considerable attention due to their high discharge capacity (> 200 mAh g-1), high operating potential (4.6-4.8 V vs. Li/Li+), low cost, hypotoxicity and high safety [9-11]. Among various Li-rich cathodes,
Li1.2Mn0.54Ni0.13Co0.13O2
(LMNC,
equivalently
0.5Li2MnO3·0.5LiMn0.33Ni0.33Co0.33O2) has been widely studied because of its large discharge capacity (~ 250 mAh g-1) and good stability [12,13]. However, Li-rich cathode materials possess a few disadvantages that limit their practical applications: (1) large irreversible capacity loss (ICL) in the 1st cycle, which is attributed to the removal of Li2O from the Li2MnO3 component during the 1st charging process; this process accompanies with the removing of oxygen-ion vacancies, leading to the reduction in Li+ ion insertion sites in the subsequent cycles [9,14,15]; (2) poor rate capability relating to the low electronic conductivity and poor lithium ion diffusion coefficient of Li2MnO3 component [9,16,17]; (3) fast capacity fading arising from the structure evolution from layered to spinel during further cycling [15,18,19]. Many approaches have been taken to solve these disadvantages of Li-rich cathodes, including structure and morphology control [20,21], mild acidic treatment [22-24], cationic substitution [25-28] and the formation of composite cathode with lithium-free insertion host materials [29,30]. Among these approaches, simple surface coating has been demonstrated to be an effective way to enhance cycling stability and rate capability. It has been proved that the coating layer could protect the active materials against the attack of hydrofluoric acid by separating them from electrolyte, suppress the oxygen loss by reducing the activity of oxygen ions, and retain the oxygen-ion 3
vacancies [31,32]. Many compounds have been explored as the coating layer, such as metallic phosphates [34,35], oxides [36-38] and fluorides [39-41]. Oxides of lanthanide series (e.g., Ce2O3, Sm2O3, etc.) have been frequently explored as effective surface modifiers for cathode materials of LIBs because of their high voltage stability and great thermal stability [36,42]. Many researchers have reported that lanthanide oxides can significantly improve electrochemical properties of cathode materials [43-46]. For instance, G.T. K. Fey et al. [47] successfully coated layered LiCoO2 with La2O3 by polymeric method and enhanced the cycling stability and thermal stability of the bare sample. L. Feng et al. [48] found that the cycling stability and rate performance of the spinel LiMn2O4 can be enhanced with the coating of 5 wt% nano-La2O3. In addition, P. Mohan et al. [49] reported that 2 wt% La2O3-coated LiNiO2 electrode has satisfied structural stability (187 mAh g-1 at 0.5 C after 60 cycles with 5% capacity loss), good reversibility and enhanced rate capability (152 mAh g-1 at 5 C) compared with the uncoated sample. It can be clearly noted that La2O3 is an excellent modifier. Therefore, it can be reasonably anticipated that the coating of La2O3 may effectively enhance the specific capacity, rate performance and cycling stability of Li-rich layered oxide LMNC. In this work, La2O3-coated LMNC materials were fabricated via a combined approach of sol-gel and wet chemical processes. The structure, morphology and electrochemical performance of the pristine and La2O3-coated LMNC samples were investigated. Our results show that the La2O3-coated LMNC electrodes exhibit much higher specific capacity, better cyclic performance and rate capability than the bare 4
material.
2. Experimental
2.1. Sample preparation LMNC powders were prepared via a sol-gel method. Stoichiometric amounts of Manganous acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99%), nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 98%) and cobaltous acetate tetrahydrate (Co(CH3COO)2·4H2O, 99.5%) were dissolved into distill water. Afterwards, the mixed solution of citric acid monohydrate (C6H8O7·H2O, 99.5%) and lithium acetate dehydrate (Li(CH3COO)2·2H2O, 99%, 5% excess) with a molar ratio of about 1:1 were slowly dripped into the former solution under continually stirring. The pH value of the resulting mixtures was adjusted to ~ 7.5 by ammonium hydroxide (25-28%). The solution was heated at 80 oC until the purple gel was obtained. The prepared gel was then heat treated at 120 oC for 12 h in the vacuum oven, sequentially preheated in a muffle furnace at 450 oC for 5 h. The resulting powders were pressed into disk samples and sintered in the furnace at 900 oC for 10 h. La2O3-coated LMNC (LMNC-L) was fabricated via a wet chemical process. Stoichiometric amounts of lanthanum nitrate hexahydrate (La(NO3)3·6H2O, 99.9%) was dissolved in distill water. The prepared LMNC powders were dispersed in the solution containing La(NO3)3·6H2O. Ammonium hydroxide was dripped into the mixture. Afterward, the mixture was stirred at 80 oC to evaporate water. The obtained 5
products were sintered at 500 oC for 5 h in air to get LMNC-L powders. The coating amounts of La2O3 were set at 0.5, 1.5, 2.5, 3 and 5 wt% and the corresponding products were referred as LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5, respectively.
2.2. Sample characterizations The crystalline structure of the LMNC and LMNC-L materials were characterized using X-ray diffraction (XRD) analysis with Cu Kα radiation (SmartLab, Rigaku, Tokyo, Japan) at a scan rate of 1o min-1 in the 2 range of 10-80o. The microstructure of LMNC, LMNC-L2.5 and LMNC-L5 powders was examined using transmission electron microscopy (TEM, JEM2100). A field-emission scanning electron microscope (FE-SEM, JSM-7500, Japan) equipped with energy dispersive spectrum X-ray detector (EDS) was used for the observation of the morphologies, element composition and distribution for all the materials.
2.3. Electrochemical measurements The cell cathodes were fabricated by dissolving active material, conductive agents (acetylene black) and PVDF binder (at a mass ratio of 8:1:1) into NMP solvent with thoroughly stirring to get a homogenous slurry. The black and ropy slurry was coated onto Al current collectors and heat treated at 120 oC for 12 h. The CR-2025 coin-type half-cells were assembled and sealed in an Ar-filled glove box (MB-Labstar, Germany). The electrolyte consists of 1 mol L-1 LiPF6 dissolved in ethylene 6
carbonate-dimethyl carbonate (at a volume ratio of 1:1). Metallic lithium plates were used as reference electrodes and Celgard 2400 were the separators. The galvanostatic charge and discharge tests were measured using a cell test system (LANHE CT-2001A, Wuhan, China) between 2.0 and 4.8 V (vs. Li/Li+) at different rates at room temperature. The cyclic voltammograms (CV) for the cells were recorded on an electrochemical station (CHI660E, Shanghai, China) at a scanning rate of 0.1mV s -1 in the voltage range of 2.0-4.8 V. Electrochemical impedance spectroscopy (EIS) measurements of the cells were carried out after 1 cycle and 30 cycles at 0.2 C and monitored at the electrochemical station (CHI660E, Shanghai, China) in the frequency range of 0.01-100000 Hz at a voltage amplitude of 5 mV. Zview2 software was used for the simulation of the EIS results.
3. Results and discussion
3.1. Structure and morphology of LMNC and LMNC-L Fig. 1 shows the XRD patterns of LMNC and LMNC-L. It can be seen that all the materials possess a characteristic XRD pattern of Li-rich layered oxides, indicating that the surface coating of La2O3 has no significant influence on the structure of LMNC materials. For LMNC and LMNC-L materials, the strong diffraction peaks are —
indexed to a hexagonal layered -NaFeO2 structure (space group symmetry: R3m). In addition, the extra diffraction peaks between 20o and 25o are corresponding to the LiMn6 cation arrangement in the transition metal layer of monoclinic Li2MnO3 with 7
C2/m space group. All the materials are proved to be with a well-layered structure by the completely splitting of (006)/(012) and (118)/(110) peaks [25,41]. In general, the I(003)/I(104) intensity ratio (R) is an important symbol to characterize the extent of cation mixing between Ni2+ and Li+ due to their similar ionic radii [38], and it was found that the serious cation mixing comes up when R < 1.2. The value of R increases from 1.17 for the bare LMNC to 1.679, 1.501, 1.734, 1.697 and 1.546 for the LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 samples, respectively, confirming that the LMNC-L samples have lower degree of cation mixing. The much lower cation mixing is favorable to the improvement of cycling stability in the La2O3-coated LMNC materials [36]. The diffraction peaks of La2O3 cannot be observed in Fig.1, which may be due to the low content or low crystalline degree of La2O3 calcined at low temperatures [50]. Similar phenomenon was observed in the CeO2-coated materials [36]. Fig. 2 presents the SEM images of LMNC and LMNC-L powders. From Fig. 2a, the bare LMNC sample is composed of nanoparticles with the sizes of 100~400 nm. After modification with 0.5~2.5 wt% La2O3, the morphology and particle size of LMNC-L0.5, LMNC-L1.5 and LMNC-L2.5 samples (Figs. 2b, c and d) have no obvious change due to the low content of La2O3. These samples with a well-defined morphology have high crystallinity and loose aggregation. When the content of La2O3 increases from 3 wt% to 5 wt%, LMNC-L3 and LMNC-L5 samples (Figs. 2e and f) exhibit inhomogeneous morphology, serious agglomeration and smaller particle size. The EDS spectra of LMNC and LMNC-L samples are given in Fig.3, while Fig. 4 8
exhibits the EDS elemental mapping images of LMNC-L2.5. As shown in Fig. 3, it can be clearly observed that lanthanum is present for all of LMNC-L. From Fig. 3a, the peaks of lanthanum are not found in the pristine LMNC. As the coating level of lanthanum element increases, the peak intensity of lanthanum element increases gradually (Figs. 3b-f). The peaks of Mn, Ni, Co and O are similar for all the samples, indicating that the surface modification of La2O3 has no obvious influence on the elemental composition of the materials. From Fig. 4, Mn, Ni, Co, O and La are homogeneously distributed in the material, which suggests that LMNC particles are successfully coated with La2O3. In order to further reveal the morphology and microstructure of LMNC and LMNC-L, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HR-TEM) and fast fourier transform pattern (FFT) images have been analyzed and shown in Fig. 5. Compared with the bare LMNC, the morphology of the La2O3-modified LMNC exhibits slight sticky (Figs. 5a1, b1 and c1). From Figs. 5a2, b2 and c2, it can be seen that the LMNC and La2O3-modified LMNC materials consist of sphere-like nanoparticles. The LMNC shows good crystallinity with clear interference fringes that can extend to the edge of the particle as shown in Fig. 5a3. A thin La2O3 layer is clearly observed on the particle edge of LMNC-L2.5 (~ 2 nm) and LMNC-L5 (~ 5 nm) in Figs. 5b3 and c3. Compared with the clear interference fringes in LMNC and LMNC-L2.5 (Fig. 5a3 and b3), the LMNC-L5 sample exhibits much lower crystallinity with blurry interference fringes (Fig. 5c3). This may be ascribed to the diffusion of some La3+ ions into the host structure of LMNC [5]. Similar 9
phenomenon has been observed in ZrF4-coated and LiAlSiO4-coated Li-rich cathode materials [50,51]. From Figs. 5a3 and b3, the lattice spacing of 0.48 nm is indexed to —
the planar spacing of (003) plane of the layered R3m phase for the LMNC and LMNC-L2.5 samples. The lattice spacings of 0.43 and 0.25 nm for LMNC-L5 (Fig. 5c3) correspond to the (020) plane of monoclinic C/2m phase and the (101) plane of —
layered R3m phase, respectively. The inset FFT images can further confirm the above results.
3.2. Electrochemical performance of LMNC and LMNC-L electrodes
Fig. 6 exhibits the initial charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C (1 C = 250 mAh g-1). From Fig. 6, all the initial charge profiles have two voltage plateaus: one is located at 3.7~4.4 V and the other is at about 4.5 V. The first plateau is connected with the extraction of Li+ from LiNi0.33Co0.33Mn0.33O2 phase along with the oxidation of transition metal ions (Ni2+ Ni4+ and Co3+ Co4+). The second plateau is related to the activation of the electrochemical inactive Li2MnO3. At 4.5 V plateau, Li2O is irreversibly removed from the Li2MnO3 lattice, which leads to the formation of the electrochemical active [MnO2] component [11]. It can be seen that the charge profiles of the LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes lie below that of LMNC electrode, while the discharge profiles of LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes are above that of LMNC electrode. The phenomenon implies that the appropriate content of La2O3 can lower the charge potential plateaus and elevate the discharge potential plateaus. The lower 10
charge potential plateaus and higher discharge potential plateaus indicate the lower polarization of 1.5 ~ 3 wt% La2O3-coated LMNC compared to the bare LMNC electrode. This should be ascribed to the improved electronic conductivity and lithium ion diffusion induced by surface coating layers [52]. Compared with the initial discharge capacity of LMNC electrode (242.2 mAh g-1), LMNC-L0.5 electrode presents a close value of 241.7 mAh g-1. However, when compared with LMNC and LMNC-L0.5 electrodes, LMNC-L1.5, LMNC-L2.5 and LMNC-L3 electrodes deliver much higher discharge capacities of 259.2, 276.9 and 250.8 mAh g-1, respectively. Since excess coating of La2O3 significantly degrades the specific capacity of the material, LMNC-L5 electrode exhibits the minimum discharge capacity of 184.7 mAh g-1. All the electrodes have a large ICL, which should be attributed to the reduction of oxygen-ion vacancies and Li+ sites along with the secondary reaction of electrolyte at high working voltages [53]. It can be concluded that the coating of 1.5 ~ 3 wt% La2O3 can effectively enhance the initial discharge capacity of LMNC due to the acceleration of the Li+ interfacial diffusion kinetics, which is verified by the increased discharge plateau [51]. In addition, the low discharge capacity of LMNC-L5 electrode is due to the excessively thick coating layer, which increases the Li+ diffusion path and decreases the electronic tunneling rate [51]. The charge/discharge profiles of LMNC and LMNC-L electrodes in the 1st, 2nd, 3rd, 10th, 50th and 100th cycles at 0.1 C are shown in Fig. 7. From Figs. 7a and b, the discharge capacities of LMNC and LMNC-L0.5 electrodes successively fade with cycle number increasing from 1 to 100. However, the first three discharge capacities 11
of LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes increase gradually, indicating that the complete activation of Li2MnO3 phase takes several cycles after coating with La2O3 [41,54]. In other words, the activation of electrochemical inactive Li2MnO3 is delayed by the La2O3 coating layer. For LMNC-L electrodes, the activation process in the subsequent cycles may be attributed to the strong interface interaction that stabilizes the electrodes in the electrochemical environment [54]. Therefore, the La2O3 coating layer can stabilize the surface structure of LMNC. After the activation process, the discharge capacities of LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes reach the maximum values of 275.9, 282.0, 260.6 and 202.0 mAh g-1 in the 3rd cycle, respectively. For all the samples, the discharge profiles move to a lower voltage plateau with increasing the cycle number, which implies the increment of polarization with cycling. As marked by a black arrow in Fig. 7a, the discharge midpoint potential for the LMNC electrode decays 0.72 V (V) from the 1st to 100th cycles. However, with La2O3 coating, the LMNC-L electrodes present smaller V values in comparison with that of LMNC electrode. It is because the La2O3 layer can stabilize the extraction/reinsertion of Li+ in the interface of electrode/electrolyte and enhance the structural stability of LMNC cathode [50,54]. Fig. 8a shows the cycling performance of LMNC and LMNC-L electrodes during 100 cycles at 0.1 C, while Fig. 8b exhibits the variations of discharge capacity and capacity retention in the 100th cycle with the content of La2O3. From Fig. 8a, it can be clearly seen that 1.5 ~ 3 wt% La2O3-coated LMNCL electrodes exhibit much higher 12
discharge capacity than the bare LMNC electrode during long-term cycling. From Fig. 8b, the discharge capacity of LMNC electrode decreases to 140.5 mAh g-1 after 100 cycles and just remains 58.3% of the initial discharge capacity. However, after coating with La2O3 layer, the discharge capacity and capacity retention of the samples after 100 cycles are significantly enhanced. Compared with the bare LMNC material, all the La2O3-coated materials deliver much higher discharge capacities of 148.1, 188.9, 201.4, 178.2 and 150.9 mAh g-1 with capacity retentions of 61.3%, 72.7%, 71%, 71.1% and 81.7% after 100 cycles for LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes, respectively. The LMNC-L2.5 electrode possesses the highest specific capacity among all samples, which implies that 2.5 wt% La2O3-coating is the optimal content for improving the discharge capacity and cycling performance of LMNC electrode. For the materials coated with La2O3, the La2O3 coating layer can hinder cathodes from directly contacting electrolyte so as to protect the cathodes against the erosion in electrolyte, thus stabilizing the structure of cathode material [55]. Consequently, the cycling stability of the materials is significantly improved. The rate capability of the LMNC and LMNC-L electrodes at various rates and the variations of discharge capacity with discharge current density are shown in Figs. 9a and 9b, respectively. The cells are charged at 0.1 C and then discharged at different rates for 5 cycles. The discharge capacity for all the electrodes gradually reduces with increasing discharge rate due to the increased polarization at high current density [56]. It can be seen that LMNC-L1.5 and LMNC-L2.5 electrodes deliver higher discharge 13
capacities at different discharge rates than other electrodes as shown in Fig. 9, reflecting that the 1.5 ~ 2.5 wt% La2O3-coated LMNCL materials exhibit excellent rate performance. The discharge capacities at 5 C of LMNC-L0.5, LMNC-L1.5, LMNC-L2.5, LMNC-L3 and LMNC-L5 electrodes are 55.2, 90.2, 77.4, 74 and 80.8 mAh g-1, respectively, which are higher than that of LMNC electrode (32.6 mAh g-1). This implies the surface modification with La2O3 can greatly improve the rate performance of LMNC material. The above results reveal that the La2O3 coating layer can accelerate the rate of Li+ transport and improve ion exchange during the charge-discharge process. Fig. 10 displays the cycle voltammograms (CV) of LMNC and LMNC-L electrodes. All the samples exhibit two anodic peaks at ~ 4.1 V and 4.6 V in the 1st cycle, relating to the two voltage plateaus in the initial charge profiles. The 4.1 V anodic peak is related to the oxidation of transition metal ions (Ni2+ Ni4+ and Co3+ Co4+), while the 4.6 V anodic peak is due to the irreversible elimination of Li2O from Li2MnO3 to form MnO2 compound. From Fig. 10a, the anodic peak at 4.6 V in the 1st charging process is strong and narrow for the LMNC electrode, but it becomes weak and broad after surface modification with La2O3 (Figs. 10b-d). The weak and broad anodic peak at 4.6 V for LMNC-L electrodes implies a relaxative oxygen loss process or dissolution of electrolyte at high potential voltages [4,25]. This phenomenon suggests that the La2O3 layer may reduce the structural change and side reactions of electrolyte, and thus improve the cycling stability. In addition, the 4.6 V anodic peak for LMNC electrode vanishes away in subsequent cycles, while it is still distinct for the LMNC-L 14
electrodes in the previous several cycles because of the continuous activation of Li2MnO3 phase. As a result, the increased discharge capacity is exhibited in the initial three cycles (Figs. 7c-f). Meanwhile, for all samples, the two cathodic peaks at 3.7 V and 4.4 V are attributed to the reduction of transition metal ions (Ni4+ Ni2+ and Co4+ Co3+, respectively). Another cathodic peak at 3.2 V appearing in the second cycle for LMNC electrode is related to the reduction of Mn4+ to Mn3+ that balances the charge of oxygen vacancies caused by the oxygen loss in the initial charge process [4]; however, it can be only observed in the 4th and 5th cycles for LMNC-L electrodes after the complete activation of Li2MnO3 phase. This phenomenon also suggests that the oxygen loss can be alleviated. In addition, the CV curves of LMNC-L electrodes in the 4th and 5th cycles are highly overlapped, demonstrating an excellent cyclic reversibility that agrees with the improvement in the cycling stability. EIS measurements are carried out to comprehend the enhanced electrochemical properties and investigate the electrochemical kinetics of the La2O3-modified LMNC electrodes. Figs. 11a and 11b present the Nyquist plots of LMNC and LMNC-L electrodes after 1 cycle and 30 cycles, respectively. The equivalent circuit shown in Fig. 11c is used to further analyze the Nyquist plots. In general, the Nyquist plot consists of three parts: a high-frequency semicircle that is attributed to the lithium ion diffusion through the surface layer (including solid electrolyte interface (SEI) film and coated layer); a mid-frequency semicircle that is assigned to the charge transfer resistance in the interface of electrode/electrolyte, and a low-frequency slope corresponding to the Warburg impedance that is correlated to the lithium ion diffusion 15
process in electrode materials [57]. The simulated electrochemical parameters from EIS spectra are shown in Table 1. In the equivalent circuit (Fig. 11c), Rs, Rsl and Rct represent the internal resistance of the cell, the surface layer resistance and the charge transfer resistance, respectively. From Table 1, all the samples have a quite small Rs, indicating that the three investigated materials have negligible ohmic polarization [58,59]. The value of Rsl for LMNC electrode increases from 228 Ω to 432 Ω after 30 cycles, suggesting the fast growth of SEI film and structural instability on the surface. However, unlike the bare LMNC, as the cycle number increases from 1 to 30, the Rsl value for LMNC-L1.5 electrode decreases from 83 Ω to 52 Ω, while that for LMNC-L2.5 electrode decreases slightly from 41 Ω to 35 Ω. Moreover, the value of Rct for LMNC electrode drastically increases from 5062 Ω to 12789 Ω (Rct = 7727 Ω), while those of LMNC-L1.5 and LMNC-L2.5 electrodes only show slight change from 2973 Ω and 3843 Ω at 1st cycle to 3048 Ω and 3912 Ω at 30th cycle (Rct = 75 Ω and 65 Ω), respectively. The sharp decrement of Rsl, Rct and Rct suggests that the La2O3 coating layer can reduce the secondary reactions between cathode materials and electrolyte by hindering the direct contract between cathode materials and electrolyte, alleviate the growth of SEI film and accelerate the Li+ interfacial diffusion kinetics between electrode and electrolyte. These would result in the significantly enhanced electrochemical performance of the La2O3-coated LMNC materials.
4. Conclusions
16
La2O3-coated LMNC cathodes with significantly enhanced specific capacity, rate capability and cycling performance were fabricated via a combined method of sol-gel and wet chemical processes. A thin La2O3 layer is evenly coated on the surface of LMNC particles. There is no distinct influence on the crystal structure of the materials after modification with La2O3. The surface modification of La2O3 leads to the great enhancement in the specific capacity, cycling stability and rate capability of the Li-rich cathode. The optimal La2O3-coating level is 2.5 wt%. The LMNC coated with 2.5 wt% La2O3 presents the highest discharge capacity, greatly enhanced cycling stability and sharply reduced charge transfer resistance, which delivers a large discharge capacity of 276.9 mAh g-1 in the 1st cycle and high capacity retention of 71% (201.4 mAh g-1) after 100 cycles. The Li-rich oxide with the coating level of 1.5 wt% La2O3 exhibits the optimal rate capability. The improved electrochemical performance of La2O3-modified LMNC can be ascribed to the La2O3 layer that reduces the side reactions between cathodes and electrolyte by hindering the direct contract of cathodes and electrolyte, alleviates the growth of SEI film and accelerates the Li+ interfacial diffusion kinetics between electrode and electrolyte. Our study shows that La2O3 is an efficient modifier to enhance the electrochemical performance of Li-rich cathode materials. The La2O3-coated Li1.2Mn0.54Ni0.13Co0.13O2 cathodes with enhanced specific capacity, rate capability and cycling stability are the most promising candidate for new generation rechargeable lithium-ion batteries.
Acknowledgement 17
This work was supported by large precision instrument projects of Sichuan Normal University (DJ 2015-48 and DJ 2015-42).
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26
Figure captions Fig. 1 XRD patterns of LMNC and LMNC-L. Fig. 2 SEM images of LMNC and LMNC-L: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 3 EDS spectrums of LMNC and LMNC-L: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 4 EDS elemental mapping images of LMNC-L2.5. Fig. 5 TEM images of LMNC (a1-a2), LMNC-L2.5 (b1-b2), LMNC-L5 (c1-c2); HRTEM images of LMNC (a3), LMNC-L2.5 (b3), LMNC-L5 (c3) (the insert is the corresponding fast fourier transform pattern). Fig. 6 Initial charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C between 2.0 and 4.8 V Fig. 7 Charge/discharge profiles of LMNC and LMNC-L electrodes at 0.1 C between 2.0 and 4.8 V: (a) LMNC, (b) LMNC-L0.5, (c) LMNC-L1.5, (d) LMNC-L2.5, (e) LMNC-L3, (f) LMNC-L5. Fig. 8 (a) Cycling performances of LMNC and LMNC-L electrodes during 100 cycles at 0.1 C between 2.0 and 4.8 V, (b) variations of discharge capacity and capacity retention in the 100th cycle with content of La2O3. Fig. 9 (a) Rate capability of the LMNC and LMNC-L electrodes at various rates between 2.0 and 4.8 V, (b) variations of discharge capacity with discharge current density. Fig. 10 Cycle voltammograms of LMNC and LMNC-L electrodes between 2.0 and 27
4.8 V, at a scan rate of 0.1 mV s-1: (a) LMNC, (b) LMNC-L1.5, (c) LMNC-L2.5, (d) LMNC-L5. Fig. 11 Nyquist plots of LMNC and LMNC-L electrodes: (a) after 1 cycle, (b) after 30 cycles and (c) the equivalent circuit for the impedance spectra.
Table 1 Simulated data from EIS spectra of the LMNC and LMNC-L electrodes after 1 cycle and 30 cycles. after 1cycle
after 30 cycles
Rs (Ω)
Rsl (Ω)
Rct (Ω)
Rs (Ω)
Rsl (Ω)
Rct (Ω)
Rct (Ω)
LMNC
3
228
5062
9
432
12789
7727
LMNC-L1.5
2
83
2973
11
52
3048
75
LMNC-L2.5
5
41
3843
5
35
3912
65
28
29
30
31
32