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3O2 cathode material for high-voltage Li-ion batteries
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CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries Y. Xie a, D. Gao a, L.L. Zhang a, J.J. Chen a,b, S. Cheng a, H.F. Xiang a,n a b
School of Materials Science and Engineering, Hefei University of Technology, Anhui Hefei 230009, PR China Department of Chemistry & Chemical Engineering, Anqing Normal University, Anhui Anqing 246011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 9 May 2016 Accepted 11 June 2016
A facile chemical deposition method has been adopted to prepare cerium fluoride (CeF3) surface modified LiNi1/3Co1/3Mn1/3O2 as cathode material for lithium-ion batteries. Structure analyses reveal that the surface of LiNi1/3Co1/3Mn1/3O2 particles is uniformly coated by CeF3. Electrochemical tests indicate that the optimal CeF3 content is 1 wt%. The 1 wt% CeF3-coated LiNi1/3Co1/3Mn1/3O2 can deliver a discharge capacity of 107.1 mA h g 1 even at 5 C rate, while the pristine does only 57.3 mA h g 1. Compared to the pristine, the 1 wt% CeF3-coated LiNi1/3Co1/3Mn1/3O2 exhibits the greatly enhanced capacity and cycling stability in the voltage range of 3.0–4.5 V, which suggests that the CeF3 coating has the positive effect on the high-voltage application of LiNi1/3Co1/3Mn1/3O2. According to the analyses from electrochemical impedance spectra, enhanced electrochemical performance is mainly because the stable CeF3 coating layer can prevent the HF-containing electrolyte from continuously attacking the LiNi1/3Co1/3Mn1/3O2 cathode and retard the passivating layer growth on the cathode. & 2016 Published by Elsevier Ltd.
Keywords: Lithium-ion batteries Cathode Surface coating High voltage
1. Introduction Due to high working voltage, large energy density, high safety and environmental friendliness, lithium ion batteries have become the most important power sources for portable electronic devices such as laptops, mobile phones and cameras. The key to the development of lithium ion batteries is the cathode materials. LiNi1/3 Co1/3Mn1/3O2, firstly proposed by Ohzuku [1], has been extensively studied as one of promising alternative cathode materials to replace LiCoO2 and have been applied in hybrid electric vehicles (HEVs) and electric vehicles (EVs) owing to their high discharge capacities and low cost [2–8]. In LiNi1/3Co1/3Mn1/3O2, the valence states of Mn, Ni and Co ions are þ4, þ2 and þ 3, respectively and the existence of Mn4 þ prevents the layered structure transferring to the spinel structure during lithium de-intercalation/intercalation. When the cells using LiNi1/3Co1/3Mn1/3O2 cathode are charged to cut-off voltages of 4.3 and 4.5 V, the discharge capacities can reach 150 and 170 mA h g 1, respectively [9,10]. Huang [9] reported that the discharge capacity of LiNi1/3Co1/3Mn1/3O2 was 149 mA h g 1 at 4.3–3.0 V, but significantly rose to 171 mA h g 1 at 4.5–3.0 V. However, there are still some limitations for the highvoltage application of LiNi1/3Co1/3Mn1/3O2, such as dissolution of active materials into the HF-containing electrolyte [11] and n
Corresponding author. E-mail address: [email protected] (H.F. Xiang).
oxidative decomposition of electrolyte at high potentials. Various strategies have been introduced to solve the aforementioned issues of LiNi1/3Co1/3Mn1/3O2 cathode materials, such as introduction of functional additives into electrolytes [12], cation doping [13,14] and surface coating for the cathode materials [10,15–17]. Recently coating with inert metal oxides (such as Al2O3, ZnO, TiO2 and LiAlO2) is an effective strategy [18–20], because the coating layers hinder the direct contact so as to suppress the side reactions between the electrode and organic electrolytes. Wu [21] reported that the CeO2-coated LiNi1/3Co1/3Mn1/3O2 exhibited the enhanced rate capability. Its capacities of 136.5 mA h g 1 at 2.0 C and 108.6 mA h g 1 at 3.0 C were much higher than the corresponding capacities of 120.6 mA h g 1 at 2.0 C and 93.7 mA h g 1 at 3.0 C for the pristine LiNi1/3Co1/3Mn1/3O2. In addition, coating the metal fluorides, such as AlF3 and CaF2 [22,23] was used to improve the electrochemical performance of the LiNi1/3Co1/3Mn1/3O2 cathode materials. However, to our best knowledge, coating CeF3 on LiNi1/3Co1/3Mn1/3O2 cathode material has seldom been reported so far. CeF3 is a kind of electrochemical inert material even at high temperatures [24], and has excellent structural stability in acidic environment such as HF-containing electrolyte solutions. Furthermore, CeF3 has been reported to have high ionic conductivity at room temperature [25,26]. So, the CeF3 would be a promising coating material for fabricating LiNi1/3Co1/3Mn1/3O2 cathodes with enhanced electrochemical performance. In this study, we fabricated CeF3-coating layers on the LiNi1/3Co1/3Mn1/3O2 (abbreviated as NCM) particles by a simple
http://dx.doi.org/10.1016/j.ceramint.2016.06.074 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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chemical deposition method. The structure and electrochemical performance of CeF3-coated LiNi1/3Co1/3Mn1/3O2 (abbreviated as CF-NCM) were investigated and the content of CeF3 coating on LiNi1/3Co1/3Mn1/3O2 was also optimized.
2. Experimental NCM (Umicore Greater China Co. LTD) was used as the start material. The CF-NCM samples were prepared by a simple chemical deposition method. In a typical reaction, Ce(NO3)3 6H2O was dissolved into deionized water and then NCM powders were dispersed into the solution and stirred for 0.5 h to form a suspension. Afterward, NH4F was fed into the suspension. The molar ratio of F to Ce element was regulated to 3:1, and the coating amounts of CeF3 were set at 0.5, 1, 2 and 3 wt% of the bulk NCM cathode material. The mixture was held at 60 °C under stirring for 4 h, and then dried at 80 °C overnight. The obtained samples were sintered at 450 °C for 2 h in air to obtain the CF-NCM products. X-ray diffraction patterns of the pristine NCM and CF-NCM were conducted on a diffractometer (D/MAX 2500 V, Cu Ka radiation) in the 2 theta range from 10° to 80°. The particle sizes and morphologies of the pristine NCM and CF-NCM were examined by field-emission scanning electron microscopy (FESEM, Hitachi SU8020) and high resolution transmission electron microscope (HRTEM, JEM-2100F). X-ray spectroscopy (EDS) analysis of CFNCM was performed with the JEM-2100F microscope. X-ray photoelectron spectroscopy (XPS, ESCALAB250) was performed to characterize the surface state of the pristine NCM and CF-NCM. Electrochemical performance of the cathode materials were evaluated by using CR2032-type cells. In order to prepare an electrode laminate, a slurry containing 80 wt% active material (NCM or CF-NCM), 6 wt% conductive graphite (KS-6), 6 wt% acetylene black and 8 wt% polyvinylidene fluoride (PVDF) dispersed in N-methyl-2-pyrrolidinone (NMP) was cast onto an aluminum current collector. After vacuum drying at 70 °C, the received laminate was punched into discs (Ф14 mm) for assembling the coin cells. Celgard 2400 polypropylene microporous membrane was used as separator. Highly pure lithium foil was used as the counter and reference electrode. The electrolyte was 1 mol L 1 LiPF6/ ethylene carbonate (EC)þ dimethyl carbonate (DMC) (1:1, w/w). The CR2032-type cells were assembled in an argon-filled glove box (MBraun). Electrochemical performance of the pristine NCM and CF-NCM was evaluated on an Arbin BT2000 multichannel battery cycler. All the cells were initially cycled twice between 2.5 and 4.3 V at a current rate of 0.1 C (1 C¼ 150 mA g 1) at room temperature. The cycling tests were performed at 1 C in the constant current-constant voltage (CC-CV) charge mode and constant current (CC) discharge mode between 2.5 and 4.3 V at room or elevated (55 °C) temperatures. For high-voltage application, the charging cut-off voltage was increased to 4.5 V. The electrochemical impedance spectroscopy (EIS) of the coin cell was measured in the frequency range from 100 kHz to 0.01 Hz at a CHI604D electrochemical workstation (Shanghai Chenhua Instruments Co. Ltd.). Cyclic voltammogram (CV) tests were performed over the potential range of 2.5–4.3 V at variety scan rates on the CHI604D electrochemical workstation.
Fig. 1. X-ray diffraction patterns of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples.
of (006)/(012) and (018)/(110) peaks indicates that all the samples possess a well-developed layered structure [27]. The crystal lattice constants of all the samples were calculated by Jade software, and the results are summarized in Table 1. The obtained lattice parameters of the pristine NCM and CF-NCM materials are very close. No evident impurity peaks are observed in the patterns of the CFNCM samples because of low contents of CeF3. In order to confirm the chemical state of Ce elements in the CFNCM samples, XPS measurements were performed. Fig. 2 shows the Ce 3d spectra of the pristine NCM and CF-NCM samples, where the signal at 901.1 eV (assigned to Ce 3d3/2) in the CF-NCM is characteristic of Ce3 þ [28]. The XPS results indicate that Ce(III) spectrum is detected and thus the coating layer is proposed to be CeF3. SEM images of the pristine NCM and 1 wt% CF-NCM particles are presented in Fig. 3. From Fig. 3a and b, both the primary particle sizes of the two samples are 0.5–1.0 mm and small particles aggregated to form spherical secondary particles with a size of 5– 10 mm in diameter. The pristine NCM particles have smooth and clean surface (Fig. 3a′), while the surface of the 1 wt% CF-NCM is distinctly covered by some “CeF3 moss” (Fig. 3b′). The surface morphology and microstructure of the pristine and 1 wt% CF-NCM samples were further analyzed by FETEM and EDS, as presented in Fig. 4. It can be seen more clearly that the pristine NCM has smooth edge lines (Fig. 4a), whereas the 1 wt% CF-NCM particle is enwrapped by a coating layer with a thickness of about 8 nm (Fig. 4b and c). In Fig. 4d, the interlayer spacing of 0.238 nm is well indexed to the (202) plane of CeF3 crystal. The element distribution of 1 wt% CF-NCM sample in Fig. 4e–j shows that the Ce and F elements uniformly distribute on the surface of the NCM particle. Table 1 Lattice parameters of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples. Samples
3. Results and discussion Fig. 1 presents the X-ray diffraction (XRD) patterns of the pristine NCM and CF-NCM samples. Diffraction patterns of all the samples can be identified as a layered α-NaFeO2 structure with a space group R-3m, indicating that the CeF3 coating process does not destroy the original layered structure of NCM. Distinct splitting
Pristine NCM 0.5% CF-NCM 1% CF-NCM 2% CF-NCM 3% CF-NCM
Lattice parameters a (Å)
c (Å)
c/a
2.8599 2.8621 2.8627 2.8648 2.8659
14.2610 14.2641 14.2666 14.3001 14.3294
4.9865 4.9838 4.9836 4.9917 5.0000
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 2. X-ray photoelectron spectroscopy (XPS) results for Ce element in the pristine NCM and CF-NCM samples.
Combined with the XPS results, it can be confirmed that CeF3 is uniformly coated in the CF-NCM samples. Electrochemical performance of the pristine NCM and CF-NCM samples was evaluated in coin-cells. The initial voltage profiles of the cells with the pristine NCM and CF-NCM cathode materials are shown in Fig. 5a. The smooth charge and discharge profiles suggest that the electrode structures are stable in the test voltage range [29]. During the initial charge process, the voltage plateau around 3.7–4.0 V corresponds to the Li þ extraction from the
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structure of space group R-3m mainly accompanying with the oxidation of Ni2 þ /Ni4 þ . The pristine sample delivers the discharge capacity of 155.8 mA h g 1. After coating with 0.5, 1 and 2 wt% CeF3, the discharge capacity does not significantly change (152.2, 155.7, 151.5 mA h g 1, respectively). However, further increasing the amount of CeF3 to 3 wt% leads to a distinct capacity loss (143.5 mA h g 1) because of the inactive nature of CeF3. The Coulombic efficiencies of the pristine NCM and 0.5, 1, 2, 3 wt% CFNCM samples are 85.8%, 80.8%, 88.9%, 90.3% and 85.3%, respectively. Although the discharge capacities are slightly lower than that of the pristine, the 1, 2 and 3 wt% CF-NCM samples exhibit less irreversible capacity loss of 19.4, 16.2 and 24.8 mA h g 1, compared to that of the pristine NCM (25.7 mA h g 1). The relatively less irreversible capacity loss and higher initial Coulombic efficiency for the CF-NCM cathode materials are attributed to the suppression of the CeF3 coating layer on the oxidative decomposition of the electrolyte. Cycling performance of the pristine and CF-NCM samples was investigated in the range of 4.3–3.0 V at 1 C after two formation cycles. In Fig. 5b, for the pristine NCM, the discharge capacity drops rapidly from 155.8 mA h g 1 in the initial cycle to only 95.3 mA h g 1 after 100 cycles. The capacity retention is as low as 61.2%, mainly due to the gradual sabotage of the pristine particles surface attacked by HF. For the 0.5, 1, 2, 3 wt% CF-NCM samples, the discharge capacities are 152.5, 152.3, 151.5, 143.5 mA h g 1 in the initial cycle, and decrease to 114.0, 130.7, 115.5, 97.8 mA h g 1 after 100 cycles, with the capacity retentions are 74.8%, 85.8%, 76.2% and 68.2%, respectively. It is noteworthy that all the CF-NCM materials show higher capacity retention than that of the pristine, especially for the 1 wt% CF-NCM. Hence, it can be speculated that the stable CeF3 coating layer can prevent the HF-containing
Fig. 3. SEM images of the pristine NCM (a, a′) and 1 wt% CF-NCM (b, b′) samples.
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 4. SEM images of the pristine NCM (a) and 1 wt% CF-NCM (b, c and d) samples; EDS elemental mapping images of 1 wt% CF-NCM sample, Ni (e), Co (f), Mn (g), O (h), Ce (i) and F (j).
electrolyte from continuously attacking the NCM cathode, and consequently the cycling performance of NCM is improved. Fig. 6a shows the discharge capacities of the pristine NCM and 1 wt% CF-NCM materials at a constant current density of 1 C in a potential region between 2.5 and 4.5 V. The reversible capacities of the pristine NCM drop from 157.7 to 110.4 mA h g 1 after 100 cycles and the capacity retention is only 70.0%. The discharge
capacities of 1 wt% CF-NCM sample reduce from 168.3 to 134.1 mA h g 1 after 100 cycles with the capacity retention of 79.7%. The discharge voltage profiles of the pristine NCM and 1 wt% CF-NCM at the 3rd, 50th and 100th cycles are compared in Fig. 6b. It is clear that the voltage fading of the CF-NCM is much less than that of the pristine NCM, which suggests that the CeF3 coating layer can suppress not only the capacity fading but also the
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 5. Initial voltage profiles at 0.1 C (a), cycle performance at 1 C of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples for 100 cycles (b) between 2.5 V and 4.3 V at room temperature.
Fig. 6. Cycle performance of the pristine NCM and 1 wt% CF-NCM samples at 1 C rate between 2.5 V and 4.5 V for 100 cycles (a), discharge capacity profiles of the two samples (b).
voltage fading. On one hand, the enhanced stability on cycling capacity and voltage plateau of the CF-NCM sample at the highvoltage range might be attributed to the suppressed oxidative decomposition of the electrolyte on the surface of the NCM cathode at high potentials. The CeF3 coating layer effectively deactivates the catalyzing effect of some transition metal ions on the NCM surface so that the oxidative decomposition of the electrolyte at high potentials could be suppressed and the cell polarization be reduced. On the other hand, the CeF3 coating layer can not only protect the NCM bulk from the attack of HF in the electrolyte, but also enhance the structural stability of the charged NCM cathode material at high potentials. Therefore, both the enhanced stabilities of the cathode material and the electrolyte result in the good cycling stability of 1 wt% CF-NCM for the high-voltage application. The rate capabilities of the pristine NCM and CF-NCM samples in the voltage range of 2.5–4.3 V from 0.1 C to 5 C are shown in Fig. 7, respectively. The pristine NCM presents a capacity of 100.8 mA h g 1 at 2 C, corresponding to 69.7% of its capacity at 0.1 C. The discharge capacity of 1 wt% CF-NCM material is 119.2 mA h g 1 at 2 C rate, 78.2% of that at 0.1 C. Moreover, the 0.5, 1, 2, 3 wt% CF-NCM materials deliver discharge capacities of 71.2, 107.1, 80.9, 48.5 mA h g 1 at 5 C compared to 57.3 mA h g 1 for the pristine NCM. Especially, the discharge capacity of 1 wt% CF-
Fig. 7. Rate capability of the pristine NCM and 1 wt% CF-NCM samples at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C rate.
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 8. Cyclic voltammetry of the pristine NCM (a) and 1 wt% CF-NCM samples (b) at variety scan rates, relationship between the peak (A1, A2) current (Ip) and the square root of scan rates (V1/2) for the two samples (c and d).
NCM is nearly twice as high as that of the pristine. Thus, the optimal CeF3 content for the CF-NCM materials is 1 wt%. The 3 wt% CF-NCM shows the low discharge capacity and capacity retention possibly due to the thick inert CeF3 coating layer that hinders Li þ insertion and extraction. The enhanced rate capability of 1 wt% CFNCM indicates that the CeF3 coating layer can not only contribute to the low polarization and also make NCM cathode materials endure great changes at high discharge current densities. More evidences on the positive contribution of the CeF3 coating is illustrated by CV tests between 2.5 and 4.3 V at various scan rates of 0.1, 0.2, 0.5 and 0.8 mV s 1. As shown in Fig. 8a and b, both the CV curves exhibit the typical pair of redox peaks between 3.6 and 4.0 V corresponding to the redox reaction of Ni2 þ /Ni4 þ [30]. The potential difference (ΔV) between anodic peak and cathodic peak is known to indicate the polarization between Li þ insertion and extraction, where the larger potential difference represents the higher electrochemical polarization [31]. The potential difference of the pristine NCM is 0.41 V, which is much larger than 0.16 V of 1 wt% CF-NCM at a scan rate of 0.1 mV s 1, proving that the CF-NCM has smaller polarization. When the scan rate is increased, the oxidation peaks move to high potentials, whereas the reductive peaks move to low potential for all the cells. But the area under curves is much larger for CF-NCM cathode material compared with the pristine at the same scan rate, which is consistent with its good capacity retention. Fig. 8c and d show the linear dependence of peak currents on
the square root of scan rates. The slope of the line indicates the Li þ diffusion coefficient. Based on the formula of the relationship of peak currents and the square root of scan rates,
Ip = 2.69 × 105 × n3/2 × A × D1/2 × C0 × V1/2 Ip: peak current, n: the amount of charge transferred, A: the contact area between the electrode and the electrolyte (approach to the area of the electrode), C0: Li þ bulk concentration in the electrode, V: scan rate, D: Li þ diffusion coefficient. The Li þ diffusion coefficient of the pristine NCM and 1 wt% CF-NCM in the anodic peak A1 are 3.65 10 11 cm2 s 1 and 4.74 10 11 cm2 s 1. The Li þ diffusion coefficient of the pristine NCM and 1 wt% CF-NCM in the cathodic peak A2 are 3.37 10 12 cm2 s 1, 1.02 10 11 cm2 s 1, respectively. Obviously, the Li þ diffusion coefficient is improved after the CeF3 coating. Usually, the surface coating should not affect the Li þ diffusion coefficient of the material bulk when no structural change occurs in the material bulk. However, as shown in Table 1, 1 wt% CF-NCM has the bigger lattice parameters than the pristine NCM. That means, either Ce or F ions in the coated layer could dope into the NCM lattice during heat treatment and the lattice volume of the NCM bulk expands. It is reasonable that the higher Li þ diffusion coefficient of 1 wt% CF-NCM is resulted from partial bulk doping from the coated CeF3 layer during the heat treatment. Electrochemical impedance spectroscopy (EIS) of the pristine
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 9. Nyquist plots of the pristine, 1 wt% and 3 wt% CF-NCM samples after 50 cycles and 100 cycles, respectively (a and b), and equivalent circuit performed to fit the Nyquist plots (c).
Table 2 Calculated Rs, RSEI, Rct1 and Rct2 values of the pristine NCM, 1 wt% and 3 wt% CF-NCM samples. Samples
Pristine NCM 1% CF-NCM 3% CF-NCM
RSEI/Ω
Rs/Ω
Rct1/Ω
Rct2/Ω
50 cycles
100 cycles
50 cycles
100 cycles
50 cycles
100 cycles
50 cycles
100 cycles
4.40 4.59 3.24
4.52 5.34 5.00
8.33 7.20 28.44
8.89 11.03 30.30
16.01 16.65 12.62
20.29 27.38 22.20
72.52 9.18 32.19
161.50 41.74 69.40
NCM and CF-NCM was shown in Fig. 9a and b. As been described in previous work [32], in the curves, the high-frequency semicircle is related to the solid electrolyte interface (SEI) resistance (RSEI), the medium-high frequency semicircle is due to the chargetransfer resistance (Rct1) at the interface between the electrolyte and the lithium anode, the intermediate-frequency semicircle is ascribed to the charge-transfer resistance (Rct2) at the interface between the electrolyte and the cathode material and the lowfrequency oblique line is attributed to the Li þ diffusion process in the electrode materials, the Warburg impedance (ZW). The point of the high-frequency semicircle intersects with the abscissa is related to ohm impedance (Rs) of lithium ion diffusion process in the electrolyte. The Nyquist plots of materials were analyzed by fitting to an equivalent electrical circuit (Fig. 9c). The fitted impedance parameters of the equivalent circuit are listed in Table 2. The value Rs of the pristine NCM cathode is relatively close to that of the CFNCM materials. The values of RSEI of the 1 and 3 wt% CF-NCM (11.03, 30.30 Ω, respectively) is higher than that of the pristine NCM (8.89 Ω) due to the formation of higher stability of the solid electrolyte interface (SEI) after CeF3 coating which is in favor of the improved cycle performance of cells. Compared to Rct2, the values of Rct1 slightly change for the pristine NCM, 1 and 3 wt% CF-NCM cells. The values of Rct2 of the pristine NCM cell increase from 72.52 Ω at the 50th cycle to 161.50 Ω at the 100th cycle. However,
the values of Rct2 of the 1 and 3 wt% CF-NCM cells only increase from 9.18 Ω, 32.19 Ω at the 50th cycle to 41.74 Ω, 69.40 Ω at the 100th cycle, respectively. Compared to the pristine NCM, the much smaller RSEI and Rct2 resistances of the CF-NCM sample indicate that the stable CeF3 coating layer prevents the HF-containing electrolyte from continuously attacking the NCM cathode and retards the interface growth due to the oxidative decomposition of the electrolyte.
4. Conclusions In summary, the LiNi1/3Co1/3Mn1/3O2 cathode materials coated with CeF3 have been successfully prepared by a simple chemical deposition method and different CeF3 contents for coating have been investigated. The electrochemical results show both cycling performance at high voltage and rate capability were improved by 1 wt% CeF3 coating. It is believed that the enhanced electrochemical performance by the CeF3 coating is attributed to the suppressed oxidation decomposition of the electrolyte on the surface of the cathode materials. EIS and CV results prove that the CeF3 coating can decrease charge transfer resistance and speed up Li þ diffusion in LiNi1/3Co1/3Mn1/3O2. Those results suggest that the appropriate amount of CeF3 coating on LiNi1/3Co1/3Mn1/3O2
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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cathode material will be a promising approach to improve electrochemical performances of LiNi1/3Co1/3Mn1/3O2 layered materials for lithium-ion batteries, especially for high-voltage applications.
Acknowledgements This study was supported by National Natural Science Foundation of China (Grant nos. 51372060 and 31501576).
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CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries Y. Xie a, D. Gao a, L.L. Zhang a, J.J. Chen a,b, S. Cheng a, H.F. Xiang a,n a b
School of Materials Science and Engineering, Hefei University of Technology, Anhui Hefei 230009, PR China Department of Chemistry & Chemical Engineering, Anqing Normal University, Anhui Anqing 246011, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 18 April 2016 Received in revised form 9 May 2016 Accepted 11 June 2016
A facile chemical deposition method has been adopted to prepare cerium fluoride (CeF3) surface modified LiNi1/3Co1/3Mn1/3O2 as cathode material for lithium-ion batteries. Structure analyses reveal that the surface of LiNi1/3Co1/3Mn1/3O2 particles is uniformly coated by CeF3. Electrochemical tests indicate that the optimal CeF3 content is 1 wt%. The 1 wt% CeF3-coated LiNi1/3Co1/3Mn1/3O2 can deliver a discharge capacity of 107.1 mA h g 1 even at 5 C rate, while the pristine does only 57.3 mA h g 1. Compared to the pristine, the 1 wt% CeF3-coated LiNi1/3Co1/3Mn1/3O2 exhibits the greatly enhanced capacity and cycling stability in the voltage range of 3.0–4.5 V, which suggests that the CeF3 coating has the positive effect on the high-voltage application of LiNi1/3Co1/3Mn1/3O2. According to the analyses from electrochemical impedance spectra, enhanced electrochemical performance is mainly because the stable CeF3 coating layer can prevent the HF-containing electrolyte from continuously attacking the LiNi1/3Co1/3Mn1/3O2 cathode and retard the passivating layer growth on the cathode. & 2016 Published by Elsevier Ltd.
Keywords: Lithium-ion batteries Cathode Surface coating High voltage
1. Introduction Due to high working voltage, large energy density, high safety and environmental friendliness, lithium ion batteries have become the most important power sources for portable electronic devices such as laptops, mobile phones and cameras. The key to the development of lithium ion batteries is the cathode materials. LiNi1/3 Co1/3Mn1/3O2, firstly proposed by Ohzuku [1], has been extensively studied as one of promising alternative cathode materials to replace LiCoO2 and have been applied in hybrid electric vehicles (HEVs) and electric vehicles (EVs) owing to their high discharge capacities and low cost [2–8]. In LiNi1/3Co1/3Mn1/3O2, the valence states of Mn, Ni and Co ions are þ4, þ2 and þ 3, respectively and the existence of Mn4 þ prevents the layered structure transferring to the spinel structure during lithium de-intercalation/intercalation. When the cells using LiNi1/3Co1/3Mn1/3O2 cathode are charged to cut-off voltages of 4.3 and 4.5 V, the discharge capacities can reach 150 and 170 mA h g 1, respectively [9,10]. Huang [9] reported that the discharge capacity of LiNi1/3Co1/3Mn1/3O2 was 149 mA h g 1 at 4.3–3.0 V, but significantly rose to 171 mA h g 1 at 4.5–3.0 V. However, there are still some limitations for the highvoltage application of LiNi1/3Co1/3Mn1/3O2, such as dissolution of active materials into the HF-containing electrolyte [11] and n
Corresponding author. E-mail address: [email protected] (H.F. Xiang).
oxidative decomposition of electrolyte at high potentials. Various strategies have been introduced to solve the aforementioned issues of LiNi1/3Co1/3Mn1/3O2 cathode materials, such as introduction of functional additives into electrolytes [12], cation doping [13,14] and surface coating for the cathode materials [10,15–17]. Recently coating with inert metal oxides (such as Al2O3, ZnO, TiO2 and LiAlO2) is an effective strategy [18–20], because the coating layers hinder the direct contact so as to suppress the side reactions between the electrode and organic electrolytes. Wu [21] reported that the CeO2-coated LiNi1/3Co1/3Mn1/3O2 exhibited the enhanced rate capability. Its capacities of 136.5 mA h g 1 at 2.0 C and 108.6 mA h g 1 at 3.0 C were much higher than the corresponding capacities of 120.6 mA h g 1 at 2.0 C and 93.7 mA h g 1 at 3.0 C for the pristine LiNi1/3Co1/3Mn1/3O2. In addition, coating the metal fluorides, such as AlF3 and CaF2 [22,23] was used to improve the electrochemical performance of the LiNi1/3Co1/3Mn1/3O2 cathode materials. However, to our best knowledge, coating CeF3 on LiNi1/3Co1/3Mn1/3O2 cathode material has seldom been reported so far. CeF3 is a kind of electrochemical inert material even at high temperatures [24], and has excellent structural stability in acidic environment such as HF-containing electrolyte solutions. Furthermore, CeF3 has been reported to have high ionic conductivity at room temperature [25,26]. So, the CeF3 would be a promising coating material for fabricating LiNi1/3Co1/3Mn1/3O2 cathodes with enhanced electrochemical performance. In this study, we fabricated CeF3-coating layers on the LiNi1/3Co1/3Mn1/3O2 (abbreviated as NCM) particles by a simple
http://dx.doi.org/10.1016/j.ceramint.2016.06.074 0272-8842/& 2016 Published by Elsevier Ltd.
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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chemical deposition method. The structure and electrochemical performance of CeF3-coated LiNi1/3Co1/3Mn1/3O2 (abbreviated as CF-NCM) were investigated and the content of CeF3 coating on LiNi1/3Co1/3Mn1/3O2 was also optimized.
2. Experimental NCM (Umicore Greater China Co. LTD) was used as the start material. The CF-NCM samples were prepared by a simple chemical deposition method. In a typical reaction, Ce(NO3)3 6H2O was dissolved into deionized water and then NCM powders were dispersed into the solution and stirred for 0.5 h to form a suspension. Afterward, NH4F was fed into the suspension. The molar ratio of F to Ce element was regulated to 3:1, and the coating amounts of CeF3 were set at 0.5, 1, 2 and 3 wt% of the bulk NCM cathode material. The mixture was held at 60 °C under stirring for 4 h, and then dried at 80 °C overnight. The obtained samples were sintered at 450 °C for 2 h in air to obtain the CF-NCM products. X-ray diffraction patterns of the pristine NCM and CF-NCM were conducted on a diffractometer (D/MAX 2500 V, Cu Ka radiation) in the 2 theta range from 10° to 80°. The particle sizes and morphologies of the pristine NCM and CF-NCM were examined by field-emission scanning electron microscopy (FESEM, Hitachi SU8020) and high resolution transmission electron microscope (HRTEM, JEM-2100F). X-ray spectroscopy (EDS) analysis of CFNCM was performed with the JEM-2100F microscope. X-ray photoelectron spectroscopy (XPS, ESCALAB250) was performed to characterize the surface state of the pristine NCM and CF-NCM. Electrochemical performance of the cathode materials were evaluated by using CR2032-type cells. In order to prepare an electrode laminate, a slurry containing 80 wt% active material (NCM or CF-NCM), 6 wt% conductive graphite (KS-6), 6 wt% acetylene black and 8 wt% polyvinylidene fluoride (PVDF) dispersed in N-methyl-2-pyrrolidinone (NMP) was cast onto an aluminum current collector. After vacuum drying at 70 °C, the received laminate was punched into discs (Ф14 mm) for assembling the coin cells. Celgard 2400 polypropylene microporous membrane was used as separator. Highly pure lithium foil was used as the counter and reference electrode. The electrolyte was 1 mol L 1 LiPF6/ ethylene carbonate (EC)þ dimethyl carbonate (DMC) (1:1, w/w). The CR2032-type cells were assembled in an argon-filled glove box (MBraun). Electrochemical performance of the pristine NCM and CF-NCM was evaluated on an Arbin BT2000 multichannel battery cycler. All the cells were initially cycled twice between 2.5 and 4.3 V at a current rate of 0.1 C (1 C¼ 150 mA g 1) at room temperature. The cycling tests were performed at 1 C in the constant current-constant voltage (CC-CV) charge mode and constant current (CC) discharge mode between 2.5 and 4.3 V at room or elevated (55 °C) temperatures. For high-voltage application, the charging cut-off voltage was increased to 4.5 V. The electrochemical impedance spectroscopy (EIS) of the coin cell was measured in the frequency range from 100 kHz to 0.01 Hz at a CHI604D electrochemical workstation (Shanghai Chenhua Instruments Co. Ltd.). Cyclic voltammogram (CV) tests were performed over the potential range of 2.5–4.3 V at variety scan rates on the CHI604D electrochemical workstation.
Fig. 1. X-ray diffraction patterns of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples.
of (006)/(012) and (018)/(110) peaks indicates that all the samples possess a well-developed layered structure [27]. The crystal lattice constants of all the samples were calculated by Jade software, and the results are summarized in Table 1. The obtained lattice parameters of the pristine NCM and CF-NCM materials are very close. No evident impurity peaks are observed in the patterns of the CFNCM samples because of low contents of CeF3. In order to confirm the chemical state of Ce elements in the CFNCM samples, XPS measurements were performed. Fig. 2 shows the Ce 3d spectra of the pristine NCM and CF-NCM samples, where the signal at 901.1 eV (assigned to Ce 3d3/2) in the CF-NCM is characteristic of Ce3 þ [28]. The XPS results indicate that Ce(III) spectrum is detected and thus the coating layer is proposed to be CeF3. SEM images of the pristine NCM and 1 wt% CF-NCM particles are presented in Fig. 3. From Fig. 3a and b, both the primary particle sizes of the two samples are 0.5–1.0 mm and small particles aggregated to form spherical secondary particles with a size of 5– 10 mm in diameter. The pristine NCM particles have smooth and clean surface (Fig. 3a′), while the surface of the 1 wt% CF-NCM is distinctly covered by some “CeF3 moss” (Fig. 3b′). The surface morphology and microstructure of the pristine and 1 wt% CF-NCM samples were further analyzed by FETEM and EDS, as presented in Fig. 4. It can be seen more clearly that the pristine NCM has smooth edge lines (Fig. 4a), whereas the 1 wt% CF-NCM particle is enwrapped by a coating layer with a thickness of about 8 nm (Fig. 4b and c). In Fig. 4d, the interlayer spacing of 0.238 nm is well indexed to the (202) plane of CeF3 crystal. The element distribution of 1 wt% CF-NCM sample in Fig. 4e–j shows that the Ce and F elements uniformly distribute on the surface of the NCM particle. Table 1 Lattice parameters of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples. Samples
3. Results and discussion Fig. 1 presents the X-ray diffraction (XRD) patterns of the pristine NCM and CF-NCM samples. Diffraction patterns of all the samples can be identified as a layered α-NaFeO2 structure with a space group R-3m, indicating that the CeF3 coating process does not destroy the original layered structure of NCM. Distinct splitting
Pristine NCM 0.5% CF-NCM 1% CF-NCM 2% CF-NCM 3% CF-NCM
Lattice parameters a (Å)
c (Å)
c/a
2.8599 2.8621 2.8627 2.8648 2.8659
14.2610 14.2641 14.2666 14.3001 14.3294
4.9865 4.9838 4.9836 4.9917 5.0000
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Fig. 2. X-ray photoelectron spectroscopy (XPS) results for Ce element in the pristine NCM and CF-NCM samples.
Combined with the XPS results, it can be confirmed that CeF3 is uniformly coated in the CF-NCM samples. Electrochemical performance of the pristine NCM and CF-NCM samples was evaluated in coin-cells. The initial voltage profiles of the cells with the pristine NCM and CF-NCM cathode materials are shown in Fig. 5a. The smooth charge and discharge profiles suggest that the electrode structures are stable in the test voltage range [29]. During the initial charge process, the voltage plateau around 3.7–4.0 V corresponds to the Li þ extraction from the
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structure of space group R-3m mainly accompanying with the oxidation of Ni2 þ /Ni4 þ . The pristine sample delivers the discharge capacity of 155.8 mA h g 1. After coating with 0.5, 1 and 2 wt% CeF3, the discharge capacity does not significantly change (152.2, 155.7, 151.5 mA h g 1, respectively). However, further increasing the amount of CeF3 to 3 wt% leads to a distinct capacity loss (143.5 mA h g 1) because of the inactive nature of CeF3. The Coulombic efficiencies of the pristine NCM and 0.5, 1, 2, 3 wt% CFNCM samples are 85.8%, 80.8%, 88.9%, 90.3% and 85.3%, respectively. Although the discharge capacities are slightly lower than that of the pristine, the 1, 2 and 3 wt% CF-NCM samples exhibit less irreversible capacity loss of 19.4, 16.2 and 24.8 mA h g 1, compared to that of the pristine NCM (25.7 mA h g 1). The relatively less irreversible capacity loss and higher initial Coulombic efficiency for the CF-NCM cathode materials are attributed to the suppression of the CeF3 coating layer on the oxidative decomposition of the electrolyte. Cycling performance of the pristine and CF-NCM samples was investigated in the range of 4.3–3.0 V at 1 C after two formation cycles. In Fig. 5b, for the pristine NCM, the discharge capacity drops rapidly from 155.8 mA h g 1 in the initial cycle to only 95.3 mA h g 1 after 100 cycles. The capacity retention is as low as 61.2%, mainly due to the gradual sabotage of the pristine particles surface attacked by HF. For the 0.5, 1, 2, 3 wt% CF-NCM samples, the discharge capacities are 152.5, 152.3, 151.5, 143.5 mA h g 1 in the initial cycle, and decrease to 114.0, 130.7, 115.5, 97.8 mA h g 1 after 100 cycles, with the capacity retentions are 74.8%, 85.8%, 76.2% and 68.2%, respectively. It is noteworthy that all the CF-NCM materials show higher capacity retention than that of the pristine, especially for the 1 wt% CF-NCM. Hence, it can be speculated that the stable CeF3 coating layer can prevent the HF-containing
Fig. 3. SEM images of the pristine NCM (a, a′) and 1 wt% CF-NCM (b, b′) samples.
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Fig. 4. SEM images of the pristine NCM (a) and 1 wt% CF-NCM (b, c and d) samples; EDS elemental mapping images of 1 wt% CF-NCM sample, Ni (e), Co (f), Mn (g), O (h), Ce (i) and F (j).
electrolyte from continuously attacking the NCM cathode, and consequently the cycling performance of NCM is improved. Fig. 6a shows the discharge capacities of the pristine NCM and 1 wt% CF-NCM materials at a constant current density of 1 C in a potential region between 2.5 and 4.5 V. The reversible capacities of the pristine NCM drop from 157.7 to 110.4 mA h g 1 after 100 cycles and the capacity retention is only 70.0%. The discharge
capacities of 1 wt% CF-NCM sample reduce from 168.3 to 134.1 mA h g 1 after 100 cycles with the capacity retention of 79.7%. The discharge voltage profiles of the pristine NCM and 1 wt% CF-NCM at the 3rd, 50th and 100th cycles are compared in Fig. 6b. It is clear that the voltage fading of the CF-NCM is much less than that of the pristine NCM, which suggests that the CeF3 coating layer can suppress not only the capacity fading but also the
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Fig. 5. Initial voltage profiles at 0.1 C (a), cycle performance at 1 C of the pristine NCM, 0.5 wt%, 1 wt%, 2 wt% and 3 wt% CF-NCM samples for 100 cycles (b) between 2.5 V and 4.3 V at room temperature.
Fig. 6. Cycle performance of the pristine NCM and 1 wt% CF-NCM samples at 1 C rate between 2.5 V and 4.5 V for 100 cycles (a), discharge capacity profiles of the two samples (b).
voltage fading. On one hand, the enhanced stability on cycling capacity and voltage plateau of the CF-NCM sample at the highvoltage range might be attributed to the suppressed oxidative decomposition of the electrolyte on the surface of the NCM cathode at high potentials. The CeF3 coating layer effectively deactivates the catalyzing effect of some transition metal ions on the NCM surface so that the oxidative decomposition of the electrolyte at high potentials could be suppressed and the cell polarization be reduced. On the other hand, the CeF3 coating layer can not only protect the NCM bulk from the attack of HF in the electrolyte, but also enhance the structural stability of the charged NCM cathode material at high potentials. Therefore, both the enhanced stabilities of the cathode material and the electrolyte result in the good cycling stability of 1 wt% CF-NCM for the high-voltage application. The rate capabilities of the pristine NCM and CF-NCM samples in the voltage range of 2.5–4.3 V from 0.1 C to 5 C are shown in Fig. 7, respectively. The pristine NCM presents a capacity of 100.8 mA h g 1 at 2 C, corresponding to 69.7% of its capacity at 0.1 C. The discharge capacity of 1 wt% CF-NCM material is 119.2 mA h g 1 at 2 C rate, 78.2% of that at 0.1 C. Moreover, the 0.5, 1, 2, 3 wt% CF-NCM materials deliver discharge capacities of 71.2, 107.1, 80.9, 48.5 mA h g 1 at 5 C compared to 57.3 mA h g 1 for the pristine NCM. Especially, the discharge capacity of 1 wt% CF-
Fig. 7. Rate capability of the pristine NCM and 1 wt% CF-NCM samples at 0.1 C, 0.2 C, 0.5 C, 1 C, 2 C and 5 C rate.
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Fig. 8. Cyclic voltammetry of the pristine NCM (a) and 1 wt% CF-NCM samples (b) at variety scan rates, relationship between the peak (A1, A2) current (Ip) and the square root of scan rates (V1/2) for the two samples (c and d).
NCM is nearly twice as high as that of the pristine. Thus, the optimal CeF3 content for the CF-NCM materials is 1 wt%. The 3 wt% CF-NCM shows the low discharge capacity and capacity retention possibly due to the thick inert CeF3 coating layer that hinders Li þ insertion and extraction. The enhanced rate capability of 1 wt% CFNCM indicates that the CeF3 coating layer can not only contribute to the low polarization and also make NCM cathode materials endure great changes at high discharge current densities. More evidences on the positive contribution of the CeF3 coating is illustrated by CV tests between 2.5 and 4.3 V at various scan rates of 0.1, 0.2, 0.5 and 0.8 mV s 1. As shown in Fig. 8a and b, both the CV curves exhibit the typical pair of redox peaks between 3.6 and 4.0 V corresponding to the redox reaction of Ni2 þ /Ni4 þ [30]. The potential difference (ΔV) between anodic peak and cathodic peak is known to indicate the polarization between Li þ insertion and extraction, where the larger potential difference represents the higher electrochemical polarization [31]. The potential difference of the pristine NCM is 0.41 V, which is much larger than 0.16 V of 1 wt% CF-NCM at a scan rate of 0.1 mV s 1, proving that the CF-NCM has smaller polarization. When the scan rate is increased, the oxidation peaks move to high potentials, whereas the reductive peaks move to low potential for all the cells. But the area under curves is much larger for CF-NCM cathode material compared with the pristine at the same scan rate, which is consistent with its good capacity retention. Fig. 8c and d show the linear dependence of peak currents on
the square root of scan rates. The slope of the line indicates the Li þ diffusion coefficient. Based on the formula of the relationship of peak currents and the square root of scan rates,
Ip = 2.69 × 105 × n3/2 × A × D1/2 × C0 × V1/2 Ip: peak current, n: the amount of charge transferred, A: the contact area between the electrode and the electrolyte (approach to the area of the electrode), C0: Li þ bulk concentration in the electrode, V: scan rate, D: Li þ diffusion coefficient. The Li þ diffusion coefficient of the pristine NCM and 1 wt% CF-NCM in the anodic peak A1 are 3.65 10 11 cm2 s 1 and 4.74 10 11 cm2 s 1. The Li þ diffusion coefficient of the pristine NCM and 1 wt% CF-NCM in the cathodic peak A2 are 3.37 10 12 cm2 s 1, 1.02 10 11 cm2 s 1, respectively. Obviously, the Li þ diffusion coefficient is improved after the CeF3 coating. Usually, the surface coating should not affect the Li þ diffusion coefficient of the material bulk when no structural change occurs in the material bulk. However, as shown in Table 1, 1 wt% CF-NCM has the bigger lattice parameters than the pristine NCM. That means, either Ce or F ions in the coated layer could dope into the NCM lattice during heat treatment and the lattice volume of the NCM bulk expands. It is reasonable that the higher Li þ diffusion coefficient of 1 wt% CF-NCM is resulted from partial bulk doping from the coated CeF3 layer during the heat treatment. Electrochemical impedance spectroscopy (EIS) of the pristine
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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Fig. 9. Nyquist plots of the pristine, 1 wt% and 3 wt% CF-NCM samples after 50 cycles and 100 cycles, respectively (a and b), and equivalent circuit performed to fit the Nyquist plots (c).
Table 2 Calculated Rs, RSEI, Rct1 and Rct2 values of the pristine NCM, 1 wt% and 3 wt% CF-NCM samples. Samples
Pristine NCM 1% CF-NCM 3% CF-NCM
RSEI/Ω
Rs/Ω
Rct1/Ω
Rct2/Ω
50 cycles
100 cycles
50 cycles
100 cycles
50 cycles
100 cycles
50 cycles
100 cycles
4.40 4.59 3.24
4.52 5.34 5.00
8.33 7.20 28.44
8.89 11.03 30.30
16.01 16.65 12.62
20.29 27.38 22.20
72.52 9.18 32.19
161.50 41.74 69.40
NCM and CF-NCM was shown in Fig. 9a and b. As been described in previous work [32], in the curves, the high-frequency semicircle is related to the solid electrolyte interface (SEI) resistance (RSEI), the medium-high frequency semicircle is due to the chargetransfer resistance (Rct1) at the interface between the electrolyte and the lithium anode, the intermediate-frequency semicircle is ascribed to the charge-transfer resistance (Rct2) at the interface between the electrolyte and the cathode material and the lowfrequency oblique line is attributed to the Li þ diffusion process in the electrode materials, the Warburg impedance (ZW). The point of the high-frequency semicircle intersects with the abscissa is related to ohm impedance (Rs) of lithium ion diffusion process in the electrolyte. The Nyquist plots of materials were analyzed by fitting to an equivalent electrical circuit (Fig. 9c). The fitted impedance parameters of the equivalent circuit are listed in Table 2. The value Rs of the pristine NCM cathode is relatively close to that of the CFNCM materials. The values of RSEI of the 1 and 3 wt% CF-NCM (11.03, 30.30 Ω, respectively) is higher than that of the pristine NCM (8.89 Ω) due to the formation of higher stability of the solid electrolyte interface (SEI) after CeF3 coating which is in favor of the improved cycle performance of cells. Compared to Rct2, the values of Rct1 slightly change for the pristine NCM, 1 and 3 wt% CF-NCM cells. The values of Rct2 of the pristine NCM cell increase from 72.52 Ω at the 50th cycle to 161.50 Ω at the 100th cycle. However,
the values of Rct2 of the 1 and 3 wt% CF-NCM cells only increase from 9.18 Ω, 32.19 Ω at the 50th cycle to 41.74 Ω, 69.40 Ω at the 100th cycle, respectively. Compared to the pristine NCM, the much smaller RSEI and Rct2 resistances of the CF-NCM sample indicate that the stable CeF3 coating layer prevents the HF-containing electrolyte from continuously attacking the NCM cathode and retards the interface growth due to the oxidative decomposition of the electrolyte.
4. Conclusions In summary, the LiNi1/3Co1/3Mn1/3O2 cathode materials coated with CeF3 have been successfully prepared by a simple chemical deposition method and different CeF3 contents for coating have been investigated. The electrochemical results show both cycling performance at high voltage and rate capability were improved by 1 wt% CeF3 coating. It is believed that the enhanced electrochemical performance by the CeF3 coating is attributed to the suppressed oxidation decomposition of the electrolyte on the surface of the cathode materials. EIS and CV results prove that the CeF3 coating can decrease charge transfer resistance and speed up Li þ diffusion in LiNi1/3Co1/3Mn1/3O2. Those results suggest that the appropriate amount of CeF3 coating on LiNi1/3Co1/3Mn1/3O2
Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i
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cathode material will be a promising approach to improve electrochemical performances of LiNi1/3Co1/3Mn1/3O2 layered materials for lithium-ion batteries, especially for high-voltage applications.
Acknowledgements This study was supported by National Natural Science Foundation of China (Grant nos. 51372060 and 31501576).
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Please cite this article as: Y. Xie, et al., CeF3-modified LiNi1/3Co1/3Mn1/3O2 cathode material for high-voltage Li-ion batteries, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.06.074i