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Enhanced electrochemical performance of ZrO2 modified LiNi0.6Co0.2Mn0.2O2 cathode material for lithium ion batteries
Ceramics International 43 (2017) 15173–15178
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of ZrO2 modified LiNi0.6Co0.2Mn0.2O2 cathode material for lithium ion batteries ⁎
MARK
⁎
Tao Taoa, , Chao Chena, Yingbang Yaoa, Bo Lianga, Shengguo Lua, , Ying Chenb a Guangdong Provincial Research Center on Smart Materials and Energy Conversion Devices, Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China b Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Cathode ZrO2 nanoparticles LiNi0.6Co0.2Mn0.2O2 Modification Lithium ion batteries
LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode has been modified by incorporating ZrO2 nanoparticles to improve its electrochemical performance. Compared to the pristine electrode, the cycling stability and rate capability of 0.5 wt% ZrO2 modified-NCM622 have been improved significantly. The 0.5 wt% ZrO2 modified-NCM622 cathode shows a capacity retention of 83.8% after 100 cycles at 0.1 C between 2.8 and 4.3 V, while that of the pristine NCM622 electrode is only 75.6%. When the current rate is set as 5C, the capacity retention of the 0.5 wt % ZrO2-modified NCM622 is 10% higher than that of the pristine NCM622. Also, the rate capability of 0.5 wt% ZrO2-modified NCM622 is better than that of the pristine NCM622 at various C-rates in a voltage range of 2.8– 4.3 V. The enhanced electrochemical performances of the ZrO2-modified NCM622 cathodes can be attributed to their high Li-ion conductivity and structural stability.
1. Introduction Ni-rich layer structured LiNix[M]1−xO2 (x ≥ 0.5, M=Co, Mn and Al) cathodes have attracted much attention as an alternative to LiCoO2 for high performance Li-ion batteries (LIBs) because of their low cost and high specific capacity under a high voltage [1–3]. However, a migration of transition metallic ions of Ni-rich layer structured cathodes usually causes their structural instability, i.e., transition from a layered phase to a spinel and a rock-salt phases during the cycling, leading to a poor cycle stability and a weak thermal stability [4–6]. Extensive efforts have been made to improve the structural stability of Ni-rich layer structured cathodes by a surface modification with metallic oxides and metallic fluorides, such as Al2O3 [7], SiO2 [8], TiO2 [9], ZrO2 [10], and AlF3 [11]. These surface coatings can suppress the side reaction in the interface between the electrolyte and the electrode, reduce the transition-metal dissolution, and hinder the decomposition of the electrolyte. LiNi0.6Co0.2Mn0.2O2 (NCM622: N=Ni, C=Co, M=Mn, and molar ratio of Ni:Co:Mn=6:2:2) is regarded as a key cathode material due to its high discharge specific capacity (about 170 mAh g−1) and operating voltage (up to 4.3 V) [12]. Only a few strategies have been proposed to enhance the electrochemical properties of NCM622, including the surface modification of NCM622 with several inorganic materials (e.g. graphite [13], a dual conductive copolymer [14], nano-Al2O3 [15] and anatase nano-TiO2 [16]), and the fluorine substitution [17].
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Corresponding authors. E-mail addresses: [email protected] (T. Tao), [email protected] (S. Lu).
http://dx.doi.org/10.1016/j.ceramint.2017.08.048 Received 11 July 2017; Received in revised form 3 August 2017; Accepted 7 August 2017 Available online 08 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Therefore, developing new strategy is necessary for preparing a high electrochemical performance NCM622 cathode. ZrO2 has been considered as an ideal coating material to enhance the electrochemical performance of LIB cathodes, such as Li2CoPO4F [18], LiMn2O4 [19], Li3V2(PO4)3/C [20], LiNi0.5Co0.2Mn0.3O2 [21], LiFePO4 [22], and Li(Ni1/3Co1/3Mn1/3)O2 [23]. A ZrO2 coating can effectively increase the interphase stability between cathodes and electrolyte and prevent the structural breakage of cathodes. Up to now, the effect of ZrO2 coating on the electrochemical performance of NCM622 cathode has not been investigated. In this paper, NCM622 cathode materials are synthesized by a co-precipitation method. The surfaces of these electrodes are modified by coating with ZrO2 in order to improve their electrochemical performance. 2. Experiment 2.1. Materials synthesis 2.1.1. Preparation of NCM622 NH4OH (1.5 M) and NaOH (4 M) were mixed with a stoichiometric amount of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (molar ratio of Ni:Co:Mn=6:2:2) solution in a reactor. The obtained solution with a concentration of 2.0 M was heated at 50 °C, and stirred for 24 h. The pH value of the solution was adjusted to about 11 for preparing the
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Ni0.6Co0.2Mn0.2(OH)2 precipitate. After heating, the Ni0.6Co0.2 Mn0.2(OH)2 precipitate was washed, filtered and dried. The dried Ni0.6Co0.2Mn0.2(OH)2 was mixed with 5 wt% excess of LiOH·H2O. Then the mixture was heated at 500 °C for 6 h, and further annealed at 820 °C for 12 h under an O2 atmosphere to synthesize the NCM622 powder. The heating rate is fixed as 2 °C min−1. 2.1.2. Preparation of ZrO2 modified NCM622. ZrO2 nanoparticles (10–30 nm) were firstly dispersed in isopropyl alcohol with sonication for 1 h, and mixed with the NCM622 powder. Then the mixture was sonicated for 0.5 h, and heated to 60 °C and stirred with a magnetic bar for 6 h. The heated sample was dried at 100 °C for over 10 h, and annealed at 450 °C for 6 h in air for the preparation of ZrO2-modified NCM622. The heating rate was set as 2 °C min−1. 2.2. Materials characterization The crystallographic structures of the samples were examined by Xray diffraction (XRD, Bruker D8) with a Cu Ka radiation from 10° to 80° at 40 kV and 40 mA. The Fullprof software was used to refine the lattice parameters for the Rietveld analysis. The surface morphology, structure and particle size of samples were analyzed using SEM (FEINova Nano 450), and transmission electron microscopy (TEM, FEI Tecnai G2 F20) with energy dispersive x-ray spectrum (EDS) captured at 200 kV, respectively. Differential scanning calorimetry (DSC, STA 449 F3 Jupiter) was used to investigate the thermal stability of samples from room temperature to 500 °C at a rate of 5 °C min−1. 2.3. Electrochemical measurements All electrodes were composed of 80 wt% active materials, 10 wt% carbon black and 10 wt% poly(vinylidene fluoride) (PVDF). Their average mass loading on the Al foils (9 mm in diameter) was about 4 mg cm−2. 1 M LiPF6 in a 1:1:1 (by volume) mixture of ethylene carbonate, diethylene carbonate and dimethyl carbonate was used as the electrolyte. Li foil was selected as a counter/reference electrode, and a Celgard 2400 porous polyethylene film as a separator. Two electrode coin cells (CR2025-type) were assembled in an argon-filled glove box. LAND battery system (Land BT 2001A,) was used to test the Galvanostatic charge-discharge cycling between 2.5 V and 4.3 V (vs. Li/ Li+) at various current rates. Cyclic voltammetry was pursued at a scan rate of 0.1 mV s−1 from 2.5 to 4.3 V. Electrochemical impedance spectroscopy (EIS) was investigated with an amplitude of 5 mV s−1 in the frequency range of 10 mHz to 100 kHz using an electrochemical workstation (VMP3, Bio-Logic SA, France). 3. Results and discussion Fig. 1a shows the XRD patterns of the pristine powder and the NCM622 powders modified with 0.5, 1.0 and 2.0 wt% of ZrO2. Their Rietveld refined XRD patterns are presented in Fig. 1b–1c. All the XRD patterns indicate the characteristic peaks of a hexagonal α-NaFeO2 structure (space group: R3m ) [14]. No diffraction peaks of any impurities are detected. The samples of ZrO2-modified NCM622 have a high intensity ratio of I(003)/I(104)(> 1.2), which shows a low degree of cationic mixing of NCM622 [24]. The α-value (2.8468) and c-value (14.2061) of the unit cells are not changed after ZrO2 coating (α = 2.8473, c = 14.2065) according to the Rietveld refinement analyses. It is obvious that low content of ZrO2 coating does not affect the structure of NCM622 significantly. A SEM image of Ni0.6Co0.2Mn0.2(OH)2 precursor is shown in Fig. 2a. It reveals that a secondary spherical particle size of Ni0.6Co0.2Mn0.2(OH)2 precursor is around 4–6 µm, which consists of aggregated primary particles with a layered structure. After annealing and coating, the surface morphology of the spherical particle is
Fig. 1. (a) XRD patterns of pristine and ZrO2-modified samples, and Rietveld refined XRD patterns of (b) pristine and (c) 0.5 wt% ZrO2-modified samples.
obviously changed (Fig. 2b). The spherical particles consist of primary particles with sizes of about 0.5–1 µm instead of sheets. The EDS mapping (inset in Fig. 2b) and TEM image (Fig. 2c) of 0.5 wt% ZrO2modified NCM622 indicate that ZrO2 nanoparticles are attached on the surface of the pristine NCM622 particle after coating. The initial charge/discharge curves show that the content of ZrO2 nanoparticles coating has an effect on the discharge capacity of NCM622 samples (Fig. 3a). The sample modified with 0.5 wt% of ZrO2 delivers higher discharge capacity of 177.7 mAh g−1 at 0.1C rate (1C = 200 mAh g−1) with a voltage range of 2.8–4.3 V as compared
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Fig. 3. (a) Initial charge/discharge curves, (b) cycle performance, and (c) rate capability for pristine and ZrO2-modified NCM622. Fig. 2. SEM images of (a) Ni0.6Co0.2Mn0.2(OH)2 precursor and (b) 0.5 wt% ZrO2modified NCM622 (Inset is the corresponding EDS mapping of ZrO2- modified sample), and (c) TEM image of 0.5 wt% ZrO2-modified NCM622.
with the pristine sample (176.8 mAh g−1) and the samples modified with 1, and 2 wt% of ZrO2 (168.8 mAh g−1 and 168.2 mAh g−1). Fig. 3b presents the cycling performance of samples at 0.1C rate in a voltage range of 2.8–4.3 V. The pristine NCM622 electrode delivers the discharge capacity of 133.7 mAh g−1, and its capacity retention is 75.6% after 100 cycles. While the 0.5 wt% ZrO2-modified NCM622 electrode exhibits better capacity retention, and its discharge capacity is 146.6 mAh g−1 (83.8% of the initial discharge capacity) after 100 cycles.
The rate capabilities of the cathodes at various C-rates (from 0.1 to 5C) in a voltage range of 2.8–4.3 V are shown in Fig. 3c. The rate capability of 0.5 wt% ZrO2-modified NCM622 is obviously better than that of the pristine NCM622, 1.0 wt% ZrO2-modified NCM622 and 2.0 wt% ZrO2-modified NCM622. The discharge capacity of 0.5 wt% ZrO2-modified NCM622 is 177.0 mAh g−1, 128.0 mAh g−1 and 94.5 mAh g−1 at 0.1C, 2C and 5C, respectively. Correspondingly, its capacity retention is 53.4% at 5C, which is higher than that of the pristine NCM622 (43.2%). These results reveal that a proper amount of ZrO2 modification can enhance the electrochemical performance of the NCM622 electrode. The surface coating layer of ZrO2 between the electrode material and electrolyte could increase the interphase stability, the structural stability and the electric conductivity of the
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Fig. 4. Cyclic voltammetry curves of (a) pristine, (b) 0.5 wt% ZrO2-modified NCM622, (c) 1.0 wt% ZrO2-modified NCM622, and (d) 2.0 wt% ZrO2-modified NCM622 between 2.5 V and 4.5 V at a scan rate of 0.1 mV s−1.
electrode during cycling, which benefit their cycling performance, rate capability and thermal stability [25–27]. However, the electrochemical performance of NCM622 could be very sensitive to the modification amount of ZrO2 nanoparticles. Excess amount (2.0 wt%) of ZrO2 exhibits a negative effect on the improvement of the capacity and cyclic stability. Low ionic and electronic conductivities of the coating layer can suppress the transition of ions between the cathode and the electrolyte. Fig. 4 shows the cyclic voltammetry (CV) curves of samples in the first three cycles between 2.5 and 4.5 V at a scan rate of 0.1 mV s−1. All samples have one pair of anodic/cathodic peaks (Ni2+/Ni4+) in the CV curves during the lithiation/delithiation process. The electrochemical reversibility of electrodes is associated with a redox reaction gap (ΔV) between the cathodic peak and the anodic peak. The cathodes show a better electrochemical reversibility, when ΔV value is smaller. Compared to the pristine, 1.0 wt% ZrO2-modified NCM622 and 2.0 wt% ZrO2-modified NCM622 samples, ΔV value of 0.5 wt% ZrO2modified NCM622 sample is smaller in the first three cycles, which is 0.14, 0.09, and 0.07 V, respectively. It indicates that 0.5 wt% ZrO2modified NCM622 sample has the better electrochemical reversibility. The suitable amount (0.5 wt%) of ZrO2 modification is in favor of the decrease of the polarization and the improvement of cycling performance for the NCM622 cathodes. EIS measurements are proceeded to study the electrochemical kinetics of the electrodes after 100th cycles at 0.1C between 2.8 and 4.3 V. Fig. 5 shows the typical Nyquist plots of the pristine and ZrO2modified NCM622 samples. Wo represents the Warburg impedance, Rs is the resistance of the electrolyte solution, Rct corresponds to the
Fig. 5. Nyquist plots of pristine and ZrO2-modified NCM622 after 100 cycles.
resistance of interfacial charge transfer, and Rf is the SEI film resistance (inset in Fig. 5). The Rf values of all ZrO2-modified NCM622 samples (0.5 wt% = 24.07 Ω, 1.0 wt% = 22.84 Ω, and 2.0 wt % = 33.27 Ω) are lower than that of the pristine sample (73.78 Ω), indicating that the ZrO2 modification can decrease the SEI film resistance of the electrodes during charge/discharge cycles. Among these samples (0.0 wt% = 921.4 Ω, 1.0 wt% = 785.4 Ω, and 2.0 wt% = 871.6 Ω), 0.5 wt% ZrO2-modified NCM622 sample shows the lowest Rct value (550.1 Ω). It suggests that the suitable amount (0.5 wt%) of
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Fig. 6. SEM images of 0.5 wt% ZrO2-modified ((a) and (b)), and of pristine ((c) and (d)) NCM622 after 100 cycles at 0.1C between 2.8–4.3 V.
ZrO2 modification can increase the electrochemical kinetic behaviors at the interface of electrode and electrolyte. In order to understand the effect of the ZrO2 modification layer on the structural stability of the NCM622 electrodes during cycling, the morphologies of the pristine and 0.5 wt% ZrO2-modified NCM622 samples after 100 cycles at 0.1 C between 2.8 and 4.3 V are investigated (Fig. 6). Most of the 0.5 wt% ZrO2-modified NCM622 particles can maintain almost intact spherical morphologies (Fig. 6a and b) after a long-term cycling. However, without the protection of the ZrO2 modification, the spherical morphology of the pristine sample suffers serious damage. Most spherical particles (4–6 µm) are broken into many smaller particles (0.5–1 µm) (Figs. 6c and d). It could be because the continuous HF attack and the oxygen release generated from electrolyte could result in the structural instability of the electrodes [10,12]. Thus, it is believed that the ZrO2 modification may act as a protecting layer to hinder the interface side reaction between the electrolyte and the electrode, and inhibit the transition-metal dissolution of the NCM622 cathodes.
4. Conclusion NCM622 cathode material has been synthesized by a combination of co-precipitation and subsequent calcination. It has been demonstrated that the suitable amount (0.5 wt%) of ZrO2 nanoparticle modification can decrease the polarization and the charge transfer resistance of the NCM622 cathodes, and enhance their structural stability during cycling. In comparison with the pristine NCM622 cathode, the 0.5 wt% ZrO2-modified NCM622 electrode shows higher cycling stability, specific capacity and rate capability. The improved electrochemical performances of the 0.5 wt% ZrO2-modified NCM622 electrode could be attributed to the ZrO2 nanoparticles modification which can suppress the decomposition of the electrolyte and protect the electrode.
Acknowledgments This work was financially supported by the Science and Technology Program of Guangzhou (Grant number 201607010110), Science and Technology Planning Project of Guangdong Province, China (Grant number 2016A010104014), the Natural Science Foundation of China (Grant number 51372042), Guangdong Provincial Natural Science Foundation (Grant number 2015A030308004), and the NSFCGuangdong Joint Fund (Grant number U1501246). References [1] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, Thermal behavior of Li1−yNiO2 and the decomposition mechanism, Solid State Ion. 109 (1998) 295–302. [2] H. Arai, M. Tsuda, K. Saito, M. Hayashi, Y. Sakurai, Nickel dioxide polymorphs as lithium insertion electrodes, Electrochim. Acta 47 (2002) 2697–2705. [3] A.M. Kannan, A. Manthiram, Structural stability of Li1−XNi0.85Co0.15O2 and Li1−XNi0.85Co0.12Al0.03O2 cathodes at elevated temperatures, J. Electrochem. Soc. 150 (2003) A349–A353. [4] S. Venkatraman, J. Choi, A. Manthiram, Factors influencing the chemical lithium extraction rate from layered LiNi1−y−zCoyMnzO2 cathodes, Electrochem. Commun. 6 (2004) 832–837. [5] Y. Cho, P. Oh, J. Cho, A new type of protective surface layer for high-capacity Nibased cathode materials: nanoscaled surface pillaring layer, Nano Lett. 13 (2013) 1145–1152. [6] Y. Kojima, S. Muto, K. Tatsumi, H. Kondo, H. Oka, K. Horibuchi, Y. Ukyo, Degradation analysis of a Ni-based layered positive-electrode active material cycled at elevated temperatures studied by scanning transmission electron microscopy and electron energy-loss spectroscopy, J. Power Sources 196 (2011) 7721–7727. [7] A.M. Wise, C.M. Ban, J.N. Weker, S. Misra, A.S. Cavanagh, Z.C. Wu, Z. Li, M.S. Whittingham, K. Xu, S.M. George, M.F. Toney, Effect of Al2O3 coating on stabilizing LiNi0.4Mn0.4Co0.2O2 cathode, Chem. Mater. 27 (2015) 6146–6154. [8] C. Chen, T. Tao, W. Qi, H. Zeng, Y. Wu, B. Liang, Y.B. Yao, S.G. Lu, Y. Chen, Highperformance lithium ion batteries using SiO2-coated LiNi0.5Co0.2Mn0.3O2 microspheres as cathodes, J. Alloy. Compd. 709 (2017) 708–716. [9] T. Subburaj, Y.N. Jo, K. Prasanna, K.J. Kim, C.W. Lee, Titanium oxide nanofibers decorated nickel-rich cathodes as high performance electrodes in lithium ion batteries, J. Ind. Eng. Chem. 51 (2017) 223–228. [10] S.K. Hu, G.H. Cheng, M.Y. Cheng, B.J. Hwang, R. Santhanam, Cycle life improvement of ZrO2-coated spherical LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion batteries, J. Power Sources 188 (2009) 564–569.
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Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Enhanced electrochemical performance of ZrO2 modified LiNi0.6Co0.2Mn0.2O2 cathode material for lithium ion batteries ⁎
MARK
⁎
Tao Taoa, , Chao Chena, Yingbang Yaoa, Bo Lianga, Shengguo Lua, , Ying Chenb a Guangdong Provincial Research Center on Smart Materials and Energy Conversion Devices, Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China b Institute for Frontier Materials, Deakin University, Waurn Ponds, Victoria 3216, Australia
A R T I C L E I N F O
A BS T RAC T
Keywords: Cathode ZrO2 nanoparticles LiNi0.6Co0.2Mn0.2O2 Modification Lithium ion batteries
LiNi0.6Co0.2Mn0.2O2 (NCM622) cathode has been modified by incorporating ZrO2 nanoparticles to improve its electrochemical performance. Compared to the pristine electrode, the cycling stability and rate capability of 0.5 wt% ZrO2 modified-NCM622 have been improved significantly. The 0.5 wt% ZrO2 modified-NCM622 cathode shows a capacity retention of 83.8% after 100 cycles at 0.1 C between 2.8 and 4.3 V, while that of the pristine NCM622 electrode is only 75.6%. When the current rate is set as 5C, the capacity retention of the 0.5 wt % ZrO2-modified NCM622 is 10% higher than that of the pristine NCM622. Also, the rate capability of 0.5 wt% ZrO2-modified NCM622 is better than that of the pristine NCM622 at various C-rates in a voltage range of 2.8– 4.3 V. The enhanced electrochemical performances of the ZrO2-modified NCM622 cathodes can be attributed to their high Li-ion conductivity and structural stability.
1. Introduction Ni-rich layer structured LiNix[M]1−xO2 (x ≥ 0.5, M=Co, Mn and Al) cathodes have attracted much attention as an alternative to LiCoO2 for high performance Li-ion batteries (LIBs) because of their low cost and high specific capacity under a high voltage [1–3]. However, a migration of transition metallic ions of Ni-rich layer structured cathodes usually causes their structural instability, i.e., transition from a layered phase to a spinel and a rock-salt phases during the cycling, leading to a poor cycle stability and a weak thermal stability [4–6]. Extensive efforts have been made to improve the structural stability of Ni-rich layer structured cathodes by a surface modification with metallic oxides and metallic fluorides, such as Al2O3 [7], SiO2 [8], TiO2 [9], ZrO2 [10], and AlF3 [11]. These surface coatings can suppress the side reaction in the interface between the electrolyte and the electrode, reduce the transition-metal dissolution, and hinder the decomposition of the electrolyte. LiNi0.6Co0.2Mn0.2O2 (NCM622: N=Ni, C=Co, M=Mn, and molar ratio of Ni:Co:Mn=6:2:2) is regarded as a key cathode material due to its high discharge specific capacity (about 170 mAh g−1) and operating voltage (up to 4.3 V) [12]. Only a few strategies have been proposed to enhance the electrochemical properties of NCM622, including the surface modification of NCM622 with several inorganic materials (e.g. graphite [13], a dual conductive copolymer [14], nano-Al2O3 [15] and anatase nano-TiO2 [16]), and the fluorine substitution [17].
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Corresponding authors. E-mail addresses: [email protected] (T. Tao), [email protected] (S. Lu).
http://dx.doi.org/10.1016/j.ceramint.2017.08.048 Received 11 July 2017; Received in revised form 3 August 2017; Accepted 7 August 2017 Available online 08 August 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Therefore, developing new strategy is necessary for preparing a high electrochemical performance NCM622 cathode. ZrO2 has been considered as an ideal coating material to enhance the electrochemical performance of LIB cathodes, such as Li2CoPO4F [18], LiMn2O4 [19], Li3V2(PO4)3/C [20], LiNi0.5Co0.2Mn0.3O2 [21], LiFePO4 [22], and Li(Ni1/3Co1/3Mn1/3)O2 [23]. A ZrO2 coating can effectively increase the interphase stability between cathodes and electrolyte and prevent the structural breakage of cathodes. Up to now, the effect of ZrO2 coating on the electrochemical performance of NCM622 cathode has not been investigated. In this paper, NCM622 cathode materials are synthesized by a co-precipitation method. The surfaces of these electrodes are modified by coating with ZrO2 in order to improve their electrochemical performance. 2. Experiment 2.1. Materials synthesis 2.1.1. Preparation of NCM622 NH4OH (1.5 M) and NaOH (4 M) were mixed with a stoichiometric amount of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (molar ratio of Ni:Co:Mn=6:2:2) solution in a reactor. The obtained solution with a concentration of 2.0 M was heated at 50 °C, and stirred for 24 h. The pH value of the solution was adjusted to about 11 for preparing the
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Ni0.6Co0.2Mn0.2(OH)2 precipitate. After heating, the Ni0.6Co0.2 Mn0.2(OH)2 precipitate was washed, filtered and dried. The dried Ni0.6Co0.2Mn0.2(OH)2 was mixed with 5 wt% excess of LiOH·H2O. Then the mixture was heated at 500 °C for 6 h, and further annealed at 820 °C for 12 h under an O2 atmosphere to synthesize the NCM622 powder. The heating rate is fixed as 2 °C min−1. 2.1.2. Preparation of ZrO2 modified NCM622. ZrO2 nanoparticles (10–30 nm) were firstly dispersed in isopropyl alcohol with sonication for 1 h, and mixed with the NCM622 powder. Then the mixture was sonicated for 0.5 h, and heated to 60 °C and stirred with a magnetic bar for 6 h. The heated sample was dried at 100 °C for over 10 h, and annealed at 450 °C for 6 h in air for the preparation of ZrO2-modified NCM622. The heating rate was set as 2 °C min−1. 2.2. Materials characterization The crystallographic structures of the samples were examined by Xray diffraction (XRD, Bruker D8) with a Cu Ka radiation from 10° to 80° at 40 kV and 40 mA. The Fullprof software was used to refine the lattice parameters for the Rietveld analysis. The surface morphology, structure and particle size of samples were analyzed using SEM (FEINova Nano 450), and transmission electron microscopy (TEM, FEI Tecnai G2 F20) with energy dispersive x-ray spectrum (EDS) captured at 200 kV, respectively. Differential scanning calorimetry (DSC, STA 449 F3 Jupiter) was used to investigate the thermal stability of samples from room temperature to 500 °C at a rate of 5 °C min−1. 2.3. Electrochemical measurements All electrodes were composed of 80 wt% active materials, 10 wt% carbon black and 10 wt% poly(vinylidene fluoride) (PVDF). Their average mass loading on the Al foils (9 mm in diameter) was about 4 mg cm−2. 1 M LiPF6 in a 1:1:1 (by volume) mixture of ethylene carbonate, diethylene carbonate and dimethyl carbonate was used as the electrolyte. Li foil was selected as a counter/reference electrode, and a Celgard 2400 porous polyethylene film as a separator. Two electrode coin cells (CR2025-type) were assembled in an argon-filled glove box. LAND battery system (Land BT 2001A,) was used to test the Galvanostatic charge-discharge cycling between 2.5 V and 4.3 V (vs. Li/ Li+) at various current rates. Cyclic voltammetry was pursued at a scan rate of 0.1 mV s−1 from 2.5 to 4.3 V. Electrochemical impedance spectroscopy (EIS) was investigated with an amplitude of 5 mV s−1 in the frequency range of 10 mHz to 100 kHz using an electrochemical workstation (VMP3, Bio-Logic SA, France). 3. Results and discussion Fig. 1a shows the XRD patterns of the pristine powder and the NCM622 powders modified with 0.5, 1.0 and 2.0 wt% of ZrO2. Their Rietveld refined XRD patterns are presented in Fig. 1b–1c. All the XRD patterns indicate the characteristic peaks of a hexagonal α-NaFeO2 structure (space group: R3m ) [14]. No diffraction peaks of any impurities are detected. The samples of ZrO2-modified NCM622 have a high intensity ratio of I(003)/I(104)(> 1.2), which shows a low degree of cationic mixing of NCM622 [24]. The α-value (2.8468) and c-value (14.2061) of the unit cells are not changed after ZrO2 coating (α = 2.8473, c = 14.2065) according to the Rietveld refinement analyses. It is obvious that low content of ZrO2 coating does not affect the structure of NCM622 significantly. A SEM image of Ni0.6Co0.2Mn0.2(OH)2 precursor is shown in Fig. 2a. It reveals that a secondary spherical particle size of Ni0.6Co0.2Mn0.2(OH)2 precursor is around 4–6 µm, which consists of aggregated primary particles with a layered structure. After annealing and coating, the surface morphology of the spherical particle is
Fig. 1. (a) XRD patterns of pristine and ZrO2-modified samples, and Rietveld refined XRD patterns of (b) pristine and (c) 0.5 wt% ZrO2-modified samples.
obviously changed (Fig. 2b). The spherical particles consist of primary particles with sizes of about 0.5–1 µm instead of sheets. The EDS mapping (inset in Fig. 2b) and TEM image (Fig. 2c) of 0.5 wt% ZrO2modified NCM622 indicate that ZrO2 nanoparticles are attached on the surface of the pristine NCM622 particle after coating. The initial charge/discharge curves show that the content of ZrO2 nanoparticles coating has an effect on the discharge capacity of NCM622 samples (Fig. 3a). The sample modified with 0.5 wt% of ZrO2 delivers higher discharge capacity of 177.7 mAh g−1 at 0.1C rate (1C = 200 mAh g−1) with a voltage range of 2.8–4.3 V as compared
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Fig. 3. (a) Initial charge/discharge curves, (b) cycle performance, and (c) rate capability for pristine and ZrO2-modified NCM622. Fig. 2. SEM images of (a) Ni0.6Co0.2Mn0.2(OH)2 precursor and (b) 0.5 wt% ZrO2modified NCM622 (Inset is the corresponding EDS mapping of ZrO2- modified sample), and (c) TEM image of 0.5 wt% ZrO2-modified NCM622.
with the pristine sample (176.8 mAh g−1) and the samples modified with 1, and 2 wt% of ZrO2 (168.8 mAh g−1 and 168.2 mAh g−1). Fig. 3b presents the cycling performance of samples at 0.1C rate in a voltage range of 2.8–4.3 V. The pristine NCM622 electrode delivers the discharge capacity of 133.7 mAh g−1, and its capacity retention is 75.6% after 100 cycles. While the 0.5 wt% ZrO2-modified NCM622 electrode exhibits better capacity retention, and its discharge capacity is 146.6 mAh g−1 (83.8% of the initial discharge capacity) after 100 cycles.
The rate capabilities of the cathodes at various C-rates (from 0.1 to 5C) in a voltage range of 2.8–4.3 V are shown in Fig. 3c. The rate capability of 0.5 wt% ZrO2-modified NCM622 is obviously better than that of the pristine NCM622, 1.0 wt% ZrO2-modified NCM622 and 2.0 wt% ZrO2-modified NCM622. The discharge capacity of 0.5 wt% ZrO2-modified NCM622 is 177.0 mAh g−1, 128.0 mAh g−1 and 94.5 mAh g−1 at 0.1C, 2C and 5C, respectively. Correspondingly, its capacity retention is 53.4% at 5C, which is higher than that of the pristine NCM622 (43.2%). These results reveal that a proper amount of ZrO2 modification can enhance the electrochemical performance of the NCM622 electrode. The surface coating layer of ZrO2 between the electrode material and electrolyte could increase the interphase stability, the structural stability and the electric conductivity of the
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Fig. 4. Cyclic voltammetry curves of (a) pristine, (b) 0.5 wt% ZrO2-modified NCM622, (c) 1.0 wt% ZrO2-modified NCM622, and (d) 2.0 wt% ZrO2-modified NCM622 between 2.5 V and 4.5 V at a scan rate of 0.1 mV s−1.
electrode during cycling, which benefit their cycling performance, rate capability and thermal stability [25–27]. However, the electrochemical performance of NCM622 could be very sensitive to the modification amount of ZrO2 nanoparticles. Excess amount (2.0 wt%) of ZrO2 exhibits a negative effect on the improvement of the capacity and cyclic stability. Low ionic and electronic conductivities of the coating layer can suppress the transition of ions between the cathode and the electrolyte. Fig. 4 shows the cyclic voltammetry (CV) curves of samples in the first three cycles between 2.5 and 4.5 V at a scan rate of 0.1 mV s−1. All samples have one pair of anodic/cathodic peaks (Ni2+/Ni4+) in the CV curves during the lithiation/delithiation process. The electrochemical reversibility of electrodes is associated with a redox reaction gap (ΔV) between the cathodic peak and the anodic peak. The cathodes show a better electrochemical reversibility, when ΔV value is smaller. Compared to the pristine, 1.0 wt% ZrO2-modified NCM622 and 2.0 wt% ZrO2-modified NCM622 samples, ΔV value of 0.5 wt% ZrO2modified NCM622 sample is smaller in the first three cycles, which is 0.14, 0.09, and 0.07 V, respectively. It indicates that 0.5 wt% ZrO2modified NCM622 sample has the better electrochemical reversibility. The suitable amount (0.5 wt%) of ZrO2 modification is in favor of the decrease of the polarization and the improvement of cycling performance for the NCM622 cathodes. EIS measurements are proceeded to study the electrochemical kinetics of the electrodes after 100th cycles at 0.1C between 2.8 and 4.3 V. Fig. 5 shows the typical Nyquist plots of the pristine and ZrO2modified NCM622 samples. Wo represents the Warburg impedance, Rs is the resistance of the electrolyte solution, Rct corresponds to the
Fig. 5. Nyquist plots of pristine and ZrO2-modified NCM622 after 100 cycles.
resistance of interfacial charge transfer, and Rf is the SEI film resistance (inset in Fig. 5). The Rf values of all ZrO2-modified NCM622 samples (0.5 wt% = 24.07 Ω, 1.0 wt% = 22.84 Ω, and 2.0 wt % = 33.27 Ω) are lower than that of the pristine sample (73.78 Ω), indicating that the ZrO2 modification can decrease the SEI film resistance of the electrodes during charge/discharge cycles. Among these samples (0.0 wt% = 921.4 Ω, 1.0 wt% = 785.4 Ω, and 2.0 wt% = 871.6 Ω), 0.5 wt% ZrO2-modified NCM622 sample shows the lowest Rct value (550.1 Ω). It suggests that the suitable amount (0.5 wt%) of
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Fig. 6. SEM images of 0.5 wt% ZrO2-modified ((a) and (b)), and of pristine ((c) and (d)) NCM622 after 100 cycles at 0.1C between 2.8–4.3 V.
ZrO2 modification can increase the electrochemical kinetic behaviors at the interface of electrode and electrolyte. In order to understand the effect of the ZrO2 modification layer on the structural stability of the NCM622 electrodes during cycling, the morphologies of the pristine and 0.5 wt% ZrO2-modified NCM622 samples after 100 cycles at 0.1 C between 2.8 and 4.3 V are investigated (Fig. 6). Most of the 0.5 wt% ZrO2-modified NCM622 particles can maintain almost intact spherical morphologies (Fig. 6a and b) after a long-term cycling. However, without the protection of the ZrO2 modification, the spherical morphology of the pristine sample suffers serious damage. Most spherical particles (4–6 µm) are broken into many smaller particles (0.5–1 µm) (Figs. 6c and d). It could be because the continuous HF attack and the oxygen release generated from electrolyte could result in the structural instability of the electrodes [10,12]. Thus, it is believed that the ZrO2 modification may act as a protecting layer to hinder the interface side reaction between the electrolyte and the electrode, and inhibit the transition-metal dissolution of the NCM622 cathodes.
4. Conclusion NCM622 cathode material has been synthesized by a combination of co-precipitation and subsequent calcination. It has been demonstrated that the suitable amount (0.5 wt%) of ZrO2 nanoparticle modification can decrease the polarization and the charge transfer resistance of the NCM622 cathodes, and enhance their structural stability during cycling. In comparison with the pristine NCM622 cathode, the 0.5 wt% ZrO2-modified NCM622 electrode shows higher cycling stability, specific capacity and rate capability. The improved electrochemical performances of the 0.5 wt% ZrO2-modified NCM622 electrode could be attributed to the ZrO2 nanoparticles modification which can suppress the decomposition of the electrolyte and protect the electrode.
Acknowledgments This work was financially supported by the Science and Technology Program of Guangzhou (Grant number 201607010110), Science and Technology Planning Project of Guangdong Province, China (Grant number 2016A010104014), the Natural Science Foundation of China (Grant number 51372042), Guangdong Provincial Natural Science Foundation (Grant number 2015A030308004), and the NSFCGuangdong Joint Fund (Grant number U1501246). References [1] H. Arai, S. Okada, Y. Sakurai, J. Yamaki, Thermal behavior of Li1−yNiO2 and the decomposition mechanism, Solid State Ion. 109 (1998) 295–302. [2] H. Arai, M. Tsuda, K. Saito, M. Hayashi, Y. Sakurai, Nickel dioxide polymorphs as lithium insertion electrodes, Electrochim. Acta 47 (2002) 2697–2705. [3] A.M. Kannan, A. Manthiram, Structural stability of Li1−XNi0.85Co0.15O2 and Li1−XNi0.85Co0.12Al0.03O2 cathodes at elevated temperatures, J. Electrochem. Soc. 150 (2003) A349–A353. [4] S. Venkatraman, J. Choi, A. Manthiram, Factors influencing the chemical lithium extraction rate from layered LiNi1−y−zCoyMnzO2 cathodes, Electrochem. Commun. 6 (2004) 832–837. [5] Y. Cho, P. Oh, J. Cho, A new type of protective surface layer for high-capacity Nibased cathode materials: nanoscaled surface pillaring layer, Nano Lett. 13 (2013) 1145–1152. [6] Y. Kojima, S. Muto, K. Tatsumi, H. Kondo, H. Oka, K. Horibuchi, Y. Ukyo, Degradation analysis of a Ni-based layered positive-electrode active material cycled at elevated temperatures studied by scanning transmission electron microscopy and electron energy-loss spectroscopy, J. Power Sources 196 (2011) 7721–7727. [7] A.M. Wise, C.M. Ban, J.N. Weker, S. Misra, A.S. Cavanagh, Z.C. Wu, Z. Li, M.S. Whittingham, K. Xu, S.M. George, M.F. Toney, Effect of Al2O3 coating on stabilizing LiNi0.4Mn0.4Co0.2O2 cathode, Chem. Mater. 27 (2015) 6146–6154. [8] C. Chen, T. Tao, W. Qi, H. Zeng, Y. Wu, B. Liang, Y.B. Yao, S.G. Lu, Y. Chen, Highperformance lithium ion batteries using SiO2-coated LiNi0.5Co0.2Mn0.3O2 microspheres as cathodes, J. Alloy. Compd. 709 (2017) 708–716. [9] T. Subburaj, Y.N. Jo, K. Prasanna, K.J. Kim, C.W. Lee, Titanium oxide nanofibers decorated nickel-rich cathodes as high performance electrodes in lithium ion batteries, J. Ind. Eng. Chem. 51 (2017) 223–228. [10] S.K. Hu, G.H. Cheng, M.Y. Cheng, B.J. Hwang, R. Santhanam, Cycle life improvement of ZrO2-coated spherical LiNi1/3Co1/3Mn1/3O2 cathode material for lithium ion batteries, J. Power Sources 188 (2009) 564–569.
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