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Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries
Author’s Accepted Manuscript Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries Zuguang Yang, Wei Xiang, Zhenguo Wu, Fengrong He, Jun Zhang, Yao Xiao, Benhe Zhong, Xiaodong Guo
PII: DOI: Reference:
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S0272-8842(16)32298-2 http://dx.doi.org/10.1016/j.ceramint.2016.12.048 CERI14358
To appear in: Ceramics International Received date: 7 November 2016 Revised date: 7 December 2016 Accepted date: 8 December 2016 Cite this article as: Zuguang Yang, Wei Xiang, Zhenguo Wu, Fengrong He, Jun Zhang, Yao Xiao, Benhe Zhong and Xiaodong Guo, Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.12.048 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.
Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries Zuguang Yang1,2, Wei Xiang3, Zhenguo Wu1**, Fengrong He2, Jun Zhang2, Yao Xiao1, Benhe Zhong1, Xiaodong Guo1,4* 1
School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China.
2
Dong guan Hec Technology Research Corporation, Guangdong Dongguan 523871.
3
College of Materials and Chemistry &Chemical Engineering, Chengdu University of
Technology, Chengdu 610059, P. R. China. 4
Institute for Superconducting and Electronic Materials, University of Wollongong,
Wollongong NSW 2522, Australia. [email protected] [email protected] *
Corresponding authors
Abstract: Key issues including poor rate capability and limited cycle life span should be addressed for the extended application of LiNi0.5Co0.2Mn0.3O2 cathode. The suppressed Li+/Ni2+ site exchange, enlarged LiO2 inter-slab space and reduced impedance, which could facilitate the structure stability, were achieved by controlled Niobium (Nb) doping and contributed to enhanced performance even at elevated temperature (55 oC). The detailed role of the doped Nb was investigated thoroughly and systematically with the help of XRD, SEM, XPS and related electrochemical tests. The full and accurate
results demonstrate that the Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 sample with appropriate Nb doping amount possess high capacity retention of 93.77% after 100 cycles at 1.0 C and improved rate performance with 125.5 mAh g-1 at 5.0 C, which are much better than that of the LiNi0.5Co0.2Mn0.3O2. Moreover, at high temperature of 55℃, Nb doping shows more remarkable effect on stabilizing the structure and 88.63% of the initial reversible
capacity could
be
retained,
which
is
~20%
higher
than
the
LiNi0.5Co0.2Mn0.3O2. This study intensively determines that controlled Nb doping could be effectively maintain the structure stability of advanced LiNi0.5Co0.2Mn0.3O2 cathode and promote the development of high energy density lithium ion batteries.
Keywords Lithium-ion battery; Cathode; LiNi0.5Co0.2Mn0.3O2; Nb doping;
1. Introduction With the development of portable electronic devices and hybrid electric vehicles, lithium-ion batteries(LIBs) with large multiplying power, high energy density, and high degree of safety are urgently required [1-3]. The layered LiNixCoyMn1-x-yO2 cathode has attracted considerable research attention because of the integrated advantages of high capacity, high energy density, low cost, and low toxicity from LiNiO2, LiMnO2, and LiCoO2 [4-6]. Contrasted to symmetrical three-component materials (such as LiNi1/3Co1/3Mn1/3O2, LiNi0.45Co0.1Mn0.45O2, etc.), the asymmetric three-component material (such as LiNi0.5Co0.2Mn0.3O2) is capable of balancing the energy density, power density, and safety, which endows the application in electric vehicles. Therefore, LiNi0.5Co0.2Mn0.3O2 is considered as an alternative cathode materials for LiCoO2 [7-9].
Nevertheless, the asymmetric three-component material suffers from poor rate capability and inferior cycling stability due to the Li+/Ni2+ site-exchange caused by the similar radius of Li+(0.076nm)/Ni2+(0.069nm) and the crystal structure degradation induced by the side reaction between electrode and electrolyte [10-12]. To decrease the cation mixing of Li+/Ni2+ and strengthen the material structure, various tactics including the structural design, metal-ion doping and surface modification have been extensively studied. Among them, doping method with Na [13], Al [14], Ti [15], Zr [16], and V[17] have been reported for improving the structural stability and electrochemical properties of LiNi0.5Co0.2Mn0.3O2. Moreover, Nb has been regarded as the most attractive candidates with mutual effect of consolidating structure stability and boosting electrical conductivity. [18-22]. Yi et al. [23] revealed that the Nb dopant with a high valence in LiMn1.5Ni0.5O4 could improve the rate capability and cycling performance ascribed to small crystallite, higher conductivity and a fast lithium diffusion. Wu et al. [24] synthesized Nb-doped LiNi1/3Co1/3Mn1/3O2 with improved capacity retention, resulting from the more stable structure and lower resistance. Considering that the bond energy of Nb-O is stronger than that of M-O (M=Ni,Co,Mn) and LiNi0.5Co0.2Mn0.3O2 is currently the focus of cathode materials research, the effect of Nb doping on the structural and electrochemical properties of LiNi0.5Co0.2Mn0.3O2 deserves a further investigation. Here, controlled Nb doping strategy was adopted to conquer the drawbacks of LiNi0.5Co0.2Mn0.3O2 including unstable structure and restricted lifespan. And the doping
amount
was
varied
and
optimized.
The
spherical
precursor
Ni0.5Co0.2Mn0.3(OH)2 and Li(Ni0.5Co0.2Mn0.3)1-xNbxO2 (x=0,0.005,0.01,0.02) were prepared by hydroxide co-precipitation and solid-state method. The electrochemical test
results
showed
that
the
rate
capability
and
cycling
stability
of
Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 were obviously improved compared with that of LiNi0.5Co0.2Mn0.3O2. Various measurements were carried out to identify the effect of Nb dopant. The study proved that the Nb-doping in LiNi0.5Co0.2Mn0.3O2 could ameliorate the performances, thus promoting the development of its industrialization. 2. Experimental 2.1. Materials preparation Ni0.5Co0.2Mn0.3(OH)2 that used as the precursor of Nb-doped LiNi0.5Co0.2Mn0.3O2 was prepared by a continuous hydroxide co-precipitation method. Typically, the aqueous solution of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (in stoichiometric ratio of Ni:Co:Mn=5:2:3) with a concentration 2.0 mol·L-1 was slowly dripped into a continuous stirred tank reactor (CSTR) with a volume of 20 L under N2 atmosphere. Meanwhile, the 8 mol·L-1 NaOH solution, and a desired amount of NH3·H2O solution as chelating agent were also separately fed into the designed reactor. The reaction temperature was controlled at 55 ºC and the pH value of solution was kept at 11.5. Finally, the precipitate slurry was filtered, washed and dried at 120 ºC for 12 h to obtain spherical Ni0.5Co0.2Mn0.3(OH)2 precursor. The bare LiNi0.5Co0.2Mn0.3O2 powders (labeled as Nb0) were obtained by solidstate calcination. Precisely, as-prepared Ni0.5Co0.2Mn0.3(OH)2 precursor and Li2CO3 in a molar ratio of 1:1.03. After milling, the mixture was dried at 100 ºC for several hours to remove the ethanol completely. Then, the mixture was annealed at 500 ºC for 5 h, and then elevated to 920 ºC for 12 h in air. The Niobium doped Li(Ni0.5Co0.2Mn0.3)1-xNbxO2
materials,
labeled
Nb0.5(x=0.005),
Nb1(x=0.01),
Nb2(x=0.02) were synthesized by follow the same process. Appropriate ratio of Ni0.5Co0.2Mn0.3(OH)2 precursor, Li2CO3 and Nb2O5 were mixed. Then, the mixture was maintained at 550 ºC for 5 h, and then annealed at 920 ºC for 12 h in air.
2.2 Materials characterization Crystalline structure of the as-prepared compounds were characterized by Rigaku D/Max-IV X-ray diffractometer using Cu Kα radiation with the operating conditions of 40 kV and 30 mA. The XRD data was collected in the range of 10°–70° (2θ). The diffraction patterns were analyzed by PDXL2.0 program. Particle morphologies and size of samples were observed by scanning electron microscope (SEM, JEOL JSM6510LV). Energy dispersive X-ray spectroscopy (EDS) was employed to identify the element on the surface of samples. X-ray photoelectron spectra (XPS) test was performed using Thermo Scientific ESCALAB 250Xi. 2.3 Electrochemical measurement Electrochemical performances of these cathode materials were evaluated with CR2016 coin cell. The positive electrodes were prepared by mixing 90 wt% active materials, 5wt.% conductive materials (mixture of ks-6 and super P in ratio of 1:1) and 5wt.% PVDF (polyvinylidene fluoride) in N-methyl-2-pyrrolidone (NMP). Subsequently, the obtained slurry was casted onto an Al foil current collector by an automatic coating machine (JFA-II, Tianjin Yonglida laboratory equipment Co., LTD). And the Al foil with the slurry was dried at 100 ºC for 20 min in an oven. Then, the dried plate was cut into disk with diameters of 15 mm; and all electrodes were dried at 120 ºC for 12 h in a vacuum oven. The average loading mass of active material in the disk was about 7 mg. The areal loading amount of the active materials is about 4mg·cm-2. Subsequently, the electrodes were assembled into cells in glove box filled with argon using Li metal as anode and a polypropylene micro-porous film (Celgard 2300) as separator. Then, 1 M lithium hexafluorophosphate (LiPF6) dissolved in the solvents of ethylene carbonate (EC) and dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in volume) was used as electrolyte. The amount of
electrolyte is about 0.2g in a coin cell. The coin cells were galvanostatically charged to 4.3 V (vs. Li/Li+) and discharged to 3.0 V at room temperature and 55 °C using a battery test system. The cycle voltammogram (CV) was conducted by AUTOLAB (PGSTAT302N) electrochemical workstation in the voltage range from 3 to 4.3 V at a scanning rate of 0.1 mV·s-1. The Electrochemical impedance spectroscopy (EIS) tests were measured over the frequency range of 100 kHz to 0.01 Hz with alternatingcurrent amplitude of 5 mV using AUTOLAB (PGSTAT302N). 3. Results and discussion The crystal structures of the as-prepared samples were analyzed by X-ray diffraction (XRD), and the results were shown in Fig. 1. The diffraction peaks of all the samples could be ascribed to layered hexagonal structure α-NaFeO2 with R-3m space group without any detectable impure peaks. The diffraction peaks at (006)/(102) and (108)/(110) were clearly split, which inferred that the Nb dopant would not affect the highly ordered layered structure. The characteristic (003) peak shifted to a lower degree with the increase of doping amount, which could be attributed to the larger radius of Nb5+ compared that of Co3+ and Mn4+ [25]. Fig. 2 shows the refinement results of XRD pattern with Rietveld method, which give detailed structure information. The reliability factor (Rwp) of samples was below 8.0%, indicating reliable results [26-28]. The structural parameters of all samples are summarized in Table 1. The values of a and c increased with the niobium doping amount rose, proving that Nb+5 has entered the crystal lattice of the materials successfully and the consistency of (003) peak shift. What’s more, the percentage of Ni3b/Nitot, which represents the ratio of Li+/Ni2+ site-exchange, decreased from 10.8% to 5.8% as the doping amount of Nb increased (Table 1), proving that Nb doped samples had a low degree of cation mixing. XRD refinement also confirmed that the
layer thickness of Li-O (ILiO2) significantly increased with Nb doping. In addition, according to the reports that the bond dissociation energy of Nb-O (Hƒ298(Nb-O) = 753 kJ·mol-1) is stronger than that of M-O (Hƒ298(Ni-O)=391.6 kJ·mol-1, Hƒ298(CoO)= 368 kJ·mol-1 and Hƒ298(Mn-O)= 402 kJ·mol-1) [16, 29]. Therefore, the substitution of Nb5+ in the host structure would effectively suppress the structure degradation and enhance the ultimate electrochemical performances. Spherical particles piled up tightly by primary nano-particles were observed in all the samples (Fig. 2), pointing out that the morphology of the bare sample was not affected by the Nb doping. Homogeneous element distribution of Ni, Co, Mn could be detected according to the EDS elemental mapping results (Fig. 3). XPS measurements of Nb0 and Nb1 were carried out to further study the influence of Nb doping on the chemical states of transition-metal ions. The prominent peaks of the binding energy at 854.3, 780.0, and 642.8 eV correspond to the binding energy positions of Ni2+, Co3+, and Mn4+, respectively [30]. The curves of the Ni2p spectra of Nb0 and Nb1 illustrated that both Ni2+ and Ni3+ could be detected as previous reports [31, 32]. The fitting spectra demonstrated that the peak area ratio of Ni2+ and Ni3+ of Nb0 is smaller than that of Nb1, which could be ascribed to charge balance and the reduction of cation mixing degree [16]. As shown in Fig. 4(b), the existence of Nb elements were detected through XPS. The prominent peaks of the binding energy at 206.3 eV and 209.05 eV are attributed to the binding energy positions of Nb3d5/2 and Nb3d3/2, respectively. These results agree with the former reported values of Nb5+ in literature [33]. To determine the influence of the Nb doping on the rate performance of the materials, the electrodes of all samples were executed charge–discharge tests by coin cells under the discharge capacity of 0.1, 0.2 C, 0.5 C, 1.0 C, 2.0 C, and 5.0 C. The rate
capabilities of all samples are shown in Fig. 5(a). Charge–discharge tests were performed for five times under each current density. As the current density increased, the discharge specific capacity of sample Nb0 decreased rapidly. And the Nb1 sample exhibits the best rate performance. The discharge capacity of Nb1 at 5.0 C is 125.5 mAh·g-1, whereas those of Nb0, Nb0.005, and Nb2 are only 108.2, 117.0, and 113.2 mAh·g-1, respectively. All these results prove that an appropriate amount of Nb doping could adequately improve the rate capability. Furthermore, the cycle performance of the as-prepared samples are shown in Fig. 5(b). After 100 cycles of loop testing at room temperature at 1.0 C, the discharge capacity of Nb0 is 126.72 mAh·g-1 and its corresponding capacity retention is 81.70 %. Under the same conditions, the results are 92.02 %, 93.77 %, and 87.19 % for Nb0.5, Nb1, and Nb2, respectively. Therefore, a appropriate amount of Nb doping could promote the material to maintain superior cycle performance. The cycle performance at 55 °C was also evaluated to estimate the structure stability
of
Li(Ni0.5Co0.2Mn0.3)1-xNbxO2
(x=0,0.005,0.01,0.02)
under
elevated
temperature with the help of a constant temperature box. As shown in Fig. 6, the initial discharge capacities at 1.0 C of Nb0, Nb0.5, Nb1, and Nb2 are 174.52, 169.70, 169.70, and 168.81 mAh·g-1, respectively. The initial discharge capacities at 55 °C are higher than that at room temperature, which indicates that high temperature could expedite the electrochemical activity. Nb1 maintained much better cycle stability with a high capacity retention of 88.63% in comparison with that of Nb0 (68.28%). And the capacity retention ratios of Nb0.5 and Nb2 are 77.32 % and 71.75 %, respectively. And the better structure stability of Nb-doped sample could be further confirmed by this result.
The cyclic voltammetry (CV) of the Nb0 and Nb1 were measured at a scan rate of 0.1 mV·s-1 between 3.0 and 4.3 V. And the first three cycles were shown in Fig. 7. According to the CV curves, only one pair redox peaks was found. No phase transformation from hexagonal phase to spinel phase occurred during Li ions intercalation/deintercalation. The oxidation peak around 3.6-4.2V was ascribed to the reaction of Ni2+ to Ni3+/Ni4+ [34, 35]. The cathodic peak potential (Epc), anodic peak potential (Epa), and corresponding potential difference (ΔEp) of the Nb0 and Nb1 for the first three cycles were listed in Table 2. As can be seen in Table 2, The ΔEp of Nb1 (0.192 V) is quite smaller than that of Nb0 (0.269 V), which suggests that 1 % Nb doping could be helpful to decrease the electrochemical polarization and result in better reaction kinetic. In order to elucidate the electrode kinetics of cathode materials, the impedance changes of the Nb0 and Nb1 samples were analyzed and compared through electrochemical impedance spectroscopy (EIS). Fig. 8 shows the EIS graph of Nb0 and Nb1 for 1 cycle and 100 cycles under a room temperature at 1.0 C. Generally, small interrupt represents to the impedance of solution (Rs). The first semicircle is attributed to the surface film resistance (Rsf) corresponding lithium ion migration through the interface between the electrolyte and the surface of particles, and the second semicircle could be regarded as the charge-transfer resistance (Rct) [24, 36, 37]. The Rsf and Rct values for the Nb0 and Nb1 electrodes are fitted by the given equivalent circuit using Zview software, and the results are shown in table 3. The increase of Rct values for both samples after 100 cycles are relatively large as compared to the Rsf values, which indicates that the cell impedance is mainly reflected by the charge-transfer resistance (Rct). The Rct of Nb1 was 101.6 after first cycle then increased to 263.9 after 100 cycles. Nevertheless, the Rct of Nb0 enlarged drastically from 137.4 after first cycle
to 769.4 after 100 cycles. The results demonstrate that 1% of niobium doping could also bring lower charge transfer resistance, thereby improving electrochemical properties.
4. Conclusions Li(Ni0.5Co0.2Mn0.3)1-xNbxO2 (x=0, 0.005, 0.01 and 0.02) cathode materials were successfully prepared via a hydroxide co-precipitation reaction with subsequent solid state sintering reaction. The Li+/Ni2+ cation mixing degree was reduced with Nb doping. Besides, the lattice parameters a, c and interslab space thickness (ILiO2) were slight increased, which facilitate the Li+ migration and structure stability. The electrochemical measurements confirm that niobium doping leads to the excellent cycling stability. When cycling at 55 °C, Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 exhibits a superior capacity retention of 88.63 % after 100 cycles compared with other samples. CV and EIS results confirmed that Nb1 sample has lower polarization and smaller charge transfer resistance. Thus, an appropriate amount of Nb doping into LiNi0.5Co0.2Mn0.3O2 could improve its electrochemical performance, as well as promote the development of its industrialization.
Acknowledgements The authors appreciate the supports of the Project funded by China Postdoctoral Science Foundation (No. 2014M562322), the Science and Technology Pillar Program of Sichuan University (2014GZ0077), the Development of Advanced Electrode and Electrolytes for LIB (AutoCRC Project 1-111) and the National Natural Science Foundation of China (No.201506133).
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[28]
B.
Song,
M.O.
Lai,
L.
Lu,
Influence
of
Ru
substitution
on
Li-rich
0.55Li2MnO3·0.45LiNi1/3Co1/3Mn1/3O2 cathode for Li-ion batteries, Electrochim. Acta 80 (2012) 187-195. [29] N.A. Lange, J.A. Dean, Lange's Handbook of Chemistry, McGraw-Hill, 1973. [30] J. Li, S. Xiong, Y. Liu, Z. Ju, Y. Qian, Uniform LiNi1/3Co1/3Mn1/3O2 hollow microspheres: Designed synthesis, topotactical structural transformation and their enhanced electrochemical performance, Nano Energy 2 (6) (2013) 1249-1260. [31] K. Amine. H.Tukamoto, H. Yasuda, and Y. Fuiita, A New Three-Volt Spinel Li1+xMn1.5Ni0.5O4 for Secondary Lithium Batteries, J. Electrochem. Soc. 14 (1996) 16071613. [32] Y.S. Lee, D. Ahn, Y.H. Cho, T.E. Hong, J. Cho, Improved Rate Capability and Thermal Stability of LiNi0.5Co0.2Mn0.3O2 Cathode Materials via Nanoscale SiP2O7 Coating, J. Electrochem. Soc. 158 (12) (2011) A1354-A1360. [33] M. Aufray, S. Menuel, Y. Fort, J. Eschbach, D. Rouxel, B. Vincent, New Synthesis of Nanosized Niobium Oxides and Lithium Niobate Particles and Their Characterization by XPS Analysis, J. Nanosci. Nanotechnol. 9 (8) (2009) 4780-4785. [34] W.H. Ryu, S.J. Lim, W.K. Kim, H. Kwon, 3-D dumbbell-like LiNi1/3Mn1/3Co1/3O2 cathode materials assembled with nano-building blocks for lithium-ion batteries, J. Power Sources 257 (2014) 186-191. [35] S.H. Park, C.S. Yoon, S.G. Kang, H.S. Kim, S.I. Moon, Y.K. Sun, Synthesis and structural characterization of layered Li[Ni1/3Co1/3Mn1/3]O2 cathode materials by ultrasonic spray pyrolysis method, Electrochim. Acta 49 (4) (2004) 557-563. [36] Y. Ding, P. Zhang, D. Gao, Synthesis and electrochemical properties of layered Li[Ni1/3Co1/3Mn1/3]0.96Ti0.04O1.96F0.04 as cathode material for lithium-ion batteries, J. Alloys. Compd. 456 (1-2) (2008) 344-347. [37] G. Yan, X. Li, Z. Wang, H. Guo, C. Wang, Tris(trimethylsilyl)phosphate: A filmforming additive for high voltage cathode material in lithium-ion batteries, J. Power Sources 248 (2014) 1306-1311.
Fig. 1 (a) XRD patterns of bare (Nb0) and Nb-doped LiNi0.5Co0.2Mn0.3O2 (Nb0.5, Nb1, Nb2)；(b) the magnified zone from 18.3 to 19.5° of XRD patterns Fig. 2 Rietveld refinement of the Nb0 (a), Nb0.5 (b), Nb1 (c) and Nb2 (d) samples with SEM images inset. Fig. 3 SEM image and Energy dispersive X-ray spectroscopy (EDS) mapping of Ni, Mn, Co, O, Nb of Nb1 sample. Fig. 4 High resolution XPS spectra with fitted results of (a) Ni2p and (b) Nb3d of Nb0 and Nb1 samples. Fig. 5 (a) Rate performance of the prepared samples at different current densities. (b) Cycling performance of the samples at room temperature. Fig. 6 Cycling performance of the as-prepared samples at 55 oC at 1.0 C in the voltage of 3-4.3V. Fig. 7 CV curves for the first 3 cycles of the prepared samples (a) Nb0 and (b) Nb1. Fig. 8 EIS plots of the Nb0 (a) and Nb1 (b) electrodes with an equivalent circuit (inset).
Table 1. Structural parameters obtained from X-ray Rietveld refinement of the prepared samples. Sample
a(Å)
c(Å)
Ni3b
Ni3b/Nitot(%)
ILiO2
Rwp(%)
Rp(%)
Nb0
2.8689
14.2370
0.054
10.8
2.6120
4.85
3.94
Nb0.5
2.8708
14.2462
0.038
7.6
2.6168
4.42
3.81
Nb1
2.8716
14.2510
0.029
5.8
2.6183
5.14
4.23
Nb2
2.8721
14.2537
0.047
9.4
2.6154
2.16
1.69
Table 2. The anodic peak potential (Epa), cathodic peak potential (Epc) and the corresponding potential difference (ΔEp) obtained from CV curves of as-prepared samples for the first three cycles. Sample
1st
2nd
3rd
Epa(V)
Epc(V)
ΔEp(V)
Epa(V)
Epc(V)
ΔEp(V)
Epa(V)
Epc(V)
ΔEp(V)
Nb0
3.948
3.679
0.269
3.848
3.682
0.166
3.838
3.679
0.159
Nb1
3.901
3.709
0.192
3.828
3.709
0.119
3.823
3.713
0.110
Table 3 Fitting results of equivalent circuit from Nyquist curves for the samples. 1st cycle Samples
100th cycle
Rs
Rsf()
Rct()
Rs
Rsf()
Rct()
Nb0
4.1
31.88
137.4
7.0
70.34
769.4
Nb1
5.0
18.96
101.6
5.7
69.34
263.9
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
PII: DOI: Reference:
www.elsevier.com/locate/ceri
S0272-8842(16)32298-2 http://dx.doi.org/10.1016/j.ceramint.2016.12.048 CERI14358
To appear in: Ceramics International Received date: 7 November 2016 Revised date: 7 December 2016 Accepted date: 8 December 2016 Cite this article as: Zuguang Yang, Wei Xiang, Zhenguo Wu, Fengrong He, Jun Zhang, Yao Xiao, Benhe Zhong and Xiaodong Guo, Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.12.048 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.
Effect of niobium doping on the structure and electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials for lithium ion batteries Zuguang Yang1,2, Wei Xiang3, Zhenguo Wu1**, Fengrong He2, Jun Zhang2, Yao Xiao1, Benhe Zhong1, Xiaodong Guo1,4* 1
School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China.
2
Dong guan Hec Technology Research Corporation, Guangdong Dongguan 523871.
3
College of Materials and Chemistry &Chemical Engineering, Chengdu University of
Technology, Chengdu 610059, P. R. China. 4
Institute for Superconducting and Electronic Materials, University of Wollongong,
Wollongong NSW 2522, Australia. [email protected] [email protected] *
Corresponding authors
Abstract: Key issues including poor rate capability and limited cycle life span should be addressed for the extended application of LiNi0.5Co0.2Mn0.3O2 cathode. The suppressed Li+/Ni2+ site exchange, enlarged LiO2 inter-slab space and reduced impedance, which could facilitate the structure stability, were achieved by controlled Niobium (Nb) doping and contributed to enhanced performance even at elevated temperature (55 oC). The detailed role of the doped Nb was investigated thoroughly and systematically with the help of XRD, SEM, XPS and related electrochemical tests. The full and accurate
results demonstrate that the Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 sample with appropriate Nb doping amount possess high capacity retention of 93.77% after 100 cycles at 1.0 C and improved rate performance with 125.5 mAh g-1 at 5.0 C, which are much better than that of the LiNi0.5Co0.2Mn0.3O2. Moreover, at high temperature of 55℃, Nb doping shows more remarkable effect on stabilizing the structure and 88.63% of the initial reversible
capacity could
be
retained,
which
is
~20%
higher
than
the
LiNi0.5Co0.2Mn0.3O2. This study intensively determines that controlled Nb doping could be effectively maintain the structure stability of advanced LiNi0.5Co0.2Mn0.3O2 cathode and promote the development of high energy density lithium ion batteries.
Keywords Lithium-ion battery; Cathode; LiNi0.5Co0.2Mn0.3O2; Nb doping;
1. Introduction With the development of portable electronic devices and hybrid electric vehicles, lithium-ion batteries(LIBs) with large multiplying power, high energy density, and high degree of safety are urgently required [1-3]. The layered LiNixCoyMn1-x-yO2 cathode has attracted considerable research attention because of the integrated advantages of high capacity, high energy density, low cost, and low toxicity from LiNiO2, LiMnO2, and LiCoO2 [4-6]. Contrasted to symmetrical three-component materials (such as LiNi1/3Co1/3Mn1/3O2, LiNi0.45Co0.1Mn0.45O2, etc.), the asymmetric three-component material (such as LiNi0.5Co0.2Mn0.3O2) is capable of balancing the energy density, power density, and safety, which endows the application in electric vehicles. Therefore, LiNi0.5Co0.2Mn0.3O2 is considered as an alternative cathode materials for LiCoO2 [7-9].
Nevertheless, the asymmetric three-component material suffers from poor rate capability and inferior cycling stability due to the Li+/Ni2+ site-exchange caused by the similar radius of Li+(0.076nm)/Ni2+(0.069nm) and the crystal structure degradation induced by the side reaction between electrode and electrolyte [10-12]. To decrease the cation mixing of Li+/Ni2+ and strengthen the material structure, various tactics including the structural design, metal-ion doping and surface modification have been extensively studied. Among them, doping method with Na [13], Al [14], Ti [15], Zr [16], and V[17] have been reported for improving the structural stability and electrochemical properties of LiNi0.5Co0.2Mn0.3O2. Moreover, Nb has been regarded as the most attractive candidates with mutual effect of consolidating structure stability and boosting electrical conductivity. [18-22]. Yi et al. [23] revealed that the Nb dopant with a high valence in LiMn1.5Ni0.5O4 could improve the rate capability and cycling performance ascribed to small crystallite, higher conductivity and a fast lithium diffusion. Wu et al. [24] synthesized Nb-doped LiNi1/3Co1/3Mn1/3O2 with improved capacity retention, resulting from the more stable structure and lower resistance. Considering that the bond energy of Nb-O is stronger than that of M-O (M=Ni,Co,Mn) and LiNi0.5Co0.2Mn0.3O2 is currently the focus of cathode materials research, the effect of Nb doping on the structural and electrochemical properties of LiNi0.5Co0.2Mn0.3O2 deserves a further investigation. Here, controlled Nb doping strategy was adopted to conquer the drawbacks of LiNi0.5Co0.2Mn0.3O2 including unstable structure and restricted lifespan. And the doping
amount
was
varied
and
optimized.
The
spherical
precursor
Ni0.5Co0.2Mn0.3(OH)2 and Li(Ni0.5Co0.2Mn0.3)1-xNbxO2 (x=0,0.005,0.01,0.02) were prepared by hydroxide co-precipitation and solid-state method. The electrochemical test
results
showed
that
the
rate
capability
and
cycling
stability
of
Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 were obviously improved compared with that of LiNi0.5Co0.2Mn0.3O2. Various measurements were carried out to identify the effect of Nb dopant. The study proved that the Nb-doping in LiNi0.5Co0.2Mn0.3O2 could ameliorate the performances, thus promoting the development of its industrialization. 2. Experimental 2.1. Materials preparation Ni0.5Co0.2Mn0.3(OH)2 that used as the precursor of Nb-doped LiNi0.5Co0.2Mn0.3O2 was prepared by a continuous hydroxide co-precipitation method. Typically, the aqueous solution of NiSO4·6H2O, CoSO4·7H2O and MnSO4·H2O (in stoichiometric ratio of Ni:Co:Mn=5:2:3) with a concentration 2.0 mol·L-1 was slowly dripped into a continuous stirred tank reactor (CSTR) with a volume of 20 L under N2 atmosphere. Meanwhile, the 8 mol·L-1 NaOH solution, and a desired amount of NH3·H2O solution as chelating agent were also separately fed into the designed reactor. The reaction temperature was controlled at 55 ºC and the pH value of solution was kept at 11.5. Finally, the precipitate slurry was filtered, washed and dried at 120 ºC for 12 h to obtain spherical Ni0.5Co0.2Mn0.3(OH)2 precursor. The bare LiNi0.5Co0.2Mn0.3O2 powders (labeled as Nb0) were obtained by solidstate calcination. Precisely, as-prepared Ni0.5Co0.2Mn0.3(OH)2 precursor and Li2CO3 in a molar ratio of 1:1.03. After milling, the mixture was dried at 100 ºC for several hours to remove the ethanol completely. Then, the mixture was annealed at 500 ºC for 5 h, and then elevated to 920 ºC for 12 h in air. The Niobium doped Li(Ni0.5Co0.2Mn0.3)1-xNbxO2
materials,
labeled
Nb0.5(x=0.005),
Nb1(x=0.01),
Nb2(x=0.02) were synthesized by follow the same process. Appropriate ratio of Ni0.5Co0.2Mn0.3(OH)2 precursor, Li2CO3 and Nb2O5 were mixed. Then, the mixture was maintained at 550 ºC for 5 h, and then annealed at 920 ºC for 12 h in air.
2.2 Materials characterization Crystalline structure of the as-prepared compounds were characterized by Rigaku D/Max-IV X-ray diffractometer using Cu Kα radiation with the operating conditions of 40 kV and 30 mA. The XRD data was collected in the range of 10°–70° (2θ). The diffraction patterns were analyzed by PDXL2.0 program. Particle morphologies and size of samples were observed by scanning electron microscope (SEM, JEOL JSM6510LV). Energy dispersive X-ray spectroscopy (EDS) was employed to identify the element on the surface of samples. X-ray photoelectron spectra (XPS) test was performed using Thermo Scientific ESCALAB 250Xi. 2.3 Electrochemical measurement Electrochemical performances of these cathode materials were evaluated with CR2016 coin cell. The positive electrodes were prepared by mixing 90 wt% active materials, 5wt.% conductive materials (mixture of ks-6 and super P in ratio of 1:1) and 5wt.% PVDF (polyvinylidene fluoride) in N-methyl-2-pyrrolidone (NMP). Subsequently, the obtained slurry was casted onto an Al foil current collector by an automatic coating machine (JFA-II, Tianjin Yonglida laboratory equipment Co., LTD). And the Al foil with the slurry was dried at 100 ºC for 20 min in an oven. Then, the dried plate was cut into disk with diameters of 15 mm; and all electrodes were dried at 120 ºC for 12 h in a vacuum oven. The average loading mass of active material in the disk was about 7 mg. The areal loading amount of the active materials is about 4mg·cm-2. Subsequently, the electrodes were assembled into cells in glove box filled with argon using Li metal as anode and a polypropylene micro-porous film (Celgard 2300) as separator. Then, 1 M lithium hexafluorophosphate (LiPF6) dissolved in the solvents of ethylene carbonate (EC) and dimethyl carbonate (DMC) and diethyl carbonate (DEC) (1:1:1 in volume) was used as electrolyte. The amount of
electrolyte is about 0.2g in a coin cell. The coin cells were galvanostatically charged to 4.3 V (vs. Li/Li+) and discharged to 3.0 V at room temperature and 55 °C using a battery test system. The cycle voltammogram (CV) was conducted by AUTOLAB (PGSTAT302N) electrochemical workstation in the voltage range from 3 to 4.3 V at a scanning rate of 0.1 mV·s-1. The Electrochemical impedance spectroscopy (EIS) tests were measured over the frequency range of 100 kHz to 0.01 Hz with alternatingcurrent amplitude of 5 mV using AUTOLAB (PGSTAT302N). 3. Results and discussion The crystal structures of the as-prepared samples were analyzed by X-ray diffraction (XRD), and the results were shown in Fig. 1. The diffraction peaks of all the samples could be ascribed to layered hexagonal structure α-NaFeO2 with R-3m space group without any detectable impure peaks. The diffraction peaks at (006)/(102) and (108)/(110) were clearly split, which inferred that the Nb dopant would not affect the highly ordered layered structure. The characteristic (003) peak shifted to a lower degree with the increase of doping amount, which could be attributed to the larger radius of Nb5+ compared that of Co3+ and Mn4+ [25]. Fig. 2 shows the refinement results of XRD pattern with Rietveld method, which give detailed structure information. The reliability factor (Rwp) of samples was below 8.0%, indicating reliable results [26-28]. The structural parameters of all samples are summarized in Table 1. The values of a and c increased with the niobium doping amount rose, proving that Nb+5 has entered the crystal lattice of the materials successfully and the consistency of (003) peak shift. What’s more, the percentage of Ni3b/Nitot, which represents the ratio of Li+/Ni2+ site-exchange, decreased from 10.8% to 5.8% as the doping amount of Nb increased (Table 1), proving that Nb doped samples had a low degree of cation mixing. XRD refinement also confirmed that the
layer thickness of Li-O (ILiO2) significantly increased with Nb doping. In addition, according to the reports that the bond dissociation energy of Nb-O (Hƒ298(Nb-O) = 753 kJ·mol-1) is stronger than that of M-O (Hƒ298(Ni-O)=391.6 kJ·mol-1, Hƒ298(CoO)= 368 kJ·mol-1 and Hƒ298(Mn-O)= 402 kJ·mol-1) [16, 29]. Therefore, the substitution of Nb5+ in the host structure would effectively suppress the structure degradation and enhance the ultimate electrochemical performances. Spherical particles piled up tightly by primary nano-particles were observed in all the samples (Fig. 2), pointing out that the morphology of the bare sample was not affected by the Nb doping. Homogeneous element distribution of Ni, Co, Mn could be detected according to the EDS elemental mapping results (Fig. 3). XPS measurements of Nb0 and Nb1 were carried out to further study the influence of Nb doping on the chemical states of transition-metal ions. The prominent peaks of the binding energy at 854.3, 780.0, and 642.8 eV correspond to the binding energy positions of Ni2+, Co3+, and Mn4+, respectively [30]. The curves of the Ni2p spectra of Nb0 and Nb1 illustrated that both Ni2+ and Ni3+ could be detected as previous reports [31, 32]. The fitting spectra demonstrated that the peak area ratio of Ni2+ and Ni3+ of Nb0 is smaller than that of Nb1, which could be ascribed to charge balance and the reduction of cation mixing degree [16]. As shown in Fig. 4(b), the existence of Nb elements were detected through XPS. The prominent peaks of the binding energy at 206.3 eV and 209.05 eV are attributed to the binding energy positions of Nb3d5/2 and Nb3d3/2, respectively. These results agree with the former reported values of Nb5+ in literature [33]. To determine the influence of the Nb doping on the rate performance of the materials, the electrodes of all samples were executed charge–discharge tests by coin cells under the discharge capacity of 0.1, 0.2 C, 0.5 C, 1.0 C, 2.0 C, and 5.0 C. The rate
capabilities of all samples are shown in Fig. 5(a). Charge–discharge tests were performed for five times under each current density. As the current density increased, the discharge specific capacity of sample Nb0 decreased rapidly. And the Nb1 sample exhibits the best rate performance. The discharge capacity of Nb1 at 5.0 C is 125.5 mAh·g-1, whereas those of Nb0, Nb0.005, and Nb2 are only 108.2, 117.0, and 113.2 mAh·g-1, respectively. All these results prove that an appropriate amount of Nb doping could adequately improve the rate capability. Furthermore, the cycle performance of the as-prepared samples are shown in Fig. 5(b). After 100 cycles of loop testing at room temperature at 1.0 C, the discharge capacity of Nb0 is 126.72 mAh·g-1 and its corresponding capacity retention is 81.70 %. Under the same conditions, the results are 92.02 %, 93.77 %, and 87.19 % for Nb0.5, Nb1, and Nb2, respectively. Therefore, a appropriate amount of Nb doping could promote the material to maintain superior cycle performance. The cycle performance at 55 °C was also evaluated to estimate the structure stability
of
Li(Ni0.5Co0.2Mn0.3)1-xNbxO2
(x=0,0.005,0.01,0.02)
under
elevated
temperature with the help of a constant temperature box. As shown in Fig. 6, the initial discharge capacities at 1.0 C of Nb0, Nb0.5, Nb1, and Nb2 are 174.52, 169.70, 169.70, and 168.81 mAh·g-1, respectively. The initial discharge capacities at 55 °C are higher than that at room temperature, which indicates that high temperature could expedite the electrochemical activity. Nb1 maintained much better cycle stability with a high capacity retention of 88.63% in comparison with that of Nb0 (68.28%). And the capacity retention ratios of Nb0.5 and Nb2 are 77.32 % and 71.75 %, respectively. And the better structure stability of Nb-doped sample could be further confirmed by this result.
The cyclic voltammetry (CV) of the Nb0 and Nb1 were measured at a scan rate of 0.1 mV·s-1 between 3.0 and 4.3 V. And the first three cycles were shown in Fig. 7. According to the CV curves, only one pair redox peaks was found. No phase transformation from hexagonal phase to spinel phase occurred during Li ions intercalation/deintercalation. The oxidation peak around 3.6-4.2V was ascribed to the reaction of Ni2+ to Ni3+/Ni4+ [34, 35]. The cathodic peak potential (Epc), anodic peak potential (Epa), and corresponding potential difference (ΔEp) of the Nb0 and Nb1 for the first three cycles were listed in Table 2. As can be seen in Table 2, The ΔEp of Nb1 (0.192 V) is quite smaller than that of Nb0 (0.269 V), which suggests that 1 % Nb doping could be helpful to decrease the electrochemical polarization and result in better reaction kinetic. In order to elucidate the electrode kinetics of cathode materials, the impedance changes of the Nb0 and Nb1 samples were analyzed and compared through electrochemical impedance spectroscopy (EIS). Fig. 8 shows the EIS graph of Nb0 and Nb1 for 1 cycle and 100 cycles under a room temperature at 1.0 C. Generally, small interrupt represents to the impedance of solution (Rs). The first semicircle is attributed to the surface film resistance (Rsf) corresponding lithium ion migration through the interface between the electrolyte and the surface of particles, and the second semicircle could be regarded as the charge-transfer resistance (Rct) [24, 36, 37]. The Rsf and Rct values for the Nb0 and Nb1 electrodes are fitted by the given equivalent circuit using Zview software, and the results are shown in table 3. The increase of Rct values for both samples after 100 cycles are relatively large as compared to the Rsf values, which indicates that the cell impedance is mainly reflected by the charge-transfer resistance (Rct). The Rct of Nb1 was 101.6 after first cycle then increased to 263.9 after 100 cycles. Nevertheless, the Rct of Nb0 enlarged drastically from 137.4 after first cycle
to 769.4 after 100 cycles. The results demonstrate that 1% of niobium doping could also bring lower charge transfer resistance, thereby improving electrochemical properties.
4. Conclusions Li(Ni0.5Co0.2Mn0.3)1-xNbxO2 (x=0, 0.005, 0.01 and 0.02) cathode materials were successfully prepared via a hydroxide co-precipitation reaction with subsequent solid state sintering reaction. The Li+/Ni2+ cation mixing degree was reduced with Nb doping. Besides, the lattice parameters a, c and interslab space thickness (ILiO2) were slight increased, which facilitate the Li+ migration and structure stability. The electrochemical measurements confirm that niobium doping leads to the excellent cycling stability. When cycling at 55 °C, Li(Ni0.5Co0.2Mn0.3)0.99Nb0.01O2 exhibits a superior capacity retention of 88.63 % after 100 cycles compared with other samples. CV and EIS results confirmed that Nb1 sample has lower polarization and smaller charge transfer resistance. Thus, an appropriate amount of Nb doping into LiNi0.5Co0.2Mn0.3O2 could improve its electrochemical performance, as well as promote the development of its industrialization.
Acknowledgements The authors appreciate the supports of the Project funded by China Postdoctoral Science Foundation (No. 2014M562322), the Science and Technology Pillar Program of Sichuan University (2014GZ0077), the Development of Advanced Electrode and Electrolytes for LIB (AutoCRC Project 1-111) and the National Natural Science Foundation of China (No.201506133).
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Fig. 1 (a) XRD patterns of bare (Nb0) and Nb-doped LiNi0.5Co0.2Mn0.3O2 (Nb0.5, Nb1, Nb2)；(b) the magnified zone from 18.3 to 19.5° of XRD patterns Fig. 2 Rietveld refinement of the Nb0 (a), Nb0.5 (b), Nb1 (c) and Nb2 (d) samples with SEM images inset. Fig. 3 SEM image and Energy dispersive X-ray spectroscopy (EDS) mapping of Ni, Mn, Co, O, Nb of Nb1 sample. Fig. 4 High resolution XPS spectra with fitted results of (a) Ni2p and (b) Nb3d of Nb0 and Nb1 samples. Fig. 5 (a) Rate performance of the prepared samples at different current densities. (b) Cycling performance of the samples at room temperature. Fig. 6 Cycling performance of the as-prepared samples at 55 oC at 1.0 C in the voltage of 3-4.3V. Fig. 7 CV curves for the first 3 cycles of the prepared samples (a) Nb0 and (b) Nb1. Fig. 8 EIS plots of the Nb0 (a) and Nb1 (b) electrodes with an equivalent circuit (inset).
Table 1. Structural parameters obtained from X-ray Rietveld refinement of the prepared samples. Sample
a(Å)
c(Å)
Ni3b
Ni3b/Nitot(%)
ILiO2
Rwp(%)
Rp(%)
Nb0
2.8689
14.2370
0.054
10.8
2.6120
4.85
3.94
Nb0.5
2.8708
14.2462
0.038
7.6
2.6168
4.42
3.81
Nb1
2.8716
14.2510
0.029
5.8
2.6183
5.14
4.23
Nb2
2.8721
14.2537
0.047
9.4
2.6154
2.16
1.69
Table 2. The anodic peak potential (Epa), cathodic peak potential (Epc) and the corresponding potential difference (ΔEp) obtained from CV curves of as-prepared samples for the first three cycles. Sample
1st
2nd
3rd
Epa(V)
Epc(V)
ΔEp(V)
Epa(V)
Epc(V)
ΔEp(V)
Epa(V)
Epc(V)
ΔEp(V)
Nb0
3.948
3.679
0.269
3.848
3.682
0.166
3.838
3.679
0.159
Nb1
3.901
3.709
0.192
3.828
3.709
0.119
3.823
3.713
0.110
Table 3 Fitting results of equivalent circuit from Nyquist curves for the samples. 1st cycle Samples
100th cycle
Rs
Rsf()
Rct()
Rs
Rsf()
Rct()
Nb0
4.1
31.88
137.4
7.0
70.34
769.4
Nb1
5.0
18.96
101.6
5.7
69.34
263.9
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8