C cathode material for lithium-ion batteries

Accepted Manuscript Ni-doping to improve the performance of LiFeBO3/C cathode material for lithium-ion batteries Bao Zhang, Lei Ming, Hui Tong, Jia-feng Zhang, Jun-chao Zheng, Xiao-wei Wang, Hui Li, Lei Cheng PII:

S0925-8388(17)34147-6

DOI:

10.1016/j.jallcom.2017.11.365

Reference:

JALCOM 44058

To appear in:

Journal of Alloys and Compounds

Received Date: 17 December 2016 Revised Date:

22 November 2017

Accepted Date: 29 November 2017

Please cite this article as: B. Zhang, L. Ming, H. Tong, J.-f. Zhang, J.-c. Zheng, X.-w. Wang, H. Li, L. Cheng, Ni-doping to improve the performance of LiFeBO3/C cathode material for lithium-ion batteries, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2017.11.365. 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 proof before it is published in its final 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.

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Ni-doping to improve the performance of LiFeBO3/C cathode material for lithium-ion batteries Bao Zhang, Lei Ming, Hui Tong, Jia-feng Zhang*, Jun-chao Zheng, Xiao-wei Wang,

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Hui Li, Lei Cheng School of Metallurgy and Environment ,Central South University, Changsha 410083, P.R. China;

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Abstract

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Ni-doping is used to improve the performance of LiFeNiBO3 /C for the first time. The structure and electrochemical properties of as-prepared Ni-doped samples (LiFe1-xNixBO3/C) are investigated by X-ray diffraction, scanning electron microscopy,

transmission

electron

microscope,

electrochemical

impedance

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spectroscopy, and galvanostatic charge-discharge tests. The results show that LiFe1-xNixBO3/C composites consisted of similar spherical particles are linked and well-coated by nano-carbon frameworks, forming a multi-layer core-shell structure.

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Moreover, the substitution content of iron by nickel has significant effect on the

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electrochemical characteristics, especially for the rate performance. Particularly, the LiFe0.94Ni0.06BO3 shows the best electrochemical property, exhibiting a discharge capacity of 201.5 mAh g-1 at a current density of 10 mA g-1. The results indicate that Ni doping in LiFeBO3/C can effectively enhance the electrochemical performance, especially at a high charge/discharge rate.

Keywords LiFeBO3; spray drying; cathode material; electrochemical property; Ni doping 1

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1. Introduction Li-contained transition metal oxides such as LiCoO2, LiMn2O4 and mixed metal analogs such as Li(Ni,Mn,Co)O2 have been widely used in commercial lithium

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batteries as cathode materials [1,2]. However, their high cost, toxicity, and other disadvantages prohibit their large-scale application [3,4]. Therefore, significant researches have been devoted to finding alternative cathode materials. Among these,

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polyanion-based materials have been developed as the most promising ones due to

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their low cost, high safety, high performance, and low toxicity [5-13].

The monoclinic lithium iron borate (LiFeBO3), firstly reported by V. Legagneur and his co-workers in 2001 [14], captured the attention of researchers due to its high theoretical capacity (220 mAh g-1), small volume change (ca. 2%) [14-16], and

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favorable chemical constituents. in 2010, Yamada and co-workers achieved a high reversible capacity of approximately 190 mAh g-1 by carefully preparing electrode materials under an inert Ar atmosphere [15]. They attributed the performance

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improvement to the prevention of air exposure. Our team has successfully prepared

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the multi-layer core-shell LiFeBO3/C with an initial discharge capacity of 196 mAh g-1, and remainded 136.1 mAh g-1 after 30 cycles [16], while its rate performance still needs further improvement. Up to present, various approaches have been proposed to optimize this new borate, mostly focus on the synthetic parameters [17-19]. Doping is a common and effective method to improve the performance of lithium ion battery materials through structural modification. Although the LiFeBO3 material modified by ion doping is rarely investigated, Yamada et al. confirmed that Mn doping had the 2

ACCEPTED MANUSCRIPT improved effects on the crystal structure of LiFeBO3/C. Furthermore, due to the high Ni3+/Ni2+ redox couple of 4.8 V, it was supposed that Ni doping could improve the working pateau of LiFeBO3 [20]. Therefore, it is worthwhile to modify LiFeBO3

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material with a tiny amount of elements to increase the Fe2+/Fe3+ redox potential and stabilize the crystal structure without degrading its energy density.

Since the radius of Ni2+ (0.69 Å) is close to that of Fe2+ (0.78 Å), Ni doping has

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been successfully proved to be an effective way to optimize the properties of

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LiFePO4 and LiFeP2O7 [21-25]. In this study, Ni doped LiFe1-xNixBO3/C (x=0.02, 0.04, 0.06 and 0.08) composites were synthesized via spray-drying methods. The effect of Ni doping on the crystal structure and electrochemical performance of LiFeBO3/C were investigated in detail for the first time.

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2. Experimental

LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) composites were prepared via spray-drying followed by heat treatment. The solution for spray-drying were obtained

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by dissolving stoichiometric ratio of LiBO2·8H2O, Fe(NO3)3·9H2O, C4H6O4Ni·4H2O

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and citric acid together in deionized water. Then, the resulting solution was dried to form a mixture of precursor by atomizing via a sprinkler at an air pressure of 0.25 MPa, and the inlet and outlet air temperatures were 230 °C and 120 °C, respectively. Finally, the as-prepared precursor was transferred into a tube furnace and heated to 300 °C at a heating rate of 2 °C min−1 for 2 h under Ar atmosphere to decompose nitrate and carbohydrate reagent, and subsequently sintered at 550 °C for 10 h, followed by cooling down to room temperature naturally. 3

ACCEPTED MANUSCRIPT The structural and crystalline phase of the LiFe1-xNixBO3/C was determined by the powder X-ray diffraction (XRD, Rint-2000, Rigaku) using Cu Kα radiation. The fourier transform infrared spectrum was obtained by a spectrophotometer (FTIR,

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Nicolet 460). The morphologies of the composites were observed by scanning electron microscope (SEM, JSM-5600LV, JEOL) and transmission electron microscope (TEM, Tecnai G12).

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The electrochemical characterizations were performed using CR2025 coin-type

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cells. The positive electrode loading was in the range of 2-2.5 mg cm-2, and the electrode diameter was 14 mm. The cathode of the two-electrode electrochemical cell was fabricated by blending 80 wt% of the powder with 10 wt% of super P and 10 wt% of polyvinylidene fluoride binder in N-methyl-2-pyrrolidone. Then, the mixed slurry

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was coated uniformly on aluminum foil, and the electrode was dried at 120 °C for 4 h in the oven. The test cell which was assembled in a Mikrouna glove box filled with high-purity argon, consisted of the positive electrode and lithium foil negative

1mol

L-1

LiPF6

in

a

mixture

of

ethylene

carbonate/dimethyl

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was

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electrode separated by a porous polypropylene film (Celgard 2320). The electrolyte

carbonate/ethylmethyl carbonate (EC/DMC/EMC) solution (1:1:1, in volume). The electrochemical measurements were galvanostatically performed from 10 to 40 mA g-1 using a battery tester (LAND CT2001A) in the voltage range between 1.5 and 4.5 V at room temperature (25 °C). The impedance spectra were recorded by applying an AC voltage of 5 mV amplitude in the 0.1-100 KHz frequency range with an electrochemical analyzer (CHI660D). 4

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3. Results and discussion The XRD patterns of LiFe1-xNixBO3/C composites with different Ni doping contents are shown in Fig.1. All the observed XRD reflections highly resembled the

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previously reported patterns [15,16,26-29], indicating a monoclinic crystal type LiFeBO3 with space group C2/c. It is clear that the structure was unchanged by Ni doping, except for some trace impurities of NiO in samples (x=0.02 and 0.04) and

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FeNi in samples (x=0.06 and 0.08), and the Rietveld refinement results indicate there

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are about 6(1)% w/w LiFeO2 impurities existing in all the samples, which was similar to the reports from Y.M. Zhao [28] and B. Zhang [16]. According to the C-S analysis, the amount of C in the composite is about 6.42wt.%.The absence of carbon peaks in all patterns indicates that the carbon from pyrolysis of citric acid was

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amorphous. The calculated results of lattice parameters are listed in Table 1. It is seen that the LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) samples had similar cell parameters compared with the data reported previously [15,28,29]. It is also found

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that the parameters of cell unit decreases with the increasing Ni doping content,

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indicating Fe ions in the lattice were partially replaced by Ni ions. The smaller radius of Ni2+ (0.69 Å) compared with that of Fe2+ (0.78 Å) leads to the reduction of lattice parameters [23-25]. This indicates that most of the Ni has been successfully doped into the M1 (Li) or M2 (Fe) sites without affecting the monoclinic structure, which is similar to Ni2+ doping in Li2FeP2O7and LiFePO4 [ 24,30, 31]. Fig. 2 shows the Fe2p and Ni2p XPS spectra of the pristine LiFeBO3/C ¼and Ni-doped LiFe0.94Ni0.06BO3/C. XPS is a common technique to detect the valency 5

ACCEPTED MANUSCRIPT state of an element in a compound. As can be seen in Fig. 2, for the XPS spectra of Fe 2p, a major peak at around 711.6 eV and the satellite peak at 725.1 eV correspond to 2p3/2 and 2p1/2 in pristine samples, respectively, confirming the

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presence of Fe is +2, which matches well with the literature values [32,33]. In comparison, the binding energy of Fe 2p for Ni doped sample slightly shifts to the low value, indicating that a small amount of Fe0 is detected in LiFe0.94Ni0.06BO3/C,

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which confirms the existence of FeNi impurity in the composite. Furthermore, the

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Ni2p spectrum of the pristine LiFeBO3/C has no obvious peaks, while the Ni2p with a spin-orbit splitting component of Ni2p3/2 at 856.8eV and Ni2p1/2 at 875.8eV are observed in sample LiFe0.94Ni0.06BO3/C, indicating the presence of Ni2+ in the composite, which illustrates the homogeneous dispersion of Ni element

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among LiFe0.94Ni0.06BO3/C and is in accordance with previous literatures [34,35]. The chemical bonding of the LiFe1-xNixBO3/C samples were investigated by FTIR as shown in Fig. 3. The FTIR spectra exhibit many prominent and narrow multiple

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vibration bands in the range of 400-1600 cm-1. There were four different types of

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vibrations of trigonal [BO3]3- with D3h symmetry [36,37], corresponding to four splitting frequency ranges: 1000-1450 cm−1 (υas, E″), 850-950 cm−1 (υs, A′), 650-850 cm−1 (γ, A″), and 500-650 cm−1 (δ, E′). The weak peaks in the region of 1400-1600 cm-1 are related to the absorbance of the residual carbon [38]. The characteristic peaks of NiO (decomposition from Ni(CH3COO)2·4H2O) are approximately at 450 and 562 cm-1 [39], not found in Fig. 3, indicating that the impurity NiO in samples (x=0.02 and 0.04) were less and most of the Ni was incorporated into the lattice of LiFeBO3. 6

ACCEPTED MANUSCRIPT Furthermore, the results were in accordance with the XRD results. The SEM images of LiFe1-xNixBO3/C composites are shown in Fig.4. All the samples show good uniformity with spherical particle shape and a size distribution

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ranging from 1 to 5 µm. The broken spherical particles marked in red in Fig.4 (a) and Fig.4 (b) showed that the big spherical particles were actually an aggregation of some smaller particles of several hundred nanometers linked by the pyrolytic carbon. This

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shows that the multi-layer core-shell structure of the composites. The rough surface of

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the spherical particles is beneficial for absorption of the electrolyte. The contact area between the particle and the electrolyte increased due to the multi-layer core-shell structure, which is helpful for diffusion of the ions and the transport of the electrons. These images show that there is no obvious change in morphology after Ni doping.

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To further investigate the surface coating and carbon distribution of the particles, the TEM and HRTEM images of LiFe1-xNixBO3/C (x=0, 0.06) are illustrated in Fig.5. It is clear that the pyrolysis carbon acted as the chelating agent to bridge the

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(LiFe1-xNixBO3) particles, forming LiFe1-xNixBO3/C composite network and

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enhancing the conductivity of this material. As can be seen in Fig. 5 (a) and 5 (b), the LiFe1-xNixBO3/C particles are well wrapped and connected by nano-sized web of amorphous carbon. Fig. 5 (c) and 5 (d) show that the thickness of the carbon coating on the external surface area of the samples was approximately 1 nm. The thin layer carbon coating prevents the air corrosion when the composites were exposed in the air. In addition, the thin carbon coating provides passways for electron transport and enhances conductivity [31]. This particular multi-layer core-shell structure has several 7

ACCEPTED MANUSCRIPT advantages such as good accommodation of volume changes without fracture during cycling, good electrical connection with the current collector, short diffusion lengths and enhances conductivity, which was supposed to be good for the electrochemical

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performance [40, 41]. Fig.6(a) shows the SEM image of LiFe0.94Ni0.06BO3/C composite, and corresponding EDS mapping for C(b) , Fe(c), Ni(d) and O(e). In Fig. 6, Li and B

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elements cannot be detected due to the reason that the X-ray fluorescence yields are

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extremely low for these two elements. It is seen that Fe, O, Ni and C elements were homogeneously distributed in the particles, and the carbon was coated on the surface of LiFe0.94Ni0.06BO3/C composite nanoparticle.

Galvanostatic charge-discharge measurements at ambient temperature condition

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were conducted by using CR2025-type coin cells. The charge-discharge profiles and rate performance of LiFe1-xNixBO3/C (x=0.00, 0.02, 0.04, 0.06 and 0.08) cathodes are shown in Fig. 7 (a) and 7 (b). It can be seen that, at the current density of 10 mA g-1,

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the discharge specific capacities of LiFe1-xNixBO3/C composites with x=0, 0.02, 0.04,

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0.06 and 0.08 were 197.7, 154.6, 166.0, 201.5 and 195.3 mAh g-1, respectively. The voltage profiles exhibited two charge and discharge plateaus, which correspond to the plateaus of LiFeBO3 (2.8-3.2 V) and LiFeO2 (2.1-2.6 V) [16]. This further confirms the existence of LiFeO2 in the composite, which is in accordance with the XRD results. The discharge capacity decreased for the samples x=0.02, 0.04, while increased for the samples x=0.06, 0.08. This may be attributed to the impurity NiO in LiFe0.98Ni0.02BO3/C and LiFe0.96Ni0.04BO3/C samples. Although NiO can be utilized as 8

ACCEPTED MANUSCRIPT anode material for LIBs [42], it hardly provides the additional capacity in the potential range of 1.5-4.0 V for the cathode. While NiO may hinder the lithium ion diffusion, and reduce the active material of Fe redox. In contrast, the trace impurity FeNi alloy

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may improve the conductivity thus result in the highest discharge capacity of LiFe0.94Ni0.06BO3/C. However, the discharge capacity decrease when the Ni doping content was increased further to x=0.08, this could be ascribed to the reason that the

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Fe in the composite contributes the mostly capacity and the existence of excessive

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Ni2+ is redox inactive[21,43,44].

The cycling and rate performance of the LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) samples in the voltage range 1.5-4.5V are showed in Fig. 7 (b). It’s clearly seen that the rate performance of the composites was improved with the increasing Ni

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doping amount significantly. The sample with suitable Ni doping content (x=0.06) exhibited the best rate performance. It delivered a capacity of 201.5 mAh g-1 (1th), 139.8 mAh g-1 (36th) and 107.2 mAh g-1 (56th) at the current density of 10 mA g-1, 20

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mA g-1,40 mA g-1, respectively, which was much better than the previously reported

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ones [26,28,36]. For all the LiFe1-xNixBO3/C samples, when the current density was increased to 20 mA g-1 and 40 mA g-1, the specific capacity decreased remarkably, whereas when the current density was decreased to 10 mA g-1 again, the discharge capacity increased again, which indicates that the structure of the multi-layer core-shell LiFeBO3/C keep stable during the cycling process. It’s noted that the cycling stability of the samples in this work is insufficient compared to those mature materials, like LiFePO4 or LiMn2O4 [31]. However, due to the intrinsic properties of 9

ACCEPTED MANUSCRIPT LiFeBO3, it is unstable at ambient temperature, which will induce severe structure transformation and degrade the electrochemical performance. Besides, according to Wang [45], the capacity fading was related to the cracking during cycling and same

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unwanted impurity. The small amount impurity of NiO has adverse impact on the electrochemical property of LiFe1-xNixBO3/C, owing to that NiO may hinder the lithium ion diffusion and induce the structure instability. In the work, compared to

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those earlier studies of LiFeBO3 [24,26,34], the Ni doping into Mn site can effectively

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enhance the stability of structure and improve the cycling performance of LiFeBO3/C. Especially for LiFe0.94Ni0.06BO3/C, the Ni doping can stabilize the structure of LiFeBO3, and improve the electrochemical performance. Meanwhile, the tiny part of FeNi impurity may improve the electron conductivity and enhance the reaction kinetic,

LiFeBO3 cathode.

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thus providing an advanced rate capability, which provides an insight to modify the

EIS was applied to further analyze the effect of Ni doping on electrode impedance.

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Fig. 8 shows the three-dimensional Nyquist plots of the LiFe1-xNixBO3/C (x=0, 0.02,

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0.04, 0.06 and 0.08) at ambient temperature. The shapes of the Nyquist plots for all the samples are similar. It is clear seen that plots were composed of a small intercept at high frequency, a semicircle at high to medium frequency and a linear part in the low frequency. The intercept impedance was almost the same, which corresponds to the solution resistance (Rel). The depressed semicircle in the middle frequency is related to the charge-transfer resistance (Rct) at the cathode/electrolyte interface and the double layer capacitance (CPE). The inclined straight line in the low frequency 10

ACCEPTED MANUSCRIPT corresponds to the Warburg impedance (Zw) [21,46-51], which is associated the diffusion of Li+ ions within the LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) particles. The Rct are 151.9, 200.4, 187.3, 74.1 and 106.6 Ω for LiFe1-xNixBO3/C (x=0,

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0.02, 0.04, 0.06 and 0.08) composites, respectively, indicating that the electrochemical properties of LiFe1-xNixBO3/C composites can be improved by an appropriate amount of Ni doping. Particularly, the obtained Rct of LiFe0.94Ni0.06BO3/C was obviously the

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smallest among the samples, indicating that the transfer and diffusion of Li+ through

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the cathode/electrolyte interface is easier than other samples, which is in accordance with the electrochemical property depicted in Fig. 7 (a) and 7(b). It’s reported that Ni doping can enhance the mobility of Lithium ion, P-O bond, and stabilized the structure of LiFeO4 [52,53], so that the charge-transfer resistance was decreased. It is

4. Conclusions

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believed that the decreased Rct of LiFe1-xNixBO3/C was due to Ni doping.

Well crystallized LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) composites

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were successfully prepared via spray-drying followed by heat treatment. Based on

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the XRD results, Ni2+ was partially incorporated into the lattice of Fe in LiFeBO3. The LiFe0.94Ni0.06BO3/C shows superior cycling stability and rate capability among all the samples. It possessed the highest discharge capacity of 201.5 mAh g-1 at 0.05 C, and the capacity retention of 90.1% after 105 cycles at 0.05 C. The results indicate that Ni doping is an effective approach to achieve excellent electrochemical performance for the lithium ion borate.

Acknowledgements 11

ACCEPTED MANUSCRIPT This study was supported by National Natural Science Foundation of China (Grant No. 51272290, 51472272 and 51502350), Postdoctoral Science Foundation of China (2016M592447), and Postdoctoral International Exchange Program of China.

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ACCEPTED MANUSCRIPT [52] Ge Y, Yan X, Jing L, Electrochimica Acta 55 (2010) 5886-5890. [53] Y. Lu, J.C. Shi,Z.P. Guo,Q.S. Tong,W.J. Huang, B.Y. Li, J. Power Sources. 194

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(2009) 786-793.

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Figure captions Fig. 1. XRD patterns of LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) composites. Fig. 2. XPS spectra of Fe2p and Ni2p core level of LiFe1-xNixBO3/C (x=0, 0.06)

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Fig. 3FTIR spectra of LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) composites.

Fig. 4. SEM images of LiFe1-xNixBO3/C; (a, b) x = 0; (c) x= 0.02; (d) x = 0.04; (e) x =

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0.06 and (f) x= 0.08.

Fig. 5. TEM and HRTEM images of the LiFeBO3/C (a,c) and LiFe0.94Ni0.06BO3/C

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(b,d).

Fig. 6. (a) SEM image of LiFe0.94Ni0.06BO3/C composite; the corresponding EDS mapping for (b) C, (c) Fe, (d) Ni and (e) O.

Fig. 7. (a) Room-temperature charge-discharge curves of LiFe1-xNixBO3/C (x=0, 0.02,

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0.04, 0.06 and 0.08) composites measured at 10mA g-1; (b) Cycling performances of LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) at different rates.

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Fig. 8. Three-dimensional Nyquist plots of LiFe1-xNixBO3/C (x=0.00, 0.02, 0.04, 0.06

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and 0.08) composites.

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Figures

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Fig. 3. 4

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(c)

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(b)

Carbon

LiFeBO3

Carbon

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(a)

LiFe0.94Ni0.06BO3

(c)

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Ni-K

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Fig. 7(a).

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0 0.02 0.04 0.06 0.08

200

0 0

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Table caption

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Table 1 Lattice parameters of LiFe1-xNixBO3/C (x=0, 0.02, 0.04, 0.06 and 0.08) composites.

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b(Å)

c(Å)

β(°)

V(Å3)

LiFeNiBO3/C

5.1625(2)

8.9179(6)

10.1672(5)

91.37 (7)

466.39(7)

LiFe0.98Ni0.02BO3/C

5.1611(3)

8.9178(2)

10.1611(1)

91.43 (5)

466.12(4)

LiFe0.96Ni0.04BO3/C

5.1601(5)

8.9160(4)

10.1589(3)

91.38 (4)

465.95(2)

LiFe0.94Ni0.06BO3/C

5.1578(3)

8.9086(7)

10.1543(7)

91.40 (3)

465.76(5)

LiFe0.92Ni0.08BO3/C

5.1544(7)

8.9012(1)

10.1523(4)

91.39 (1)

465.34(7)

LiFeBO3a

5.1681(6)

8.8687(8)

10.1656(9)

91.514 (8)

465.77(8)

LiFeBO3b

5.166(2)

8.919(2)

10.135(3)

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Data taken from Ref.[27]; b Data taken from Ref.[26]

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Highlights 1. Well-crystallized Ni doped LiFeBO3/C composites have been successfully prepared via spray-drying followed by heat treatment.

LiFeBO3/C is systematically investigated.

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2. The influence of Ni doping on crystal structure and reaction kinetic of

performance at a high current rate.

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3. Ni doped LiFeBO3/C can effectively enhance the electrochemical

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4. The LiFe0.94Ni0.06BO3 showes the best electrochemical property with a

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discharge capacity of 201.5 mAh g-1 at 10 mA g-1.