Effect of Cu doping on the structural and electrochemical properties of lithium-rich Li1.25Mn0.50Ni0.125Co0.125O2 nanopowders as a cathode material

Author’s Accepted Manuscript Effect of Cu doping on the structural and electrochemical properties of lithium-rich Li1.2Mn0.50Ni0.125Co0.125O2 nanopowders as a cathode material Y. Gh. Sorboni, H. Arabi, A. Kompany www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)32925-0 https://doi.org/10.1016/j.ceramint.2018.10.122 CERI19834

To appear in: Ceramics International Received date: 27 August 2018 Revised date: 9 October 2018 Accepted date: 15 October 2018 Cite this article as: Y. Gh. Sorboni, H. Arabi and A. Kompany, Effect of Cu doping on the structural and electrochemical properties of lithium-rich Li1.2Mn0.50Ni0.125Co0.125O2 nanopowders as a cathode material, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.122 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 Cu doping on the structural and electrochemical properties of lithium-rich Li1.2Mn0.50Ni0.125Co0.125O2 nanopowders as a cathode material Y. Gh. Sorbonia, H. Arabia,b,*, A. Kompanyc a

Renewable Energies, Magnetism and Nanotechnology Research Lab.; Department of Physics,

Ferdowsi University of Mashhad, Mashhad, Iran b

Research center for hydrogen storage and lithium-ion batteries; Faculty of Science, Ferdowsi

University of Mashhad, Mashhad, Iran c

Materials and Electroceramics Lab.; Department of Physics, Faculty of Science, Ferdowsi

University of Mashhad, Mashhad, Iran.

*

Corresponding author’s : Tel: +989151615356; fax: +985138796416, Arabi-h @um.ac.ir

Abstract Lithium- manganese-rich oxides are believed to be suitable candidate for cathode materials in the next generation of lithium-ion batteries. However, they have some disadvantages such as low initial coulombic efficiency, low rate capacity, and deficient cyclability. Different approaches such as elemental doping and surface coating have been adopted to overcome these shortcomings. In this study, Cu-doped Li1.2Mn0.5Ni0.125Co0.125O2 was synthesized by sol-gel method. The prepared samples were characterized using thermal analysis, X-ray diffraction, Fourier transform infrared spectroscopy, surface area analysis, and field emission scanning electron microscopy. Galvanostatic charge-discharge measurement and electrochemical impedance spectroscopy were also conducted to investigate the electrochemical performance of the prepared samples. The XRD patterns revealed that all the samples had two phase structures. By doping Cu, the lattice parameters and the volume of the samples changed. The first discharge capacity of the doped samples was found to be lower than that of the undopped sample. In comparison with the pristine material, Cu-doped ones displayed better cycling performance and rate capability. The Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 sample (with x = 0.05 mol), which delivers 1

an initial discharge capacity of 225.2 mAhg-1 at 0.1C, was found to have the highest capacity retention with the best cycling performance (207.4 mAhg-1), after 50 cycles. The optimum performance of the doped samples could be related to its lower charge transfer resistance and better structural stability. Keywords: Li-Rich, Cathode materials, Cu - doping, Discharge capacity

1. Introduction The commercial lithium-ion batteries presented for the first time in 1990 and since then have been investigated and used in many applications such as electric vehicles and energy storage devices [1,2]. Due to the low practical capacity, high cost, and toxicity along with safety issues of lithium cobaltite as cathode material for Lithium ion battery, it has been replaced with other cathodic materials such as liMn2O4, LiFePO4, LiNiO2, LiNi0.8Co0.15Al0.05O2 (NCA) as well as LiNi1/3Mn1/3Co1/3O2(NMC) in lithium-ion batteries. However, these new batteries, with the alternative cathodic materials, still show low specific capacity which makes them incapable of providing the increasing demand for high energy density batteries used in electric vehicles (EVs) and other applications [3-6]. Among the various types of cathodic materials which have examined, the Li-rich Mn-based layered oxide cathodes, with discharge capacity higher than 250 mAh g-1, have become a suitable candidate to be used in high energy lithium-ion batteries [7-10]. These compounds are composed of two phases and formulated as xLiMO2. (1-x) Li2MnO3, which is a mixture of monoclinic Li2MnO3 and trigonal LiMO2 (M= Ni, Co, Mn or their mixture) phases. However, despite having high discharge capacity, deficiencies such as low initial coulombic efficiency, weak rate capacity, and deficient cyclability have impeded their practical applications [11-13]. In order to overcome these problems, different attempts like synthesis methods, surface coating, and doping of elements have been applied [14-16]. Among these, the doping of certain elements were found the most effective method since it stabilizes the layered structure through formation of stronger M–O bond (Mn, Co, Ni, etc.), resulting in higher conductivity, and expanding the unit cell of the cathodic materials.

2

Numerous metallic elements including Mg, Al, Ti, Ru, Zn, and Zr have been used as the dopants [16-21]. It has been reported that copper dopant [22] is an excellent alternative to Cobalt in the LiNi1/3Co1/3Mn1/3O2 compound which leads to better cycling capacity and rate performance [23]. In comparison with the undoped sample, Cu-doped LiNi0.5Mn1.5O4 shows to have more capacity at high c-rates[24]. It has been shown that Cu-doped LiFePO4/C composites have better high rate capability than the undoped samples [25]. However, up to our knowledge, there are no reports on Cu-doped Lithium-rich cathode materials i.e. [Li0.25Mn0.5Ni0.125Co0.125] O2. Copper is a cheap and abundant metal and therefore Cu-doped cathode materials seem to be better than those undoped

samples.

The

primary

objectives

of

this

study were

the

synthesis

of

[Li0.25Mn0.5Ni0.125Co0.125-x Cux] O2 (x= 0.025, 0.05, 0.075, 0.1) samples through the sol-gel method and then analyzing the structural and electrochemical properties of synthesized samples.

2. Experimental

2.1 Materials and synthesis The cathode materials, Li1.2Mn0.50Ni0.125Co0.125-xCuxO2, were synthesized by sol-gel method. Proper amounts of Mn(CH3COO)2·4H2O, Co(CH3COO)2·4H2O, Ni(CH3COO)2·4H2O, and Li(CH3COO)·2H2O, and Cu(CO2CH3)2.H2O were used as the raw materials with no further purification which were dissolved in 50 mL of deionized water with 5% excess of lithium acetate. Then an appropriate amount of citric acid solution was added to the above solution (Citric acid/Transition metal molar ratios were 1:1). The obtained solution was heated at 80 °C while stirring until a hydrogel was formed. Then the hydrogel was placed in a vacuum oven for 12 hours at 120 °C. The resulting xerogel was ground using a pestle and mortar, which was then transferred into a furnace at 500 °C for three hours. Subsequently, the obtained powders were ground again and re-calcined at 850 °C for 12h, which were used as the active cathode material.

3

2.2 Characterizations Thermal analyses were conducted using the thermogravimetric and differential scanning calorimetry (TG/DSC) (TA Instrument -Q600). The structural analysis of the prepared samples was performed by XRD (PHILIPS, model PW1730) using Cu K radiation ( =1.5406 Å). FTIR spectroscopy was carried out using Nicolet AVATAR370 equipment. The BET method was applied to measure the specific surface area (BEL model BELSORP MINI II). Also, FESEM, (MIRA3 TESCAN-XMU equipped with an energy dispersive spectrometer) was employed to characterize the morphology of the prepared samples.

2. 3 Electrochemical analysis The cathode materials which prepared in this research were made up of three major components: Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 as the active material, carbon black, and polyvinylidene fluoride (PVDF) were all mixed together at the ratio of 8:1:1 respectively. After obtaining a homogeneous mixture of the ingredients, the resultant slurry was coated on an aluminum foil and then dried at 100 °C in a vacuum oven. The aluminum foil was then pressed uniformly and punched in the form of a disk (14 mm diameter). Coin cells 2032 (from Gelon Co., China) were used to construct the cells with the aforesaid prepared material as cathode and lithium metal as the anode. The mixture of 1 M LiPF6 in ethylene carbonate (EC) and dimethyl carbonate (DMC) (50/50) was our electrolyte and Celgard 2300 as the separator. All the components were assembled in a dry argon-containing glove box (TOB-XY240-SP). Neware multi-channel instrument (CT-3008) was employed to test the performance of constructed coin cells. The voltage of the system set between 2.0 - 4.8 V and the cells charged and discharged at different C-rates (1C = 200 mAhg-1). The EIS measurements were carried out using OrigaFlex-OGF500 impedance analyzer (0.01 Hz to 0.1MHz frequency range).

4

3. Results and Discussion 3.1 Thermal analysis In order to find out the optimum calcination temperature, differential scanning calorimetry (DSC), as well as thermal gravimetry (TG), were carried out. Fig. 1 shows the TG/DSC curves of the pure and Cu-doped samples. There are two major distinct regions of mass loss in the TG curves. The first mass loss happened below 290 °C is related to water evaporation, which used for premixing of precursors and decomposition of the acetates [26]. Subsequently, in the range of 300 to 425 °C a fast weight loss occurs accompanied by a sharp exothermic peak on DSC diagram. This weight loss can be associated with the formation of preliminary particles and decomposition of the reactants [27-29]. There is no significant change in TGDSC graphs above 500 °C, indicating that the crystal structure is formed at around 450 °C. Therefore, the precalcination temperature was chosen 500°C. Heating above 500 °C improves the crystallinity of the prepared samples. The total mass loss in this temperature range was about 80wt%.

3.2 Crystal structure analysis The XRD patterns of the prepared samples are shown in Fig. 2. Since the Li-rich cathode active material consists of two integrated components (LiMO2 and Li2MnO3), there are two groups of constructive reflective lines in the X-ray diffraction patterns. All the significant spectra matched with the layered α-NaFeO2 structure (

Space group) which seems to be a sign of LiMO2

phase. The weak peaks between 20-25° are ascribed to the monoclinic phase (space group: C2/m) corresponding to the Li+ ions arrangement in the transition metal layers [3,30-33]. The little detachment between (108)/(110) and (006)/(102) couples indicates that the prepared samples lead to a better-layered structure [34, 35]. When the amount of the dopant exceeded x= 0.5, the CuO phase impurity peak appeared between 35° and 36°, indicating that a small amount of Cu has not fully entered into the crystal structure. The lattice parameters and the volume of the samples were calculated from the high symmetry ( ) space group which are listed in Table 1. By Cu doping, the lattice parameters, and the cell volume increase, because the Cu2+ ionic radius (0.73 Å) is bigger than those of Mn4+ (0.53 Å), Co3+ (0.54 Å), and Ni2+ (0.69 Å) [36]. On the other hand, lower covalency of Cu ions increases the Cu–O bond length which leads to expansion of the unit cell. The increase of lattice parameter 5

“c” can lower the lithium ions migration obstacles, resulting in easier Li intercalation/deintercalation process during charge/discharge cycling [16,37]. The solid solubility limit of Cu in Li[Li0.2Mn0.50Ni0.125Co0.125] O2 occurs when the lattice parameters decrease at x  0.075 due to the formation of cupric oxide (CuO). Furthermore, since the ionic radii of Cu2+ (0.73 Å) and Li+ (0.76 Å) are almost similar to each other, the occupation of Cu in the Li layer is possible. As a result, the presence of Cu in the Li layer may reduce the inter-layer spacing and when the content of Cu increases to 0.075 and 0.1, the lattice parameters become smaller than that of the samples with the content of Cu for 0.05 and less. All the prepared samples had the c/a ratio more than 4.899 which implies a good layered structure [34]. The higher intensity ratio (I

(003)/I (104))

of the Cu-doped samples compared to the

pristine sample indicates the lower cation mixing in Cu-doped samples [38, 39]. Based on the data presented in Table 1, Li1.2Mn0.50Ni0.125Co0.075Cu0.05O2 sample should have the best electrochemical performance.

3.3 FTIR spectroscopy The FTIR spectra of the undoped and Cu-doped Li-rich materials are shown in Fig. 3. Two vibrational peaks around 530 and 624 Cm-1 can be associated with asymmetric stretching of Li-O and M-O (M= transition metals) bonds respectively [40, 41]. The peaks around 870, 1430, 1490 Cm-1 likely correspond to the Carbonate ions CO32- (C-O or O-C-O) [42,43]. There is no characteristic vibration spectrum associated with unwanted Cu-O phase, which can be attributed to low cupric ion content. By increasing the copper doping, the vibration frequency at 537 Cm-1 slightly shifts to lower wavenumber regions that might be related to variation of M–O covalency [44].

3.4 BET analysis The results of Brunauer–Emmett–Teller (BET) measurements are presented in Table 2. The samples with Cu= 0, Cu=0.025 and Cu=0.1 have almost the same specific surface area in the range of 3.02 -3.9 m2 g−1. This suggests that specific surface area does not change remarkably with the increase of Cu concentration. On the other hand, the mean pore diameter (15.6 -18.1 nm) confirms the mesoporous structure of the prepared nanoparticles. Nano-micro mesoporous

6

structure results in better electrochemical performance by reducing the lithium-ion migration path, as well as decreasing volume variations during different charge/discharge cycles.[20, 45].

3.5 Surface morphology FESEM images of the prepared Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 powders are shown in Fig. 4. All the samples were showing the same morphology with polyhedral sub-micron particles. Also, the average size and the size distribution of the particles were not changed significantly by Cu doping. Clear edge and smooth sides with a low degree of agglomeration are the typical characteristics of all these particles. EDX spectroscopy was also performed to study the elemental compositions of the samples. The EDX spectra, Fig. 5, show the presence of Ni, Co, Mn and Cu in the synthesized specimens. The results of the XRD and EDX spectroscopies both prove the presence of copper ions in the structure of the samples.

3.6 Electrochemical performance Galvanostatic battery cell tests were performed to study the electrochemical behavior of the prepared samples. The first charge/discharge curves of the samples are illustrated in Fig. 6. The voltage range and current density of charge/discharge processes were 2.04.8 V and 0.1C (20mAhg-1), respectively. The charge curves of all samples were similar to each other consisting of two different parts: a sloping region around 3.8 V and a flat area at higher voltages. The sloping part is associated with the removal of lithium ions from LiMO2 component and the redox reaction of nickel and cobalt ions [3, 9]. The smooth region is regarded as the release of lithium and oxygen from Li2MnO3 phase [5,31]. Table 3 shows the initial charge/discharge information of Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 (x = 0, 0.025, 0.05, 0.075, 0.1) cathodes at 0.1 C rate. All the samples, except for x = 0.075 and 0.1, have discharge capacity greater than 300 mAhg-1. The first discharge capacity was recorded as 253.2, 230.1, 225.2, 220.7, and 155.6 mAhg-1 for x = 0, 0.025, 0.05, 0.075 and 0.1, respectively [Table 3]. The first discharge capacity of the samples decreases gradually with the increase of cupric ion doping, which can be associated with two main factors. Firstly, due to the proximity of Cu2+ 7

ionic radius (0.73 Å) to Li+ (0.76 Å ) [20], some copper ions may occupy lithium sites and prevent the natural diffusion of Li+ ions in charge and discharge processes. Secondly, by substitution Cu2+ for Co3+, some Ni2+ ions change to Ni3+ to keep charge neutrality. However, the Ni2+/Ni4+ pair is responsible for the redox reactivity as well as the capacity of the cathodic material, Therefore, with a minor reduction of Ni2+, the initial discharge capacity of doped materials is also reduced [22, 23, 46]. Reduction of initial discharge capacity by cationic doping has been already reported [47, 48]. The first cycle efficiencies of the prepared samples (table 3.) are 70.4%, 76.2%, 73.7%, 75.0% and 65.3%, respectively which clearly shows the improvement of coulombic efficiency in doped samples. The cycling performances of the prepared samples are shown in Fig. 7 and Table 4. It is clear that the capacity retention of Cu doped samples is better than that of the undoped sample. The samples with Cu = 0.05 was found to have the best performance. After 50 repetitive chargedischarge cycles, under 0.1C, capacity retention of the doped sample was 92.09 %, while the undoped sample could only maintain 78.3% of its first capacity. The weak performance of the pure sample can be ascribed to the gradual destruction of the cathode material structure during recurring charge/discharge processes. The presence of copper, which is electrochemically inactive, leads to the stability of the material structure. In some articles, the better cyclic performance of the Cu-doped material (LiCoO2) is believed to be attributed to the movement of copper ions toward the electrode’s surface and formation of a solid solution layer. This protective layer prevents the dissolution of cobalt ions in the electrolyte [49, 50]. The lower cation mixing of Cu-doped samples can also be another reason for the better cycling performance of these samples. The EIS measurements were conducted to investigate the kinetic properties of the prepared cathode materials. The corresponding diagrams, as an example, demonstrate in Fig. 8; indicating the standard Nyquist diagram, consist of a straight line and a semicircle. The straight line is attributed to the process of lithium diffusion and the semicircle region is related to charge transfer resistance [51]. However, charge transfer resistance has a crucial role in cell impedance [52]. Measurement of the diameter of the semicircles showed that charge transfer resistance of the samples are close to each other at the initial stage, but after 50 successive cycles, the resistance in the Cu-doped sample is getting far less than the pristine one. This fact implies that 8

the Cu-doped sample has better structural stability than that of the undoped sample. As a result, the Cu-doped samples exhibit better capacity after 50 cycles. Fig. 9 exhibits the rate performance of Cu = 0 and Cu = 0.05 samples at different rates. By increasing the current density, discharge capacity will reduce in both samples. However, the level of the reduction would be higher in the pure sample comparing to the copper-doped samples. Despite lower discharge capacity of doped samples at 0.1C, which can be attributed to the substitution of Co ions by electrochemically inactive Cu ions, the Cu-doped sample capacity surpasses at current densities larger than 0.1 C. This improvement of the rate capability may be ascribed to the improved ionic conductivity, better structural stability, and less cation mixing. After a period of different charge/discharge rates, the Cu-doped sample can achieve 210.2 mAhg-1 nearly 92.8% of the capacity at previous 0.1C rate stage, while the capacity retention for pure sample is only 80.7% (193.2 mAhg-1). Based on obtained results the structural stability and rate performance of the Cu-doped sample are better than the undoped one.

4. Conclusion We prepared the Li1.2Mn0.5Ni0.125Co0.125-xCuxO2 nanoparticles successfully via the sol-gel route. TG/DSC plots revealed that the pre-Calcination temperature must be around 500 °C. To achieve better crystallinity, the second calcination temperature selected to be 850 °. The effect of Cu doping on the structural and electrochemical properties of the prepared samples was investigated. The XRD patterns showed that pure and Cu-doped samples have both integrated hexagonal and monoclinic structure. XRD and EDX analyses also revealed that Cu was incorporated into the lattice. There was no additional CuO phase before Cu content reached 0.075. The samples were found to have almost similar surface morphology with a sub-micron grain size and fairly good distribution. The first discharge capacity of samples gets smaller, with the increase of Cu doping. The results of electrochemical characterization showed an improvement in cyclic stability and rate performance of Cu-doped samples, which arises from a better movement of Li ions due to enlargement

of

the

unit

cell

and

reduction

of

charge-transfer

resistance.

The

Li1.2Mn0.50Ni0.125Co0.075Cu0.05O2 sample had lower cation mixing, better specific surface area (3.9 m2 g−1), and the best capacity retention which was 92.09% after 50 charge-discharge cycles.

9

Acknowledgments We, the authors, hereby acknowledge Ferdowsi University of Mashhad, Iran for supporting this research via grand no.3/45214.

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14

Fig.1 TG and DSC analyses of the synthesized Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 (with x = 0 and x= 0.05). Fig. 2 XRD patterns of all the samples with different Cu-doping. Fig.3 FT-IR spectra of the synthesized Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 (x=0, 0.05 and 0.1) Fig.4 FESEM images of the prepared Samples (a) x =0, (b) x =0.025, (c) x= 0.05 (d) x = 0.075, (e) x= 0.1 Fig.5 EDX spectra of the prepared samples (f) x= 0, (g) x= 0.5, (h) x= 0.1. Fig.6. Initial charge/discharge profiles of the pristine and Cu-doped samples. Fig.7 Cycling performance of the Cu = 0 and Cu = 0, samples at 0.1 C. Fig. 8 EIS plots of the undoped and Cu-doped samples, at fresh stage (a) and after 50 cycle galvanostatic charge/discharge cycles at 0.1C (b). Fig. 9 Rate capabilities of the undoped and Cu-doped sample.

15

Table1 Lattice parameters of the prepared samples with different Cu-doping

Sample

a ( Å)

c (Å)

c/a

x=0

2.824(2)

13.888

4.917

1.29

95.92

x=0.025

2.841(5)

14.09(3)

4.959

1.38

98.44

x=0.05

2.838(9)

14.14(5)

4.982

1.51

98.62

x=0.075

2.824(1)

13.888

4.917

1.45

95.92

2.824

13.888

4.917

1.15

95.92

x=0.1

16

V(

)

Table 2 BET surface area analysis for the Li1.2Mn0.50Ni0.125Co0.125-xCuxO2 Sample composition

BET (m2/g)

Average pore diameter(nm)

x=0

3.022

15.664

x = 0.05

3.888

15.767

x= 0.1

3.528

18.106

17

Table 3 The first charge/discharge data of the pristine and Cu-doped samples

Sample Composition

Charge capacity (mAhg-1)

Discharge Capacity (mAhg-1)

Coulombic Efficiency (%)

x=0

359.3

253.2

70.4

x = 0.025

301.6

230.1

76.2

x = 0.05

305.2

225.2

73.7

x = 0.075

294.5

221.0

75.0

x= 0.1

238.1

155.6

65.3

18

Table 4 Cycling performance of the pure and Cu-doped samples at 0.1 C

First Discharge Capacity (mAhg-1)

50th Discharge Capacity(mAhg-1)

Capacity Retention (%)

x=0

253.2

198.3

78.3

x = 0.025

230.1

202.7

88.0

x = 0.05

225.2

207.4

92.09

x = 0.075

221.0

191.7

86.7

x = 0.1

155.6

120.1

77.1

Sample Composition

19

Fig. 1.

Fig. 2.

20

Fig. 3.

21

Fig. 4.

22

Fig. 5.

23

Fig. 6.

24

Fig. 7.

25

Fig. 8.

Fig. 9.

26