Structural, Morphological, and Vibrational Studies on Li(Li_0.05Ni_0.7-xMn_0.25Fe_x)O₂ (x=0,0.1,0.3,0.5,0.7) Cathode Material for Li-Ion Batteries

Structural, Morphological, and Vibrational Studies on Li(Li_0.05Ni_0.7-xMn_0.25Fe_x)O₂ (x=0,0.1,0.3,0.5,0.7) Cathode Material for Li-Ion Batteries

1. Nichelson1, , , and X.Sahaya Shajan1,*

   1 Centre for Scientific and Applied Research, PSN College of Engineering and Technology, Tirunelveli-627 152, Tamil Nadu, India.

   1. Karthickprabhu2

      2 K.Ramakrishnan College of Technology, Samayapuram, Tiruchirappalli-621112, Tamilnadu,India

      K. Karuppasamy3

      3 Polymer Materials Lab,

      Department of Chemical and Biomolecular Engineering, Sogang University,

      Mapo gu, Seoul, South Korea

      Abstract – Lithium rich layered Li(Li0.05Ni0.7-xMn0.25Fex)O2 (x = 0,0.1,0.3,0.5,0.7) cathode materials were prepared citric acid assisted sol-gel technique. The prepared samples were subjected to characterization techniques such as X-ray diffraction studies, scanning electron microscopy, energy dispersive spectra analysis, FTIR, Raman spectra. The structural and morphological behavior was studied using XRD and SEM. Li(Li0.05Ni0.7-xMn0.25Fex)O2 possess the hexagonal -NaFeO2. The crystallite size of the Li(Li0.05Ni0.7- xMn0.25Fex)O2 cathode material were around 50 nm, which supports the movement of lithium ion throughout the material. The metal oxygen interaction of the prepared cathode material was analyzed by FT-IR spectroscopy.

      Keywords – Sol-gel; crystallite; lithium ion batteries; cathode material

      1. INTRODUCTION

         Lithium ion batteries (LIBs) occupied an important place in the production of electrical appliances for the past two decades. Cellular phones, digital cameras, laptops and LIBs are few of them. These gadgets consumed LIBs due to their excellent properties such as high specific energy, high adoptability and slow discharge rate. Cathode, anode, electrolyte and separator, are the components of LIBs whereas the cathode material with less toxicity and more safety has more impact on to the cost of LIBs [1]. The crystal structure of the cathode material has a significant role because of its necessity for feasible lithium ion movement. The different crystal structure of the cathode materials are olivine, layered and spinel phased structure [2,3]. Owing to high discharge capacity, high electrical conductivity and capacity retention layered cathode materials have opted more for LIBs [4]. Among the layered oxides, the first material which was investigated and implemented as cathode material for LIBs is LiCoO2, and the LIBs with LiCoO2 as cathode and graphite as anode have been developed at a high level. However, the cell performance showed criticality which indeed need some innovations in the materials, both in preparation and performance [5 ]. Meanwhile, its low thermal stability and high cost is main barrier for large sized cells in hybridelectric vehicles (HEV).

         The usage of LiMO2 (M=Co, Ni, Mn) leads to a new dimension of research with the combination of metal ions in 3d transition state.

         The reports on lithiumnickel-manganese-cobalt oxides (LNMCs) showing their significance because of their capacity and capacity retention [6] Ohzuku and his co-workers have done a detailed study on LiNi1/3Co1/3Mn1/3O2 [7-10]. Afterwards, various stoichiometric ratios of LNMC have been studied. Amongst them the lithium rich LNMC is in focus due to their good thermal stability and capacity retention [11]. In addition, the surface modification and effect of dopants is being under study. Zn-doped LiCo0.3Ni0.4xMn0.3ZnxO2 cathode materials were synthesized via co-precipitation method by Chen et al. They found that LiCo0.3Ni0.4xMn0.3ZnxO2 had stable layered structure with – NaFeO2 type with x up to 0.05. Zn-doping has improved the high rate discharge capability and thermal stability [12].Tang et al successfully prepared Titanium doped cathode material LiNi0.8Co0.2xTixO2 (0x0.1) powders with a high tap density [13]. The element substitution such as Al, Fe, Ga, Cr, Mg, Ti instead of cobalt in Lithium manganese nickel cobalt oxide also displayed good results [14]

         Various synthesis methods are there to prepare the cathode

         materials such as co precipitation method, combustion method, sol gel method and solid state reaction method. Solgel method has its own significance in the preparation of cathode material since the materials prepared by this method possess uniform particle size and good stoichiometry [15,16]. In the present study, the compound Li(Li0.05Ni0.7-xMn0.25Fex)O2 compound was prepared by citric acid assisted modified sol-gel method where, citric acid acted as a chelating agent. The chelating agents like citric acid act on nitrate ions to decompose it and the heat evolved from the decomposition of nitrate is utilized to decompose the remaining organic constituents [17]. The cathode material Li(Li0.05Ni0.7- xMn0.25Fex)O2 prepared by sol-gel method has been subjected to subjected to X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopic analysis (FTIR), Energy dispersive analysis by X-rays (EDX) and Raman Spectroscopic analysis. The observed results were discussed and reported herein.

      2. EXPERIMENTAL DETAILS

The materials used are of AR grade purchased from Merck. Lithium nitrate (LiNO3), Manganese Nitrate Mn(NO3)2.6H2O, Nickel Nitrate (Ni(NO3)2.6H2O and Iron

III nitrate Fe(NO3)3 9H2O, Citric acid (C6H8O7). The cathode material Li(Li0.05Ni0.7-xMn0.25Fex)O2 were prepared by sol-gel method . The precursor materials were dissolved in distilled water and stirred for 7h. Now, citric acid is added as a chelating agent, drop wise to the solution. The pH was maintained between 6 and 7 by the addition of ammonium hydroxide. The solution was heated to 80 C under continuous stirring to form a gel. The gel was preheated to 500 C for 4 h. The obtained powder was grounded well for 2 h and again calcined at 850 C for 15 h.

The powder X-ray diffraction (PANalytical-XPERT PRO, Netherlands) measurement using CuK radiation was employed to characterize the synthesized material. The morphological behavior and elemental analysis were done using SEM analyzer (JEOL-JSM.6400 with acceleration voltage of 5 kV). The FTIR spectrum was recorded using JASCO FTIR/4100 spectrophotometer (Japan) in the region 400-1100 cm-1 at room temperature with a signal resolution of 8 cm-1. The Raman spectra were recorded by Laser Raman Spectroscopy with RENISHAW via Raman microscope using the 514 nm line of Argon laser source

The cathodic material Li(Li0.05Ni0.7-xMn0.25Fex)O2 was prepared by sol-gel method. The crystalline structure of the material is identified using X-ray diffraction. The surface morphology was studied with the help of Scanning electron microscope. The presence of elements in the compounds was identified using EDS analysis. The complexation behaviour of the

prepared samples was studied using the FTIR spectroscopy.

Figure 1: The XRD pattern of Li(Li0.05Ni0.7-xMn0.25Fex)O2 calcined at 850

C

From the X-ray diffraction data, the value of interplanar spacing dhkl was calculated using the Braggs relation

Raman spectroscopy was used for the phase identification

of the samples.

= 2

(3.1)

1. x -Ray Diffraction Studies

   The XRD pattern of Li(Li0.05Ni0.7-

   The lattice parameters a and c were calculated using the

   equation mentioned below [24].

   xMn0.25Fex)O2 calcined at 850 C is shown in Fig.1. The

   1 = 4 [2++2

   2

   diffraction peaks observed are indexed based on the hexagoal -NaFeO2 structure with a space group of R3m

   2

   3 2 ] + [2]

   (3.2)

   (166) [JCPDS file no 44-0145] [18,19]. This shows the

   presence of alternate layers of Li atom and MO6 octahedra (M= Ni, Fe, Mn) indicate pure phase layered crystal structure [20].But the crystalline phase is absent in the case of Li(Li0.05Ni0.7-xMn0.25Fex)O2 with x=0.7.The peak splitting observed near 38(006/102) and 65(018/110), shows that the layered structure is well developed as reported by Kim et al[21]. In the -NaFeO2-type structure, the oxygen sub lattice forms a close-packed face centered cubic (fcc) lattice with a distortion in the c direction, results in clear splitting between the (0 0 6)/(1 0 2) and (1 0 8)/(1 1

   0) peaks in the XRD patterns. When this distortion in the c direction is absent, the (0 0 6)/(1 0 2) and (1 0 8)/(1 1 0) peaks combine to form single peaks in the XRD pattern [22].

   The unit cell is the parallelepiped built on the

   vectors a, b, c of a crystallographic basis of the direct lattice. Its volume is given by the scalar triple product, V = (a, b, c) and corresponds to the square root of the determinant of the metric tensor[25]. The total volume of the unit cell (V) is calculated using the following Equation (3.3)

   = 32

   2

   (3.3)

   The lattice parameter a and c value changes with respect to the increase in value of x in the stoichiometric compound Li(Li0.05Ni0.7-xMn0.25Fex)O2 as given in Table .1. The lattice parameters a and c changes with respect to the increase in x value, which is reflected in the c/a ratio and in the unit cell volume V. The intensity ratio of the peaks with lattice planes I003 and I104 gives the information about cationic mixing in the cathode material. The critical value of I003/ I104 ratio is 1.2 for the layer structured material [26]. In general, I003/ I104 ratio is less than the critical value, there is significant amount of cationic mixing and the cationic mixing is absent for the I003/ I104 values greater than critical value. The ratio of I003/ I104 is found to have lower values as the x value increases from 0 to 0.947, which is observed from Table .1.

   Table .1. Crystal parameters of Li(Li0.05Ni0.7-xMn0.25Fex)O2 for

   x = 0,0.1,0.3,0.5,0.7

   Sample	Crystallite size(nm)	a()	c()	c/a	V(3)	I003/I104
   Li(Li0.05Ni0.7Mn0.25)O2	53	2.885	14.07	4.876	101.42	0.947
   Li(Li0.05Ni0.6Mn0.25Fe0.1)O2	45	2.896	14.160	4.889	102.87	0.557
   Li(Li0.05Ni0.4Mn0.25Fe0.3)O2	62	2.901	13.799	4.755	100.63	0.591
   Li(Li0.05Ni0.2Mn0.25Fe0.5)O2	44	2.932	14.407	4.912	107.20	0.261
   Li(Li0.05Mn0.25Fe0.7)O2	—	—	—	—	—	—

   The crystallite size was estimated using Scherrers formula as given in the following equation,

   =

   (3.4)

   where, the constant K is 0.9, is the wavelength of X-ray used ( = 1.5406 Ã…), and is the full width at half- maximum of the diffraction peak corresponding to 2. Using above equation the crystallite sizes obtained were found to be in the nanometer range. The crystallite size was measured by taking the average of three main line widths from the XRD patterns. The average crystallite size of the samples calculated using the Scherrer formula is in the range between 44 to 62 nm excluding the sample Li(Li0.05Mn0.25Fe0.7)O2, which can be observed from Table .1.

2. FTIR SPECTROSCOPIC STUDIES

   The corresponding values are assigned to asymmetric stretching modes of M-O (M = Ni/ Fe/ Mn). The peak shift in the spectra may be due to the increase in elemental concentration of Iron. The weak peaks obtained around 530 cm-1 may be ascribed to Li-O stretch which suggests the presence of Li in the material. Further, a peak in the far infra-red region 271 cm-1 is expected, but it was not covered in the present study. Moreover, the corresponding less intense peaks observed at 419 cm-1 , 421 cm-1 , 418 cm-1 , 413 cm-1 and 415 cm-1 in Fig.2 (a-e) could be assigned to the bending modes of O-M-O chemical bonds [27,28]. The corresponding peaks mentioned in Fig.2 (b-e) are also correlate with the values of Fe-O stretching and bending modes [29]. From the vibrational spectra analysis, it is concluded that, there is a good interaction between metal and oxygen.

3. Raman Spectroscopy Analysis

   Raman spectroscopy of the prepared samples has been analyzed using Renishaw InVia Raman spectro photometer. Raman spectra of Li (Li0.05Ni0.7-xMn0.25Fex)O2 samples prepared at various compositions are shown in Fig.3.

   It is observed from Figure 3.3(a-e) the observation of peaks around 575 cm-1 and 480 cm-1 may correspond to the Metal Oxide symmetric stretching and bending modes, respectively. For the layered structure cathode material with a

   space group R3m which is similar to the compound LiCoO2 [30].

   Figure 2: FTIR spectra of Li(Li0.05Ni0.7-xMn0.25Fex)O2 (a) x = 0 (b) x = 0.1 (c) x = 0.3 (d) x = 0.5 (e) x = 0.7

   The vibrational spectra of the samples Li(Li0.05Ni0.7-xMn0.25Fex)O2 synthesized were recorded in the wave number range 400 – 4000 cm-1. In this study the metal oxide peaks are expected to be present in the range between 400 and 1000 cm-1 and the same is presented in Fig.2. The strong peak are obtained at 570 cm-1, 568 cm-1, 576 cm-1, 618 cm-1, 620 cm-1 respectively for the synthesized material Li(Li0.05Ni0.7- xMn0.25Fex)O2 with (i) x = 0 (ii) x = 0.1 (iii) x = 0.3 (iv) x = 0.5 (v) x = 0.7.

   Figure 3: Raman spectra of Li(Li0.05Ni0.7-xMn0.25Fex)O2 (a) x = 0 (b) x = 0.1 (c) x = 0.3 (d) x = 0.5 (e) x = 0.7.

   Therefore the corresponding optical vibration modes are: = 2 + + 22 + 2 [31]. The first two modes are visible only by Raman while the second two modes can be seen by IR. Theoretically, two Raman bands and four IR bands should be visible for the layer structured material with space

   group R3m. Fig.3(a-c) shows the peaks around 575 cm-1 and 480 cm-1 which corresponds to the active M-O symmetrical stretching

   and bending vibrational Raman modes of A1g and Eg. The samples are clearly showing the two active Raman modes except the compounds with x = 0 and x = 0.7, which shows additional peaks other than the two modes. The additional peaks observed for the Li (Li0.05Ni0.7-xMn0.25Fex)O2 compound with x = 0 reflects the display of single peak at the 2 values 38 and 68. This may correspond to the cubical sub lattice of oxygen in the -NaFeO2 type structure. For x = 0.7, the humps observed around 420 cm-1 may be due to the absence of nickel in the compound [32].

4. Scanning Electron Microscope And Energy Dispersive Analysis

Figure 4: SEM images of Li(Li0.05Ni0.7-xMn0.25Fex)O2 (a) x = 0 (b) x = 0.1 (c) x = 0.3 (d) x = 0.5 (e) x = 0.7

Surface morphology of Li(Li0.05Ni0.7- xMn0.25Fex)O2 was studied by scanning electron microscopy. SEM image of Li(Li0.05Ni0.7-xMn0.25Fex)O2 is presented in Fig.4. The agglomeration of particles is more in the Li(Li0.05Ni0.7-xMn0.25Fex)O2 for x = 0.7 as it is non crystalline as observed from the XRD pattern. Similar pattern of particles are observed for the samples Li(Li0.05Ni0.7-xMn0.25Fex)O2 (x = 0, 0.1, 0.3 and 0.5).

Furthermore, the particles are observed to be formed in

different sizes mostly in the range of 200-250 nm for the samples mentioned in Fig .4 (a-d).

Figure 5: Energy Dispersive Spectra of Li(Li0.05Ni0.7-xMn.25Fex)O2 (a) x = 0 (b) x = 0.1 (c) x = 0.3 (d) x = 0.5 (e) x = 0.7

The addition of citric acid as chelating agent, while preparing the precursor leads to the even distribution of particles, which in turn helps to keep the particle size small after calcination. The large surface area of the material will be obtained from smaller particle size, which could shorten the diffusion path of lithium ion and be helpful to improve the electrochemical performance of the cathode material [33]. The EDS spectra of the synthesized Li(Li0.05Ni0.7-x Mn0.25Fex)O2 are presented in Fig.5.

The spectra confirm the presence of Ni, Mn, Co and O in the prepared samples. Lithium is not detected in the EDS spectrum due to its low atomic number [34]

CONCLUSION

XRD analysis of Li(Li0.05Ni0.7-x Mn0.25Fex)O2 (x=0,0.1,0.3,0.5,0.7) possessed layered hexagonal – NaFeO2 structure. The crystallite size of the samples was between 44 62 nm. From FTIR and Raman spectroscopic analysis it was observed that the samples exhibit the vibrational modes of asymmetric stretching of M-O and bending modes of O-M-O revealed the formation of layered structure compounds. In FTIR spectra the peaks appeared between 420 cm-1 & 450 cm-1 were allotted to the asymmetric stretching mode of M-O and the peaks between 570 cm-1 & 650 cm-1 were attributed to the bending modes of O-M-O. Whereas in Raman spectroscopy it displayed two Raman modes of vibration around 480 cm-1 & 575 cm-1 were assigned to the stretching and bending modes of metal and oxygen except the lower and higher values of x. The SEM analysis showed a good morphological behaviour and the particle sizes between 200 300 nm which favours the lithium ion motion. EDS displayed the presence of elements in the corresponding compounds.

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