Characterization of Nanocathode Material for Rechargeable Lithium Ion Battery

Characterization of Nanocathode Material for Rechargeable Lithium Ion Battery

,

,

V. Moniha1 N.Vijaya2, T. Jeniffer Rani3

1,2,3Department of Physics

S.F.R College for Women Sivakasi-626 123, Tamil Nadu, India

M. Premalatha4

4Materials Research Center Coimbatore-641 045, Tamil Nadu, India

Abstract Li-ion batteries are one of the most commercialized solutions to store electrochemical energy, but until now their broad use is limited to small electronic devices. During the past few years, much attention was focused on cathode materials with either high voltage or high capacity coupled with high stability. In the present work, we focused on the synthesis and characterization of LiNiPO4 and lanthanum-doped LiNiPO4 prepared by Pechini- type precursor method. The X-ray diffraction analysis confirms the formation of compounds LiNiPO4 and lanthanum- doped LiNiPO4 with orthorhombic structure. The Fourier transform infra-red analysis has been made to confirm the formation of Li1-yLayNiPO4 (y = 0.05, 0.07,

1. mol%). From AC impedance analysis, it is found that the undoped LiNiPO4 has ionic conductivity of 8.13×10-09 S cm-1 and the La-doped sample (Li0.93La0.07NiPO4) has maximum conductivity of 3.83×10-08 S cm-1 at ambient temperature. This value is one order greater than that of the undoped LiNiPO4.

   Keywords Cathode Material; Lanthanum; Lithium Ion Battery; Nickel Phosphate; Rare Earths

   1. INTRODUCTION

      Currently the energy economy is in constant hazard due to the dependence on non-renewable fossil fuels. In recent years, substantial interest is being paid to exploit cathode materials for lithium ion batteries with high capacity, safety and dependability. Lithium orthophosphates LiMPO4 (M=Fe, Mn, Co, Ni, etc.) have been a lot of interest as cathodes for use in Li-ion batteries [1]. These olivine phosphate cathode materials have been intensively studied as high energy and high potential cathodes for lithium ion batteries due to their non toxic, cost effectiveness, environmental friendly, stable even at over charge and better thermal stability during charging and discharging [2]. However, one of the main problems of olivine phosphate cathode materials lies in its poor rate capability, which is attributed to its low electronic and ionic conductivity and slow kinetics of lithium ion diffusion through the LiMPO4

      – MPO4 interfaces [3]. Various approaches have been attempted to overcome this problem. Particle size reduction by modifying synthesis conditions, carbon coating and lattice doping with supervalent metal ions are considered as effective ways to improve the electrochemical properties of lithium metal phosphates [4-8]. Chung et al [7] showed that controlled cation non-stoichiometry combined with solid-solution doping by metals supervalent to Li+ increases the electronic conductivity of Li0.99D0.01FePO4 (D = Nb, Zr, Mg, Ti). Rare earth materials such as yttrium, lanthanum, europium, etc. have been used as dopants in the preparation of LiMPO4

      (M=Fe, Mn, Co, Ni, etc.) cathode materials to improve their ionic conductivity. The research of Jiezi Hu et al [9] has shown that substitution of rare earth ions at Li site of (LiY)FePO4 improves the ionic conductivity. LUO et al [10] added a small amount of lanthanum ions in LiFePO4 by solid state synthesis, and found that these ions doping could improve its capacity delivery and cycle performance. Prabu et al carried out investigation on the undoped LiNiPO4 and yttrium-doped LiNiPO4 [11] and europium-doped LiNiPO4 [12] cathode materials prepared by the Pechini- type polymerizable precursor method and found that the ionic conductivity of LiNiPO4 (9.34×10-9 Scm1) is enhanced by around two orders of magnitude by doping yttrium and one order of magnitude by doping europium at room temperature.

      In the present study an effort has been made to investigate the effect of the rare earth material lanthanum (La) on the structural, vibrational, and electrical properties of the LiNiPO4 cathode material prepared by Pechini- type precursor method.

   2. EXPERIMENTAL

      1. Preparation of La-doped LiNiPO4

         Cathode materials, bare LiNiPO4 and LiNiPO4 doped with different concentrations of lanthanum in mol% have been prepared by the Pechini–type precursor method using high purity raw materials. To prepare LiNiPO4, LiNO3, Ni (NO3)3.6H2O and NH4H2PO4 (Loba Chemie) are dissolved in double distilled water. In case of lanthanum doped LiNiPO4 then lanthanum nitrate hexahydrate (LaNO3.6H2O) is added in different molar concentrations as reduced in lithium nitrate part. Citric acid (Loba Chemie) and Ethylene glycol (Spectrum) in the molar ratio of (0.2:0.2) have been added to the above salt solution for gelation with vigorous stirring. The solution is evaporated on a hot plate; the black powder recovered is ground into fine powder and calcinated at 800 0C by keeping it in silica crucible for 8 hours in muffle furnace to form yellow powder. The following compositions of cathode material have been prepared.

         Li1-yLayNiPO4 (y = 0, 0.05, 0.07, 0.09 mol%)

         These samples are characterized using X-Ray diffraction (XRD), Fourier transform infra-red (FTIR) and complex impedance spectroscopy techniques. The results obtained from the instruments are explained with relevant literature, and presented in this paper.

      2. Characterization Techniques

      The prepared undoped LiNiPO4 and La-doped LiNiPO4 powders are yellow in color. The diffraction patterns for the cathode materials are recorded on XPERT-PRO X-ray diffractometer equipped with CuK radiation (=0.154 nm) with a scan step time [s] 3.1750. Infra-red (IR) spectra is recorded in transmission mode by using SHIMADZU IR Affinity 1 spectrophotometer at ambient temperature. For conductivity measurements, the powder has been pressed (~1.2 ton pressure) to form a pellet (~1 mm thick and 10 mm diameter). AC measurements have been carried out using impedance analyzer HIOKI 3532 LCR HiTESTER at room temperature in the frequency range of 42 Hz to 1 MHz.

   3. RESULTS AND DISCUSSION

      1. XRD Analysis

         The XRD is used to analyze structure parameters such as,

         The unit cell parameters (lattice constants) and unit cell volumes for the series of Li1-yLayNiPO4 (y=0, 0.05, 0.07, 0.09 mol%) are calculated by using the software of cellcalc. These values are consolidated in Table 1 and displayed in Figure 2

         (a) & (b). Doping effect will sometime increase or decrease lattice parameters depending on the nature of doping ion and amount of doping [5,10,11,13,14]. It is seen that doping of lanthanum does not change the crystal structure but decreases the lattice parameter along a-axis and increases along c-axis in LiNiPO4 as the position of Li+ in the lattice is partially replaced by La3+, which is favorable for the transmission of Li ions along c-axis [11]. This is because; the radius of Li+ (0.076 nm) is smaller than that of La3+ (0.1016 nm).

         Table 1 Lattice parameters and volume of the unit cell for all compounds Compound a (Ã…) b (Ã…) c (Ã…) Volume of

         the unit

         cell (Ã…)3

         unit cell parameters and unit cell volumes. The knowledge of

         such parameters especially for new materials is essential to	LiNiPO4	5.869	10.034	4.681	275.6
         find their technical applications. Figure 1 shows the X-ray	Li0.95La0.05NiPO4	5.860	10.027	4.686	275.3
         diffractin patterns of undoped LiNiPO4 and LiNiPO4 doped	Li0.93La0.07NiPO4	5.861	10.044	4.682	275.6
         with various concentrations of La. The diffraction patterns of all the samples have shown single phase LiNiPO4 with an	Li0.91La0.09NiPO4	5.855	10.017	4.684	274.7
         orthorhombic olivine structure with the space group Pmnb. It

         can be observed from the patterns that all diffraction peaks are very strong, which indicated that the sample has good crystal structure [11]. The major diffraction peak values are well matched with the reported values (JCPDS file, 032-0578).

         Figure 2 Variation of (a) unit cell parameters and (b) unit cell volume as a function of y in Li1-yLayNiPO4

         Figure 1 XRD patterns of (a) undoped LiNiPO4 and (b – d) La-doped LiNiPO4

      2. FTIR Analysis

         Figure 3 shows the FTIR spectra for undoped LiNiPO4 and LiNiPO4 doped with various concentrations of La with wavenumber ranging from 400 to 4000 cm1 recorded at room temperature. The bands observed in the region 1400400 cm1 are characteristic of phosphate ion [PO4]3- which are in free- state existing in tetrahedral (Td) symmetry.

         The peak at 393 cm-1 is essentially due to the translation modes of the Ni-O bonds. The peaks at 471-478 cm-1 are attributed to vibrations of Li-O bonds. The intensity of this peak decreases considerably upon Li extraction from the olivine lattice [15]. The peak at 548 cm-1 is attributed to the component originating to the 2(PO4) mode and the peaks at 648-656 cm-1 are accredited essentially to the components originating to the

         4 (PO4) mode, 2 and 4 involve the symmetric and antisymmetric bending modes of O-P-O bonds [16]. The peaks observed between 964-1150 cm-1 are related to stretching mode of the tetrahendral 3 (PO4), 3 involves the antisymmetric stretching mode of P-O bonds [1]. The characteristic peaks at 471, 648, 964 cm-1 observed for the undoped LiNiPO4 are found

         to be shifted to 478, 656, 972 cm-1 respectively for the La- doped LiNiPO4. Changes in the intensity of peaks have been observed from the FTIR patterns of the La-doped LiNiPO4 with respect to undoped LiNiPO4. Thus the FTIR analysis confirms the formation of La-doped LiNiPO4.

         Figure 3 FTIR spectra for undoped LiNiPO4 and LiNiPO4 doped with various concentrations of La

      3. Impedance Analysis

      The Cole-Cole plots or Nyquist plots (Z' versus -Z'') for the series of samples Li1-yLayNiPO4 (y=0, 0.05, 0.07, 0.09 mol%) prepared by Pechini- type precursor method at room temperature are shown in Figure 4. These plots consist of depressed semicircles for La-doped LiNiPO4 and a high frequency semicircle and a low frequency spike for undoped LiNiPO4. These semicircles are due to parallel combination of bulk resistance (Rb) and bulk capacitance (Cb) of the materials. The depressed semicircle is an indication of the non-ideal nature of material and the spike is due to the effect of blocking electrodes. The equivalent circuit for the impedance plot of LiNiPO4 is given as inset of Figure 4. Bulk resistance values (Rb) are obtained from the low frequency intercept of the semicircle on the real Z-axis using the EQ software developed by Boukamp for the analysis of impedance data [17]. The ionic conductivity () values are calculated using the equation:

      = L/RbA .. (1)

      where L, A, Rb are the thickness, area and bulk resistance of the pellet, respectively.

      The conductivity values for all the samples at 303 K are presented in Table 2. The conductivity of the undoped LiNiPO4 at room temperature is found to be 8.13×10-09 S/cm which agrees well with that of the same sample reported by Prabu et al. [11]. In Pechini- type precursor method, the samples LiNiPO4 doped with different concentrations of La on Li site have shown improved conductivity over an order. The reason is that lanthanum doping can reduce the grain size, decrease the Li+ diffusion resistance. Also, the impregnation

      of lanthanum leads to creation of vacancies of lithium ions which is beneficial to the diffusion of lithium ions and lanthanum can reduce the interaction force between Li+ and Li+, which makes the Li+ intercalation/de-intercalation easier. Similar effect is reported by LUO et al [10] in lanthanum- doped LiFePO4, Prabu et al [11] in yttrium- doped LiNiPO4 and Tian et al [18] in yttrium- doped LiFePO4. The sample with 0.07 mol% La-doped LiNiPO4 on Li site has maximum conductivity of 3.83×10-08 S/cm at 303 K. It is noted that lanthanum doping improves the conductivity of LiNiPO4.

      Figure 4 Nyquist plots for all samples at room temperature (303 K) Table 2 Conductivity values of all the cathode materials at room temperature

      Li1-y LayNiPO4	Conductivity (S/cm)
      y
      0	8.13×10-09
      0.05	1.38×10-08
      0.07	3.83×10-08
      0.09	7.19×10-09

   4. CONCLUSION

Nanocathode materials based on LiNiPO4 and La-doped LiNiPO4 on Li sites have been prepared by Pechini- type precursor method and investigated for structural, vibrational, and electrical properties. The X-ray diffraction patterns of the samples confirm single phase formation and orthorhombic crystal structure. FTIR spectra of the samples show characteristic peaks for [PO4]3- ion, Li-O symmetric bending and Ni-O bending, shifts and changes in the intensity of the characteristic absorption peaks for the La-doped LiNiPO4 with respect to undoped LiNiPO4 confirming the formation of La- doped LiNiPO4. Complex impedance spectroscopy reveals that the 0.07 mol% of La-doped on Li-site of LiNiPO4 has improved the conductivity over an order. It can be seen that the sample Li0.93La0.07NiPO4 shows better conductivity (3.83×10-08 S/cm at 303 K) than all other samples. This optimized sample Li0.93La0.07NiPO4 proves to be a better choice for cathode material in Li-ion batteries.

REFERENCES

1. Ben Bechir M, Ben Rhaiem A, Guidara K, A.c. conductivity and dielectric study of LiNiPO4 synthesized by solid-state method, Bull. Mater. Sci., 2014, 37(3):473.

2. Karthickprabhu S, Hirankumar G, Maheswaran A, Sanjeeviraja C, Daries Bella RS, Structural and conductivity studies on LiNiPO4 synthesized by the polyol method. J Alloys and cmpds 2013, 548:65-69

3. Yamada A, Hosoya. M, Chung. S.C, Kudo. Y, Hinokuma. K, Liu. K.Y, Nishi. Y, Olivine-type cathodes: Achievements and problems, J. Power Sources, 2003,119-121:232.

4. Gangulibabu D.B, Kalaiselvi N, Jayapraksh N, Periyasamy P. CAM solgel synthesized LiMPO4 (M=Co, Ni) cathodes for rechargeable lithium batteries. J Sol-gel Sci Technol., 2009, 49:137.

5. Yang R, Song X.P, Zhao M.S., Wang F, Characteristics of Li0.98Cu0.01FePO4 prepared from improved co-precipitation, J. Alloys cmpds, 2009, 468: 365-369.

6. Huang H, Yin S.C, Nazar L.F, Approaching Theoretical Capacity of LiFePO4 at Room Temperature at High Rates, Electrochem Solid- State Lett 2001, 4(10):A170-A172.

7. Chung S.Y, Blocking J.T, Chiang Y.M, Electronically conductive phospho-olivines as lithium storage electrodes, Nat Mater 2002, 1:123-128

8. Ni J.F, Morishita, M, Kawabe Y, Watada M, Takeichi N, Hydrothermal preparation of LiFePO4 nanocrystals mediated by organic acid, J.Power Sources 2010, 195:2877-2882.

9. Jiezi Hu, Jian Xie, Xinbing Zhao, Hongming Yu, Xin Zhou, Gaoshao Ca Jiangping Tu, Doping Effects on Electronic Conductivity and Electrochemical Performance of LiFePO4, J. Mater. Sci. Technol. 2009, 25:3.

10. LUO Shaohua, TIAN Yong, LI Hui, SHI Kejie, TANG Zilong, ZHANG Zhongtai, Influence of lanthanum oping on performance of LiFePO4 cathode materials for lithium-ion batteries, J. Rare Earths, 2010, 28:439.

11. Prabu M, Selvasekarapandian S, Kulkarni A, Karthikeyan S, Hirankumar G, Sanjeeviraja C. Structural, dielectric, and conductivity studies of yttrium-doped LiNiPO4 cathode materials. Ionics, 2011, 17:201.

12. Prabu M, Selvasekarapandian S, Kulkarni A.R, Karthikeyan S, Sanjeeviraja C, Influence of europium doping on conductivity of LiNiPO4, Trans. Nonferrous Met. Soc. China, 2012, 22:342-347.

13. Shanmukaraj D, Murugan R, Synthesis and characterization of LiNiyCo1yPO4 (y= 01) cathode materials for lithium secondary batteries. Ionics, 2004, 10: 88.

14. Wen Wang, Yuqing Qiao, Li He, Ludovic Dumée, Lingxue Kong, Minshou Zhao, Weimin Gao, Study on LiFe1xSmxPO4/C used as cathode materials for lithium-ion batteries with low Sm component,

    Ionics, DOI 10.1007/s11581-015-1397-z

15. Walrafen G E, Irish D E, Young F. Raman Spectral Studies of Molten Potassium Bisulfate and Vibrational Frequencies of S2O7 Groups.J.

    Chem. Phys.,1962, 37 :662

16. Julien C M, Mauger A, Zaghib K, Veillette R, Groult H. Structural and electronic properties of the LiNiPO4 orthophosphate, Ionics, 2012, 18: 625.

17. Boukamp B A. A package for impedance/admittance data analysis, Solid State Ionics, 1986, 18-19(1):136.

18. Tian Y, Kang X, Liu L, Xu C, Qu T Research on cathode material of Li-ion battery by yttrium doping (2008) J Rare Earths 26:279