Synthesis of surface modified LiFePO4 cathode material via polyol technique for high rate lithium secondary battery

Accepted Manuscript Title: Synthesis of Surface modified LiFePO4 cathode material via polyol technique for high rate Lithium Secondary battery Author: M. Sivakumar R. Muruganantham R. Subadevi PII: DOI: Reference:

S0169-4332(15)00415-8 http://dx.doi.org/doi:10.1016/j.apsusc.2015.02.100 APSUSC 29779

To appear in:

APSUSC

Received date: Revised date: Accepted date:

28-12-2014 9-2-2015 13-2-2015

Please cite this article as: M. Sivakumar, R. Muruganantham, R. Subadevi, Synthesis of Surface modified LiFePO4 cathode material via polyol technique for high rate Lithium Secondary battery, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.02.100 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.

Synthesis of Surface modified LiFePO4 cathode material via polyol technique for high rate Lithium Secondary battery

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M. Sivakumar*, R. Muruganantham, R. Subadevi

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# 120, Energy Materials Lab, School of Physics, Alagappa University, Karaikudi-630 004, Tamil Nadu, India.

*Corresponding author.

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Dr.M.Sivakumar,

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School of Physics, Alagappa University,

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Karaikudi – 630 004. Tamil Nadu, India.

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E-mail: [email protected]

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Tel.: 91-04565-225205; Fax: +91-04565-225202. Mobile : +91 9842954116

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Abstract The NiO coated LiFePO4 composite cathode materials were prepared by simple tailored polyol technique, which has orthorhombic olivine structure without NiO phase. It delivers the

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reverse capacity of 164 mAh g-1 at 0.1 C at ambient temperature. The material exhibits stable reverse capacity for several cycles even for the higher C-rates. Increasing the rate, the reverse

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capacity is almost stable over several cycles. Furthermore, at a high rate viz, 10, 20 and 30 C,

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the discharge capacity has been observed for the optimized NiO coated LFP as 135, 120 and 69 mAh g-1, which demonstrates the excellent rate performance that can be useful for high power

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lithium secondary battery.

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Keywords: LiFePO4, Rate performance, Olivine structure, Polyol technique, Reverse capacity,

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Lithium secondary battery.

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1.Introduction Day by day, our society meets two major challenging issues, such as energy crisis and environmental issues.

Hence, a safe, low-cost, high efficiency and environmentally benign

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alternative power sources have become a most urgent need. Past decades, Lithium battery has

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been fulfilled the requirement and played a key role in the consumer electronic market. Nowadays, it is moving towards electronic automotive transportation (electric vehicles, full

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hybrid electric and plug-in electric vehicles) [1-2].

Electrode plays a crucial role in electrochemical performance of batteries and ion

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transmission between positive and negative electrodes. It holds an important implication for

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battery capacity, circulation, and security properties. At present, thermal stability of LiFePO4 enables the safety of the batteries especially for large-scale applications. It has a highly stable

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three-dimensional frame work due to strong P–O covalent bonds in the PO43− polyanion, which

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prevents the liberation of oxygen and keeps the stable framework during the charge and discharge processes [3]. This property provides an excellent safety, stable operation of lithium

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batteries even under unusual conditions. Recently, some approaches have been explored to enhance the electrochemical performances of LiFePO4 through particle size reduction, conductive elements or metal oxide modification on the surface of LiFePO4 and lattice modification with other metals [4-7]. Among these, few works have been established to develop the LiFePO4 via coating with oxide materials, such as RuO2 [8], ZrO2 [9], TiO2 [10], CuO [11], SiO2 [12], Al2O3 [13], V2O3 [14], CeO2 [15], ZnO [16], SnO2 [17], MoO2 [18] and WO2 [19] and improved its electrochemical performances. In connection, NiO is one of the suitable materials for energy storage applications owing to its high theoretical capacity, low cost, and high chemical and thermal stability [20]. The NiO nanoflakes enhance the diffusion of electrolyte and 3 Page 3 of 24

provide more paths for diffusion of ions leading to improvement in the performance of the electrode material [21]. The NiO coated LiFePO4 active material, provides a better channel for the Li+ diffusion from the electrolyte to the surface of the active materials during intercalation

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and deintercalation. Ge et al. [22] and Qing et al. [23] reported that the Ni doped LiFePO4/C by solid state reaction method; Also Novikova et al. [24] proposed Ni, Co and Mg doped

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LiFePO4/C using sol-gel method. They have obtained an excellent rate capability and cycling

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performance. The first polyol technique prepared LiFePO4 has been reported by Kim and Kim [2, 25] and recently Carbon nanotube and graphene nanosheet co-modified LiFePO4 via polyol

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process reported by Wu et al. [26]

To the best of our knowledge, no reports are available in the literature about the NiO

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coated LiFePO4 prepared via polyol technique. Hence, in the present investigation, an effort has

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been made to prepare NiO coated LiFePO4 composite and to scrutiny their structural,

2. Experimental

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morphological and electrochemical performances.

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2.1. Material preparation via polyol process Stoichiometric molar ratio of iron (II) sulphate heptahydrate (FeSO4.7H2O, 99.9 % of Alfa-Aesar) and lithium dihydrogen phosphate (LiH2PO4, 99.9 % of Alfa-Aesar) were dissolved in a polyol solvent of Diethylene glycol (DEG, Aldrich). The mixed solution was refluxed near to the boiling point of the polyol solvent (245 °C) for 18 h. After that, the reacted solution was washed several times with ethanol and acetone. The resulting particles were separated and dried in a vacuum oven at 150 °C for 48 h. [27] Finally, the distinguished uncoated-LiFePO4 sample was obtained.

The uncoated LiFePO4 particles were mixed with 1 to 3 wt. % of Ni

(CH3COO)2.4H2O (nickel (ii) acetate tetrahydrate, 99+%, for analysis, Alfa-Aesar) in distilled

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water and then allowed it to be gellified. Finally, the gel particles dried in vacuum oven at 150 °C for 5 h and then calcined at 500 °C for 1 hr under Ar environment. These samples were labeled as NiO-coated LiFePO4.

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2.2. Structure and morphological characterization. The powder X-ray diffraction (PXRD) analysis was performed for NiO coated LiFePO4 samples using PANalytical X’-pert pro

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diffractometer with Cu Kα radiation operated at 40 kV and 30 mA and the radiation of

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λ =1.54060 Å in the range 2θ=10-80°. The functional group vibration was analyzed form 4000-400 cm-1 using Thermo Nicolet 380 FT-IR spectrophotometer using KBr pellets.

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Morphological studies were performed using field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM) (Techni G2 SChemical valence states of the elements were

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TWIN, FEI, The Netherlands) techniques.

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analyzed by X-ray photoelectron spectroscopy (XPS, PHI model 5802).

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2.3. Electrochemical characterization. The uncoated and NiO coated LiFePO4 electrodes were fabricated by mixing the active materials with super P and binder poly (vinylidene fluoride)

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(PVdF) with the weight ratios of 80: 10: 10. The mixed powder was dispersed in N-methyl Pyrrolidone (NMP) solvent to form homogeneous slurry and uniformly coated on Alumina-foil. Those electrode sheets were dried at 120 °C under vacuum for 6 h and then roll pressed, further those samples were punched into circular discs. The coin cell (CR2032) was fabricated using the circular disc as cathode and lithium metal as a counter electrode in between the separator Polypropylene. The electrolyte solution used in the cells was 1 M solution of LiPF6 in a mixture 1:1 volume ratio of ethylene carbonate (EC) and diethyl carbonate (DEC).

Polypropylene

separator (Celgard 2400, Hoechst Celenese Corp) was soaked in the electrolyte for 24 h prior to use. All the coin cell assembling procedures were performed using Ar-filled glove box by

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keeping both the oxygen and moisture levels less than 1 ppm.

The galvanostatic charge-

discharge analysis was performed using a BTS-55 Neware battery testing system between the potential 2.5 to 4.5 V (vs. Li/Li+) at ambient temperature with different C-rates.

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3. Results and Discussion 3.1 Phase structure analysis by XRD

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Figure 1 shows XRD patterns of bare and NiO coated LiFePO4 powders. It is observed

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that the diffraction peaks of bare LiFePO4 were indexed to the ordered orthorhombic olivine crystal structure with space group of Pnma. Even after, the NiO coating there are no diffraction

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lines corresponding to NiO (JCPDS 89-7390, 89-7130 and 89-7131) phase appeared in the XRD patterns of the coated LiFePO4 samples. The absence of diffraction peaks correspond to NiO

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may be of the following reasons:

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The coated material may be in the form of thin layer coating on the surface of the core

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material and the NiO may be presented or attached on the core material as amorphous particles as well. Also, the content of NiO may also be very low [17, 28, 29]. The lattice parameters of

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the prepared samples were calculated and furnished in Table 1. The average crystallite sizes (D) of both LiFePO4 samples were calculated from the Scherrer equation [30]. Wang et al. [31] reported that there was a little change in lattice parameter when coated with AlF3 on LiNi1/3Co1/3Mn1/3O2, which suggests that the phase of AlF3 coated layer nearly close to the surface of LiNi1/3Co1/3Mn1/3O2 could form the solid solution. Studies on CoO coated o-LiMnO2 by Cho and team [32, 33] showed that Co is incorporated into the surface layers forming a solid solution with the compound. Similar observations were made in the present study that the NiO has been formed as a solid solution layer on the surface of LiFePO4. However, as the coating procedure has been carried out at 500 °C; few amorphous particles may have been attached on

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the surface of the core material. Also, Ge et al. [22] observed a decrement in the second decimal of lattice parameter ‘a’ when doping with Nickel on LFP/C due to its ionic radius is small when compared to iron ion. The same effect has been dealt by Qing and co-workers [34] as lattice

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parameters a and b decreased linearly with the increasing ratio of nickel due to the smaller Shannon radii of [Ni2+] (0.69˚A) com-pared to [Fe2+] (0.78˚A); Lattice parameter c remained

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relatively unchanged, for LiFePO4 and LiNiPO4 and their composites. From the analysis, one

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can suggest that the NiO may have been formed as thin layer and also may have been attached on LiFePO4. The slight change in cell parameter ‘a’ values in third digit indicates the presence of

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NiO on LFP surface and doesn’t affect the structure of bare LFP. It indicates that NiO did not diffuse into LiFePO4 lattice; it only locates on the surface of LiFePO4 particles. Also, average

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crystallite sizes are slightly increased with increase of coating content and do not change the

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3.2 FT-IR analysis

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phase structure of LiFePO4, similarly as other coating materials [17, 35].

FT-IR spectra have been recorded for the additional information on their local structures

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and impurities of the bands of prepared samples. Figure 2 represents the FT-IR spectra of uncoated and coated LFP samples in the range of 4000-400 cm-1. It is observed the wave numbers around 1140 to 950 cm-1 demonstrate the vibrations of symmetric and antisymmetric PO- stretching mode of LiFePO4 for the characteristic bond of [PO4]3-. The bands at 635, 577 and 550 cm-1 denote the symmetric and antisymmetric bending vibrations of O-P-O modes [36]. The bands near 500 and 465 cm−1 are assigned to Li+ ion “cage modes” [37]. The band at 445 cm−1 corresponds to the Ni-O in this region overlap with the Li+ ion cage modes [38]. The bands between 1400-1630 cm-1 corresponding to the symmetric and antisymmetric vibration of CO2

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present on the prepared samples, which is due to the decomposition of precursor and polyol solvent [39]. 3.3 Morphological Observations

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Figure 3 (a-d) shows the SEM images of the pure LiFePO4 and 1-3 wt. % of NiO coated LiFePO4 samples with the magnification of 40 k. The uncoated LiFePO4 particles are smaller

The average particle sizes were observed as length× breadth (350×110),

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shaped particles.

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than the NiO-coated LFP samples. Figure 3 (a-d), it is clear that the samples have plate like rod

(410×210), (690×250) and (550×270) nm respectively for the pure LiFePO4, 1, 2 and 3 wt. % It is understood that from the sizes of the particle higher

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NiO coated LiFePO4 samples.

dimensioned (length × breadth) samples provide very good electronic conductivity to bare

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LiFePO4, thereby improved the electrochemical properties for the coated samples. However,

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further increase of NiO to 3 wt. % causes excess breadth and lower length. This enabled as to

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have more attention on 2 wt. % of NiO coated samples and TEM analysis was performed. Hence, the coating of NiO, the particle sizes are slightly increased, when increased the coating

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content of the particles to form a faintly agglomeration. TEM images of the 2 wt. % NiO-coated LiFePO4 sample has been depicted in Figure 4 (a-d); Figure 4(a, c) clearly represents the LiFePO4 particles have been surrounded intermittently by NiO particles. The observed HR-TEM lattice fringes images (Fig. 4 (b, c & d)) demonstrate that NiO has been coated as discontinuous layer on the surface of LiFePO4 particles. It is also observed from the Figure 4 a, there are few amorphous particles were present on the surface of the core material as well. From the lattice fringes images, it may clinch the confirmation of coating and discontinuous layer formation of NiO on the LFP surface. The HR-TEM image illustrates the lattice fringes of NiO-coated on LFP sample with coating thickness of ~3 nm.

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Since, the thin NiO-coating structure may enhance the high rate capability, which is in agreement with the literature [40]. The SAED pattern of the 2 wt. % NiO-coated LiFePO4 sample is shown in Figure 4(e). The clear diffraction spots reveal that the single-crystalline nature of the rod-like

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particles and the corresponding planes are indexed to the LiFePO4 particles, which are in line with XRD analysis [41].

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3.4 Elemental analysis using XPS

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Figure 5 (a-f) depicts the X-ray photoelectron spectrum (XPS) of 2 wt. % NiO-LFP sample. The full range XPS spectrum of 2 wt. % NiO-LFP is shown in Figure 5a, and its core

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XPS spectra in the binding energy range of Li 1 s, Fe 2p, P 2p, O 1s and Ni 2p are showed in Figure 5 (b–f). In the XPS full spectrum peaks corresponding to elements Li, Fe, P, O, Ni and C

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were found, which indicates the presence of elements Li, Fe, P, O, Ni and C on the surface of the

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prepared sample. From Figure 5a, the low intensity binding energy peak of 284.86 eV represents

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the carbon present on the surface of prepared material, which is due to polyol solvent used for the preparation. The binding energy of Li 1s spectrum peak is observed at 55.9 eV is shown in

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Figure 5b. The Fe 2p core level spectrum is shown in Figure 5c. Spin-orbit splitted doublet 2p3/2 and 2p1/2 peaks and satellite structures appear in XPS spectrum. The peak corresponding to Fe2+ is found at binding energy 710.91 eV and their satellite peaks of 724.5 eV for the binding energy of 2p3/2, which indicates that the oxidation state of Fe is +2 in the NiO coated LiFePO4 sample [42, 43]. Figure 5( d & e) represents the core spectra of P 2p and O 1s peaks with binding energies of 132.85 and 531 eV [44]. The Ni 2p3/2 and Ni 2p1/2 spectra are presented in Figure 5f and depict 5 discernible peaks: 2p3/2 (855.37 eV) and its satellites (861.62 and 867.9 eV), 2p1/2 (872.8 eV) and its satellite (880.5 eV), all of which may be attributed to the Ni2+ and the oxygen

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with Ni binding energy correspond to the oxidation state of Ni2+ are in agreement with the literatures [45, 22]. 3.5 Electrochemical studies

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Figure 6a, illustrates the charge/discharge performance of uncoated and coated LFP materials which were tested between 2.5-4.5 V under room temperature at 0.1C rate. The

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uncoated LFP based cell shows a discharge capacity of 147 mAh g-1 with capacity retention of

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92 %, whereas the capacities of NiO-coated LFP cells were increased among the uncoated sample. The initial discharge capacity of 1, 2 and 3 wt. % of NiO-LFP cells were increased to

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154, 164 and 158 mAh g-1 with capacity retention of 98, 98.8 and 96.9 %, respectively. The discharge capacity has been increased upon increasing the NiO content until 2 wt. %. This is due

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to the plate like rod shaped particles possess the average dimensions of 690×250 nm as length ×

Further increment of NiO content causes shorter length particles and

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discharge analysis.

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breadth, which provides more conductivity towards the better performances during charge-

increased width coating on LFP, which obstructs the Li+ diffusion. Yao et al. [46] reported that

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the discontinuity attachment of CeO2 particles on LFP/C materials has enhanced the charge/discharge performance under 20 to -20 °C. An incomplete carbon network has been repaired by the nano-sized CuO on LFP particles which was studied by Cui et al. [10]. They suggested that the nano-sized CuO and carbon co-coating significantly reduced the polarization of the cathode and improved the electrochemical performances. Also, Quan et al. [47] reported that the CePO4 particles attached with LFP morphology has been improved the electrochemical performance with carbon coating on LFP particles. Obviously, some portion of NiO may have been formed as layer and some portions may be formed as particles on LFP surface as observed in the TEM results. It is beneficial to enhance the capacity and rate performance of LFP due to

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the discontinuity attachment of particles to ionic conductivity with stable structure in electrolytes solution. Figure 6b, depicts the charge-discharge profile of 2 wt. % NiO-coated LiFePO4 sample at

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different rates. The discharge capacity values were observed as 159, 146, 132, 125 and 68 mAh g-1 respectively 0.5, 1, 10, 20 and 30 C rates. This is demonstrating an estimable high rate

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performance for high power lithium ion battery. The improvement of the rate performance of the

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LiFePO4 with NiO coating is mainly due to the nano NiO coating, which helps to prevent LiFePO4 particles may be direct contact to the electrolyte, and thus reduces iron dissolution of Figure 6c shows the cycling

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LiFePO4 during intercalation/deintercalation process [48].

performance of 2 wt. % of NiO-coated LFP at the current of 0.1 C under room temperature.

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Obviously, NiO modification of LiFePO4 can help to obtain an excellent stable cycle life over

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100 cycles with capacity retention of 98 %. This enhanced performance is represent the lower

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stability of NiO may react faster with acidic species which are produced from electrolytic decomposition, comparing with stable materials [49]. Therefore, NiO coated LFP via polyol

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process is an effective economic with energy efficient method to fabricate the electrode materials for lithium-ion batteries.

Figure 6d shows the rate performance with cyclic behaviour of 2 wt. % of NiO-coated LFP was performed under various current rates (0.5 C, 1 C, 10 C, 20 C and 30 C) at room temperature. It can be seen that the capacities decreased along with the increased current rate. It is because higher current rate means more lithium ions extract and reinsertion during relatively short time. When lithium ion migration cannot meet the requirement, the discharge capacity will decay significantly [50]. This enhancement of rate performance is beneficial to the high rate lithium-ion battery applications as EVs and HEVs.

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4. Conclusion The NiO coated (1, 2 and 3 wt. %) LiFePO4 nanocomposite cathode materials were successfully synthesized by simple polyol technique. The surface modification through coating

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did not alter its orthorhombic olivine structure of pure LiFePO4 for the studied composition of NiO. The coating of NiO favours higher electronic conduction until the optima content of NiO

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reaches 2 wt. %. This enhances the electrochemical properties of bare LiFePO4 exhibited the

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discharge capacity of 164 mAh g-1 at 0.1 C rate under ambient temperature. The surface modification of LiFePO4 with NiO provides plate like nano rod formation an excellent discharge

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capacity and cycle life, especially the high rate performance. The development of the nano metal oxide coating on LiFePO4 cathode materials can be used for high-power and high rate capability

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Acknowledgements

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of advanced lithium batteries capture promising place in the electric and hybrid vehicle markets.

The authors M.Sivakumar and R.Muruganantham gratefully acknowledge for the financial

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support to carry out this work by Department of Science and Technology (DST), New Delhi, Govt. of India under DST-SERC major research project whose contract number is SR/S2/CMP0049/2008.

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Figure Captions Fig. 1: XRD patterns of bare and NiO coated LiFePO4. Fig. 2: FT-IR spectra of LiFePO4 and NiO-coated LiFePO4 samples in the range of

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400-4000 cm-1.

Fig. 3: SEM images of (a) uncoated (b) 1 wt. % of NiO (c) 2 wt. % of NiO (d) 3 wt. % of NiO-

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coated LiFePO4 samples.

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Fig. 4: (a, c) HR-TEM images (b, d) lattice fringe images (e) SAED pattern of 2 wt. % NiOcoated LiFePO4 sample

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Fig. 5: (a) wide range XPS spectra (b) Li 1s (c) Fe 2p (d) P 2p (e) O 1s and (f) Ni 2p XPS spectra of 2 wt. % of NiO-coated LiFePO4 sample

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Fig. 6: (a) charge/discharge curve of uncoated coated LFP samples at 0.1 C rate under room

d

temperature (b) charge/discharge curves with various rates under the room temperature of 2 wt.%

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of NiO-coated LFP (c) cyclic behavior of 2 wt. % NiO-coated LiFePO4 sample under 0.1 C rate and (d) high rate capability of 2 wt. % NiO-coated LiFePO4 sample under room temperature.

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Table caption:

Lattice parameters and crystallite sizes of uncoated and coated samples from XRD data

16 Page 16 of 24

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cr

Table 1. The lattice parameters and crystallite sizes of uncoated and coated samples

M

Lattice parameters (Å)

Cell volume V (Å3)

Crystallite size (D) (nm)

b

(JCPDS)Standard LFP

10.33

6.010

4.693

291.470

--

Uncoated LFP

10.327

6.006

4.700

291.463

38

1 wt. % of NiO-LFP

10.335

6.008

4.694

291.469

42

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te

a

d

Materials

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us

from XRD data

c

2 wt. % of NiO-LFP

10.345

6.010

4.688

291.513

43

3 wt. % of NiO-LFP

10.343

6.013

4.704

292.553

46

17 Page 17 of 24

Highlights:  NiO coating on LiFePO4 via polyol technique may be the first attempt.  Surface coating using NiO on LiFePO4 did not affect its orthorhombic olivine structure.

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 Surface modification using NiO on LiFePO4 delivers the discharge capacity of 164

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d

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cr

mAh/g at 0.1 C rate.

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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