Temperature effect on electrochemical properties of lithium manganese phosphate with carbon coating and decorating with MWCNT for lithium-ion battery

The increasing demands for higher energy density and higher power capacity of Li-ion secondary batteries have led to a search for electrode materials whose capacities and performance are better than those available today. One promising candidate is lithium manganese phosphate, and it is necessary to understand its transport properties. These properties are crucial for designing high-power Li-ion batteries. The effect on the electronic conductivity is analyzed with a conductor material, carbon nanotubes multi-walled, and glucose was used as a carbon source. Here, the transport properties of LiMnPO4, LiMnPO4/C, and LiMnPO4/MWCNT are investigated using impedance spectroscopy. The electronic conductivity is found to increase with increasing the temperature from 2.92 × 10−5 S cm−1 to 6.11 × 10−5 S cm−1. The magnetization properties are investigated, and antiferromagnetic behavior below 34 K is reported for the three compositions. The structural characterizations were studied to confirm the phase formation of material with XRD, TEM, and SEM.


Introduction
Zero emissions and renewable energy technologies have become important options to mitigate greenhouse gas emissions and form an independent carbon cycle to reduce global warming. Solar, wind, and tidal powers have been used as alternative energy sources. These power systems are generally used in the electrical supply grid for the generation and supply of auxiliary power. The support mechanism has enhanced the efficiency and quality of the production and storage of renewable energy sources [1]. Olivine-structured lithium transition-metal phosphate has attracted broad attention as a potential Li-ion battery cathode material to replace transition metal oxide-based materials such as LiCoO 2 [2,3]. Lithium-ion batteries have been intensively investigated in recent years. The reason is that there has been increasing demand in the portable electronics market and its application in electric vehicles (EVs) and hybrid electric vehicles (EHVs). The phospho-olivine LiMPO 4 (M = Fe, Mn, Co, Ni) has been investigated and used in cylindrical batteries since the pioneering work of Goodenough et al. as a candidate positive electrode material for use in lithium rechargeable batteries [4][5][6][7]. This family of phosphate compounds has a three-dimensional framework stabilized by strong covalent bonds between oxygen ions and P 5+ , resulting in PO 4 3− tetrahedral polyanions. These compounds are promising as a cathode material due to their high safety, environmental tolerability, and low cost. Olivine structured lithium manganese phosphate (LiMnPO 4 ) as a cathode material for lithium-ion batteries is having a high capacity (170 mAh g −1 ) and high redox potential (4.1 V vs Li + ), respectively. Moreover, LiMnPO 4 has a 20% greater energy density compared to LiFePO 4 [8], working ability at high temperatures, low cost, and is eco-friendly which makes LiMnPO 4 a successful cathode candidate for lithium-ion batteries [9]. In spite of these properties, poor electrical conductivity is a crucial issue to be addressed [10]; to overcome it, one of the good ways is through surface coating with a carbon source [11,12]. Luo et al. obtained LiMnPO 4 /C via citric acid-assisted sol-gel method [13]. Their work demonstrated that LiMnPO 4 /C obtained at 500 °C for 10 h has good structural ability and electrical properties. Kwon et al. adopted the sol-gel method to prepare LiMnPO 4 /C using anhydrous ethanol as solvent, and graphene nanoplates (GNP) were added into terminal material by ball milling [14]. Zhong et al. performed different compositions of LiMnPO 4 electrodes with addition of multi-walled carbon nanotubes (MWCNTs) [15]; Zong and Liu reported hydrothermal approach and solid-state reaction to synthesis of LMP compound and carbon-coated LMP samples with carbon nanotubes [16]. Thermal stability is a characteristic of the degradation of the battery due to high temperatures generated during charge-discharge cycles; the cathode after several cycles tends to decompose and release oxygen which promotes runaway reactions with the electrolyte. But there is still lacking information on solid-state analysis which is an alternative to improving the electrochemical properties of this material. The majority of studies of LiMnPO 4 have focused on electrolyte systems; in this work, we think that it is a good approach to analyze the previous functionalization of electrolyte systems. In the present study, the analysis and comparison of the crystal structure, morphology, impedance values, and electrochemical properties of the conductive LiMnPO 4 LiMnPO 4 /C and LiMnPO 4 / MWCNT composites are addressed.

Experimental procedure
The sol-gel method was used to synthesize the powders. As starting precursors, manganese (II) acetate tetrahydrate (C 6 H 6 MnO 6 × 4H 2 O, 99.9% Aldrich 229,776), ammonium phosphate dibasic ((NH 4 ) 2 HPO 4 , 98% Aldrich 7783-28-0), and lithium acetate dihydrate (CH 3 COOLi × 2H 2 O, Aldrich, 6108-17-4) are separately dissolved in deionized water at 60 °C in beakers 1:1:1:1 molar ratio under constant stirring. Afterward, all solutions are mixed under constant stirring for 1 h and then dried at 150 °C for 24 h in the oven. The obtained xerogel is grinded in a mortar and pestle, and then fired under reducing atmosphere with 90% argon and 10% hydrogen at 500 °C for 6 h. For the carbon coating, the same procedure is followed; the only difference is the moment when all the solutions are mixed, the glucose is added as a carbon source in a 1:2 molar ratio. To decorate with MWCNT, carbon nanotubes and powders LiMnPO 4 are mixed in a 1:2 molar ratio under stirring. Then, 0.75 ml of H 2 SO 4 and 10 ml of distilled water, 50 mg of malic acid, and KMnO 4 are added and put under constant stirring at 60 °C, then it is washed several times with distilled water to reach pH = 7. It dried at room temperature for 24 h.
The sample was characterized using X-ray diffraction analysis (XRD) in a Panalytical X'pert Pro with X-rays source of Cu K-alpha. The structural characterization was performed by scanning electron microscopy, using SEM JSM 5800-LV and transmission electron microscopy, using TEM JEM-2200FS. Magnetic behavior is analyzed through room temperature vibrating sample magnetometer studies. The structural magnetic behavior was studied using an LDJ VSM9600 Cryogen-free Physical Property Measurement System, and curves were measured from 5 to 300 K. Compacted pellets were prepared from the powder by pressing at 340 MPa for 60 s, forming cylindrical samples of 14 mm in diameter; the pellets were put under heat treatment under controlled atmosphere (90% argon and 10% hydrogen) at 500 °C for 6 h. For electrochemical properties, the pellets were polished on both sides and painted with silver paste on both surfaces forming the cell configuration Ag║LiMPO║Ag. The pellets were subsequently heated at 80 °C for 5 h in order to remove the organic solvent. Electrochemical impedance spectroscopy was employed to measure the electrical conductivity using CS series electrochemical Workstation in the frequency range 0.01 Hz-100 kHz. The measurements were performed at temperatures 45, 50, 55, and 60 °C. The sample temperature was measured by a thermocouple placed near the sample. The ionic diffusivity was measured by electrochemical impedance spectroscopy. EIS was measured over a range of frequency 0.01 Hz-100 kHz at AC amplitude of 10 mV with the same range of temperatures as in the electrical measured. The measure was performed in a Swagelok-type cell, polyethylene oxide was used as an electron blocking, lithium conducting layer. Figure 1 shows the XRD patterns for pure LiMnPO 4 , LiMnPO 4 /MWCNT, and LiMnPO 4 /C composites. The three patterns can be indexed to an orthorhombic olivine-type structure with space group Pnmb, and there is no secondary phase in each pattern. For LiMnPO 4 /C samples, the carbon phase is amorphous, and then it only affects the X-ray pattern background. As the added carbon amount is small, it only has a small influence on the background. The peaks for MWCNT are localized for 2θ less than 10° with very low intensity [17], and the present XRD patterns were run for values higher than that. In order to find out if each carbon source affected the crystal structure, the crystallite size (D) was calculated by Scherrer formula D = 0.9 λ/β cos Ɵ, where λ is the X-ray wavelength, β is the width and half maximum (FWHM), and Ɵ is the Bragg angle. To estimate the FWHM values, the four highest peaks corresponding to (101), (201), (211), and (311) directions were used. The calculated values are 55, 58, and 46 nm for pristine and composites with glucose and MWCNT, respectively. It is believed that the small particle size is useful for the intercalation/de-intercalation process of lithium ions [18]; in this work, we used glucose and MWCNT carbon source to increase electronic conductivity and promoted better diffusion which was demonstrated in the electrochemistry properties further. Table 1 summarizes some lattice parameters in which those of the pristine phase coincide with previous reports [19].

SEM
The morphological structure of the samples was analyzed by SEM and TEM micrographs which are shown in Fig. 2. The pristine sample (Fig. 2a, b) is composed of a few agglomerates up to several microns and many submicron grains. Carbon coating can be seen in Fig. 2c and here clearly shows the difference in several microns in the agglomerates where the particles are interconnected by the carbon resulting from glucose. Figure 2e depicts the TEM image where the carbon layer on the surface of the sample can be clearly identified. In Fig. 2f-h, TEM images for LiMnPO 4 /MWCNT sample are shown. The use of sulfuric acid leads to chemistry oxidation and permits carboxylic groups to be formed; through this way, the LiMnPO 4 and carbon nanotubes could be connected through an ester bond, and this process has demonstrated to create moieties to bond new reactive chains that improve compatibility with other materials and allow taking advantage of MWCNT properties [20,21]. This kind of treatment may create defects in the sidewalls of the MWCNT also decreasing the oxidized surface which allows adherence of the powders and MWCNT. The addition of KMnO 4 and malic acid was to achieve better dispersion of MWCNT on the surface of LiMnPO 4 [22]. Furthermore, porosity and interconnected networks of carbon nanotube are clearly noted. This framework enhances ion diffusion during the insertions and extraction process.

Magnetic structure behavior
The magnetization curves are linear over the investigated temperature and magnetic field ranges. The deviations from such behavior are indicative of structural defects and/or impurities [23]. The LiMnPO 4 compound is a Curie-Weiss paramagnet at room temperature and undergoes antiferromagnetic transitions with collinear magnetic structures, but different spin orientations [24]. The magnetic transition metal ions occupy the distorted octahedral M2 site forming corner-sharing MO 6 units which are separated by the PO 4 tetrahedra and edgesharing LiO 6 octahedra. The magnetization of LiMnPO 4 , LiMnPO 4 /C, and LiMnPO 4 /MWCNT is shown in Fig. 3 recorded over a range of fields, and the magnetization increases linearly with the applied field. The alignment of the spins to the applied field promotes the paramagnetic behavior, and the magnetic susceptibility is positive. The lack of ferromagnetic behavior indicates the absence of different valance states which suggest negligible contribution from the electronic part, and it indicates the nature of the ion conductivity of these materials. The temperature dependence of the inverse magnetic susceptibility was analyzed. All compounds exhibit a Curie-Weiss type dependence temperature for an antiferromagnetic ground state below the Nee temperature that is reported T N = 34 K (Fig. 4). Values These results are consistent with previous reports [25]. Clearer evidence of defects is antisite defect; when Mn and Li switch sites, magnetic properties should be sensitive to these structural modifications; for example, dilution of the transition metal sublattice with non-magnetic ions may lead to a decrease of T N and absolute Ɵ values [26]. This analysis lets us know that our samples are out of impurities that many times cannot be detected by classical characterization.

Electrochemical properties
The electrochemical impedance spectra were analyzed in the frequency range 0.01 Hz-100 kHz, and the Nyquist plot is shown in Fig. 5 of pristine, carbon-coated, and multiwalled carbon nanotube decorated samples. The impedance spectra were measured at 45 °C, 50 °C, 55 °C, and 60 °C temperatures with the configuration Ag║LiMPO║Ag. The absence of an additional polarization process at low frequencies (i.e., a second semicircle) indicates that the electronic carries conduction predominately, and it also suggests negligible ionic conductivity. All the samples were evaluated with the ideal equivalent circuit; the semicircle is assigned in this work as R1 which represents the interfacial charge transfer resistance, and CPE (constant phase element) stands as the typical electrical double-layer capacitance (Fig. 5d). As shown in Table 2, the composition LiMnPO 4 /MWCNT exhibits the highest electronic conductivity. Previous works Fig. 5 a, b, c Electrochemical impedance spectral characterization of the samples LiMnPO 4 , LiMnPO 4 /C, and LiMnPO 4 /MWCNT, and d equivalent circuit to evaluate the impedance spectra reported the electrochemical reactions on different heat treatment temperatures of LiMnPO 4 materials synthesized by several approaches determined by EIS [27][28][29]. The pure LiMnPO 4 phase presents very low electronic intrinsic properties as well as low ionic conductivity, manifesting this in poor electrochemical properties [30]. This poor response is attributed to the low diffusion of lithium atoms between particles. A carbon source provides the conductive path for the diffusion of the ions [31]; among the several options as a carbon source, the MWCNT is getting attention due to its high symmetry which provides an excellent path for ionic and electronic conductivity. The conductivities for the three compositions measured at a given temperature increase with respect to the sample sequence: LiMnPO 4 , LiMnPO 4 /C, and LiMnPO 4 /MWCNT. The result of the pristine sample is in good agreement with a previous report [32]. For the samples with carbon source and MWCNT, we found a remarkable increase in the conductivities. The values of activation energy, calculated using Arrhenius law, are displayed in Fig. 6; these results suggest a polaron conduction mechanism that is supported by temperature dependence [33]. It is well known that carbon coating is a good approach to improving electronic conductivity. The lowest value of 0.03 eV of activation energy is reported for decorated MWCNT, and this facilitates the conductivity. The analysis with electron blocking cells takes into account the temperature-dependence of ionic diffusivity. Figure 7 shows the impedance spectra of LiMnPO 4 (Fig. 7a), LiMnPO 4 /C (Fig. 7b), and LiMnPO 4 /MWCNT (Fig. 7c) measured at 45 °C, 50 °C, 55 °C, and 60 °C. In contrast to the ion-blocking cells above reported, these impedance spectra consist of one semicircle at high frequencies, followed by a Warburg response at low frequencies. The high-frequency semicircle represents the total resistance to the charge (R1), which means electronic and ionic motion. The Warburg response is indicative of the diffusion of ions polarization owing to block electrons (W), and CPE represents the constant phase of the electrical double-layer capacitance; the impedance spectra were fitted with the equivalent circuit as shown in Fig. 7d. The lithium diffusion coefficient D can be calculated using the following equation [34]: where T is the room absolute temperature, R is the gas constant, A is the surface area of the electrode, F is the Faraday constant, n is the number of electrons per molecule attending the electronic transfer reaction, and C is the concentration of lithium ion in the LiMnPO 4 electrode. The plots of Z re against ω 1/2 are in Fig. 8, according to Eq. (2), and σ is the slope of the straight line:   Table 2, using the above-obtained σ with the plots in Fig. 8. Previous works reported the diffusion coefficient of LiMnPO 4 materials. Xiao et al. [35] and Han et al. [36] reported lithium diffusion coefficients of 5 × 10 −14 cm 2 s −1 and 8 × 10 −14 cm 2 s −1 , respectively, for solvothermal prepared LiMnPO 4 materials. Pan et al. [37] D Li+ of 2 × 10 −14 cm 2 s −1 obtained in solid-state approach. Wang et al. [38] investigated the effect of the high-energy ball milling time on the lithium diffusion properties in LiMnPO 4 synthesized by a soft template method. They showed that milling time affects the electrochemically evaluated lithium diffusion coefficient, reporting D Li+ within 10 −16 -10 −13 cm 2 s −1 range.
The results herein reported agree with current literature works suggesting the hindered kinetics of lithium insertion/ deinsertion for LiMnPO 4 . This severe issue represents the main problem limiting the effective employment of Mnbased olivines as cathodes in lithium batteries [39,40].

Conclusions
Electronic and lithium ionic transport in LiMnPO 4 , LiMnPO 4 /C, and LiMnPO 4 /MWCNT have been measured using ion and electron blocking cell configurations. SEM and TEM revealed the well and uniformly decorated with MWCNT. The LiMnPO 4 pristine presented a higher charge transfer resistance, and the electronic conductivity and diffusivity of LiMnPO 4 /MWCNT composite in the function of the temperature applied are higher than that of pure LiMnPO 4 and LiMnPO 4 /C. The added MWCNTs not only increase the electronic conductivity and the lithiumion diffusion coefficient but also decrease crystallite size and the charge transfer resistance. A comparison of these values with literature reports suggested that the lithium diffusion and conductivity evaluation is strongly affected by the structural and morphological characteristics and the adopted experimental setup used for evaluation. This evaluation could give a good sight previous to the use the electrolyte in future works.