Characterization and electrochemical performance of Manganese oxide with carbon nano tubes as negative electrode for lithium batteries

Different ratios of Mn3O4/CNT (carbon nano tubes) samples were prepared by ultrasonic and hydrothermal reactions. The added ratios of CNT: Mn wt./wt. were 0, 5, 15, 25, 35 and 50%. The phases of the samples were identified by X-ray diffraction (XRD). The particle morphology was studied with a field emission scanning electron microscope (FESEM). The lowest charge transfer resistance, Rct was 19.18 Ω with cell of Mn3O4/35% CNT. For Li / Mn3O4-35% CNT cell, the specific discharge capacity was about 688 mAhg−1 after 100 cycles, which was the highest value among the other cells at discharge current density 0.02Acm−2.


Introduction
Small and Portable electronic devices such as laptops, mobile telephones, camcorders, etc. use Lithium-ion battery (LIB). They are considered the essential electric power component for these devices due to their excellent properties like long durability, high energy density, easily handling and safe for the environment. A variety of metals and metal oxides are intensively studied in order to obtain high-capacity anode materials for LIBs [1][2][3][4].
It was reported that Mn 3 O 4 nano-particles anchored on continuous super-aligned carbon nanotube (SACNT) films were obtained by decomposition of Mn(NO 3 ) 2 [9]. The electrochemical performance of the Mn 3 O 4 /SACNT composite electrode is greatly affected by the grain size of Mn 3 O 4 . For smaller particle size, the electrode displayed smaller voltage hysteresis and much improved reversibility and rate capability. Mn 3 O 4 with particle size of less than 10 nm inside the Mn 3 O 4 /SACNT electrodes showed a capacity of 342 mAh g -1 at 10 C. This composite material has more advantages than the commercial graphite anode and promising in practical applications. Carbon nanofiber/Mn3O4 (CNF/Mn 3 O 4 ) coaxial nano-fibers with a three-dimensional (3D) structure were prepared for lithium ion batteries by electrophoretic deposition on an electrospun CNF cathode followed by heat treatment in air [11]. The CNF/Mn 3 O 4 coaxial nanocables showed a high reversible capacity of 760 mAh g -1 at 100 mA g -1 even after 50 charge discharge cycles without capacity fading.
A facile and green chemistry route was developed to prepare a Mn 3 O 4 nanoparticles (NPs)/ exfoliated graphite (EG) composite by artfully tuning the traditional fabrication process of EG [12]. During this treatment, Mn 3 O 4 -NPs with high crystallinity and uniform dimension of 7 nm were found to be homogeneously and firmly anchored on the surface of EG. The composite as an anode material of Li-ion batteries exhibited suitable specific discharge capacity of 655 mAhg -1 extending to more than 100 cycles.The aim of this work is to study the effect of different ratios of CNT on the performance of Mn -oxide in lithium cell as there is a need to study effect of the CNT ratios according to the reported literature.

Materials preparation and characterization
Different ratios of carbon nano tubes (CNT), (99%, Ad-Nano Technologies Private Limited, India) 0, 5, 15, 25, 35 and 50% were added to stoichiometric amounts of (CH 3 CO 2 ) 2 Mn.4H 2 O (Aldrich, 99.5%) dissolved in distilled water to prepare the samples. The ratios of CNT were calculated as Mn: C. The suspended CNT of each mixture solution was stirred and heat-treated at 80C for 2 hours. Furthermore, the mixture was exposed toultrasonic vibration for one hour. After that, the mixture was transferred to an autoclave vessel (250 cm 3 ) and heat-treated at 200°C for 24 hours.
The crystalline phases of the samples were identified by X-ray diffraction (XRD) using a step size of 0.02° and at a scanning rate of 0.16°/min on a Brucker axis D8 diffractometer with crystallographic data software Topas 2 using Cu Kα (λ = 1.54056 Å) radiation operating at accelerating voltage and applied current were 40 kV and 80 mA, respectively. The diffraction data was recorded for 2θ values between 10° and 80°. The particle morphology was studied with a field emission scanning electron microscope (FESEM QUANTAFEG 250). The magnetic properties of some of the samples powders were measured at room temperature using a vibrating sample magnetometer (VSM, 7410 Lakeshore, USA) in a maximum applied field of 20 kOe.

Electrochemical measurements
The homogeneous slurry used to form the electrodes was composed of 85 wt% active materials, 10 wt% acetylene black, and 5 wt% sodium carboxy methyl cellulose (SCMC) as binder dissolved in distilled water. It was then spread onto Cu foil substrates. The area of each coated electrode was 1 cm 2 . The electrodes were dried in a vacuum oven at 110°C for 12 h. The electrodes were then pressed at a pressure of 2000 kg/cm2. The active material loading was about 5 mg for each individual electrode. CR2032 coin cells were then assembled in an argon filled glove box (Mbraun, Unilab, Germany), using lithium metal foil as the counter electrode. The electrolyte was 1M LiPF 6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (1:1 by volume, provided by MERCK). The cells were galvanostatically charged and discharged over a voltage range of 0.0-3 V using a current of 0.02 A/ cm 2 for both processes. Cyclic voltammetry (CV) measurements were performed using a Multi-stat CHI660 Electrochemical Workstation at a 0.1 mVs -1 scanning rate, and the potential windows were 0 and 3 V versus Li/Li+ electrode. The Galvanostatic charge-discharge measurements were performed on a Neware Battery Tester. The frequency range of the AC impedance measurement was 1MHz -10 mHz and10 mV amplitude.

Structural Characterization
X-ray diffraction patterns of the manganese oxide and manganese oxide with CNT samples show suitable crystallinity as shown in Figure 1. Samples diffraction peaks exhibited good crystalline structures. Their structures were indexed to the tetragonal system with space group of I41/amd (JCPDS No. 24-0734). The peak at 2θ =26.4° can be achieved to (002)  (1) where θ and w are the Bragg angle and the full width at half maximum peak, measured in radians, of each diffraction peak, respectively. Also, λ is the X-ray wavelength (1.54056Å), and L is the effective particle or grain size.

Electrochemical Impedance Spectroscopy and magnetic properties measurements
Electrochemical impedance spectroscopy (EIS) may be considered as one of the most sensitive tools for the study of differences in electrode behavior due to surface modification. The electrochemical impedance spectra of the cells, as presented in Figure 3, show an intercept at high frequency on the real axis Z′ for the resistance of the electrolyte, R e , followed by a semicircle in the high-middle frequency region, and a straight line in the low frequency region. The numerical value of the diameter of the semicircle on the Z real axis is approximately equal to the charge transfer resistance, R ct , and therefore, it can be seen that there is a marked decrease in Rct with the addition of CNT until 35% (R ct =127 ), but 50% CNT increases R ct , 928 . So, the increase of CNT percentage decreases the R ct until 35% CNT and after that the charge resistance increases again with the increase of CNT. i.e. 35% CNT is the optimum condition of these experiments.  The straight line in the low frequency region is attributed to the diffusion of the lithium ions into the bulk of the electrode material, or the so-called Warburg diffusion. The plot of the real part of the impedance, Z re, versus the reciprocal root square of the lower angular frequencies is displayed in Figure 4.    The straight lines are attributed to the diffusion of the lithium ions into the bulk of the electrode material, or the so-called Warburg diffusion. This relation is governed by equation 2. It is observed that the Warburg impedance coefficient,  w is 339 .s 0.5 for cell composed from Mn 2 O 3 with 35% CNT cell, and it is the lowest value among the other cells. The parameters of the electrochemical impedance spectroscopy are presented in Table 1. Also, the diffusion coefficient values of the lithium ions for diffusion into the bulk electrode materials have been calculated using Equation (3) and are recorded in Table 1.
Z re = R e + R ct + 2 w 2 . C dl (4) The double layer capacitance is given by equation (5).  = 1 / R ct .C dl (5) where: R ct : charge transfer resistance, R e : electrolyte resistance, : angular frequency in low frequency region, D: diffusion coefficient, R: the gas constant, T: the absolute temperature, F: Faraday's constant, A: the area of electrode surface, and C: molar concentration of Li + ions [2,15]. The obtained diffusion coefficient, 3.09x10 -13 cm 2 s -1 for Mn 3 O 4 with 35% CNT cell explains the higher mobility for Li + ion diffusion than in the other cell. Furthermore, the exchange current density is given by the following formula:i° = RT/nFR ct , where n is the number of electron involved in the electrochemical reaction. It is observed that C dl for cell " Mn 3 O 4 with 35% CNT " has the highest value: 1.66 x 10 -8 F.  Figure 5. It is observed that the magnetization of Mn3O4 with 35% CNT sample is higher than Mn 3 O 4 one. The magnetic coercivities (Hc) are 322.98 and 365.39 G for samples without CNT and with 35% CNT, respectively as shown in Figure 5. This means that the presence of 35% C increases the H c . i.e. Mn 3 O 4 with 35% CNT is more ferromagnetic than Mn 3 O 4 itself.

Potentiodynamic and galvanostatic measurements
Cyclic voltammetric (CV) measurements were carried out between 0 and 3 V as shown in Figure 6. The cyclic voltammogram of the investigated Mn 3 O 4 sample shows two cathodic reduction peaks at 0.9 (broad) and 0.3 V vs. Li+. The first peak for formation of a solid electrolyte interphase (SEI) film owing to the decomposition of electrolyte on the Mn 3 O 4 particle surface and reduction of Mn 3+ to Mn 2+ [8], while the second one (0. 3V) for Mn 2+ to Mn(0) and the formation of amorphous Li 2 O based on the electrochemical conversion reaction as follows: On the other hand, the anodic oxidation peaks take place at 1.3 and 2.6V vs. Li+. These peaks are attributed to the oxidation of Mn 0 to Mn 2+ and the other is assigned to the re-oxidization of Mn 2+ to higher oxidation state manganese [13]. It is observed that the addition of the CNT with 25 or 35% increases the current intensities of the peaks. The first discharge/charge capacity plateaus versus the working voltage between 3 and 0.01V vs. Li + are shown in Figure 7. The first discharge capacities of the cells are nearly about 1050 mAhg -1 . The profiles for the first discharge curves look fairly similar for the investigated cells. There is a small transient and slope plateau between 1.5 and 0.25V vs. Li + for the formation of solid electrolyte interface (SEI) film and the reduction of Mn 3 O 4 to Mn 2+ , respectively. A plateau at 0.25 V is observed that corresponded to further reduction of Mn 2+ to Mn(0) [7][8][9]16]. Figure 8 shows the curves of the specific discharge capacity vs. the cycle number of the different cells samples in the discharge voltage range of 3 and 0.01 V. For Li/ Mn 3 O 4 -35% CNT cell, the capacity can be retained to about 90% of the discharge capacity after 100 cycles with respect to 2 nd cycle and the specific discharge capacity is about 688 mAhg -1 , which is the highest value among the other cells. The initial capacity loss is attributed to the surface film formation, which is known as solid electrolyte interface (SEI) film. This is due the formation of irreversible reaction as: 8Li + + 8e − + Mn 3 [17,18]. It is observed from CV measurements ( Figure 6) that the 2nd and the followed cycles of Mn 3 O 4 have no cathodic reduction peaks at 1V butMn 3 O 4 with 25 and 35% CNT have this type peak. This means that quasi-reversible of Mn 3 O 4 /CNT composite is formed as reported with similar Mn 3 O 4 /Graphite composite [17]. It can be explained that the reaction is an intercalation reaction when the discharge voltage was limited at 1.5V, the frame structure of material almost was not destroyed, and however, the reaction was a probable redox reaction when the discharge voltage went to lower potential (i.e. 1.5-0.01 V), then the structure of the materials is entirely destroyed at very low potential and cannot be recovered. It can be expected that the charge/discharge voltage is the key factor to affect the cyclic performance.

Conclusion
Different ratios of Mn 3 O 4 /CNT (carbon nano tubes) samples were prepared by hydrothermal reaction. Their crystal structures of the samples are tetragonal system. The cell of Mn 3 O 4 /35% CNT has lowest charge transfer resistance, R ct , 19.18 . The magnetic coercivities (H c ) are 322.98 and 365.39 G for samples without CNT and with 35% CNT, respectively. For Li/ Mn 3 O 4 -35% CNT cell, the capacity can be retained to about 90% of the discharge capacity after 100 cycles and the specific discharge capacity is about 688 mAhg -1 , which is the highest among the other cells at discharge current density 0.02Acm -2 .