1. INTRODUCTION
Spinel LiMn2O4 has become one of the most expected cathode materials for lithium ion battery due to its high power density, low cost, environmental friendliness, and high abundance [1 - 3]. The fatal shortcoming of LiMn2O4 can be seen in previous literatures [4 , 5].
Most surface modification of LiMn2O4 just considered enhancing the cycle performance by restraining Jahn-Teller distortion of Mn3+, reducing the dissolution of Mn2+ and decreasing the electrolyte solution to decompose on the electrode [6 , 7]. D. Arumugam et al. [8] coated LiMn2O4 cathode materials with various wt.% SiO2 by a polymeric process, the SiO2 surface coating on LiMn2O4 controlled the formation of a passive layer film during electrochemical cycling; Dongqiang Liu et al. [5] used AlPO4-coated LiMn2O4 to increase cycling stability.
Electronic conduction was always considered as the crucial factor on cathode materials, Sanghan Lee et al. [9] got LiMn2O4 micrometer-sized particles that consist of aggregated nanoparticles, but exhibit a large electric resistance, so they coated spinel LiMn2O4 nanoclusters with a thin carbon layer using sucrose as the carbon source. But Byoungwoo Kang and Gerbrand Ceder [10] gave us a new concept: creating a fast ion-conducting surface phase on the surface of LiFePO4 to improve ionic conductivity, the rate capability enhanced significantly. Lu-Lu Zhang et al. [11] prepared SiO2-modified Li3V2(PO4)3/C composites significantly improved electrochemical performance of materials. SiO2 modification significant improved materials’ structural stability, Li-ion conductivity, and capacity retention. The ionic conduction of Li2SiO3 had been researched by Hirotsohi Yamada, Shin-ichi Furusawa [12 , 13]. In our work, we consider using coating layer to restrain Jahn-Teller distortion without performance reducing. The sucrose as source of carbon and metasilicic acid lithium (Li2SiO4) were used as electronic conductive agent and ionic conductive agent respectively. The synergistic effect of them coated on micrometer-sized spherical spinel LiMn2O4 consisted of aggregated nanoparticles was investigated.
2. EXPERIMENTAL
2.1. Spherical LiMn2O4 Preparation
Spherical MnCO3 was synthesized by liquid phase precipitation method as the precursor of spherical spinel LiMn2O4. MnSO4∙H2O (A. R. 99%) and NH4HCO3 (A. R. 99%) were dissolved in distilled water to get 0.3 mol/L and 0.1 mol/L solution respectively, Nh4HCO3 solution were dropped into MnSO4 solution slowly and kept stirring at 30˚C with sodium dodecyl sulfate (SDS) as grain size control agent. Spherical MnCO3 then mixed with Li2CO3 by Stoichiometric ratio 4:1.03, and the mixture was calcined to produce LiMn2O4 by heat-treatment as follow [14]: 1) heating from room temperature to 560˚C by heating rate 5˚C/min and holding for 4 h; 2) heating from 560˚C to 750˚C by rate 5˚C/min and holding for 10 h; 3) cooling to room temperature spontaneously.
2.2. Coated LiMn2O4 Preparation
1 w% Li2SiO3 dissolved in 10 mL distilled water, 5 g bare LiMn2O4 added into and ultrasonic dispersed 30 min, then magnetic stirred until dry, and subsequently annealed at 600˚C under air for 2 h in a furnace, Li2SiO3 coated LiMn2O4 (LMO/LSO) obtained. Herein, we referenced a very special characteristic of Li2SiO3: After dried and formed a thin film, cannot dissolved in water again. LMO/LSO (2 g) was dispersed in water and ethanol (1:3 v/v; 12 mL), and then sucrose solution (0.1 g sucrose/10 mL distilled water) was added. The mixed solution was dispersed by ultrasonication for 10 min and then concentrated to dryness, fired at 600˚C for 10 min and cooled quickly to room temperature [10], LMO/LSO/C obtained. LMO/C/LSO was prepared by similar methods, just exchanged coating order. But the step of Li2SiO3 coating was in tube furnace full of nitrogen and kept 600˚C for 2 h, in order to avoid carbon oxy-genated.
2.3. Samples Detection
Phase analysis of prepared samples was carried out by X-ray diffraction (XRD, Rigaku D/max 2500 PC). The morphology of bare LiMn2O4 was observed by scanning electron microscope (SEM, JSM-6360LA) and high resolution projection electron microscopy (HRTEM). Images of coated samples were obtained by a JEOL-JEM 2100 Electron Microscope equipped with an X-EDS analysis system.
For the fabrication of the cathode, a slurry with 80 wt.% synthesized materials, 10 wt.% acetylene black and 10 wt.% Polyvinylidene fluoride (PVDF) was prepared and rolled onto aluminum foil, then dried at 40˚C for 4 h, moved into vacuum drying oven and dried at 80˚C under vacuum overnight. The size of cathode pole piece was 0.5 × 0.5 cm2. The experimental model batteries were assembled in an argon-filled glove box using Lithium foil as counter electrode and the electrolyte ethylene carbonate (EC): dimethylcarbonate (DMC) 1:1-1 M LiPF6. The cycling tests and ratio tests were performed using Neware Battery Testing System, the cyclic voltammogram were tested by Electrochemical workstation (Chenhua Chi 660D).
3. RESULTS AND DISCUSSION
3.1. XRD Analysis
Figure 1 shows that the carbon and Li2SiO3 coating did not damage the crystal structure of LiMn2O4. All the diffraction peaks are indexed to a cubic spinel structure with a space group Fd3m. However, impurities founded in LMO/C/LSO, this may because a small part of LiMn2O4 had been deoxidated by carbon, Mn2O3 and Mn3O4 generated.
3.2. Morphology Characterization
Spherical particles could increase the tap density of material. Figure 2(a) shows the SEM image of prepared spherical LiMn2O4, the particles dispersed uniformly and the partical size about 1 μm. From Figure 2(b), we could see the particle is compactly made up of a large number of sheet crystalline grains of spinel LiMn2O4.
Figure 2(b) and Figure 2(c) show the TEM images of LMO/C/LSO and LMO/LSO/C. We chose two areas analyzed with X-EDS in each image, the results shown in Figure 3 and confirmed that we have coated two layers.
3.3. Electrochemical Investigation
Figure 4(1) summarizes cycling performances of bare LiMn2O4, LMO/C/LSO and LMO/LSO/C as cathode cycled at a current density of 22 mAh∙g−1 (0.2C, 1C = 110 mAh∙g−1) in a potential range of 3 - 4.3 V (vs. Li/Li+). The initial discharged capacity was 105.987 mAh∙g−1, 100.733 mAh∙g−1, and LMO/LSO/C is 107.321 mAh∙g−1 respectively. LMO/C/LSO delivers the lowest initial discharged capacity and the worst cycling capacity could be expected by XRD analysis, In the LMO/C/LSO phase a part of Mn4+ was reduction into Mn3+ by carbon, which lead to a much more obvious Jahn-Teller distortion and the decrease of active materials. The excellent performance of LMO/LSO/C is attributed to the electron conduction ability and the inhibition of the reduction of Mn4+ to Mn3+ provided by the Li2SiO4 coating layer. At the same time, the carbon coating layer improves the electron conduction ability of LMO/LSO. Part (2) of Figure 4 shows the rate capabilities of three samples. It is obvious that LMO/LSO/C exhibits better rate capability than the other two samples. At a 1C rate, LMO/LSO/C gives a discharge capacity of 80 mAh∙g−1, which is higher than those of LMO/C/LSO (about 75 mAh∙g−1) and bare LiMn2O4 (about 69 mAh∙g−1). It should be noted that when the current rate is decreased from 5C to 1C rate, the discharge capacity of LMO/LSO/C can be recovered to 105 mAh∙g−1, indicating a good reversibility upon cycling. Herein, LMO/C/LSO shows better rate capability than bare LiMn2O4, this may because the protection of Li2SiO3 dominant.
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Figure 1. XRD patterns of prepared samples.
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Figure 2. SEM images of (a) spherical spinel LiMn2O4; TEM images of (b) LMO/C/LSO and (c) LMO/LSO/C.
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Figure 3. Two areas of images of Figure 2(b) LMO/C/LSO and Figure 2(c) LMO/LSO/C analyzed by X-EDS respectively.
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Figure 4. (1) Cycling performances of (a) bare LiMn2O4 cathode, (b) LMO/C/LSO cathodes and (c) LMO/LSO/C cathodes. (2) Discharge capacity with cycling number at different current rates of LMO, LOM/C/LSO, LMO/LSO/C.
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Figure 5. (1) Initial cyclic voltammogram curves for LMO, LMO/C/LSO, LMO/LSO/C at the scan rate was 0.1 mV/s and (2) initial discharged capacity of three materials.
Figure 5(1) show all of samples have two reversible redox peaks based on Mn4+/Mn3+. It is obvious that the samples which coated have more sharp peaks and even more symmetrical，at least indicate that the coated samples still keep spinel structure very well, and LMO/LSO/C shows the most stable structure. Figure 5(2) gives us informations of discharged voltage plats and initial discharged capacity. The two discharged voltage plats corresponding to the two reduction peaks of cyclic voltammogram curve.
4. CONCLUSIONS
1) Spherical spinel LiMn2O4 was synthesized by solid-state reaction combined with liquid phase precipitation.
2) Li2SiO3 and carbon were both coated on the surface of LiMn2O4 by different orders and obtained LMO/LSO/C and LMO/C/LSO.
3) Compared with LMO/C/LSO and bare LiMn2O4, LMO/LSO/C exhibited the best capacity retention and rate performance due to the inside Li2SiO4 provided a certain degree of ionic conductivity and inhibited the direct contact between LiMn2O4 and electrolyte. At the mean time, the outside carbon insured the electronic conductivity.