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.

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.

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).

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.

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.

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.

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.