Synthesis And Characterizations Of A 3V Cathode Materials For Lithium Secondary Batteries: Li_0.33MnO₂ And Li₂MSiO₄

Finite fossil-fuel supplies, nuclear waste and global warming linked to CO2 emissions have made the development of alternative/"green"methods of energy production, conversion and storage popular topics in today's energy-conscious society. These crucial environmental issues, together with the rapid advance and eagerness from the electric automotive industry (for example, electric vehicles and hybrid electric vehicles) have combined to make the development of radically improved energy storage systems a worldwide imperative.Most of the commercial lithium ion batteries use LiCoO2 as the cathode materials. But, the high cost and toxic properties, along with the safety problems hamper the applications of LiCoO2 in large-scale lithium ion batteries. In recent years, great efforts have been devoted to develop alternative cathode materials to replace LiCoO2. Manganese and iron based oxides are among the most attractive candidates because of their economical and environmental advantages. In this paper, We carried out a series of research on preparation and characterization of two types of materials Li_0.33MnO₂ and Li₂MSiO₄, made some valuable results.1. for Li_0.33MnO₂:We determined the optimum temperature by the TG-DTA curve of the raw materials. The material was prepared by conventional solid-sate reaction. MnO2 (EMD, Aldrich) was pretreated at 250 oC for 5 h to remove the adsorbed and chemically bonded water, and then ball milled together to ensure their homogeneous mixing. This resultant mixture was heat treated at 360 oC for 20 h at the ambient condition to obtain the Li_0.33MnO₂ powder. The X-ray diffraction pattern of the Li_0.33MnO₂ sample shows this material have a monoclinic structure with the space group of C/2m. The mean coherent domain size of the material was about 10nm estimated by the Scherer formula whereβis the full width half maximum (FWHM) of the strongest (211) reflection and the constant K = 0.9.The crystal structure of the material was refined. The lattice parameters were calculated to be a = 13.798 (5) ?; b = 2.839 (6) ?; c = 4.925 (2) ? andβ= 88.3°. From the sketch of the crystal structure of Li_0.33MnO₂ constructed by the Rietveld refinement parameters, we could see that the crystal structure of Li_0.33MnO₂ is composed of an ordered arrangement of [1×2] and [1×1] tunnels. The Li ions were believed to reside in the [1×2] holes since they are much larger than the [1×1] ones.The SEM image of the Li_0.33MnO₂ sample shows that the material was composed of irregular shaped and sized particles. The geometric particle size ranged randomly from several tens to hundreds nanometers. The BET surface area was measured to be 11.9 m2/g, which is beneficial to the electrochemical properties of materials. The HRTEM image clearly shows that the material was polycrystalline. The mean coherent domain size determined from HRTEM was about 10 nm, which is consistent with that estimated by X-ray diffraction. The selected-area electron diffraction pattern (SAED) indicates that the domain particles were well crystallized. The lattice fringe of 4.05 ? corresponds to the (200) plane, which is consistent with the X-ray diffraction result.The cyclic voltamograms of Li_0.33MnO₂ between 3.5 and 2.0V in the initial seven cycles shows that the oxidation peak shifted towards a higher potential after five cycles. This was attributed to the formation of a thin Li0.5MnO2 (LiMn2O4) spinel layer on the Li_0.33MnO₂ particle surface taking place during Li+ insertion to x=0.5. This spinel layer might be very thin so that it didn't affect the reduction potential during the whole measurement.The cyclic voltamograms of Li_0.33MnO₂ in the initial seven cycles between 4.3 and 2.0 V shows that a new redox couple appeared at 3.28/3.88V, which was not observed when cycled between 3.5 and 2.0 V. This is attributed to Li+ insertion/extraction in LixMnO2 with x ranging between 0 and 0.33. There is only one oxidation peak. The reduction peak at 2.84 V is known as the reduction peak of spinel LiMn2O4. This indicates that a thicker spinel layer was formed in the potential range of 4.3-2.0 V than in 3.5-2.0 V.Galvanostatic charge-discharge cycling in the potential region of 3.4-2.0V shows the material has an initial discharge capacity of 168 mAh·g-1, which is corresponding to 0.59 mol of Li+ insertion into Li_0.33MnO₂. A relatively fast capacity fading was observed in the initial several cycles, perhaps due to the formation of Li0.5MnO2 spinel layer. Then the material showed good capacity retention and recorded a reversible discharge capacity of 140 mAh·g-1 over 30 cycles. The material showed a discharge plateau at 2.9 V and a corresponding charge plateau at 3.1 V and the shape of the potential profiles didn't change much upon cycling, highlighting the good cyclability of the material.The cycling performance of Li_0.33MnO₂ between 4.3 and 2.0 V showed an initial discharge capacity of 198 mAh·g-1. This is much larger than that cycled in 3.4-2.0 V because of the contribution from 00.33MnO₂. This is different from those of LiMnO2 and Li0.45MnO2. In these cases, the phase transformation took place in the bulk phase. It is suggested that if the formation of the spinel layer was effectively depressed by some techniques such as surface coating, Li_0.33MnO₂ could be as a good candidate for 3 V cathode materials for lithium ion batteries.2. For Li₂MSiO₄:The material was prepared by the ultrasonic-assisted sol-gel process. Stoichiometric amounts of LiCH3COO·2H2O, FeC2O4·2H2O, and tetraethyl orthosilicate (TEOS) were mixed in ethanol, and dispersed in a Ultrasonic Cell Crusher, the mixture were stirred and then the ethanol was evaporated. The resulting powder was pressed into pellets. The pellets were heated in a horizontal quartz tube oven and calcinated at 650°C for 10 h with nitrogen.The X-ray diffraction pattern shows this material have an orthorhombic structure with the space group of Pmn21, there is no other phases in both two materials. The refined lattice parameters showed that the admixture of Mn increased a and c, this may due to the larger ionic radius of Mn2+ ion than Fe2+ ion, and there's no increasement of b, it may due to the MO4 tetrahedron is on the XOZ layers.The FT-IR result showed Li2CO3 was formed by the surface adsorption of CO2; the peak at 900cm-1 and 550 cm-1 of the SiO4 tetrahedron split to two peaks because of the impact of Fe2+ and Mn2+, and the peak of Fe-O and Mn-O are out of FT-IR range, so the figure two curves are basically the same.Galvanostatic charge-discharge cycling shows both two materials have an obviously different plateau near 3.0V from the 1st cycle, indicating that the material took a phase transition after the first cycle, which is consistent with Nyten's report. As a result of the polarization of materials from low conductivity material, the charge-discharge plateau drops down, the capacity reduced, and with the number of the cycle capacity increased. For Li2Fe0.9Mn0.1SiO4, a new plateau appeared at 2nd to 5th cycle, and remained after the 5th cycle, this may due to the redox couple of Mn3+-Mn4+, as the oxidation peak at 3.88V of LiMnO2, and there's a fast capacity fading in the 1st cycle, like most of the manganese-included materials.Because of raw material-rich, low price, harmlessness to environment, Li₂MSiO₄ could be a new generation cathode material for lithium ion batteries with potential application if the electrical conductivity was increased by some techniques such as surface coating.