Structural and Electrochemical Studies of Rhodium Substituted Li2MnO3

          Commercial realization of rechargeable Li-ion batteries has revolutionized portable electronic industry, especially microelectronics in recent years. However, issues such as safety, high temperature performance, cost, and environmental concerns need to be addressed, especially for high power applications. Among the cathode materials investigated, lithium manganese oxides have been exploited due to the abundance of Mn in earth, low toxicity and safety. In this family of oxides, Li₂MnO₃ possess monoclinic structure having c2/m space group symmetry with a layered arrangement of Li[Li_1/3Mn_2/3]O₂, thus resembling the ideal layered structure of LiCoO₂. Li₂MnO₃ is known to be electrochemically inactive due to the presence of Mn in tetravalent oxidation state. Substitutions of ions in the structure allow the reduction of the valance state of manganese, electrochemically activating the material. In the present study, we investigated the electrochemical and structural properties of Li₂MnO₃at room temperature by partially substituting Mn site with rhodium via the solid state synthesis method.

          Li₂MnO₃, Li₂RhO₃ and Li₂Mn_0.5Rh_0.5O₃ compositions were prepared by a solid state synthesis method. For the individual compounds the synthesis was carried out using lithium carbonate [Li₂CO₃], manganese oxide [MnO₂] and rhodium chloride [RhCl₃] as precursor materials. Stoichiometric ratios of the metals corresponding to each composition were measured and carefully ground using a pestle and mortar for 1 hour. The resulting mixture was calcined in air at a ramp rate of 100^oC h^-1 to a final temperature of 600^oC for Li₂CO₃/MnO₂, 850^oC for Li₂CO₃/RhCl₃ and 900^oC for Li₂CO₃/MnO₂/RhCl₃. The final heat treated powder samples were mixed with polyvinylidenefloride (PVDF) and carbon black in the wt % ratio of 80:10:10, respectively. A slurry paste was prepared using 1-methyl 2-pyrolidone as solvent and was coated on Al foil substrates to form a homogeneous layer. Electrodes were punched out for the fabrication of 2032 coin cells. Metallic Li was used as the counter electrode along with 1.2M LiPF₆in EC:DMC (1:1) as electrolyte. The charge and discharge analysis was performed at 10mAh/g within a potential window of 2.0V- 4.8V.

          The structural phase formation of the materials and presence of any crystalline impurities in the compounds were studied with X-ray diffraction (XRD). Figure 1 shows XRD spectra for Li₂MnO₃ 600^o C -8h, Li₂Mn_0.5Rh_0.5O₃ 900^o C -24h and Li₂RhO₃ 850^o C-24h. X-ray photoelectron spectroscopy (XPS) was obtained to study the valance state of the manganese ion after rhodium substitution. Figure 2 shows XPS pattern for Mn 2p of pristine and partially Rh-substituted samples. The Mn 2p_1/2 and Mn 2p_3/2 peaks for Li₂MnO₃ are 654.25eV and 642.50eV, respectively, being in agreement with those reported in literature [3]. A slightly shift to a lower binding energy is observed for Li₂Mn_0.5Rh_0.5O₃, indicating a decrease in the tetravalent state of manganese. Therefore, an improvement in electrochemical performance is expected.

          Preliminary results shown in Figure 3 indicate that partial Rh-substitution enhances the cycleability of Li₂MnO₃ and increases the capacity by 64mAh/g. Electrochemical performance of Li₂MnO₃, Li₂RhO₃ and Li₂Mn_0.5Rh_0.5O₃ cathodes will be presented in detail.