Structural and Electrochemical Studies of Rhodium Substituted Li2MnO3
Li2MnO3, Li2RhO3 and Li2Mn0.5Rh0.5O3 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 100oC h-1 to a final temperature of 600oC for Li2CO3/MnO2, 850oC for Li2CO3/RhCl3 and 900oC 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 600o C -8h, Li2Mn0.5Rh0.5O3 900o C -24h and Li2RhO3 850o 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 2p1/2 and Mn 2p3/2 peaks for Li2MnO3 are 654.25eV and 642.50eV, respectively, being in agreement with those reported in literature . A slightly shift to a lower binding energy is observed for Li2Mn0.5Rh0.5O3, 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 Li2Mn0.5Rh0.5O3 cathodes will be presented in detail.