Effect of Mg Substitution in the Structural and Electrochemical Properties of Li2MnO3
Authors: Torres-Castro Loraine , Katiyar Ram S. , Manivannan A. 
Published: February 2020

The revolutionary concept of energy storage is constantly changing due to the increasing demand of high energy density devices. However, issues such as safety, high temperature performance, cost, and environmental concerns need to be addressed, especially for the 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[Li1/3Mn2/3]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 possible reduction of the valance state of manganese and electrochemically activating the material. In the present study, we investigated the electrochemical and structural properties of Li2MnO3at room temperature by partially substituting lithium with magnesium using the Pechini synthesis method.

Li2MnO3, Li1.5Mg0.25MnO3, Li1.0Mg0.50MnO3 and Li0.5Mg0.75MnO3 compositions were prepared by the Pechini synthesis method. These substitutions were chosen balancing the charge of the cations. For the individual compounds the synthesis was carried out using lithium nitrate [LiNO3], manganese nitrate [Mn(NO3)2], magnesium nitrate [Mg(NO3)2•6H2O], ethylene glycol [C2H6O2] and citric acid [H3C6H5O7] as precursors materials.The molar ratio of M:CA:EG used was 1:1.2:1.2, where M is the sum of all metal ions. Stoichiometric ratios of the metals corresponding to each composition were measured and dissolved in deionized water (H2O) along with citric acid (CA), which behaves as a chelating reagent. The resulting compound was placed in an oven at 130oC for 24 h to complete the drying process, followed by annealing of the powder at 750oC for 8h in air to obtain desire phase. The final heat treated powder samples were mixed with polyvinylidenefloride (PVDF) and carbon black in the wt % ratio of 80:1010, 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 patterns for Li2MnO3, Li1.5Mg0.25MnO3, Li1.0Mg0.50MnO3 and Li0.5Mg0.75MnO3 synthesized at 7500C for 8h. The formation of single phase material was confirmed for Li2MnO3 and a combination of monoclinic/spinel phases (Li2-xMn1-yMgx+yO3+z) was observed for the Mg-substituted compounds.

X-ray photoelectron spectroscopy (XPS) was used to study the valance state of the manganese ion after magnesium substitution. Figure 2 shows XPS pattern for Mn 2p of pristine and partially Mg-substituted samples. A shift to a lower binding energy is observed for Li1.0Mg0.50MnO3 and Li0.5Mg0.75MnO3, indicating a decrease in the tetravalent state of manganese. Therefore, an improvement in electrochemical cycling performance is expected.

Preliminary results shown in Figure 3 indicate that partial Mg-substitution enhances the cycleability of Li2MnO3 and increase the capacity to ~90mAh/g. All the electrochemical performance of Li2MnO3, Li1.5Mg0.25MnO3, Li1.0Mg0.50MnO3 and Li0.5Mg0.75MnO3 cathodes will be presented in detail.

Figure 1