Effect of Al and Mg Doping on the Electrochemical Performance of Pure Li2MnO3

          In the last three decades, lithium manganese oxides have been exploited as potential cathode materials for lithium ion batteries. In this family of oxides, LiMnO₂ possess very high discharge capacity while LiMn₂O₄ possess high operating voltage. These materials suffer from serious structural change after certain number of cycles due to the presence of Mn³⁺ ion, which is known to be a Jahn-Teller active ion. Side reactions such as 2Mn³⁺ → Mn⁴⁺ + Mn²⁺, also lead to the battery failure upon cycling, as Mn²⁺ is known to be soluble in the non-aqueous electrolytes. Li₂MnO₃ belongs to the same family of oxide materials but is more stable due to the presence of Mn⁴⁺ instead of Mn³⁺ as in LiMnO₂ or LiMn₂O_4. Li₂MnO₃ possess monoclinic structure having c2/m space group symmetry with a layered arrangement of Li[Li_1/3Mn_2/3]O₂.

          Li₂MnO₃ is known to be electrochemically inactive because of the presence of Mn in +4 oxidation state which cannot be oxidized further. However, it can be made electrochemically active by extracting lithium and oxygen simultaneously from the structure by chemical or electrochemical means [1]. It has been reported that potentials higher than 4.5V causes the partial removal of Li and O from the structure leading to electrochemical activation[2].  This activation lead to the rearrangement of the ions and hence vacancies in the host structure for the reversible intercalation of  Li. In the present study we aim to improve manganese dissolution in Li₂MnO₃using aluminum and magnesium doping at the Li site.

          Li₂MnO₃, LiAlMnO₃ and LiMgMnO₃ were prepared by sol-gel Pechini method. For the individual compounds the synthesis was carried out using lithium nitrate [LiNO₃], manganese nitrate [Mn(NO₃)₂], aluminum nitrate [Al(NO₃)₃•9H₂O], 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 130◦ C for 24 h to complete the drying process, followed by annealing of the powder at 750^oC for 8h in air to obtain desire phase. The obtained powder samples were mixed with polyvinylidene (PVDF) and carbon black in the wt % ratio of 80 : 10 :10, respectively. Slurry was prepared using 1-methyl 2-pyrolidone as solvent and was  spray coated on Al foil substrates to form a homogeneous electrode. Electrodes were punched out for the fabrication of 2032 coin cells, where Li was used as a counter electrode along with 1.2M LiPF₆in EC:DMC (1:1) as electrolyte. The charge and discharge analysis was performed using 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 was studied with X-ray diffraction (XRD). Figure 1 shows XRD spectra for (a)Li₂MnO₃, (b)LiAlMnO₃ and (c)LiMgMnO₃. Phase formation for the samples was obtained at 750C-8h. In the case of  LiAlMnO₃, the characteristic peak between 20-25 degrees vanishes, possibly due to the Mn site substitution by Al.

          X-ray photoelectron spectroscopy (XPS) were obtained to study the valance state of the manganese ion after aluminum and magnesium doping. Figure 2 shows XPS pattern for Mn 2p of the samples. The Mn 2p_1/2 and Mn 2p_3/2 peaks for Li₂MnO₃ are 654.15eV and 642.45eV, respectively, being in agreement with those reported in  literature[3]. A slightly shift to the right is observed for LiAlMnO₃ pattern, indicating a decrease in the tetravalent state of manganese. Therefore, an improvement in electrochemical performance is expected. In the case of LiMgMnO₃ the Mn_3/2 peak is shifted to the left, with an increase of approximately 0.1 eV,  demonstrating a considerably increase in the binding energy indicative of a higher valance state.

          Preliminary results, shown in Figure 3, indicates that Al-doping enhances the cyclability of Li₂MnO₃and increase the capacity by 30mAh/g. The Mg-doped cells also have better cycle stability as compared to the pristine material, but the capacity faded by ~15mAh/g.  Electrochemical performance of the pristine, Al-doped and Mg-doped cells will be presented in detail.

References

1. MH Rossouw, MM Thackeray

Materials Research Bulletin, Volume 26, Issue 6, 1991 (463)

2.N.Tran, L. Croguennec, M. Menetrier, F. Weill, P. Biensan, C. Jordy, C. Delmas

Chemistry of Materials 20  , 2008 (4815)

3. Xin Dong, Youlong Xu, Liong Xiong, Xiaofei Sun, Zhengwei Zhang

Journal of Power Sources 243, 2013 (78)