Effect of Al and Mg Doping on the Electrochemical Performance of Pure Li2MnO3
Authors: Torres-Castro Loraine , Shojan Jifi , Katiyar Ram S. , Manivannan A. 
Published: February 2020

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 LiMn2O4. Li2MnO3 possess monoclinic structure having c2/m space group symmetry with a layered arrangement of Li[Li1/3Mn2/3]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 750oC 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 2p1/2 and Mn 2p3/2 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 Mn3/2 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)