Synthesis, Characterization and Electrochemical Performance for a Series of Al- Substituted Li2MnO3

Lithium manganese oxides have been exploited due to the abundance of Mn in Earth, low toxicity and safety. In this family of oxides, LiMnO₂ and LiMn₂O₄ have shown promising electrochemical features, such as high discharge capacity and high operating voltage, respectively. However, these materials suffer from serious structural changes after certain number of cycles due to the presence of Mn³⁺, which is known to be a Jahn-Teller active ion [1]. Side reactions such as 2Mn³⁺ → Mn⁴⁺ + Mn²⁺, also lead to battery failure upon cycling, as Mn²⁺ is known to be soluble in 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.

The drawback of Li₂MnO₃ is the presence of Mn in +4 oxidation state, which makes the material electrochemically inactive. However, it can be made electrochemically active by extracting lithium and oxygen simultaneously from the structure by chemical or electrochemical means [2].  Electrochemical activation has been achieved after exposing the electrode to potentials higher than 4.5 V vs. Li⁰/Li⁺, which causes the partial removal of lithium and oxygen from the structure. This process leads to the rearrangement of the ions and hence vacancies in the host structure for the reversible intercalation of lithium. In the present study, we aim to improve manganese dissolution in Li₂MnO₃ using aluminum substitution at the Lithium site. 

Li₂MnO₃ and Al-substituted materials 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], 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^oC 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 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 LiPF₆ in EC:DMC (1:1) as electrolyte. The charge and discharge analysis was performed using 10mAh/g within a potential window of 2.8V- 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 (a) Li₂MnO₃, (b) 25% Li_, (c) 50% Li and (d) 25% Li. Phase formation for the samples was obtained at 750C-8h without impurity.

X-ray photoelectron spectroscopy (XPS) was obtained to study the valance state of the manganese ion after aluminum substitution. 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 654.35eV and 642.55eV, respectively, being in agreement with those reported in literature [3]. A slightly shift to a lower binding energy is observed for 50% Li and 25% Li, indicating a decrease in the tetravalent state of manganese. Therefore, an improvement in electrochemical performance is expected. In the case of 75% Li, the Mn 2p peaks are shifted to the left, demonstrating a considerably increase in the binding energy which is indicative of a higher valance state. 

Preliminary results shown in Figure 3 indicate that Al-substitution for 75% of Lithium enhances the cycleability of Li₂MnO₃ and increase the capacity by 60mAh/g. Electrochemical performance of the pristine and Al-substituted cathodeswill be presented in detail.

References:

[1] A. J. Paterson, A. R. Armstrong, P. G. Bruce, Stoichiometric LiMnO₂ with Layered Structure: Charge/Discharge Capacity and the Influence of Grinding, J. Electrochem. Soc 151 (2004) A1552-A1558.

[2] M.H. Rossouw, M.M. Thackeray, Lithium Manganese Oxides from Li₂MnO₃ for Rechargeable Lithium Battery Applications, Mat. Res. Bull. 26 (1991) 463-473.

[3] Xin Dong, Youlong Xu, Liong Xiong, Xiaofei Sun, Zhengwei Zhang, Sodium substitution for partial lithium to significantly enhance the cycling stability of Li₂MnO₃ cathode material, J. Power Sources (2013) 78