High capacity, long cycle life, environmentally friendliness, and safety are the main requirements for advanced lithium-ion batteries (LIBs), that have found a broad application in our daily life. Among different cathode materials for LIBs, Li2MnSiO4 has been attracting considerable attention due to its possibility to meet the above-mentioned requirements [1].
Electrospinning is relatively new and promising technique recently applied to this material, and can produce composite nanofibers with advantageous physical-chemical properties [2, 3]. However, few study was done on the synthesis of Li2MnSiO4 directly from a homogeneous precursor solution by electrospinning with heat treatment. In this work, we have prepared Li2MnSiO4 from homogeneous precursor solutions by electrospinning followed by heat treatment and studied its physical and electrochemical properties.
Experimental details
The precursor solutions used were prepared by dissolving stoichiometric amounts of lithium nitrate (LiNO3), manganese acetate (Mn(CH3COO)2) or manganese nitrate hexahydrate (Mn(NO3)2･6H2O), and tetraethyl orthosilicate (TEOS) in a mixture of ethanol and distilled water. Polyvinylpyrrolidone (PVP) was added as electrospinning template to the precursor solutions, and they were magnetically stirred for 2 h. The mass ratio of the precursor salts to PVP was optimally varied to obtain uniform fibers by electrospinning. The viscous homogeneous solutions were placed into a glass syringe and electrospun at a high voltage of 15 kV. A feed rate of the precursor solutions is fixed at 0.9 mL h-1. The electrospun fibers were collected on an Al foil placed 10 cm away from the tip of stainless steel nozzle connected to the glass syringe. The as-spun uniform fibers were dried at 110 oC for 8 h by a vacuum oven to remove the solvent. The dried fibers were heated at 360 oC for 1 h in air flow, and further heat treatment was conducted at 700 oC for 1 h in a reduced N2+H2 (97/3 v.v %) atmosphere. A heating rate was 5 oC min-1. The electrochemical properties of the samples as cathode materials for lithium batteries were investigated by galvanostatic measurements with a coin-type cell (CR2032).
Results and discussion
Figure 1 shows the XRD pattern of the sample prepared from the homogeneous precursor solution, which is obtained by dissolving a stoichiometric ratio of LiNO3, Mn (CH3COO) 2 and TEOS into the mixture of ethanol and distilled water with PVP, by electrospinning followed by heat treatments. The main peaks of the XRD pattern correspond to orthorhombic Li2MnSiO4 with a Pmn21 space group, and some small impurity peaks attributed to monoclinic Li2MnSiO4 with Pn are also detected. From the SEM micrograph of the sample in the inset of Fig. 1, formation of porous structure with interconnected spherical particles can be observed. Diameter of particles ranges from 200 to 400 nm. Although the drastic morphology change from the fiber to the porous structure occurs in the present synthesis process, it can help to obtain Li2MnSiO4 with small impurities in a short heat treatment time, which is 10 times shorter than that previously reported [2, 3].
References
[1] Q. Cheng, W. He, X. Zhang, M. Lia and L. Wang, J. Mater. Chem. A, 5 (2017) 10772
[2] S. Zhang, Y. Li, G. Xu, S. Li, Y. Lu, O. Toprakci, X. Zhang, J. Power Sources, 213 (2012) 10
[3] H.J. Song, J.-C. Kim, M. Choi, C. Choi, M. A. Dar, C. W. Lee, S. Park, D.-W. Kim, Electrochim. Acta 180 (2015) 756