PVP-Functionalized Nanometer Scale Metal Oxide Coatings for. Cathode Materials: Successful Application to LiMn 2 O 4 Spinel.

Transcription

1 PVP-Functionalized Nanometer Scale Metal Oxide Coatings for Cathode Materials: Successful Application to LiMn 2 O 4 Spinel Nanoparticles Hyesun Lim, Jaephil Cho* Department of Applied Chemistry Hanyang University Ansan, Korea Synthesis: 20g of manganese sulfate (MnSO 4 H 2 O) and an equal amount of ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) were dissolved in 30ml of distilled water, which was then transferred into a Teflon-lined stainless steel autoclave. This was sealed and maintained at a temperature of 150 o C for 20 h. After the reaction was complete, the resulting solid products were filter and washed using distilled water several times. They were finally washed with ethanol and annealed at 120 o C overnight. Obtained MnO 2 was annealed at 700 o C for 2h. These sentences were added in supplementary information. MnO 2 nanoparticles with a size <90 nm were used for preparing spinel powders, and lithium acetate C 2 H 3 O 2 Li 2H 2 O (12.25g) was dissolved in 100 ml of distilled water and mixed with the MnO 2 nanopraticles (20.6g) for preparing LiMn 2 O 4. After drying at 120 o C, the solid products were further annealed at 700 o C for 10 h in air and slowly cooled to room temperature. For coating with MgO or Al 2 O 3, 1 g of polyvinyl pyrrolidone (PVP, Mw = 55,000, Aldrich) was dissolved in 20 ml of distilled water, and 100g of spinel powders was added to the mixed solution, which was thoroughly mixed as the temperature was increased slowly to 40 o C and maintained for 10 min. After that, 3 g of Mg 2 C 2 O 4 or Al nitrate was added in this mixture. Finally, the powders were filtered to remove the dissolved NO 3 - and C 2 O 4 -. The filtered powder was annealed at 600 o C for 3 h in air to remove the PVP, and the total coating concentration was estimated as 1 wt% of the spinel material based upon ICP-MS (Inductively coupled Plasma-Mass Spectroscopy) analysis. During the slow heating to 600 o C, the PVP burnt-out slowly, and metal ions were believed to be converted to uniform metal oxide 1

2 coating layer on the spinel particles simultaneously. In addition, it is highly possible that PVP facilitated wetting of metal oxide on the surface or inhibited Oswald ripening of the metal oxide. Characterization: The electrolyte for the coin-type half cells (2016R type) was 1 M LiPF 6 with ethylene carbonate/diethylene carbonate/ethyl-methyl carbonate (EC/DEC/EMC= 30: 30: 40 vol %) (Cheil Industries). The electrode was composed of 90 wt% active material, 5 wt%. polyvinylidene fluoride binder, and 5 wt% Super P carbon black. HRTEM samples were prepared by the evaporation of the dispersed naoparticles in acetone or hexane on carbon-coated copper grids. The field-emission electron microscope was a JEOL 2100F operating at 200 kv. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analysis were performed on JOEL 2100F, and field scanning electron microscope (FE-SEM) images were taken on a Philips XL-30 equipped with an energy dispersive x-ray (EDX) spectrometer. 2

3 (311) (111) Intensity (a.u.) w/ PVP w/o PVP (222) (440) (331) (221) Uncoated Scattering angle (2 θ)/degree Fig. S1. XRD patterns of the uncoated, and 1wt% MgO-coated LiMn 2 O 4 spinel nanoparticles functionalized with or without PVP. 3

4 Fig. S2. TEM images of (a) 1 wt% Al 2 O 3 and (b) MgO-coated spinel nanoparticles without using PVP. 4

5 Fig. S3. TEM images of 2 wt% MgO coated spinel nanoparticle using PVP (a) and without using PVP (b). 5

6 In order to investigate structural changes of the samples after cycling at 65 o C, cathode electrodes were extracted from the cell after cycling, and X-ray diffraction (XRD) analysis was performed, as shown in Figure 4. Structural changes can occur at the spinel cathode because of the instability of the lithiated and delithaited Li- Mn-O phases in acidic, nonaqueous electrolytes. For example, as proposed by Aurbach and coworkers, the acidity of electrolytes containing LiPF 6 salt can be attributed to the hydrofluoric acid that is formed by the reaction of LiPF 6 with residual water in the organic solvent, according to the following reaction LiPF 6 + H 2 O LiF + POF 3 + 2HF [1,2]. Hunter reported that acid treatment of LiMn 2 O 4 resulted in the defect spinel product λ- MnO 2, according to the reaction 2LiMn 2 O 4 3λ-MnO 2 (solid) + MnO (solution) + Li 2 O (solution) [3]. The exact composition of the resulting defect spinel structure was dictated by the relative solubility of MnO and Li 2 O at the surface. Under ideal conditions, MnO and Li 2 O dissolution rates may not be constant. All defective spinel compounds have lattice constants smaller than those of LiMn 2 O 4 because LiMn 2 O 4 has the highest concentration of the relatively large Mn 3+ ion (ionic radius of Mn 3+ = 0.65 Å; ionic radius of Mn 4+ =0.53 Å[4]). Therefore, in an XRD pattern, all of the spinels within this tie triangle will have diffraction peaks that lie to the right of those of LiMn 2 O 4. For bare cathodes, a shift in the spinel peaks to higher 2θ values (a = 8.108± Å) is be observed (s4), accompanied by peak broadening (the lattice constant a of the bare sample before cycling was ± Å) after 65 o C cycling. In the case of the MgO-coated spinels functionalizing with PVP, peaks shift slightly to the upper angles, although the lattice constants are ± Å (the lattice constant a of the coated cathodes before cycling is ± Å). Al foil Intensity (a.u.) Uncoated MgO coating w/o PVP MgO coating w/ PVP Before charging Scattering angle (2 )/degree θ Fig. S4. Ex situ XRD patterns of the electrodes after 100 cycles at 65 o C. Alternatively, the lattice constant a of the coated cathode functionalizing without PVP after cycling is ± Å. Therefore, the uncoated LiMn 2 O 4 spinel and spinel nanoparticles without PVP functionalization are more defective under storage at 65 o C. References [1] D. Aurbach, Y. Gofer, J. Electrochem. Soc. 1991, 138, [2] D. Aurbach, A. Zaban, A. Schlecter, Y. Ein-Eli, E. Zenigard, B. Markowsky, J. Electrochem. Soc. 1995, 142,

7 [3] J. C. Hunter, J. Solid State Chem. 1981, 39, 142. [4] R. D. Shannon, Acta Crystallogr. 1976, A32,