Supplementary Figures

Transcription

1 Supplementary Figures Supplementary Figure 1 SEM/EDS mapping of LiNi 0.4 Mn 0.4 Co 0.18 Ti 0.02 O 2. The experimental error of the mapping is ±1%. The atomic percentages of each element are based on multiple SEM/EDS mapping results.

2 Supplementary Figure 2 N 2 adsorption and desorption isotherms. N 2 adsorption and desorption isotherms of NMC materials with varied specific surface areas (i.e., particle sizes). This study was primarily performed on the material with larger particle size (9.0 m 2 /g).

3 Supplementary Figure 3 Pristine NMC particle. EELS line scan profile for a pristine NMC particle along the <001> direction: (a) Mn L-edge EELS spectra along the scanning pathway, (b) 2D EELS map visualizing no peak shift, and (c) the simultaneously acquired subpixel mosaicked ADF-STEM image. The onset energy for Mn L-edge spectra remains constant along the scanning pathway indicating that the Mn oxidation state is constant at +4.

4 Supplementary Figure 4 As-made pristine composite electrode. Cross-section SEM image of a typical composite electrode (84 wt% active material, 8 wt% polyvinylidene fluoride, 4 wt% acetylene carbon black and 4wt% SFG-6 synthetic graphite on an aluminum current collector, visible on the left side of the image.

5 Supplementary Figure 5 Ti L-edge spectra of NMC materials and standard anatase TiO 2. The pre-edge features and splitting of e g /L 3 are circled. The splitting of the L 3 doublet e g (circled) is due to the tetragonal distortion in anatase TiO 2. The different shape of L 3 doublet e g in the NMC materials suggests that Ti is incorporated in the lattice and is not phase separated to either anatase or rutile 1.

6 Supplementary Figure 6 NMC materials with different particle size distributions. (a) Comparison of XRD patterns for NMC materials with two different particle sizes. XAS spectra of the NMC material with smaller particle size (note that the one with larger particle size is presented in the main text): (b) Ni L-edge, (c) Mn L-edge and (d) Co L- edge using Auger electron yield (blue), total electron yield (red) and fluorescence yield (green) modes.

7 Supplementary Figure 7 XAS Ni L-edge before and after cycling. XAS Ni L-edge spectra for the samples in the pristine state (red) and after 20 charge-discharge cycles in a voltage range of 2.0 V- 4.7 V vs Li/Li + (green): (a) AEY mode and (b) TEY mode.

8 Supplementary Figure 8 NMC particles with 20 cycles. EELS line scan profile for an NMC particle along <001> direction: (a) STEM image to indicate scanning pathway, where a surface reaction layer is labeled in the image. (b) Mn L-edge EELS spectra along the scanning pathway, (c) 2D EELS map to enhance the visualization of peak shift, and (d) concentration profiles for Mn 2+ and Mn 4+ using a linear combination method. The measurements were performed on electrodes in the discharged state after 20 cycles between a voltage range of 2.0 V- 4.7 V vs Li/Li +.

9 Supplementary Figure 9 NMC particles with 20 cycles. EELS line scan profile for an NMC particle along the <001> direction: (a) Mn L-edge EELS spectra along the scanning pathway, (b) 2D EELS map, (c) the simultaneously acquired subpixel mosaicked ADF-STEM image, and (d) concentration profiles for Mn 2+ and Mn 4+ using a linear combination method. The measurements were performed on electrodes in the discharged state after 20 cycles between a voltage range of 2.0 V- 4.3 V vs Li/Li +.

10 Supplementary Figure 10 Buildup of surface reduced layer dependent on particle sizes. Comparison of XAS/TEY spectra for NMC materials with two different particle sizes after various numbers of charge-discharge cycles: (a) Co L-edge and (b) Mn L- edge.

11 Supplementary Figure 11 Comparison of charge-discharge profiles for different particle sizes. Charge-discharge profiles of lithium cells containing (a) NMC material with large particle size. (b) NMC materials with small particle size between V for 20 cycles at a rate of C/20. Note that (a) is also presented in Fig. 2a.

12 Supplementary Figure 12 Surface reaction layer after high-voltage cycling. Investigation of the surface reaction layer for NMC particles cycled twenty times between 2.0 V- 4.7 V vs Li/Li + and rinsed completely with DMC. (a) STEM images of NMC particles showing the surface reaction layer indicated by arrows in the images. (b) A broad EELS spectrum from 260 ev to 800 ev, where C, O and F are the major elements detected in the range for the surface reaction layer. (c) High-resolution (i) Li K- edge, (ii) O K-edge and (iii) F K-edge spectra suggest that the dominant inorganic species of the surface reaction layer is LiF 2,3.

13 Supplementary Figure 13 Surface reaction layer after low-voltage cycling. STEM images of NMC particles cycled twenty times between 2.0 V- 4.3 V vs Li/Li + and rinsed completely with DMC.

14 Supplementary Figure 14 NMC particles after exposure to electrolytic solution. STEM images of NMC particles after being exposed to the electrolytic solution (1 M LiPF 6 in EC-DMC) for 7 days and rinsed completely with DMC.

15 Supplementary Figure 15 Charge-discharge profile. (a) Charge-discharge profiles ( V vs. Li/Li + at C/20) after 20 high-voltage cycles and after being refreshed with new electrolytic solution. (b) Charge-discharge profiles ( V vs. Li/Li + at C/20) after 20 high-voltage cycles. (c) Charge-discharge profile ( V vs. Li/Li + at C/50) after 20 high-voltage cycles. Note that the impedance rise imposed less effect on the capacity when the electrode was cycled at the extremely slow rate, such as C/50.

16 Supplementary Figure 16 Atomic resolution ADF/STEM images. The results show NMC particles with orientation-dependent characteristics. The blue and green arrows indicate the thick and thin surface reconstruction layers, respectively. Some thin regions are almost free of reconstruction layer, for example, the thin regions in (b) and (c).

17 Supplementary Figure 17 DFT calculation. Formation energies of rock salt structure in the NMC structure as functions of lithium stoichiometry and oxygen partial pressure.

18 Supplementary Table Supplementary Table 1 Lattice parameters for NMC materials. Materials a (Å) b (Å) c (Å) V (Å 3 ) χ 2 Large (9) Small (7)

19 Supplementary References 1. Zhou, J. G. et al. Electronic structure of TiO 2 nanotube arrays from X-ray absorption near edge structure studies. J. Mater. Chem. 19, (2009). 2. Wang, F. et al. Chemical distribution and bonding of lithium in intercalated graphite: identification with optimized electron energy loss spectroscopy. ACS Nano 5, (2011). 3. Stoebe, T. G. et al. An electron energy loss spectroscopy (EELS) study of thermal effects in LiF: Mg, Cu, P phosphors. J. Phys.: Condens. Matter 11, L341 L347 (1999).