Acta Phys. -Chim. Sin. ›› 2017, Vol. 33 ›› Issue (11): 2293-2300.doi: 10.3866/PKU.WHXB201705294

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Dual Electron Energy Loss Spectrum Imaging of the Surfaces of LiNi0.5Mn1.5O4 Cathode Material

Ya-Dong LI1,2,Yu-Feng DENG1,Zhi-Yi PAN1,2,Yin-Ping WEI1,2,Shi-Xi ZHAO1,Lin GAN1,2,*()   

  1. 1 Department of Energy and Environment, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong Province, P. R. China
    2 Electron Microscopy Laboratory, Materials and Devices Testing Center, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, Guangdong Province, P. R. China
  • Received:2017-04-24 Published:2017-08-25
  • Contact: Lin GAN E-mail:lgan@sz.tsinghua.edu.cn
  • Supported by:
    Guangdong Natural Science Foundation for Distinguished Young Scholars, China(2016A030306035);Shenzhen Basic Research Program, China(JCYJ20160531194754308)

Abstract:

Acquiring the spatial distribution of Li and the valence state of transition metals (TMs) in lithium ion battery (LIB) electrode materials is critical for understanding their electrochemical performances. Electron energy loss spectrum (EELS) is in principle optimum for analyzing light elements; however, quantitative analysis of Li, the lightest solid element in the periodic table, using EELS remains challenging. This is not only because of the overlap of the Li-K edge and the M23 edge of TMs but also due to the normally large particle sizes of LIB electrode materials (hundreds of nm), leading to significant plural scattering effect in the EELS spectra. Using LiNi0.5Mn1.5O4 (LNMO) as the cathode material, we obtained the spatial distribution of Li, Ni, and Mn by dual EELS spectral imaging, which allows us to simultaneously acquire the zero loss and core loss spectra, thus eliminating both the energy drift and plural scattering effects. Our results reveal that the as-prepared LNMO particles have a Mn/Ni-enriched and Li-poor surface layer of thickness 1-2 nm, and the valence of Mn gradually changed from +4 in the bulk to +2 in the surface layer. Given that the low-valent Mn2+ dissolution is a critical reason for structure damage and capacity degradation of LNMO, our results indicate that rational synthesis of LNMO with decreased low-valent Mn2+ content could be a previously neglected approach to enhance their electrochemical performance.

Key words: Lithium ion battery, Li mapping, Valence state distribution, Electron energy loss spectroscopy, Spectrum imaging

MSC2000: 

  • O649