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Title: Transition-metal redox evolution in LiNi 0.5Mn 0.3Co 0.2O 2 electrodes at high potentials

The mixed transition-metal layered compound, LiNi 0.5Mn 0.3Co 0.2O 2 (NMC532), is a promising high-energy cathode material. However, the required high-voltage (>4.3 V) cycling is accompanied by a rapid capacity fade associated with a complex redox mechanism that has not been clarified. Here in this paper, we report soft x-ray absorption spectroscopy of NMC532 electrodes, both pristine and those charged to 4.2, 4.35, or 4.5 V in graphite/NMC532 cells. A quantitative sXAS analysis shows that about 20% of the nickel exists as Ni 4+ in the as-synthesized NMC532. The Ni redox reaction contributes only to the experimental capacity obtained below 4.2 V, while Co redox reactions take place throughout the entire electrochemical cycling up to 4.5 V. In contrast to the changing ratio of the well-defined Ni 2+, Ni 3+ and Ni 4+ ions, Co always displays ill-defined intermediate valence states in the charged NMC532 electrodes. This indicates an itinerant electron system in NMC electrodes related to the improved rate performance through Co doping. Furthermore, about 20% of Ni 2+ is found on the electrode surface at the high potential, which suggests that the electrode surface has either gone through surface reconstruction or reacted with the electrolyte at high voltage.
Authors:
 [1] ;  [2] ;  [3] ;  [3] ;  [3] ;  [3] ;  [3] ;  [4] ; ORCiD logo [1]
  1. Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Advanced Light Source
  2. DuPont Company, Experimental Station, Wilmington, DE (United States)
  3. DuPont Company, Experimental Station, Wilmington, DE (United States)
  4. New York Univ. (NYU), NY (United States). Dept. of Physics
Publication Date:
Grant/Contract Number:
AC02-05CH11231
Type:
Accepted Manuscript
Journal Name:
Journal of Power Sources
Additional Journal Information:
Journal Volume: 360; Journal Issue: C; Related Information: © 2017 Elsevier B.V.; Journal ID: ISSN 0378-7753
Publisher:
Elsevier
Research Org:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
Language:
English
Subject:
25 ENERGY STORAGE; 36 MATERIALS SCIENCE; Lithium-ion batteries; LiNi0.5Mn0.3Co0.2O2; High voltage; Soft x-ray spectroscopy
OSTI Identifier:
1437956