Direct Observation of Nano-scaled Chemical Redox on LLNMO High-Capacity Cathode Materials
Hung Wei Shiu1*, Ming-Hsien Lin2, Yu-Ling Lai1, Ming-Wei Lin1, Tolek Tyliszczak3, Bing Joe Hwang2, Yao-Jane Hsu1
1Nano Science Group, National Synchrotron Radiation Research Center, Hsinchu, Taiwan
2Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan
3Advance Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
* presenting author:Hung Wei Shiu,
Lithium-rich cathode nano-materials such as Li[NixLi(1-2x)/3Mn(2-x)/3]O2 (0 ≤ x ≤ 0.5) (LLNMO) have generated a lot of interest due to their high reversible capacities (270 ~ 350 mAh/g).[1,2] However, these Li cathodes suffer from voltage-fade, transition metal dissolution, structure change and the formation of new species on the cathodes with increasing number of cycles.[3] A strong correlation was found between the reversible migration of transition-metal ions from the tetrahedral sites to octahedral sites and the irreversible movement of the metal ions into local cubic sites during discharging (voltage-fade process).[4] Besides, previous study implied the extraction of Li2O from the LLNMO and formed on the cathode and anode during the plateau region.[5,6] In spite of that, the micro-origin and direct evidence are still uncertain. Understanding the effect of the activating reactions on the cathodes during the charging/discharging cycles can lead to the improvements in stability and performance of the batteries.
By using X-ray absorption spectroscopy (XAS) the “average” electronic structural changes that occur at different charged states can be examined. Particular attention in this study will be given to the redox reaction process at each single nano-materials during electrochemical cycling thus the truly mechanism can be established. Scanning Transmission X-ray Microscopy (STXM) is the most promising technique to understand and investigate nanoscaled electronic structure. In this study, the nano-scaled redox reaction has been determined. With increasing the state of charge, some of the particles change its valence states from Ni2+ to Ni4+. However, some of them remain unchanged. Besides, during the redox reaction, the intermediated states of Ni3+ were observed.
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[3] Gu, M.; Belharouak, I.; Zheng, J.; Wu, H.; Xiao, J.; Genc, A.; Amine, K.; Thevuthasan, S.; Bare, D. R.; Zhang, J.-G.; Browning, N. D.; Liu, J.; Wang, C. ACS Nano 2012, 7, 760.
[4] Gallagher, K. G.; Croy, J. R.; Balasubramanian, M.; Bettge, M.; Abraham, D. P.; Burrell, A. K.; Thackeray, M. M. Electrochem. Commun. 2013, 33, 96.
[5] Lu, Z.; Dahn, J. R. J. Electrochem. Soc. 2002, 149, A815.
[6] Hy, S.; Felix F.; Rick, J.; Su, W.-N.; Hwang, B. J. J. Am. Chem. Soc. 2014, 136, 999.

Keywords: Lithium , Battery, STXM, XAS