title: Evolution of lattice structure and chemical composition of the surface reconstruction layer in Li(1.2)Ni(0.2)Mn(0.6)O2 cathode material for lithium ion batteries.
abstract: Voltage and capacity fading of layer structured lithium and manganese rich (LMR) transition metal oxide is directly related to the structural and composition evolution of the material during the cycling of the battery. However, understanding such evolution at atomic level remains elusive. On the basis of atomic level structural imaging, elemental mapping of the pristine and cycled samples, and density functional theory calculations, it is found that accompanying the hoping of Li ions is the simultaneous migration of Ni ions toward the surface from the bulk lattice, leading to the gradual depletion of Ni in the bulk lattice and thickening of a Ni enriched surface reconstruction layer (SRL). Furthermore, Ni and Mn also exhibit concentration partitions within the thin layer of SRL in the cycled samples where Ni is almost depleted at the very surface of the SRL, indicating the preferential dissolution of Ni ions in the electrolyte. Accompanying the elemental composition evolution, significant structural evolution is also observed and identified as a sequential phase transition of C2/m → I41 → Spinel. For the first time, it is found that the surface facet terminated with pure cation/anion is more stable than that with a mixture of cation and anion. These findings firmly established how the elemental species in the lattice of LMR cathode transfer from the bulk lattice to surface layer and further into the electrolyte, clarifying the long-standing confusion and debate on the structure and chemistry of the surface layer and their correlation with the voltage fading and capacity decaying of LMR cathode. Therefore, this work provides critical insights for design of cathode materials with both high capacity and voltage stability during cycling.
authors.affiliation: Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory , 902 Battelle Boulevard, Richland, Washington 99352, United States.
authors.forename: Pengfei
authors.initials: P
authors.lastname: Yan
authors.forename: Anmin
authors.initials: A
authors.lastname: Nie
authors.forename: Jianming
authors.initials: J
authors.lastname: Zheng
authors.forename: Yungang
authors.initials: Y
authors.lastname: Zhou
authors.forename: Dongping
authors.initials: D
authors.lastname: Lu
authors.forename: Xiaofeng
authors.initials: X
authors.lastname: Zhang
authors.forename: Rui
authors.initials: R
authors.lastname: Xu
authors.forename: Ilias
authors.initials: I
authors.lastname: Belharouak
authors.forename: Xiaotao
authors.initials: X
authors.lastname: Zu
authors.forename: Jie
authors.initials: J
authors.lastname: Xiao
authors.forename: Khalil
authors.initials: K
authors.lastname: Amine
authors.forename: Jun
authors.initials: J
authors.lastname: Liu
authors.forename: Fei
authors.initials: F
authors.lastname: Gao
authors.forename: Reza
authors.initials: R
authors.lastname: Shahbazian-Yassar
authors.forename: Ji-Guang
authors.initials: JG
authors.lastname: Zhang
authors.forename: Chong-Min
authors.initials: CM
authors.lastname: Wang
keywords: LMR cathode; Ni surface segregation; ion migration; lithium ion battery; surface reconstruction; voltage fading