Abstract
The distribution of cations in Li-ion battery cathodes as a function of cycling is a pivotal characteristic of battery performance. The transition metal cation distribution has been shown to affect cathode performance; however, Li is notoriously challenging to characterize with typical imaging techniques. Here laser-assisted atom probe tomography (APT) is used to map the three-dimensional distribution of Li at a sub-nanometre spatial resolution and correlate it with the distribution of the transition metal cations (M) and the oxygen. As-fabricated layered Li1.2Ni0.2Mn0.6O2 is shown to have Li-rich Li2MO3 phase regions and Li-depleted Li(Ni0.5Mn0.5)O2 regions. Cycled material has an overall loss of Li in addition to Ni-, Mn- and Li-rich regions. Spinel LiNi0.5Mn1.5O4 is shown to have a uniform distribution of all cations. APT results were compared to energy dispersive spectroscopy mapping with a scanning transmission electron microscope to confirm the transition metal cation distribution.
Highlights
The distribution of cations in Li-ion battery cathodes as a function of cycling is a pivotal characteristic of battery performance
Compositional segregation of Ni to surfaces and grain boundaries within some particles and partitioning of Mn away from Ni-rich regions in layered Li1.2Ni0.2Mn0.6O2 has been shown by energy dispersive spectroscopy (EDS) tomography[10,11,12]
By comparison of high-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) images to multislice simulations, the Ni-rich regions were shown to be consistent with an R-3m phase and the Ni-deficient regions a C2/m phase[12]
Summary
The distribution of cations in Li-ion battery cathodes as a function of cycling is a pivotal characteristic of battery performance. Understanding nanoscale distribution of all of the elements that makeup Li-ion battery cathodes—especially Li ions—as a function of different synthesis procedures and extents of electrochemical cycling is a critical step towards developing new materials. Layered Li1.2Ni0.2Mn0.6O2 is considered to be a phase mixture of the trigonal LiMO2 (R-3m) and monoclinic Li2MO3 (C2/m ) phases (M 1⁄4 Ni, Mn) Both of these structures can be represented as repeating layers of transition metal ions, O and Li. Recently, compositional segregation of Ni to surfaces and grain boundaries within some particles and partitioning of Mn away from Ni-rich regions in layered Li1.2Ni0.2Mn0.6O2 has been shown by energy dispersive spectroscopy (EDS) tomography[10,11,12]. Despite substantial characterization efforts in previous studies, quantifying lithium depletion and subtle changes in oxygen concentration in the surface layer have been proven to be elusively challenging
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