On the right the cube represents the structure of lithium- and manganese- rich transition metal oxides. The models on the left show the structure from three different directions, which correspond to the STEM images of the cube. Click to enlarge.

Layered transition metal oxides (LiMO₂, where M is usually Co, Mn, Ni or some combination thereof) are presently used as cathode materials for secondary lithium-ion batteries in consumer electronics, as they provide reversible capacities from about 140 to 190  mAh g⁻¹. It is known that increasing the lithium and manganese contents in these materials to a general composition of Li_1+xM_1−xO₂ can lead to much higher capacities, typically >250  mAh g⁻¹. Although this class of Li- and Mn-rich transition metal oxides (referred to as LMRTMOs hereafter) was introduced nearly a decade ago, their commercial application has been hindered by severe shortcomings in the materials, including a large first-cycle irreversible capacity loss, voltage and capacity fade, d.c. resistance rise at a low state of charge and transition metal dissolution.

Studies have attributed these issues to the structural changes occurring during first-cycle activation and prolonged cycling, yet the crystal structure of the pristine oxides is still a matter of debate. Much of the research efforts have been focused on electrochemistry and phase transformation studies, and several attempts that have been made to solve the crystal structure of the pristine material in the past have led to conclusions in three categories: (1) intermixed nano-domains of two phases that are trigonal LiMO₂ (where M represents all the transition metals present in the compound) and monoclinic Li₂MnO₃, (2) a single monoclinic phase in the entire sample and (3) a single trigonal phase or a ‘solid solution’ with the presence of a superstructure.

—Shukla et al.

The research team applyied complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li_1.2(Ni_0.13Mn_0.54Co_0.13)O₂ crystals with two different morphologies as well as two commercially available materials with similar compositions to image the materials at atomic resolution.

Because previous studies have been ambiguous about the structure, the researchers minimized ambiguity by looking at the material from different directions, or zone axes.

Misinterpretations from electron microscopy data are possible because individual two-dimensional projections do not give you the three-dimensional information needed to solve a structure. So you need to look at the sample in as many directions as you can.

—Alpesh Khushalchand Shukla

Scientists have been divided on whether the material structure is single trigonal phase, double phase, or defected single monoclinic phase. The “phase” of a material refers to the arrangement of the atoms with respect to each other.

The two-phase and one-phase model are very closely related. It’s not like comparing an apple to an orange—it’s more like comparing an orange and a grapefruit from very far away. It’s hard to tell the difference between the two.

—Colin Ophus

In addition to viewing the material at atomic resolution along multiple zone axes, the researchers viewed entire particles rather than just a subsection.

By systematically observing the entire primary particles along multiple zone axes, the team concluded that the particles are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets.

More specifically, they showed that the bulk of the oxides can be described as an aperiodic crystal consisting of randomly stacked domains that correspond to three variants of monoclinic structure, while the surface is composed of a Co- and/or Ni-rich spinel with antisite defects.

Our paper gives very strong support for the defected single-phase monoclinic model and rules out the two-phase model, at least in the range of compositions used in our study.

—Colin Ophus

In addition to solving the structure of the bulk material, which has been studied by other research groups, the Berkeley Lab team also solved the surface structure, which is different from the bulk and consists of just a few layers of atoms on select crystallographic facets. Because the intercalation of lithium starts at the surface, understanding the surface of the pristine material is very important, Shukla said.

On top of the STEM (scanning transmission electron microscopy) imaging that they used for the bulk, they had to use additional techniques to solve the surface, including EELS (electron energy loss spectroscopy) and XEDS (X-ray energy dispersive spectroscopy).

This study not only solves the ambiguity in the structure determination of these materials but it also highlights the importance of atomic-resolution imaging and spectroscopy performed on multiple zone axes when carrying out such investigations. It should be emphasized that these results are relevant to LMRTMOs, and it will be interesting to see whether and how the bulk structure changes as the amount of Li and Mn is decreased and we plan to study this effect in the future. We hope that results from this study on pristine LMRTMOs will provide new directions to researchers studying the phase transformations in this class of materials and can be applied to potentially solve the problems such as voltage and capacity fade that are related to the surface and bulk structure.

Indeed, the voltage profiles of initial charge/discharge of LMRTMOs have been so far conveniently divided into two regions where Li is removed from the two ‘components’, namely, LiMO₂ and Li₂MnO₃. The substantial proof on a single phase presented in this study calls for revisiting the interpretation of the voltage profiles. Furthermore, with a now clearer understanding of the surface structure, the facets where they are formed and their orientation relationship with the bulk, we can attempt to understand the effect of delithiation and cycling on different facets by using electron- and X-ray-based tomography techniques that can provide both chemical and morphological information in a wide length scale. Ultimately, such studies can guide us to engineer primary particles with tailored facets that would minimize the change of structure and hence the stress and strain in the cycled material.

—Shukla et al.

The work was funded by the Vehicle Technologies Office under the U.S. Department of Energy. The Molecular Foundry is a DOE Office of Science User Facility.

Resources

• Alpesh Khushalchand Shukla, Quentin M. Ramasse, Colin Ophus, Hugues Duncan, Fredrik Hage and Guoying Chen (2015) “Unravelling structural ambiguities in lithium- and manganese-rich transition metal oxides” Nature Communications 6, Article number: 8711 doi: 10.1038/ncomms9711