ABSTRACT
Although Li- and Mn-rich transition metal oxides have been extensively studied as high-capacity cathode materials for Li-ion batteries, the crystal structure of these materials in their pristine state is not yet fully understood. Here we apply complementary electron microscopy and spectroscopy techniques at multi-length scale on well-formed Li1.2(Ni0.13Mn0.54Co0.13)O2 crystals with two different morphologies as well as two commercially available materials with similar compositions, and unambiguously describe the structural make-up of these samples. Systematically observing the entire primary particles along multiple zone axes reveals that they are consistently made up of a single phase, save for rare localized defects and a thin surface layer on certain crystallographic facets. More specifically, we show 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.
ABSTRACT
Instabilities resulting from side reactions between the high-voltage cathode and the electrolyte are major barriers to meeting the calendar and cycle life requirements in lithium-ion batteries for vehicular applications. The present study reports a new approach for minimizing the effect of these reactions. LiMn1.5Ni0.5O4 (LMNO) with two distinct morphologies, octahedron with (111) and plate with (112) surface facets, were synthesized in a similar size and investigated for structural changes and electrochemical stability during long-term cycling and storage in the presence of a liquid carbonate electrolyte. Bulk and surface analyses using ICP, XRD, FTIR, soft and hard XAS revealed that in the charged state, the high-valent transition metals in Mn1.5Ni0.5O4 (MNO) oxidatively decompose the electrolyte which results in electron transfer from the electrolyte to the cathode. As a compensation mechanism, Li(+) ions are re-inserted into MNO and the cathode self-discharges. Surface facets where the local redox reactions occur heavily influence the reaction kinetics and selectivity which ultimately determine the nature of the products and rate of self-discharge. Significantly lower self-discharge was observed on octahedra with the (111) facets, benefiting from their ability for promoting sufficient passivation after the initial interaction with the electrolyte. The importance of particle engineering reported in this work has a broad implication in the development of next generation cathode materials with improved performance.
ABSTRACT
Intercalation compounds such as transition metal oxides or phosphates are the most commonly used electrode materials in Li-ion and Na-ion batteries. During insertion or removal of alkali metal ions, the redox states of transition metals in the compounds change and structural transformations such as phase transitions and/or lattice parameter increases or decreases occur. These behaviors in turn determine important characteristics of the batteries such as the potential profiles, rate capabilities, and cycle lives. The extremely bright and tunable x-rays produced by synchrotron radiation allow rapid acquisition of high-resolution data that provide information about these processes. Transformations in the bulk materials, such as phase transitions, can be directly observed using X-ray diffraction (XRD), while X-ray absorption spectroscopy (XAS) gives information about the local electronic and geometric structures (e.g. changes in redox states and bond lengths). In situ experiments carried out on operating cells are particularly useful because they allow direct correlation between the electrochemical and structural properties of the materials. These experiments are time-consuming and can be challenging to design due to the reactivity and air-sensitivity of the alkali metal anodes used in the half-cell configurations, and/or the possibility of signal interference from other cell components and hardware. For these reasons, it is appropriate to carry out ex situ experiments (e.g. on electrodes harvested from partially charged or cycled cells) in some cases. Here, we present detailed protocols for the preparation of both ex situ and in situ samples for experiments involving synchrotron radiation and demonstrate how these experiments are done.
