An XPS Study of Electrolytes for Li-Ion Batteries in Full Cell LNMO vs Si/Graphite

Two different types of electrolytes (co-solvent and multi-salt) are tested for use in high voltage LiNi0.5Mn1.5O4||Si/graphite full cells and compared against a carbonate-based standard LiPF6 containing electrolyte (baseline). Ex situ postmortem XPS analysis on both anodes and cathodes over the life span of the cells reveals a continuously growing SEI and CEI for the baseline electrolyte. The cells cycled in the co-solvent electrolyte exhibited a relatively thick and long-term stable CEI (on LNMO), while a slowly growing SEI was determined to form on the Si/graphite. The multi-salt electrolyte offers more inorganic-rich SEI/CEI while also forming the thinnest SEI/CEI observed in this study. Cross-talk is identified in the baseline electrolyte cell, where Si is detected on the cathode, and Mn is detected on the anode. Both the multi-salt and co-solvent electrolytes are observed to substantially reduce this cross-talk, where the co-solvent is found to be the most effective. In addition, Al corrosion is detected for the multi-salt electrolyte mainly at its end-of-life stage, where Al can be found on both the anode and cathode. Although the co-solvent electrolyte offers superior interface properties in terms of the limitation of cross-talk, the multi-salt electrolyte offers the best overall performance, suggesting that interface thickness plays a superior role compared to cross-talk. Together with their electrochemical cycling performance, the results suggest that multi-salt electrolyte provides a better long-term passivation of the electrodes for high-voltage cells.

High Entropy and Low Symmetry: Triclinic High-Entropy Molybdates

Metal molybdates constitute a promising class of materials with a wide application range. Here, we report, to our knowledge for the first time, on the preparation and characterization of medium-entropy and high-entropy metal molybdates, synthesized by an oxalate-based coprecipitation approach. The high-entropy molybdate crystallizes in a triclinic structure, thus rendering it as high-entropy material with the lowest symmetry reported so far. This is noteworthy because high-entropy materials usually tend to crystallize into highly symmetrical structures. It is expected that application of the high-entropy concept to metal molybdates alters the material's characteristics and adds the features of high-entropy systems, that is, tailorable composition and properties. The phase purity and solid solution nature of the molybdates were confirmed by XRD, Raman spectroscopy, TEM, XPS, and ICP-OES.

Lithium containing layered high entropy oxide structures

Layered Delafossite-type Lix(M1M2M3M4M5…Mn)O2 materials, a new class of high-entropy oxides, were synthesized by nebulized spray pyrolysis and subsequent high-temperature annealing. Various metal species (M = Ni, Co, Mn, Al, Fe, Zn, Cr, Ti, Zr, Cu) could be incorporated into this structure type, and in most cases, single-phase oxides were obtained. Delafossite structures are well known and the related materials are used in different fields of application, especially in electrochemical energy storage (e.g., LiNixCoyMnzO2 [NCM]). The transfer of the high-entropy concept to this type of materials and the successful structural replication enabled the preparation of novel compounds with unprecedented properties. Here, we report on the characterization of a series of Delafossite-type high-entropy oxides by means of TEM, SEM, XPS, ICP-OES, Mössbauer spectroscopy, XRD including Rietveld refinement analysis, SAED and STEM mapping and discuss about the role of entropy stabilization. Our experimental data indicate the formation of uniform solid-solution structures with some Li/M mixing.

In Situ X-ray Diffraction and X-ray Absorption Spectroscopic Studies of a Lithium-Rich Layered Positive Electrode Material: Comparison of Composite and Core-Shell Structures

Lithium- and manganese-rich transition-metal oxide (LMR-NMC) electrodes have been designed either as heterostructures of the primary components ("composite") or as core-shell structures with improved electrochemistry reported for both configurations when compared with their primary components. A detailed electrochemical and structural investigation of the 0.5Li₂MnO₃-0.5LiNi_0.5Mn_0.3Co_0.2O₂ composite and core-shell structured positive electrode materials is reported. The core-shell material shows better overall electrochemical performance compared to its corresponding composite material. While both configurations gave the same initial charge capacity of ∼300 mAh/g when cycled at a rate of 10 mA/g at 25 °C, the core-shell sample gives a discharge capacity of 232 mAh/g compared to 208 mAh/g delivered by the composite sample. Also, the core-shell sample gave better rate capability and a smaller first-cycle irreversible capacity loss than the composite sample. The improved performance of the core-shell material is attributed to its lower surface reactivity and limited structural change since the more stable Li₂MnO₃ shell screens the more reactive Ni-rich core material from interacting with either air or electrolyte at high potentials, thereby preventing electrode surface modification. In situ X-ray diffraction correlated with electrochemical data revealed that the composite sample shows stronger volumetric changes in the lattice parameters during charging to 4.8 V. In addition, X-ray absorption spectroscopy showed an incomplete Ni reduction process after the first discharge for the composite sample. From these results, it was shown that this leads to a more severe degradation in the composite material that affects Li⁺ intercalation in the subsequent discharge, thereby resulting in its poorer performance. Furthermore, to confirm these results, another LMR-NMC material with a different composition (having a Ni-poor core)-0.5Li₂MnO₃-0.5LiNi_0.33Mn_0.33Co_0.33O₂-was investigated. The core-shell structured positive electrode material also gave an improved electrochemical performance compared to the corresponding composite positive electrode material. These results show that the core-shell configuration could effectively be used to improve the performance of the LMR-NMC materials to enable future high-energy applications.

Spinel to Rock-Salt Transformation in High Entropy Oxides with Li Incorporation

High entropy oxides (HEOs) constitute a promising class of materials with possibly new and largely unexplored properties. The virtually infinite variety of compositions (multi-element approach) for a single-phase structure allows the tailoring of their physical properties and enables unprecedented materials design. Nevertheless, this level of versatility renders their characterization as well as the study of specific processes or reaction mechanisms challenging. In the present work, we report the structural and electrochemical behavior of different multi-cationic HEOs. Phase transformation from spinel to rock-salt was observed upon incorporation of monovalent Li+ ions, accompanied by partial oxidation of certain elements in the lattice. This transition was studied by X-ray diffraction, inductively coupled plasma-optical emission spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, and attenuated total reflection infrared spectroscopy. In addition, the redox behavior was probed using cyclic voltammetry. Especially, the lithiated rock-salt structure HEOs were found to exhibit potential for usage as negative and positive electrode materials in rechargeable lithium-ion batteries.

High electrochemical performance of 3D highly porous Zn0.2Ni0.8Co2O4 microspheres as an electrode material for electrochemical energy storage

3D highly porous Zn_0.2Ni_0.8Co₂O₄ microspheres unveil superior electrochemical energy storage properties.