Surface coating of cathode materials with AlO has been shown to be a promising method for cathode stabilization and improved cycling performance at high operating voltages. However, a detailed understanding on how coating process and cathode composition change the chemical composition, morphology, and distribution of coating within the cathode interface and bulk lattice is still missing. In this study, we use a wet-chemical method to synthesize a series of AlO-coated LiNiCoMnO and LiCoO cathodes treated under various annealing temperatures and a combination of structural characterization techniques to understand the composition, homogeneity, and morphology of the coating layer and the bulk cathode. Nuclear magnetic resonance and electron microscopy results reveal that the nature of the interface is highly dependent on the annealing temperature and cathode composition. For AlO-coated LiNiCoMnO, higher annealing temperature leads to more homogeneous and more closely attached coating on cathode materials, corresponding to better electrochemical performance. Lower AlO coating content is found to be helpful to further improve the initial capacity and cyclability, which can greatly outperform the pristine cathode material. For AlO-coated LiCoO, the incorporation of Al into the cathode lattice is observed after annealing at high temperatures, implying the transformation from "surface coatings" to "dopants", which is not observed for LiNiCoMnO. As a result, AlO-coated LiCoO annealed at higher temperature shows similar initial capacity but lower retention compared to that annealed at a lower temperature, due to the intercalation of surface alumina into the bulk layered structure forming a solid solution.

Triethlylphosphite (TEP) and tris(2,2,2-trifluoroethyl) phosphite (TTFP) have been evaluated as electrolyte additives for high-voltage Li-ion battery cells using a Ni-rich layered cathode material LiNi0.5Co0.2Mn0.3O2 (NCM523) and the conventional carbonate electrolyte. The repeated charge/discharge cycling for cells containing 1 wt % of these additives was performed using an NCM523/graphite full cell operated at the voltage window from 3.0-4.6 V. During the initial charge process, these additives decompose on the cathode surface at a lower oxidation potential than the baseline electrolyte. Impedance spectroscopy and post-test analyses indicate the formation of protective coatings by both additives on the cathode surface that prevent oxidative breakdown of the electrolyte. However, only TTFP containing cells demonstrate the improved capacity retention and Coulombic efficiency. For TEP, the protective coating is also formed, but low Li(+) ion mobility through the interphase layer results in inferior performance. These observations are rationalized through the inhibition of electrocatalytic centers present on the cathode surface and the formation of organophosphate deposits isolating the cathode surface from the electrolyte. The difference between the two phosphites clearly originates in the different properties of the resulting phosphate coatings, which may be in Li(+) ion conductivity through such materials.

A new class of electrolyte additives based on cyclic fluorinated phosphate esters was rationally designed and identified as being able to stabilize the surface of a LiNiMnCoO (NMC532) cathode when cycled at potentials higher than 4.6 V vs Li/Li. Cyclic fluorinated phosphates were designed to incorporate functionalities of various existing additives to maximize their utilization. The synthesis and characterization of these new additives are described and their electrochemical performance in a NMC532/graphite cell cycled between 4.6 and 3.0 V are investigated. With 1.0 wt % 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEOP) in the conventional electrolyte the NMC532/graphite cell exhibited much improved capacity retention compared to that without any additive. The additive is believed to form a passivation layer on the surface of the cathode via a sacrificial polymerization reaction as evidenced by X-ray photoelectron spectroscopy (XPS) and nuclear magnetic resonsance (NMR) analysis results. The rational pathway of a cathode-electrolyte-interface formation was proposed for this type of additive. Both experimental results and the mechanism hypothesis suggest the effectiveness of the additive stems from both the polymerizable cyclic ring and the electron-withdrawing fluorinated alkyl group in the phosphate molecular structure. The successful development of cyclic fluorinated phosphate additives demonstrated that this new functionality selection principle, by incorporating useful functionalities of various additives into one molecule, is an effective approach for the development of new additives.

Tris(trimethylsilyl) phosphite (TMSPi) has emerged as an useful electrolyte additive for lithium ion cells. This work examines the use of TMSPi and a structurally analogous compound, triethyl phosphite (TEPi), in LiNi 0.5 Mn 0.3 Co 0.2 O 2 -graphite full cells, containing a (baseline) electrolyte with 1.2 M LiPF 6 in EC:EMC (3:7 w/w) and operating between 3.0-4.4 V. Galvanostatic cycling data reveal a measurable difference in capacity fade between the TMSPi and TEPi cells. Furthermore, lower impedance rise is observed for the TMSPi cells, because of the formation of a P-and O-rich surface film on the positive electrode that was revealed by X-ray photoelectron spectroscopy data. Elemental analysis on negative electrodes harvested from cycled cells show lower contents of transition metal (TM) elements for the TMSPi cells than for the baseline and TEPi cells. Our findings indicate that removal of TMS groups from the central P-O core of the TMSPi additive enables formation of the oxide surface film. This film is able to block the generation of reactive TM-oxygen radical species, suppress hydrogen abstraction from the electrolyte solvent, and minimize oxidation reactions at the positive electrode-electrolyte interface. In contrast, oxidation of TEPi does not yield a protective positive electrode film, which results in inferior electrochemical performance. The electrolyte plays a vital role in lithium ion battery (LIB) cells -not only must it enable Li+ ion transport between the electrodes, it should also form passivating layers (such as the solid electrolyte interphase (SEI) on graphite), and remain stable under the oxidizing and reducing conditions at the positive and negative electrodes, respectively. While conventional carbonate-based electrolytes meet the requirements of commercial LIB cells, the next generation of cathode materials will require high-voltage operating conditions (>4.5 V vs. Li/Li + ) in order to meet the demands of higher energy and power. At these higher voltages, the conventional electrolytes experience severe oxidation leading to rapid performance loss and cell failure. Approaches to mitigate electrolyte oxidation at the positive electrode (cathode) include the (i) use of electrolytes with higher oxidation potentials which are intrinsically stable at higher voltages;

The stimuli--responsive properties of a series of aromatic conjugated monoalkoxynaphthalene--naphthalimide donor--acceptor dyads were studied. Two of the dyads, dyads 1 and 4, showed a difference in solid--state color between relatively faster (yellow) and slower (yellow--orange or orange) evaporation from solution, while the other dyads, dyad 2 and 3, only showed one color (yellow--green) for both evaporation rates. Importantly, highly solvatochromic dyad 4 dis-played thermochromic (orange to yellow), mechanochromic (orange to yellow) and vapochromic (yellow to orange) stim-uli--responsive behavior in the solid--state with repeatable cycles of color changing. Structural and spectroscopic studies indicated that the stimuli--responsive behavior of dyad 4 is the result of a 180° molecular rotation wherein the thermody-namically more stable head--to--head stacked orange crystalline solid interconverts with a head--to--tail stacked soft-crystalline yellow mesophase. The thermochromic transition of 4 from a presumably more stable crystalline state (or-ange) to a metastable soft crystalline mesophase state (yellow) that persists at room temperature unless exposed to sol-vent vapor is particularly noteworthy.

No abstract

Both experiments and theory show that TTFP does not passivate NMC532 cathode surface towards electrolyte oxidation.

The thermally induced conformational switching of a stacked dialkxoynaphthalene- naphthalenetetracarboxylic diimide (DAN-NDI) amphiphilic foldamer to an NDI-NDI fibril aggregate is described. The aggregated fibril structures were explored by UV-Vis, CD, AFM and TEM. Our findings indicate that the aromatic DAN-NDI interactions of the original foldamer undergoes transformation to a fibrillar assembly with aromatic NDI-NDI stacked interactions. These structural insights could help inform new molecular designs and increase our understanding of the fibrillar assembly and aggregation process in aqueous solution.