Among the most challenging issues in electrochemical energy storage is developing insightful in situ probes of redox processes for a working cell. This is particularly true for cells that operate on the basis of chemical transformations such as Li−S and Li−O 2 , where the factors that govern capacity and cycling stability are difficult to access owing to the amorphous nature of the intermediate species. Here, we investigate cathodes for the Li−S cell comprised of sulfur-imbibed robust spherical carbon shells with tailored porosity that exhibit excellent cycling stability. Their highly regular nanoscale dimensions and thin carbon shells allow highly uniform electrochemical response and further enable direct monitoring of sulfur speciation within the cell over the entire redox range by operando X-ray absorption spectroscopy on the S K-edge. The results reveal the first detailed evidence of the mechanisms of sulfur redox chemistry on cycling, showing how sulfur fraction (under-utilization) and sulfide precipitation impact capacity. Such information is critical for promoting improvements in Li−S batteries. SECTION: Energy Conversion and Storage; Energy and Charge Transport

Combination of a solvent–salt complex [acetonitrile(ACN)2–LiTFSI] with a hydrofluoroether (HFE) co-solvent unveils a new class of Li–S battery electrolytes that possess essentially no solubility for lithium polysulfides, yet exhibit excellent capacity and very good rate capability..

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2−/Sen−) and (S2−/Sn−) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.

The influence of the initial Li/Co stoichiometry in LiCoO (LCO) (1.00 ≤ Li/Co ≤ 1.05) on the phase-transition mechanisms occurring at high voltage during lithium deintercalation ( V > 4.5 vs Li/Li) was investigated by in situ X-ray diffraction. Even if the excess Li in LiCoO does not hinder the formation of the H1-3 and O1 phases, the latter are obtained at higher voltages and exhibit larger c parameters compared with their analogues formed from LiCoO. We also showed that for the stoichiometric LiCoO the deintercalation process is more complex than already reported, with the formation of an intermediate structure between H1-3 and O1.

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The surface reactivity of Ni-rich layered transition metal oxides is instrumental to the performance of batteries based on these positive electrode materials. Most often, strong surface modifications are detailed with respect to a supposed ideal initial state. Here, we study the LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode material in its pristine state, hence before any contact with electrolyte or cycling, thanks to advanced microscopy and spectroscopy techniques to fully characterize its surface down to the nanometer scale. Scanning transmission electron microscopy–electron energy loss spectroscopy (STEM-EELS), solid-state nuclear magnetic resonance (SS-NMR), and X-ray photoelectron spectroscopy (XPS) are combined and correlated in an innovative manner. The results demonstrate that in usual storage conditions after synthesis, the extreme surface is already chemically different from the nominal values. In particular, nickel is found in a reduced state compared to the bulk value, and a Mn enrichment is determined in the first few nanometers of primary particles. Further exposition to humid air allows for quantifying the formed lithiated species per gram of active material, identifying their repartition and proposing a reaction path in relation with the instability of the surface.